Abstract: Preliminary Fluid Inclusion Evidence for the Mechanism of Dolomitization in the Lower Permian Chase Group, Hugoton Embayment, Southwest Kansas

Bryan J. Bergmann, John A. Luczaj

Primary fluid inclusions in dolomite from the lower Krider Limestone of the Mobil Nix #1 Unit #3 well and the base of the Winfield Limestone of the Mobil Clair Curry Unit #3 well in southwestern Kansas were analyzed to determine the salinity, temperature, major ions, and gas content of fluids responsible for dolomitization.

Dolomite rhombs show growth zonation with inclusion-rich cores and clear, inclusion-free rims. Both one-phase and two-phase aqueous fluid inclusions are present in the inclusion-rich cores. Melting temperatures of ice range from -17.5°C to -22.8°C (20.6 to 24.2 weight % NaCl equivalent). Homogenization temperatures range from 56.9°C to 128°C, with 85% between 75°C and 95°C. Eutectic melting temperatures were observed at -57°C for the Nix well and -52°C for the Clair Curry well indicating model compositions of Na-Ca-Mg-Cl-rich and Na-Ca-Cl-rich fluids, respectively. Intermediate melting temperatures between ~- 42° and -38°C were observed but were not interpretable. Crushing runs reveal the vapor phase contains an exsolved gas of unknown composition at pressures between 5 to 35 bars at room temperature.

Cathodoluminescence petrography reveals three growth zones that are especially well developed in dolomite of the Clair Curry well. Zone 1 (inclusion- rich core) is dull but contains some bright areas suggestive of recrystallization of an originally dull rhomb. Zone 2 (inner zone of clear rim) is bright and exhibits polyhedral crystal growth. Zone 3 (outer zone of clear rim) is dull and rhombic.

The presence of one-phase and two-phase inclusions within the cores of dolomite crystals suggests that highly saline fluids were entrapped below about 50 degrees C and subsequently re-equilibrated (by leakage and refilling or by recrystallization). A possible-model responsible for the dolomitization involves refluxing of brines followed by re-equilibration of inclusions by warm brines migrating cratonward from the Anadarko Basin.

AAPG Search and Discovery Article #90957©1995 AAPG Mid-Continent Section Meeting, Tulsa, Oklahoma