Fluid Inclusion Evidence for Fluid Interactions During the Filling of Hydrocarbon Reservoirs

Eadington, Peter ¹; Bourdet, Julien ¹; Kempton, Richard ¹; Volk, Herbert ²; Liu, Keyu ¹
(1) Earth Science & Resource Engineering, CSIRO, Kensington, WA, Australia. (2) Earth Science & Resource Engineering, CSIRO, Sydney, NSW, Australia.

Oil inclusion data show that gas zones, oil zones and residual oil zones often overlie palaeo-oil zones. Mapped palaeo-fluid boundaries often differ from the current fluid boundaries; indicating they are a snap shot of hydrocarbons, prior to the current disposition of hydrocarbons in the reservoir-trap.

The attributes of oil inclusions within palaeo-oil zones are usually too complex to interpret by inspection. Their many attributes form multiple layers of data concerned with (1)identifying assemblages of fluid inclusions that were trapped at the same time, (2) superposition and relative timing of assemblages, (3) fluorescence colour and size of the gas bubble reflecting the composition of the oil and its gas oil ratio, (4) the abundance of oil inclusions which locates palaeo-fluid boundaries, and (5) the biomarker composition indicating source, maturity, and secondary alteration. Mapped attributes of oil inclusion assemblages (OIAs) show palaeo-oil zones often have internal structure. Attribute boundaries often coincide with the boundaries of residual oil zones or gas-condensate zones.

In many cases OIAs in gas-condensate zones have variant attributes and superficially the inclusions appear unrelated. This is the case at a well in the Cuu Long Basin where OIAs have covariance in which inclusions with large gas bubbles have a rim of blue fluorescing oil and inclusions with yellow fluorescing oil contain small gas bubbles. Some inclusions contain asphaltene. Below the gas-oil contact the OIAs have uniform near-white fluorescence. All the features of OIAs in the gas-condensate zone are reproduced by experimentally equilibrating gas and oil in sealed capillaries. This suggests the inclusions were formed in a zone where molecules had partitioned between oil and gas.

OIAs that formed where water imbibed into residual oil zone may not have variant attributes. However their attributes may differ from those of OIAs in the oil zone. At a well in the Vulcan Sub-basin an OIA that is only in a palaeo-oil zone coincides with a large residual zone. This OIA has uniform near-white fluorescence and uniform small gas bubbles. This differs from OIAs in the current oil zone which have near-blue fluorescence and uniform small gas bubble. These data are consistent with the OIA with near-white fluorescence having been formed when water imbibed into the reservoir and fluid inclusions trapped oil that had interacted with water.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.