--> ABSTRACT: Biological Marker Geochemistry and Organic Petrology of Chattanooga Shale, by Jannie P. Adams, Michael A. Kruge, and David F. Bensley; #91023 (1989)

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Biological Marker Geochemistry and Organic Petrology of Chattanooga Shale

Jannie P. Adams, Michael A. Kruge, David F. Bensley

The Upper Devonian Chattanooga Shale is an extremely organic-rich unit containing up to 13% organic carbon, thus making it an important potential oil shale and/or petroleum source rock. Samples of the Chattanooga Shale from the periphery of the Nashville dome in central Tennessee and south-central Kentucky have been examined and grouped by maturity and organofacies, using biological marker geochemistry of extractable bitumen and organic petrology.

All samples were at minimum incipient maturity for oil generation, as the standard C29 sterane maturity parameters were at, or close to, equilibrium values, but not overmature, as the Methylphenathrene Index values were all low. Thus, although the samples were largely within the oil window, they still showed considerable variations in maturity, well documented by the ratio of low to high molecular weight triaromatic steroids and the relative concentration of methyldibenzothiophene (meDBT) isomers. For example, the ratio of 4-meDBT/(4-meDBT + 1-meDBT) showed a smooth increase from 0.51 to 0.78 for these samples. In particular, the meDBT ratios appeared to be sensitive maturation parameters for samples within the oil window. Vitrinite reflectance data, ranging from 0.53 to 0. 1% Ro, support the biomarker maturity trends. In regional terms, there is a gradual increase in maturity to the south and east.

As to organofacies, the Chattanooga samples are overall very similar geochemically, however subtle but significant differences were noted. For example, although C29 steranes consistently predominate over the C27 in all samples, one subgroup (facies A) has a lower relative concentration of C27 steranes compared to the other (facies B). Facies A also has lower C26/C28 triaromatic steroid and tricyclic/pentacyclic terpane ratios. Petrographically, the organic matter of facies A is primarily tasmanitid telalginite, whereas facies B is characterized principally by liptodetrinite. These facies distinctions are independent of maturity and must be due to changes in depositional environment and in the type of organic matter. This study sho s the advantages of using both organic petrology and organic geochemistry in characterizing source rocks.

AAPG Search and Discovery Article #91023©1989 AAPG Eastern Section, Sept. 10-13, 1989, Bloomington, Indiana.