--> ABSTRACT: Factors Affecting Porosity Formation and Preservation Beneath Regional Unconformities in the Lisburne Field, Prudhoe Bay, Alaska, by ; #91020 (1995).

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Factors Affecting Porosity Formation and Preservation Beneath Regional Unconformities in the Lisburne Field, Prudhoe Bay, Alaska

Jeremy Jameson

Five major subaerial unconformities are present within the Pennsylvanian age Lisburne Group carbonates in Prudhoe Bay. These range from angular unconformities attributable to major tectonic events to more localized exposure events at the close of high-frequency cycles. All five unconformities show erosional truncation, but differ in origin, amount of erosion, overlying sediment composition, length of exposure, diagenesis and extent of porosity formation.

Economically significant porosity is associated with the pre-upper Permian unconformity (PUPU), the lower Cretaceous Unconformity (LCU) and perhaps with peri-tidal dolomites beneath a mid-Mississippian Unconformity (MMU). LCU and PUPU are high-angle, erosional surfaces formed during long periods of regional exposure, erosion and tectonism. Neither surface preserves signs of early porosity development, probably because of later erosion. Petrological and geochemical data suggest porosity is of late origin, due to burial expulsion of fluids from side and top sealing shales above the unconformities. LCU is a case of burial diagenetic enhancement of a lithosome with little or no previous porosity.

The other unconformities occur at tops of high frequency cycles and are more likely to be of eustatic origin. These differ greatly in sediment response and length of exposure. Few show significant porosity formation, except perhaps the MMU where peri-tidal, spiculitic wackestones and mudstones have been dolomitized. Unconformites of eustatic origin in the Lisburne field are more likely to preserve signs of exposure, but less likely to have economically significant porosity. Major tectonic unconformities are sealed by major shales that have focussed fluid flow into Lisburne structure during burial. Local structural setting during burial and the sealing capacity of shales above the unconformities are key factors in porosity formation within Lisburne field carbonates.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995