Seafloor expression of fluid and gas expulsion from deep petroleum systems, continental slope of the northern Gulf of Mexico

Roberts, H. H., Coastal Studies Institute, LA State University, Baton Rouge, LA, Roger Sassen, and A. V. MiIkov, Geochemical & Environmental Research Group, Texas A&M University, College Station, TX

Abstract

In the northern Gulf of Mexico slope province, complex structural relationships resulting from dynamic adjustments between large volumes of sediments and salt provide numerous faulted pathways for deep subsurface fluids and gases to migrate from the deep petroleum-generating subsurface to the modern seafloor. These migration pathways are concentrated along the margins of intraslope basins where they are directed by a spectrum of salt and salt migration geometries. Both geological and biological responses are highly variable and dependent on rate of delivery as well as fluid and gas composition. Qualitatively, rapid expulsions of gas-charged fluids (including fluidized sediment) result in the deposition of sediment sheets or localized core-like buildups, mud volcanoes. Both of these products of rapid expulsion vary greatly in scale. The sheet-like flows may extend over many square kilometers of the slope while mud volcanoes vary from a few meters to several kilometers in diameter. Hydrocarbons associated with rapid flux systems reflect little biodegradation during migration. Samples from the seafloor are remarkably similar to hydrocarbons produced from the parent, deep subsurface reservoirs that are directly connected to the surface by faults. High accumulation rates and limited hydrocarbon storage capacity characterize sediments of flux systems. Lucinid-vesycomyid clams dominate in these settings.

At other end of the flux rate spectrum, slow seepage promotes lithification and mineralization of the seafloor, processes resulting in precipitation of a variety of mineral species. The most important of these processes involves the microbial utilization of hydrocarbons and precipitation of ¹³C-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-l0m relief are most common. Mound-building carbonates represent mixed mineral phases of aragonite, Mg-calcite, and dolomite; Mg-calcite is 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.

Intermediate flux settings seem best exemplified by areas where gas hydrates occur at or very near the seafloor. Intermediate flux environments display considerable variability with regard to surficial geology and on a local scale have elements of both rapid and slow flux settings. However, this dynamic setting 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 at or near the modem seafloor 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 sets suggest that the expulsion process is highly influenced by migration pathways dictated by salt geometries that change from canopy structures of the mid-slope to lower-slope nappes.