Origin of the Micropore Pore Network within the Lower Cretaceous Stuart City Tight-Gas-Carbonate Play in the Pawnee Field area in South Texas

Robert Loucks, Jerry Lucia, and Lowell Waite
[email protected]

The Stuart City Trend has been gas productive over the last 50 years. Recently Pioneer Natural Resources in the Pawnee field have revitalized this deep, tight-gas-carbonate trend by drilling numerous horizontal productive wells. The Stuart City is a shelf-edge complex showing forereef slope facies, thick reef-core boundstone facies, and backreef wackestone through grainstone facies. Porosity is generally less than 7% and permeability less than 1 md. The major pore network consists of micropores associated with the development of microrhombic calcite, which ranges between 2 and 6 microns in size and pore throats sizes generally a micron or less. The pore network at thin-section scale shows development of micropores within grains, in grain rims, and in very fine micritic, peloidal 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, foraminifera, micrite rims, and very fine peloids. The process of formation is by dissolution of original Mg-calcite nanocrystallites (averaging 30–40 nanometers in size) in allochems and carbonate mud and the reprecipitation of microrhombic calcite, which shows competitive growth with other microrhombic crystals. These crystals may totally fill local pore space, occluding all porosity. In the Pawnee field, this micropore network is developed in all facies over 1,000 feet of section. This is an example of a tight-gas reservoir where macropores are cemented during burial diagenesis and micropores, being more resistant to cementation, remain open to depths of 13,000 to 15,000 ft.

AAPG Search and Discovery Article #90167©2013 GCAGS and GCSSEPM 63^rd Annual Convention, New Orleans, Louisiana, October 6-8, 2013