A study of reservoir characteristics of the Nanushuk and Colville groups, Umiat test well 11, National Petroleum Reserve in Alaska

Cretaceous sandstones in the Umiat Anticline contain the largest volume of oil discovered to date in the National Petroleum Reserve in Alaska. Umiat test well 11, although dry and abandoned, penetrated the most complete sequence of Cretaceous rocks in the Umiat area. Cretaceous formations cored (oldest to youngest) were the Grandstand, Chandler, and Ninuluk Formations of the Nanushuk Group and the Seabee and Prince Creek Formations of the Colville Group. Cores from sandstone beds in each of the formations penetrated were studied to identify the factors influencing porosity and permeability.

Based on lithologic, textural, sedimentary-structural, faunal and floral, and regional paleogeographic evidence, the Cretaceous stratigraphic sequence in the Umiat area can be described as complexly interbedded delta-front and delta-plain facies (named the Umiat delta). The Grandstand Formation and Killik Tongue of the Chandler Formation represent one thick progradational sequence of delta-front and delta-plain facies, respectively. This sequence was followed by deposition of transgressive marine facies of the Ninuluk and Seabee Formations, which were in turn overlain by another progradational delta-plain facies, the Tuluvak Tongue of the Prince Creek Formation.

The delta-front sandstone of the Grandstand Formation is well-sorted, fine-grained to very fine grained, angular to subangular chert arenite and phyllarenite. Authigenic cements include dolomite, calcite, siderite, quartz overgrowth, kaolinite, chert, pyrite, and possibly some small flakes of chlorite. The source terrane was southwest of Umiat and, on the basis of the aforementioned petrographic evidence, consisted of low-grade metamorphic rocks and possibly sandstone and cherty limestone. The weighted average porosity, based on well-log analyses, for the lower part of the Grandstand Formation is 15.1 percent and for the upper part is 15.6 percent; the weighted average permeability is 58.6 md for the lower part and 167 md for the upper part. The average size of visible pores is about 50 ?m. A linear relationship was established between permeability and porosity for sandstone samples from depths less than 405 m and greater than 644 m; the average permeability of these intervals can be estimated with reasonable accuracy not only for the Grandstand but also for the Tuluvak, Seabee, and Killik.

The delta-front, delta-plain marginal facies of the Killik Tongue of the Chandler Formation includes sandstone, siltstone, shale, and coal. Sandstone samples studied petrographically are similar to those of the Grandstand Formation; the delta prograded northeasterly across the Umiat area and the source terrane for the sediments remained the same. Well-log analyses indicate that the weighted average porosity for the Killik Tongue is 16.4 percent and the weighted average permeability is 96.2 md.

The delta-front sandstone of the Ninuluk Formation is similar to that of the Grandstand and Killik, but is moderately sorted and contains less detrital quartz and significantly more metamorphic rock fragments than chert fragments. Sandstone in the upper part of the formation contains a considerable amount of calcite. Visible pores average about 35 ?m in size. The weighted average porosity for the Ninuluk Formation, based on well-log analyses, is 12.6 percent; the weighted average permeability is 10.7 md.

The Seabee Formation is primarily shale deposited in an open-marine environment; sandstone units in the Seabee overlie and are overlain by black marine shale. The mineralogy of these sandstone units differs markedly from that of the older formations. The sandstones are characterized by an abundance of volcanic rock fragments, high content of volcanic plagioclase feldspar, and low content of detrital quartz. Quartz, chert, phyllite, and metaquartzite all appear to be the same petrographically as in the Nanushuk Group. Abundant chlorite and smectite reduce permeability and make sandstones