Wescott and Hood (1991) and Thompson (1991) made oral presentations of similar studies that were later published (Wescott and Hood, 1994; Thompson, 1994). In 1991 we were in agreement concerning the existence of discrete Upper and Lower Cretaceous oil families in the East Texas basin, and the migration paths followed by the former that charged the East Texas field. We differ, however, in our interpretations of the nature of Smackover-reservoired petroleums, particularly in the matter of data relating to their sulfur and nitrogen contents.

In Figure 9 of Wescott and Hood (1994), the authors employed a proposed scheme of petroleum classification (Thompson, 1991, 1994) that is based on the ratio of total organic sulfur to total organic nitrogen (S:N). Their application preceded the publication of the classification and the attendant rationale.

Petroleum sulfur and nitrogen data show that genetic groups of oils, or families, are characterized by a nearly constant S:N ratio (Thompson, 1991, 1994), regardless of maturation, which removes both sulfur and nitrogen, or biodegradation. The data also show that specific ranges of S:N ratio values distinguish lacustrine-, clastic-, carbonate-, and hypersaline-carbonate-sourced oils. In those instances where increasing maturity is accompanied by thermochemical sulfate reduction (TSR), sulfur and nitrogen are lost from oil in the usual fashion, but original organic sulfur is replaced by larger quantities of sulfate-derived sulfur (Orr, 1974), resulting in increasing S:N ratios in the oils and gas condensates of highest maturities. TSR generates hydrogen sulfide and carbon dioxide in ac ompanying natural gases. I suggest (Thompson, 1994) that TSR has operated in the instance of numerous Smackover petroleums in the East Texas basin.

Wescott and Hood (1994) suggest (caption, their figure 9) that Jurassic Smackover Formation-sourced oils in the East Texas basin represent two families, those having sulfur:nitrogen (S:N) ratios of 20, and those having S:N ratios greater than 100. The continuity between groups observed by them was attributed to mixing. No substantiating evidence was presented. In both my 1991 and 1994 studies I examined Smackover sulfur and nitrogen data that are comparable, or identical, to those later presented by Wescott and Hood, and developed the diagram reproduced here as Figure 1. However, I interpret the curvilinear trend evident in Figure 1 as representing maturation accompanied by progressive thermochemical sulfate reduction. The nearly linearly correlated data representing petroleums with g eater than 0.02% nitrogen reflect only the operation of maturation. At nitrogen concentrations below 0.02%, varying and competing degrees of maturation and of TSR are suggested to account for constant, or increasing, sulfur contents, and the scatter of data. The original S:N ratio, close to 20, suggests (Thompson, 1994) derivation of the oil from carbonate source rocks deposited in a hypersaline environment.

An interpretation of S:N data representing high-API-gravity Smackover petroleums in terms of TSR is corroborated by the extensive occurrence, and sometimes extremely high levels, of hydrogen sulfide in East Texas basin Smackover gases. Numerous authors (e.g., Claypool and Mancini, 1989; Wade et al., 1989; Sassen et al., 1990; Chung et al., 1991) concur in the conclusion that TSR has operated in the cases of certain Smackover-reservoired oils.

Carbon isotopic data (Thompson, 1991) fail to suggest the presence of more than one genetic group (family) of Jurassic oils in the East Texas

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basin. The modal ^dgr¹³C value (vs. PDB) of the saturated hydrocarbon fraction of East Texas basin Smackover petroleums previously published (Thompson, 1991) is -25.0^pmil. The range of Smackover values in the Gulf Coast region is unusually wide for a single family (Chung et al., 1991) and extends, minimally, from -26.2 to -21.1^pmil [the latter in the Copeland gas-condensate field, Alabama; data of Claypool and Mancini (1989)]. Claypool and Mancini (1989) also documented the occurrence of isotopically heavy sulfur in the Copeland and similar condensates, conclusively demonstrating that such fluids have experienced TSR. The process is also responsible for their unusually heavy carbon isotopic values (Chung et al., 1991). Wide ranges of carbon isotope ratios might likewise o cur in East Texas basin Smackover fluids because of the occurrence of TSR, but such variability could not be construed in terms of genetic (source rock) differences.

Fig. 1. Relationships of concentrations of sulfur and nitrogen, and of API gravity, in petroleums of the Smackover Formation in the East Texas basin and southern Arkansas. Progressive maturation of a single family of petroleums is revealed, accompanied, beyond approximately 40° API gravity, by increasing levels of thermochemical sulfate reduction and sulfur incorporation. An initial S:N ratio equal to, or slightly greater than, 20 is postulated to indicate petroleum derivation from a carbonate source rock deposited in a hypersaline environment. Reprinted from Organic Geochemistry, v. 21, K. F. M. Thompson, "Classification of Petroleum on the Basis of the Ratio of Sulfur to Nitrogen," p. 877-890, copyright 1994, with kind permission from Elsevier Science td., Kindlington, OX5 1GB, United Kingdom.