Studies of Interbedded Shales in Tango Fluvial Sandstones, Ivishak Sandstone, Prudhoe Bay: Implications for Depositional Setting and Reservoir Architecture

By

J.J. Hickey (Applied Reservoir Petrology) and B.A. Burns (Phillips Alaska, Inc.)

New interpretations of significant marine influence in the Tango interval of the Ivishak reservoir at Prudhoe Bay carry important implications for shale geometry and continuity, vertical compartmentalization, and dynamic fluid distribution. A broad study of shale characteristics within the Tango and other members of the Ivishak was undertaken in order to investigate the possible presence of regional flooding surfaces. Thin-section petrography, X-ray diffraction, and whole-rock geochemistry techniques were employed to document features diagnostic of syndepositional conditions, such as patterns of clay mineral distribution (reflecting preferential flocculation in a proximal marine setting), presence of oxidation and other pedogenic processes, early diagenetic minerals (especially siderite and pyrite), and patterns of trace metal distribution (also a reflection of salinity-induced changes in clay adsorption properties). These features occur in varying combinations and degrees of development in the samples examined, and permit the definition of a series of gradational categories (petrofacies) ranging from fully non-marine to mixed/transitional through fully marine. The stratigraphic and geographic distribution of the various shale petrofacies, integrated with other lines of evidence such as core descriptions and sand maps, indicate the location and nature of marine incursions during the Tango interval. Thicker shale beds typically record a sequence of changing environmental conditions, from initial marine to eventual nonmarine deposition. On balance these shale studies confirm the idea of multiple episodes of marine influence punctuating Tango fluvial sand deposition, but usually suggest small-scale, localized events such as ephemeral bays and lakes rather than regional transgressions.

AAPG Search and Discovery Article #90008©2002 AAPG Pacific Section/SPE Western Region Joint Conference of Geoscientists and Petroleum Engineers, Anchorage, Alaska, May 18–23, 2002.