Abstract

We have developed process-based models for early grain coats and their impact on deep reservoir quality in the Jurassic eolian Norphlet Formation, Alabama, with implications for exploration and development in other conventional and tight-gas continental reservoirs. The Norphlet, a major gas reservoir to depths of 21,800 ft (6645 m) and temperatures of 419°F (215°C), displays contrasting intervals of high and low reservoir quality within compositionally similar cross-bedded eolian sands. Study results show that grain coats formed soon after deposition are responsible for differences in deep Norphlet porosity of up to 20% and permeability up to 200 md. Three types of grain coats were identified in Norphlet dune sands, each formed in a different part of a shallow groundwater system, and each with distinctive impact on deep reservoir quality. Diagenetic chlorite coats, formed where dunes subsided into shallow hypersaline groundwater, preserve good deep porosity (to 20%) and permeability (to 200 md). Continuous tangential illitic coats, formed in the vadose zone of stabilized dunes exposed to periodic fresh-water influx, preserve good deep porosity (to 15%) associated with poor permeability (<1 md) due to linked formation of later high-temperature diagenetic illite. Discontinuous grain coats, formed in active dunes where grains were abraded by eolian transport, are associated at depth with tight zones of pervasive quartz cement, low porosity (<8%), and low permeability (<1 md). These concepts plus data from 60 wells were used to derive bay-wide predictive tight and porous-zone isopachs that can be used for well placement, geologic models, and field development.

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