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

Thomas R. Taylor

The Triassic Ivishak 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 op of the Ivishak, directly below the organic-rich, marine Shublik Shale, and is a result of bacterial sulfate reduction in the presence of marine pore fluids. Siderite cement has depleted ^dgr¹⁸O_PDB(-5.9 to -6.6 ^pmil) and ^dgr¹³C(-3.4 to -8.3% ^pmil) and highly radiogenic ⁸⁷Sr/⁸⁶Sr (0.71064), reflecting the influx of meteoric waters and precipitation at 25 - 45 °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 (~7.5%) and associated kaolinite (~2%). Calcite; forms a pervasive cement in discrete 4 - 25 foot thick beds and is characterized by moderately depleted ^dgr¹⁸O_PDB(-6.3 to -7.1 ^pmil , positive ^dgr¹³C(0.9 to 4.3 ^pmil) and highly radiogenic ⁸⁷Sr/⁸⁶Sr (0.71088). These isotopic data are consistent with precipitation from continentally-derived meteoric waters at moderate burial depths.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California