Light hydrocarbons in subsurface sediments

The major features and numerous compositional details of the indigenous C₂–C₇ hydrocarbon suites of argillaceous sediments are systematically temperature dependent. The relative concentrations of alicyclic compounds exhibit a consistent maximum at subsurface temperatures close to 170°F (77°C) without regard to the chemical nature of the bulk of the kerogen, whether rich or poor in hydrogen, though this strongly affects the specific yield. A continuous increase in relative alkane content follows at higher temperatures. Indices of paraffinicity may be devised. One such, termed the ‘heptane value’ (essentially the percentage of n-heptane in the b.p. range 80.7–100.9°C), possesses a linear association with temperature, provides an index of catagenesis, and frequently provides a means of appraising paleotemperatures. Regressions of heptane value on temperature are compared in two composite stratigraphic sections dominated by kerogens representing two extremes of composition. The regression coefficients differ by 7%. Yields of light hydrocarbons increase exponentially in these sections by more than three orders of magnitude along sub-parallel, temperature-dependent curves. These similarities infer universally similar generating reactions and compositionally similar suites of light hydrocarbons at given subsurface temperatures, regardless of kerogen type, particularly for sections which underwent burial and heating during the Tertiary period.