Abstract

Structurally and stratigraphically entrapped hydrocarbons, as well as the trends of reservoir facies and hydrocarbon migration pathways, appear to be strongly influenced by the wrench fault systems present in basins throughout the Rockies and elsewhere. At least three types of basement structural features influence production: 1) basement structural highs; 2) relatively short basement faults, which mostly define the margins of basement structural highs, and 3) regional crosscutting wrench faults, which define and create major structural and compositional discontinuities. These three structural feature types are most readily interpreted from patterns seen on the Second Vertical Derivative and SUNMAG displays, as well as structures defined by line profile analysis. Integration of these data with log, facies, hydrocarbon show, and production information indicate that motion along wrench faults is instrumental in controlling where entrapment of hydrocarbons takes place.

Production bears a direct and obvious relationship to either the juncture of basement structural highs, with the cross-cutting wrench faults that can be interpreted from discontinuity of patterns present on the aeromagnetic displays, or to certain structural features orthogonal to these wrench faults. Essentially, all fields occur on the tops or immediate flanks of mapped basement structures, an indication that even subtle structures at basement level are important in the stratigraphic entrapment of hydrocarbons. In basins such as the Williston, Big Horn, Powder River, Piceance, Uinta, and Greater Green River, fields are located on mostly northeast trending wrench faults, or on orthogonal structures limited by these cross-cutting wrench faults. Major fields such as Little Knife, Billings Nose, Fryburg, the Dickinson-Eland Wausortian Mound Fields, Oregon Basin, Cottonwood Creek, Jonah, Highlight, and the Rulison-Parachute-Grand Valley fields bear a distinct relationship to these major, but mostly subtly defined faults.

Production commonly ends abruptly or changes trend at wrench fault discontinuities. Consequently, these faults appear to be of critical importance in controlling not only structural development, but also the updip productive limit of many stratigraphic entrapments, whether being caused by diagenetic pore throat entrapment, or a change in facies. In either case, these relationships are an indication that regional wrench faults were active during deposition or influenced diagenetic fluid movement through the reservoir system at a later time.