Two Upper Cretaceous marine sandstones have produced 36 million bbl of oil and 164 bcf of gas. Offshore bars enclosed in finer grained rocks of the middle Pierre Shale. Sea-level fluctuations; structurally controlled sea-floor topography. Some of the structures that influenced Pierre deposition are believed to have controlled later emplacement of oil and gas. Four phases of diagenesis have occurred. Early diagenetic events largely destroyed initially good primary porosity and permeability. Later diagenetic events of calcite solution to form secondary porosity and partial occlusion of the secondary porosity by authigenic kaolinite and minor illite reflect a change in pore-fluid chemistry believed to be related to vertical migration of acidic fluids from below. Both the acidic fluids and the oil and gas are postulated to have migrated vertically into the reservoir along tensional faults and fractures related to a paleostructural element at depth. Spindle Field represents a combination of stratigraphic and diagenetic trapping.--Modified journal abstract.

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