Cretaceous Oceanic Anoxic Events'as Causal Factors in Development of Reef-Reservoired Giant Oil Fields'

Large amounts of organic carbon were de­ posited and preserved in marine sediments of late Bar- remian through middle Albian and late Cenomanian- earty Turonian age owing to the development of poorly oxygenated oceanic water masses and expanded oxy- § en minimum zones during "oceanic anoxic events." ediments rich in organic carbon which were depos­ ited 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 reser­ voirs 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 pro­ pose 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 fades buildups. We propose that the oil in such reservoirs originated as follows: (1) dur­ ing middle Cretaceous marine transgressions, equable world cHmate with high sea-surface temperatures pre­ vailed and led to carbonate reef buildups on shallow shelves and marginal platforms; (2) at the same time marine oxygen-nilnlmum zones expanded and deeper basinal oxygen deficits were Intensified owing to the lack of strong ocaanic mixing because of stable densi­ ty stratificatio n and possible low oceanic thermal gra­ dients; (3) these oxygen deficits enhanced the preser­ vation of organic carbon in basin, slope, and some shelf fades; (4) later Cretaceous transgression result­ ed in the drowning of the carbonate buildups which were then sealed under a cap of fine-grained hemipe- iagic sediment; interim regressions resulted In en­ hancement of porosity of reef fades and may have al­ lowed deposition of interbedded sand bodies in some sections; (6) many of these carbonate complexes and source beds were buried to suitable depths by Late Cretaceous and Tertiary clastic wedges and, depend­ ing on local geothermal gradients, hydrocarbon matu­ ration in the black stiale basinal fades occurred. These hydrocartions niigrated to shelf-edge reservoirs, such as platform cart)onate rocks, through intermediate fa­ des. This scenario with its combination of oceano- graphic and geokigic events led to development of fields such as those in the Golden Lane in Mexico. Thus, prime exploration targets are deeply buried ru­ dist buMuBs that are stratigraphically linked to basinal black shaw source tieds. 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.