Comparison of Microbial and Sorbed Soil Gas Surface Geochemical Techniques with Seismic Surveys from the Southern Altiplano, Bolivia

Oscar R. Aranibar, James D. Tucker, Daniel C. Hitzman

Yacimientos Petroliferos Fiscales Bolivianos (YPFB) undertook a large seismic evaluation in the southern Altiplano, Bolivia in 1994. As an additional layer of information, sorbed soil gas and Microbial Oil Survey Technique (MOST) geochemical surveys were conducted to evaluate the hydrocarbon microseepage potential. The Wara Sara Prospect had 387 sorbed soil gas samples, collected from one meter depth, and 539 shallow soil microbial samples, collected from 15 to 20 centimeter depth. The sorbed soil gas samples were collected every 500 meters and microbial samples every 250 meters along geochemical traverses spaced 1 km apart. The presence of anomalous hydrocarbon microseepage is indicated by (1) a single hydrocarbon source identified by gas crossplots, (2) the high gas val es with a broad range, (3) the high overall gas averages, (4) the clusters of elevated samples, and (5) the right hand skewed data distributions. The gas ratios, from worldwide empirical observations, indicate the anomalous hydrocarbons to be from an oil or condensate source. Likewise, the clusters of elevated microbial populations identified by MOST surveys confirmed the same anomalous hydrocarbon microseepage trends. Three discrete targets are identified by both surface geochemical techniques. These targets indicate the structure may be more complex than identified by previous seismic surveys. Both surface geochemical exploration tools also suggest the hydrocarbon anomalies extend further to the northeast and southwest from the original sample grid. The preliminary 1994 seismic results indicate the structure to be more complex with the anomalous geochemical highs aligning with discrete structural events.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995