Abstract: Petroleum Source Beds on Continental Slopes and Rises

Wallace G. Dow

Continental slopes and rises commonly are the sites of high organic productivity because of nutrients supplied by deep-water upwelling and river runoff. This marine organic matter has a much higher petroleum convertibility than terrestrial material which predominates on continental shelves. Incorporation of organic matter into sediments prior to oxidation is essential and requires reducing conditions like those in which irregularities in bottom topography result in closed anoxic basins. Slope sediments average 0.6 to 1.0 weight % organic carbon and are typically the most organic-rich continental margin deposits.

Conversion of organic matter to petroleum requires appropriate time-temperature conditions influenced by the geothermal gradient, the sedimentation rate, and the age of the source section. Deep wells on continental shelves have shown that on pull-apart margins, 3 to 4 km of burial is required for oil generation and 4 to 7 km for gas generation. Warmer compressional margins need less burial, but this is compensated in part by much younger rocks. Most of the sedimentary section under present slopes and rises is not mature enough to have generated petroleum and many of the deeper intervals were deposited in unfavorable source environments.

The section beneath slopes and rises contains oil and gas source beds only where organic content and maturity requirements are met. The most effective generation and expulsion occur in areas of rapid burial or high geothermal gradients. Migration and accumulation are most efficient where reservoir sequences prograde over deeply buried source intervals and in areas of structural complexity. Source beds for large amounts of oil and gas are present on slopes and rises, but they are the exception rather than the rule. The challenge is to find them.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC