ABSTRACT

This study discusses the influence that sedimentology, diagenesis, and basin subsidence history have on porosity and permeability of the Upper Cretaceous Terry sandstone of Spindle field in the Denver basin.

The Terry sandstone consists of lithic subarkoses and feldspathic litharenites deposited in a marine bar complex. The reservoir facies, which was deposited in a marine bar crestal position, is a cross-bedded sandstone with a mixture of primary and secondary macropores. Approximately half of the pore volume is microporosity associated with clay minerals and lithic fragments. The nonreservoir facies was deposited on the bar-flank margin as lower-energy, alternating layers of mud and sand, which were extensively bioturbated to produce muddy sands.

Ductile deformation of lithic fragments is a major factor in porosity loss for the reservoir facies. Chlorite and calcite are the most important mineral cements. Authigenic quartz, dolomite, ferroan calcite, kaolinite, feldspar, pyrite, and tourmaline are present in minor to trace amounts. Precipitation of these minerals is believed to be related to the evolution of marine water. Secondary porosity formed by dissolution of pore-filling and replacement calcite and framework feldspars. Sandstones with pervasive calcite cement have no secondary porosity developed, which suggests some permeability may be needed to develop secondary porosity.

Plagioclase feldspars become more albitized with increasing depth. A temperature-depth subsidence profile predicts a maximum temperature of only 85°C, at least 15° lower than in Tertiary rocks, which suggests that kinetic considerations may also be important to albitization. Vitrinite reflectance (0.65%) and interbedded shales with ordered mixed-layered illite/smectite with up to 50% expandability also support the lower temperature regime. Calculations suggest that seawater can supply sufficient sodium to account for albitization. Albitization proceeded more rapidly in permeable sandstone, where the plagioclase was in direct contact with the pore fluids. Plagioclase in relatively impervious sandstones eventually became albitized, probably through diffusion.