Quantitative Evaluation of Oil-Leg Potential in Gas Reservoirs

Mark Lisk, Geoffrey W. O'Brien, Peter J. Eadington, and Frank W. Krieger

Oil bearing fluid inclusions in sandstone represent hidden oil shows. The frequency of quartz grains containing these inclusions (GOI number) reflects maximum palaeo-oil saturation irrespective of the present fluid phase. In this way fluid inclusion data can be used to both identify palaeo-oil columns and to map original oil water contacts (OWC) in wells where oil has been displaced by later gas charge. Studies conducted on gas fields from the North West Shelf of Australia have shown that substantial oil columns were once present. Moreover, detailed GOI mapping has been used to define the location of the original OWC in these reservoirs allowing the height of the palaeo-column to be determined and an estimate to be made of original oil in place (OOIP).

At Oliver-1 in the Timor Sea the reservoir is presently filled to spill with a 164 m gas, and 14.5 m oil, column. GOI mapping has, however, delineated a 96 m thick palaeo-oil column within the gas leg. This is almost seven times thicker than the present oil leg which suggests that perhaps 170-190 million barrels of oil were displaced from this structure.

In the Pepper gas field in the Carnarvon Basin GOI mapping has demonstrated the presence of a gross palaeo-oil column between 15 and 30 m thick, suggesting that between about 50 and 70 million barrels of oil has been displaced. This is more oil than that reservoired in any of the surrounding oil discoveries, which emphasises the importance, from an exploration standpoint, of defining these remigration pathways.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California