Reservoir Description, Kf Sandstone, Redwash Field, Utah
Authors: Webb M.G. 
Published: December 2007

The Kf sandstone in the main area of the Red Wash field consists of a complex barrier bar system composed of multiple lobes. Reservoir quality is variable, but the variability can be related to subenvironments of this system. Most of the flow will be controlled by the foreshore, upper shore face, and backshore subenvironments.

The complexity of the model discourages expensive tertiary recovery methods but encourages advocates of infill drilling for improving total recovery. The model also provides insight into the orientation of directional permeability.

Introduction

This study covers Sections 13 through 15 and 22 through 27 of Township 7S, Range 23E, Uintah County, Utah (Fig. 1). It was performed as an aid to the evaluation of a proposed tertiary recovery pilot in the Eocene, Green River Kf sandstone. The Kf sand was chosen among many of the productive Green River sands because it was relatively thick and presented no correlation problems. It was a tertiary recovery candidate as it was believed to be behind the waterflood front in the area of the proposed pilot. The objective of this study was to determine the three-dimensional distribution of porosity and permeability through the construction of a three-dimensional physical model based on the environment of deposition. physical model based on the environment of deposition. This model would identify natural boundaries to any displacement process. Ideally, it would allow us to optimize the design of the assisted recovery pilot.

The paper documents the study of the results of one small area, but it should be useful in evaluating similar depositional sedimentary environments in other sands in the Red Wash field. It illustrates that barrier bar systems can possess considerable internal heterogeneity. This study was keyed to two recently cored wells, RWU #220 and RWU #225 (Fig. 1). The wells were drilled behind the waterflood front, using a fresh-water mud system. Plugs were cut for both routine (porosity, permeability, and saturation) and restored pressure measurements. Plugs were taken for flow tests also.

Plugs and cores were photographed together in both color and color-ultraviolet light. Cores from Well 225 were slabbed prior to photography.

Tests made in support of detailed log analysis included formation factor and cation exchange capacity. X-ray diffraction measurements of mineralogy were obtained on a foot-by-foot basis in order to document changes in mineralogy, as an aid to log analysis, and as necessary background material for the scanning electron microscope work performed on RWU #225.

Initial production from RWU #220 was 26 BOPD and 5 Mcf/D of gas and from RWU #225 was 29 BOPD, 19 BWPD, and i Mcf/D of gas. This suggests that either the waterflood front has not passed HWU #220 and only just touched RWU #225, or the areal conformance of the waterflood was low.

In both wells, coring did not start until the Kf sandstone had been penetrated, so the upper contact was not observed. Interpretation of the environment of deposition was keyed to an analysis of primary sedimentary structures. Previous work had primary sedimentary structures. Previous work had identified the over-all environment of deposition as lacustrine and included the locale and orientation of the shoreline together with source areas and distributions of major sedimentary units. The shoreline is oriented NE-SW and the source of the clastics was the Uintah mountains.

Reservoir parameters then were related to specific subenvironments as determined from core examination. An early study by Jackson et al. provided regional information regarding major provided regional information regarding major tectonic-structural features of the area along with general descriptions of the reservoir beds.

A report by Sanborn et al. 2 provided information regarding the basin shape and orientation, source area, and transport direction.