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Abstract: HC Migration in Thrusted Area Using Basin Modeling Application to the Bolivian Sub Andean Zone

Schneider, F. and I. Moretti - IFP

Integrated basin modeling provides a strategy for optimizing exploration in frontier areas and evaluating new plays within well-explored basins. Ideally, a basin simulator should span the entire process of source-rock burial, hydrocarbon generation, expulsion, migration into a potential trap, and assessment of trap integrity throughout the evolution of a basin.

Today?s IFP state-of-the-art sedimentary basin models are able to handle complex geometry: faults with significant offsets, creeping salt or mud, formation of diapirs. In addition, fluid flow and convective heat transfer do handle the permeability evolution of faults. A fault can be a migration pathway or act as a seal, and it can change its behavior through time. This software (Ceres 2D) has been used to study a petroleum system in the Bolivian foothills where faults are the most important discontinuities affecting hydrocarbon migration. Extensive field analysis has been carried out to describe the characteristics of the faults.

The Sub-Andean Zone of Bolivia corresponds to the eastern frontal part of the Andean belt, thrusted eastward on the Oligocene to Recent foredeep-fill. The thrust sheets are constituted by Paleozoic-Mesozoic sequences and Tertiary foredeep sediments. In the southern part, the main detachment level is located at the bottom of the Silurian shaly units, but an important intermediate décollement, the Devonian Los Monos Fm., is responsible for numerous duplexes. The source rock are the Paleozoic shale (especially the Los Monos Fm.) and the various sandstone beds, from Devonian to Miocene ages, are reservoirs. Oil seeps are numerous and most of them (90%) are located along outcropping thrust faults or extrado faults in the hinges of the anticlines. Hot mineral waters also leak in the same structural positions. The very high present and paleo-temperatures of these mineral waters indicate that the faults are long and fast migration pathways.

The damaged zones associated with the thrust-faults are a few meters to tens of meters thick, and are characterized by small-scale faults, folds and mineral veins (quartz, carbonate or anhydrite/gypsum). At the microscopic scale, the faults consist of cataclastic shear bands which fracture porosity is either open, or sealed by quartz or carbonate. Mineralogy and isotope geochemistry of carbonate cements indicate that fluids inside and outside the faults zones were different.

These observations, as well as the frequent occurrence of active oil-seeps along thrust-faults show that the damaged zones associated to the faults form preferential drains for hydrocarbon migration through the thrust system. On the other hand, there is no indication that fluids may have crossed the fault zones. This anisotropy of the fault permeability leads to compartmentalization of the drainage area, and has huge influence on the potential petroleum charge of each anticline. Using these observations, as well as the well data, to calibrate the parameters, a model of hydrocarbon migration in the Bolivian foothills is presented. The size of the potential reserves in the structures is quantified as a function of the fault behavior.

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