Large amounts of organic carbon were deposited and preserved in marine sediments of late Barremian through middle Albian and late Cenomanian-early Turanian age owing to the development of poorly oxygenated oceanic water masses and expanded oxygen minimum zones during “oceanic anoxic events.” Sediments rich in organic carbon which were deposited during such events are thick sequences of basinal black shale or mudstone, thin black beds in shelf chalks, and thin beds and lenses in rudist reef and associated limestones. Analysis of the stratigraphic distribution of both known oil and giant oil-field reservoirs by many workers has indicated that a large part of the world’s oil is of Mesozoic age and that most giant reservoirs are in rocks of Mesozoic age. We propose that the relative abundance of Mesozoic oil in the world oil picture is in part the result of maturation of organic carbon deposited during Cretaceous oceanic anoxic events.
Many giant fields of Cretaceous age have reservoirs of shallow-water carbonate complexes such as rudist reef and associated facies buildups. We propose that the oil in such reservoirs originated as follows: (1) during middle Cretaceous marine transgressions, equable world climate with high sea-surface temperatures prevailed and led to carbonate reef buildups on shallow shelves and marginal platforms; (2) at the same time marine oxygen-minimum zones expanded and deeper basinal oxygen deficits were intensified owing to the lack of strong oceanic mixing because of stable density stratification and possible low oceanic thermal gradients; (3) these oxygen deficits enhanced the preservation of organic carbon in basin, slope, and some shelf facies; (4) later Cretaceous transgression resulted in the drowning of the carbonate buildups which were then sealed under a cap of fine-grained hemipelagic sediment; interim regressions resulted in enhancement of porosity of reef facies and may have allowed deposition of interbedded sand bodies in some sections; (5) many of these carbonate complexes and source beds were buried to suitable depths by Late Cretaceous and Tertiary clastic wedges and, depending on local geothermal gradients, hydrocarbon maturation in the black shale basinal facies occurred. These hydrocarbons migrated to shelf-edge reservoirs, such as platform carbonate rocks, through intermediate facies. This scenario with its combination of oceanographic and geologic events led to development of fields such as those in the Golden Lane in Mexico.
Thus, prime exploration targets are deeply buried rudist buildups that are stratigraphically linked to basinal black shale source beds. Such buildups should be found on subsided passive margins of low to middle latitude regions; the source beds formed where nearly contemporaneous low-oxygen conditions obtained in adjacent basins.