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Lonnee, Jeff1 
(1) University of Alberta, Edmonton, AB

ABSTRACT: Complex Fluid Evolution in a Hydrothermal Dolomite Reservoir - Devonian Clarke Lake Gas Field, Canada

The Clarke Lake gas field, one of the largest gas fields in western Canada, is hosted in pervasively dolomitized Middle Devonian carbonates of the Slave Point Formation. The reservoir displays a highly complex paragenetic sequence. In addition to multiple phases of dolomite (matrix and saddle dolomite), late diagenetic phases included sphalerite, galena, quartz, and sparry calcite cement. A proper characterization and interpretation of dolomitization and commensurate porosity-permeability development should improve exploration success and enhance gas recovery throughout the region, and may lead to a new model for dolomitization. 
Both dolomite phases have identical oxygen and Sr isotope compositions: 18O range from –16 to –13‰ VPDB, and 87Sr/86Sr ratios range from 0.7087 to 0.7167. Fluid inclusions contain complex Ca-Cl brines and have Th values between 140 and 190ºC. Th and 18O values together indicate precipitation from evaporated seawater with 18O between -1 and +4‰ VSMOW. 
The present formation fluids bear seemingly incompatible ionic and/or isotopic characteristics. Their composition suggests a modified seawater component that is depleted in Mg and enriched in Ca, but D values range from –135 to –60‰ VSMOW, which is characteristic of mixing between meteoric and metamorphic/basement fluids. Furthermore, the 87Sr/86Sr ratios suggest a contribution of at least two distinct fluids, one that travelled through underlying clastics, and another that has interacted with basement rocks. Apparently, the fluid(s) responsible for dolomitization were flushed from the system, and the present formation fluids are of mixed origin of at least two end members. 


AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.