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Abstract: Source Rock in the Lower Tertiary and Cretaceous, Deep-Water Gulf of Mexico

Bruce E. Wagner, Zvi Sofer, Brenda L. Claxton

Amoco drilled three wells in the deep-water Gulf of Mexico in 1993. One well, in Mississippi Canyon Block 84 (W.D. 5200 ft), drilled a structural feature. The well penetrated Cretaceous section and crossed the middle Cenomanian unconformity. Six sidewall cores from 14,230-15,200 ft (subsea) contained TOC values from 2.6 to 5.2% with hydrogen indices from 360 to 543 ppm in lower Tertiary and Cretaceous shales.

All six cores were thermally immature, for oil generation, based on biomarker ratios and vitrinite reflectance measurements. Organic extracts from cores in the Cretaceous had biomarker characteristics similar to oil reservoired in the Miocene. The oil was probably generated from a similar, but more mature, source rock.

The high structural position of the well prevented the lower Tertiary and Upper Cretaceous section from entering the oil window at this location. There are over 2000 ft of structural relief and an additional 6000-8000 ft of Lower Cretaceous section below the level penetrated by the well. It is probable that an equivalent section off structure is in the oil window.

Prior to drilling, estimates of expected thermal maturities and temperatures were made using BASINMOD, a hydrocarbon generation/expulsion modeling package. The model predicted higher well temperatures (e.g., 225° F vs. 192° F) and lower vitrinite maturity (0.44% vs. 0.64%) than encountered in the well.

Vitrinite reflectance equivalents of 0.41% and 0.43% were calculated from biomarker ratios of the Cretaceous core extracts, matching the BASINMOD predicted value of 0.44%.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994