Geologic Factors Affecting Reservoir Analysis, Anschutz Ranch East Field, Utah and Wyoming
Authors: Lelek Jeffrey J. 
Published: December 2007

Introduction
The Anschutz Ranch East field, a retrograde condensate accumulation discovered in late 1979, was soon recognized as the jewel of the U.S. Western Over thrust Belt and as one of the most significant recent discoveries in the entire U.S. At this early stage of development, many questions are not vet answered, including the extent of the structures and the magnitude of reserves. Recent interpretations shown in this study are in some ways speculative and are expected to change as new data become available. Regionally, as shown in Fig. 1, the field is within the Absaroka thrust sheet, one of four major northeast/ southwest-striking, rock, packages believed to have been thrust eastward during Cretaceous and early Tertiary time. Total crustal shortening across these major thrust faults is estimated to be nearly 60 miles (96.6 km). The field lies along the narrow northeast/southwest-striking Mesozoic liquids trend that presently extends northward from Pine view field to Painter reservoirs and Ryckman Creek fields. This Mesozoic trend is parallel to and east of the Paleozoic sour gas trend producing from fields such as the Whitney Canyon-Carter Creek field. The source for Nugget hydrocarbons in the liquids trend is believed to be sub thrust Cretaceous. Migration may have coincided with the structural evolution of the overriding thrust sheet and probably continued after cessation of movement. This paper presents an overview of geologic factors that influence initial reservoir analysis and performance prediction at Anschutz Ranch East. A more detailed prediction at Anschutz Ranch East. A more detailed geologic discussion is presented in Ref. 5. In early stages of development, gross structural interpretation and the prediction of reservoir size and orientation is paramount. prediction of reservoir size and orientation is paramount. As development proceeds, geologic input becomes more specialized to include small-scale structural, stratigraphic, and diagenetic factors. At Anschutz Ranch East, pressure maintenance must be instituted before any pressure maintenance must be instituted before any retrograde condensate is produced. Therefore, geologic data normally associated with secondary recovery studies are in this case needed for initial reservoir development. Because reservoir pressure is so close to the dew point pressure, the field is essentially shut-in until pressure pressure, the field is essentially shut-in until pressure maintenance in the form of hydrocarbon gas and nitrogen injection is begun. Currently, the working interest owners, including Amoco Production Co., The Anschutz Corp., Champlin Petroleum, BWAB Inc., Tom Brown Inc., Mesa Petroleum, BWAB Inc., Tom Brown Inc., Mesa Petroleum Co., and Gulf Oil Exploration and Production Petroleum Co., and Gulf Oil Exploration and Production Co., are collaborating to form a development unit. Amoco Production Co. will be the major interest holder and the field operator.

Evolution of Structural Interpretation
The size and orientation of the reservoir is of major importance in evaluating a hydrocarbon accumulation. At this time, more than 2 years after the discovery, and with more than 20 additional pay penetrations, delineation of the field remains a critical and dynamic issue. To illustrate the importance of gross structure mapping, several historic interpretations are presented. These interpretations were made from the best subsurface well-control and seismic data available at the time. At the surface, Tertiary terrestrial sediments overlie the field. These surface beds are structurally detached from underlying rock units and therefore provide little information on deeper structures.
JPT
P. 1539