Abstract: Investigation of Shales as Source of Authigenic Phases in Shale-Encased, Valley-Fill Sandstones, Muddy Formation, Wind River Basin, Wyoming

Ward A. Whiteman, Wendy J. Harrison

Detailed sampling of sandstone/shale contacts was undertaken in four cores within and nearby Grieve and North Grieve fields. The cores from this study area are presently at significantly different depths indicating each may have been exposed to significantly different temperatures. The cores are also absent of any feldspar which limits the formation of secondary porosity and one of the main in-situ sources of authigenic phases within sandstone. Therefore this sandstone/shale content investigation is unique.

Multiple lines of evidence indicate shales source and control the distribution of authigenic siderite, calcite, and chlorite. We propose a mass transfer model that explains the distribution of authigenic phases by the rate of transfer from shales to sandstones relative to the fluid migration through the sandstone. Thin-section scale transfer results from rapid availability and transfer of elements (calcium and iron) from the shale to the sandstone relative to fluid migration through the sandstone (calcite and siderite) causing precipitation near the sandstone/shale contact. Regional transfer results from a rapid fluid migration through the sandstone relative to the availability and transfer of elements from the shale to the sandstone (quartz?) causing no precipitation or precipitation at distances away from the sandstone/shale contact. Local transfer results from variable rates of availability and transfer of elements (iron) relative to fluid migration through the sandstone (chlorite).

In this study the main cause of porosity reduction was quartz cementation. The source(s) of quartz cement could not be identified, but evidence suggests the distribution is strongly controlled by the presence of preexisting authigenic phases (especially clays). Prediction of these earlier-precipitated authigenic phases (especially chlorite) could result in a better prediction of porosity due to the absence of significant quartz cement.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994