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Data clearly demonstrate that a near-surface geochemical anomaly over the Cognac field, offshore Louisiana, is either the result of upward migration along well-defined paths (faults) or is a false anomaly.
A pre-sale (1974) hydrocarbon survey in the Cognac area was discussed at the 1978 annual AAPG convention. Analysis of 6 ft (2 m) deep sediment samples resulted in the delineation of hydrocarbon anomalies that included the discovery well of Cognac field. A part of this survey included the determination of methane ^dgrC13 values (^dgrC1) on three sediment samples having anomalous concentrations of hydrocarbons. The ^dgrC1 values (-38.1, -39.2, and -37.3 ppt PDB) plus the methane/ethane ratios (C1/C2 = 7 to 15) are excellent evidence for thermal hydrocarbons.
Later, during the drilling of developmental wells, gas pressure buildup was encountered in the casing annulus of several wells. This gas probably enters the casing annulus at casing shoes located about -2,000 and -4,000 ft (-610 and -1,220 m; subsea). The casing annulus gas is nearly pure methane (98.3 to 99.6%), with a C1/C2 ratio of nearly 2,000 and ^dgrC1 values around -68 ppt PDB. Thus this gas is of low temperature, bacterial origin. It is probably related to gas shows found between -2,200 and -3,500 ft (-671 and -1,067 m) in these wells.
Thus if the near-surface gas anomaly is the result of leakage from a deep reservoir, the leakage must have developed along well-defined migration paths (faults), so that it did not become mixed with the shallow bacterial gas. Diffusional migration would have resulted in a mixing of bacterial gas and thermal gas.
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