The Effects of Tectonism and Eustasy on Sequence Stratigraphic Framework and the Distribution of Play Elements, Upper Cretaceous Foreland Basin, Southwestern Wyoming

DEVLIN, WILLIAM, J., and KURT W. RUDOLPH, Exxon Production Research Co., Houston, TX

Larger scale ("second-order") stratal patterns in the Upper Cretaceous foreland succession record the alternation of active thrusting and relative quiescence in the adjacent thrust belt. Periods of active thrusting are characterized by a rapid increase in subsidence-related accommodation. During these times, proximal foreland strata consist of aggradation deposits with poor preservation potential. Sediments in medial to distal parts of the basin are relatively starved, display an abruptly deepening to retrogradational stacking pattern, and become more source- and seal-prone further from the mountain belt. Periods of tectonic quiescence are characterized by a decrease in subsidence-related accommodation. Accommodation space in proximal regions is quickly filled by sediment, and the loc s of coarser grained sedimentation shifts with the basin through progradation. Consequently, strata in medial portions of the basin display a progradational stacking pattern and become more reservoir-prone.

Smaller scale, third- and fourth-order sequences are interpreted as eustatic, but their expression is controlled by the larger scale (tectonic) patterns. In general, transgressive and highstand systems tracts tend to be enhanced during rapid subsidence. Lithofacies are source- and seal-prone, and condensed sections will be enhanced; valley incision and lowstand reservoir development (including deep-water sand deposition) will be inhibited. Conversely, times of quiescence favor progradation of reservoir facies into the basin, enhance lowstand deposition, and inhibit eustatically related transgressive systems tracts.

Intra-foreland, basement-involved movements further affect the distribution of play elements. Such effects include deposition of widespread, reservoir-prone, nonmarine facies, development of localized, depositional shelf edges, and the creation of subbasins that serve as depocenters for basinally restricted reservoir deposits.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)