| Summary: | The relations between overpressuring, diagenesis, and fluid flow in sedimentary basins are complex and multifaceted. The Matagorda Island 519 field (MI 519), offshore Texas, provides an excellent area for investigating some of these relations. The top of overpressure at MI 519 occurs at a depth of 3.5 to 3.8 km in a Lower Miocene deltaic sequence. On the basis of log-derived lithostratigraphy, the pressure seal does not appear to be lithologic in origin. Geochemical, mineralogical, and cuttings information indicate instead that the precipitation of diagenetic calcite and possibly quartz cements has been the major factor in seal development. Stratigraphic variation in mudstone chemistry indicates diagenesis has been an open-system process, with significant loss of Ca, Si, Mg, and Fe and gain of K in sediments below the pressure seal. Fluid pressures calculated from shale resistivities provide evidence for several vertically-stacked overpressured compartments at MI 519. Lateral sealing within the overpressured section may be provided by faults and precipitation of diagenetic cements within faults.
In contrast to other areas of the Gulf of Mexico Basin, overpressure development at MI 519 does not appear to be due to compaction disequilibrium because of the lack of significant post-Miocene deposition and a lack of a reversal in mudstone porosity below the top of overpressure. More likely causes of overpressuring are clay mineral dewatering, petroleum generation, and the presence of a large column of natural gas.
At least six stages of fluid flow and/or diagenetic development have occurred at the field: 1) calcite cementation within preferred intervals from fluids that originated by dissolution of updip salt domes, 2) deep overpressure development and upward focused flow of underlying Mesozoic brines and the development of secondary porosity in reservoir beds by carbonate dissolution, 3) precipitation of a seal by mixing of deeply-sourced and updip-sourced fluids, 4) hydrocarbon generation and shallow overpressure development, with hydrocarbons filling in porosity created by calcite dissolution, 5) hard overpressure development from smectite dehydration, and 6) development of a shallow freshwater lens during the Pleistocene lowstand.
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