Examining Detailed Facies and Rock Property Variation in Upper Cretaceous, Tight Gas Reservoirs, Pinedale Field, Wyoming
The giant Pinedale gas field, Green River Basin, Wyoming, produces from ~5500 ft of the Upper Cretaceous fluvial sandstones of the Lance Formation, Upper Mesaverde Group, and the Paleocene “Unnamed” unit. Typical porosities for the field are <12% with micro-Darcy permeability. Over 3500 feet of core have been examined to better characterize facies for correlation to rock properties for reservoir modeling and development decisions.
Two main types of reservoir sandstones include river channel deposits and overbank splay or sheet sands. Channels display fining-up sequences typical of bar deposits. These sequences have been sub-divided into four facies: (1) channel base lags, (2) basal bar or active channel fill, (3) upper bar, and (4) soil-modified bar top. Splay sandstones are typically finer grained and more cemented, with lower reservoir quality. Despite an overall similarity in facies character, there are variations in facies and stacking pattern both vertically and laterally around the anticline. The lower “Mesaverde” interval has less sand percent, thinner channel sands, thicker intervals of splay sands, and more carbonaceous and bioturbated overbank mudstones. This suggests a higher-accomodation, lower coastal plain setting with poorly drained floodplains. The Lance Formation contains thicker channel deposits with varying amalgamation. Especially prominent is the gradation from thick, amalgamated “Middle Lance” sandstones in the north, to mud-dominated Middle Lance towards the south. Lance Formation overbank muds range from very carbonaceous at the base, to more oxidized with common caliche nodules at the top indicating transition from poorly to well-drained floodplains. The Unnamed Formation channel deposits contain arkosic gravel bars, indicating exposure of the granitic core of the Wind River range.
Despite significant compaction and cementation leading to low porosity and permeability, we can demonstrate good correspondence of core and log petrophysical properties to facies - with larger grain size and higher energy facies having generally greater porosity and permeability. Subtle variations in log responses can be explained by detailed characterization of core properties, such as calcite cement and presence of mud clasts. Investigations of storey thickness, stratification types and rock properties in single-storey and multi-storey channels have provided guidelines for populating detailed reservoir models for performance prediction.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009