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San Andres Play in the Northwest Shelf: A New Insight on Its Petroleum Systems From Oil Geochemistry

Abstract

The San Andres Formation is the most prolific reservoir of hydrocarbons in the Permian Basin of west Texas and eastern New Mexico, with a hydrocarbon production that has exceeded 3.9 billion bbl in the Northwest Shelf (Dutton et al., 2004). In this region, the San Andres Fm. represents a series of regressive cyclic deposits of shallow-water carbonates and evaporates that prograded southward across a broad, low-relief, shallow-water shelf. The main reservoir facies are subtidal porous dolostones; these successive porosity zones are offset basinward and occur in increasingly younger strata to the south. The trapping mechanism for the San Andres play in the Northwest Shelf results mainly from porosity pinch-outs defined by an increase in the anhydrite content or in the degree of the dolomitization (Cowan and Harris, 1986). The source for this play has been proposed to be Wolfcampian basinal clastics in the northern Midland Basin (Ramondetta, 1982). This author has also proposed that the biodegradation from anaerobic sulfate-reducing bacteria could explain the association of highly sulfurous and aromatic oil in the anhydrite-rich reservoirs of the Northwest Shelf. In this study, a number of oils and core extracts have been analyzed and the results have shown high sulfur, very low pristine/phytane ratios, and relatively normal alkane and isoprenoid distribution in the whole oil gas chromatographic traces. The integration of these geochemistry results together with data from public databases and a geological model of the depositional environment indicates that a carbonate-rich (Type IIs or Class A) source rock could explain the characters found in the oils. Moreover, the increase in the aromatics could also be explained by a process of evaporative fractionation from oils trapped in stratigraphic traps and updip migration of a gassier and lighter phase. Finally, Basin and Petroleum Systems Modeling has also been applied to analyze the timing of generation and charge, and the generated volumes for the different potential source rock intervals of Permian age. A detailed understanding of the source rock facies that have charged the San Andres reservoirs in the Northwest Shelf and the integration into a BPSM are critical steps in the regional evaluation of the oil signatures and, eventually, in the assessment of the differences in the productivity of the fields.