--> Abstract: Characterization and Source of Oil Impregnating a Sandstone in the Fort Union Formation, Castle Garden Area, Wind River Basin, Wyoming, by J. G. Palacas, R. M. Flores, C. W. Keighin, and D. E. Anders; #91017 (1992).

Datapages, Inc.Print this page

ABSTRACT: Characterization and Source of Oil Impregnating a Sandstone in the Fort Union Formation, Castle Garden Area, Wind River Basin, Wyoming

PALACAS, J. G., R. M. FLORES, C. W. KEIGHIN, and D. E. ANDERS, U.S. Geological Survey, Denver, CO

The Paleocene Fort Union Formation in the Wind River basin produces mainly gas, and lesser quantities of oil. An oil-saturated, exhumed, fluvial channel sandstone is present near the top of the formation in the Castle Garden area. Volumetric calculations, based on an extract yield of nearly 50,000 ppm of the rock, indicate an in-place resource of roughly 1100 bbl/ac-ft.

The sandstone is somewhat heterogeneous throughout the outcrop, but grain-size variation does not appear to influence the degree of oil saturation. Modal (thin-section) porosity reaches 26%; most pores are clay free, even though clay-rich matrix is common. The abundance of clay could cause problems during production if this sandstone formed a reservoir.

Gas chromatographic analyses indicate that the oil has sustained severe biodegradation, water washing, and evaporation. All n-paraffins and nearly all low molecular weight (m.w.) hydrocarbons (C[15] or lesser) are absent. Therefore, prior to degradation, the nondegraded oil-in-place could have amounted to approximately 1500 to 2000 bbl/ac-ft. Biomarker analysis revealed (1) a predominance of pentacyclic terpanes (m/z 191) and minor amounts of tricyclic terpanes (m/z 191), (2) relatively small amounts of steranes (m/z 217) dominated by the C[29] regular steranes, (3) the presence of low-m.w. triaromatic steroids (m/z 231) but the absence of high-m.w. triaromatics and all monoaromatics (m/z 253), and (4) a Tm/Ts ratio much less than 1. Based on biomarker signatures, the "tar-sand" oil i mature and very similar to oil hosted in the Fort Union Formation in the Fuller Reservoir field approximately 20 mi northwest of the tar-sand locality.

Because the Fuller reservoir oil is believed to be derived from Tertiary rocks rich in higher plant, coaly-type organic matter, it is suggested that the tar-sand oil also has a similar source. If the underlying Tertiary source rocks are significantly less mature than the oil show at the surface, then the other likely source would be from the more deeply buried (mature) Tertiary rocks in the depocenter of the basin north of the Castle Garden area.

 

AAPG Search and Discovery Article #91017©1992 AAPG Rocky Mountain Section Meeting, Casper, Wyoming, September 13-16, 1992 (2009)