The Role of Conventional Core in Characterizing Deep Sea Turbidite Reservoir Sands in the Iris Discovery, NCMA-4 Offshore Trinidad and Tobago

Abstract

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.