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Abstract: Integration of Surface Geology and Geochemistry with Geophysical Surveys for Hydrocarbon Exploration in Southern Bolivia.

Françolin, J. B. L. - PetrobrasICenpes; Soares Filho, J. R. S. - Braspetro

Petrobras Bolivia and partners are exploring blocks in the Southern Bolivian Sub-Andean Zone using integrated studies of Satellite Images, Surface Geology, Geophysical Surveys and Surface Geochemistry.

The dominant structural style in the zone is N-S trending folds and related thrust faults created by thin skin tectonics. Two detachment levels, corresponding to Silurian and Devonian marine shales, control faulting in the area. Anticlines, the main target for hydrocarbon exploration, are generally expressed on the surface as parallel N-S mountain ranges. Offset of the shallow from the deep structural axes is frequently observed, related to the tectonic transport and to the geometry of faults that merge with fish-tail geometry in the Devonian Los Monos Fm. In some cases, deep anticlines are buried under shallow synclines, with the Los Monos Fm. acting as a disharmonic layer that disconnects the upper structure from the one below. The Devonian Huamampampa Fm. sandstone is the main gas reservoir in the basin, occurring at depths from -2.000 to -3.000 meters.

Satellite Image Interpretation and Surface Geologic Mapping were the initial studies carried out in the area, where outcropping rocks range in age from Devonian to Recent. Field work defined the structural style of the area and was important to constrain the interpretation of geophysical data. Cross-sections based on surface geology may be partially extrapolated to depth using kink band methods of fold construction, but the presence of the Los Monos disharmonic layer require further control of cross-sections using seismic and well data (Fig. 1).

Surface Geochemical Prospection was based on soil sampling along seismic lines and was collected in holes 1 to 2 meters deep. The gas contained in these samples was extracted and analyzed by high-resolution chromatography to find microseeps. Results were statistically treated to define background and standard deviation values. Third order anomalies were defined as values higher than the background plus three times the standard deviation. The third order anomalies observed coincided with anticline axes (Fig. 2).

A 2-D Seismic Survey showed good quality over the synclines and fair over the anticlines. The interpretation of this dataset, constrained by geologic cross-section balancing, permitted structural mapping at reservoir level and prospect definition. Previously unknown prospects and leads were also identified, all of them coincident with the surface geochemical anomalies.

The integration of these exploration tools was fundamental to assess the hydrocarbon potential of the area. With the help of these exploratory methods, a well site was chosen and an exploratory well is now been drilled.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil