Origin and Description of the Micropore Network within the Lower Cretaceous Stuart City Trend Tight-Gas Limestone Reservoir In Pawnee Field in South Texas

摘要

The Stuart City Trend has been gas productive over the last 50 years. Pioneer Natural Resources Co. recently revitalized this deep, tight-gas carbonate trend by drilling numerous horizontal productive wells in Pawnee Field. The Stuart City is a shelf-edge complex showing forereef-slope facies, thick reef-core boundstone facies, and back-reef wackestone through grainstone facies. Porosity is generally less than 8%, and permeability is less than 1 md. The major pore network consists of micropores associated with development of microrhombic calcite, which ranges between 1-6 μm in diameter. Pore-throat sizes are generally 1 μm or less. The pore network at thin-section scale shows development of micropores within grains, in grain rims, and in very fine micritic, peloi-dal matrix. Most macropores have been cemented by equant calcite and radiaxial fibrous calcite. The microrhombic calcite formed by stabilization of Mg-calcite, especially in Lithocodium-Bacinella, foraminifera, micrite envelopes, and very fine peloids. The process of formation is by dissolution of original Mg-calcite nanocrystallites (averaging 50 to 200 nm in diameter) in allochems and carbonate mud and reprecipitation of microrhombic calcite, which shows competitive growth with other microrhombic crystals. This transformation process also increased the size of pore throats between crystals, improving permeability. In Pawnee Field, this micropore network is developed in all facies over 1000 ft of section. This field is an example of a tight-gas reservoir in which macropores are cemented during burial diagenesis and micropores, being more resistant to cementation, remain open to depths of 13,000 to 15,000 ft.