Porosity, Cement and Clay Relationships within the Major Channel Facies of the Lance Formation, Pinedale Anticline, Wyoming
The Pinedale Field, Green River Basin, Wyoming, is the third largest gas field currently producing in the United States, with production at ~1.1 BCF per day and an estimated 25-30 TCF of recoverable reserves. With the increased exploration and exploitation of similar tight-gas sands in the Rocky Mountains and elsewhere, it has become important to have well-defined reservoir characteristics.
The Pinedale Field currently produces from ~5000-6000 ft of Upper Cretaceous fluvial sandstones of the Lance Formation and Upper Mesaverde Group. Many well-developed reservoir sandstones display fining-up sequences typical of river 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. Typical porosities for the field are <10% with permeabilities in the micro-darcy range.
No published studies now exist that examine variations in porosity, cementation and clay content of the Lance Formation with respect to depositional framework. This study looks for diagenetic variations with respect to primary channel facies, both vertically through the Lance sequence and laterally across the field. Thin section, SEM, XRD, and whole rock geochemical samples were taken from the four major bar facies. Samples were taken from the Upper Lance and Lower Lance from 5 wells along the axis of the anticline. Additional Upper Lance samples were taken from 2 off-axis wells.
Examination of facies compositions indicates that the lithic fraction decreases with decreased grain size. A similar trend is observed with porosity, which suggests a relationship between lithic fraction grain size and porosity. Preliminary thin section observations may indicate that the dominant type of porosity changes with depth, more specifically, suggesting that secondary porosity increases with depth, and thus may have an impact on how gas is stored and produced.
Currently work is underway to quantify porosity types by thin section analysis and SEM. Whole rock geochemical data are used to examine the transport of components associated with cement precipitation and generation of secondary porosity. Quantitative XRD data are used to refine the whole rock mineral composition and also quantify clay content and its impact on porosity preservation.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009