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ABSTRACT: Delaware Basin/Central Basin Platform Margin: The Development of a Subthrust Deep-Gas Province in the Permian Basin

William John Purves, Sam C. Ting

A deep-gas-prone province was identified along the Delaware basin/Central Basin platform margin, a margin conventionally interpreted to be bounded by high-angle normal or high-angle reverse structures. Redefinition of the tectonic style between the Delaware basin and the adjacent platform resulted in the identification of this Delaware basin/Central Basin platform subthrust province and a giant prospect within it.

Definition of a giant-sized gas prospect in northern Pecos County, Texas, revealed that portions of this margin may be characterized by shingled, low-angle (35-40°), eastward-dipping, basement involved thrust faults. Interpretations suggest that hidden, subthrust footwall structures may trend discontinuously for greater than 100 mi along this structural margin. Subthrust footwall structures formed as basinal buttress points for the Central Basin platform to climb over the Delaware basin. In this area, structural relief of over 19000 ft over a 10-mi width is believed due to stacking of low-angle thrust sheets. Seismic resolution of this subthrust margin has been complexed by allochtonous hanging-wall gravity-glide blocks and folds and by velocity changes in overlying syn- and post ectonic sediments associated with basin-to-shelf lithofacies changes.

Statistical studies indicate that this deep-gas province has a play potential of greater than 10 tcf of gas, with individual prospect sizes exceeding 1 tcfg. The prospects defined along this trend are deep (approximately 20,000 ft) subthrust structural traps that are indigenously sourced and reservoired by dual-matrix porosity. Vitrinite supported maturation modeling suggests that these subthrust structures formed prior to catagenic conversion of the oldest source rocks to oil and later to gas. Tectonically fractured Ordovician Ellenburger and Devonian sediments are considered the principal reservoirs. Shales overlying reservoir intervals form vertical seals. Traps consist of fold closure combined with juxtaposition of reservoir to low-angle thrusted sediments and basement.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990