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ABSTRACT: Controls on Porosity Development in the Devonian Reservoir at Dollarhide Field

Arthur Saller, Donna Van Horn, James Miller

Devonian strata in the Dollarhide anticline originally contained 138 million bbl of oil. Porosity distribution is controlled by depositional facies. Devonian strata contain five main lithologies (from bottom to top). (1) Nonporous, microcrystalline chert and dolomite deposited as carbonate and siliceous mud in a basinal environment. (2) Burrowed chert-dolomite (10-35 ft thick) with variable porosity accumulated as siliceous sponge spicules and carbonate mud in a low-energy slope environment. (3) Laminated tripolitic chert (0-65 ft) deposited as a spiculitic sand in submarine channels in a slope environment. It has excellent porosity, mainly molds of sponge spicules. (4) Bioclastic limestone (47-92 ft) accumulated as a crinoidal grainstone in a moderate- to high-energy, ou er shelf environment. Calcite cementation occluded virtually all porosity. (5) Upper dolomite (50-90 ft) consists of l-3-ft-thick sequences of dolomitized peloidal grainstone capped by fine-grained chert. The grainstone was deposited in high-energy, shallow-subtidal and tidal-flat environments and has variable porosity.

Deposition of the 200-ft-thick Devonian section occurred during a major progradation of shelf and slope facies. Porosity occurs in two stratigraphic zones separated by tight bioclastic limestone. The upper porosity zone (0-90 ft in upper dolomite) is heterogeneous with stratified porosity that bifurcates, coalesces, and pinches out. The lower porosity zone (laminated tripolitic chert and burrowed chert-dolomite) is homogeneous, with high porosity (commonly 25-35%), moderate permeability (5-20 md), and, in the southern part of the field, no distinct vertical permeability barriers. Thickness of the lower reservoir is variable (0-100 ft) with elongate thicks where sponge spicule sands filled channels. The lower reservoir in the southern part of the field is ideal for uniform sweep during CO2 flood, whereas thinner, less-continuous pay zones in the upper reservoir will cause less uniform CO2 sweep.

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