Controls on Hydrocarbon Distribution in the Subandean Foldbelt, Colombia to Argentina
Authors: McGroder M.F. , Lease R.O. , Pearson D.M. 
Published: May 2016

Abstract
Inherited structures are responsible for the segmentation of hydrocarbon resources in the Cenozoic retroarc belt of South America. We use an 8000km-long strike cross-section from northern Colombia to central Argentina to illustrate how these inherited elements influence structural styles and resource distribution. Three events are responsible for segmentation of retroarc sub-basins. Late Ordovician terrane accretion in Argentina and Chile created a crustal welt oriented obliquely to the modern-day Andes. This long-lived Transpampean arch lacks thick Paleozoic sediments and Bolivian-style hydrocarbon occurrences, which are the most significant accumulations in the central Subandes. Important source rocks in the Chaco, Bene, Madre de Dios and Ucayali basins were deposited in middle and late Paleozoic time to the northeast of this arch. The arch shortens by thick-skinned contraction of rift basins and basement. A widespread mid-Carboniferous event rejuvenated pre-existing regional highs and created new contractional uplifts, most conspicuously in Peru. Lastly, protracted Mesozoic rifting is responsible for the second volumetrically important quantity of discovered hydrocarbons in the Subandean belt. In Argentina, the Neuquén (Jurassic), Cuyo (Triassic) and Salta (Cretaceous) rifts trend at moderate to high angles to the Andes. The Neuquén, following a long history of conventional production, is now being evaluated for unconventional resource potential from upper Jurassic and lower Cretaceous shales. The Cuyo basin produces from several reservoirs in inverted fault blocks. Several small fields are also present within and flanking the Salta rift. In central Peru, the Jurassic Pucara Formation has elevated TOCs due to deposition in restricted half-graben basins, but may have matured prior to formation of Subandean traps in the Late Cenozoic. In Colombia, Early Cretaceous normal faulting lead to thickening of section from the Llanos basin westward into the Eastern Cordillera. Fault blocks became covered by well-known, world class Cenomanian-Santonian sources, and later inverted, creating and filling several large closures in the Llanos foothills. In comparing the Subandes to foldbelts elsewhere, it is the rule rather than the exception that petroleum systems robustness varies on a scale of a few hundred kilometers in the strike direction, as most continents possess heterogeneous basement, superposed deformations, and sub-basin stratigraphy that vary over roughly that dimension.

Introduction
Innumerable descriptions have been published on the geometry and kinematic evolution of fold-thrust belts around the globe, utilizing various data types. To a lesser degree, attempts have been made to relate those structural styles and attributes to the distribution of hydrocarbon resources in petroleum basins (e.g. Cooper, 2007). Both types of synthesis have focused mainly on two-dimensional, cross-sectional evaluations relating, for example, sedimentary wedge taper to wavelength of structural imbrication or mechanical stratigraphy to decollement preferences.
In this paper we extend our focus to the third (strike) dimension of the Subandean foldbelt, in order to address a) over what length scales do the structural styles of foldbelts change and for what reasons?, b) how does the original sedimentary configuration and mechanical stratigraphy of a basin influence or control subsequent structural development of the contractional system?, and c) how do these stratigraphic and structural changes in the strike direction conspire to yield boundaries or transitions between more and less robust petroleum systems?.