Core, brine, and oil from a faulted Plio-Pleistocene deltaic sand-shale sequence comprising the South Eugene Island Block 330 field tell different parts of a story of long distance (kms) vertical migration of fluid, significantly aided by a minibasin-bounding growth fault. Core through the shale-dominated main growth fault shows oil staining in only the part of the fault that was active during oil migration; fluid pressure was never sufficient to hydraulically fracture the sediments. Vitrinite reflectance and geochemical studies of samples from several wells identify the main growth fault as a paleothermal and mass flux anomaly. In one well, a fluid pulse produced a thermal anomaly in the fault which is modeled as representing ascent of 5 million barrels of fluid in 150 years, compatible with a fault permeability that episodically reached values on the order of 200 millidarcy. Permeability increased as effective stress in the fault decreased.

Previous studies (Whelan et al, 1994, Organic Geochem.) indicated a uniform Mesozoic carbonate source for the SEI330 oils. Brine major element and radioisotope geochemistry point to several km of vertical ascent, with participation of brine in albitization and calcite cementation at depth. Further work shows that different reservoirs contain oils from source rocks characterized by a range of chemical conditions; the reservoirs also have different brine salinities. In addition, oils from different reservoirs have been variably affected by phase fractionation prior to reservoir filling. Fluid phase equilibria modeling of the effect of pressure on phase fractionation indicates that the SEI330 oils interacted with methane at a variety of depths, and a fractionation/filling history for the reservoirs is deduced. Different reservoirs appear to have different “plumbing systems”, but the main growth fault is the common link between them.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah