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Numerical Identification of Microseeps in Surface Soil Gases of Western Venezuela, and Its Significance for Hydrocarbon Exploration

By
Angel Francisco Callejón
Angel Francisco Callejón
PDVSA Exploración y Producción Laboratorio Geológico Puerto La Cruz, Venezuela
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Hans H. von der Dick
Hans H. von der Dick
ChemTerra Exploration (CTE) Roetgen, Germany
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Published:
January 01, 2002

Abstract

A soil-gas survey was conducted over a number of prospects in the plains (“llanos”) and the Andean foothills regions in the Barinas-Apure Basin of western Venezuela. Initial attempts to numerically distinguish a seep-gas signal from background gases failed.

Basic statistics allow only a tenuous differentiation between background gases and possible seep-gas populations having slightly anomalous hydrocarbon-gas concentrations. Further data processing on this specific population using advanced vector analysis facilitated the discrimination within a variety of soil-gas sources and distinguished suites of samples with weak, seepage-related surface-gas patterns that resemble normal condensate/wet-gas sources at depth.

An important observation from this surface geochemical program was the presence of a condensate/wet-gas trend in the foothills, confirming subsurface geochemical model predictions of the existence of wet reservoir gases in this area.

Plots of the distribution of the various soil-gas sample suites on base maps of identified exploratory prospects show distinct clustering of seepage-related sample suites over areas with structural highs. Further scrutiny of the distribution of these “geochemical exploration leads” (GEL anomalies), in terms of coincidence with oil-productive areas, reveals that the GEL anomalies occur predominantly over now-productive wells. This observation also prompted the reevaluation of an old well recently associated with a surface GEL anomaly but originally abandoned and classified as dry in the 1930s. Reinterpretation of the well-log information now indicates 30 m of net pay in sandy reservoirs. Future drilling should provide more information on the extent and impact of this surface-gas technology on exploration decision making and risk analysis.

Overall, the active seepage in the two work areas is relatively weak, although higher levels are observed in the foothills region because of favorable gas/oil ratios (GORs), normal reservoir pressure, and intense tectonic fracturing. These factors provide more favorable vertical or near-vertical gas migration than occurs in the plains region, where low GORs, reservoir underpressure, and less intense tectonic fracturing diminish vertical migration.

In addition, initial (presurvey) concerns of intense in-situ biological soil-gas formation in a tropical environment, which would mask or diminish possible seep signals at the surface, could be resolved: Deep soil-gas sampling below a shallow zone of intense soil microbial activity, in combination with numerical surface-gas data processing, permits recognition and evaluation of subtle surface-seep trends in the area.

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Contents

AAPG Studies in Geology

Surface Exploration Case Histories: Applications of Geoschemistry, Magnetics, and Remote Sensing

Dietmar Schumacher
Dietmar Schumacher
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Leonard A. LeSchack
Leonard A. LeSchack
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American Association of Petroleum Geologists
Volume
48
ISBN electronic:
9781629810591
Publication date:
January 01, 2002

GeoRef

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