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Chapter 23: An Integrated Approach to De-Risking Exploration and Appraisal in Structurally Complex Fold-and-Thrust Belts: Application to the Incahuasi Field (Bolivia)
The south sub-Andean zone of Bolivia is characterized by east-verging thrusts, detaching within Silurian and Devonian series, and associated to a series of north-south to north-north-east-south-southwest regional anticlines and synclines holding large gas accumulations. This chapter is a case history of the multi-TCF (trillion cubic feet) Incahuasi discovery and how an integrated, multidisciplinary approach allows a progressive improvement in the understanding of the trap geometry, reducing the uncertainty during exploration and field appraisal.
While-drilling structural interpretation and biostratigraphic analyses suggested that the discovery well, prognosed mainly by surface geology and targeting the top of the Huamam-pampa Formation as a bright discontinuous seismic reflector on a 2-D seismic test line, was drilling the overturned limb of the surface anticline instead of penetrating the core of the structure. The top reservoir was found deeper by a side track well.
The appraisal wells all contributed to progressively constrain the complex geometry of the trap while leaving several possible geometric interpretations of the reservoir structure. The Huamampampa Formation is repeated and overturned by a series of thrusts that increase the internal complexity of the field. These results also confirmed that the shales of the Icla Formation behave as an effective detachment below the reservoir, contributing to the structural complexity of the field.
A calibrated/constrained inversion of 3-D magnetotelluric acquisition performed over the structure shows the near-top Huamampampa Formation resistivity anomaly plunging to the north, which helps to identify the possible structural spills. In map view, the variation of orientation provides information on local bends of the target and hence is an important element to be considered for optimizing future appraisal wells.
The Incahuasi discovery provided the opportunity to test the validity and efficiency of many geological and geophysical tools while noting their limitations in a context where only a few hundred meters of horizontal error can lead to missing the reservoir and trap.
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