ABSTRACT

Detailed organic geochemical analyses were made of the fine-grained rocks from the 6.9-km-deep McNair No. 1 well, Hinds County, Mississippi. Analyses 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 Late 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 1550 ppm) C₁₅⁺ hydrocarbons are present in Jurassic rocks at temperatures of 200°-225°C. Substantial amounts (80-180 mg/g) of pyrolyzable hydrocarbons remain on the kerogen in these same rocks. Temperature and/or duration of heating appears to have been inadequate for complete generation and thermal destruction of hydrocarbons to have taken place 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- and iso-paraffins decrease with an increase in depth and that the three- 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, which we present here, suggest that some of the accepted concepts regarding generation and maturation of petroleum hydrocarbons may need further refinement.