ABSTRACT

The northern margin of the offhore Gulf of Mexico Basin is a mixed gas-oil province. Natural gas pools are commonly found that consist largely of bacterial methane enriched in isotopically-light carbon (¹³<None>C (-55 per mil). But some bacterial gas accumulations are much wetter than might be expected-- they produce commercial amounts of condensate. The wet fraction of these gas accumulations typically consists of crude oil that has dissolved into dry bacterial methane. We illustrate these concepts using field data collected at two wet bacterial gas fields in the northern Gulf of Mexico.

The Popeye Field (EUR = 320 Bcf, plus 3 MMB condensate) is located along the Louisiana continental slope c. 40 km (25 mi) east of the Bullwinkle Field. This field consists of two commercial gas pools in Pleistocene turbidites (G pay Sands). The two pools are separated by a structural saddle. A few thin, sub-commercial oil zones (H sand series) also occur at Popeye. Conventional cores from the G sand that were obtained in the southern gas pool generally do not fluoresce. But a thin fluorescent zone immediately above the seat seal of the G sand contains remnant oil. The natural gas at Popeye consists largely (c.90%) of bacterial methane. The geochemistry of Popeye condensate indicates that it represents dissolved sour oil that probably was similar to the crude oil produced at the Bullwinkle Field.

The Peccary Field (EUR = 105 Bcf) is located on the Louisiana Shelf c. 40 km (25 mi) northeast of Popeye. The pay sands at Peccary also contain wet bacterial gas. During re-development of this field, one well unexpectedly produced waxy sweet oil when it was recompleted in an intermediate objective (the C₆ sand). Indirect evidence suggests that a wet gas accumulation in the shallower C₅ pay sand (which generally yielded colorless condensate) was underlain by a black oil rim - just before a gas well in the C₅ pay sand watered out, it produced a waxy, amber-colored liquid that appears to have been a mixture of gas-condensate and waxy crude similar to the oil encountered in the deeper C₆ sand.

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