TULUVAK FORMATION (UPPER CRETACEOUS): RESERVOIR CHARACTERIZATION OF FLUVIAL-DELTAIC OUTCROP SANDSTONE

Tuluvak Formation outcrop samples are characterized by excellent porosity (average ~15%; n=36) and permeability (0.5-8,000 millidarcy), and range from quartzose-rich quartz arenite to arkosic litharenite (n=26 point count samples). The typical Tuluvak sandstone contains more than 75 percent quartzose grains and is classified quartz arenite or sublitharenite. The framework grain composition of sandstones in the Tuluvak includes quartz (10 to 70 %) and chert (15 to 70 %) and lesser sedimentary and volcanic lithic grains. Quartz overgrowths typically line pores along with locally significant minor siderite and calcite. Tuluvak Formation outcrops reported on here occur on Alaska's North Slope from the Anaktuvuk River to the Chandler- and Ayiyak rivers, some 24 km to 74 km south of Umiat (about 1,300 square km). The Tuluvak Formation is a discontinuously exposed, seismic-scale, coarsening-up sandstone package at least 180 m thick. The Tuluvak is conformably underlain by the Upper Cretaceous (Cenomanian and Turonian) age Seabee Formation and conformably overlain by the Santonian to Maestrichtian age Schrader Bluff Formation and records coarsening-up and thickening-up deposition in near shore, shallow marine, shelf environments, including shoreface and foreshore beach, with lesser delta plain, delta-front, and prodelta deposits. To address reservoir characteristics, lateral continuity, facies distribution, and regional characteristics, four stratigraphic sections were sampled for porosity, permeability and petrography. The Shale Wall Bluff section is 140 m thick and contains five coarsening and thickening up cycles. The Ayiyak Mesa syncline section, 24 km south, is 60 m thick with evidence of subaerial exposure near its base. A poorly exposed 180 m section of Tuluvak in May Creek syncline on the Nanushuk River is organized in six thickening-up successions, and includes an apparent Glossifungites demarcated discontinuity surface near its base. This section is 17 km southeast of the Ayiyak Mesa section. The fourth section is located near the east end of May Creek syncline is a discontinuously exposed 60 meter thick succession of well-sorted conglomeratic shoreface deposits capped by coarse fluvial sandstone. The May Creek syncline section is 10 km due east of the Nanushuk River-May Creek syncline section.