Multi-Disciplinary Approach in Foothills Exploration; a Bolivian Case Study
J. F. Ballard, J. C. Ringenbach, J. M. Moron, and F. Clement
TOTAL EP, Pau, France
As illustrated by the example of Incahuasi field in the Bolivian Subandean domain, exploration in foothills areas rely on an integrated multi-disciplinary regional approach.
The foothills of southern Bolivia and northwestern Argentina are characterized by a succession of sub-parallel NNE-SSW trending mountain ranges. Each of these ranges corresponds to a tightly folded elongated anticline structure up to 300 km long. Within the south Bolivian foothills, the non-deformed stratigraphic section is approximately 10.000 to 12.000 meters thick, ranging from Silurian to Tertiary (Pliocene). This section consists almost exclusively of clastics with only minor presence of carbonates and evaporites. The primary objective is the tight, fractured Devonian Sandstone of the Huamampampa formation. The equally tight, fractured sandstones of the underlying Middle Icla and Santa Rosa Formations are secondary objectives. Depth of the low porosity (3%) gas reservoirs is around 5000mMD.
The geometry of the fold-and-thrust belt is mainly controlled by lithology and the presence of three potential detachment layers with detachment potential controlled by shaliness, partly disconnect deformations above and below them. The main detachment (Los Monos, middle Devonian) disconnects the deformations observed at surface (Tertiary to Lower Carboniferous series) from what happened at depth near reservoir level (Devonian sandstones Huamampampa).
In this case the reservoir is highly deformed inside a complex detachment fold, where seismic imagery (2D lines or sparse 3D) is poor. Structural complexity of the trap is not the only parameter to consider. Due to high dips, rough topography, weathered zone, the seismic image in Subandean domains, either 2D or 3D, is very often of low quality and doesn't allow to properly imaging the reservoir.
In the absence of correct seismic imaging prior to drilling, the geological sections have to consider and reflect the uncertainties concerning the depth, location and structural complexity of the reservoir. It use surface data as far as can be helpful when upper deformation is disconnected from deep part. All other relevant data, as Magneto-Telluric (MT) that may give additional information but with a low resolution (+ 500m) are used to better constrain the building of a 3D model mainly based on the geological sections. In order to better anticipate the probable variations of reservoir at depth in relation with changing of geometry of the surface anticline, a series of 3D analogue models of the trend have also been realised and analysed by X-ray tomography.
Exploration and delineation require a good cooperation between driller and geologist to steer the well toward the target using dipmeter interpretation but also palaeontology. An effort for biostratigraphy synthesis has been done prior to drilling in order to get some keys markers to help steering the wells.
Altogether, exploring FTBs requires skilled structural geologists with a long experience in the area and the full integration of all existing field data as well as conventional and non conventional geophysical data. In the absence of seismic imaging breakthrough like what happened in the deep offshore sub-salt domains, it will remain a risky business.
AAPG Search and Discovery Article #120140© 2014 AAPG Hedberg Conference 3D Structural Geologic Interpretation: Earth, Mind and Machine, June 23-27, 2013, Reno, Nevada