Comparative Anatomy and Petrophysical Property Structure of Seaward- and Landward- Stepping Deltaic Reservoir Analogs, Ferron Sandstone, Utah

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1995

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The recovery of natural gas from fluvial-deltaic reservoirs is governed by complex internal architectures. To aid in the translation of outcrop geology to reservoir equivalents, all existing Ferron outcrop, petrophysical, and subsurface data have been integrated into a geologic model of reservoir heterogeneity that compares and contrasts seaward- and landward-stepping stratigraphic cycles. Reservoir architecture varies in a predictable fashion between seaward- and landward-stepping stratigraphic cycles. Within seaward-stepping units, delta-front strata are highly compartmentalized by marine and marginal marine shales coincident with stratigraphic cycle, parasequence, and mouth-bar bounding surfaces. Coeval distributaries are volumetrically a minor component and are preserved as ribbon-like sand bodies encased in finer-grained strata. By contrast, within landward-stepping units, parasequences and component mouth-bar deposits are amalgamated into a lithologically homogeneous strike-elongate sand body. Coeval distributaries are volumetrically a major component and are preserved as a complex network of interconnected, lithologically diverse sand bodies. Internal heterogeneities, related to floodplain, abandoned channel fill, and mud-clast lag deposits, severely disrupt lateral and vertical continuity. Analysis of the Ferron gas field reveals that favorable sites for stratigraphic entrapment occur where proximal and distal portions of parasequences pinch out into lagoonal and marine mudstones, respectively.

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