Petrology, Diagenesis, and Reservoir Quality of Lower Cretaceous Kuparuk River Formation Sandstone, Kuparuk River Field, North Slope, Alaska: ABSTRACT

The Kuparuk River formation consists of upper and lower members separated by an intraformational unconformity. Marine sandstone in each is distinct in terms of depositional environments, sand-body geometry, texture, composition, diagenesis, and reservoir quality. Sandstone in the upper member is very fine to very coarse-grained sublitharenite to lithic arenite with an average quartz-feldspar-lithic (QFL) of 75-1-24. Glauconite constitutes 10-50% of framework grains. Chert, muscovite, heavy minerals and mudstone, limestone, siderite, and metasedimentary rock fragments are less abundant. The diagenetic sequence is: aragonite or high-Mg calcite-collophane-pyrite-siderite-ankerite-calcite-(dissolution of carbonate cements and glauconite)-quartz-kaolinite-illite/smectite-pyrite. Sandstone in the lower member is very fine to fine-grained quartz arenite to subarkose with an average Q-F-L of 92-5-3. Mudstone fragments, chert, muscovite, heavy minerals, and glauconite are less abundant. The diagenetic sequence is: pyrite-siderite-ankerite-calcite-(dissolution of ankerite and feldspar)-quartz-kaolinite-illite/smectite-pyrite. Early diagenesis in upper and lower member sandstones is different, whereas burial diagenesis is similar. Early siderite cemented sandstones in the upper member but did not significantly affect sandstones in the lower member. Subsequent changes in pore fluid chemistry during burial resulted in precipitation of the cement sequence siderite-ankerite-calcite in both upper and lower member sandstones. Stable isotope trends in carbonate cements parallel those of cement texture and composition. Upper member porosity (mostly secondary) and permeability average 23% and 130 md, with upper limits of 28-33% and 500-1,500 md, respectively. Reservoir quality is heterogeneous and controlled by grain size, distribution of primary and secondary porosity, and fractures. Both horizontal and vertical permeability are similar except where fractures enhance horizontal permeability. Lower member porosity (mostly primary) and permeability average 23% and 100 md, with upper limits of 28-30% and 400-500 md, respectively. Reservoir quality is homogeneous. Ankerite locally eliminates porosity, and shale beds and laminations reduce vertical permeability. End_of_Article - Last_Page 664------------