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Abstract: Surface Exploration Geochemistry: Numerical Data Processing of Near Surface Hydrocarbon Gases and Application in the Llanos Basin, Venezuela

Hans von Der Dick, Angel Callejon-Gimenez, Dane Bosman

Traditional exploration techniques in near surface gas surveys usually apply hydrocarbon (HC) gas concentrations and HC gas ratios to extract information relevant to exploration. Although gas surveys are sometimes used to define exploration fairways or to evaluate prospects, nearsurface data are often misleading and prone to misinterpretation. This is largely the result of a poor understanding of the origin, distribution and fate of HC gases in near surface regime.

We report new numerical techniques based on unmixing algorithms that allow for the discrimination and determination of individual gas sources in near surface sediments. Results clearly show a variety of HC gas sources that build up complex gas compositions and gas associations in this shallow environment. The bulk of non-seepage related HC gases is discriminated from a (usually) small fraction of shallow HC gases related to seepage. Model calculations are used to systematically search for a defined seepage signal in these gas data sets.

This technique, termed GEL (Geochemical Exploration Lead), is described and exploration examples are provided from the Llanos area, Venezuela. Drilling results on some anomalies show commercial deep oil reservoirs with very low gas/oil - ratio. The prognosis of the well status clearly points to an increased success ratio when GEL data are incorporated into seismic/geologic information.

AAPG Search and Discovery Article #90951©1996 AAPG International Conference and Exhibition, Caracas, Venezuela