Quantitative evaluation of the oil-leg potential in the Oliver gas field, Timor Sea, Australia

Abstract

Oil-bearing fluid inclusions in sandstone cores and cuttings represent hidden oil shows. The frequency of quartz grains containing these inclusions (the GOI number) reflects the maximum paleo-oil saturation experienced in a sandstone reservoir irrespective of the present fluid phase. Samples that have been exposed to high oil saturation have GOI numbers at least one order of magnitude greater than samples that have demonstrably low oil saturation. In this way, these fluid inclusion data can be used to identify paleo-oil columns and to map original oil-water contacts in wells where oil has been displaced by a later gas charge. Moreover, the use of detailed GOI mapping to accurately define the location of the original oil-water contact allows the height of the paleocolumn to be determined and an estimate to be made of original oil in place. The Oliver oil and gas discovery, located in the Timor Sea, Australia, presently contains a hydrocarbon column of 178.5 m, composed of 164 m of gas over a 14.5 m oil leg, and is filled to spillpoint. In well Oliver-1, GOI mapping has delineated a gross paleo-oil column of between 99 and 132 m within the present gas leg. This corresponds to original oil in place of up to 200 million bbl, considerably greater than the 45 million bbl of oil presently reservoired. The displacement of up to 155 million bbl of oil from this structure has high-graded the prospectivity of tilted fault blocks updip from the Oliver structure. GOI mapping is an innovative approach to reservoir characterization that can reliably detect paleo-oil accumulation in hydrocarbon traps that are presently filled by gas. These data allow the oil-leg potential of both gas discoveries and nearby untested structures to be addressed in a quantitative manner before additional drilling is commissioned.