The gasoline range compositions of petroleum light hydrocarbons (LHC) extracted from fresh cuttings or side wall cores
are determined by kerogen maturation and type. Four stages of compositional evolution are illustrated in Figures 30 and 31, annotated by maximum
attained subsurface temperature in sequences undergoing Cretaceous-Tertiary-Recent burial. These stages were described in
Thompson, (1979), Organic Geochemistry, p.659 .
Figure 30 illustrates LHC occurring at low temperatures, in the left panel at 41C,
prior to the occurrence of significant generation, and in the right panel, at 66C in an early generative stage with high concentrations of naphthenes.
Figure 31 (left panel) represents generation at a stratal temperature of 103C,
an equivalent Ro value of 0.55% in the US Gulf Coast Miocene - Pliocene section, an immature condition. The right panel represents
compositions at a stratal temperature of 193C, an equivalent Ro value of approximately 1.9%. At this level, substantial secondary
cracking of hydrocarbons has occurred.
Table 5 presents relative concentrations by weight percent of the compounds
identified and enumerated in Figures 30 and 31.
RATIOS: INDEXING PROGRESSIVE LHC COMPOSITIONAL CHANGE WITH MATURATION
A set of inter-compound ratios were adopted (Thompson, 1983, Geochimica et Cosmochimica Acta, 47, p 312) to track and investigate
compositional change. Ratios were developed under three headings: paraffinicity, aromaticity and branching, illustrating, respectively, the ratio
of paraffins to naphthenes, of aromatics to paraffins and of branched to unbranched alkanes, both paraffinic and naphthenic.
Designations and definitions are given in the Subsection linked under Ratios in the
introductory Section "CONCEPTS"
Table 6 provides the values of these inter-compound ratios defining the four stages
represented in Figures 30, 31 and Table 5.
Figure 32 illustrates the group of compounds employed in the definition of the
Isoheptane Value, I, at two arbitrary stages of maturation.
Figure 33 illustrates the group of compounds employed in the definition of the
Heptane Value, H, at two stages of maturation.
RATIO VALUES IN SEDIMENTS
Figure 34 illustrates the exponential increase in I with increasing maximum attained
subsurface temperature in Type III kerogens.
Figure 35 illustrates the linear increase in H with increasing maximum attained
subsurface temperature in sediments bearing Type II, petroleum precursor kerogen.
Figure 36 illustrates the exponential increase in F, n-heptane/methylcyclohexane, with
increasing maximum attained subsurface temperature.
Figure 37 shows the relationship between H and I with increasing maturity in a
number of fresh sediment samples without regard to kerogen type. Type II and Type III relationships are distinguished in Figures 38 and 39.
Figure 38 illustrates H vs I in sediments bearing Type III kerogen at increasing levels
of maturity. The relationship is expressed by the equation H = 11.331 * I^(0.57803).
Figure 39 shows H vs I in sediments bearing Type II source rock kerogen at increasing levels
of maturity. The relationship is expressed by the equation H = 19.07 + 24.7log(I).
Figure 40 presents H and I values encountered in the well H.H. Burns No. 1, SW Louisiana, in
which sediments bearing Type III kerogen transition into a source rock interval bearing Type II kerogen, and revert to Type III at greater depths.
LHC CONCENTRATIONS IN SEDIMENTS
Figure 41 illustrates the rate increase in concentration of light
hydrocarbons in sediments during diagenesis, employing n-heptane as an index compound for the entire suite, and a relative measure
of concentration based on carbon concentration and quantity of eluant (helium). Yields in Type II kerogens are approximately 100-fold
greater than in Type III. Destruction of in-situ LHC ensues at a stratal temperature of approximately 160C.