Abstract

This project studies Ordovician, Mississippian, and Pennsylvanian rocks of the USA Midcontinent to demonstrate a three-stage evolution of hydrothermal systems that was important in controlling reservoir properties. The first stage is regional in extent and recognized from fluid inclusions in megaquartz, indicating advective fluid flow of low-salinity connate fluids and gas. The second stage is recognized in baroque dolomite cements. Fluid inclusions in the baroque dolomite contain concentrated brines and oil. The Sr isotope data indicate extensive rock-water interaction leading to the interpretation of regionally advective fluid flow. The oxygen isotope data indicate higher temperatures at the top of the Mississippian, suggesting that the Mississippian-to-Cambrian-Ordovician section acted as a regional aquifer, and the Pennsylvanian section acted as a leaky confining unit. Reservoir porosity and thermal maturation is partially controlled by hydrothermal fluid migration enhancing the porosity and heating areas where fractures and faults led to preferred hydrothermal fluid flow, especially close to the top of this regional hydrothermal aquifer. The third stage of hydrothermal fluid flow is characterized by fluid inclusion, strontium and oxygen isotope data in calcite. The data show well-to-well variation, a highly radiogenic end member, and indications of vertical fluid flow. This stage differs from previous stages in that faults and fractures may have led to highly localized hydrothermal systems associated with deformation events.

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