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Thin Bedded Reservoirs in the Plio-Pleistocene of the Columbus Basin, Offshore Trinidad: Challenges of Reservoir Architecture, Quantification and Characteristics

Abstract

The Columbus Basin is a mature gas producing basin with a number of major fields now in decline. Focus for infield exploration and production is shifting, with thin bedded sands as a secondary pay target. This basin is exceptional as age equivalent analogues to the subsurface reservoir are exposed along the coast of Mayaro Bay in southeast Trinidad (16 to25m cliff faces). The study aims to examine three main aspects of thin bedded sands: 1) the large-scale architecture i.e. macroscopic characteristics, using outcrop and core description; 2) facies and microfacies architecture using thin sections, X-ray diffraction, spectral gamma ray, scanning electron microscopy and bulk porosity measurements which reveals mineralogy, fabric and reservoir quality and 3) pore system micro morphology framework using X-ray tomography at the Manchester X-Ray Imaging Facility (MXIF). Sedimentary logging revealed that thin beds are highly interbedded units with thicknesses of 1mm-10cm and have a lenticular geometry. Their lateral extent, controlled by their exposure, varies from 1-2m to 10'sm. Field sampling and microfacies analysis, revealed five distinct lithofacies types and five microfacies types that make up two principal facies associations (FA): (FA1) axial distal delta front facies and (FA2) lateral distal delta front facies. The main feature that separates these two FA's is the degree of bioturbation; FA1 is sparsely bioturbated whereas in FA2 it is abundant. This may be the result of deposition occurring in the same sequence stratigraphy systems tract (either HST or FSST) but at different locations on the delta. A bulk porosity measurement experiment was used to measure porosity as the rocks are very friable and do not stand up well in laboratory tests. It yielded an average of 22%; in range with porosities from RCA data on basin core. However this does not illustrate the degree of connectivity or pore morphology as well as the nature of the clays present as the XRD analysis only confirmed the rocks were quartz rich with >5% other mineral material. Using the Xradia CT at MXIF, samples 2-3mm in size were scanned, reconstructed and their micro porosity framework skeletonised and quantified using Avizo and MatLab respectively. This has yielded an average pore size of 14 microns, max thickness of 45 microns and minimum of 2 microns. These results are of direct application to the exploration and development of secondary reservoirs.