--> Abstract: Petrographic Heterogeneity, Pore Throats, and Reservoir Quality of Tertiary Sandstones, Wind River Basin, Wyoming, by C. W. Keighin and R. M. Flores; #90987 (1993).

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KEIGHIN, C. WILLIAM, and ROMEO M. FLORES, U.S. Geological Survey, Denver, CO

ABSTRACT: Petrographic Heterogeneity, Pore Throats, and Reservoir Quality of Tertiary Sandstones, Wind River Basin, Wyoming

Approximately 80 thin sections from outcrops and subsurface cores of Tertiary Fort Union sandstones were examined to determine effects of post-burial diagenetic reactions on quality of potential sandstone reservoirs. Cores examined were from oil and gas fields which have a cumulative production of at least 2.5 million bbl of oil and condensate, and 223 bcf of gas. Selected samples were also examined by SEM to determine morphology and distribution of clay minerals and other potential cementing agents which may influence fluid flow. Approximately 20 mercury injection and pore pressure/capillary pressure measurements were conducted to determine distribution of pore throats and effects of confining stress on porosity and permeability.

Reservoir petrography indicates wide variation in quantity and distribution of cementing agents, both between and within, outcrops and core samples. Variation in outcrop samples is attributed primarily to effects of near-surface alterations; however, in core samples variation is due to different fades types, and their grain size and mineralogy. SEM analyses and mercury injection/capillary pressure tests show that small (< 1.0 micrometer) pore throats, rather than microfractures are dominant in controlling fluid flow in these sandstones. Carbonate cements, although limited in occurrence and distribution, may largely occlude porosity. Clay minerals, primarily smectite and kaolinite, are widely distributed. Smectite is largely inherited from older detrital rocks; kaolinite appears to e neoformed and inherited. These clays influence fluid flow, but do not appear to significantly reduce either porosity or permeability.

Knowledge of reservoir petrography, diagenesis, and nature of pore throats assists in mitigating formation damage during well completion and recovery. This information is significant in developing a strategy for changing an otherwise uneconomic prospect to an economic play.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.