Modes of Organic Carbon Accumulation in Late Jurassic Sediments: Insights from the Northcentral Gulf of Mexico

Ezat Heydari, Laurie C. Anderson

Twenty-five percent of the world's petroleum was sourced by Late Jurassic organic-rich sediments, predominantly from four basins: (1) the Persian Gulf (11.5%), (2) West Siberia (5.5%), (3) the Gulf of Mexico (3.25%), and (4) the North Sea (2.75%). Deposition of organic-rich sediments in these basins was not synchronous.

In the northern Gulf of Mexico, hydrocarbons were derived from lime mudstones at the base of the Late Oxfordian Smackover Formation. Three lime mudstone lithofacies, each up to 50 m thick, are recognized. In ascending order they are: (1) laminated, (2) thin-bedded, and (3) burrowed. Organic carbon is concentrated in siliciclastic-rich lamina-sets and beds of the laminated and thin-bedded facies. The burrowed mudstone is devoid of organic carbon. Laminated mudstones were deposited in a deep basin under anoxic conditions. Thin-bedded mudstones formed in a slope that experienced alternating anoxic and oxic conditions. Burrowed mudstones were deposited in well oxygenated mid- to outer shelf environments.

Concentration of organic matter in siliciclastic-rich layers may have been linked to freshwater influx rates and related migration of the pycnocline within the water column. Organic- and siliciclastic-poor layers were deposited during dry periods when freshwater influx was low and the pycnocline was located deep in the water column. The deep basin was anoxic while slope and outer shelf were oxygenated. Organic matter would have been destroyed due to water column oxygenation.

Organic- and siliciclastic-rich layers were deposited during wet periods when freshwater influx was high and the pycnocline migrated into shallower waters resulting in the expansion of anoxia from the basin to the slope and outer shelf. Organic matter was preserved due to water column anoxia. This model may be applicable to other Late Jurassic basins.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995