On 25th October, Tina Lohr, Staff Structural Geologist, presented on 'An Integrated approach to develop a prospective sub-thrust model in the Sub-Andes, Bolivia' at the geology of Fractured Reservoirs conference hosted by the Petroleum Group at the Geological Society of London. Below is a summary of the presentation.
Working with Echo Energy, Pluspetrol and independent associates, ERCE have taken a multi-disciplinary approach and integrated geological data, fracture and pore-pressure data, and structural modelling in order to understand the exploration history, and to investigate further the prospectivity at depth of an anticlinal structure.
The area of study is located in southern Bolivia, within the Sub-Andean fold and thrust belt, a highly prospective area for gas. The belt is characterised by elongated anticlines that strike NNE-SSW forming several continuous parallel ranges. The main detachment level is in Silurian Kirusillas shale, which is an efficient decollement level at depth, along which the fold and thrust belt propagates from west to east. Important intermediate detachment levels in Devonian shales generate lift-off structures and decoupling of the lower and upper structural levels.
Hydrocarbons are found in the tight sandstone layers of the Huamampampa, Icla and Santa Rosa formations. All are of Early Devonian age, and have low matrix porosity and permeability. Fracture enhancement is required for productivity. The Huamampampa formation is the main reservoir rock in all the deep gas fields in the Sub-Andes in southern Bolivia and northern Argentina.
Seven wells have been drilled into the surface anticline, evident in the area of study, but only a small gas accumulation has been encountered in the Huamampampa-Icla formations. Reservoir rocks were penetrated unexpectedly shallow and are less overpressured than anticipated.
We have constructed several cross-sections tied to surface geology and well data. Through data integration and kinematic modelling, we have developed a model that explains the current well results and proposes a deeper structure beneath the currently drilled anticline. This model suggests a large imbricated sub-thrust structure at depth greater than four kilometres, that involves Upper Silurian to Lower Devonian rocks which are detached via a roof thrust within the overlaying Los Monos shale.
The lack of adequate seismic data is a major limitation in defining the sub-thrust structure at depth. However, the presence of this structure is reasonable and consistent with the structural understanding of the region. There is an active petroleum system at present day, and several producing gas fields nearby where the reservoir rocks are at similar depths.
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