Development of a Mature Giant Offshore Oil Field

Abstract

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