ABSTRACT: Distribution of Porosity in Three Dimensions within the Upper Cretaceous Sussex Sandstone, House Creek Field, Wyoming

HIGLEY, DEBRA K., and DAVID K. VAUGHAN, U.S. Geological Survey, Lakewood, CO

Oil in the House Creek field is produced from the Sussex Sandstone Member of the Upper Cretaceous Cody Shale, mainly from trough cross-bedded, fine to medium-grained, marine-ridge sandstone. This major reservoir facies is present as one to three discrete lenses near the top of the Sussex Sandstone, and contains the greatest porosity of all Sussex facies (6.9% by volume, thin-section porosity). Porosity-distribution results are based on sedimentologic, geochemical, and petrologic analyses of core data from 21 wells within and proximal to the field. A three-dimensional (3-D) computer model used down-hole core porosity for 15 of these wells, plus compensated neutron-density porosity logs for an additional 60 wells-all of these wells were located in the southern one-third of the House Cre k field. Porosity was gridded in 3-D space using the Dynamic Graphics, Inc. interactive volumetric modeling program. The resulting 3-D model of porosity in the Sussex Sandstone shows the generalized distribution of sandstone porosity within the field.

Variation in porosity is great; productive wells generally contain one to three discrete lenses of reservoir-grade porosity that are laterally discontinuous. Also shown by the study are (1) a general upward increase in porosity, which corresponds to the upward-coarsening sequence of shelf to ridge sandstones, (2) complex porosity consisting of primary, secondary, and microporosity, and (3) highly variable amounts, scales, and distributions of porosity within reservoir sandstones. Interbedding of thinly-bedded (porous) reservoir and low-porosity nonreservoir facies is common and results in stacked zones of porous and tight sandstones. The major petrologic factors affecting porosity distribution in reservoir facies are cementation by calcite (11% average by volume) and, to a lesser exte t, quartz cementation (8%).

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AAPG Search and Discovery Article #91017©1992 AAPG Rocky Mountain Section Meeting, Casper, Wyoming, September 13-16, 1992 (2009)