Abstract: Paleothermal Anomalies Associated with a Growth Fault System on the Northern Margin of a Salt Withdrawal Mini-Basin in Block 330, South Eugene Island, South Edition

Lorraine Eglinton, Steven Losh, M. Luo, J. Wood

Vitrinite reflectance and smectite-illite ratios in fault zone rocks from South Eugene Island Block 330 are significantly elevated relative to reservoir rocks of the same age and depth 1000 ft or more from the fault. Fault rocks were collected as sidewall cores in the A6ST well, which crosses a large-displacement growth fault that bounds one side of Pennzoil's Block 330 reservoirs. Reservoir rock samples are from the 1.4 Ma KE sand at depths of 6500 to 7600 ft. Mean vitrinite reflectance in five epoxy-mounted samples of fault zone rock is 0.58% Ro, whereas mean reflectance of vitrinite in four thin sections from the KE reservoir sand away from the fault is 0.28% Ro. In the same fault zone samples, the mean illite/illite+smectite ratio is 0.44, significantly elevated relative to averag values of 0.2-0.3 from rocks of similar age and depth elsewhere in the area. Simultaneous solutions for kinetic models for vitrinite maturation and smectite-illite transformation indicate the fault rocks have been subjected to heating relative to present-day temperatures. The magnitude of the heating event may have been as much as 40°C over the present-day ambient temperature of 80°C. This heating event is best explained as resulting from a pulse or pulses of hot fluid ascending the fault from depth.

In order to evaluate the spatial extent and magnitude of the thermal anomaly related to the growth fault system, vitrinite reflectance and smectite-illite ratios are analyzed in samples at varying distances from faults and from depths of 4500 to 10,000 ft throughout Block 330. In addition, petrographic and isotopic data from the fault zone core will also be compared with similar data from Pennzoil reservoirs in order to assess the connection between fluids in the fault zone and those in adjacent reservoirs.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994