Abstract: Near-Surface Evidence of Hydrocarbon Movement from Depth

By

Leo Horvitz

Tens of thousands of near-surface sediment samples, taken from both onshore and offshore areas, have been analyzed for the light, saturated hydrocarbons, methane through pentane. Many of the samples were collected over gas and oil fields, but most were gathered in unproven areas. Recognizable hydrocarbon-distribution patterns were observed over known fields, and similar patterns were also found in the unproven areas. A high percentage of the anomalies that developed in the latter areas have subsequently been found to be associated with petroleum deposits.

One of the land surveys includes the Flomaton-Jay- Blackfoot Creek area. It was conducted shortly after the Jay discovery, but before the Blackfoot Creek field was known. Hydrocarbon-distribution patterns 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 yet clear, but the phenomenon has been validated by evidence beyond that provided by empirical data of near-surface surveys. Carbon-isotope data are part of this evidence. Methane, desorbed from a soil sample taken at 124 depth from an anomalous area observed over the Francitas field in Texas, yielded a dC¹³‰ value of -44.0 relative to the PDB standard. Interstitial methane extracted from a 124 sample taken at another location within the same hydrocarbon halo yielded a value of -40.8. Reservoir methane from the Francitas field showed dC¹³‰ values ranging from -41.0 to -43.8, almost identical to 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 anticipated oil and gas accumulations. Such a log was responsible for the discovery of the East Bernard field in Texas.

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