ABSTRACT: Stratigraphic and Areal Distribution of Overpressuring, Northern Green River Basin, Wyoming

Stephen E. Prensky

Gamma-ray logs from 175 wells and drilling mud-weight histories from over 400 wells have been collected and digitized and a structure map on the "second Frontier sandstone" was prepared for use in a two-phase study of regional overpressuring in the northern Green River basin. In the first phase, the stratigraphic and areal distribution of overpressuring across the basin was examined. A mud weight greater than, or equal to, 10 lb/gal was used as an indicator of the onset of significant overpressuring.

A series of cross sections shows that stratigraphically, the top of overpressuring on the western flank of the basin, on the Moxa arch-La Barge platform, is restricted to the "second Frontier sandstone." Moving eastward, overpressuring gradually rises through the stratigraphic section reaching uppermost Cretaceous rocks along the Wind River thrust on the Pinedale anticline and Pacific Creek areas. In contrast, formation pressures on the Rock Springs uplift are generally hydrostatic through the entire Cretaceous section.

Maps of mud weight at depth intervals of 5000, 10,000, and 15,000 ft, and the depth to 10 lb/gal mud, show the areal distribution of the top of overpressuring with depth. Anomalies on the Moxa arch and Pinedale anticline, where individual wells were successfully drilled using only hydrostatic mud weights, may be related to variations in permeability. The overall regional distribution of the top overpressuring may be related to variations in the depth to the onset of hydrocarbon generation, as others have suggested for the Pinedale anticline in this basin and for the Bakken shale in the Williston basin.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990