Predictive Porosity Models for the Lower Cretaceous J Sandstone, Denver Basin, Colorado

SCHMOKER, JAMES W., and DEBRA K. HIGLEY, U.S. Geological Survey, Denver, CO

Porosity of the J sandstone decreases with increasing thermal maturity. Regression lines of porosity upon vitrinite reflectance [R(0)] of the form Phi = A[R(0)](B) (where B is a negative number) define trends for the 10th, 25th, 50th, 75th, and 90th porosity percentiles of the J sandstone. Correlation coefficients vary between -0.76 and -0.88. These regression lines, taken together, can be used to predict the magnitude and range of porosity in the J sandstone throughout the Denver basin. Quartz cementation and/or pressure solution are

mechanisms primarily responsible for the overall decrease of J sandstone porosity with increasing thermal maturity.

Much of the porosity variability of the J sandstone at a given level of thermal maturity relates to the amount of clay present and the extent of carbonate cementation. Carbonate cement is not common in the thin sections studied but, where present, is typically pervasive and significantly reduces porosity. In addition, carbonate cement might formerly have been more widespread, temporarily filling the pore network in places and causing porosity variability by inhibiting for a time compaction, quartz cementation, and the formation of authigenic clays. Abundant detrital or authigenic clay reduces porosity by occupying pores; low clay content indirectly reduces porosity of the J sandstone because the inhibiting effects of clay upon quartz cementation and pressure solution are reduced.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)