Anatomy of a Tight Gas Sand: Upper Lance Core from Pinedale Field, Green River Basin, Sublette County, Wyoming

Mauro, Laura A. and Mark W. Longman
Questar Exploration and Production, Denver, CO

     Upper Cretaceous Lance and Upper Mesaverde sandstones are the major producing intervals in Pinedale field. Gas is produced from stacked fluvial sandstones and siltstones. The productive interval in the Lance Formation is approximately 4000 ft thick; the productive interval in the Upper Mesaverde is about 1200 ft thick. The Lance Formation lies below the arkosic sandstones of the lower Fort Union Formation and above the sublitharenite sandstones of the Mesaverde Group.
     The upper Lance was cored from 9010 to 9130 ft in the Mesa 4CB-20D well. The shallower part of the core penetrated approximately 60 ft of stacked fluvial channels containing mud rip-up clasts, massive to cross-stratified, very fine- to medium-grained chertarenite sandstones, and flakes of organic material. The deeper part of the core contains a mix of overbank mudstones and siltstones, crevasse splay deposits, and floodplain shales.
     Lance sandstones are composed of 79-86% quartz, 0-4% feldspar, 3-6% carbonate, and 6-14% clay minerals, mainly illite and chlorite. Rock fragments are dominantly chert and carbonate clasts derived from Upper Paleozoic deposits. Plutonic, volcanic, and metamorphic rock fragments are absent, which indicates that Precambrian basement rocks were not exposed in the source terrain during Lance deposition.
     Measured core porosities in the sandstones range from 3.1 to 12% and average 7.4%. Klinkenberg permeabilities in the sandstones range from 0.001 to 0.094 md with a median value of 0.01 md. Measured gas saturations in the sandstones range from 1.4 to 63.7% and average 41.4%. Sandstones in the cored interval were perforated and hydraulic fracture stimulated and contribute to the well's initial production of 9,577 MCFGPD.

AAPG Search and Discovery Article #90071 © 2007 AAPG Rocky Mountain Meeting, Snowbird, Utah