Abstract: Reservoir Characterization and Sequence Stratigraphic Analysis of Turbidite Sandstones in Catshill Field, Trinidad

Heeralal, Sookdeo and Rambaran, Vishram - Petroleum Company of Trinidad and Tobago Limited

Forty-seven years after initial completion, the discovery well for the Catshill Field continues to flow dry oil (28° API) and gas from 25 ft of laminated sandstone pay. Sustained production performance suggests a reservoir that is laterally continuous and areally extensive. Indeed, it is possible to map sand packages for several thousand feet, with individual thin beds being correlatable over distances of hundreds of feet. The integration of rock samples, well logs, biostratigraphy and eustatic curves resulted in the productive interval of the Karamat-Herrera Formation in Catshill being classified as slope fan deposits within the Low Stand Systems Tract. The section displays a subdued, "ratty" log pattern and comprises stacked sequences of thin, monotonously interbedded fine grained Bouma Tb/c sandstone and mudstone. Reservoir rock varies from almost pure quartz arenite to sublitharenite. Bed thickness ranges from 1 inch to 12 feet, with thinner beds being prevalent distally. Likewise, sand/shale ratios vary stratigraphically from 60% sand in proximal areas to 30% sand at the fringes. Petrophysical analysis of thin sections indicates that there has been some reduction in reservoir quality due to compactional effects, pore occluding clays and cements such as quartz, calcite and siderite. In such instances, a higher than normal formation resistivity factor may have obvious implications for log interpretation in this field.

Reservoir characterization of the slope fan complex is enhanced by the use of high resolution logging. The Formation Micro Scanner (FMS) and Dipmeter have been particularly useful for the following reasons: (a) dip patterns characteristic of thin-bedded turbidites, with microresistivity curves showing sharp erosional bases at interbed boundary; (b) more accurate net pay counts; (c) "whole core" electric imagery which provides vital information on sedimentary structures and bedding type. Visible turbidite associated features on the FMS include: load cast with contorted bedding, "pinch-and-swell" structures, erosive mud lined scours, graded bedding and amalgamated sand on sand contacts.

The Catshill accumulation is relatively small by international standards. It is considered to be an underfilled trap with hydrocarbons reservoired in the crestal position of a thrust faulted anticline. A thorough understanding of the reservoir architecture and depositional geometry is paramount to optimal exploitation of such limited reserves. In this regard, the following key points are noted: (a) Beds are laterally continuous, however vertical connectivity is low. Therefore the Kv/Kh ratio will be low. (b) A common oil/water contact may not necessarily hold true. Each bed may have its unique fluid distribution and composition. Such wells may have to be selectively perforated. (c) With thin beds a slant well may lead to more areal distribution for accelerated recoveries. Thin beds also demand the use of higher shots per foot during completion, otherwise there can be significant bypassed pay (d) X-Ray diffraction of rock samples in Catshill shows all clay types present. Therefore in computing the Elain Log to determine water saturation, a model should be used to reflect the entire clay spectrum and not just the traditional illite model. (e) Finally, for reliable pay identification in thin beds, high resolution logging is a definite requirement in the Catshill Field.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil