Lithologic and Diagenetic Controls on Intergranular Porosity of the Ivishak Sandstone, Northstar Unit, Beaufort Sea, Alaska: ABSTRACT

Abstract

The Triassic lvishak Sandstone has a bimodal pore system consisting of (1) intergranular pores and (2) intragranular microporosity within white chert grains. Capillary modeling indicates that throughout the Ivishak at Northstar (offshore Beaufort Sea) microporous chert is water saturated, and only the intergranular pore system is available to hydrocarbons. The volume of intergranular porosity is largely related to depositional sorting. Well-sorted sandstones retain an average of 4% greater intergranular porosity than poorly-sorted sandy conglomerates despite extensive diagenesis. Paragenesis follows the sequence: pyrite > apatite > siderite > siderite dissolution > quartz > kaolinite > barite > calcite. Early diagenetic pyrite is concentrated near the top of the Ivishak, directly below the organic-rich, marine Shublik Shale, and is a result of bactedal sulfate reduction in the presence of marine pore fluids. Siderite cement has depleted [delta][sup 18]O[sub PDB] (-5.9 to -6.6 [per thousand]) and [delta][sup 13]C (-3.4 to -8.3[per thousand]) and highly radiogenic [sup 87]Sr/[sup 86]Sr (0.71064), reflecting the influx of meteoric waters and precipitation at 25 -45[degrees]C. Partial dissolution of siderite cement restores only minor amounts of porosity. Continued invasion by meteoric water led to the formation of abundant quartz cement ([approximately]7.5%) and associated kaolinite ([approximately]2%). Calcite forms a pervasivemore » cement in discrete 4 - 25 foot thick beds and is characterized by moderately depleted [delta][sup 18]O[sub PDB] (-6.3 to -7.1 [per thousand]), positive [delta][sup 13]C (0.9 to 4.3 01.), and highly radiogenic [sup 87]Sr/[sup 86]Sr (0.71088). These isotopic data are consistent with precipitation from continentally derived meteoric waters at moderate burial depths.« less