Abstract: Upper Almond (Mesaverde Group) Paleoshoreline Trends, Eastern Green River Basin, Wyoming

CLUFF, ROBERT M., and CATHERINE M. MURPHY

A regional grid of stratigraphic cross-sections was constructed as part of an evaluation of the geology and petrophysics of tight gas fields in the Washakie and Red Desert basins. East-west sections were constructed in every tier of townships from the Rock Springs uplift to the Rawlins uplift, with north-south tie sections every two to three townships. The stratigraphic interval includes the uppermost Lewis through the Almond- Ericson Formations to the top of the Rock Springs Formation.

A consistent and simple numerical nomenclature was adopted for the Upper Almond shoreline trends. Shorelines were numbered UA-1 through UA-16, beginning with the youngest shorelines at the western edge of the basin through consecutively older shorelines to the east. The numbering system was tied to established terminology in the Patrick Draw area. Each shoreline trend is easily mapped at 2- 3 mile resolution from the cross-sections alone.

All of the major Upper Almond gas accumulations are defined by eastward updip pinchouts of shoreline sandstones into offshore Lewis shales. On the western side of the Washakie basin, where the shoreline sands interfinger updip with Main Almond coastal sandstones and shales, no significant gas fields have been discovered. We believe gas migrating through the Upper Almond sandstones leaked updip into the permeable coastal plain sediments. As a result early hydrocarbon accumulations failed to form and the sandstones suffered continuing diagenetic loss of porosity and permeability. Consequently, the major exploration target on the west side of the basin will be in Main Almond stratigraphic traps and not Upper Almond shoreline sands as have traditionally been pursued.

AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado