Structural Uncertainties in the Subsurface Interpretation of Complex Thrust Belts: An Example from the Southern Subandean Belt in Bolivia

Abstract

The southern Subandean ranges of Bolivia consist of a thin-skinned fold and thrust belt that have shortened the stratigraphic post-Ordovician-Neogene succession (~10 km) along the Silurian to upper Ordovician detachment. Historically oil accumulations in this fold-thrust belt were discovered in the shallower Carboniferous and Cretaceous reservoirs. Today, the principal targets for hydrocarbon exploration are deeper structures involving older Devonian reservoirs where giant gas fields are located. These deep structures display a range of different geometries that in most instances are based on evolving interpretation of very low quality seismic images, relying heavily on structural models. These model driven interpretations are frequently changed during drilling activities making the trap definition and prediction of subsurface structure extremely uncertain. All these factors have contributed to a large number of drilling surprises during hydrocarbon exploration in Bolivia and in this work we attempt to embrace uncertainty, addressing some of these challenges and discussing practices to support the subsurface interpretation of these structures. In order to capture the range of structural uncertainties, we present several structural models that match the same data (surface geology and seismic data) built using different balanced and forward modelled structural scenarios. The challenge of the work is not only to build a model for trap definition and volumetric calculations, but also to capture all the possible drilling outcomes and help managing expectations for the entire spectrum of stakeholder involved in these very expensive projects.

AAPG Datapages/Search and Discovery Article #90326 © 2018 AAPG Latin America & Caribbean Region, Optimizing Exploration and Development in Thrust Belts and Foreland Basins. Santa Cruz de la Sierra, Bolivia, June 6-8, 2018