Integrated characterization and modeling of reservoir lithofacies and reserves of the Sussex Sandstone, House Creek North area, Powder River Basin, Wyoming

Edmund (Gus) Gustason and Matthew Pranter

House Creek, North House Creek, and Triangle-U fields in the central Powder River Basin produce stratigraphically trapped oil and associated gas from the Upper Cretaceous Sussex Sandstone. The Sussex Sandstone is divided into three stratigraphic intervals (in ascending order, Sussex B, Sussex A1, and Sussex A). The Sussex A produces at Triangle-U Field and the Sussex B produces at House Creek North Field. The Sussex B is 'wet' in Triangle-U Field. The integration of production data and a sequence-stratigraphic framework indicates that the Sussex A in Triangle-U Field is stratigraphically higher (younger) than the Sussex A1 in House Creek North Unit. The Sussex A and Sussex A1 sandstones form northwest-southeast trending, discrete, elongate sandstone bodies that merge laterally into mudstone. The Sussex B interval is widespread over the field area and contains northwest-southeast-trending zones of higher net-to-gross ratio and porosity. Based on facies analysis of 27 cores and adjacent outcrops, the Sussex A, A1, and B sandstones are interpreted as shelf sandstones. The Sussex contains three main lithofacies: (1) high- and low-angle, cross-stratified sandstone; (2) heterolithic or interbedded, ripple-cross-laminated sandstone and laminated mudstone; and (3) mudstone. The heterolithic facies exhibits the highest reservoir quality and is further subdivided based on sandstone-mudstone percentage into streaky, lenticular, lenticular-wavy, wavy, and flaser bedding. Carbonate cement has occluded most pore space in the cross-stratified sandstones, which originally were the higher energy lithofacies. The heterolithic facies, especially where bioturbation is moderate to pervasive, retains moderate porosity and permeability. Core and log data from 230 wells were used to build a 127 mi2 (330 km2) three-dimensional reservoir model of the Sussex Sandstone for House Creek North Unit area. Porosity, permeability, and water-saturation data from routine-core analysis were integrated with core lithofacies and log character to create 4 'log facies' for use in the 'facies' model. Three-dimensional lithofacies and petrophysical models (porosity and water saturation) were generated to estimate original oil-in-place, determine optimum locations for future development drilling, design optimum water-flood pattern / orientation, and explain past production performance in the area.

AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012