Abstract: Middle Cretaceous "Oceanic Anoxic Events" as Causal Factors in Development of Giant Oil Fields

Michael A. Arthur, Seymour O. Schlanger

Large amounts of organic carbon (Corg) were preserved in marine sediments of late Barremian-middle Albian and late Cenomanian-early Turonian age as a result of the development of poorly oxygenenated oceanic water masses and expanded oxygen-minimum zones during "oceanic anoxic events." Sediments rich in Corg are thick sequences of basinal black shale, thin black beds in shelf chalks, and thin beds and lenses in rudistid reef and associated limestones. We propose that the abundance of giant oil fields having reservoirs of middle Cretaceous age is linked directly to the Early to middle Cretaceous peaks of Corg preservation in sediments. Some giant fields which have reservoirs of shallow-water carbonate rock developed as follows: (1) during middle Cretaceous marine transgress on, an equable climate and high sea-surface temperatures led to carbonate-reef buildups on shelves and marginal platforms; simultaneously (2) the oxygen-minimum zone and some deeper basinal oxygen deficits were intensified and expanded because of the lack of strong oceanic thermal gradients; (3) the oxygen deficits enhanced Corg preservation in basin, slope, and some shelf facies; (4) with further rapid transgression of seas, the carbonate buildups were drowned and sealed under a cap of fine-grained hemipelagic sediment, although interim regression may have allowed local deposition of overlying sand bodies; (5) burial to suitable depth, depending on local geothermal gradient, allowed petroleum generation from black shale basinal facies. This migrated to shelf-edge reservoirs, either plat orm carbonate rocks or regressive sandstones.

We propose that the preceding combination of oceanographic and geologic factors led to development of a field such as the Poza Rica in Mexico. Thus, prime exploration targets are rudistid buildups on subsided passive margins of low to middle-latitude regions where nearly contemporaneous low-oxygen conditions also obtained.

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