Kinematic deformation model of the south-central subandean zone, Bolivia

Abstract

The Bolivian Subandean region is characterized as a thin-skinned fold-thrust belt with structures persisting for 50–100 km along strike in a broadly North-South direction. The evolution of the Subandean structures is related to shortening and mass transfer. The stratigraphy comprises ~12 km of Ordovician to Recent sediments, which are dominated by the Paleozoic section including Devonian source and reservoir horizons capped by the Middle Devonian Los Monos regional seal. Detachments occur at top of basement, dipping 2° to the west, within the Silurian and two Devonian levels, including the Los Monos shale.The fold-belt contains the Mega Gas Fields of Bolivia. This study focused on an exploration target in the Igüembe structure, located nearby the Margarita and Incahuasi Gas Fields. In addition to the standard methodology of integrating seismic lines, surface geology and well data, this study also utilized magnetotelluric (MT) analysis to create regional structural sections that were balanced and restored, recording ~40% shortening between 12Ma and the present. The compression was propagated from west to east along a basal detachment along top basement at ~12 km depth, creating four different and superimposed structural units. The Ordovician to Middle Devonian section deformed as antiformal stacks at ~15 km spacing, with the highly ductile Los Monos horizon overlying it and acting as a major detachment and seal. The Upper Devonian to Cretaceous section was buttressed by over 5 km of overlying Neogene sequence; in such a way, that it was forced to propagate to the surface directly above the antiformal imbricates in a sequence of triangle zones above and along the Los Monos detachment. The 5+ km thick Neogene section is preserved as parallel bedded open synclines between the triangle zone culminations. The Middle Devonian source and reservoir may have been buried 6–8 km prior to deformation, which has reduced porosity, but also generated hydrocarbons. It is plausible that during the Siluro-Devonian Cordilleran compressional phase, gentle regional anticlines or inversion structures might have been formed, perhaps with ~15 km spacing. These may have trapped the early-formed hydrocarbons and preserved porosity. These anticlines would be the nuclei for subsequent contractional deformation in the Neogene, with hydrocarbons re-migration and some hydrocarbons remnants trapped in the original structures.