Application of organic geochemistry to sequence stratigraphic analysis: Four corners platform area, New Mexico, U.S.A.

Abstract

The sequence stratigraphic model of sedimentation postulates the existence of a time-condensed section, deposited at very low sedimentation rates, occurring in the uppermost transgressive systems tract and the lowermost highstand systems tract. This condensed interval has of often been equated with zones of increased hydrocarbon source potential, and invoked as a petroleum source rock, often in the absence of supporting geochemical data. In this study, isotopic, molecular, lithostratographic and paleontological data are presented for 51 outcrop samples from a Cenomanian-Turonian section at Red Wash, New Mexico, U.S.A.; interpretations are made within a sequence stratigraphic framework (ramp model). Sedimentation rates are estimated in order to define the location of the codensed intervalm for comparison with the zone of best source potential. Approximately 80 m of section were sampled. The interval of maximum flooding and westernmost progression of the Cretaceous Seaway shoreline, as defined by paleontological data, is stratigraphically immediately below the occurrence of discontinuities in total organic carbon content, hydrogen index and fluoramorphinite concentration in the kerogen. The overall transgressive event is accompanied by increases in relative concentrations of 24- n-propylcholestane, total steranes, and rearranged steranes. These changes are consistent with a simultaneous decrease in the ratio of terrigenous organic matter to marine algal input as westward flooding proceeded. The simultaneous occurrence of discontinuities in kerogen-related parameters and continuous trends in bitumen-related parameters probably implies mobilization of soluble organic matter across the flooding event boundary. Sedimentation rate estimates for six intervals in the Red Wash section show that the best oil source rock potential occurs immediately above the condensed interval and maximum flooding surface, within the highstand systems tract. Previous studies of Cretaceous sections in other portions of the Western Interior Basin have located the best source rock potential below the condensed zone. We conclude that the equivalence between source rock occurence and condensed sections as applied through the sequence stratigraphic model requires modification before the model may be used to predict the location of oil source rocks in a ramp setting. In this respect, it is critical that the data in sequence stratigraphic studies be used to define the model, and not allow the model to dictate the interpretation of the data.