BRISTER, BRIAN S., New Mexico Bureau of Mines and Mineral Resources, Socorro, NM; WILLIAM C. STEPHENS and GREGG A. NORMAN, Gunn Oil Company, Wichita Falls, TX

ABSTRACT: Hydrocarbon system related to an Ancestral Rocky Mountains (lower Pennsylvanian) pull-apart graben, north-central Texas

Natural gas fields in Cottle and King counties, Texas, comprise a hydrocarbon system sourced from the Gunn graben, an elongate 180 km2 pull-apart basin in southeast Cottle County. The graben was created by overstepping of major faults with leftlateral strike-slip motion within the Red River-Matador tectonic zone. a component of the 'greater Ancestral Rocky Mountains. The graben formed within a piedmont region dominated by net south-oriented transport of arkosic detritus from the AmarilloWichita uplift towards the Knox/Baylor marine basin. Episodic graben subsidence accommodated syntectonic, cyclicallystacked Atokan (lower Pennsylvanian) alluvial fan-delta and braided stream deposits. Subsequent transgression capped the Atokan non-marine to marine source and reservoir rocks with marine shale. The thick (>1.3 km) graben fill contains mixed Type 11 and 111, wet gas prone kerogen. Total organic carbon content from 16 samples averages 4.2% (range 1.05 to 20.01), and is considered to be sufficient for hydrocarbon generation. Time-temperature modeling demonstrates that the dark gray to black Atokan source-rock mudstone units in the graben have been thermally mature for gas generation since Late Permian, primarily due to deep burial (>2.4 km). Generated gas migrated within and outside of the graben, following non-sealing fault zones and channelized fluvial pathways to fill traps in five general play types. Recent exploration efforts have benefited from delineation of this hydrocarbon system which has yielded over 100 BCF of produced gas to date.

AAPG Search and Discovery Article #90915©2000 AAPG Rocky Mountain Section, Albuquerque, New Mexico