eature by L d i J. wood I ~ t ~ d Gmlyn Roberts ' Article 'Bureau of Economic Geology, University of Texas at Austin, Austin, T m (I& wood@beg.ut~~~(.eduJ

'Ministry of Energy and Energy Industries, Trinidad 6 Tobago, Port-ofispain, Trinidad (mhrm@energy.gov. ttl

Opportunity in a World-class ~ydrocarbon Basin: Trinidad and Tobago's Eastern Offshore Marine Province

lnaoductlon The eastern offshore marine province (EOM) of the eastern East Venezuela Basin is one of the most prolific hydrocarbon provinces in the world with proven reserves of over 486 MMbl and 23.4 TCF of natural gas (Fig. 1). Combined proven, proba- ble and possible reserves in the offshose basin are calculated at over 1.5 billion bbl of oil and 33.5 TCF of gas. Development of liquified natural gas (LNG) processing facilities on the island of Trinidad over the past 5 years has turned the s n d island nation into a leading world supplier of LNG. Trinidad currently exports to Spain, as well as providing over 40% of the LNG needs of the United State's east coast custonlers (Barrionuevo, 2001). Some estimate exploration potential to total 50-70 TCF of gas in a country where companies experience an 80% success rate of wells (Thomas, 2001). Rising U.S. gas prices in early 200 1

have many U.S. industries considering relocating or expanding to Trinidad. The combination of a stable government, synchronous design of hydrocarbon markets, and new explo- ration and development opportunities has forced conlpanies to take notice of the region's potential alongside that of west Africa, eastern South America and the US. Gulf of Mexico (Thomas, 2001). An ongoing cycle of bid rounds (1996, 1998 and 2001 [dosing on September 4, 20011) in both onshore and offshore acreages will open up new development opportunity in existing fields and exploration opportunity in the unexplored deep and ultradeep water acreage.

The Graveyard of Geologists The majority of current offshorc production occurs in the region defined by Leonard (1983) as the Columbus Basin. Almost 100% of the current wells are drilled in water depths trf less than 150 meters and are located less than 100 kilometers from onshore processing facilities. Often referred to as the

"graveyard of geologists" the basin is developed along the oblique transpressional margin of the Caribbean and South American plates. The basin is bounded on the south by the sta- ble Orinoco Offshore Platform and on the north by the thrust-cored Darien Ridge (Fig. 1). It is the eastern extent of a transpressional deformation front that has advanced diachro- nously toward the southeast in direct response to the eastward migration of the Caribbean plate (Erikson and Pindell, 1993; Lugo and Mam, 1995; Parnaud and others, 1995).

The most recent oblique foreland basin, the Columbus Basin is filled with over 12 kilometers of clastic, late Tertiary age sediments (Wood, 2000). The Orinoco River has been the primary source of sediment for the basin, having established an outlet to the Atlantic sometime during the early Miocene (Diaz de Garnero, 1995). The Orinoco River and Delta advanced onto the eastern shelf during the high-frequency shoreline changes of the Pliocene and Pleistocene. Its supply-dominated nature meant that periods of rising shoreline had little effect on its progradational character. The Cretaceous age passive margin, which dips steadily to the north h-om the Venezuelan margin, formed a stable platform across which pre-Pliocene distal marine shales were deposited (DiCroce, Vail and Bdy, 1999). Rapid deposition of Pliocene and Pleistocene clastics that tollowed resulted in significant mobilization of underlying under-compacted shales. Compressional plate boundary stresses combined with depositional loading to produce a signif- icant number of mud volcanoes throughout the late Tertiary. This diapiric activity continues today on the present-day seafloor and in subaerial portions of the modern delta (Huyghe and others, 2000; Brami and others, 2000; Aslan and others, 1998). Thc combination of active tectonics, strong

June 2001 Houston Geological Soc~ety Bulle!~n

0 10'mi 0 100 km After Pocknall and ather@ (1 998)

%7000. Water depth

A 7 Strike-slip fautt Thrust fault

U Normal fault w 7 5 4 ( b ) c

F@m 1. Teetom-gmgmphic map showing regumal mudurn1 hhrm northern South America, including tha island 4 Rnidad and Tobago, and dre exfzmional, minibasin and wmprernonal pmvinces associated with Trinidad's eastern more marine (EOM) hydmrbon barin.

oceanographic processes, late Tertiary sea level changes and enormous sediment supplies makes for a cnmplex hydrocarbon systems that justifies its label as the graveyard of geologists.

IntegmUon for Answen, Exploration and production activity has increased in the basin throughout the past decade, spurred by competition between companies anxious to grow their involvement in the region's hydrocarbon industry. This growth has been driven by the need to prove reserves in anticipation of LNG development and main- tain the domestic energy supply, including that vital to downstream methanol and ammonia industries. These activities resulted in the publication of several papers regarding the sequence stratigraphy of the basin (Wood, 2000; P&d and others, 1999; Pocknall and others, 1998), application of new seismic technology (Sharp and others, 1998; Day and others, 2000), and hydrocarbon maturation and migration (Heppard and others, 1998). The majority of present-day public knowl- edge is confined to &r offshore extensional province; however, recent drilling and data acquisition in deepwater acreage has

begun to increase understanding of the more distal provinces u l the eastern offshore marine area (Fig. 2; Patterson and others, in

press; Brami and others, 2000). Although varymgly complex in their structure and stratigraphy, these distal provinces are referred to here as the minibasin and the compressiond tectonomorphic provinces for simplicity.

Prh.ary stmcturaJ elements in the extensional province of the basin include: 1) a series of transpressional northeast-southwest trending ridges and 2) northwest-southeast oriented, down-to- the-northeast normal faults. Major hydrocarbon fields trend along the basin's four principle ridges. They are defined as the Sarnaan-Galeota Ridge, Poui-%&-Omega, Ibis-Pelican-Oilbird and Cassia-East 'Mayaro anticlinal trends (Fig. 2). Normal faults are generally younger to the northeast as are the 'hydrocarbon traps and reservoir ages. Hydrocarbons are trapped in a series of downthrown four-way, bow-tie anticlinal structures with east-west closure formed by two-way subsidencedriven rollover, and north-south closure formed by upIift along the bust-cored ridges (Fig. 3).

Houston Geological Society Bulletin June 2001

Figure 2. Major structural features of the EOh4 area including regional roller farrlts, right lateral strike slip faults and offshore anticli- nal ridges. A4ajor hydrocarbon fields as shown distributed along these ridges.

Fauna and flora extinctions and evolutions, as well as some abundance data, derived from over 41 wells were combined with abundant seismic data, outcrop, and log facies analysis by Wood (2000) and Pocknall and others (1998 and 1999) to establish a chronostratigraphic framework for the extensional province of the EOM. Ten progradational clastic wedges form the Pliocene- Pleistocene stratigraphic architecture of the area (Fig. 4). These wedges prograded from southwest to northeast and are youngest to the east. They were sourced primarily by the wave-dominated paleo-Orinoco delta and can show a great degree of lateral conti- nuity in facies along depositional strike. Within each wedge, the depositional facies deepen progressively from southwest to northeast, changing from ( 1 ) terrigenous fluvialtestuarine to (2) progradational o r aggradational shoreface to (3) middle and outer neritic shelf to (4) slope and finally to (5) basinal facies over distances of a few kilometers. The distal toes of these sequences downlap o n t o distal submarine shale ridges. Depositional sequences average 10,000 to 12,000 Ii in thickness and accumulated over relatively short periods of time (300 to 500 kYr). These sequences are considered third and fourth-order based on the definition of Mitchum and Van Wagoncr (1991). The sequences are bounded at their bases by sequence bound- aries of basinwide extent that include, from oldest to youngest,

( 1 ) NBP (3.8 Ma), (2) GLS (3.6 Ma), (3) B8LS (3.0 Ma), (4) BB2LS (2.3 Ma), (5) NLS (2.0 Ma), JLS (1.78 Ma), HLS (1.5 Ma), ELS (1.0 Ma), DLS (0.8 Ma) and BLS (0.5 Ma) (Fig. 4).

Bow-tie Anticlines The pre-Pliocene mobile shales exert a significant influence on the geometry and kinetics of hydrocarbon traps forming in asso- ciation with prograding megasequences. Megasequences are cut by a series of down-to-the-northeast growth faults that appear to sole out above the underlying Mesozoic stratz (Fig. 3; i.e., roll-

over faults of Rowan and others, 2000). Regional extension initiates normal faulting and diapiric rise (Jackson and others, 1994). Sediment accumulation in the developing accommoda- tion space exacerbates shale evacuation beneath the prograding wedge. Shales evacuate to the east along the gently dipping

Mesozoic detachment surface to form toe-of-slope diapirs. Sediments thicken to the west in response to accommodation developing in the hanging wall of the normal roller faults, and they thicken to the east in reponse to accommodation develop- ing along counter-regional glide plane surfaces associated with the evacuated shale diapirs. Once the mobile substrate ic exhausted and a shale weld occurs, the supply-dominated deltaic depositional systems fill any remaining accommodation space.

June 2001 Houston Geological Society Bulletin

The entire system then steps basinward to the next developing f - b - .' normal fault and the process begins again. The result of these

series of events is bi-directional off-crest stratigraphic thickening .: and bi-directional rollover resulting in the formation of a bow- tie shaped anticline (Figs. 3 and 5). The entire system resembles

3 ,+ the classic Gulf of Mexico-style Roho fault systems classified by c3 Rowan and others (2000) as a Roller Fault Family. Significant

& rollover fault development can be seen in these extensional province, bow-tie anticlinal structures with younger rollover fault segments migrating basinward and upward. Such basin- ward migration of roller faults is in response t o basinward migration of the anticlinal hinge over time caused by seaward evacuation of shale in the distal shale roller (M. Jackson, pers. comm.). When employing a fill-and-spill model of hydrocarbon charge (Gibson and Bentham, 2000), the occurrence of such rollover faulting has implications for the secondary migration and distribution pattern of hydrocarbons throughout these bow-tie anticlinal structures.

Although exploration in the extensional province of the EOM has been ongoing for nearly half a century, significant discoveries continue to be made by companies active in the area. Recent successes by EOG Resources with its Osprey- l and Tanager- l

wells and BP discoveries at Red Mango (Farmer, in press) and Manakin-1 make current blocks open for bid in the present bidround more prospective. Major technical issues in the exten- sional province continue to be seal presence and in deeper facies, reservoir sand volumes, as well as drilling problems (ix., high pore pressure) associated with deeper stratigraphic horizons.

Opportunity in New Provinces Significant industry and government focus in the past several years has been placed on deep water opportunities in the EOM area of Trinidad. Exploration activity by most companies throughout the late 1990s has been focused on the minibasin

tectonomorphic region (Patterson and others, in press; Brami and others, 2000; Ramkhelawan and others, 2000) with an eye toward future seismic acquisition and drilling opportunities in the ultra-deep, compressional province (Fig. 1).

The minibasin province occurs basinward of the present-day shelf-slope break and extends -120 km northeastward. At least three major shoreline lowstands (1.0, 0.8, and 0.5 my., Fig. 4)

have transferred sediments out to and beyond this province. Mud diapirs and toe thrusts bound basins that can be asymmetric to either the basin or the shelf (Fig. 6). Escape sedimentation dominates this province occuring in and around these bathyrnetric highs (Fig. 7). Hydrocarbons may migrate along the shale diapirlflank faults o r within the overpressured and fractured diapir itself to charge these basins at depth, then continue migra- tion updip until limited by petrophysical o r lithologic pinchout. It was generally accepted by most geoscientists that diapirs post-date thrust faulting, with faults acting as fluid conduits tu

the surface and encouraging diapiric rise. However, recent physi- cal modeling of diapiric processes suggest that diapiric rise, driven by regional extension, may predate thrusts, and can pro- vide weaknesses across which thrusts detach when acted on by compressional plate forces (7 . De Chalvez, pers.comm.,).

Principle technical issues in the monoclinal province appear from early drilling results to be hydrocarbon presence and migration (Ramkhelawan and others, 2000). Yeilding and Apps (1994) documented that in the Gulf of Mexico, it was approxi- mately 113 km from the blocky, lobate sand bodies of the lower slope to the coeval updip shelf margin. In the EOM area of Trinidad, transport of the late Pleistocene (1.0, 0.8 and 0.5 m.y. lowstands) coeval shelf margins 100 kilometers east, would place well-developed lower slope reservoirs significantly farther east

than the current exploration blocks. Companies currently exploring in the paleoslope bypass province recognize the need to move basinward to find quality reservoir sands.

The compressional province occurs approximately basinward of the 1600-m bathymetric contour and extends to the front of the modern accretionary prism (Fig. 1). Older, pre-Pliocene section shallows stratigraphically eastward, away from the axis and

depocenter of the EOM basin (Sanchez, in press; Cresini, 1998). Therefore, this province may offer significant opportunity for exploration success in older strata and traps. Here, sediment sinks form in piggyback basins between thrusts associated with accretionary wedge subduction. Depocenter axes in asymmetric- to-the-basin piggybacks migrate toward the shelf as progressive

thrusting backrotates the older host sheet (Fig. 8). Upper Pleistocene, deep-marine, basin floor turbidites most likely exhibiting sheet geometries (similar to their modern counter- parts; Faugeres and others, 1993; Huyghe and others, 2000) show mounded and shingled offlap seismic facies. They are perched along the rising eastward basin margins and intermingle with large slumped debris flow deposits. Hydrocarbons can migrate into these basins along deep, bounding thrusts and associated faults. In the backrotated basin, hydrocarbons may migrate updip into traps formed in perched, basin floor turbidite deposits o r be found in later thrust-fault traps associated with progressive subduction of the wedge (Fig. 9).

The principle technical concerns in this province is most likely hydrocarbon migration and presence. Reservoirs are deposited as debris flows around and off the flanks of growing structures. There is some uncertainty about sand content in these deposits. However, the strong deep currents, well documented off this continental margin, probably rework deposits and may result in clean, concentrated and well-sorted reservoirs in certain deep-water locations. The active tectonic nature of this province creates concerns of trap integrity, but opportunities exist to produce late structural traps as younger thrusts cross-cut older debris flows. Proximity to the continental/oceanic crust boundary

Houston Geological Society Bulletin June 2001

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June 2

001

Northeast

-r-ran Deep matine slow deposition and minor erosion

PluviaVestuarine sands and shales - Sequence bourrdary

- - - Matine Rooding surtace

- - - - - . . Correlative mnformity

-746(a)c

Figure A Chronostratigraphic chart of the Enidad eastern offshore marinepmvince (Columbus Basin of Leonard, 1982). Note the uverall continuous progmdarioml character of megasequences in the basin

creates hydrocarbon source and generation concerns that will be resolved only through well tests.

Summary The EOM area of Trinidad and Tobago is a world-class gas and oil province with profitable markets and prolific reserves. Although current proven, probable and possible reserves in the entire country currently are 1.5 billion bbl of oil and over 33

TCF of gas, much of the available acreage has yet to be explored. Current and developing markets both on the island and in the United States make this a very attractive E & P region, combined with a stable government and good infrastructure.

The eastern marine basin is located near the juncture of the Atlantic, Caribbean and South American plates and its character and fill are strongly influenced by the oblique collision between the Caribbean and South American plates, as well as compres- sional tectonics from the Caribbean plates overriding the more eastward Atlantic plate. The basin can be divided into at least three major tectonogeomorphic provinces: extensional, mono- clinal minibasin and compressional, each with its own unique processes for generation of accomodation space and basin Wing.

Within the extensional province, extension, compression and sediment loading combine to produce a regional normal, roller fault system that are younger to the east, and abundant shale diapirs with associated counter-regional glide planes. Ten large

mega-sequences of Pliocene-Pleistocene age form the basin's fill with fluvial facies transitioning westward to deep marine fans and slope deposib. These wedges thicken across normal fadts and down into basinward counter-regional surfaces to form bow-tie anticlines with crests that migrate basinward as one moves stratigraphically higher in the section. Secondary roilover faults, developing in response to this crestal migration influence, charge pathways and segregate hydrocarbons across the larger anticha1 structures.

The minibasin province, currently the focus of exploration activ- ity in the area is dominated by abundant shale diapir growth around which interbasin fill-and-spill processes occur. Several significant deep-marine, slope leveed-channel features are [den- tifiable on seismic from the area. This province serves as a sand conduit to deeper marine regions. Significant remaining techni- cal issues in this province are hydrocarbon migration and source rock maturation.

The compressional province, located distal of the 1600-111

bathymetric contour to the front of the present day accretionary prism, is a region of plggybadc basin formation with depocenters that translate basinward or landward in response to loading by proximal thrust sheets. Documented strong oceanographic processes offer the opportunity to rework debris flow deposits into quality reservoirs. However, active tectonics increase the risk of seal breach and loss of hydrocarbons into younger, shallow sehen t s .

Houston Geological Society Bulletin June 2001

Rollover

Bow-tie anticlinal structure

Bi-directional thickening both landward and basinward Well-developed shale railers

CR f a u l t Secondary rollover faults partion hydrocarbon Rollover faults migrate basinward indicating

OAc8612(b)c

Flgure 5. Schetnatic illustration o f n bow-tie anticlinal structure developing in the hanging wall of a roller fault. Sediments thicken in tht. landward directions along normalgrowth faults and in the basinward direction along colrnter-regionalfbtilts associated with large toe-of-slope shale diapirs. Signijicnnt secondary rolloverfarrlts fonning from structural crest migration basinward will have a sig- nificant impact on hydrocarbon distributiot~ as fluidsfill-and-spill their way across the structrtre.

L-- - -- -- -- --- -- - .. - - --

I- Southwest Northeast 1

Seisrnic provided by the Ministry of Energy and Energy Industries, Trmidad and Tobago CJAc7554c ( Figure 6. Seismic and line drawlrrg from the minibasin province in Trinidad's EOM arm showing the seismic Figures 7- 9

facies and structrrrr across the area. on page 45 )

Biographical Sketches DR. LESLI J. WOOD is a research scientist at the University of Texas Bureau of Economic Geology. Her expertise is in basin analysis, risk assessment, clastic sedimentology and sequence stratigraphy, and evolution of tectonically active basins. After completing her PhD at Colorado State University, Dr. Wood

worked for six years in the hydrocarbon industry, first with

A R C 0 then Amoco. Her o ther research interests include Caribbean plate tectonics m c t hydrocarbon systems, Quaternary stratigraphy, shale diapirism, deep marine sedimentation and seismic sedimentology.

CAROLYN ROBERTS is a senior geologist a t the Ministry ( t i Energy & Energy Industries, Trinidad & Tobago. In that capaci- ty she is involved in numerous energy projects that vary from negotiations for production sharing contracts to log analysis. She has a Master's degree from Imperial College, UK. Her special interests include petrophysics, hydrocarbon systems, geochemistry, shallow and deep marine sedimentation and Caribbean tectonics.

~ -- -- -- - - - ~.

1 NOTE: Due to space considerations references are available upon request. I I - - - . - - - - -

June 2001 Houston Geological Society Bulletin 43

I Source

Ponded turbidites Vertically stacked, minor fi debris flows

a Large, HA slope leveed channel system Pure vertical accommodation

I Rgwe 7. Schematicfigure of the interbasin, escape sedimtarion processes active ncross the minibasin tectorwmorphic province. I

Seismlc provided by the Ministry of Energy and Energy Industries, Tdnidad and Tobago QAca818(a)c

Figwe 8. Seismic line showing a pzggybncf: basin from the compressional province associated with the Barbados Accretionary Prism illustrating the migrating depocenm c-hamcterisCic qf the piggyback basin type shown.

Source Landward-migrating dep-era Domlnant north tranawrt s h e e t tur~iites inter6edd.d with ddtts ~ o w s Oblique vertical accommodation

ngure 9. Schematic fiurr of the compressiorrul piggyback basin province of the Trinidad eastern ofisbore marine area.

June 2001 Houston Geological Society Bulletin