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GEOREF RECORD

Geochemical relationships of petroleum in Mesozoic reservoirs to carbonate source rocks of Jurassic Smackover Formation, southwestern Alabama

George E. Claypool and Ernest A. Mancini
Geochemical relationships of petroleum in Mesozoic reservoirs to carbonate source rocks of Jurassic Smackover Formation, southwestern Alabama
AAPG Bulletin (July 1989) 73 (7): 904-924

Abstract

Algal carbonate mudstones of the Jurassic Smackover Formation are the main source rocks for oil and condensate in Mesozoic reservoir rocks in southwestern Alabama. This interpretation is based on geochemical analyses of oils, condensates, and organic matter in selected samples of shale (Norphlet Formation, Haynesville Formation, Trinity Group, Tuscaloosa Group) and carbonate (Smackover Formation) rocks. Potential and probable oil source rocks are present in the Tuscaloosa Group and Smackover Formation, respectively. Extractable organic matter from Smackover carbonates has molecular and isotopic similarities to Jurassic oil. Although the Jurassic oils and condensates in southwestern Alabama have genetic similarities, they show significant compositional variations due to differences in thermal maturity and organic facies/lithofacies. Organic facies reflect different depositional conditions for source rocks in the various basins. The Mississippi Interior Salt basin was characterized by more continuous marine to hypersaline conditions, whereas the Manila and Conecuh embayments periodically had lower salinity and greater input of clastic debris and terrestrial organic matter. Petroleum and organic matter in Jurassic rocks of southwestern Alabama show a range of thermal transformations. The gas content of hydrocarbons in reservoirs increases with increasing depth and temperature. In some reservoirs where the temperature is above 266 degrees F (130 degrees C), gas-condensate is enriched in isotopically heavy sulfur, apparently derived from thermochemical reduction of Jurassic evaporite sulfate. This process also results in increased H2S and CO2 in the gas, and depletion of saturated hydrocarbons in the condensate liquids. Thermochemical sulfate reduction probably depends on the mineralogic composition of the reservoir rock as well as temperature, because some deep (18,000 ft or 5.5 km) and hot (320 degrees F or 160 degrees C) Smackover and Norphlet reservoirs contain low-sulfur petroleum.


ISSN: 0149-1423
EISSN: 1558-9153
Coden: AABUD2
Serial Title: AAPG Bulletin
Serial Volume: 73
Serial Issue: 7
Title: Geochemical relationships of petroleum in Mesozoic reservoirs to carbonate source rocks of Jurassic Smackover Formation, southwestern Alabama
Affiliation: U. S. Geol. Surv., Denver, CO, United States
Pages: 904-924
Published: 198907
Text Language: English
Publisher: American Association of Petroleum Geologists, Tulsa, OK, United States
References: 24
Accession Number: 1989-052382
Categories: Economic geology, geology of energy sourcesIsotope geochemistry
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 5 tables, strat. col., sketch maps
N30°00'00" - N35°00'00", W88°30'00" - W85°00'00"
Secondary Affiliation: Geol. Surv. Ala., USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute.
Update Code: 1989
Program Name: USGSOPNon-USGS publications with USGS authors
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