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ABSTRACT

Condensed section deposits in carbonates and siliciclastics are generally fine-grained rocks that commonly contain relatively high concentrations of organic matter; therefore, these rocks may have the potential to be petroleum source rocks if buried under conditions favorable for thermogenic hydrocarbon generation. Of the condensed section deposits in the Mesozoic strata of southwest Alabama, only the Upper Jurassic Smackover Formation carbonate mudstones from the condensed section of the lower Zuni A Gulf Coast-4.1 depositional cycle have sufficient organic carbon and were subjected to burial and thermal conditions in which this potential has been realized. These condensed section and transgressive carbonate mudstones contain total organic carbon contents of algal and amorphous kerogen of as much as 2.19% and exhibit thermal alteration indices of 2− to 3+. The laminated carbonate mudstones of the Smackover Formation have apparently served as the hydrocarbon source for the majority of Mesozoic reservoirs throughout southwest Alabama. The Upper Cretaceous Tuscaloosa Group marine claystones from the condensed section of the upper Zuni A Gulf Coast-2.5 depositional cycle are rich (total organic carbon values of as much as 2.91%) in herbaceous and amorphous organic matter but have not been subjected to burial and thermal conditions favorable for thermogenic hydrocarbon generation. These claystones exhibit thermal alteration indices of 1+ to 2. The Jurassic Norphlet shales of the condensed section of the lower Zuni A Gulf Coast-3.1 depositional cycle are low in total organic carbon content (0.1%). These rocks have experienced burial and thermal conditions favorable for thermogenic hydrocarbon generation, but depositional conditions have limited their potential as source rocks because of the paucity of organic carbon preserved in these deposits. No well-developed condensed sections are recognized in the Upper Jurassic Haynesville lower Zuni A Gulf Coast-4.2 depositional cycle or the Upper Cretaceous Tuscaloosa upper Zuni A Gulf Coast-2.3 and upper Zuni A Gulf Coast-2.4 depositional cycles. Although condensed sections within depositional sequences, in general, should have the highest source rock potential, specific environmental, preservational and/or burial and thermal history conditions within a particular basin dictate whether or not this potential is realized. This relationship is shown by the condensed sections of the Mesozoic depositional sequences in southwest Alabama. Therefore, petroleum geologists can use sequence stratigraphy as a tool to help identify stratigraphic intervals that might have potential to contain hydrocarbon source rocks; however, only through geochemical analyses can the actual source rock potential be determined.

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