Pennsylvanian Fan-Delta Deposition Resulting from Tectonic Uplift Along Southwestern Margin of Anadarko Basin
Alan R. Carroll
Pennsylvanian sedimentation on the southwestern margin of the Anadarko basin was dominated by a clastic wedge of conglomeratic alluvial fan-delta deposits, referred to locally as "washes." Chert, carbonate, and granite (arkosic) washes were shed basinward in response to thrusting and uplift of the Ancestral Wichita Mountains. Chert and carbonate washes were deposited in the Early Pennsylvanian, as lower Paleozoic limestones and dolomites eroded from the Wichitas. Arkosic washes predominated during the Middle and Late Pennsylvanian, as drainage basins composed primarily of Precambrian-Cambrian crystalline rocks eroded.
The detailed stratigraphy available from well control in the Anadarko basin permits relatively precise timing of orogenic events along the Ancestral Wichitas. Wash deposition in the basin began in the Morrowan and continued episodically in seven major pulses through the Missourian. These wash sequences range from 200 to 1,500 ft thick; each terminates with an unconformity. Wash sequences are further subdivided into as many as nine conformable subsequences, 100-400 ft thick and correlative for 10-20 mi, which appear to record individual drainage-basin erosional cycles. Subsequences generally coarsen upward abruptly, then gradually fine upward to the next subsequence.
Basinal marine shales interfinger with the wash sequences and are a possible source of structurally and stratigraphically entrapped gas and condensate. Optimal reservoir quality occurs in braided channels and fan-delta fronts. Reserves average 2-5 bcf and 50,000-200,000 bbl/well, making granite wash an economically attractive target at present Anadarko drilling costs.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.