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Petroleum Source Rock Potential of Mesozoic Condensed Section Deposits in Southwestern Alabama

MANCINI, ERNEST A., Geological Survey of Alabama and University of Alabama, Tuscaloosa, AL, and BERRY H. TEW and ROBERT M. MINK, Geological Survey of Alabama, Tuscaloosa, AL

Because condensed section deposits in carbonates and siliciclastics are generally fine-grained lithologies often containing relatively high concentrations of organic matter, these sediments have the potential to be petroleum source rocks if buried under conditions favorable for hydrocarbon generation. In the Mesozoic deposits of southwestern Alabama, only the Upper Jurassic Smackover carbonate mudstones of the condensed section of the LZAGC-4.1 cycle have realized their potential as hydrocarbon source rocks. These carbonate mudstones contain organic carbon concentrations of algal and amorphous kerogen of up to 1.7% and have thermal alteration indices of 2- to 3+. The Smackover carbonate mudstone source potential was so substantial that these rocks apparently have served as the source or the Mesozoic reservoirs throughout southwestern Alabama. The Upper Cretaceous Tuscaloosa marine claystones of the condensed section of the UZAGC-2.5 cycle are rich (up to 2.9%) in herbaceous and amorphous organic matter but have not been subjected to burial conditions favorable for hydrocarbon generation. These claystones exhibit thermal alteration indices of 1+ to 2-. The Jurassic Pine Hill/Norphlet black shales of the condensed section of the LZAGC-3.1 cycle and the Upper Jurassic Haynesville carbonate mudstones of the condensed section

of the LZAGC-4.2 cycle are low (0.1%) in organic carbon. These rocks have experienced burial conditions favorable for hydrocarbon generation, but depositional conditions have limited their potential as source rocks because of the paucity of organic carbon preserved in these deposits. Although condensed sections within depositional sequences should have the highest source rock potential, specific environmental, preservational, and/or burial history conditions within a particular basin will dictate whether or not the potential is realized as evidenced by the condensed sections of the Mesozoic depositional sequences in southwestern Alabama. Therefore, petroleum geologists can use sequence stratigraphy to identify potential source rocks; however, only through geochemical analyses can the uality of these potential source rocks be determined.


AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)