Origin and Distribution of Microrhombic Calcite and Associated Micropores in the Lower Cretaceous Stuart City Tight-Gas-Carbonate Play in South Texas

The Stuart City Trend has been gas productive over the last 50 years. Recently Pioneer Natural Resources and others have revitalized this deep, tight-gas-carbonate trend by drilling numerous horizontal wells. The Stuart City is a shelf-edge complex showing forereef slope facies, thick reef-core boundstone facies, and backreef wackestone through grainstone facies. The major pore network consists of micropores associated with the development of microrhombic calcite, which ranges between 2 and 6 microns, associated pores ranging between 1 and 10 microns. 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 and may totally fill local pore space, occluding all porosity. The pore network at thin-section scale shows development of micropores within grains, in grain rims, and in very fine micritic, peloidal matrix. Permeability is generally less than 1 md. In the Stuart City tight-gas carbonate trend, this micropore network is present through 1,000 feet of section and along strike for several hundreds of miles. It is developed in all facies. Similar micropore networks are common in other tight-gas-carbonate reservoirs, and diagenetic microrhombic calcite is an extremely common diagenetic product following the stabilization of Mg-calcite. Micropores also appear to be able to stay open to greater depths (<15,000 feet) than macropores.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California