Abstract:
Despite the knowledge that billions of barrels of oil had been pumped from sandstones interpreted as turbidites in the Los Angeles basin, California, and elsewhere, and Sullwold's (1961) persuasive arguments on behalf of so doing, petroleum geologists did not immediately add turbidites and associated deep-water coarse sediments to their preferred list of exploration targets. In addition to citing all the oil from the inferred turbidite reservoirs, Sullwold emphasized three other aspects of deep-water sands that ranked as positives with respect to petroleum possibilities: (1) many turbidity currents deposited not only sands but also transported into deep water abundant allochthonous organic matter from shallow-water marine organisms and land-plant material; (2) turbidity currents and associated gravity-driven, bottom-following sediment-transport processes brought potential reservoir sands into close juxtaposition with basin-floor anoxic muds, ideal "source beds," thus combining two petroleum prerequisites and in a setting requiring a minimum amount of "primary migration" of the petroleum from source to reservoir; and (3) turbidites and associated sands contain abundant information for reconstructing ancient paleogeography, a distinct exploration advantage. A fourth positive factor was added later by others: the critical petroleum precursor, kerogen, was found to be recycled in sands on the floor of the Antarctic Ocean. Any recycled immature kerogens in a sand could provide a further possible basis for "indigenous petroleum." Part of the explanation for the petroleum industry's initial lack of enthusiasm for targeting deep-water sands may have resulted from the 1950's research emphasis on turbidites as muddy sands--the modern analogues of graywackes. However, two other developments helped to dispel this petroleum-negative "muddy-sand" viewpoint. (1) In the 1960's, laboratory analyses of modern deep-sea sands showed that many lack the supposedly characteristic primary detrital-fines matrix. (2) Cummins (1962) suggested that the matrix of many graywackes is not a primary detrital matrix, as had been widely presumed, but is of diagenetic origin and typically resulted from the dissolution of framework particles (especially feldspars and/or volcanic rock fragments) and subsequent authigenic growth of clay minerals. Further research, both on turbidites and their associated sediments in the modern oceans and on exposed examples of inferred ancient analogues, has supported- and elaborated on the systematic relationships among the three associated suites of deep-water coarse clastic sediments that Sullwold outlined: (a) submarine-canyon-axis channel deposits, (b) sea-floor fans at the mouths of submarine canyons, and (c) the deposits underlying basin-floor plains. The results from the Deep Sea (sic) Drilling Project (DSDP) reversed the generally negative outlook about petroleum prospects in the deep sea. In the second hole drilled, on the Sigsbee Knoll in the Gulf of Mexico, liquid hydrocarbons (immature crude oil) were found in sediments of Jurassic/Cretaceous age associated with a salt diapir. On Leg 11, the first of many examples of black, anoxic sediments of Cretaceous ages were drilled off eastern North America. Organic-geochemical analyses of deep-sea sediments support the interpretation that many qualify as being potential petroleum "source beds." One of the developments of seismic stratigraphy has been the recognition of low-stand wedges composed of turbidites and associated basin-floor fans. During the low stands of the sea that accompanied the spread of Pleistocene continental glaciers in the Northern Hemisphere, turbidity currents and associated gravity-driven, bottom-following sediment-transport processes deposited huge quantities of sand around the margins of modern ocean basins. In places such as the Gulf of Mexico, plans are underway to test the petroleum potential of these deep-sea sands. All of these research developments have converged at last to take turbidites and related deep-sea sands off the "no-no" list and to place them on the list of high-priority petroleum-exploration targets.