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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 68 (1984)

Issue: 4. (April)

First Page: 539

Last Page: 539

Title: Salt Domes, Anoxic Brines, and Organic-Rich Sediments in Intraslope Basins: ABSTRACT

Author(s): Douglas F. Williams


The origin of organic-rich sediments and their potential relationship to petroleum source beds remains problematic despite the development of models relying primarily on oceanographic mechanisms. We outline instead a model that involves salt dome development under a prograding shelf-slope and has a modern analog in the hypersaline anoxic Orca basin on the Texas-Louisiana continental slope. We believe the model may explain the occurrence of some ancient black shale sequences by a mechanism which is not dependent on either intensification of the mid-water oxygen minimum, enhanced marine productivity, or circulation-induced bottom water anoxia. In our working model, salt diapir movement structurally produces sedimentary basins in the intraslope region. Dissolution of the sal inhibits oxygen replenishment in the deep parts of the basin. As oxygen levels and bioturbation in the basin are diminished, finely laminated sediments with high organic carbon contents result. Gradual evolution of the basin to complete anoxic conditions leads to the accumulation of fine-grained, fluid-rich, black muds which, upon burial and dewatering through compaction, resemble black anoxic shales. Infilling and burial of the basin with clastic and hemipelagic sediments from glacioeustatic-driven climatic episodes subsequently provide source beds and reservoir rocks for potential exploration plays. Lerche and O'Brien have recently shown that the high thermal heat capacity of salt domes (^approx 5 times > sediment) produce predictable thermal anomalies in the surrounding sediments. Thermal maturation rates of the organic-rich source beds are enhanced near the top of the salt dome and suppressed on the lower flanks. The negative thermal anomaly on the lower flanks of the salt dome inhibits overmaturation and thus enlarges the hydrocarbon window. As overburden increases, the density contrast between the salt (^approx 2.2 gm/cm3) and surrounding compacted sediments (2.6-2.7 gm/cm3) eventually becomes such that upward diapirism resumes. The present model, although preliminary, may have wide applicability in sedimentary basins other than the Gulf of Mexico. Further work on the Orca basin and other sedimentary sections near salt displacement should prove beneficial in future modeling of organic-rich sediments.

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