The Southwest Texas Heavy Oil Province—a Ten-Billion-Barrel Resource

Thomas E. Ewing
Frontera Exploration Consultants, San Antonio, TX

The Southwest Texas Heavy Oil Province is the largest heavy oil resource in the Gulf Coast region, perhaps the largest in the United States. It extends for 4300 km2, at depths of 0-1000m. The province includes early to middle Campanian asphalt-saturated carbonate grainstone shoals (Anacacho Formation), and late Campanian - Maastrichtian sandstone-hosted asphalt and heavy oil (San Miguel, Olmos, and Escondido Formations).

Anacacho asphalt has been used for paving material since 1888, with two active quarries. The published figure of 550 million barrels in place (from 94.7 km2 in the Anacacho limestone) underestimates the likely resource (due to oxidation of natural outcrops). Stratigraphy indicates that the entire 380-km2 outcrop area contains asphalt.

The San Miguel ‘D' sandstone (640m depth, 663 km2) was targeted for heavy oil research in the early 1980s, when Exxon and Conoco produced 417,673 barrels from pilot plants. This oil is extraordinarily heavy (10o to -2o) and viscous. Published work indicates 3.2 billion barrels in place -- the second-largest identified reservoir in the United States. Many other Upper Cretaceous sandstones within the province also contain heavy oil, with local dry gas accumulations.

The province lies on the northeastern margin of the rapidly-subsiding Late Cretaceous Maverick Basin, and the southwest flank of the coeval San Marcos Arch. Unconformities in the Maastrichtian and the Paleocene regional traps to long-distance migration.

The province-wide resource is poorly known, but may exceed ten billion barrels in place. This is a tempting target for advanced technology, despite daunting obstacles; it deserves another look.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005