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ABSTRACT

A quantitative basin analysis model has been applied to a proposed drilling site in the San Marcos Arch area, Dewitt County, Texas. Geologic events from the Triassic to Recent are modeled to quantify the geologic evolution and temperature, pressure and hydrocarbon generation histories of the area.

The optimized heat flow values showed that the heat flow was 0.90 HFU until the end of the Triassic, when it stabilized at 0.85 HFU. A slight increase to 0.90 HFU occurred during the Cretaceous. During the Late Miocene, the heat flow increased from 0.90 to 1.15 HFU owing to lateral heat flow from the deeper part of the Gulf Coast. The temperatures measured at present are the highest ever recorded in the area.

The study area was normally pressured until the deposition of Lower Cretaceous shales, when the system became abnormally pressured. Pressures increased continuously until the end of Wilcox deposition. Owing to lower sedimentation rates after Wilcox deposition, abnormal pressures started to decrease and this continued to the present.

Abnormal pressures led to higher geothermal gradients in the area especially by increasing the temperatures of the Cretaceous and Tertiary sediments. The rapid changes in the temperatures resulted in three different maturity gradients, with a lower gradient from the surface to the base of Wilcox, a high gradient from the base of Wilcox to the top of Cotton Valley and a lower gradient down to the Triassic red beds.

Oil generation started during the Middle Cretaceous, followed by gas generation during the Early Tertiary. Due to higher temperatures during the Late Cretaceous to present, the depth of oil and gas generation was continuously shallower.

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