ABSTRACT

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 and core measurements have shown three zones of organic maturity for source materials: (1) an immature zone above 6000 ft, (2) a peak-generation and accumulation zone from 6000 to 7000 ft, and (3) a mature, expulsion and migration zone below 7000 ft. The response of common well logs identifies these zones. True resistivity (Rt) is low in the immature zone, increases to a maximum in the accumulation zone, and decreases to intermediate values in the migration zone. Density and neutron porosities are different in the immature zone but are nearly equal in the accumulation and migration zones. Correlations with conventional core analyses indicate that Rt values between 9 and 40 ohm-m in the migration 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 log evaluation of moveable oil could be important in exploration.