Nina Harun¹

(1) The University of Texas at Austin, Fairbanks, AK

ABSTRACT: Porosity Development in a Detrital Chert Reservoir: Diagenesis and Provenance of the Sadlerochit Reservoir (Lower Triassic Ivishak Sandstone), Prudhoe Bay, Alaska

The Ivishak Sandstone is the major producing formation at Prudhoe Bay. Porosity development is primarily controlled by dissolution of framework chert clasts and siderite cement. Previous workers have associated dissolution with the Lower Cretaceous unconformity (LCU). Petrographic and geochemical data suggest that major dissolution occurred at deep meteoric depths and is not associated with the LCU.

Petrographic data indicates that microporosity development in chert grains is related to textural features in chert that are a function of provenance. Spiculitic chert dissolves more readily than other types of chert. Higher intragranular porosities in zones 2 and 4 reflect the greater abundance of spiculitic chert.

Petrographic evidence shows multiple episodes of chert dissolution. The most effective episode of chert dissolution is associated spacially with kaolinite precipitation. Oxygen isotopic data from kaolinite suggests precipitation from hot meteoric fluids. The major phase of chert dissolution is interpreted to have occurred by deep meteoric fluids.

A pargenetic sequence constructed through petographic, SEM, and electron miroprobe analyses, and stable isotopes analyses of d13C and d18O indicates that siderite is the first major diagenetic phase to form. Variability in siderite elemental and isotopic compositions (d13C and d18O of -13.7 to +17.4 PDB and -8.8 to 0.0 PDB, respctively) reflects multiple cementation episodes inculding a later post-Triassic episode. Quartz and kaolinite cement followed later siderite cementation. Siderite dissolution occurred late in the paragnetic sequence. Later in the paragenetic sequence, massive pyrite, ankerite and Fe-calcite cementation reduced porosities at deep burial depths, followed lastly by oil emplacement.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado