unlocking the remaining potential of the thayem and...

AbstractAbstractThe Thayyem and Al Nishan oil fields are located in north-eastern Syria, where commercial quantities of oil were discovered in the 1980s. However, since 1991 productionfrom these fields has been declining. An integrated core and outcrop study was carried out on the Lower Miocene Dhiban Formation in order to develop a depositionalmodel, which in turn would lead to a better understanding of the facies distribution and reservoir quality within these fields.

Outcrop and core studies indicate that the transgressive and regressive part of the Dhiban Formation are each characterized by different facies associations and twodepositional models are required to describe this interval. The transgressive interval is dominated by moderate energy, good reservoir quality subtidal mollusk sands andlittoral deposits closely associated with supra-tidal sabkha. This may suggest the presence of a poorly developed protective barrier and that the inter-tidal and sub-tidalenvironments may have been isolated by growth of beach bars and spits. In core, the regressive unit displays a more restricted facies association of thick oil stainedstromatolites, salina, muds and marls and supra-tidal sabkha. Here the protective barrier is better developed with a low energy inter- to sub-tidal area between the barrierand the supra-tidal sabkha. The two depositional models have different implications for the lateral continuity and stacking trends of the reservoir and sealing intervals.

1. Objectives

The objective of this study is to provide adepositional model for the Lower Miocene

Dhiban Formation in support of 3-D staticreservoir modeling and an assessment of

the lateral extent of reservoir and sealing

units. The study involved describing a totalof 250m of Miocene cores from Thayyem

and Al Nishan Fields. The results were inturn compared to Miocene outcrops in the Al

Bishri and Al-Raqqa areas (NE Syria). This

poster documents 1) the observations madein the field and cores, 2) two depositional

models for the Dhiban Formation aresuggested.

Unlocking the Remaining Potential of The Thayem and Nishan

Fields From Outcrop and Core Studies, Syria

OMAR AL JA′AIDI1, PETER HOMEWOOD1, HENK DROSTE1, JAMAL AL JUNDI2, MOHAMMED ABOU SHAKER2, & JÜRGEN GRÖTSCH2

1 Carbonate Research Centre, SULTAN QABOOS UNIVERSITY, P.O. BOX 36 PC 123, AL KHOD, OMAN2 Al Furat Oil Company, DAMASCUS, SYRIA

2. Outcrop Study

The outcrops studied are located on the Al Bishri Mountains in north-eastern Syria (Figure 1). The Al Bishri uplift is a northeastern closure of foldsof the Palmyra range and is made up of Upper Cretaceous and Paleogene rocks. Miocene strata are exposed on the flanks of the uplift. The Bishri

uplift started to form in Late Eocene time related to the beginning of the final collision phase of the Arabian Plate with Eurasia ( Brew et al. 2001)and since then influenced sediment thickness and distribution in the area. Major uplift and deformation of the area occurred in the Pliocene

associated with basement block movements and basalt lava flows related to the terminal suturing of the northern margin (Brew et al. 2001). The

structural setting of the Bishri block significantly differs from that of the adjacent Euphrates Graben area where the Thayyem and An Nishan Fieldsare located. This is also reflected in the stratigraphy, the Lower Miocene consisting of clastics in the Bishri outcrops while carbonates predominate

in the Euphrates Graben..eses

3. Stratigraphy

Syria is host to several carbonate Tertiary reservoirs including the Jeribe, Dhiban and Eupahrates Formations (Figure 2). The stratigraphicsubdivision of the Neogene deposits used on the geological map for the study area is summarized in Table 1. Compared to the subsurface

stratigraphy in the Thayyem and Al Nishan Fields the ‘Lower Miocene’ is stratigraphically equivalent to the Euphrates and Dhiban Fm, the‘Helvetian’ to the Jeribe Fm. and the ‘Tortonian’ to the Fars. In depositional setting, however, the ‘Helvetian’ carbonates show great similarity to

those of the Dhiban Fm and can be used as an analogue for subsurface modeling.

Stratigraphic

Unit

Thickness Age Lithology

‘Tortonian’ Upper

Part

> 500 m Tortonian based on

forams

Greenish gray marls and, in

the upper part, quartz sandstones and limestones

interbedded with evaporites

‘Tortonian’ Lower

part

100 to 110 m in

Wadi Aj-Jir

Tortonian based on

forams

Alternation of gypsum and

limestones intervals. Limestone intervals 0.5 to a

few meters thick, gypsum a few meters in the lower part to > 10 m in the upper part

of the sequence.

‘Helvetian’ 25 m in the Shjiri

area

Helvetian based on

forams

Light bedded occasionally

arenaceous limestones overlain by gray compact

arenaceous limestones in the Sjiri area limestone coquinas with gypsum and

dolomite predominate

‘Lower Miocene’ 12 –13 m in the

Shjiri area according to

geological map (we estimate at least 80 m)

Early Miocene

assumed from its stratigraphic

position

Barren sand series above

Oligocene strata poorly defined on geological map

(Table 1)(Table 1)

Tertiary

Cretaceous

Jurassic

Triassic

Perm

.Carbonif.

Dev.

Sil.

Ord.

Late

Early

M u lu s sa G r .

S h ir an is h F m

E r e k F mR ’m a hD e r roPo s t-J ud e a S and sJ ud e aR ut b ah

G

F

E

D

C

B

U p p e r

L ow e r

D o ub a yat F m I .C .D .

A bb a Fm

K ha b ou r Fm

F a r sJ e r ib eD h ib anE up h r ate sC h ilo uJ ad d a laA a li j i

Pre-Rift

Syn-Rift

Post-Rift

B K L

B KU

R e s.

Re s .

R e s.

SR /(S ea l)

S R

Se a l

S e a l

S e a l

Tertiary

Cretaceous

Jurassic

Triassic

Perm

.Carbonif.

Dev.

Sil.

Ord.

Late

Early

M u lu s sa G r .

S h ir an is h F m


G

F

E

D

C

B

U p p e r

L ow e r


A bb a Fm

K ha b ou r Fm


Pre-Rift

Syn-Rift

Post-Rift

B K L

B KU

R e s.

Re s .

R e s.

SR /(S ea l)

S R

Se a l

S e a l

S e a l

Tertiary

Cretaceous

Jurassic

Triassic

Perm

.Carbonif.

Dev.

Sil.

Ord.

Late

Early

M u lu s sa G r .

S h ir an is h F m


G

F

E

D

C

B

U p p e r

L ow e r


A bb a Fm

K ha b ou r Fm


Pre-Rift

Syn-Rift

Post-Rift

B K L

B KU

R e s.

Re s .

R e s.

SR /(S ea l)

S R

Se a l

S e a l

S e a l

Intra-Carboniferous

Dolomite

Mulussa CDE

Judea

Rmah

Erek

Upper Shiranish

Lower Shiranish

Jeribe, Dhiban, Euphrates

Lower Fars (TZCA)

Intra-Carboniferous

Dolomite

Mulussa CDE

Judea

Rmah

Erek

Upper Shiranish

Lower Shiranish

Jeribe, Dhiban, Euphrates

Lower Fars (TZCA)

StratigraphyCarbonate Reservoirs

(Figure 2)(Figure 2)

Poster 1 of 5

(Figure 1)(Figure 1) Outcrop Stops

Wadi Dufina-

Wadi Shijiri

Wadi Al Jir-

Wadi Rattla-

1 2

4

3

1 2

3

4

Wadi Al Jir 1Wadi Shijiri 2

Wadi Rattla 3

Wadi Dufina 4

Poster 2 of 5





Wadi Ajir N35o 27.156’ E039o 15.427’

In this wadi a more or less complete section can be viewed from the Oligocene (tar-bearing) sandstones up to the upper ‘Tortonian’ evaporites andmarls. The marly and evaporite intervals however are poorly exposed. The ‘Upper Miocene’ consists of green marls and sandstones with burrows andflaser bedding deposited in a shallow marine depositional setting. Higher up in the section this interval becomes sand dominated withwhite/yellow/reddish sands with chert and limestone pebble lags, low angle cross laminations and Ophiomorpha burrows. This interval is interpretedas a high-energy beach / shoreface facies. These are capped by a partially reworked soil horizon overlain by a brecciated stromatolic limestone andanhydrite, which forms the base of the ‘Helvetian’. Within the Helvetian limestone large stromatolites, m scale in diameter and up to 50 cm relief, occurinterbedded with grainstones with abundant bivalve fragments. Small, cm-sized stromatolites cover some of the larger forms (Figure 3). Thesecarbonates are overlain by thick, > 10 m, thin to thick-bedded anhydrites of the lower part of the ‘Tortonian’. Some dissolution surfaces can beobserved along the bedding planes filled in by thin laminated limestone intervals. Large, m-size polygons can be observed along exposed beddingplanes at the top of the anhydrite packages. The evaporites are interbedded with green clays and marls and limestones. In the upper part of thesection the anhydrites are interbedded with shales and this interval probably belongs to the upper part of the ‘Tortonian’. The evaporite layers oftenshow buckling and internal deformation related to volume changes as a result of recrystallisation between gypsum an anhydrite.

Exposure of some 4 m interbeddedanhydrite and grainstones of a thickanhydrite sequence. The limestonescontain mollusk fragments and crossbedding. The uppermost limestone ofthis interval is brecciated and containsthrombolitic like mounds andstromatolites. The thrombolitic faciesshow elongated shapes, which mayreflect the current direction. Theposition between thick anhydrite unitssuggests that this section belongs tothe lower part of the ‘Tortonian’.

Wadi Shijiri N35o 26.162’ E039o 25.914’

Good outcrops of the ‘Lower Miocene’ and the ‘Helvetian’; the ‘Tortonian’ is poorly exposed. The ‘Lower Miocene’consists of m –scale fine to med-grained quartz sandstones interbedded with thick (several to > 10m) poorlyexposed marls. The sandstones contain abundant (Ophiomorpha?) burrows and low angle cross laminations. Inone of the cycles within the sandstone interval a 1-2 m deep channel incision was observed. The ‘Helvetian’consists of some 15 m of limestones with dm thick intercalations of nodular anhydrite. Some of the anhydriteslayers pinch out laterally over only a few tens of meters. The limestones contain abundant mollusk fragments. Theuppermost bed contains stromatolites. The Helvetian is overlain by at least three evaporite limestone cycles. Thelimestones are about 1 to 2 m thick have a grainy texture with stromatolites. The evaporite intervals are poorlyexposed and > 10 m in thickness.

Wadi Al Ratla N35o 52.454’ E039o 02.940’

Thick, > 50 m, packages of poorly exposedevaporites and marls with a few meter thickintervals of limestones. The limestones aremedium bedded and show cross beddingand burrowing and contain common molluskfragments. Several erosional surfacesoverlain by thin intraclast lags are present.The thick anhydrite units suggests that thissection belongs to the lower part of the‘Tortonian’.

Wadi Daphina N35o 06.602’ E039o 37.207’

Some 40 meters of a poorly exposed marl and evaporite alternation (3 cycles) capped bya brownish marl layer overlain by a muddy stromatolitic limestone with quartz grains. Thelithology suggests this is part of the upper part of the Tortonian.

4. Field Work

Wadi Ajir-2 N35o 27.118’ E039o 18.205’

STRATI

GRA

PHY

DEPTH

(m)CEMENTS POROSITY

GRA

PHIC

LITH

OLO

GY

TREN

DS

&

SURFA

CES

LITHOFACIES REMARKS

SEDIMENTARY DATA SHEET WELL:

GEOLOGIST:JVRCCS LOGGING SCALE

823

822

821

820

819

818

817

816

815

814

813

812

811

810

809

808

THAYYEM-C

m

m

Algal Laminated

Mullusc grainestone

Mullusc packstone

Algal Laminated

Mullusc grainestone

Vuggy Filament Packstone

No stain

Filamental grain Algal Laminated? Clayey at top

Domal StromatolitesGrainestoneMollusc LAG Bivalve + Gastropod

Filamental grain vuggy (fine) small leachesLeaches Bioclasts

Rudstone, Corals large shells, Rhodoliths

Mullusc grain (no strain)

Marl

Leaches shells

1:100

Leaches shells

Fine mollusc grainstone, small cherty

and anhydrite nodules

Mollusc marl / Clayey Wackestone

Hetrolithic interval, reworked algal lamination

Intra-Formation conglomerate

Mollusc grainestone (no stain)

V

V

V

V

V

V

V

VV

V V VV VVV V

V

V

V

V

V

V V

V

M W P G F R B C

CARBONATE TEXTURES

CLA

Y0

.01

6

SILT

0.0

62

VER

Y FI

NE

0.1

25

FIN

E0

.25

0M

EDIU

M

0.5

00

CO

ARSE

V. C

OA

RSE

1.0

0

Rubble

Vug

gy

HC

Sta

in

Do

lom

itic

ce

me

nte

d p

atc

he

s

Poster 3 of 5





5. Core Investigation

5 c

m

Karst

811 m

845 m

5 c

m

Hummocky cross stratification

5 c

m

Evaporite overlain by shelly marl

818.2 m

5 c

m

827 m

Evaporite crystal fans

833.5 m

Vuggy grainstones

5 c

m

Dhiban Formation

Begins with a few m thick mainly muddy interval with brecciated gray dolomitic patches andabundant cm to dm sized anhydrite nodules, large shell fragments, finger corals andrhodoliths capped by a hardground surface. In other wells only a gravel lag with cm-sizedpebbles is present (THM-A). Basal part m-scale beds of bioturbated molluskpack/grainstones interbedded with dm thick algal laminated to stromatolitic beds. Someintervals thrombolites? Common shell (sometimes algal coated) lags and scattered fingercoral fragments. Scattered anhydrite nodules (interpreted to be diagenetic).In An Nishan-002 an anhydrite layer occurs at the base of this interval which may be equivalent to thebrecciated beds in Thayyem. The overlying section in An Nishan is coarser grained andmore vuggy.

The middle part of the Dhiban Fm. consits of a more heterolithic interval with marls, algallaminites, anhydrite beds and occasionally faintly laminated grainstones. Anhydrite bedsare dm to m thick, bedded at the base to more massive nodular at the top. In places somefaint Xmas tree like structures are present that could represent relicts of gypsum crystals.Stacked packages of anhydrite occur, beds separated by dissolution surfaces, clay layersor thin lags with cross bedding. Algal intervals are cm to m thick and consist of laminated tostromatolitic mudstones. The marls are strongly bioturbated and contain fine bioclasts(shells sometimes show borings). In the equivalent interval of An Nishan-B the evaporiteunits are thicker developed and the sediments are coarser grained (grainstones replacingmarls).

The upper part of the Dhiban consists of m-scale intervals of fine bioclastic marls/mudswith wavy to low angle cross bedding, in places burrowed. Within these marlserosional/scour surfaces are present overlain by cm-sized intraclasts and lags (shells,mud, clay, flat pebble), clasts sometimes contain borings. In places mudcracks (THM-A)and scattered anhydrite and chert nodules occur. In Thayyem-002 a 1.5 m thick bed withdispersed pebble and granule breccia and cavity filling sediment is present within this unit.In the equivalent interval of An Nishan-B the marly facies is much less developed, insteadmore grainstones and additional anhydrite beds are present.At the top of this interval athick (6 m) evaporite unit is present which internally consists of a stacking of 1 to 2 m thickpackages, each with bedded evaporites at the base grading to nodular to massive at thetop. These packages are separated by dissolution surfaces overlain by lags of reworkedevaporite clasts. In Thayyem-D a 2.5 m thick internally chaotic unit with dispersedintraclasts and broken beds overlies this evaporite.

THM-A

v v

?

v

D

D

Rubble

vv

vv

v

vvvvv

vv

Clay seams

vv

v

vv

vv

vv

v

vv

vvv

v

vv

v vv

v v

vv

v

vvv

vv vvvvv

v

v v

v

v v

v v

v

v v

vv

vv

vv

vv

vv

vvv

v

vv

vv

v v v v

v vvvvvv

v

v

v

vvv

v

v

v

v

v

v

vvv

v

813

814

815

816

817

818

819

820

821

822

823

824

825

826

827

828

829

830

812

811

810

809

808

807

806

STRATI

GRA

PHY

DEPTH(m)

CEMENTSPOROSITY LITHOFACIES REMARKSTREN

DS

&

SURFA

CES

CLAYEY

MF

MF

MF

MF

MFHeterolithic

MF

MF

MF

MF

MF

MF

MF

vv v

v

v

v vv

v

vv

Vv

v v

vv

v

vv

vv

v

vv

v

vv v

v

v vv vv v

v

BRECCIA

Heterolith

Karst above upper layer

Dispersed granule + pebbles breccciaCavity filled with sediments

v

v

vv

v

v

Chert nodules

831

832

833

834

835

836

837

838

839

840

841

842

843

844

845

846

847

848

Chert

Shelly lag

Shelly lag

Laminated facies

Laminated marls

Salina pond ?

Coarse grainstone, leached grains, cm-scale lamination

Patchy dolomite

Heterolith with scoured surfaces

Relict gypsum crystals?

Heterolithic interval with stromatolitesMarls + gypsum crystals

Top heterolithic

Heterolithic

Heterolithic + stromatolites

Large bivalves

Streaks of coarser sediment

Heterolithic interval, mud to grainstone + stromatolites

Gravel lag, cm sizes pebbles

Leached shelly bed, dolomitic

Shelly rockLeached gastropods + bivalves

Vuggy

Fining and coarsening upward bedding

Cm lamina of coarser sedimentWavy structures

Reworked anhydrite nodules ?

Medium grainstone with bivalve fragments

SEDIMENTARY DATA SHEET WELL: THAYYEM AGEOLOGIST: JVRCCS LOGGING SCALE: 1:100

M W P G F R B C

CARBONATE TEXTURES

CLA

Y0

.01

6

SILT

0.0

62

VER

Y FI

NE

0.1

25

FIN

E0

.25

0M

EDIU

M

0.5

00

CO

ARSE

V. C

OA

RSE

1.0

0

D

D

D

HCS

Heterolithic

Vuggy cross-bedded grainstone

Vug

gy

Do

lom

itic

HC

Sta

in

LEGEND

5 c

m

821 m

Chaotic bed

5 c

m

821.3 m

Stromatolite / thrombolitic texture

848

847

846

845

844

843

842

841

840

839

838

837

829

828

827

826

825

824

823

822

821

819

814

813

812

811

810

809

808

802

801

800

799

STRATI

GRA

PH

Y

DEPTH

(m) CEMENTSPOROSITY

TREN

DS

&

SURFA

CES

LITHOFACIES REMARKS

SEDIMENTARY DATA SHEET WELL: PROJECT:

CORE WIDTH/CUT:

GEOLOGIST: JVRCCS

LOGGING SCALE: 1:100THAYYEM-B

LITH

OLO

GY

Fine Bioclasts marl + Anhydrated Nudular

Cm sized mudstone clasts in shelly PackstoneMudstone Faint Ripple ? Laminated + burrowed

Bivalve Packstone

Core 1 798 - 803

Fine Bioclasts

5cm Fine Packstone/Grainestone Clutted structure

Muddy Laminated

Fine graine Algal Laminated + clutted structureFine Packstone/grainstone Filaments Algal

G stromatolite at top with new clay clasts

Grey clay

100cm

Bioclasts made up of crinkly filaments algal

Chotic structureChautic FR GRF?

Stromatolites/Thromb layer

Bioclasts grainestone

Chautic mix of coarse grains Leaches Algal/corals/

rreoncan/cem pachesSmall shell + gastropod Fragments, Algal coated grainestone.Thrombolite?Brecciated Algal/clutted

Bivalves/Gastrop Packstone to grainestone

Reworked clasts? Coral fragments Dolomitic Clasts large shellsDolomitic

Coarse bioclasts grainestone white unstained !Grey dolomitic shell lag

Fine bioclastic Packstone? (Cemented grains not clear)

Black crinkly structure burrows?

Core 3 813 829.92

Core-log shift of several meters within

some vuggy intervals

Thick/Lem. Calcite cement

Silty/Fine grainestone?

Dark Grey Clay

Bioclasts Packstone/grainestone shell streinkern

Dark Grey marls burrows Filled with fine grainestone

Fine bioclasts Packstone-grainestone Tight cemented

Wackestone Fine bioclasts on grey

Grey mudstone, tight

Fine Bioclasts Packstone, GreyGraiestone + leaches bioclatsVery coarse bioclasts grainestone shells

Grainestone to Packstone Fine bioclasts leaches

No sedementary Structures visiblePackstone

Calcite cemented nodules?

Core 4 837 - 848.37

Bivalve packstone

Clay pebble LAG Fine grainestone to packstone

Core 808-814.5

Shells + gastropods

V

V

V

V V V VV V V

VVV

V VV VV V V

V V VV V V

V V VV V V

V V V

V V

VV

V

V

V

V

V

V

V V VV V V

V V

V V V

V V

V VV V

M W P G F R B C

CARBONATE TEXTURES

CLA

Y0

.01

6

SILT

0.0

62

VERY

FIN

E

0.1

25

FIN

E0

.25

0M

EDIU

M

0.5

00

CO

ARS

E

V. C

OA

RSE

1.0

0

820

Vuggy mollusc grainstone

5 c

m

Coral rhodolith rudstone

821.5 m

5 c

m

5 c

m

Stromatolites

5 c

m

809.56 m

Mollusc packstones to marls819.55 m

THM-B THM-C

Poster 4 of 5





Lithofacies Description Core Outcrop

This facies consists of dm to m-scale bedded sands that are composed of bivalve and

gastropod fragments. In the lower part of the Dhiban Fm. also some scattered coral

and rhodolith/coated bioclasts fragments are present. These sands are structureless or

mottled and bioturbated, in places some faint parallel and cross lamination is visible.

The sands are interpreted as beach and shoreface sands of a protective barrier of lime

sand shoals that separated the tidal flats and evaporites from more open marine shelf

or basin. The more burrowed and slightly muddy (packstone) intervals represent

deeper water lower shoreface setting. The presence of coral fragments and rhodoliths

suggest that the barrier complex is locally associated with reefs.

Thinner sand beds within the lagoonal muds and evaporitic beds may represent storm

deposits that pushed some sands from the sand shoals into the lagoon and onto the

tidal flats.Mouldic pores after leaching of

shelly fragments

Tortonian Wadi Shijri area

5 c

m5 c

m5 c

m

Lagoonal clays

Playa muds

Karst breccia

Wadi Ajir stratified Tortonian evaporites

Nodular anhydrite

Wadi Ajir ‘Helvetian carbonate’

Evaporites

Molluscan grainstone

Stromatolites

Anhydrite occurs as scattered nodules (cm to dm size) in the sediments or in layers

of up 6 m thick. The latter can consist of cm size anhydrite nodules in a muddy

bedded (algal laminated?) sediment, nodular ‘chicken wire fabric or massive

anhydrite. Internally these layers consist of a stacking of 1 to 2 m thick packages,

each with bedded evaporites at the base grading to nodular to massive at the top.

These packages are separated by dissolution surfaces, in places overlain by thin

beds of gray clays or lags of reworked evaporite clasts. Rarely some faint vertical

elongated growth structures can be seen in the anhydrite that may represent relicts

of gypsum crystals.

The fabric of the anhydrites suggests they are the result of authigenic growth of

evaporites within the sediment from hypersaline groundwater in a very arid climate

(Figure 6). This process took place either above the groundwater table in the

intertidal zone or above as well below the water table in the supratidal zone

(James, 1984). As they grow within the sediment, the evaporites represent a very

early diagenetic overprint of the host sediment which could be from various

depositional environments not directly related to the setting were the anhydrite was

formed. The scattered anhydrite nodules represent later diagenetic features and

are not related to the depositional setting of the host sediments.

Chaotic intervals of up to 2 m thick with dispersed intraclasts, broken beds and

sediment filled cavities. These occur both in the upper part of and just above the

Dhiban Fm.

The association with the evaporite beds suggests that these are collapse breccias.

Percolating groundwater of low salinity (probably meteoric) dissolved evaporites

creating a void leaving no support for overlying sediments. Like the evaporites

these structures represent an early diagenetic event that is related to a stratigraphic

surface shallower than the affected interval.

Algal intervals are cm to m thick and consist of laminated to stromatolitic mudstones.

The algal mats show irregular to even laminations and may vertically change into

more domal shapes. Outcrops in the Bishri area suggest that the small stromatolites

and algal laminations visible in the core may actually be part of much larger

stromatolites. In the lower Dhiban Fm. the stromatolitic beds are often associated

with mollusk grainstones.

Algal mats suggest en intertidal zone possibly even extending into the subtidal zone

in case the waters are hypersaline while the stromatolites more higher energy

environment with active sediment movement (James, 1984).

This facies is characterized by fine bioclastic marls/muds with wavy to low angle

cross bedding, mud drapes, mud cracks and only some rare burrows. Common

erosional/scour surfaces are present overlain by lime sand beds with cm-sized

intra clasts and lags (shells, mud, clay, flat pebble); the clasts sometimes contain

borings. Scattered anhydrite and chert nodules are common. This facies occurs as

a 6 m thick unit in the Upper Dhiban Fm. in the Thayyem Field. In Thayyem-2 a

brecciated intervals occurs just above this unit.

The lack of bioturbation in this facies suggest deposition in a highly restricted,

probably hypersaline environment. These may have been restricted lagoon/ponds

(coastal salina) behind the barrier complex. The association with dissolution

breccias (salt dissolution?) and the presence of elongated structures that may

suggest in-situ growth in the evaporites just below and above this interval also

point to a coastal salina type of setting.

These consist of strongly bioturbated marls/lime mud - packstones with common

bivalve fragments (shells sometimes bored). These form intervals of 1 to 2 m thick

and are associated with the mollusk sands. This facies commonly occurs in the

lower to middle Dhiban.

These sediments were deposited in a low energy, subtidal non-restricted

environment. The close association with the mollusk sands and the evaporites

suggest a lagoonal environment between the semi protective shoal complex and

the tidal flats.

5 c

m

6. Linking Outcrop & Core

Poster 5 of 5





?18 Ma

15 Ma

Top Dhiban Karst

Sabkha/Playa Salinaclay influx

Mod/low energy lagoonal

Base Dhiban

Lower energy open marine

Eu

ph

rate

sD

hib

an

Je

rib

e

ng20

ng30

Euphrates Graben Bishri Block

Sequence Stratigraphic Model

Lower Dhiban Transgressive Systems Tract

Flat time correlation lines on a field scale

Very high lateral continuity of flow units

Evaporites will form horizontal baffles but will have limited

lateral extend Transgressive lags may form High K streaks

Upper Dhiban Highstand Systems tract

Flat time correlation lines on a field scale

More frequent intercalated baffles (evaporates and

mud/marls) which are laterally more continuous, reducing Kv

Possible incisions (channeling) near top

Stronger early diagenetic overprint than in lower Dhiban,

impact of early freshwater diagenesis on rock fabric related to

karst at top Dhiban

7. Dhiban Formation Sequence Stratigraphic & Depositional Models

8. Reservoir Implications 9. Conclusions

Figure XX shows a model of the sequence stratigraphic framework based on the core observations and thewell log information of the Euphrates, Dhiban and Jeribe Formation. Major flooding surfaces occur in thelower Euphrates and Jeribe Fm, the lower one has been tentatively dated at 18 Ma (Burdigalian) by Sharlandet al. (2001, ng 20), the upper one has a more firm date based on planktonic foraminifera of 15 Ma (Langhian,ng 30 Sharland et al. 2001). Another regional flooding event occurs in the middle of the Dhiban Formationcorresponding to a shift to a more open lagoonal setting within the tidal flat interval.

Sequence boundaries associated with exposure and karst occur at the base of the Dhiban in the Thayyemfield (the top of the ‘chaotic rubble bed’) and on top of the uppermost evaporite unit. The lower boundary isnot well defined in the An Nishan area but may correspond to the top of the lowermost anhydrite. Severalother levels associated with exposure occur in the upper Dhiban (karst breccias) and at the top of theevaporitic units, these represent higher order sequence boundaries.

The surfaces define three sequences that correspond to the lithostratigraphic units each made up bytransgressive and regressive systems tract. It is possible that the Dhiban represents a higher order sequencethat those of the Euphrates and the Jeribe (e.g. if the rubble beds at the base Dhiban represent justtransgressive reworking, see discussion in stacking trend of the Thayyem Field. In this case the Dhiban andthe Euphrates form one sequence with a transgressive and highstand systems tract capped by the UpperDhiban exposure surface.

A similar sequence stratigraphic pattern can be observed in the time equivalent outcrops of the Bishri High: amajor sequence boundary associated with soil horizons at the base of the Helvetian Limestone (base Jeribeequivalent) and a sequence boundary associated with a channel incision into the Lower Miocene sandsequivalent to the top Euphrates. The maximum flooding surfaces at the base of the marly section in the LowerMiocene (equivalent middle Dhiban) and within the Helvetian carbonate (equivalent basal Jeribe). The similarstratigraphic patterns in these two areas despite different depositional and structural setting suggest that therelative changes in sea level involved are regional (eustatic?) events.

The regressive unit shows a more restricted facies

association of thick stromatolites, salina (salt ponds) muds and

marls and supratidal sabkha. In this setting the protectivebarrier is better developed with a low energy inter- to subtidal areabetween the barrier and the supratidal sabkha. As the systemprograded out parts of the lagoon became more distant from theopen sea and changed into hypersaline ponds/salinas within thesabkha plain (see also Kendall, 1984). Strong progradation resultsin laterally more continuous evaporite units.

The transgressive and regressive part of the Dhiban Fm each arecharacterized by different facies associations and two depositionalmodels are required to describe this interval. The transgressiveinterval is dominated by moderate energy subtidal mollusk sandsand littoral deposits closely associated with supratidal sabkha. Thismay suggest that the protective barrier is relatively poorlydeveloped and that the intertidal and subtidal environments areisolated by growth of beach bars and spits (Kendall, 1984). Thesupratidal sabkha sediments are less continuous as a result of thebackstepping stacking pattern (see also Pratt et al. 1992).

?18 Ma

15 Ma

Sabkha/Playa Salinaclay influx

Eu

ph

rate

sD

hib

an

ng20

ng30

Euphrates Graben

Depositional Model

• regressive scenario

– salina (salt ponds)

– playas

– stromatolites

– supratidal sabkha• transgressive scenario

– lagoonal moderate energy grainstones

– littoral deposits

– supratidal sabkha

Top Dhiban Karst

SabkhaSabkhaSabkhaSalinaRestricted

lagoonProtective barrier

SabkhaBeach bar/spitMollusc sands

In the Thayyem and An Nishan area both the Jeribe and Euphrates Formationsconsists of open marine to shoreface sediments while the Dhiban Fm. consists ofperitidal carbonates and evaporites.

Each of the above formations consist of a transgressive regressive cycle. As thetransgressive and regressive parts are characterized by different faciesassociations, at least two depositional models were required to describe eachformation.

Each depositional model had also different implications for the lateral continuityand stacking trends of the reservoir and sealing intervals.

Seismic not shown here, shows the presence of mounded features in theEuphrates Fm. which may represent coralgal reefs and suggest that adifferentiated topography was present before deposition of the Dhiban. Possiblythe peritidal carbonates and evaporates are filling in remnants this topographyfollowing a relative drop in sea level.

The hydrocarbons saturation shows a very irregular distribution in the reservoirsand no structural closure of the Thayyem Field can be mapped to the west whilethe updip wells are dry. These accumulations may represent unconventionaltraps or may be on a migration pathway. An overview of different trappingmechanisms and a required evidence has been provided.

unlocking the remaining potential of the thayem and...

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