Petrology and Reservoir Properties of Point Lookout Sandstone, Southern Ute Indian Reservation, La Plata County, Colorado: ABSTRACT

Abstract

Natural gas is produced from regressive marine Point Lookout Sandstone of Late Cretaceous age underlying much of the Southern Ute Indian Reservation. The sandstones comprising the Point Lookout are quartz rich, fine to very fine grained, and they contain moderately variable quantities of potassium feldspar and lithic fragments. Locally, the sandstone is tightly cemented by carbonate cement; clays are not important as cementing agents, although they significantly reduce permeability of some samples. Though dissolution of grains and lithic fragments created some secondary porosity, most pores appear to be primary. Petrophysical and petrographic (thin section and SEM) analyses indicate that pores in the Point Lookout Sandstone are very small; many occur as intergranular micropores between crystals of authigenic clay. Measured ambient porosity varies between 4 and 20%; reduction of porosity by increasing confining stress is variable but is typically small. Ambient permeability ranges between 0.03 and 82 md. Permeability in some samples is very sensitive to increasing confining stress, especially in samples in which initial permeability is low. In these cases under confining stress approximating reservoir conditions, in-situ permeability may be less than 5% the ambient permeability. Even though the gross appearance of the samples from two core holes penetrating themore » Point Lookout Sandstone is similar, measurements indicate that reservoir properties vary significantly within the sandstone.« less