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ABSTRACT: Petrology and Reservoir Quality of the Katakturuk Dolomite, Arctic National Wildlife Refuge, Alaska

A. K. Armstrong, J. S. Kelley

The Katakturuk Dolomite comprises up to 2500 m of cyclically deposited upper Proterozoic(?) dolostones underlying the Sadlerochit and Shublik mountains and at least part of the adjacent arctic coastal plain. The bulk of the Katakturuk Dolomite consists of carbonates deposited in shoaling-upward subtidal to supratidal cycles 5 to 50 m thick.

Early dolomitization of matrix, allochems, and early cements preserves original depositional fabric, but together with later cementation greatly reduces porosity and permeability. Dolomicrites are predominantly peloids, cryptalgal fragments, and algal-laminated clasts. Allochems include ooids, pisolites, oncolites, peloids, mud pellets, coated grains, fragments of algal mats and stromatolites, and grains formed by diminution dolomicrite envelopes. Dolomite rhombs (2 to 6 µm) replace calcium carbonate allochems, micriter, and cements. Internal molds in ooid, pisoid, and fenestral cavities in algal mats and stromatolites are filled with acicular dolomite and dolospar, silica, and spar calcite. Intercrystal pores are filled with acicular dolomite, columnar dolomite, dolospar, and si ica. Much of the upper half of the formation is massive dolostone composed of 100- to 500-micron rhombs of xenotropic dolomite, the intercrystal pore space of which is mostly filled with dolospar, calcite, and silica.

Pervasive fracturing associated with Brooks Range thrust faulting may enhance reservoir characteristics of the Katakturuk Dolomite. The formation is fractured and cut by conjugate high-angle faults that sole in a basal thrust fault in the Sadlerochit Mountains. Earlier workers reported low porosity and permeability values from outcrop and core samples, values that probably reflect reservoir characteristics related to depositional fabric and diagenetic history. Additionally, other workers also reported a calculated flow rate of 4800 bbl/day of water from two wells located about 10 km west of the outcrop of Katakturuk Dolomite in the Sadlerochit Mountains, a flow rate that likely reflects reservoir enhancement by fracturing. Intensity of fracturing and high-angle faulting in the Kataktu uk Dolomite increases from west to east in the Sadlerochit Mountains. Reservoir enhancement of the Katakturuk Dolomite under the coastal plain probably is greatest where the formation is thrust-ramped into place.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990