Abstract Abnormal pore pressure is widespread in the Tertiary through upper Mesozoic, clastic-dominated section of the Eastern Venezuelan Basin and the eastern extension of the basin into Trinidad. Some of the largest oil and gas columns are found within abnormally pressured sandstones which account for 43 million bbl (6.8 million m 3 ) of oil in Poui field and 882 billion ft 3 (24.98 billion m 3 ) of gas in Cassia field. Abnormal pressure within the Tertiary to Upper Cretaceous rocks resulted from the transfer of overburden stress to the pore system during the rapid subsidence and infilling of the foredeep basin during the Miocene and Pliocene. Primary migration from thick, Upper Cretaceous source rocks and secondary migration through the thick Tertiary elastics occurred principally through hydraulically induced fractures within a highly overpressured section. Final migration out of the overpressured section and charging of present-day reservoirs off the east coast of Trinidad occurred during the late Pliocene to Pleistocene uplift and associated complex normal faulting. The multiple pressure compartments within the six fields studied are separated by relatively thin, abnormally pressured shale. The shale seals are most effective in trapping hydrocarbons when the pressure difference across the shale is less than 4 psi/ft (90 kPa/m) regardless of the shale thickness. Normal faults form effective pressure seals throughout the basin, separating porous sandstone pressure compartments with pressure differences as great as 1,856 psi (12.8 MPa). The oil and gas fields of offshore Trinidad reveal a widely varying depth to the top of abnormal pressure, large pressure differences across faults, pressure reversals, and a narrow zone of transition from mild abnormal pressure (<11 PPG [lb/gal] equivalent) to highly overpressured conditions (>14 PPG equivalent).

Abstract Normal faults commonly represent one of the principal controls on the origin and formation of sedimentary rock-hosted mineral deposits. Their presence within rift basins has a profound effect on fluid flow, with their impact ranging from acting as barriers, causing pressure compartmentalization of basinal pore fluids, to forming conduits for up-fault fluid flow. Despite their established importance in controlling the migration and trapping of mineralizing fluids, we have yet to adequately reconcile this duality of flow behavior and its impact on mineral flow systems within basinal sequences from a semiquantitative to quantitative perspective. Combining insights and models derived from earthquake, hydrocarbon, and mineral studies, the principal processes and models for fault-related fluid flow within sedimentary basins are reviewed and a unified conceptual model defined for their role in mineral systems. We illustrate associated concepts with case studies from Irish-type Zn-Pb deposits, sedimentary rock-hosted Cu deposits, and active sedimentary basins. We show that faults can actively affect fluid flow by a variety of associated processes, including seismic pumping and pulsing, or can provide pathways for the upward flow of overpressured fluids or the downward sinking of heavy brines. Associated models support the generation of crustal-scale convective flow systems that underpin the formation of major mineral provinces and provide a basis for differences in the flow behavior of faults, depending on a variety of factors such as fault zone complexities, host-rock properties, deformation conditions, and pressure drives. Flow heterogeneity along faults provides a basis for the thoroughly 3D flow systems that localize fluid flow and lead to the formation of mineral deposits.

Abstract The southeastern Caribbean is a geologically challenging region, and nowhere is this more evident than off the east coast of Trinidad. Rapid and prolific deposition in a deltaic setting and the effects of tectonic interaction between the Caribbean, North American, and South American plates serve to make this a unique area. The unpredictability that characterizes petroleum exploration in this region bears witness to this intricate geologic scenario. Similarities between this extended shelf and the Barbados accretionary prismto the north have been assumed mostly based on scant seismic coverage over this region. Analysis of newly acquired seismic data has revealed a subregion of intermittent shale mobilization, and vast sedimentary deposition exists eastward of the currently producing area. Although a great similarity to the shale belt in the region off the Darien Ridge is observed, this subregion exhibits its own distinct character. Shale flow features with associated bottom-simulating reflectors, syndepositional and postdepositional intrusives, which constrain subbasin development, and extrusive features are apparent. A distinguishing characteristic of this subregion is the association of mobilized shales with thrust faulting within the thick sequence of unconsolidated overburden, providing pathways for shale movement; this has the potential of initiating flow not only of shale but also of hydrocarbons from deeper source rocks. This may explain the presence of thermogenic hydrocarbons shown in this region and may have implications for petroleum exploration in this deep-water area.

ABSTRACT Oil accumulations in Trinidad were sourced by the Upper Cretaceous calcareous siliceous shales deposited along the Cretaceous passive margin of northern South America. Maturation of these source rocks, oil generation, migration and re-migration occurred in a foreland uplift-foredeep setting that resulted from interaction between Caribbean and South American plates during Lower Miocene to Recent times in the Trinidad area. During Lower Miocene-Recent times, the foreland basin experienced strong compressional events, which controlled generation, migration, and accumulation of oil in Trinidad. A series of mature source rock kitchens formed in Lower Miocene-Recent times in the Southern and Columbus Basins southward of the Central Range Thrust which trends generally NE/SW. The thrusts and associated fractures first developed around 22 m.y.b.p. and served as near vertical migration paths for the oil generated in penecontemporaneous kitchens. This oil migrated into submarine fans deposited in the foredeep basin axis and older reservoirs deformed into structural traps. Further generation and migration of oil, and re-migration of earlier oil took place during Pliocene-Holocene times, when later thrusting and wrench faulting served as the migration paths. Extremely high sedimentation rates in Pliocene-Pleistocene time, concurrent with active faulting, was responsible for very rapid generation of oil and gas. Vertically migrating gas often mixed with earlier migrated oil in overlying reservoirs. This caused depletion of oil in light hydrocarbons with accompanied fractionation among hydrocarbon types resulting in heavier oil in lower reservoirs, and enrichment of light hydrocarbons and accumulation of gas-condensates in upper reservoirs. This process led to an oil-gravity stratification within about 10,000 feet of section. These concepts enable prediction of petroleum occurrences onshore and offshore Trinidad.