Characterization of Permeability Reducing Spheroidal Dolomite Cement from the Karachaganak Isolated Carbonate Platform: Carboniferous of the Pricaspian Basin, Kazakhstan

Katz, David A.¹; Macalello, Steven ²; Humphrey, John ²; Ducea, Mihai ^3
1 ETC, Chevron, San Ramon, CA.
² Geology and Geological Engineering, Colorado School of Mines, Golden, CO.
³ Geology, University of Arizona, Tucson, AZ.

Dolomite cement significantly reduces permeability of otherwise high-quality dolomitized reservoirs. Paragenetically early replacement by dolomite and associated dissolution of calcite matrix initially improves permeability of the system, while simultaneously exposing it to subsequent fluids that are responsible for negatively impacting the permeability of the system. This interplay of early reservoir enhancement and later porosity occlusion challenges our ability to determine the location and shape of high-quality dolomitized regions. Therefore, using geochemical tools to characterize the origins of the dolomite phases within the system is key to understanding the resulting diagenetically altered pore-system heterogeneity. Stable carbon and oxygen isotope data, together with Sr isotopic analysis and U-Pb dating, are used in conjunction with standard and cathodoluminescence petrography to constrain the absolute age and the diagenetic conditions under which the separate phases formed and to aid the modeling of the migration pathways for the fluids responsible for diagenesis. Sr isotope analyses and absolute U-Pb dates suggest both cement and replacive dolomite phases occurred paragenetically early during the Carboniferous and from marine-derived pore fluids. Stable carbon isotope data suggest a closed-system carbon source, while oxygen results indicate effects of burial recrystallization of the dolomite.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009