Technical Sessions
Technical sessions for 1999 also got underway on Wednesday 27, January 1999 with a Poster presentation by Sookdeo Heeralal and Vishram Rambaran. The title of the paper is "Reservoir Characterization and Sequence Stratigraphic Analysis of Turbidite Sandstones in Catshill Field, Trinidad." This Poster presentation was recently presented at the AAPG International Convention held in Rio de Janero, Brazil last November. The following is the Abstract of this presentation:
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 (Cum:– 218 mbo, 3 bcfg). Sustained production performance suggests a reservoir that is laterally continuous and areally extensive. Indeed, in this field it is possible to map sand packages over distances of several thousand feet, with individual thin beds being correlatable for hundreds of feet. The productive interval of the Karamat-Herrera Formation in Catshill is interpreted as Slope Fan Deposits within the Low Stand Systems Tract. This is based on the integration of rock samples, well logs and biostratigraphy. The zone displays a subdued, ‘ratty’ log pattern and comprises stacked sequences of thin, rhythmically interbedded, fine grained, Bouma Tb/c sandstones and mudstones. 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. Thin section analysis indicates that there has been some deterioration in reservoir quality due to compactional effects, pore occluding clays and cements such as quartz, calcite and siderite.
Reservoir characterization is enhanced by the use of high resolution logging. The Formation Micro Scanner (FMS) and Dipmeter (SHDT) have been particularly useful for the following reasons:
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dip patterns characteristic of thin-bedded turbidites, with microresistivity curves showing sharp erosional bases at interbed boundary.
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more accurate net pay counts ± 'whole core' electric imagery which provides vital information on sedimentary structures and bedding type.
Visible turbidite associated features on the FMS include: parallel laminated sand-shale sequences, load cast with contorted bedding, 'pinch-and-swell' structures, erosive mud lined scours, graded bedding, ‘bullseye’ patterns and amalgamated sand on sand contacts.
The Catshill accumulation is relatively small. It is considered to be an underfilled trap with hydrocarbons reservoired in the crestal position of a compressive fold, that was later subjected to tectonic overprinting. The field itself is located at the distal edge of an elongated northeast – southwest syntectonically deposited sand fairway. A thorough understanding of the reservoir composition, architecture and depositional geometry, within the context of the overall tectonic evolution of the basin, is paramount to effective and efficient exploration and exploitation of such limited reserves. In this regard, the following key points are noted:
(a) The productive interval is comprised of laminated sand–shale sequences. Beds are laterally continuous, however vertical connectivity is limited. This type of facies model must be reflected in the flow units chosen for Reservoir Simulation.
(b) A common oil/water contact may not necessarily hold true for this field. Each thin bed is sufficiently vertically partitioned to have its unique fluid distribution and composition i.e. oil vs. water. Such wells may have to be selectively perforated.
(c) In thin beds small throw on faults can cause sand–shale juxtaposition with associated reservoir compartmentalization. Higher well density may be necessary to significantly effect recovery from such reservoirs.
(d) A high angle deviated well may lead to more areal exposure for accelerated recoveries. Thin beds also demand the use of higher shots per foot during completion, otherwise there can be significant bypassed pay.
(e) Deductions from X-ray Diffraction Studies and Thin Section Analyses are as follows: (i) All clay types are present. Therefore in computing the Elan Log to determine hydrocarbon potential, a mineralogical model should be used to reflect the entire clay spectrum, and not just the traditional illite model.
(ii) Sandstones are cemented and have undergone compactional effects. An appropriate cementation exponent (‘m’) should be applied in order to derive a realistic picture of water saturation. (iii) Acidizing should be done judiciously in this formation. Analyses indicate appreciable amounts of pore filling calcite which can react with hydrofluoric acid (HFl) to precipitate calcium fluoride (CaFl
2), an insoluble compound which armours the well bore and restricts flow.
Chlorite present in this reservoir will dissolve readily in hydrochloric acid (HCl) and the iron liberated will re-precipitate as ferric hydroxide (Fe[OH]
3), which contains large crystals that can block the pore throats and impede the flow of hydrocarbons.
(f) Finally, for reliable pay identification, high-resolution logging is a definite requirement in the Catshill Field. Logs should be recorded at high sample rate and at a logging speed that will allow for thin bed analysis.