No abstract

Six well logs and two seismic lines are used to reconstruct the paleo-seafloor surface onto which was deposited the Middle Miocene age Golconda Marl Member (Globorotalia fohsi peripheroronda to G. fohsi robusta planktonic foraminiferal Zones, = Zones N9/M6 to N12/M9) of the Cipero Formation of Trinidad, western tropical Atlantic Ocean, The reconstruction reveals five N-S oriented, south-vergent, reverse thrust fault sheets within the underlying Princes Town Marl Member (Catapsydrax dissimilis through Globigerinatella insueta Zones, N5/M2 – N7/M4) and Retrench Sandstone Member (Globigerinatella insueta planktonic foraminiferal Zone, N7/M4) onto which the Golconda Marl Member was deposited. The occurrence of sandy sections, determined from well-log signatures and illustrated on the reconstructed surface, can be used as an aid in locating potential hydrocarbon reservoirs within the Golconda Marl Member. Micropaleontology is used to infer (a) paleodepth and (b) changing paleoenvironmental conditions throughout the Cipero Formation. Foraminiferal investigation of 21 Cipero Formation samples collected at the La Fortune Anticline (LFA) places the Golconda Marl Member of the Cipero Formation at this site in the Globorotalia fohsi peripheroronda planktonic foraminiferal Zone (N9/M6). Planulina wuellerstorfi, recovered in abundance, places the LFAsection and remaining Golconda Member at water depths >700 m. The statistical similarity of the benthic foraminiferal assemblages in the >63 um and >106 um fractions suggests winnowing of the LFA section, which is thus hypothesized to comprise a contourite. The statistical similarity of previously published data from Zones N6/M3 and N12/M9 (Shannon’s H, the Berger-Parker Dominance Index, Evenness E, a Nutrient Flux Index [NFI = sum of proportional abundances of Bolivina + Bulimina + Uvigerina], and the Benthic Foraminiferal Dissolved-Oxygen Index [BFOI]) suggests that these were deposited in comparable paleoenvironments with, in contrast to Zone N9/M6 at the LFA, relatively little bottom current action.

In 1971, the Bluewater III semi-submersible rig drilled the first wildcat well, KK6-1, some 40km north west of Tobago's Buccoo Reef, in the Paria Sub-Basin of the North Coast Marine Area, resulting in the Orchid discovery. The subsequent appraisal well, KK6-2 discovered gas in a series of stacked Late Miocene (NC30) – Pliocene (NC80) sandstones. The lowermost and thickest of these gas charged sandstones was found unconformably overlying Cretaceous metamorphic basement, and has since been termed the Basal Sand. In 2011 a 4370sqkm 3D seismic survey was jointly acquired over NCMA and Block-22 by Niko Resources and Centrica Energy, allowing for extensive regional mapping and the potential to resolve the Orchid Basal Sand gas discovery. Detailed mapping on basement revealed extensive areas with pronounced dendritic drainage, potential islands and shelfal platforms. Seismic amplitude extractions at this interval point to a number of low impedance anomalies, one of which ties very well to the KK6-2 gas discovery. In the absence of Basal Sand core at Orchid, present day analogues were selected for reservoir sampling and analysis. Integration of regional well data, sea bed samples, ocean current data, seismic impedance volumes, thin sections from side wall cores and cuttings as well as palinspastic restorations resulted in the generation of a regional NC30 Basal Sand gross depositional map, depicting the drowned mountain ridge of ‘Isla de la Asumpción', carbonate mounds and potential Orchid Basal Sand look-alike plays within the Paria Sub-Basin.

This paper focuses on the role that conventional core has played in the reservoir characterization of the Iris discovery, located in licence block NCMA-4, offshore Trinidad and Tobago. Despite the prolific gas-producing fields from early Pliocene reservoirs in this region, confirmed gas pay within the younger Pleistocene succession is only found in Iris and one other well. The generation of reservoir models is therefore hampered by a lack of analogue data making the role of core even more fundamental in the integration with seismic for field evaluation. Iris was discovered in 1975 by well LL9-1, drilled by Deminex and planned on the basis of 2D seismic. It encountered two thin sand units, 20 and 30 ft thick, within a claystone dominated package interpreted originally as being of Pliocene age. No conventional core and wireline pressure data were obtained. Acquisition of high quality 3D seismic in 2011 was followed by the drilling of the Iris-1 appraisal well in 2013. The well was specifically designed to acquire high resolution wireline logs and full conventional core across the entire reservoir package. It is now known that the reservoir is of early Pleistocene age and comprises three main sands, informally termed UP5a-c, deposited in a close-to base of slope, deep sea environmental setting. The post well subsurface evaluation in support of a pre-development project initially focused on the sedimentological analysis of the slabbed core. Quantitative data extracted from high resolution, helical CT scans of the whole core was also integrated with wireline logs as part of the petrophysical formation evaluation. The conventional core data has been fundamental in the reservoir geological interpretation and integration with seismic amplitude anomaly interpretation as the basis for a 3D geological model covering the entire field area. Based on the core studies, the UP5 reservoir sands are characterized as deep water, base of slope, turbidite deposits which in the case of the lowermost UP5b and UP5a sands may have been confined and deposited in a lobe setting. The topmost UP5c sands are more extensive, perhaps less confined, and show evidence of channelling, both on seismic and in core.

Determining whether a seismic re-processing and inversion project has delivered its technical objectives is fairly straightforward. A comparison and analysis of the structural and stratigraphic imaging, reflector continuity, bandwidth, noise content and synthetic well ties can all be used to determine technical success. It is sometimes more difficult, however, to quantify the value derived from the new seismic products. The value proposition is particularly relevant when seeking approval for future re-processing projects especially in the current difficult economic climate. We present an example from offshore Trinidad where the value of a seismic reprocessing and inversion project have been quantified by reviewing the impact decisions made as a direct result of the new data has had on the cost and Net Present Value of the project. The Cassra and Iris fields are Pliocene and Pleistocene aged gas sands deposited in a shallow marine environment and basin floor setting respectively and are located 15-20km off the north coast of Tobago. Both fields are covered by 3D seismic data and are currently in the pre-development project phase. A post migration gather conditioning and inversion project has recently been completed over both fields with the primary objective of improving the pre-stack data and subsequent inversion products for reservoir characterization, dynamic simulation and development well planning. A full re-interpretation on the new data is ongoing but initial observations have already dramatically influenced field development decisions. Detailed mapping of the reservoir on improved resolution data allows for a confident assessment of reservoir architecture, mapping of individual sedimentary clinoforms and determining the areal extent of individual sand bodies. This has changed our views on the risk of reservoir compartmentalization and when integrated with other data types have influenced the development concept and well count required to develop these fields. A seismically derived net pay estimate on both fields has also influenced project volumes and an assessment of the Net Present Value (NPV) impact of the volume change has also been used to quantify the value of the new data. The financial impact of the new data as a result of changes to the field development plan may be either positive or negative in terms of overall development cost, but provides an indication of the true value of the data. This is based on the premise that if the new data merely confirms the existing interpretation and has not impacted any business decisions, then no additional value has truly been achieved. Specifically, we aim to answer the question most managers will pose…‥"You have spent a lot of time, money and effort on this project and delivered some imaging improvements, but what value has it brought and why should I spend any more money on another project?" We hope to answer that question!