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The Sub-Andean Fold and Thrust Belt in Southern Bolivia: Integrated Methodology and Regional Structural Implications

Abstract

We present the impact on prospect definition, incorporating outcrop data and iterative structural balancing on thrust traps of the southern sub-Andean mountain belt of Bolivia. In this region, the fold and thrust belt has two main detachments: the basal detachment is interpreted to be shales of the upper Silurian, Kirusillas Formation; and the upper detachment is interpreted within the Devonian, Los Monos Formation shales. The lower Devonian, Huamampampa Formation forms the fractured sandstone reservoir, that lies between these two detachments. There are three classes of thrusts in the region: - blind thrusts that ramp up from the basal detachment and flatten into the upper detachment; - thrusts that ramp from the basal detachment and cut to surface, with large displacement; - thrusts that ramp from the upper detachment and cut to surface, with small displacement. Distinguishing between the thrust classes has direct business value as the blind thrusts form the anticlinal structural traps and the large displacement, to surface thrusts modify trap geometry and sealing potential. Due to the mountainous terrain and structural complexity in southern Bolivia, seismic data often poorly image the thrust fault-bend-folds, making thrust and trap geometry interpretation uncertain on seismic sections alone, whilst the synclines and foreland are well imaged. Geological field mapping provides two critical datasets: a geological outcrop map; and structural orientation data of the outcropping formations and types of contacts. The outcrop map and orientation data constrain the range of possible seismic interpretation and are used to define the structural geometry to depth within limits, using section construction techniques. The construction workflow generates excellent near-surface models, which are increasingly unconstrained with depth. To increase the model constraint at depth, multiple strike and dip sections are constructed and balanced. Outcrop data only identify thrusts cutting to surface, not the blind thrusts that form the fault-bend-fold traps. The process generates a structurally-constrained geometry above the upper detachment; and defines the space available between the upper and basal detachments to fill with thrust sheets associated with the blind thrusts. Iterative structural balancing, in combination with seismic interpretation, is carried out to define the blind thrust sheet geometries, which a direct impact on risk and hydrocarbon volume estimation, i.e. being able to predict one, two or three fault-bend-folds. The subsurface interpretation must honour the geological map and outcrop orientation data, well dip data, be balanceable, and match the seismic image where appropriate. The repeatable workflow has identified robust traps that are being matured into drill-ready prospects, directly leveraging the combination of outcrop data and structural balancing.