ABSTRACT: Estimating Organic Maturity from Well Logs, Upper Cretaceous Austin Chalk, Texas Gulf Coast

Gregory A. Hines, Robert R. Berg

The Austin Chalk is both a source rock for oil and a fractured reservoir, and the evaluation of its organic maturity from well logs could be an aid to exploration and production. Geochemical measurements have shown three zones of organic maturity for source materials: (1) an immature zone to depths of 6000 ft, (2) a peak-generation and accumulation zone from 6000 to 6500 ft, and (3) a mature, expulsion and migration zone below 6500 ft. The response of common well logs identifies these zones. True resistivity (R_t) is low in the immature zone, increases to a maximum in the peak-generation zone, and decreases to intermediate values in the expulsion zone. Density and neutron porosities are different in the immature zone but are nearly equal in the peak generation a d expulsion zones. Correlations with conventional core analyses indicate that R_t values between 9 and 40 ohm-m in the expulsion zone reflect a moveable oil saturation of 10 to 20% in the rock matrix. The moveable saturation provides oil from the matrix to fractures and is essential for sustained oil production. Therefore, the evaluation of moveable oil from well logs could be important in exploration.

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990