Geologic and Hydrologic Controls on Reservoir-Scale Variability in Formation-Water Compositions

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1993

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Subsurface formation waters exhibit regional trends in measured chemistries, but the data also exhibit marked local variance that has not been adequately described or explained. An integrated study of chemical, petrologic, and fluid-pressure data from a well-characterized natural gas field in the Gulf Coast basin will allow us to determine reservoir-scale controls on chemical and diagenetic variability. Understanding the controls on chemistry can provide insight into fluid flow and rock-water interactions in similar geologic settings. Knowledge of solute distributions will aid in the assessment of compartmentalization within reservoirs and fluid communication between reservoirs. Such assessment is relevant not only to improved hydrocarbon exploitation but also to the safe injection of chemical wastes. Finally, understanding small-scale chemical changes would further the interpretation of regional variations in water chemistry, diagenetic facies, and fluid flow within the Cenozoic section of the Gulf Coast basin. This interpretation is potentially important in the study of hydrocarbon migration and entrapment.

We propose to sample in detail formation waters from Stratton Field in Nueces and Kleberg Counties, Texas, in order to map and interpret chemical variations within and between individual reservoirs. The results of water analyses will be mapped with respect to facies and reservoir geometries and features such as faults in order to determine stratigraphic, structural, and hydraulic controls on chemical variability. Hydrochemical data will be compared with mineralogic analyses of core, and geochemical modeling will be conducted. Results will be assessed in terms of the extent of rock-water equilibration to determine plausible reaction and mixing sequences along flow paths.

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