Interaction of Rivers and Oceans--Pleistocene Petroleum Potential

Abstract

The common association of a river (with or without a delta), submarine canyon, and abyssal fan implies a genetic relation. The formation of these features must be considered as one dynamic system and not as independent events. A river entering the ocean may deposit most of its load near its mouth. Factors such as fluctuating discharge and saltwater encroachment, resuspension of material by current action, and other processes may cause much of this material to be conveyed to the deeper parts of the ocean basins. These sediments, commonly transported through submarine canyons or sea valleys, ultimately will be deposited on the abyssal plains, either in the form of a submarine cone or redistributed by deep currents to form a continental rise. The Los Angeles basin and some fields in the Gulf Coast contain prolific petroleum reserves in deep-water turbidites. The accumulations are in reservoirs of different Cenozoic ages. Oil and gas fields have been discovered in sediments of Pleistocene age; however, the known accumulations are only modest. As exploration moves seaward, greater volumes of late Cenozoic sedimentary strata and petroleum accumulations can be anticipated. During the Pleistocene, the regimens of rivers were altered significantly. For long periods of time, lowered sea level, increased river gradients, and voluminous glacial meltwater in flood-swollen rivers prevented the formation of large deltas. The net result was transportation of great sediment volumes across the continental shelf and slope with attendant canyon cutting and the deposition of the material at the base of the slope as submarine fans. The associated reservoir rocks of these fans may contain petroleum reserves as great as those of their shoreline equivalents, the deltas.