Numerical Identification of Microseeps in Surface Soil Gases of Western Venezuela, and Its Significance for Hydrocarbon Exploration
Angel Francisco Callejón, Hans H. von der Dick, 2002. "Numerical Identification of Microseeps in Surface Soil Gases of Western Venezuela, and Its Significance for Hydrocarbon Exploration", Surface Exploration Case Histories: Applications of Geoschemistry, Magnetics, and Remote Sensing, Dietmar Schumacher, Leonard A. LeSchack
Download citation file:
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.
Figures & Tables
Surface Exploration Case Histories: Applications of Geochemistry, Magnetics, and Remote Sensing provides an overview of successful applications of surface exploration methods. Through a series of independent case histories, this volume presents clearly documented evidence that demonstrates how surface exploration methods can significantly reduce exploration risk and finding costs: geochemical, magnetic, and remote sensing. The 19 chapters in this volume reflect the broad scope of applications for these methods: frontier basin reconnaissance, prospect development, prospect evaluation, and field development and production. The case histories span the globe: 1. North America 2. Africa 3. South America 4. Europe 5. Middle East 6. Australia. This book will interest explorationists and managers who seek to get the most out of each exploration dollar.