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Two Upper Cretaceous marine sandstones have produced 36 million bbl of oil and 164 bcf of gas at Spindle field in the Denver basin of Colorado. The sandstones are offshore bars enclosed in finer grained rocks of the middle Pierre Shale. They were deposited in response to western interior sea-level fluctuations that affected water depths, energy levels, and sediment distribution across structurally controlled sea-floor topography during several intervals of Cretaceous sedimentation. Some of the structures that influenced Pierre deposition are believed to have controlled later emplacement of oil and gas.
Preliminary petrographic analysis of the sandstones indicates that four phases of diagenesis have occurred. Early diagenetic events of compaction and formation of authigenic chlorite rims, silica overgrowths, and calcite cement within the sandstones (phases I and II) largely destroyed initially good primary porosity and permeability in these well-sorted sandstones. These early events approximately coincide with the energy-of-formation (^Dgr G°f) related paragenetic sequences in fine to medium-grained, well-sorted marine rocks as reported by other workers. However, later diagenetic events (phases III and IV) clearly record separate phases with no sequential relationship to earlier events.
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. Acid water, resulting from dissociation of the CO2 gas produced during degradation of organic matter in the source rocks, is considered a likely solvent for the calcite. Secondary porosity has created the reservoir at Spindle field; elsewhere, thick, originally well-sorted sandstones are tight. Both the acidic fluids and the oil and gas are postulated to have migrated vertically, under pressure related to petroleum generation, into the reservoir along tensional faults and fractures related to a paleostru tural element at depth. Spindle field hydrocarbons represent a combination of stratigraphic and diagenetic trapping.
Diagenetic sequences record aspects of the total burial history of a rock that may be combined with other geologic information to give a better understanding of the total burial history. Predictability of timing and mechanism of hydrocarbon emplacement may be enhanced by relating diagenetic sequences to the structural features and history of an area. The diagenetic sequence observed in the sandstones at Spindle field is recognized as a response to structural influence on fluid migration.
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