Fluid Inclusion Evidence for Origin and Diagenesis of Sparry Calcite Cement in Capitan Limestone, McKittrick Canyon, Permian Basin, West Texas

Bonnie L. Crysdale

Sparry calcite cement is responsible for major porosity loss in the reef and fore-reef facies of the Capitan Limestone, Permian basin. This cement occurs throughout the massive reef facies where it fills large vugs as much as 0.5 m in diameter, with individual crystals up to several centimeters in length. Sparry cement in the fore-reef facies is less extensive and fills smaller vugs than in the reef, although individual crystals can be as large. The cement is composed of coarse, equant, clear to milky calcite that is rich in fluid inclusions.

Homogenization temperatures (T_h) of primary fluid inclusions trapped during cementation record formation temperatures from 55°-85°C. A relatively low geothermal gradient of 2°C/100 m suggests cementation occurred at moderate burial depths of 1-2 km.

Precipitation of sparry cement in fractures and brecciated areas also indicates cementation postdates minor burial compaction. Freezing temperatures (T_f) for primary fluid inclusions range from about -1° to 0°C (1.7 wt. % NaCl equivalent or less), suggesting the cement was formed in fresh to brackish waters. This range of T_f data is consistent with cement formation in a relatively deep lens of meteoric to mixing-zone waters.

Secondary fluid inclusions with T_f of about -12°C (> 16 wt. % NaCl equivalent) indicate that highly saline brines may have migrated along secondary fractures in the cement during later deposition of evaporites in the adjacent Castile and Salado Formations.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.