Tens of thousands of near-surface sediment samples, from both onshore and offshore areas, have been analyzed for the light, saturated hydrocarbons, methane through pentane. Many samples were collected over gas and oil fields, but most were from unproven areas. Recognizable hydrocarbon-distribution patterns were observed over known fields, and similar patterns also were found in the unproven areas. Many anomalies that developed in the latter areas subsequently have been found to be associated with petroleum deposits.
One of the land surveys includes the Flomaton-Jay-Blackfoot Creek area and was conducted shortly after the Jay discovery, but before the Blackfoot Creek field was known. Hydrocarbon distribution patterns developed which reflected Flomaton and Jay production and indicated Blackfoot Creek as prospective. A hydrocarbon survey, conducted offshore Louisiana prior to the March 1974 Gulf of Mexico sale, produced a hydrocarbon anomaly which now contains the discovery well of the Cognac field.
The mechanism by which the lighter hydrocarbons move from a deposit to the surface is not clear, but the phenomenon has been validated by evidence beyond that provided by empirical data or near-surface surveys. Carbon isotope data are part of this evidence. Methane, desorbed from a soil sample taken at 12 feet (4 m) from an anomalous area over the Francitas field in Texas, yielded a δC13 per mil value of -44.0 relative to the PDB standard. Interstitial methane extracted from a 12-ft (4 m) sample, from another location within the same hydrocarbon halo, yielded a value of -40.8. Reservoir methane from the Francitas field showed δC13 per mil values ranging from -41.0 to -43.8, almost identical with those of the near-surface methane.
Additional evidence of upward movement of hydrocarbons from petroleum accumulations is supplied by analyses of well cuttings. Hydrocarbon buildups, observed in cuttings logs of test wells, have preceded discoveries of oil and gas accumulations. Such a log was responsible for the discovery of the East Bernard field in Texas.