Source Rock Potential and Maturation of Deep Wilcox from South-Central Texas

Wallace G. Dow, P. K. Mukhopadhyay, T. Jackson

Organic-rich shales (TOC > 0.7%) and coals from the Eocene Wilcox Group between depths of 5,000 and 24,000 ft from south-central Texas are analyzed to evaluate organic facies, source rock potential, and maturation. The lowest and highest vitrinite reflectance and bottom-hole temperature recorded are 0.4% R_o and 122°F (68°C) at 5,000 ft (1,750 m) and 4.5% R_o and 460°F (256°C) at 24,000 ft (8,000 m).

Organic petrography reveals four types of organic facies which corroborates well-log analysis. Three phases of types IIA and IIB oil-prone kerogens are present and are related to depositional environment. The change in n-alkane distribution pattern and the hydrogen index of these sediments are controlled partly by facies and partly by maturation. These data suggest variability of organic facies and establish deltaic coal and deep-sea fan or pro-delta shales in the lower Wilcox as probable source rocks for liquid hydrocarbons in the south-central Texas Gulf Coast. Generation and expulsion of crude oil took place during deposition of overlying middle Miocene reservoir facies.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.