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The AAPG/Datapages Combined Publications Database
Detailed organic geochemical analyses of the fine-grained rocks from the 6.9-km-deep McNair 1 well in Hinds County, Mississippi, include organic carbon, extraction and elution chromatography, gas chromatography and mass spectrometry of C15+ saturated and aromatic hydrocarbon fractions, rock pyrolysis, and kerogen analysis. Rocks range in age from Paleocene to Early Jurassic. The well had an estimated bottom-hole temperature of 225°C, although paleotemperatures were probably higher.
Some data from this well are inconsistent with currently accepted organic geochemical hypotheses concerning the generation and thermal destruction of hydrocarbons. For example, appreciable (up to 1,550 ppm) C15+ hydrocarbons are present in Jurassic rocks at temperatures of 200 to 225° C. Substantial amounts (80 to 180 mg/g) of pyrolyzable hydrocarbons remain on the kerogen in these same rocks. Temperatures and/or duration of heating appear to have been inadequate for complete peak generation and thermal destruction of hydrocarbons to have occurred in these rocks. In addition, unexpected trends (versus depth) are present in the C15+ saturated hydrocarbons as well as in the kerogen composition. Mass spectrometric analyses show that the n-paraffins and iso-para fins decrease with an increase in depth and that the three-ring to five-ring naphthenes increase with increase in depth; both trends occur over the depth range from 1.8 to 5.2 km. Further, elemental analyses of kerogen show that the H/C, O/C, and N/C atomic ratios of kerogen increase over the depth range from 5.3 to 7.0 km.
Significant changes in the organic geochemical characteristics of this well bore correlate with changes in lithology.
The data from this and other wells studied, suggest that some of the accepted concepts regarding generation and maturation of petroleum hydrocarbons may need further refinement.
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