Abstract The Pompano salt canopy has been penetrated by six wells between 2001 and 2003 to test and develop various subsalt and extra-salt plays. Data from these wells provide insight into subsalt geologic models that may be applicable to other subsalt prospects in the Gulf of Mexico. The wells are aligned in a dip direction along the length of the salt canopy from the upturned section (which onlaps the flank of the salt), to beneath the salt, 5,600 feet back from the leading edge of the salt. In addition, almost the entire Miocene section has been tested below the salt. Pressure, dipmeter, and paleontological data indicate a zone about 400 feet thick below the base of salt, roughly conformable with the base of salt, and which terminates rapidly towards the leading edge of the salt canopy. These characteristic features are interpreted to be a shear or fault zone immediately below the base of salt. The Pompano salt canopy is anomalous among salt sheets because permeable sands occur immediately beneath, and juxtaposed against, the base of salt in a zone referred to as the ‘basal shear’ or ‘disturbed’ zone. This provides a rare opportunity to measure the pore pressure within the zone, and to generate pressure profiles below the salt. Within the high-pressured section near the base of salt, anomalous structural dips were encountered which were generally parallel to the base of salt. Migrated, thermogenic petroleum fluids having varying maturity and gas-oil ratio are found near the base of salt. Biostratigraphic data collected immediately beneath salt can indicate that the sequence is upright or overturned and consistently older than the rock found below, in addition to being fairly constant in thickness. The transition between the section adjacent to the salt and the country rock is abrupt and has all the attributes of a major fault. The section in the shear zone can be up to 1,000 feet structurally higher than the estimated cutoff on the footwall. In light of these observations, the preferred subsalt model for Pompano is a base of salt shear zone.

Abstract Intense faulting in the northern Gulf of Mexico slope province results from complex interactions between subsurface salt and the deposition of large volumes of sediment. Many of these faults provide pathways for subsurface fluids and gases to migrate from deep petroleum-generating zones to the modern seafloor. These migration pathways are concentrated along the margins of intraslope basins where they are directed by a spectrum of salt geometries. Both geological and biological responses are highly variable and dependent on rate of delivery as well as on fluid and gas composition. Qualitatively, rapid expulsions of gas-charged fluids (including fluidized sediment) result in the deposition of sediment sheets or, mud volcanoes. Both products of rapid expulsion vary greatly in scale. The sheet-like flows may be localized or extend over many square kilometers of the slope while mud volcanoes vary from < 1 m to several km in diameter. Hydrocarbons associated with rapid flux systems reflect little biodegradation during migration. Sediment samples from these seafloor expulsion areas frequently contain hydrocarbons that are remarkably similar to those that are produced from the parent deep subsurface reservoirs that are directly connected to the surface by faults. High accumulation rates, thin depositional units, and limited hydrocarbon storage capacity characterize sediments of rapid flux systems. Lucinid-vesycomyid clams and bacterial mats are the chemosynthetic communities that dominate in these settings. At the other end of the flux rate spectrum, slow hydrocarbon seepage results in lithification and mineralization of the seafloor. Microbial utilization of hydrocarbons promotes the precipitation of 13 C- depleted Ca-Mg carbonates as by-products. These products occur over the full depth range of the slope. Mounded carbonates can have relief of up to 30m, but mounds of 5-10m relief are most common. Mound-building carbonates represent mixed mineral phases of aragonite, Mg-calcite, and dolomite with Mg-calcite being the most common. Barite is another product that is precipitated from mineral-rich fluids that arrive at the seafloor in low-to-moderate seep rate settings. Hydrocarbons analyzed from these slow-flux settings are highly biodegraded and chemosynthetic organisms are generally limited to bacterial mats. Below water depth of approximately 500 m, intermediate flux settings seem best exemplified by areas where gas hydrates occur at or very near the seafloor. These environments display considerable variability with regard to surficial geology and on a local scale have elements of both rapid and slow flux. However, this dynamic setting apparently has a constant supply of hydrocarbons to promote gas hydrate formation at the seafloor even though oceanic temperature variation (primarily on the upper slope) cause periodic shallow gas hydrate decomposition. In the northern Gulf, gas hydrates contain both thermogenic and biogenic gas. The presence of these deposits provides the unique set of conditions necessary to sustain dense and diverse chemosynthetic communities. The cross-slope variability of seafloor response to fluid and gas expulsion is not well known. However, present data indicate that the expulsion process is highly influenced by migration pathways dictated by salt geometries that change downslope from isolated salt masses to canopy structures to nappes.

Abstract A deep, hot subsurface petroleum system in the Green Canyon area of the Gulf slope has generated oil and gas synchronously with salt deformation and fault activation, producing vertical migration conduits that charge traps in the subsurface. Trapping efficiency is poor. Much oil and gas is lost to venting and seepage at the sea floor. Sea floor vent and seep environments show hydrocarbon flux that ranges from rapid venting to slow seepage, affecting hydrocarbon geochemistry and gas hydrate abundance. Active mud volcanoes that vent oil and gas are the high flux end-member. Oil and gas rapidly bypass the sediment and enter the water column. The oil from the active vent sites shows only limited bacterial oxidation. Venting is episodic and may cease altogether, and warm brines may be present, potentially destabilizing gas hydrate. It appears that gas hydrate is only indirectly associated with high flux mud volcanoes. Massive gas hydrate is most often found at sites of moderate flux. Gas hydrate is associated with smaller but steady vents of relatively unaltered thermogenic gas, chemosynthetic communities, and authigenic carbonate rock. Oil-related structure II gas hydrate is most abundant. Oil and free gas in sediment are bacterially oxidized in moderate flux environments, leading to accumulation of abundant authigenic carbonate rock and to H 2 S, favoring complex chemosynthetic communities. Exposed gas hydrate is transiently unstable because of changes in seawater temperature, but this is a thin-skin process and more deeply buried gas hydrate appears to be stable and accumulating. Mineralized seep sites with low hydrocarbon flux do not appear to be important with respect to gas hydrate accumulation.