Abstract: Reservoir Characterization of a Complex Impact Crater: "Ames Crater," Northern Shelf, Anadarko Basin

M. D. Kuykendall, C. L. Johnson, R. A. Carlson

The concentric structural feature known as the "Ames Hole," or "Ames Crater," on the northern shelf of the Anadarko basin contains several heterogeneous and uniquely associated hydrocarbon reservoirs within extensively brecciated, fractured, and faulted Cambrian-Ordovician Arbuckle Group dolomites, Precambrian granites, and pseudopyroclastic (impact melt) rocks. Critical diagnostic structural features and petrographic evidence from cores and cuffings strongly support an impact origin of the structure. Numerous trapping mechanisms exist in and around the crater, which is estimated to contain ultimate reserves of 50 million bbl of oil and 20 bcf of gas.

A detailed reservoir characterization study was conducted in the Ames Crater, which included subsurface data from all available wells (65 as of June 1993), including three horizontal wells. Petrophysical evaluation included digital log analysis and petrographic and standard core analyses of whole cores, rotary sidewall core plugs, and cuttings. Correlation and calibration of log-rock characteristics of distinct reservoir lithofacies from key wells provided the basis for field-wide inferences about rock and pore type, permeability, fluid saturations and contacts, and related production characteristics.

The crater is 8-10 mi in diameter and buried at 8500-9500 ft. Morphologically distinct areas, based on analogy with well-documented complex-type impact craters, include a central rebound feature composed of a concentric peak-ring horst and graben structure and associated collapse zone, a steep crater wall, and an uplifted inner rim and hummocky outer rim. Reservoir porosities and permeabilities are quite variable, but can be exceptional, as shown by the tremendous production and reserve potential of some wells. For example, individual granite breccia wells may contain up to 285 ft of net pay and estimated primary oil reserves of 10 million bbl. Geochemical data suggest that the principle source rock is contained in the lower portions of the organic-rich black shales of the overlying ( rater-filling) Middle Ordovician Oil Creek Formation.

The Ames Crater contains unique structural features and complex heterolithic reservoirs. Future development and exploration success must be based on a thorough understanding of the genesis, distribution, and petrophysical properties of the various reservoir lithofacies. The Ames Crater may prove to be the most extensively explored and hydrocarbon-productive impact crater in the world, as well as a future exploration and development analog for similar impact structures.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994