bach ho field, a fractured granitic basement …upper triassic – cretaceous plutonic rocks whose...

129Journal of Petroleum Geology, Vol. 32(2), April 2009, pp 129-156

© 2009 The Authors. Journal compilation © 2009 Scientific Press Ltd

BACH HO FIELD, A FRACTURED GRANITICBASEMENT RESERVOIR, CUU LONG BASIN,OFFSHORE SE VIETNAM: A “BURIED-HILL” PLAY

Trinh Xuan Cuong* and J. K. Warren**

A combination of seismic, wireline, FMI and core data shows that Bach Ho field in the Cuu LongBasin, offshore SE Vietnam, is an unusual “buried hill” reservoir. There is little or no production fromassociated siliciclastic “grus” or granite wash, and the fractured reservoir matrix is largely made upof unaltered acid igneous lithologies (mostly granites and granodiorites). A major NE-SW lateOligocene reverse fault system cross-cuts the field, with about 2000 m of lateral displacement inthe highly productive Central Block. The associated fracture meshwork greatly enhances reservoirquality. Transpressional wrench faulting in the late Oligocene in this part of the field emplaced ablock of brittle granitic rock on top of organic-rich Eocene – Oligocene mudstones, and facilitatedthe early migration of hydrocarbons into the fracture network.

Structure, not erosion, set up the 1000 m column of liquids in the fractured granodioriteswhich form the reservoir at Bach Ho. Faulted intervals with associated damage zones create anenhanced secondary porosity system in the granodiorite; effective porosities range from 3-5% andoccasionally up to 20%. Some associated fractures are partially blocked by authigenic calcite andkaolinite.

Features that degrade reservoir quality at Bach Ho include: (i) a thin, low-permeability clay-plugged “rind” created by surface-related (meteoric) Eocene – Oligocene weathering — this rindvariably overprints the uppermost 10-40 m of exposed basement throughout the Cuu Long Basin;and (ii) widespread hydrothermal cements which largely predate late Oligocene wrench faulting;cementation mostly took place during post-magmatic cooling and precipitated zeolites, carbonatesand silica in fractures which cut across both the igneous and the country rocks.

Porosity-occluding hydrothermal and authigenic precipitates developed in pre-existing fracturesin the Bach Ho granodiorite. These pre– late Oligocene mineral-filled fractures acted as zones ofstructural weakness during and after subsequent late Oligocene structural deformation. Togetherwith new fractures formed during thrusting, the older fractures may have reopened during thrustemplacement, and subsequent gravitational settling of, the Central Block.

*Exploration and Production Dept, Vietnam NationalOil Group, Petrovietnam, 22 Nyo Quyem St, Hanoi,Vietnam.** Shell Chair, Sultan Qaboos University, Muscat,Sultanate of Oman. previously : Dept. PetroleumGeoscience, Universiti Brunei. Present address:Chulalongkorn University, Bangkok, Thailand. author forcorrespondence: [email protected]

INTRODUCTION

Vietnam has produced almost 1 billion brls of crudeoil and 300 billion cu. ft of natural gas. The country’s

estimated resources, both on- and offshore, total some6.5-8.5 billion brls of oil and 75-100 trillion cu. ft ofgas (AAPG Explorer, February 2005). Most of theproduction in the Cuu Long Basin, offshore SEVietnam, has comes from fractured and weatheredgranitic basement reservoirs at fields including BachHo (White Tiger) (the largest field), Rang Dong(Aurora), Rong Tay (Dragon West) and Hong Ngoc(Ruby). Use of the term “basement” in this paper

Key words: Bach Ho, Cuu Long Basin, Vietnam, fracturedbasement, buried hill, basement high, Vung Tau, granite.

130 Bach Ho field, offshore SE Vietnam: A fractured basement reservoir

follows that of Landes (1960) who defined it as anycombination of metamorphic or igneous rocks(regardless of age), which are unconformably overlainby a sedimentary sequence. North (1990; p. 216) gavea broader definition of “basement” that includes rockswith a sedimentary origin, providing they haveessentially little or no matrix porosity.

The discovery well for Bach Ho was drilled in 1975by Mobil Oil Co. but the company left Vietnam in thesame year. The Bach Ho “buried-hill” was notdeveloped until the mid-1980s by Vietsovpetro. Today,the field’s average daily production (about 140,000b/d) is declining; in 2008 it was about 25,000 b/d lessthan in the previous year (www.upstreamonline.com/live/article169696.ece). This rate of decline issomewhat higher than that predicted by Vietsovpetro,which envisaged an annual decline of about 20,000b/d for the period 2010-2014, but it can perhaps beexpected in a field producing from an open fracturesystem with little matrix storage.

At Bach Ho, matrix between hydrocarbon-filledfractures is largely composed of unaltered igneous andmetamorphic rocks. Some production wells in

fractured granodiorite in the Central Block of the fieldproduce as much as 15,000 b/d, although rates of2000-5000 b/d are more typical (Cuong, 2000; Dienet al., 1998). Matrix storage in the reservoir is low tonon-existent, yet near-constant production rates havebeen maintained for more than five years, implyingsignificant recharge to the fracture network. To date,the fine-grained Tertiary fluvio-deltaic sedimentaryrocks which encase the productive basement blockhave not produced sufficient volumes of hydrocarbonsto make them a target.

Not all basement highs in the Cuu Long Basin havesuch outstanding reservoir and productioncharacteristics. A contrast to the success of Bach Hois given by the development and production historyof Dai Hung (Big Bear) field, where hydrocarbonsare hosted in a similar granite/granodiorite matrix. In1993, BHP led a consortium that won the bid todevelop Dai Hung and predicted that the field couldproduce up to 250,000 b/d. By the mid-1990s, some25,000 b/d were being produced but then outputdeclined rapidly and BHP left the consortium in 1997announcing that the field was not profitable. Petronas

Fig. 1. Generalized map of the major Cenozoic sedimentary basins and current tectonic elements in theSouth China (East Vietnam) Sea.

0 200

km

Basin boundary

Major fault

reading ridge

Extensional crustal margin

Song Hong

Basin

Mae Ping FaultDong Nai Fault

Con Son High Nam Con Son Basin

3 Pagodas Fault

Ho Chi Minh City

Vung Tau

Red River FaultN

Cuu LongBasin

131Trinh Xuan Cuong and J. K. Warren

then took over as operator but failed to raise outputbeyond 10,000-15,000 b/d and left in 1999. In 2000,Vietsovpetro (the operator of Bach Ho and Rongfields) took over Dai Hung but was only able toproduce some 5400 b/d.

There is therefore a need to investigate the geologyof Bach Ho field and to understand why successfuleconomic production comes from this unusualreservoir. Previous studies of the granitic basementreservoirs in the Cuu Long Basin include papers byAreshev et al. (1992), Dmitriyevskiy et al. (1993),Tran et al. (1994), Koen (1995), Tuan et al. (1996),Dong (1996), Dong et al. (1996, 1997, 1999), Dongand Kireev (1998), Quy et al. (1997), Tandom et al.(1997), Tjiia et al. (1998), and Vinh (1999). However,these studies are based on relatively old data orfocused on particular areas of research. In this paper,

we attempt to integrate available data in order tocompile a predictive exploration model for the BachHo type of “buried-hill” play. We attempt to evaluatefracture distribution in the basement reservoir and toassess factors influencing reservoir quality.

REGIONAL STRUCTURE ANDSTRATIGRAPHY OF BACH HO FIELD

The Cuu Long Basin is filled with 6 to 8 km ofsedimentary rocks which overlie pre-Tertiarybasement. The sedimentary fill rests on an undulatingbasement surface which divides the basin into a seriesof sub-basins trending parallel to the main basin axis(Fig. 2) (Hall and Morley, 2004; Lee et al., 2001).The pre-Tertiary basement is mostly composed ofUpper Triassic – Cretaceous plutonic rocks whose

B.Middle-Late Miocene

Early Miocene

Eocene

15G-1X15C-1XBH-4BD-1XTD-1XDD-1X

0200040006000 D

epth

(m)

10° 00’

9° 40’

107° 20’ 107° 40’ 108° 00’

R-4

BV

10° 20’

Isodepth (km)

Fault

Well

107° 00’

Bach Ho Basement

0.5

1

1.5

20.5 3

5

6

5

7

5

0 25km

A.

R-3

1

1.5

1.2

1

0.5

21.5

3

23

4

65.5

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15C-1x

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BH-4

BH-17 8

6

BH-12BH-10

6.3

86.56

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54

332.52

3

4.5443

2.5

2.52

22

32.5

3

342

2.5

314

3

2.5

45 5.5

66

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5.5

5

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53

4

4.5 4.5

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4

32

2.5

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1.5

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6

54

321.5

3.5

3.5

3.2

65.5

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3.2

108° 20’

221.5

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3

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4 5 4 56

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3.5 432

3.2

1

Fig. 2. Basement distribution in the Bach Ho region, Cuu Long Basin. (A) Top of basement structure map;rectangle indicates position of the Bach Ho oilfield and the various shaded areas indicate basement highs.(B) Regional geological section showing variation in basement depth along line indicated by dashed line in A.(after Que, 1994).


composition ranges form basic (diorite) to acidic(granite) (Table 1; Dong and Kireev, 1998).

The shallowest part of the highly fractured “buried-hill” at Bach Ho lies at a depth of 3050 m (Fig. 2),while the effective base for liquids production in thefield is typically shallower than 4650 m. However, thedeepest producing interval in the field is a fracturedzone at 5013 m, with a flow rate around 100 b/d at atemperature of 162oC. Productive intervals in Bach Howells are invariably fractured zones.

Production data indicates that Bach Ho field is madeup of a number of separate basement blocks withdiffering hydrodynamic properties. For productionpurposes, the field is divided into the Northern, Centraland Southern regions. Communication betweenfractured basement and onlapping sandstones is notseen in current production and pressure data.

The current geothermal gradient at Bach Ho fieldincreases with formation age. It is 2.2-2.5oC/100m inQuaternary – middle Miocene sediments; 3.4oC/100min the lower Miocene and Oligocene; and 3.9oC/100min the basement. Temperatures in the basement atdepths of 3-4 km range from 120 to 165oC. It has beenobserved that 90°C formation waters from Bach Hocontain thermophilic sulphate-reducing archaea/bacteria (Rozanova et al., 2001). Geothermal gradients

were locally higher in the Mesozoic and early Tertiaryduring magmatism.

Tectonic evolutionThe structural evolution of the extensional Cuu LongBasin can be explained in terms of (Figs 1, 2): (i) thepropagation of the South China (East Vietnam) Searift around 17-32 Ma (Taylor and Hayes, 1983;Hutchison, 2004), and (ii) the collision of India andEurasia beginning in the Palaeogene, which drovethe extrusion of Indochina along the Mae Ping andRed River Faults (Tapponier et al., 1982, 1986),although more recent studies have shown thatcollision-driven strike-slip extrusion, and itsinfluence on basement tectonics throughoutIndochina (including offshore Vietnam) has probablybeen over-emphasized (Morley, 2004; Hall andMorley, 2004; Searle, 2006). Local variations intectonic activity have had a significant influence onbasement structure in the Cuu Long and Nam ConSon Basins, resulting in subsidence and uplift withdifferent intensities and styles at various times sincethe Cretaceous. Hall and Morley (2204.) concludedthat the basement “grain” in northern Sundaland(including offshore Vietnam) is not related to theHimalayan collision but is due to localised collisional

Geologic Age

Period Ma Formation

MIO

CE

NE

OLI

GO

CE

NE

EOCENE

QUAT.

PLIOCENE

LATE

MID

DLE

EA

RLY

LATE

EA

RLY

10.4

16.3

23.3

29.3

56.5

35.4

5.2

1.64

Stratigraphy

Lithology Thick(m)

Seis.Hor.

200 -

2700

PalaeobathymetricChange

Structure

HydrocarbonAssociations

TectonicsOpenmarine Lacustrine

Sourcerock

Reser-voir

RIF

TED

SE

DIM

EN

TARY

BA

SIN

Seal

Con

tinen

t.sh

elf c

over

RE

AC

TIV

E

Pos

trift

trou

gh

DIV

ER

GE

NC

E

Syn

rift

grab

en

Ther

mal

sag

Rift

ing

pull

apar

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PR

ER

IFT

CO

NV

ER

GE

NC

EO

F C

ON

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EN

TS

Description

Only in the western part,dominated by interbedded conglomerates and medium-grained sandstones, minor mudstones.

Predominantly lacustrine sediments, mostly in the centre and the east of the basin. Sediments are mostly black mudstones intercalated with grey-brown, fine to medium-grained sandstones. In the west, where fluvial deposition dominated, sediments show a higher percentage of sand and coarser grain sizes. Volcaniclastics are also common in this unit.

Mainly shallow marine with a strong fluvial influence in its lower part. Mostly sandstones, interbedded with grey, green or brown mudstones. It is absent in the west of the basin. Its upper part (the Rotalia Bed) is composed of green mudstones.

Coastal setting with fluvial dominance. Mostly in the basin centre and in east where coarse and medium-gained sandstones alternate with grey mudstones and several interbeds of lignite.

Coastal environment to the west, river-mouth and shallow marine sedimentation in the basin center and to the east. Mostly fine to medium-grained sandstones, intercalated with grey siltstones and mudstones.

Fine-grained marine sands interbedded with siltstones

BA

SE

ME

NT

CO

MP

LEX

Mostly hydrothermally-altered Late Cretaceous granites, lesser Late Triassic & Late Jurassic intrusives (basic and weakly acidic). Together with the generation of the Late Cretaceous granite, some andesitic and dacitic dykes were emplaced with patterns controlled by conjugate faults and fractures. Country rocks were metamorphosed by both mechanical and thermal processes especially during Late Cretaceous.

LATE

ME

SO

ZOIC

CO

NTI

NE

NTA

LE

NV

IRO

NM

EN

T

HE

TER

OG

EN

OU

SB

AS

EM

EN

T

100 -

300

C

B1

B2

B3

B4

800 -

1400

600 -

900

500 -

600

400 -

700

TRACU

TRATAN

BACHHO

CONSON

DONGNAI

BIENDONG

Fig. 3. Cuu Long Basin stratigraphy (after Dien et al., 1998).


events which preceded the main India–Asia collision.These smaller-scale events included left-lateralmovements on the Mae Ping and Three PagodasFaults. Initial movement on some of the major faultswhich are still active at the present day therefore tookplace in the Cretaceous, and not all movements weredriven by the mid- to late Tertiary uplift of the TibetanPlateau.

The Cuu Long Basin is located along-strike fromthe youngest spreading centre in the South China (EastVietnam) Sea. Since spreading began at around 32Ma, its dominantly SW propagation has influencedstructural grain in the Cuu Long Basin basement (Fig.2; Hutchison 2004). The regional structural grain isNW–SE to north-south (Hall and Morley, 2004;Morley, 2004). Extension-derived stresses from thisspreading centre may still result in fault movementsin the Cuu Long Basin (Shi et al., 2003).

Stratigraphic evolution of the Bach Ho coverThe sedimentary succession at Bach Ho comprisesEocene, Oligocene and lower Miocene deposits (Fig.

3). Sands in the mud-dominated Eocene and lowerOligocene succession are mostly immature arkoses,with lesser lithic-arkoses and some feldspathiclitharenites deposited in alluvial/fluvial and lacustrineenvironments. Upper Oligocene sediments are fine-grained lacustrine muds, while the overlying lowerMiocene succession passes from muddy delta-frontdeposits up into marine shales (Que, 1994).Sandstones in the lower Oligocene and lower Miocenesuccessions are possible secondary reservoir targetsat Bach Ho. Late Oligocene lacustrine and lowerMiocene marine shales constitute regional seals. Oilsthroughout the Cuu Long Basin are sourced fromlacustrine muds (ten Haven, 1996; Reid, 1997). Sourcerocks in Bach Ho are present in the Eocene andOligocene intervals (Canh et al., 1994; Areshev et al.,1992), with the upper Oligocene being the mostprolific (Hung et al., 2003). None of the hydrocarbonsoffshore Vietnam are derived from a deep mantlesource, contrary to a suggestion which was madeduring early field development studies (Dmitriyevskiyet al., 1993).

Fig. 4. Character of Bach Ho basement.(A, above). Classification of acid igneous rocks in thebasement of well BH-12 (see Fig. 6 for well location).Open circles indicate core samples, solid circlesindicates side-wall core samples.(B, right). Cores of typical fractured granodioritebasement. Sub-vertical tectonic fractures (0.1-1.5cm wide) are filled with zeolites, silica and calcite.There is residual oil in some of the fractures(appears brown and is indicated by red arrow).Drilling-induced fractures (light blue arrow) are alsopresent. Scale bars are 5 cm long. These cores comefrom deeper intervals with few open fractures,hence the higher recovery efficiency.


Igneous and hydrothermal evolutionWidespread Mesozoic plutonism on- and offshore SEVietnam was dominated by Jurassic – Late Cretaceousgranites and granodiorites (Hall and Morley, 2004).At a regional scale, plutonism was driven by theNWward subduction of the Proto-Pacific Plate beneathEast Asia. Lateral forces associated with the northwardmovement of India gave rise to stress changes withinIndo-china which resulted in crustal thickening andpluton emplacement in Vietnam and Thailand (Morley,2004; Hutchinson, 2004).

At Bach Ho, three phases of igneous intrusion arerecognised (Late Triassic, Late Jurassic and LateCretaceous) and correspond to the Hon Khoai, DinhQuan and Ca Na complexes, respectively (Table 1)(Areshev et al. 1992; Dong and Kireev, 1998; Morley,2004). Late Triassic and Late Jurassic intrusives consistof basic and weakly acidic rocks. These are a minorcomponent of the Bach Ho basement and occur mainlyin the northern part of the field. Elsewhere, thebasement is composed of Late Cretaceous acidic rocks(mostly granitic) which are characterised by their brittlebehaviour under stress (Fig. 4a). Andesitic and daciticdykes were emplaced with orientations controlled byconjugate faults and fractures. Thermal haloesassociated with each phase of magma emplacementaltered country rocks into greenschist meta-sediments.Cements precipitated during associated hydrothermalcirculation tended to occlude any porosity in thegranites and adjacent Mesozoic sediments (Fig. 4b).During subsequent episodes of tectonism, regions of

hydrothermally-cemented and fractured zones tendedto be weaker than adjacent zones dominated byunaltered and unfractured granites.

Bach Ho Basement – seismic characterSeismic responses can be used to divide the Bach Hobasement into three depth-related seismic zones (Fig.5, line positions shown in Fig. 6; see also theschematic diagram in Fig. 16). Interval velocities inthe upper, middle and lower zones typically rangefrom 3500 to 3800 m/s, 3900 to 5200 m/s and 5000to 5500m/s, respectively.

The upper zone is characterized by high continuityof the basement-top reflector immediately below thepurple reflector in all three lines in Fig. 5. Its characteris created by the sharp acoustic impedance contrastbetween the weathered top of the basement, theoverlying fine-grained sediments and the underlyingfractured igneous basement. This upper zonesignature is best developed at the highest parts of theBach Ho structure and loses thickness and definitionaway from the crest.

Below is a zone characterized by groups of lowerfrequency, higher amplitude and lower continuitysignatures within a chaotic reflection background (theapproximate vertical extent of this zone is indicatedby the yellow arrows in Fig. 5a-c). Its presence in allthe Bach Ho seismic lines implies that there arelaterally-developed velocity contrasts between therelatively brecciated and altered zones (or dykes) inthe upper part of the basement high and the

Table 1. Primary minerals, composition and K-Ar age ranges in igneous rocks that constitute the reservoir inBach Ho field, offshore Vietnam (After Dong and Kireev, 1998). See Fig. 6 for the location of the Central andNorthern blocks.

Oligoclase andesine

Oligoclase andesine AndesineDominant type

of plagioclase Oligoclase Oligoclase andesine

Oligoclase andesine

0-2 0-2 -

Hastingsite - - - Jun-18 Jun-18 05-Oct

Hornblende - 0-3 0-2

Oct-13 May-13 13-15

Muscovite 0-3 - - - - -

Biotite 02-Oct 06-Oct 05-Oct

30-45 Oct-27 03-Dec

Quartz 25-35 18-24 Dec-17 May-18 May-17 <5

K-feldspar 20-40 14-28 17-28

Quartz biotite

monzonite

Plagioclase 25-40 40-54 47-58 30-40 43-52 62-68

GranodioriteBiotite quartz

monzonite

Quartz biotite

monzonite

Biotite quartz monzonite

Central Block Centre of Northern Block Eastern part of the

Northern BlockCa-Na

complex (108-115

Ma)

Dinh Quan complex

(135-154 Ma)

Hon Khoai complex

(194-261 Ma)

Granite


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surrounding granitic rocks. There is a suggestion ofpervasive faulting in this interval (indicated by thered lines in Fig. 5). but individual faults cannot belocated accurately due to the relatively low resolutionof the seismic data.

Seismic attribute contrasts in lines 1 and 2, andcomparison with outcrop analogues at Vung Tau (seebelow), imply that the upper part of the Bach Ho highis more altered and brecciated compared to theunderlying zones. Cores and sidewall cores show thelower part of the high are more weakly altered and,although fractures and breccias are still common, theyare typically occluded and cemented by hydrothermalminerals (Fig. 4b). An underlying third zone (beneaththe yellow arrows in Fig. 5a-c) is characterized by

chaotic reflections and is thought to possess few openfractures; it is not considered to be economic.

Seismic, image logs and core data can be used todivide the Bach Ho basement into three blocks (North,Central, and South), each bounded NE-SW faultsegments (Fig. 6). The reverse fault to NW of theCentral Block locally has a lateral displacement of upto 2000 m (Dong, 1996). The “chaotic” seismic zone,described above as indicating a high degree offracturing and deformation, is particularly associatedwith this reverse fault (see Figs 5a, b). The reversefault dies out along strike and passes into a linkednormal fault to both NE and SW (Fig. 6). Thisrelationship implies that the reverse-faulted marginof the Central Block is an inverted portion of a pre-

Fig. 6. Main fault and fracture system in Bach Ho field (after Cuong, 2000). The highest basement, shown byyellow and green shading, is in the Central Block adjacent to the region with most obvious displacement onthe reverse fault (approx. 2000 m; see line 1 in Fig. 5). The Central Block shows the most intense deformationand wells here have higher production capacities.


existing normal fault. FMI analyses (see below) showthat fracture intensity is greatest in the Central Block.Production wells in the Central Block consistentlyshow production rates as high as 4000-6000 b/d.

By contrast, the seismic profiles indicate thepresence of normal faults along the SE flank of thefield, dipping to the south and SE. Seismic characterof the basement in this part of the field is characterizedby lower amplitudes, and reservoir quality is lowercompared to the NW margin of the Central Block.

Most fault sets at Bach Ho, as interpreted fromseismic profiles, are confined to the basement and donot appear to continue into the overlying sedimentarycover; nor can they be resolved in deeper parts of thebasement (Fig. 5a-c). The lower transitional zones ofboth the weathering profile and the faulted interval inthe basement are difficult to define on seismic profiles.Comparison with the Vung Tau outcrops (see below)suggest that the lower transition of the basementreservoir zone follows an irregular boundary, probablycontrolled by variations in fracture intensity in thebasement.

Seismic analyses indicates a significant associationbetween late Oligocene reverse faulting and the BachHo Central Block. NW displacement of up to 2000 mon this fault emplaced a block of igneous basementon top of Tertiary mudstones. Subsequent gravitationalcollapse of the block and stress relaxation served bothto open uncemented fractures within the block and toreopen pre-existing zeolite- and kaolinite-filledfractures. Intense fracturing of basement rocks in theCentral Block and enhanced reservoir characteristicsare reflected in the high well outputs.

THE VUNG TAU GRANITES:ANALOGUES FOR BACH HO BASEMENT

Triassic-Cretaceous granites and granodiorites outcropon the coast near Vung Tau, about 100 km SE of HoChi Minh City (Fig. 1). Weathered outliers can befound up 0.5-1km away from the main outcrops whichextend for several km along the coast. A number ofgranite quarries and road cuts were visited, mappedand photographed in order to better understandfracture style, timing and host lithologies. The outcropstudies defined a number of fracture styles andexposure-related features which are discussed in thefollowing paragraphs.

Contractional and surface-related fracturesStress-release sheet fractures (exfoliation fractures)in granodiorites at the Big Mountain location, VungTau, are sub-parallel to the topography of the basementblock. Dip angles tend to be low – sub-horizontal to10-15° (Fig. 7a). Near the present-day land surface,the spacing between stress-release sheet fractures

ranges from several centimetres to 3 m and increasesprogressively with depth.

Exfoliation fractures form in the shallowsubsurface as a result of stress release as a plutonexhumes and associated overburden stresses firstdecrease then disappear (e.g. Younes et al., 1998).The fractures are formed due to changes in thermalstress during annual or daily ambient temperaturecycles in combination with meteoric-drivenweakening of matrix strength due to mineraltransformations (Holzhausen, 1989). The lateral extentof individual sheet fractures could not be measured atthe Vung Tau exposures. Nermat-Nasser and Horri(1992) noted that open sheet fractures in granites canreach widths of 200m. Tandom et al. (1997) showedthat at depths of up to 200-250 m, exfoliation sheetfractures in an igneous host can be 25-30 m or moreapart. By analogy, sub-horizontal exfoliation fracturesmay therefore be present at Bach Ho in the top 50-100 m of basement rocks.

At the Vung Tau outcrops, exfoliation fractureswere observed to become blocked at depths of 10-30m below the surface with fine clay soils and otherweathering products including kaolin and Fe-oxides.The contribution of these types of fracture to reservoirporosity at Bach Ho will therefore probably be low.

Sub-horizontal features (“temperature reductionfractures”) can also form as a magma body cools andcontracts. The intensity of this type of fracturing varieswith the relative proportions of quartz, K-feldspar andplagioclase minerals. Contractional fractures are morelikely to occur in acidic rocks with elevatedproportions of quartz (Osipove, 1974). Rocks withminor quartz but more feldspar (quartz-monzonitesor diorites) show less volume shrinkage and hencefewer contractional fractures. The acidic lithologieswhich dominate the Central Block at Bach Ho willbe susceptible to contractional fracturing. However,the contribution of early thermal reduction fracturesto effective primary porosity will probably be low. Atthe Vung Tau outcrops, these fracture types wereobserved to be filled by hydrothermal mineral cementssuch as zeolites or coarse calcite spar.

Tectonic fracturesTectonic fractures in the Vung Tau outcrops dip atangles of 50-90o and are typically composed of cross-cutting conjugate sets (Fig. 7a, b). Dominantorientations are NE-SW and north-south; the NE-SWtrend is aligned with the trends of the regional DongNai and Mae Ping faults, and parallels the regionalstructural “grain” associated with opening of the SouthChina Sea (Morley, 2004; Hall and Morley, 2004; Tjiiaet al., 1998; Dong et al., 1999).

At a quarry at Round Mountain near Vung Tau,tectonically-induced fracture apertures are mostly mm-


Fig. 7. Field photographs of basement outcrops at Vung Tau (see Fig. 1 for location)(A) A weathered zone, 2-5 m thick, overlies the top of basement rocks at Small Mountain, Vung Tau. Sub-vertical (red) and sub-horizontal (green) fractures with varying densities occur beneath the weathered zone.Sub-horizontal fractures are a combination of sheet and exfoliation fractures with fracture spacing varyingfrom 0.05-0.50m (yellow arrows) and 0.5-3.0m (light blue arrows).(B) Basement rock (at Small Mountain, Vung Tau) is deformed by tectonic fractures showing high dip angles(red). A narrow igneous dyke (ca.0.8m wide) cuts across the basement. The dyke runs along a fault andseparates hanging- and foot-walls. Near the dyke, fractures are mostly parallel to the dyke margin and thereare vugs or aperture fractures at the contact with the country rocks. Rocks to the left of the dyke are morehighly fractured than those to the right.(C) Plan view of basement (at Small Mountain, Vung Tau) showing conjugate, relatively steeply-inclinedfractures.(D) A weathered zone (0.5 to 2.0 m thick) caps the basement rocks which are cross-cut by conjugate, steeplydipping fractures and faults (Small Mountain, Vung Tau).(E) Close-up of Fig. 7d showing deformation and brecciation in the fault damage zone, especially at theintersection of two faults where a cave has formed (indicated by white arrow). Numerous steeply inclinedfractures are associated with the breccia. There are two fracture sets: a tectonic (sub-vertical) set and arelease (sub-horizontal) set.(F) Close-up of breccia in Fig. 7e, showing composition of broken basement fragments (0.3 to 3.0 cm across)in a matrix dominated by the products of weathering and diagenesis. There are two sets of fractures: a steeplyinclined tectonic set and a set of sub-horizontal release fractures.


scale but range up to 1-2 cm wide, with rare aperturesup to tens of cm across. About 30% of measuredfracture apertures were between 0.2 cm and 1.0 cm.Most fractures were filled, or partially filled, byvarying combinations of modified magma andhydrothermal precipitates and clays. They areinterpreted as outcrop analogues of the mineral-filledfractures illustrated in Fig. 4b.

Fault damage zone widths ranged from tens ofdecimetres to several metres, depending on theassociated fault displacement (Fig. 7b, f). The intensityof fault-related fracturing varies from very high nearto major faults (100-150 fractures/metre), anddecreases at greater distances (e.g. 35-50 fractures/metre at a distance of 1.5 m from the fault in Fig. 8).Major shear zones with high associated fractureintensities are up to 35 m or more wide at Vung Tau(Tjiia et al., 1998).

Igneous dykesAndesitic and rhyolitic dykes cut across the granitesand granodiorites exposed along the Vung Tau coast(Fig. 7b). The dykes are in general oriented parallelto nearby faults and fractures, with widths varyingfrom 0.5m to several metres. The dykes arecharacterized by higher densities of fractures (about5-10 cm spacing) than the surrounding granites (10-50 cm spacing). Fracturing in the dykes is typicallyoriented parallel to the dyke axis. Mineral-filled (clayand zeolite) mm-cm scale vugs and contractionalfractures occurred at contacts between dykes and

adjacent granitic rocks. Analogous features in theBach Ho subsurface contained zeolites and otherhydrothermal products. Oil staining in such fracturesindicated later re-opening. The mineral-filled fractureconstituted zones of relative structural weaknesscompared to the adjacent granodiorite (Fig. 4b).

Weathered zonesA weathered and lateritized soil profile blankets theVung Tau outcrops, and soil-related muds infiltrateexfoliation, tectonic and contractional fractures (Fig.7a, d). Soil penetrations into open fractures, andassociated weathering haloes, constitute a zone up to40-50 m thick on the flanks of Vung Tau outcropblocks; the zone is 10-20 m thick over the crests ofthe blocks. Inland from the coast, in areas where soilsare less well developed, weathered and bleachedgranite domes up to tens of metres across are exposedsubaerially. Basment blocks are Bach Ho werelikewise subaerially exposed. By analogy with theVung Tau outcrops, weathering and the infiltration ofclay-sized fines will have occluded open fractures inthe subsurface (Vinh, 1999).

FRACTURE CHARACTER IN BACH HOBASED ON IMAGE LOGS

Image log analysis at Bach Ho detected severalfracture types and their interpretation was aided bycalibration against outcrop analogues (Fig. 7) andcore. An interpreted image suite from well BH-12 is

Fig. 8. Fault-damage fracture map showing distribution of fracture density in a brecciated fault zone, based onmeasured fractured separations collected from the Vung Tau outcrop at sites shown in Figs 7e, f. The faultcentre consists of a breccia (0.1 – 10m wide; Fig. 7f), while zones of high fracture density occur adjacent to thebrecciated zone (10-35 m; Fig. 7e). Mineral precipitates, and less commonly sediments, fill or partially filldissolution voids equivalent to those in the subsurface but more labile minerals are leached at outcrop.


shown in Fig. 9 (fracture identification follows theclassification scheme of Schlumberger, 1992, 1997a,b). This interval illustrates the utility of running acombination of imaging tools (DSI, FMI, UBI, ARI)when defining the nature of fractures intersecting aborehole.

The FMI (Fullbore Formation MicroImager) toolproduces a high-resolution microresistivity image ofthe borehole wall, covering 80% of the wall in an 8.5-inch borehole. Each microconductivity button can readup to 1 cm into the formation and in combination withadjacent buttons is capable of distinguishingconductivity anomalies smaller than the tool’seffective resolution of 0.2 cm. Of currently availableimaging tools, the FMI tool provides the most detailfor analysis of the Bach Ho reservoir. It sees mostnatural and induced open fractures intersecting theborehole wall together with matrix variations in bulkformation textures, but is has problems seeing fineropen fractures and in separating open from closedmineral-lined fractures at the sub-millimetre scale.

The DSI (Dipole Shear Sonic Imager or Stoneleywave) tool, in contrast to the FMI tool, has a 15 cm(6-inch) depth of investigation and a vertical resolutionof 1 m. The presence of open fractures strongly affectsthe waveform, so this tool is better suited to locatingbroader-scale intervals (m-scale) characterized bynumerous open fractures.

The ARI (Azimuthal Resistivity Imager) toolrecords lateral resistivity up to 60 cm (24 inches) intothe formation. The increased depth of investigationattained when using a combination of DSI and ARIimages allows for intervals with numerous openfractures to be distinguished from intervals withnumerous but perhaps smaller, probably cemented andunproductive fractures. As with the FMI images, it ispossible to orient fractures picked from ARI images(Fig. 10).

The UBI (Ultrasonic Borehole Imager) tool is anacoustic imaging log providing a 360° scan of theborehole wall. Although it has a lower resolution thanthe FMI tool, the full-bore coverage of the UBI toolprovides a detailed caliper of the borehole, and canseparate larger mineral-lined and closed from openfractures. This is invaluable in confirming fracturedintervals seen in FMI, and is also useful for stressanalysis and in confirming good hole conditions forsetting MDT packers.

The following discussion, together with the resultsshown in Fig. 10 and the individual well fracture rosesillustrated in Fig. 6, are based on the application ofthese imaging tools to the Bach Ho reservoir. Muchof the image-to-rock calibration was based on adetailed core study of well BH 12 (Fig. 11), this wasalso the well where a complete multi-image log suitewas run across the reservoir (Fig. 9).

Natural fracturesContinuous fracturesThese fractures show a continuous conductiveresponse in the electrical image (e.g. 3705 m in wellBH-12: Fig. 9). They also have a strong correspondingStoneley wave reflection coefficient. The amount ofenergy lost in the fracture is thought to correspond tohow open the fracture is (Horby and Luthi, 1992).However, zones with extremely rugose or washed-out profiles can generate similar acoustic lossanomalies (above 3590 m).

Discontinuous fracturesPartly-filled mineralized fractures generatediscontinuities which in the image log appear as onlypartly conductive intervals (e.g. 3715m in Fig. 9). Thisstyle of fracture development is not laterally extensiveand does not drain as effectively as a grouping ofcontinuous fractures intersecting the well bore.

Boundary fracturesThese fractures are normally associated with changesin the lithology and/or texture of the rock (e.g. 3696m, 3719.5 m, 3722 m in Fig. 9).

Brecciated fracturesThese are commonly associated with major faults,creating brecciated zones with polygonal textures onthe electrical image and more conductive signaturesin the deep resistivity log (e.g. 3690-3696m in Fig.9). The thickness of the brecciated zones can varyfrom several decimetres to tens of metres. In theStoneley waveform image, these zones show obviousintersections of downgoing and upgoing wave fields(e.g. 3719.5-3722m in Fig. 9). Ties to resistivity logsand production data show that these brecciated zoneswithin the basement reservoir act as significanthydrocarbon storage systems and have highpermeability. In well BH-12, major breccia intervalsoccur at 3597-3604 m, 3617-3625 m and 3704-3723m.

Vuggy fracturesStrong Stoneley wave and corresponding lowresistivity responses can be used to indicate the likelypresence of vugs in the reservoir. They can occur inunaltered granite but are more common withinfractured and altered zones (e.g. 3705m in Fig. 9).Typically they form the large cm-scale “cavities,” asseen in the FMI and UBI, but these logs indicateresistivity or acoustic contrast and so are indirectindicators of fracture style. The logs may beresponding to mineral fills, not fluid content. Even ifthe vugs are fluid-filled, they may be isolated and sucha vuggy zone may not be capable of generatingsubstantial fluid interconnection to the well bore.


Fig. 9. Interpreted image log suite from 3690 – 3730 m in well BH-12, showing naturally induced fractures andbreakouts, togetgher with dykes and brecciated zones (see Fig. 6 for well location).

FaultsFaults are often indicated by associated rapid increasesin fracture intensity, truncation features or brecciatedzones on the image log, as can be expected byanaology with observations from Vung Tau (Fig. 7).

Drilling induced fractures:Borehole enlargements are a common feature alongthe axis in well BH-12 and other wells at Bach Hoand are obvious in the caliper response. They arerelated to stress release failure, and FMI shows they

are typically parallel to the least principal horizontalstress. Long, straight drill-induced fractures areusually perpendicular to the direction of boreholeenlargement, as seen in the image logs (e.g. 3725-3730m in Fig. 9). Recognizing induced fractures andborehole breakout is invaluable in determining thecurrent orientation of the principal stress field. Suddenrotations of the image tool travelling across basementsections indicates that principal stress directions varywith reservoir depth and reflect the complex tectonichistory of the basement.


Fig. 10. Fracture pattern and distribution from FMI/FMS interpretations of wells in Bach Ho field. Dip anglesvary from 20 to 45o in Group 1 fractures (release and exfoliation) and 45 to 75o in Group 2 fractures(tectonic). Dominant strike angles vary from block to block across the field (see Fig. 6 for fracture roses forthis data).

Enhanced fracturesIn addition to natural fractures, pre-existing fractureswhich are extended or reopened in the borehole bydrilling are classified as “enhanced fractures”(Standen, 1991). It is difficult to differentiate enhancedfrom other fractures using image and conventionallogs. For example, possible enhanced fractures mayoccur below a depth of 3750m in FMI images in wellBH-12, but production test data shows they do notaffect the volume or rate of hydrocarbon productionin this interval.

Fracture identification using conventional logsFracture identification using conventional logs isimportant in Bach Ho wells where there are no imagelogs. Many authors have discussed the use ofconventional logs for fracture detection (e.g. Stearnsand Friedman, 1972; Aguilera, 1993; Ershaghi, 1995;Schlumberger, 1997b). In well BH-12, image logscould be used for fracture calibration and verificationof the interpretations arising on conventional logapproaches (Figs 11, 12, 13). The vertical resolutionof conventional logs is typically lower than that of


3599.0

3598.0

3597.0

3601.0

3600.0

DepthLi

thol

ogy

Sam

ple

Mas

sive

Schi

stos

e

Coa

rse

Med

ium

Fine

Very

fine

Go o

dM

oder

a te

Poor

Crystal size Porosity Struct.

Description

Schist: Dark grey to lightgrey, strong schistoselayering, very fine to finelycrystalline, hard. Bulk of therock is crosscut by fracturemeshworks, usually filled byzeolites, quartz, and calcite.White to white pinkish grey,friable. Locally, there aresome caverns of 1 mm to 3mm width, which are partiallyfilled by zeolites and othervein minerals. Abundantwhitish feldspar, black biotite.Very finely crystallineyellowish pyrite is common.Intense fracturing. Size offractures is variable, rangingfrom 0.05 mm to 1-2 mm inwidth and commonly > 1-5mm in length. Moderate tofair fracture porosity.

Schist with the samefeatures as above, intenselyfractured. Common zeoliteveining (0.5-2.5 mm wide,locally 2.5 cm). Some openfractures and caverns morethan 1 mm in width. Fair togood fracture porosity. Poreconnectivity is good.

Schist, similar to above, withmedium sized crystals (3600 to3600.1 m) and whitish greypatches (>1 cm diameter) ofpossible relict lithology.

Schist: dark grey, friable, softthin schistose layering.Abundant black biotite,common pyrite, trace whitishzeolite veins (>1 mm in width)developed parallel toschistosity. The rock has beenstrongly deformed, compacted,fractured and locallyweathered.

3832.0

3831.0

3830.0

Depth

Lith

olog

y

Coa

rse

Med

ium

Fine

Very

fine

Goo

d

P oor S a

mpl

e

Mas

sive

Schi

stos

e

Crystal size Porosity Struct.

Description

Granite: White and verylight grey with darkgrayish olive speckles,very hard, mediumgrained, slightly coarserlocally at 3830.80 m,massive structure.Abundance of quartz andfeldspar crystals, minorblack mica, trace ofyellowish pyrite, trace offractures.

Poor fracture porosity

Granite: similar to abovebut is medium to coarsegrained. Bulk of rock iscrosscut by a finefracture meshworkgenerally fi l led withzeolite(?) and quartz(white grayish). Somecaverns 5 mm wide withgrey white euhedralcrystals of 1 mm to 2 mmin diameter growing insome cavern pores.

Poor to moderate fractureporosity.

Mod

erat

e

Core

1

Core

2

3598.49 0.68 0.09 0.13 1.37 2.733598.55 1.70 0.16 0.37 1.71 2.723599.83 0.06 0.01 0.01 0.91 2.753599.70 to base core highly fragmented

3830.25 0.25 0.09 0.01 0.72 2.653830.65 0.25 0.01 0.02 1.22 2.662831.92 0.35 0.19 5.10 0.77 2.63

Permeability(Kair md)

Max. 90° Vert. Poro

sity

(hel

ium

%)

Gra

inde

nsity

(gm

/cc)

Depth(m)

Fig. 11. Core descriptions of cores 1 and 2 from well BH-12. Core 1 sampled a strongly altered rock (SARelectrofacies), while core 2 sampled the unaltered granite (FR electrofacies) from 3830-3830.8m and themoderately altered rock (MAR electrofacies) from 3830.8-3832m. Inset at lower right shows standard core-plug derived permeability, porosity and grain density. The listed plug permeabilities are maximum (plugdrilled parallel to mineral-filled microfracture), at 90° to microfracture and vertical (with respect to coreaxis). Definitions of the various electrofacies were given by Cuong (2000).


Fig.

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image logs and the depth of investigations is broader,so that conventional logs define fractured intervalsrather than individual fractures.

A higher gamma signal is typical of fracturedintervals in Bach Ho and reflects precipitation ofradiogenic hydrothermal minerals along fractures(although these fractures only contribute to reservoirporosity when re-opened as a result of laterdeformation) Electric logs, especially themicrolaterlog and the proximal log, can indicatefractures in Bach Ho basement. However, conductiveminerals, such as pyrite and other metal sulphides,and drilling-induced fractures, reduce the accuracyof natural fracture determination using these logs.

Porosity-logging tools, neutron, density and sonic,are the most useful conventional logs when definingfractured zones. Our studies show the sonic log is themost useful porosity log in Bach Ho field as it notonly indicates fractures by elevated values on shearand compressional responses, but also differentiatessecondary and primary porosity. Caliper logs reliablydefine fractured intervals by enlargements of thewellbore (e.g. 3600m, 3626m, 3705m, 3720m, and3721m in well BH 12: Fig. 12), while associated holerugosity tends to create anomalous values in mostconventional logs, especially pad-mounted porositylogs. This is an important factor in deriving reliableporoperm core-to-wireline transforms (see below).

In general, individual wireline logs do not providereliable definitions of fractured intervals at Bach Ho.Logs in combination, such as cross-plots, give morereliable results but still show variable quality outputsthat are subject to error. Based on conventional logand cross-plot approaches, we attempted to definefractured zones in well BH-12. We then comparedthese outputs to image-log based fractureinterpretations (compare Figs 12 and 13). The resultswere variable and only the M-N, M-PEF and DT4S-DT4P cross-plots could be considered partiallyreliable fracture indicators. The M-N, M-PEF cross-plots detect fracture zones through the presence ofhydrothermal minerals in the fractures. They do notclearly distinguish between zones with open versusclosed fracture networks. Comparison of the fracturedzones defined by the three cross-plots is shown in Fig.13. These zones are generally coincident and giventhe resolution problems of conventional logs to imagelogs, the outputs are considered to reasonably indicatethe intensity of fracturing in Bach Ho basement.

Fracture presence or absence can be estimated fromthe wireline determined compressional and shearvelocities (Sibbit, 1996), but again the result is notquantifiable in Bach Ho due to washout in almost allthe fracture zones. This also strongly influences thereliability of sonic readings (e.g. 3617-3628m, 3704-3723m in well BH-12).

PROBLEMS WITH OBTAININGREPRESENTATIVE POROSITYMEASUREMENTS IN CORES

During core-based petrophysical studies, it was notpossible to quantify porosity distribution in thefractured basement at Bach Ho, even using whole coredeterminations. Core and plugs of suitable quality forcore analysis tended to come from relativelyunfractured intervals, while fractured zones tendedto be recovered as rubble without little coherency.Studies showed that neither water saturation norpermeability could reliably be estimated from eitherlogs or core (Fig. 14). This reflects both the inherentlimits of conventional wireline log interpretation inthe high resistivity environment of the Bach Hoigneous and metamorphic basement, and also thelimited and biased core recovery.

Total porosities in Bach Ho basement rocks arelow, typically between 0 and 3%, this is less than thestandard deviation or error range of tool-basedporosity estimates (e.g. inset in Fig. 11). Therefore,calculated values of porosity from conventionalwireline logs, even in fractured intervals, are at bestsemi-quantitative at Bach Ho. Simple equations ofwireline porosity calculation were applied andappeared to give consistent results, but are influencedby poorly controlled variations in matrix mineralogyand fracture intensity. An assumption was made thatunaltered rocks have zero effective porosity. Thereforeporosities calculated by different logging tools had tobe corrected to zero for intervals of unaltered matrix,even where apparent porosity is listed in log outputsdue to lithology effects within these intervals. Multiplemineral models can remove some of these lithologicaleffects, but are only accurate when volumetric inputsfor each mineral are known. Wireline porositiesillustrated in Fig. 12 were calculated as best-estimatesbased on combinations of neutron, density and soniclogs, and verified against core data and image logs.

In many fractured reservoirs, especially incarbonates, the difference between porosities obtainedfrom neutron/density logs (i.e. total porosity) and sonicporosity can be used to indicate intervals of secondaryporosity. But such wireline-determined intervals ofsecondary porosity in Bach Ho basement typically didnot correspond to open fractures or vugs. The intervalsalso encompassed zones of non-effective porosity dueto hydrogenation by authigenic minerals (read asporosity by the neutron log) or microporosity inauthigenic clays and zeolites. These features werecreated during hydrothermal alteration and episodesof pore occlusion, and were not tied to effectiveporosity in the reservoir.

The matrix in Bach Ho has extremely highresistivities as it is composed of non-porous igneous


rock. This means a formation factor exponent “m”could not be measured accurately in the laboratory,making water saturation calculations (via ArchiesEquation) tentative.

FRACTURE DISTRIBUTION AT BACH HO

Fracture analysis shows that there is no consistentfield-wide relationship between the faults andfractures mapped by seismic methods and the naturalfracture orientation defined by image logs (Figs 6 and15). In the Southern Block (well BH8), the strike ofthe FMI-defined fractures are sub-perpendicular or ata high angle to strikes of seismically-mapped faults(Fig. 6). FMI-defined fractures in the Central Block(wells BH2, BH3, BH4) show a less consistent set ofstrike orientations. In wells to the north of the CentralBlock (wells BH1, BH5 and BH6), and wells to theeast but downdip of the Central Block (well BH9),the strike of the FMI-defined fractures is less welloriented than in wells in the Southern Block, but againis sub-perpendicular to the general NNE-SSW

Fig. 13. Fracture interpretation usingconventional logs in well BH-12. Likelyfracture zones interpreted fromconventional cross-plots are indicated byred highlights in the column to the left ofeach of the three conventional log tracks.(A) is based on an M-N cross-plot,(B) uses M-PEF, while (C) is based on aDT4S-DT4P cross-plot. All three showonly moderate agreement and matchesto fracture interpretation based on FMI(see Fig. 12).

orientation of the seismically-defined faults (Fig. 6).The lack of a general trend in fracture strike is mostmarked in wells in the Central Block compared withthe surrounding regions (Fig. 6).

Regions to the north, south and east of the centralhigh show a reasonably consistent tie between seismicand image-log data, whereby FMI fracture strikes areoriented sub-perpendicular to seismically-definedfault trends. This signature is consistent with theregional-scale early-mid Tertiary extension tied to theopening of the proto-South China Sea (see Hall andMorley, 2004). The region with the poorest tie betweenFMI- and seismically-defined fault orientation is theCentral Block. This is the structurally highest blockin the field and unlike the adjacent blocks, waseffected by late Oligocene thrusting. Blocks to thenorth, south and east of the Central Block showevidence of extensional block faulting but no reversefaulting (Fig. 6). As discussed above, seismic analysisshows the Central Block was translated by lateOligocene reverse faulting, which laterally displacedit by up to 2 km to the WNW.


intersects an open fracture system tied to a deeper-seated; as study of the Vung Tau outcrops showed,fracture density and porosity increase significantly inthe damage zone (Figs 7, 15b).

In summary, open fractures tend to be mostsignificant in the Central Block of Bach Ho, wherethrust faulting emplaced a brittle basement block ontop of Eocene-Oligocene mudstones. Thrusting,together with subsequent gravity-driven extension,served to open or re-open tectonic fracture networks.Cross-linkage of such fractures was probably aidedby the formation of exfoliation fractures.

RESERVOIR ZONATION TIED TOGEOLOGIC HISTORY

Combining all the available geological, petrophysicaland seismic data, the Bach Ho basement can bedivided into three reservoir zones, A-C (Fig. 16). Zoneboundaries are not horizontal, but are probablyirregular and fracture-controlled.

Zone A is strongly altered and fractured due to acombination of tectonism and fluid flushing. It iscapped by a relatively thin, intensely weathered andclay-plugged interval, similar to that observed atoutcrop at Vung Tau, which varies in thickness andcan be unconsolidated, even at depths of 4000 m. Thisinterval can be considered as a regional-scale sealcapping the underlying basement reservoirs.

The remainder of Zone A is characterized by thepresence of open to partially-open conjugate fracturesand brecciated zones, and has good but variable

Fig. 14. Fragment size controls the scale of measurement volume in a fractured basement sample in Bach Hoand so influence measured permeability (after Tuan et al., 1996). Standard plug values for porosity andpermeability in well BH-12 are listed in Fig. 11.

The less ordered orientation of fracture strike inthe Central Block is perhaps related to a combinationof buckling and subsequent stress release.Compression-driven fracturing has partiallyoverprinted pre-existing extensional fractures in theblock but also opened new set of compression-relatedfractures. During the Miocene, the thrust-emplacedCentral Block was subject to gravity-inducedextension, a process that opened pre-existing fractures.

Fracture sets in Bach Ho exhibit structurallyconsistent variations in dip angle; dominant dips offractures in all wells are 50-75° (Fig. 10). This isconsistent with stress patterns created both byextension or compression, and the associated fracturescannot be differentiated in the FMI data (seediscussion in Aydin, 2000). A subordinate set offractures have moderate dips of less than 40°. Byanalogy with the Vung Tau outcrops, these fracturesare interpreted as exfoliation fractures. They are mostobvious in wells located at some distance from themost highly tectonised part of the field (the CentralBlock); compare, for example, the dip angles of wellBH 3 with those of other wells listed in Fig. 10.

Across Bach Ho field the fracture density recordedin FMI and core data shows two trends: (i) porositytends to decline with depth (Fig. 15a). and (ii) porositytends to increase in zones of faulting and fracture (Fig.15b). These trends outline the most highly deformedupper portion of the basement. Alhough the basementreservoir is a granodiorite, it can be modelled as afracture system with the greatest flow potential in theuppermost interval. High flow will also occur if a well

30 mm50 mmFull core

0-1 1-10 10-100 100-1000Gas permeability (md)

0

10

20

30

4050

60

>1000

70Permeability from different diameter and whole core in Bach Ho basement

Per

cent

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(%)

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r 6: 1

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r 7: 1

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r 8: 1

25m

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r 9: 2

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0

Fig.

15.

Fra

ctur

e di

stri

buti

on v

aria

tion

s w

ith

dept

h an

d fa

ulti

ng a

t B

ach

Ho

field

. (A

) Fr

actu

re d

ensi

ty in

Bac

h H

o w

ells

cha

nges

wit

h de

pth

belo

w t

op-b

asem

ent.

Frac

ture

s te

nd t

o be

mos

t co

mm

on in

the

upp

er p

art

of t

he b

asem

ent

bloc

k (Z

one

A)

but

also

incr

ease

in t

he v

icin

ity

of fa

ults

in d

eep

wel

ls. L

ayer

ing

(1-1

0) is

bas

edon

FM

I. Fo

r re

ason

s of

con

fiden

tial

ity

the

wel

l loc

atio

ns a

re n

ot g

iven

, but

all

wel

ls a

re lo

cate

d in

the

reg

ion

show

n in

Fig

. 6.

(B)

Frac

ture

den

sity

and

por

osit

y di

stri

buti

on in

crea

ses

in t

he c

ored

dam

age

zone

nea

r a

faul

t. T

here

was

no

core

rec

over

y in

the

faul

t zo

ne it

self.

The

figu

re is

bas

edon

cor

e in

wel

l BH

-420

but

for

reas

ons

of c

onfid

enti

alit

y th

e lo

cati

on o

f thi

s w

ell i

s no

t gi

ven.

Thi

s tr

end

of fr

actu

re in

crea

sing

tow

ard

a br

ecci

ated

zon

e is

sim

ilar

toth

e tr

end

seen

in o

utcr

op n

ear V

ung

Tau

(see

Fig

. 7).


reservoir qualities. Zone A forms the principalreservoir interval at Bach Ho. Pores and fractures mayhave been locally enlarged by meteoric dissolution.However, most of the effective porosity is associatedwith fractures of tectonic origin. New fractures wereformed, and pre-existing zeolite-filled fractures werere-opened, during late Oligocene thrust-relatedemplacement of the Central Block. This combinationof fracture origins explains the wide spread of fractureorientations seen in FMI data (Fig. 6).

Regionally, the base of the fractured interval inZone A at Bach Ho is at a depth of around 3800-3900m. This depth is approximately equivalent to the topof the Eocene sedimentary interval where it onlapsthe basement block. The top-Eocene also correspondsregionally to the base of the upper part of the secondseismic zone (roughly at the level of the blue reflectorin Fig. 5 and part way into the zone defined by yellowarrows). In well BH-12, the boundary between ZonesA and B is coincident with the wireline-definedboundary separating Zones II and III.

Zone B is less intensely fractured and altered thanZone A, and many fractures are closed or filled byauthigenic or hydrothermal precipitates. The base ofthe zone lies at a depth of around 4000-4600m andprobably coincides with the lowermost extent of

effective production. Zone B reservoir quality isvariable and patchy. The zone corresponds to thewireline-defined Zone II in well BH-12, and the lowerpart of the second seismic zone in Fig. 5.

Zone C is weakly altered and fractured, and almostall pores and fractures are filled by zeolites and otherhydrothermal minerals. Reservoir quality is very poorand the zone is not therefore considered an explorationtarget. Zone C is equivalent to wireline-defined ZoneI in well BH-12 and to the third seismic zone locatedbelow the yellow arrows in Fig. 5.

DISCUSSION: BACH HO AND OTHER“BURIED HILL” PLAYS

In the oil-industry the term “buried hill” is widely usedto describe a reservoir associated with apalaeotopographic high, where onlap by subsequentsedimentary layers indicate that the feature was onceexposed at the landsurface (e.g. Luo et al., 2005).Reservoirs are in general located in the uppermost(fractured and weathered) parts of the “hill” or in theimmediately surrounding sediment apron. Table 2 andFig. 17 summarise how the reservoir characteristicsof the Bach Ho structure compare with those at other“buried-hill” type traps. Table 2 does not list discovery

Fig. 16. Reservoir model for the Central Block in Bach Ho field based on the integration of outcrop, seismic,wireline and core data. Reservoir zones A, B and C are discussed in the text.

Weathered rind (zone IV in BH-12 basement, and from outcrop, seismic data)

Late Cretaceous granite (from core analyses, BH-12 wireline logs and seismic data)

Triassic - Jurassic intrusions (from core analyses, BH-12 wireline logs and seismic data)

Andesite - Dacite dykes (from core analyses, BH-12 wireline logs and seismic data)

Metasediments (Pre Late Triassic)

Early Oligocene shales (source rock)

Eocene lacustrine shales (source rock)

Eocene and Oligocene sands (immature and cemented - zeolites and calcite)

Release, exfoliation and contractional sheet fractures (from core analyses, BH-12 wireline logs and seismic data) Conjugate faults, associated fractures and breccias (from FMI data, seismic data and outcrop analogues) Late Oligocene reverse fault in Central Block, associated fractures and breccias (from seismic data)

Zone A-B boundary (coincides with boundary of wireline -defined zones II and III in BH-12 and the lower part of the upper second seismic zone and is around 3800-3900m in Central Block)

in BH-12 and the top of the third seismic zone)

B

Not to scale

C

A


Fig. 17. “Buried-hill” plays (see text and Table 2 for detail):(A) Cross-section through the Amarillo-Kansas Arch, USA, which hosts giant oilfields (Panhandle-Hugoton) inreefal carbonates which developed on the basement high and which are sealed by subsequent platformevaporites. “Granite wash” derived from the eroded basement block (see inset) hosts local accumulations.(B) Cross-sections through two “buried-hill” fields, Clair and Xinlungtai. At Clair, the reservoir is dominated bysediments reworked from the exposed granite high; at Xinlingtai, the reservoir is a combination of a fracturedbasement block, a weathered sediment “halo” and an apron of reworked grus.(C) Cross section through La Paz Field, Venezuela, where the reservoir is hosted in a fractured thrust-block.Hydrocarbons are sourced from the underlying La Luna Formation.

wells in fractured basement which did not pass intoproduction (e.g. the Lago Mercedes No. 1 well inChile: Wilson et al., 1993; see also wells listed bySchutter 2003a, b); nor does it list wells and fieldsproducing from fractured volcaniclastics or shallowgabbroic and serpentinite intrusives which weresynchronous with nearby sedimentation (e.g. Kalanet al., 1994; Harrelson, 1989).

In most “buried-hill” type reservoirs within igneousor metamorphic matrix, two main processes contributeto effective porosity: (a) weathering and mechanicalreworking, which can produces a “granite wash”; and(b) open fracture and joint networks (Powers, 1932).A classic “granite wash” play occurs at Elk City field,

Oklahoma (Sneider et al., 1977). Fracture-associatedporosity tends to be tied to structurally-inducedpermeability fairways, e.g at La Paz field, Venezuela,where production comes from a combination offractured Mesozoic carbonates and granitic basement(Nelson et al., 2000).

A common characteristic of all the fracturedbasement fields listed in Table 2 is that the originalreservoir matrix was tight and had high resistivities,as in Bach Ho. Effective porosity in such a reservoiris secondary, and is related to tectonic deformation orfluid flow. Tectonically-produced porosity is relatedto stress release during faulting and uplift/unroofing,and comprises open faults, fractures, microfractures

Two-

way

trav

el ti

meTop Eocene Top EoceneMisoa Fm.Top Paleo.Guasera Fm.

Top SocuyBASEMENT

0.0

1.0

2.0

La Luna Fm.La Luna Fm.(source rock)

Reservoir (fractured carbonate)

Reservoir (fractured basement)La Paz

Field

Sea level

0 2km

Sea level

Cenozoic

Cretaceous

NW SE

Oil & gas

Clair Field,Offshore Shetland

After Ridd, 1981

After Nelson et al., 2000

Basement(Proterozoic granite)

Breccia

Paleogene congl.(wash) Tertiary lake

muds (seal)

NS

After Guangming & Quanheng, 1982

Cla

ssic

“Bur

ied

Hill

” Ana

logs

Ras

ervo

ir is

mos

tly in

rew

orke

d “g

rus”

with

min

or c

ontri

butio

n fro

mfra

ctur

ed b

asem

ent

Com

bine

d fra

ctur

e &

rew

orke

d “g

rus”

Res

ervo

ir is

tect

onic

ally

indu

ced

set o

f fra

ctur

es,

“min

or s

edim

ento

logi

cal

influ

ence

on

qual

ityA

C

B

Granite wash playAmarilloArch, Texas

Xinlungtai Field,North China Basin

La Paz-Mara Field Venezuela

gas oil

Panhandle FieldPalo DuroBasin Anadarko Basin

S N

3 km

080 km

granite (fresh)

OilH2O

grus wash

mud

After Sorenson, 2005

Oil & gas

Palaeozoic

Basement


and joints produced at a variety of scales ranging fromkilometres to millimetres, and with variable degreesof matrix damage ranging across the same scales.Secondary porosity produced by fluid circulationincludes dissolution via meteoric waters in weatheringzones, and deeper-seated alteration and leachingproduced by hydrothermal circulation. The two typesof secondary porosity development can be interrelated,where, for example, pre-existing tectonic fractures actas conduits for meteoric waters or hydrothermal fluids.

A combination of deeply-circulating meteoricwaters moving along tectonic fractures, and areworked sedimentary “halo”, characterizes rift-associated basement plays in China and the Gulf ofSuez, as well as at the Clair field, offshore Shetlands(Fig. 17b). However, weathering or “granite wash”associations do not occur in the Cuu Long Basin. Thelimited dataset in Table 2 suggest that meteoric-enhanced basement plays tend to occur at depths ofless than 3000 m, suggesting that diagenesis associatedwith deeper burial causes a reduction in porosity. Thismay reflect the dominance of immature first-cyclearkoses and litharenites which weather from upliftedbasement blocks in rift settings.

An uplifted basement block in a rift setting willsubsequently undergo thermally-induced subsidence,and can commonly be overlain by rapidly-depositedlacustrine or marine mudstones, which can be over-pressured. The mudstones may however includesource rock intervals, and a build-up of overpressurewill cause hydrocarbons, once generated, to migrateinto the sediment “halo” or into the fractured basement(e.g. Fig. 17a).

A third group of “fractured basement” plays,including that at Bach Ho, is characterized by adominance of tectonically-induced fractures with littleor no overprint by meteoric dissolution, and anabsence of sediment “haloes” associated with surface-driven weathering. These plays tend to occur insystems characterized by active (Neogene) tectonicsin thrust-wrench fault settings, and include the La Paz/Mara fields of Venezuela and perhaps theSarkadkeresztúr field, Hungary (Fig. 17c; Table 2).At La Paz, thrusting emplaced a block of fracturedgranitic basement and associated limestone cover ontop of the prolific La Luna source rock (Nelson et al.,2000).These plays may have effective porosity atdepths below 3000 m, related to brittle fracturing ofthe recently deformed basement matrix.

Many basement highs have been drilled in the CuuLong Basin; none, however, have the productioncapacity of Bach Ho. Historically, geologists workingin the Cuu Long Basin have used the weathered“buried hill” plays of China as appropriate analogues.However, the weathered crust at Bach Ho is less than30 m thick. Also, sediment “haloes” are absent from

Bach Ho, and there is little evidence that surface-driven meteoric alteration was responsible forgeneration of secondary porosity. Nor can thehydrothermal circulation, as inferred by Areshev etal. (1992), explain the porosity. Both meteoricleaching and hydrothermal alteration tend to degradeeffective porosity at Bach Ho, not enhance it. Rather,porosity at Bach Ho is largely tied to zones of brittlefracturing associated with late Oligocene thrusting andsubsequent gravity-driven extension. A moreappropriate structural analogy for Bach Ho wouldtherefore be the La Paz field.

CONCLUSIONS

Compared to many other “buried hill’ playsworldwide, Bach Ho field is unusual in that thefractured reservoir matrix is largely made up ofunaltered acid igneous lithologies (mostly granites andgranodiorites). Unlike classic “buried-hill’ plays, a“halo” or “granite wash” of reworked basementmaterial is absent. A large-scale NE-SW trending, lateOligocene reverse fault system cross-cuts the field,with approximately 2000m of lateral displacement inthe Central Block, where a block of basement rockwas emplaced on top of organic-rich Eocene-Oligocene mudstones.

Using a combination of core-calibrated wirelineand seismic data, the igneous matrix at Bach Ho canbe divided vertically into three zones (A-C): the twolower zones (below 3800-3900m) are relativelyweakly deformed and are in general sub-economic;the upper zone, which constitutes the main reservoirin the Central Block, is strongly fractured. Fractureshave a variety of origins, and include pre-existingstructures reopened during late Oligocene deformationand more recent syntectonic fractures.

Future exploration offshore Vietnam should notjust define “buried hills”, but should attempt toidentify basement highs where fractured basementblocks have been thrust onto mudstone successions. .

PostscriptThis study, based on the results of more than 30 yearsof exploration and development by Vietnamese andother earth scientists, underlines a number of appliedgeological principles known to explorationists formany years.

(1) You will not fully understand a complexreservoir before you must produce from it.

(2) Seismic collected across possible fracturedbasement reservoirs gives a reliable indicator ofstructural culminations in a basin and so defines thefirst-order drilling targets. Subsequent higher seismicresolution obtained from a 3D survey may betterdefine additional targets, and have regional


Table 2. Summary of oil- and gasfields known to have produced economic volumes of hydrocarbons fromfractured and variably weathered igneous and metamorphic basement highs, where the unaltered host rockhad little or no permeability.

Field and Setting

Age and depth

General reservoir character Seal References

Utikuma & Red Earth fields, Peace River Arch, Alberta, Canada

Middle to Upper Devo-nian clastics ≈ 1500-2500 m

Granite wash is organized into two systems of partially superimposed clastic wedges (alluvial fan deltas & estuaries) that interfinger with, and are onlapped by, the evaporitic Keg River and Muskeg Formations. Lower and upper Granite Wash clastic wedges reach a combined maximum thickness of 70 m. In the Devonian the Peace River Arch (Pre-cambrian granitic/cratonic core) formed an active series of parallel horsts (with exposed crests) and graben depressions (basins).

Devonian Muskeg Fm (marine-fed basinwide evaporite)

Cant, 1988; Dec et al., 1996

“Gra

nite

was

h” p

lay

- mos

t of t

he re

serv

oir r

esid

es in

a c

last

ic a

pron

de

rived

from

wea

ther

ing

and

mec

hani

cal r

ewor

king

of m

ater

ial e

rode

d fro

m a

n ad

jace

nt o

r und

erly

ing

igne

ous-

met

amor

phic

hig

h.

Elk City, Hall-Gurney, Gor-ham, Panhan-dle and other fields along the Kansas Uplift and the Amarillo Arch, USA

Precambrian basement, Pennsylva-nian clastics (wash) and Permian car-bonates (re-gional reser-voir) ≈ 1000 -2700 m

Minor, deeper reservoir in fractured, variably-weathered pink granites and quartzites in some fields, but clastic production is mostly within a drape made up of a mechanically reworked Paleozoic grus and arkose “halo” and in syn-high reefal build-ups. The high/arch formed in a pre-Pennsylvanian collision event. There is some 120 m of fault displacement at the top of the Precambrian granite on the SW flank on the regional (10 km long) Gorham anticline.

Permian salts and mudstones, (marine-fed platform saltern* and mudflat)

Landes, 1960; Sneider et al., 1977; Walters, 1991; Sorenson, 2005

Clair field, offshore Shetlands, UK

Precambrian (Lewisian) basement 1850 m

Reservoir is elongate NE-SW trending, normally faulted horst made up of a combination of fractured Devonian-Carboniferous redbeds and Lewisian granites. Most storage is in clastics but basement fractures are higher permeability con-duits

Cretaceous mudstone (marine)

Coney et al., 1993

Xinglongtai field, Western de-pression of the Liaohe Basin, China

Precambrian ≈ 2700 m

Highly fractured and weathered Archean granites and Mesozoic volcanics with a clastic halo in and about a horst block bound by normal faults. Relief on the Archaean-cored “hill” is 1000 m and the oil column in the field is 700 m high.

Tertiary mudstone (lacustrine)

Luo et al., 2005; P’An, 1982

Dongshenpufield, Da Ming Tun Depres-sion, China


Produces from fractured and weathered Archean metagranites and Proterozoic metasediments. Structure is the higher part of normally faulted set of basement blocks (edge of half graben) and is onlapped by Tertiary sediments (source and seal).


Xiaoguang & Zuan, 1991

Yaerxia field, Western part of the Liuxi Basin, China

Silurian (Caledonian) basement ≈ 2200 me-tres

Highly fractured and weathered Palaeozoic meta-morphic host making up the crestal zone the normal-faulted (wrench-faulted?) Yaerxia horst block. Situated along trend from the thrusted asymmetric anticline of Laojunmiao field, which is broken up by imbricated minor thrust faults on forelimb. Intense thrusting during Miocene.


Guangming & Jianguo, 1992; P’An, 1982

Cla

ssic

“Bur

ied-

Hill

” fie

lds

whe

re th

e re

serv

oir i

s a

com

bina

tion

of fr

actu

red

wea

ther

ed b

asem

ent a

nd a

var

iabl

y de

vel-

oped

cla

stic

apr

on. M

uch

of th

e ab

ility

of t

he b

asem

ent t

o st

ore

and

flow

hyd

roca

rbon

s is

due

to e

xpos

ure

and

wea

th-

erin

g of

the

fra

ctur

ed b

asem

ent

whe

n it

was

sub

aeria

lly e

xpos

ed a

nd s

uppl

ying

cla

stic

s to

the

adj

acen

t se

dim

ent

apro

n

Zeit Bay & Geisum fields, Gulf of Suez, Egypt

Precambrian basement ≈ 1200 me-tres

Highly fractured and weathered basement granitic basement horst forming a fault-defined high within Gulf of Suez rift. The 250 m thick oil column in Zeit Bay field extends from Mesozoic sandstones and granite wash down into the fractured and weath-ered basement, which contains one-third of oil in the field.

Miocene evaporites (basinwide* salt & evaporitic mudstone)

Salah & Al-sharhan, 1998 Younes et al., 1998


significance for future exploration (e.g. the importanceof the cantilevered blocks in controlling reservoirquality at Bach Ho), but still may not be capable ofreliably defining variation in quality in the knownreservoir blocks within the survey area.

(3) Often it is the simplest wireline tools, whichmake direct physical measurements through a complexreservoir, which are the most reliable in terms ofindicating variations in reservoir quality. Thus caliperlogs are the most reliable indicators of fracture densityand reservoir quality in wells at Bach Ho.

“Experience is a hard teacher because she gives thetest first, the lesson afterwards”

ACKNOWLEDGEMENTS

The wireline, rock property, and seismic data fromBach Ho field was provided by Petrovietnam, as was

access to the Bach Ho core. XRD analytical serviceswere kindly donated by Core Laboratories, Malaysiaand the various software modules used to manipulatethe seismic and wireline data were kindly donated bySchlumberger. Comments by J. Gutmanis (GeoscienceLtd, UK) and G. H. Lee (Pukyong NationalUniversity) on a previous version of the MS areacknowledged with thanks.

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ARESHEV, E. G., T. L. DONG, N. T. SAN and O. A. SNIP, 1992.Reservoirs in fractured basement on the continental shelfof southern Vietnam. Journal of Petroleum Geology, 15, 451-464.

AYDIN, A., 2000. Fractures, faults, and hydrocarbon entrapment,migration and flow. Marine and Petroleum Geology, 17, 797-

Augila-Nafoorafield, Sirte Basin, Libya

Late Precam-brian or early Cambrian basement, ≈ 2400 m

Rotated and exposed rift blocks highs with some production from fractured and weathered granites beneath the main Cretaceous Lower Rakb car-bonate reservoir.

Late Creta-ceous mudstone (marine)

Williams, 1972

La Paz-Mara fields, Venezuela

Palaeozoic ≈ 4000 m

Inverted (thrusted) wrench-fault block with frac-tured granite/granodiorites and schists beneath fractured Mesozoic carbonate (initial anticlinal reservoir prior to deepening in early 1950s). Thrusting/inversion began prior to Eocene but was most intense in Miocene-Pliocene. The total hydrocarbon column is ≈ 915 m high, 305 m of which is in a basement reservoir made up of frac-tured basement with little evidence of matrix weather.

Late Creta-ceous mudstone (marine)

Nelson et al., 2000 Smith, 1956

Zdanice-Krystalinkum field, Czech Repub-lic


Fractured granites and granodiorites subjected to exposure and weathering prior to Miocene when the Upper Eocene to Oligocene pelitic seal was thrust into place by Zdanice-Subsilesian Nappe event (structure is not well documented).

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Krejci, 1993

Res

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edim

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es re

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oxim

ity to

sou

rce

rock

.

Sarkadkeresz-túr field, Pan-nonian Basin, Hungary

Precambrian & Tertiary? ≈ 2700 m

Majority of oil is stored in a narrow horst block composed of fractured Precambrian metamorphics. Oil column is 370 m thick (structure is not well documented)

Tertiary mudstone (Lacus-trine?)

Dank, 1988

Bach Ho field Offshore Viet-nam, Cuu Long Basin

Cretaceous (mostly) ≈ 3500 m

Inverted (thrusted) rift block of fractured gran-ite/granodiorite. Thrusting occurred mostly in the Late Oligocene. Oil column height is around 1000 m.

Upper Oli-gocene muds (lacus-trine?)

This paper

Table 2 (continued).


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bach ho field, a fractured granitic basement …upper triassic – cretaceous plutonic rocks whose...

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