Overpressure history of fractures, West Maracaibo Basin, Venezuela

Prediction of density of natural fractures in reservoir rocks requires evaluation of many factors, including the effective stress and thus fluid pressure conditions through time. In this study we use analyses of fluid inclusions in calcite-filled fractures with burial and thermal history models to assess the fluid pressure history and the causes of overpressure in the West Maracaibo Basin. We analyzed abundant oil-filled and rare aqueous fluid inclusions in calcite-filled fractures in the La Luna Formation source rock and in the underlying Cogollo Gp. carbonate reservoir. Our analyses of fluid inclusion homogenization temperatures and fluorescence properties lead us to the interpretation of near-lithostatic paleo-fluid pressures in La Luna Formation and near-lithostatic to hydrostatic fluid pressures in the Cogollo, Gp. fractures. Maturation and expulsion of oil from the La Luna Formation source rock is required to generate the large inferred excess pressures as compaction disequilibrium and thermal expansion of pore fluids from rapid Miocene burial are insufficient to achieve near-lithostatic conditions. This hypothesis is supported by the observation of decreasing paleo- and modern fluid pressures with depth beneath the La Luna Formation. Thus based on the wide occurrence of oil-filled inclusions in calcite-filled fractures and the high fluid pressures associated withmore » trapped oils, we infer extensive rock fracture under overpressured conditions near maximum Miocene burial, during inferred late source rock yield, and during Miocene growth of structural traps.« less