Summary The Teak field, a mature oil field offshore southeast Trinidad, has produced266 million bbl oil since 1972. Production is from Pliocene sandstonereservoirs in a Pliocene sandstone reservoirs in a complexly faulted anticline. This paper summarizes the history and geology of Teak field, describes itsoil-producing reservoirs, and discusses the mature-field development strategyused since 1988, including case histories of two 1989 developmentprospects. Introduction The Teak field, the fourth largest oil field in Trinidad, is located 25miles off the south-east coast of the island in 190 ft of water (Fig. 1). Discovered by Amoco Production Co. in 1969, Teak has produced 266 million bblof oil and 1.1 trillion scf of gas from 1972 through 1990. Teak is a mature oilfield with significant depletion after its first 19 years of production. Despite a steep natural decline rate of 25%/yr, the field has respondedfavorably to renewed development drilling since 1989. A steady downward trendin production rate reversed, and the field achieved its highest rate in almost5 years in July 1990. A primary factor in the reversal of the decline rate wasa deliberate multidisciplinary team approach to reservoir development. Geology Structural Setting. The Teak field is located in a regionalcompressional-wrench terrain about 40 miles south of the current Caribbean and South American plate boundary along the El Pilar fault (Fig. 1). The fieldproduces hydrocarbons from Pliocene sandstones from 4,000 to 14,000 ft belowsea level. Hydrocarbons are trapped in a steep anticlinal structure along aneast/west-trending compressional ridge bisected by a large fault (Fig. 2). Thisfault dips east-northeast at 35 to 45 degrees, with vertical offset of 2,000 to3,000 ft, splitting the field into two separate hydrocarbon accumulations: oiltrapped to the west on the upthrown side of the fault and gas trapped to theeast on the downthrown side of the fault. In addition to normal offset, Fault Fexhibits left-lateral wrenching, indicated by displacement of the downthrownstructural crest about 1 mile north of the upthrown crest. This paper focusesprimarily on the upthrown oil reservoirs, which are arranged as stackedthree-way closures behind the major sealing Fault F and are between 4,000 and12,000 ft subsea (ss) (Fig. 3), informally called the Teak A-C-E field. The Teak structure is characterized by fairly steep bed dips that average 10degrees and steepen locally to more than 30 degrees. Bed dips are locallyvariable in both direction and magnitude. The structural geometry iscomplicated by myriad cross-cutting normal faults that segment the field intonumerous compartments. The oil accumulation up-thrown to Fault F is separatedinto two major fault blocks by Fault F (800 to 1,500 ft vertical offset) andseveral minor fault blocks. About 90% of the Teak field oil produced to datehas been trapped upthrown to both Faults F and F2, although deeper reservoirshave been (discovered and produced between these faults. Stratigraphy and Petrology. Teak oil reservoirs span an 8,700-ft-thickstratigraphic section in the Pliocene that contains approximately 60 % sand and 40% shale. Teak sands are typically very fine-grained (from 0.06 to 0.12 mm ingrain size) and extremely well-sorted, and average more than 90% quartz withlittle or no visible cement. Porosities average 27 to 33%. Teak shales Porosities average 27 to 33%. Teak shales exhibit a variable resistivity andgamma ray log response, unlike clean shales typical of the U.S. Gulf of Mexico. Shale lithologies vary from argillaceous shales to arenaceous mudstones, andtextures vary from soft to well-indurated and phyllitic. Oil-producing sandsexhibit no appreciable stratigraphic thickness variation across theoil-productive area. The Pliocene producing section is 10 to 20% thicker on thedownthrown side of Fault F, but individual sands maintain an excellentcorrelation across the field. The environment of deposition for the Pliocenesands is probably a broad, shallow Pliocene sands is probably a broad, shallowmarine shelf proximal to the Orinoco delta, where strong, steady coastalcurrents provided a uniform energy regime. Oil Reservoirs Reservoir and Fluid Parameters. High-quality crude oil, 27 to 33 degrees API, has been produced from 17 different sandstone reservoirs (Fig. 3 and Table1). Individual sands vary from 10 to 440 ft in thickness. Sand porosities rangefrom 18 to 33%, decreasing slightly with depth. Initial water saturationsaverage 25%, with residual oil saturation of 20%. Hydrocarbon columns rangefrom 170 to 900 ft of oil with thin gas caps. April 1991 P. 392

La constitution de la marge Ifni/Tan-Tan a commencé lors de la distension triasicoliasique et s'est poursuivie pendant l'ouverture de l' Atlantique central, qui a eu lieu au Jurassique, jusqu'a l'actuel. Située au Sud du linéament sud atlasique entre deux domaines géotectoniques variés : un domaine au Nord OU l'effet de la tectonique atlasique et alpine est tres spectaculaire et un domaine au Sud OU cet effet est tres discret, la marge Ifni/Tan-Tan est caractérisée par des séries post-rift peu déformées et perturbées, en mer, par des montées diapiriques de sel. Ces séries, qui masquent en totalité les structures du rift, ont pu etre étudiées grace al' exploitation de nombreuses données acquises par l'exploration pétroliere (sismique réflexion et forage) et a l'analyse de données de champ de potentiel (gravimétrie et magnétisme).L'étude des structures du rift, aterre et en mer, met en évidence 1'0pposition entre deux secteurs de plate-formes : (i) un secteur au NE ou le toit du substratum anté-rift est peu profond et ou le style structural dominant est celui de demi-grabens séparés par des horsts et (ii) un secteur au SW ou dominent les grabens.Cette étude montre que deux types structuraux et deux modes d'extension peuvent coexister a une centaine de kilometres l'un de l'autre sur une meme marge passive. Ces différences sont imposées par les structures héritées de l'évolution hercynienne.Mots·c1és: Maroe ,. Marge passive ,. Rift ,. Sismique réflexion ,. Magnétisme ,. Gravimétrie ,. Méso-zoi"que. ABSTRACTThe formation of the Ifni/Tan-Tan margin started during the Triassic-Liassic extensional event, and continued during the Jurassic to present opening of the central Atlantic. This margin is located to the S of the South-Atlas lineament, between two contrasted geotectonic scenarios: the N, where the effect of the atlasic and alpine tectonics is spectacular, and the S, where this does not occur. The Ifni/Tan-Tan margin is characterized by a series of post-rift deposits only slightly deformed and disturbed by salt diapirism towards the ocean. These series completely mask the structures of the rift, and they have been studied with the abundant information gathered by the oil exploration campaigns (seismic reflection and drilling), and with the analysis of gravimetrical and magnetic field-potential data.The study of the rift structures (both inland and offshore), highlights the difference between two sectors of the platform: (i) a NE sector, where the upper part of the pre-rift substratum is not very deep, and where the dominating structural style is constituted by half-grabens and horsts; and, (ii) a SW sector where grabens dominate. This paper shows that two structural styles and modes of extension can coexist on the same passive margin, one hundred kilometers aparto The differences described here are imposed by the structures inherited from the Hercynian evolution.

In October 1999, the Brighton Marine Block was awarded to Venture Production Company Trinidad Limited and Petrotrin, the Trinidadian National Oil Company, with Venture as operator.The challenges to improved field recovery posed by such a complex field, with more than 50 years of production history, are many. The field has potential, but requires new interpretations and ideas based on both new and old datasets, a phased programme of rehabilitation and a long-term development plan that will generate immediate value without compromising long-term oil production.Discovered in 1951, the Brighton Marine Field has produced almost 58 10 6 STB from an oil in place of over 500 10 6 STB. Production peaked at 14 000 BOPD in 1967 but had not exceeded 1000 BOPD since 1983.Some 275, mostly deviated, wells were drilled into the field from 11 onshore pads and 9 offshore platforms. In the year since Venture assumed operations, production rose to nearly 2000 BOPD through field infrastructure rehabilitation, well workovers and the implementation of new field management processes. In early 2001, the first new well proved the presence of a commercial quantity of petroleum in a hitherto undepleted part of the field.It is very early in the rehabilitation process and much remains uncertain, but significant progress has been made in understanding the field and tapping its full potential. The field's first 3D seismic survey, acquired in 1997, has provided a significant enhancement to the understanding of a structurally and stratigraphically complex field. The field compartmentalization and 50 years of production, have complicated the distribution of pressures, sweep and compaction. In order to mitigate risks, a phased strategy was implemented. This process focuses initially on the most straightforward, cost-effective opportunities. These redevelopments are designed to maximize data collection, test new models and help finance more extensive developments. Already, this approach is starting to yield results, although it is still early days.