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Along-Strike Variation in Structural Styles and Hydrocarbon Occurrences, Subandean Fold-And-Thrust Belt and Inner Foreland, Colombia to Argentina

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Along-Strike Variation in Structural Styles and Hydrocarbon Occurrences, Subandean Fold-And-Thrust Belt and Inner Foreland, Colombia to Argentina The Geological Society of America Memoir 212 2015 Along-strike variation in structural styles and hydrocarbon occurrences, Subandean fold-and-thrust belt and inner foreland, Colombia to Argentina Michael F. McGroder Richard O. Lease* David M. Pearson† ExxonMobil Upstream Research Company, Houston, Texas 77252, USA ABSTRACT The approximately N-S–trending Andean retroarc fold-and-thrust belt is the locus of up to 300 km of Cenozoic shortening at the convergent plate boundary where the Nazca plate subducts beneath South America. Inherited pre-Cenozoic differences in the overriding plate are largely responsible for the highly segmented distribution of hydrocarbon resources in the fold-and-thrust belt. We use an ~7500-km-long, orogen- parallel (“strike”) structural cross section drawn near the eastern terminus of the fold belt between the Colombia-Venezuela border and the south end of the Neuquén Basin, Argentina, to illustrate the control these inherited crustal elements have on structural styles and the distribution of petroleum resources. Three pre-Andean tectonic events are chiefl y responsible for segmentation of sub- basins along the trend. First, the Late Ordovician “Ocloyic” tectonic event, recording terrane accretion from the southwest onto the margin of South America (present-day northern Argentina and Chile), resulted in the formation of a NNW-trending crustal welt oriented obliquely to the modern-day Andes. This paleohigh infl uenced the dis- tribution of multiple petroleum system elements in post-Ordovician time. Second, the mid-Carboniferous “Chañic” event was a less profound event that created mod- est structural relief. Basin segmentation and localized structural collapse during this period set the stage for deposition of important Carboniferous and Permian source rocks in the Madre de Dios and Ucayali Basins in Peru. Third, protracted rifting that lasted throughout the Mesozoic provided the framework for deposition of many of the source rocks in the Subandean belt, but most are not as widely distributed as the Paleozoic sources in Bolivia and Peru. We attribute variations in the style of Andean deformation and distribution of oil versus gas in the Subandes largely to differences in pre-Cenozoic structure along *Current address: U.S. Geological Survey, Anchorage, Alaska 99508, USA. †Current address: Department of Geosciences, Idaho State University, Pocatello, Idaho 83209, USA. McGroder, M.F., Lease, R.O., and Pearson, D.M., 2015, Along-strike variation in structural styles and hydrocarbon occurrences, Subandean fold-and-thrust belt and inner foreland, Colombia to Argentina, in DeCelles, P.G., Ducea, M.N., Carrapa, B., and Kapp, P.A., eds., Geodynamics of a Cordilleran Orogenic System: The Central Andes of Argentina and Northern Chile: Geological Society of America Memoir 212, p. 79–113, doi:10.1130/2015.1212(05). For permission to copy, contact [email protected]. © 2014 The Geological Society of America. All rights reserved. 79 80 McGroder et al. the fold belt. The petroleum occurrences and remaining potential can be understood in the context of three major geographic subdivisions of the Subandes. Between Colombia and central Peru, rich, late postrift Cretaceous source rocks occur beneath Upper Cretaceous–Cenozoic strata that vary signifi cantly in thickness, yielding large accumulations around the Cusiana fi eld in Colombia and within the Oriente Basin in Ecuador, but weak or nonviable petroleum systems elsewhere, where the cover is too thin or too thick. The central Subandes of southern Peru and Bolivia evaded signifi cant Mesozoic rifting, which kept Paleozoic sources shallow enough to delay primary or secondary hydrocarbon generation. After post-Oligocene propagation of the fold belt, however, numerous accumulations were formed around Camisea, Peru, and in the Santa Cruz–Tarija fold belt of central Bolivia. In the southern Subandes of Argentina, Paleozoic crustal thickening caused by terrane accretion and associ- ated arc magmatism created a tectonic highland that precluded deposition and/or destroyed the effectiveness of any Paleozoic source rocks. Here, all accumulations in the foreland and in the fold belt rely on Triassic and younger source rocks deposited in both lacustrine and marine environments within narrow rifts. The trap styles and structurally restricted source rocks in the southern segment have yielded a much smaller discovered conventional resource volume than in the northern and central Subandean segments. In comparing the Subandean system to other global fold belt petroleum systems, it is undoubtedly the rule rather than the exception that the robustness of hydrocar- bon systems varies on a scale of a few hundred kilometers in the strike direction. Our work in the Andes of South America suggests that this is because most continents possess heterogeneous basement, superposed deformation, and subbasin stratigraphy that vary on roughly this length scale. INTRODUCTION In this paper, we build on the deep knowledge base of dip- oriented structural characteristics and extend our focus to the Innumerable descriptions have been published on the third (strike) dimension of the Subandean fold belt to address geometry and kinematic evolution of fold-and-thrust belts three questions: (1) Over what length scales do the structural around the globe, utilizing outcrop observations, industry seis- styles of fold belts change and for what reasons? (2) How do the mic-refl ection data, structural analysis techniques, and various original sedimentary confi guration and mechanical stratigraphy other data types (e.g., Rich, 1934; Bally et al., 1966; Dahlstrom, of a basin infl uence or control subsequent structural development 1970; Boyer and Elliott, 1982; Pfi ffner, 1986; DeCelles et al., of the contractional system? (3) How do these stratigraphic and 2001). To a lesser degree, attempts have been made to relate structural changes in the strike direction conspire to yield bound- those structural styles and attributes to the distribution of hydro- aries between more and less viable petroleum systems? The fi rst carbon resources in petroleum basins (e.g., Cooper, 2007). two questions have been addressed in South America in several Both types of syntheses have been aimed in large part at two- historically important syntheses, including Allmendinger et al. dimensional, cross-sectional evaluations relating, for example, (1983), Mpodozis and Ramos (1989), and Kley et al. (1999). sedimentary wedge taper to wavelength of structural imbrica- This paper leverages those and other earlier works in order to tion, mechanical stratigraphy to décollement preferences, infl u- generate new insights about the factors that control the distribu- ence of rift faults or other preexisting basement architecture on tion and quality of petroleum systems in retroarc fold belts. contraction styles, and the relation of source rock burial history We created a megaregional strike section near the eastern and hydrocarbon charge to structural evolution. Some simple terminus of substantial Neogene deformation, near the bound- observations have been made about petroleum systems in these ary between the modern Andean wedge top and foreland basin settings; for example, petroliferous fold belts commonly occur (DeCelles and Giles, 1996). This location was chosen to traverse at the margins of petroliferous basins (e.g., the Zagros fold belt the traditional habitat for fold belt petroleum traps. Our compila- and the Arabian platform; Western Canada fold belt and basin). tion draws on roughly 100 dip-oriented two-dimensional (2-D) In other cases (e.g., Papua New Guinea), however, the hydro- structural cross sections of the Subandes between latitudes 8°N carbon endowment is greater in the fold belt than in the adjacent and 40°S (Fig. 1). Our analysis relied most heavily on recently basin, partly because the fi eld sizes can be much larger in the published sections that incorporated observations from indus- fold belts than in the forelands. try seismic-refl ection profi les, utilized ties to wells, and applied Along-strike variation in structural styles and hydrocarbon occurrences, Colombia to Argentina 81 80°W 70°W 60°W Panama ∗ Venezuela ∗ ∗ Colombia ∗∗ ∗∗ ∗∗∗∗ Ecuador ∗∗ 0° ∗∗∗∗∗ Figure 8 0° ∗∗∗∗∗ ∗∗ ∗∗ Figure 9 Brazil Peru 10°S 10°S Figure 1. Map of South America show- ing the locations of cross sections dis- cussed in this paper, and the location of Figure 10 geologic maps in Figures 8–12. Gray ∗ and red lines show locations of cross ∗∗ Bolivia sections used in the construction of our ∗∗ along-strike cross section, the full ex- ∗∗∗ ∗∗∗ tent of which is shown in teal. Red lines ∗∗∗∗∗ are locations of cross sections shown in ∗∗∗∗∗∗ Figure 7. Asterisks represent active vol- ∗∗∗∗ canoes. Their absence in Peru and north- 20°S∗ 20° S ern Argentina has been attributed to fl at ∗ ∗∗∗∗ slab subduction. ∗∗∗∗∗∗ ∗∗∗∗∗ ∗∗∗ ∗∗ ∗∗∗∗∗ ∗ ∗∗∗ ∗∗∗ ∗ ∗ ∗ Figure 11 ∗∗∗∗ Chile∗∗ Argentina 30°S N 30°S ∗∗ ∗ ∗ ∗∗ ∗∗ Figure 12 ∗∗∗ 0125 250 500 750 1,000 ∗∗∗∗ ∗∗∗ ∗ ∗ ∗ Kilometers ∗∗ ∗ ∗∗∗ ∗∗∗∗ 40°S ∗∗ 40°S 80°W 70°W 60°W 82 McGroder et al. modern structural balancing approaches in section construction. tive study of the anatomy of the 7500-km-long heterogeneous Other cross sections were not included in our compilation if they mountain front in South America. Our goal is to clarify how the appeared overly schematic or were too close to other chosen sec- structural-stratigraphic architecture of fold belts at the subbasin tions. The resultant strike section presented in this paper extends
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