STONE, W. NAYLOR, and RAYMOND SIEVER, Harvard University, Department of Earth and Planetary Sciences, Cambridge, MA

ABSTRACT: Diagenesis of Deeply Buried Quartz Arenites from the Greater Green River Basin Wyoming: Implications for Porosity Prediction

Simultaneous cathodoluminescence and polarizing microscopy give high-precision data on the compositions and grain packing arrangements of deeply buried quartz arenites. Samples from the Tensleep and Weber Sandstones (Pennsylvanian), and the Nugget Sandstone (Jurassic) were taken from core with a wide range of current burial depths (4000-22,500 feet) andthermal maturities (0.4-2.1 R<o>). Quantitative determination of composition, grain size, contact index", and "tight packing index" (TPI) were obtained by point counting, supplemented by electron microscopy.

The intergranular volumes (IGV) (cement + matrix + intergranular porosity) vary relatively little (sigma = 3.8) from an average of 23% IGV. There is no relation between IGV and burial depth or R<o>, indicating that most mechanical compaction and intergranular pressure solution are occurring during the early and middle stages of diagenesis. The presence at great depth of a number of moderate to high porosity (8-18%) sandstones with IGV's of more than 20% indicates that stabilization of the grain framework occurred with small amounts of grain-supporting cement or pressure solution. Extrapolation of the linear regression of the TPIs and the IGVs indicates that quartzose sandstones mechanically compact to an IGV of approximately 31%; the remainder of the IGV decline is a result of p essure solution. Total porosity is a function of the abundance of quartz cement, which, in turn, relates to thermal maturity. Understanding the controls on the variation in cement abundance at a given thermal maturity is the key to successful porosity prediction.

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