Reservoir Characterization and Modeling of the Jurassic Smackover and Norphlet Formations, Hatter’s Pond Unit, Mobile County, Alabama
Elliott P. Ginger, Andrew R. Thomas, W. David George, Emily L. Stoudt, 1995. "Reservoir Characterization and Modeling of the Jurassic Smackover and Norphlet Formations, Hatter’s Pond Unit, Mobile County, Alabama", Hydrocarbon Reservoir Characterization: Geologic Framework and Flow Unit Modeling, Emily L. Stoudt, Paul M. Harris
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Hatter’s Pond Field, discovered in 1974, has produced over 241 billion cubic feet of wet gas from the Upper Jurassic Smackover and Norphlet Formations. From 1974 until 1988, the field was produced by pressure depletion. In 1985, the field was unitized to facilitate a gas injection program that began in 1988, aimed at retarding pressure depletion and thus delaying the onset of liquid condensation in the reservoir. This paper describes the geological and reservoir engineering studies involved in building an integrated reservoir model for Hatter’s Pond Unit, a model that has been used to evaluate, monitor, and predict reservoir performance under a variety of reservoir management scenarios.
The Norphlet Formation at Hatter’s Pond Unit consists of feldspathic lithic arenites deposited mostly in an eolian environment as dunes and interdune facies. Stratigraphically, dune successions are essentially flat lying and correlate across the Unit, and are terminated at the top and bottom by fairly sharp dune succession boundaries. There was essentially no relief on the Hatter’s Pond structure at the time of Norphlet Formation deposition. This influenced the way that the reservoir model and subsequent simulation units were constructed. The current distribution of gross reservoir properties in the Norphlet Formation at Hatter’s Pond Unit is controlled by this sedimentary template. However, that distribution has been severely modified by diagenetic processes. The quantity of authigenic illite decreases upward away from the gas/water contact, and the process is governed by the water saturation of the sandstone. Illite grows at the expense of feldspar dissolution. Norphlet intergranular volume (IGV) ranges from 16% to 27%. Zones of high IGV usually show low intergranular cement volume, low frequencies of stylolites/foot, and packing heterogeneities suggestive of secondary porosity. A recently dissolved intergranular cement (dolomite or anhydrite) is interpreted to have inhibited compactional processes in high porosity, high IGV zones.
The overlying Smackover Formation carbonates were deposited during the first major marine transgression after the opening of the Gulf of Mexico Basin. At Hatter’s Pond Unit, the Smackover Formation consists of shallow marine carbonate deposits; however, texturally destructive dolomitization has obliterated most original rock fabrics and this strong diagenetic overprint makes recognition of original depositional environments difficult. Seven distinctive lithofacies units are recognized, and they occur in three styles of vertical successions. The lower lithofacies are present in all wells in the Unit. Porosity and permeability in the lower units are largely the result of dolomitization and dissolution, and although reservoir quality is somewhat variable, it is significantly improved on the crest of the structure. Tremendous lateral variations occur within the upper Smackover lithofacies, mostly due to early diagenetic events controlled by paleotopography. Reservoir quality in these upper Smackover lithofacies is largely a function of fabric selective, meteoric dissolution of carbonate grains. This dissolution was controlled by the occurrence of localized islands and therefore is unrelated to present-day structure.
A quantitative model of reservoir properties for Hatter’s Pond Unit was constructed using Stratamodel’s SGM™. Core, log, and production data were used from 27 wells within the Unit to build a stratigraphic framework, a framework that had a robust geological characterization as its foundation. A template was necessary in the upper Smackover Formation in order to bias the attribute interpolation to best match the conceptual geologic model and production data. Four reservoir properties were exported from SGM: porosity, permeability, net pay thickness, and initial water saturation. They were imported into Western Atlas’ GeoLink® and vertically averaged into thirteen simulation layers: three for the Smackover Formation and 10 for the Norphlet Formation.
The Hatter’s Pond Unit reservoir simulation was conducted using Western Atlas’ VIP™ Compositional Model. The system can simulate gas condensate flow behavior, taking into account the fact that fluid properties and phase behavior vary strongly with fluid temperature, pressure, and composition. Fluid properties and phase equilibrium are governed by a generalized cubic equation of state in which reservoir fluid is treated as a mixture containing an arbitrary number of hydrocarbon and non-hydrocarbon components.
Accuracy and usefulness of any reservoir model is heavily dependent on the quality of the underlying geologic model, both qualitative and quantitative. Furthermore, it is critical to retain the character from the geologic model when scaling-up to the chosen simulation grid. Development of a geologically-based reservoir model formed a solid foundation for the Hatter’s Pond reservoir simulation model.
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This collection of papers presents documentation for (1) approaches to be taken in developing a geologic framework for explaining layering, heterogeneity, and compartmentalization of a reservoir; (2) the value of outcrop data in improving understanding of reservoir performance; (3) methods for integrating, analyzing, and displaying geologic, petrophysical rock property, and engineering data to be used during field evaluation, management, and simulation; (4) geostatistical approaches that are being used to characterize the spatial distribution of reservoir properties and augment geologic descriptions, and (5) methods of displaying quantitative models of reservoir properties and reservoir simulation in three dimensions.