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ABSTRACT: The impact of oil emplacement on diagenesis: A novel solution from fluid inclusion observations

Lisk, Mark , CSIRO Division of Petroleum, Perth, Australia

The impact of petroleum accumulation on diagenesis has been a contentious issue for some time. In this study we present typical examples of fields where hydrocarbon charge has clearly inhibited the development of diagenetic minerals. Conversely, examples of fields where hydrocarbon charge has no discernible impact on diagenesis are also presented for comparison. A novel solution to this question is provided by an innovative fluid inclusion technique that elegantly links the abundance of oil bearing fluid inclusions with the distribution of the pore network in sandstones. Rather than recording the absolute number of oil inclusions the GOI method documents the percentage of quartz and feldspar grains that contain oil inclusions, with each grain representing a sample point for an adjacent pore space. The order of magnitude difference in the level of oil saturation associated with a migration path (So > 5%) compared to an oil accumulation (So 30-90%) equates with a similar variation in GOI values. In this way GOI data is used as an indicator of palaeo-oil saturation and when linked to particular diagenetic minerals allows the oil saturation level during diagenesis to be assessed. GOI values calculated to include only those quartz grains with oil inclusions within diagenetic overgrowths can have GOI values of >50%. This strongly suggests that a high proportion of the available pore space contained oil as quartz overgrowths crystallised. Cementation of authigenic minerals under high oil saturation is therefore considered likely where the necessary components can be internally sourced through localised pressure solution.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia