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Petroleum tectonic comparison of fold and thrust belts: the Zagros of Iraq and Iran, the Pyrenees of Spain, the Sevier of Western USA and the Beni Sub-Andean of Bolivia

JEROME KENDALL1*, JAUME VERGÉS2, RENAS KOSHNAW3,4 & MELANIE LOUTERBACH5 1Department of Earth and Planetary Sciences, University of New Mexico-Albuquerque, USA 2Group of Dynamics of the Lithosphere, Institute of Earth Sciences Jaume Almera ICTJA CSIC, Barcelona, Spain 3Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, TX 78712, USA 4Institute of Geological Sciences, University of Bern, Baltzerstrasse 3, CH-3012, Bern, Switzerland 5Repsol Exploration S.A., Madrid, Spain J.K., 0000-0001-6382-4435; J.V., 0000-0002-4467-5291; M.L., 0000-0001-8987-2208 *Correspondence: [email protected]

Abstract: The genetic analysis of fold and thrust belts is facilitated by tracking the evolution of their organic endowment (petroleum tectonics). Petroleum tectonic analysis of convergent orogenic systems provides an audit of the processes that control the deformation and kinematics of orogenic belts. The distribution and defor- mation paths of the organic endowment intervals are key factors in determining the petroleum system evolution of fold and thrust belts. This comparison of orogenic systems illustrates the importance of flexural v. dynamic processes, orogenic wedge taper, mechanical stratigraphy and inherited architecture on the creation, preserva- tion and destruction of petroleum accumulations. The Zagros, Pyrenees, Sevier and Beni Sub-Andean conver- gent systems share key characteristics of fold and thrust belts, with major differences in scale, degree of incorporation of organic endowment in evolution of the fold and thrust belt and its foreland, and preservation of fold and thrust belt wedge-top deposits. The Zagros is an orogen dominated by flexural processes that is a perfect storm for hydrocarbon generation and preservation. Its multiple stacked sources ensure continuous hydrocarbon generation while stacked detach- ments foster a low taper and thick wedge-top basins. The Pyrenees is also a flexurally dominated orogen, but the early consumption of its source rocks led to minimal survival of hydrocarbon accumulations during exhumation in a long lasting, high-taper orogenic wedge. The Sevier was initially a flexural orogen that was later dominated by dynamic uplift of the fold and thrust belt and distal foreland subsidence with foreland deformation. The con- sumption of its pre-orogenic sources during the early low-taper phase indicates a probable robust petroleum system at that time. However, the late high-taper phase exhumed and destroyed much of the early petroleum system. The addition of syntectonic foreland sources to be matured by both local and dynamic subsidence cre- ated an additional later set of petroleum systems. Post-orogenic events have left only remnants of world-class petroleum systems. The Beni segment of the Sub-Andean Orogen is a flexural system with probable dynamic overprints. Its most robust petroleum system probably occurred during its early low-taper flexural phase, with dynamic subsidence enhancement. Its late high-taper phase with possible dynamic uplift shuts down and stresses the petroleum systems. Comparison of these orogenic systems illustrates the importance of flexural v. dynamic processes, orogenic wedge taper kinematics, mechanical stratigraphy, distribution of source rocks relative to shortening and inherited architecture on the creation, preservation and destruction of petroleum accumulations in fold and thrust belts.

Resource studies show that 14% of world-wide dis- (Cooper 2007). Oil and gas exploration in fold and covered hydrocarbon reserves are within the fold thrust belts is risky due to the complexity in tectonic and thrust belts along convergent plate boundaries style, structural architecture and petroleum system

From:HAMMERSTEIN, J. A., DI CUIA, R., COTTAM, M. A., ZAMORA,G.&BUTLER, R. W. H. (eds) Fold and Thrust Belts: Structural Style, Evolution and Exploration. Geological Society, London, Special Publications, 490, https://doi.org/10.1144/SP490-2018-102 © 2019 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0/). Published by The Geological Society of London. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://sp.lyellcollection.org/ by guest on March 31, 2020

J. KENDALL ET AL. survival potential during the growth of fold and were formed on the lower plate while the others thrust belts. Cooper (2007) presented a statistical evolved in the upper plate (Fig. 1). compilation of 55 examples of fold and thrust belts, In this analysis many of the details of these listing a large number of parameters related to both complex orogenic systems are oversimplified for deformation style and petroleum systems world- the purpose of comparison. The cross-sections ana- wide. The results show that fold and thrust belts are lysed were chosen because they are sequentially rich in traps and the most prolific are rich in organic restored with sufficient published petroleum system matter. His analysis indicated that the deformational modelling along their lengths to allow high-level style has less significance for the hydrocarbon integrated analysis. They do not capture all the endowment of fold and thrust belts than other ele- major elements of the entire fold and thrust belts they ments of the petroleum system. The genetic analysis represent but are sufficient to illustrate the potential in our study of four fold and thrust belts indicates that usefulness of the applied analysis. The focus is an the inherited architecture and details of timing are integrated high-level petroleum and tectonic genetic critical in fold and thrust belt petroleum systems. analysis of orogens. The tectonic analysis informs The objective of this study is to introduce petro- the understanding of petroleum potential while the leum tectonic analysis (PTA), as described below, petroleum analysis informs the tectonics understand- and apply it to document and compare the roles of ing. Traditional hydrocarbon analysis of fold and multiple orogenic processes on the petroleum sys- thrust belts like that of Cooper (2007) take a more tems in four fold and thrust belts: Zagros, South static approach based on present-day configurations Pyrenees, Sevier and Beni Sub-Andean. The thrust and status. The PTA approach is more dynamic, belts vary in scale, age, organic endowment and focusing on the full genetic evolution of the petro- degree of petroleum exploration activity. They leum systems. This analysis provides a method for occur in different tectonic settings: the Zagros pro- auditing the role of various competing orogenic tracted subduction-related continental collision processes in fold and thrust belts while providing a between Eurasia and Arabia; the Pyrenean intracon- context for examining their petroleum potential. tinental collision between Eurasia and Iberia; the The petroleum tectonic synthesis from this study Sevier fold and thrust belt related to the long-lived highlights the impact on the petroleum systems in subduction of the Pacific plate beneath North Amer- convergent orogens of: (1) presence and distribution ica; and the Beni Sub-Andean fold and thrust belt of source rocks; (2) shortening relative to organic related to active subduction of the Pacific plate distribution; (3) pre-orogenic architecture and the beneath South America. The Zagros and Pyrenees mechanical stratigraphy that impacts orogenic wedge

Sevier South Pyrenees

Fig. 7 C-C’ Fig. 6 B’-B

Fig. 5 A’-A Zagros

Beni Sub-Andean Fig. 8 D-D’

Fold and thrust belts Cross-sections

Fig. 1. Topographic map (Amante & Eakins 2009) showing location of fold and thrust belts. The boxes show locations of Figures 5–8. The black lines represent the transects addressed and the red lines represent the portions of the fold and thrust belts to which the transects apply. Downloaded from http://sp.lyellcollection.org/ by guest on March 31, 2020

PETROLEUM TECTONIC OF FOLD-THRUST BELTS kinematics; and (4) the relative dominance of oro- been involved in the deformation? The same analysis genic processes of flexural basin advancement v. is completed for the syntectonic section. A subsi- dynamic uplift/subsidence as the pre-orogenic sec- dence pattern analysis of the syntectonic section pro- tion is incorporated into the orogenic wedge. vides a sense of the dominant tectonic processes. Does the depositional centre progressively shift tracking the deformation front, suggesting strong Petroleum systems in fold and thrust belts flexural processes; or is it broad and relatively sta- tionary, suggesting a dominance of dynamic pro- A petroleum system, as described by Magoon & cesses? Do the subsidence patterns and dominant fi Dow (1994),isde ned as all the essential elements processes vary through time? and processes needed to create an oil and gas accu- The next step is a structural analysis concentrat- mulation. The elements are source, reservoir, seal ing on the geometric evolution of the area. Sequen- and overburden. The processes are trap formation tial restoration is required. Existing sequential and the generation-migration-accumulation of petro- cross-sections can be used but ideally the following leum. In successful petroleum systems all these ele- steps are involved. The present-day geometry is ments and processes must favourably align in space defined, focusing on levels of erosion and preserved and time to enable both formation and preservation growth sections. Structural analysis begins with the of hydrocarbon accumulations. pre-orogenic restoration, which helps define the dis- The four fold and thrust belts analysed have tribution of pre-orogenic source rocks and constrain indications of all the petroleum-system elements their thermal states. Using the orogenic processes (Table 1). In fold and thrust belts where all elements indicated by the subsidence analysis as a guide, are present, their progressive deformational histories sequential reconstructions of the fold and thrust dictate the generation and destruction of petroleum belt are generated, focusing on times of possible pro- systems (Roure & Sassi 1995). The occurrence of cess and/or taper change. Integration of organic geo- petroleum accumulations is dependent on a narrow chemical, thermochronological and geochronologic range of deformation pathways (Magoon & Dow data with the stratigraphic and structural analysis is 1994). Understanding the limited windows of time used to develop more robust sequential restorations. when all elements and processes align for hydrocar- This analysis enables identification of potential areas bon accumulations to form in fold and thrust belts, and critical time frames for generation and preserva- and the challenges of preservation of those accumu- tion or destruction of hydrocarbon accumulations. lations, is the essence of PTA. The key step in PTA is auditing the evidence of hydrocarbons relative to the critical time frames PTA defined above in order to infer the potential for pres- ervation or exhumation of accumulations and/or PTA is the integrated analysis of petroleum-system generative areas. This includes analysis of the defor- evolution and tectonic processes. It tracks the trans- mation paths of both accumulations and failed fea- formation of the organic matter relative to hydrocar- tures. Additional sample collection and analysis is bon migration and trapping geometries. PTA often required to define the deformation paths of suc- assesses the potential for and status of petroleum cessful and unsuccessful traps. PTA is presented accumulations in a tectonic context. As part of the here as a linear process; however, in practice, PTA analysis, indications of hydrocarbons are used to is an iterative process as data and understanding constrain tectonic models. The process of PTA evolve. helps identify key unknown variables and the poten- Integrated petroleum-system thermal modelling tial petroleum system impacts of the uncertainties. is an integral part of auditing fold and thrust belts The kinematic and petroleum system appraisal that is beyond the scope of this comparison paper. method (Roure & Sassi 1995) is built on by adding Potential generative areas discussed in this paper a tectonic context. PTA can be applied at a local are identified from referenced publications, or trap or orogenic scale. Fold and thrust belt petroleum where source intervals are inferred to be buried systems are very sensitive to the temporal and spatial ≳2 km and receiving significant additional burial. alignment of processes, making them excellent can- Where overburden increases to >5 km, a risk of didates for PTA. In this paper PTA is being applied being past peak generative potential is recognized. at an orogenic scale to compare and contrast the Detailed sampling, analysis and modelling are diverse fold and thrust belts analysed. required to confirm the potential generative areas. PTA of fold and thrust belts begins with a strati- graphic analysis of the pre-orogenic section. What is Orogenic processes and petroleum systems its shape and mechanical layering? What is the distri- bution of potential source intervals within the stack? DeCelles & Giles (1996) define foreland basin sys- When and how much of the pre-orogenic section has tems as having accommodation driven by flexural Downloaded from http://sp.lyellcollection.org/ by guest on March 31, 2020

J. KENDALL ET AL.

Table 1. The pre-orogenic architecture of the Zagros, southern Pyrenees, Sevier and Beni Sub-Andean fold and thrust belts

Zagros Southern Pyrenees Sevier Beni Sub-Andean

Petroleum system 11 main intervals, Inferred Jurassic Pre-orogenic sources Pre-orogenic element: Silurian, Mid and type II, TOC in Devonian, Devonian organic source Late Jurassic, c. 3% above the Carboniferous and type II and Early, Mid and basal detachment Permian type II Carboniferous/ Late Cretaceous, (Quesada et al. and I, TOC c. 1– Permian type II, Eoc/Oligocene, 1993; Biteau 3% primarily in TOC c. 3.5% in mostly type II et al. 2006) thick western upper third half of with TOC 1.5– Isolated Tertiary segment and the pre-orogenic 6.5% (Bordenave type II (Cámara syntectonic sedimentary 2014) & Klimowitz sources in wedge (Baby 1985) Cretaceous type II, et al. 1995; TOC c. 1% & III Louterbach et al. (coals) and 2018) Paleocene type I, TOC >5% foreland (Anna et al. 2007) Petroleum-system 4 Paleozoic, 5 Inferred potential 5 Paleozoic, 3 2 Paleozoic, 2 elements: Mesozoic and 1 Jurassic and Mesozoic and 1 Mesozoic and 1 reservoir and Cenozoic Cretaceous; 9 Cenozoic Cenozoic seal reservoirs, and 5 reservoirs and 6 reservoirs and 3 reservoirs, and 3 Paleozoic, 3 seals (Biteau Paleozoic, 3 Paleozoic, 2 Mesozoic and 1 et al. 2006) Mesozoic and 2 Mesozoic, and 3 Cenozoic seals Cenozoic seals, Cenozoic seals (Bordenave 2014) (Anna et al. 2007) (Baby et al. 1997; Louterbach et al. 2018) Pre-orogenic Thick and broad Thick and narrow Thick and broad Thin and narrow sedimentary wedge, inherited wedge, inherited changing at a wedge, inherited wedge shape, initial β = c. 0.2° initial β = c. 6.0° hinge to thinner initial β = c. 1.4° inherited and broad, initial β inherited initial β = c. 3.7° Mechanical 6 major pre-orogenic Basal pre-orogenic Pre-orogenic Major Paleozoic stratigraphy detachments and detachment under detachments detachment in multiple beams a thick beam and limited to thick upper third of the (Bordenave 2014; foreland segment, foreland pre-orogenic English et al. syntectonic syntectonic section (Baby 2015) detachments detachments et al. 1997; (Meigs & (Constenius et al. Louterbach et al. Burbank 1997) 2003) 2018) Tectonic Dominantly flexural Dominantly flexural Early flexural, late Inferred dominantly subsidence (Pirouz et al. (Vergés et al. dynamic (Painter flexural with processes 2017) 1998) & Carrapa 2013; possible dynamic Yonkee & Weil enhancements 2015) (Louterbach et al. 2018) Shortening c. 0.5 × source c. 2.5 × source c. 0.6 × source c. 0.8 × source v. source length length length length length load and far-field subsidence due to interactions organic matter to hydrocarbons while creating ideal between the subducted slab and the mantle (Fig. 2). geometries for accumulations. As the flexural pro- Flexural subsidence progresses systematically with cess progresses, it eventually destroys the previously the advancing orogenic load. Flexural forelands accumulated hydrocarbons. In contrast, dynamic are excellent settings for hydrocarbons. The system- topography, defined by Hager & Richards (1989) atic progression of subsidence and uplift converts as dynamic subsidence and uplift that creates longer Downloaded from http://sp.lyellcollection.org/ by guest on March 31, 2020

PETROLEUM TECTONIC OF FOLD-THRUST BELTS

Fig. 2. Flexural foreland systems modified from DeCelles & Giles (1996). The figure illustrates the sections of a progressing flexural system. The dark green polygons represent potential hydrocarbon generative areas in a wedge-top basin and foot wall. The green arrows represent direct migration routes. Wedge-top generative areas to adjacent structural traps are more robust than foot wall generative areas that do not directly link to traps. Active wedge top petroleum systems have the advantage of having generation, migration and trap in the same structural compartment. wavelength (100s–1000s km) effects, is less system- width and thickness (Chapple 1978; Davis et al. atic as it is controlled by deep earth processes. When 1983). Erosion of the top of the deforming wedge dynamic uplift and subsidence are superimposed on (α) and the mechanical architecture of the section the flexural system, they can enhance or perturb the being incorporated into the deformation are impor- petroleum systems of the orogen, depending on the tant influences on the deformation front (DeCelles phase and magnitude of the dynamic processes & Mitra 1995). Fold and thrust belts tend to advance (Painter & Carrapa 2013). intermittently, trying to reach an equilibrium angle (DeCelles & Mitra 1995; Van der Pluijm & Marshak 2004; Ford 2004). The width of the wedge-top zone at Critical taper and petroleum systems of fold the front of the deforming orogenic wedge influences and thrust belts the robustness and preservation of petroleum systems within fold and thrust belts. Figure 3 shows that wide Petroleum systems in fold and thrust belts are most wedge-top zones with limited erosion are out of sync frequently successful at the extreme leading edge with deposition in the foreland portion of the system. of orogenic wedges. In flexurally controlled basins, Increasing taper tends to erode the wedge-top zone the wedge-top zone is the overlap between the oro- providing sediments to the foreland. At the super- genic wedge and the foreland basin (Fig. 2). The critical point, the thrust front tends to rapidly advance kinematics of the wedge-top zone is critical to the in order to lower the taper (Davis et al. 1983). The petroleum tectonics of fold and thrust belts. An presence of good detachment intervals enhances the important aspect is whether the wedge-top portion low-angle step-out, or jump forward, of deformation of the fold and thrust belt is dominated by erosion (DeCelles & Mitra 1995). The jump forward to lower or preservation as the deformation progresses. Ero- taper with more limited erosion and wider wedge-top sion tends to decapitate the traps and shut down gen- zone minimizes exhumation and extends the dura- eration in wedge-top basins. tion and effectiveness of the associated petroleum Critical taper (Fig. 3) is the concept that the systems in the wedge-top portion of the fold and fronts of orogenic wedges taper toward their unde- thrust belt (Bordenave 2014). In the analyses below, formed forelands and advance when the sum (f) local palaeotopographic slope is poorly constrained of their basal (β) and upper (α) slopes reaches a crit- (α). Well documented dramatic jumps forward of ical value (fc)(Chapple 1978; Davis et al. 1983; deformation with preserved growth section, as in DeCelles & Mitra 1995)(Fig. 3). Critical taper is the southern Pyrenees (Meigs & Burbank 1997; influenced by climate, erosion, mechanical stratigra- Vergés et al. 1998), and with both preserved growth phy and the quality and distribution of detachment section and thermochronological data in the Sevier intervals (e.g. Chapple 1978; Davis et al. 1983; (Constenius et al. 2003; DeCelles & Coogan 2006; DeCelles & Mitra 1995; Van der Pluijm & Marshak Peyton et al. 2011; Yonkee & Weil 2015) and Zagros 2004; Ford 2004). Critical taper is commonly (Homke et al. 2004; Emami et al. 2010; Koshnaw applied to entire orogens but in this analysis it is et al. 2017), have been used to infer local lowering applied at a smaller scale to the dynamics at the of taper. The highly successful petroleum system of deformation front with respect to the width of the the middle Zagros fold and thrust belt (Bordenave wedge-top zone. 2014) analysed below is interpreted to be enhanced According to critical taper concept, the angle of by a taper-lowering step-out of deformation on an the deforming wedge controls the wedge-top zone excellent detachment. Downloaded from http://sp.lyellcollection.org/ by guest on March 31, 2020

J. KENDALL ET AL.

σ α σ β φ = α + β σ c 1

α

φ φ > c

α

φ c

α 3

φ φ < c

α 1

φ c

Fig. 3. Critical taper, wedge tectonics and petroleum systems. Fold and thrust belts advance forward as a deforming wedge. The lower angle of the wedge (β)isinfluenced by the quality and distribution of detachment intervals. The top of the wedge (α1) is the topographic slope controlled primarily by erosion. The wedge is attempting to get to a balanced state (fc)(a). When an imbalance arises possibly due to a drying of climate affecting (α1) or advancement to the edge of a detachment interval (β) the wedge will internally thicken, through internal thrusting, back-thrusts and duplexing. The deforming wedge will expand to a super-critical state (b). When the wedge crosses a threshold, it will step forward with new thrust slices at the toe (c). If the wedge angle gets sub-critical, too low for the situation, it will return to internally thickening to re-establish the critical angle (fc)(d). Fold and thrust belts are continually in a state of imbalance attempting to establish an evolving critical state (orange line). Fold and thrust belt petroleum systems are more successful in lower-taper settings where the tendency for wider wedge-top basins extends the generation opportunities (green polygons) and the traps and migration routes are more direct. Modified from Van der Pluijm & Marshak (2004). Downloaded from http://sp.lyellcollection.org/ by guest on March 31, 2020

PETROLEUM TECTONIC OF FOLD-THRUST BELTS

The more extensive wedge-top basins associated consumption of the pre-orogenic sedimentary wedge with local lower taper can enable a larger volume of and key characteristics of the remaining pre-orogenic organic matter to remain at temperatures where it can architecture. Figure 5 shows the pre-orogenic wedge, actively generate hydrocarbons. These active wedge- main detachments, source intervals and the location top basins are directly linked to growing structural of the deformation front for each orogen. Table 1 traps, thus maximizing the potential for the genera- summarizes the distribution of petroleum-system tion and preservation of hydrocarbon accumulations. elements and the pre-orogenic architecture of each As described below a wide wedge-top zone with lim- fold and thrust belt. The Zagros has a thick pre- ited erosion is favourable for petroleum systems in orogenic section with multiple reservoir, seal, source fold and thrust belts assuming the additional load and detachment intervals (Bordenave 2014; Aqrawi does not over-mature the source. & Badics 2015; English et al. 2015). Less than half of the Zagros pre-orogenic sedimentary wedge Pre-orogenic architecture and inherited has been incorporated into the fold and thrust belt. β The Pyrenees, by contrast, incorporated its entire initial pre-orogenic sedimentary wedge. The characteristics The presence and distribution of organic matter in of the southern Pyrenees pre-orogenic section are the pre-orogenic sedimentary wedge is a first-order inferred from the less deformed portions of the driver of petroleum tectonics. If a source is con- related pre-Pyrenean basins (Cámara & Klimowitz sumed in an orogen, it is likely to create a petroleum 1985; Vergés & García-Senz 2001; Biteau et al. system. A lack of source results in no petroleum sys- 2006). For this analysis the largely eroded pre- tem. The distribution of the source dictates the lon- orogenic section of the southern Pyrenees is inferred gevity of the petroleum system. The shape of the to have multiple reservoir-seal pairs above a primary pre-orogenic depositional wedge and distribution source interval. The Sevier has multiple pre-orogenic of detachment intervals are significant controls on source, reservoir and seal intervals (Anna et al. the early evolution of critical taper (Hughes this vol- 2007). The Sevier has incorporated more than half ume, in review; Butler et al. this volume, in review) of its pre-orogenic sedimentary wedge, including and consequently on the preservation of hydrocar- Paleozoic sources and reservoir-seal pairs (Burtner bon systems in the active fold and thrust belt. Pre- & Nigrini 1994). In addition, the Sevier contains orogenic sedimentary wedges with a steep basal sur- multiple syntectonic sources and reservoir-seal pairs. face (high β in Fig. 3) have a predisposition to ini- The Beni segment of the Sub-Andean fold and thrust belt has incorporated most of its pre-orogenic sedi- tially have a higher critical taper (fc)(DeCelles & Mitra 1995; Meigs & Burbank 1997). Pre-orogenic mentary wedge including most of the known Paleo- sedimentary wedges with high aspect ratio (low ini- zoic source intervals (Louterbach et al. 2018). tial β) and multiple detachments promote lower taper Figure 5 is a vertically exaggerated, same-scale (DeCelles & Mitra 1995; Meigs & Burbank 1997). comparison of the pre-orogenic architecture of the four fold and thrust belts based on the pre-orogenic portions of the cross-sections shown in Figures 6–9, PTA of selected fold and thrust belts which are analysed below. In addition to the obvi- ous scale differences, the variations in shapes of the fi This PTA generalizes and simpli es aspects of the pre-orogenic sedimentary wedges, distribution of selected fold and thrust belts in order to focus on detachments and source intervals are shown. Sum- / themes that can constrain and or can be constrained maries of the Zagros petroleum systems are in Bor- by analysing the deformation path of organic matter. denave (2014) for the Iranian segment and Aqrawi fi Speci cs of certain areas will naturally vary but we et al. (2010), Aqrawi & Badics (2015) and English believe the major themes of the approach are broadly et al. (2015) for the Iraqi segment (Table 1). The applicable and useful in comparing diverse fold and Zagros, with a low initial β, thick broad pre-orogenic thrust belts. While the four fold and thrust belts ana- sedimentary wedge, multiple reservoir-seal pairs and lysed have differing scales, tectonic settings and 11 major sources and detachment intervals (Table 1) degrees of petroleum exploration, PTA provides an was predisposed to be a robust petroleum system that fi insight into their genetics and a uni ed method for favoured a lower taper. The southern Pyrenees, in their comparison and contrast. contrast with the Zagros, has a mostly eroded pre- orogenic sedimentary wedge. The southern Pyrenees Stratigraphic analysis and pre-orogenic is inferred to have a high initial β and a thick and architecture narrow pre-orogenic sedimentary wedge (Meigs & Burbank 1997). Using the northern Pyrenean Aqui- A comparison of the stratigraphic section at the taine basin (Biteau et al. 2006) as an analogue, the present-day deformation front of each studied fold eroded pre-orogenic section of the southern Pyrenees and thrust belt (Fig. 4) illustrates the amount of is inferred to contain possible petroleum-system Downloaded from http://sp.lyellcollection.org/ .KENDALL J. TAL. ET byguestonMarch31,2020

Fig. 4. Simplified stratigraphic section at present-day thrust front for each fold and thrust belt. (Fig. 5 shows the location of the deformation front relative to the pre-orogenic architecture.) The sections are hung on the pre-orogenic stratigraphy. The thickness of the pre-orogenic section (blue) is indicative of how far the fold and thrust belt has advanced. The Zagros has a c. 10 km-thick pre-orogenic section with multiple source intervals (green ovals) preserved. The Pyrenees has completely consumed its pre-orogenic section and has no remaining source intervals. The mechanical stratigraphy is illustrated by the strong (red-brown) intervals and weaker detachment intervals (pink arrows = evaporite, brown arrows = mudstones). The Zagros has the most dynamic mechanical stratigraphy. The Beni has the least dynamic mechanical stratigraphy. The syntectonic sections (tan) reflect the dominant subsidence processes during the fold and thrust belt deformation. Downloaded from http://sp.lyellcollection.org/ by guest on March 31, 2020

PETROLEUM TECTONIC OF FOLD-THRUST BELTS

Fig. 5. Comparison of pre-orogenic architecture. Same scale and 3:1 vertically exaggerated pre-orogenic sedimentary wedges with major source rock (green lines) and detachment (brown line = mudstone and pink lines = evaporites) intervals indicated. Based on pre-orogenic phase of Figures 6–9 below. The brown arrows indicate the deformation front and the location of the stratigraphic columns shown in Figure 4. The distribution of detachment intervals relative to the shape of the pre-orogenic sedimentary wedge heavily influences the evolution of the orogenic wedge. The position of source intervals within the evolving orogenic wedge strongly influences the success or failure of the petroleum systems. elements (described below). It has a detachment understand the basin’s genetics (Xie & Heller near its base and a stiff section above (Meigs & Bur- 2009; Lee et al. 2019). Subsidence analysis of syn- bank 1997; Grool et al. 2018). This architecture tectonic sections is reflective of the subsidence made it predisposed to be a higher-taper system mechanism. Flexural subsidence moves in front of (Meigs & Burbank 1997). The Sevier pre-orogenic an approaching orogenic load. Subsidence rates section has an inflection point where the sedimentary accelerate as the load moves into an area (DeCelles wedge thins and initial β changes from moderate to & Giles 1996). Dynamic subsidence is over a larger low. This inflection point coincides with the loss of area and does not systematically vary or move. It is Paleozoic detachments and source intervals and believed to be driven by deep lithospheric processes becomes a focus of orogenic change and petroleum (Painter & Carrapa 2013). Figure 4 shows the syntec- system exhumation. In the Beni Basin, Paleozoic tonic section at the thrust front of each orogen. The source and detachment in the middle of the pre- Zagros (Pirouz et al. 2017) and southern Pyrenees orogenic sedimentary wedge may foster a lower- (Vergés et al. 1998) appear to have flexural basin taper step-out at the leading edge of the fold and fill of 4–5km(Fig. 4). The marine foreland sedimen- thrust belt. The near complete incorporation of the tation in the southern Pyrenees allows high resolu- Beni system source intervals is a petroleum system tion subsidence analysis; Vergés et al. (1998) and preservation concern (Louterbach et al. (2018). Grool et al. (2018) documented the 65 km advance of a 3–5 km deep flexural wave over a c. 30 myr Orogenic processes and syntectonic section period. The syntectonic section of the Sevier is indic- ative of a flexural system with a late dynamic subsi- Subsidence analysis studies the sedimentary filling dence overprint (Painter & Carrapa 2013; Yonkee & of a basin through geologic time and space to Weil 2015). The less constrained, inferred thick Downloaded from http://sp.lyellcollection.org/ .KENDALL J. TAL. ET byguestonMarch31,2020

Fig. 6. Sequential reconstructed cross-sections through the Zagros fold and thrust belt based on the restored cross-sections through Lurestan by Vergés et al. (2011) and Alavi (2007) and in Kurdistan by Koshnaw et al. (2017). Vertical exaggeration is 1:1. The pre-Zagros and proto-Zagros had significant potential hydrocarbon generative areas (green lines) (Aqrawi et al. 2010; Bordenave 2014; English et al. 2015). The middle Zagros low-taper system with large wedge-top basins (long blue arrow) was the optimal time for petroleum system formation. The late Zagros increase in taper, most dramatic at the Mountain Front Flexure (MFF), exhumed most of the generative wedge-top area (Bordenave 2014). Downloaded from http://sp.lyellcollection.org/ by guest on March 31, 2020

PETROLEUM TECTONIC OF FOLD-THRUST BELTS

Fig. 7. Sequential reconstructed cross-sections through the southern Pyrenees. Vertical exaggeration is 1:1. The optimal hydrocarbon generative time was at the initiation of the orogen. The source rocks, primarily near the base of the pre-orogenic section just above the basal detachment, would have shut down early. Any accumulations would have had to survive the exhumation and disruption of a high-taper orogenic wedge. Modified from Beaumont et al. (2000). syntectonic section used in this analysis for the Zagros fold and thrust belt Beni Sub-Andean fold and thrust belt is speculated to be a dynamically-enhanced flexural-basin system The Zagros Orogen contains the most successful fold (Louterbach et al. 2018). and thrust belt petroleum systems on the globe, with Detachment intervals in the syntectonic sections c. one-fifth of global proved reserves (Cooper 2007; shown in Figures 4 and 5 and noted in Table 1 can BP plc 2018)(Fig. 1). The Zagros fold and thrust promote rapid step-out of the deformation front belt is a young, active taper-building flexural system and potential lowering of local taper (Chapman & that has consumed approximately half of its pre- DeCelles 2015). This is similar to the impact of orogenic sedimentary wedge (e.g. Mouthereau et al. detachments in the pre-orogenic architecture dis- 2012; Saura et al. 2015)(Figs 5 & 6). Its exceptional cussed above. Syntectonic source intervals greatly organic endowment and favourable architecture for expand the petroleum potential of a system as will robust wedge-top petroleum systems, and its rela- be seen in the late Sevier system discussed below. tively early phase in the incorporation of its organic Downloaded from http://sp.lyellcollection.org/ by guest on March 31, 2020

J. KENDALL ET AL.

Fig. 8. Simplified schematic sequential reconstructed cross-sections through the Utah–Wyoming Sevier fold and thrust belt. This section is schematic, not to scale to illustrate the synchronous structural and stratigraphic changes. The early Sevier foreland was dominated by flexural subsidence. Maximum accommodation was at the thrust front. By middle Sevier the maximum accommodation had shifted c. 200 km into the foreland. By late Sevier the maximum accommodation shifted c. 400 km into the foreland and basement structures were reactivated, creating foreland traps. The early Sevier flexural-dominated lower-taper fold and thrust belt (long blue arrow) was the optimal time for the fold and thrust belt petroleum systems (long green line under fold belt) (Anna et al. 2007). The late Sevier foreland traps adjacent to deep depositional centres with syntectonic source intervals represent the optimum time for the foreland petroleum system (green line shifts to the foreland) (Anna et al. 2007). Post-orogenic extension and exhumation destroyed most of these accumulations. The dashed black line represents the level of post-fold and thrust belt exhumation. endowment in the orogen, contribute to its ex- Summaries of the Zagros petroleum systems are ceptional success (Bordenave & Hegre 2010). Its in Aqrawi et al. (2010), Bordenave (2014) and inherited architecture, with abundant source and Aqrawi & Badics (2015) for the Iranian segment, detachment intervals, and dominance by flexural and in Aqrawi et al. (2010) and English et al. processes fostered phases of extensive wedge-top (2015) for the Iraqi segment (Table 1). basins that created and preserved robust fold and thrust belt petroleum systems. Based on the ana- Pre-Zagros. The pre-orogenic architecture (Turo- lysis below, the Zagros was probably even more nian at c. 90 Ma) is a thick and broad sedimentary successful from a petroleum-systems standpoint wedge with multiple source and detachment inter- before the most recent, mostly destructive phase of vals (Fig. 5 and Table 1). The broad sedimentary its evolution. wedge and abundant detachments favour rapid Downloaded from http://sp.lyellcollection.org/ by guest on March 31, 2020

PETROLEUM TECTONIC OF FOLD-THRUST BELTS

Beni-Sub Andean Subande 50 km Fold and Amazonian Beni Foredeep thrust belt Foreland an zone (SAZ) Discovery SHows Eastern Cordillera Dry

D’

0 Ma Late Beni

Syntectonic Potential deposits hydrocarbon generative areas Pre-tectonic

Top of early dynamic basin Basement ~8 Ma (?) Middle Beni

Defornation front advance Dynamic ?? subsidence / uplift

10 km VE= 1:1 ~16 Ma (?) Early Beni

SW NE D D’

~40 Ma

Modified from Louterbach et al. (2018)

Fig. 9. Sequential reconstructed cross-sections through the Beni segment of the Sub-Andean fold and thrust belt. Vertical exaggeration is 1:1. The early Beni low-taper orogenic system (long blue arrow) was most likely the optimum time for petroleum systems (Louterbach et al. 2018). The very thick syntectonic section implies both flexural and dynamic subsidence. The location of the source at the detachment level is good for migration into adjacent traps but the extreme depth has a risk of both over-maturing the source rocks and degrading reservoir quality. The internal deformation, increasing taper and possible dynamically driven uplift shuts down most generative areas and complicates and exhumes the traps. step-outs of deformation and lower-taper orogenic hydrocarbons that migrated west from the fold and wedge phases. The sedimentary wedge is as much thrust belt (Fig. 6). Several of the structurally trapped as 10 km thick, which would have driven the deeper foreland accumulations shown on the map in Figure 6 source intervals into, and possibly through, their were initiated at this time. generative phases before any major shortening took Bordenave (2014) describes the petroleum sys- place (Bordenave 2014). Green lines in Figure 6 tems in the Iranian Zagros segment. A significant represent the potential generative areas defined by portion of the orogen’s organic endowment probably Bordenave (2014). was driven through its major hydrocarbon generative potential before the Zagros flexural system started. Proto-Zagros. The proto-Zagros obduction event The proto-Zagros petroleum systems can be audited (c. 70–40? Ma) created a local narrow, thick foreland in the proto-Zagros foreland traps shown on the map system (Homke et al. 2009; Saura et al. 2011; Kosh- in Figure 6. Evidence of modified early hydrocarbon naw et al. 2017), from 2 km to as much as 4 km thick accumulations is recorded in altered and tilted hydro- in NE Kurdistan. This event, which formed the carbon rims at some of the older accumulations Amiran Basin in Iran, is distinct from the primary in foreland traps. Dukhan Field of Qatar has a record Zagros event discussed below. The tectonic and fore- of these early petroleum systems (Fig. 6). An inte- deep loads would have driven additional pre- grated structural and geochemical analysis of the orogenic source intervals through their main genera- Dukhan structure (Norlund et al. 2009) shows folded tive potential by this time. The broad activation of degraded oil rims that restore back to horizontal foreland structures (Abdollahie Fard et al. 2006; when the Zagros growth section is unfolded. The Saura et al. 2011) at this time created traps for the implication is that proto-Zagros oil was trapped, Downloaded from http://sp.lyellcollection.org/ by guest on March 31, 2020

J. KENDALL ET AL. altered and then during the Zagros event was invol- deeper into the basement and an increase in eleva- ved in additional folding. The orogen-perpendicular tion. The MFF (see map, Fig. 6) is the surface expres- axis of the Dukhan Field also acts as an 80 km-long sion of the increasing taper (Emami et al. 2010; spirit level, tracking the flexural tilt of the structure Gines et al. this volume, in press). In Lurestan the into the fold and thrust belt foreland basin. MFF is at the deformation front and has been docu- Some of the proto-Zagros foreland traps have a mented to be active 3–2Ma(Homke et al. 2004). second life as they are later caught up in the fold Figure 6 shows the late Zagros deformation front and thrust belt (Bordenave 2014). The presence in (ZDF) in Lurestan does not advance much (short the Fars salient of presumably pre-Zagros generated blue arrow) while the slope (α) increases. Addition- Paleozoic source hydrocarbons in post-generation ally, to the north in Iraqi Kurdistan, Koshnaw et al. Zagros traps implies a ‘hotelling’ of hydrocarbons (2017) and Tozer et al. (2019) have documented a in foreland traps to be re-migrated and trapped c. 5 Ma activation of the MFF, c. 80–100 km behind again when the fold and thrust belt incorporates the the low-taper deformation front. Furthermore, drain- accumulations (for example, the gas accumulations age reorganization and changes in the channel fill in the fold and thrust belt on the extension of the characteristics have likely been created by the Qatar Arch shown near the label ‘Fars’ on the map increasing taper (Vergés 2007; Mouthereau et al. in Fig. 6). The presence of proto-Zagros foreland 2012). The detrital zircon record shows the loss of structural traps is another aspect that makes the Zag- a distinct hinterland signal in relation to this youn- ros an exceptional petroleum province. gest deformational phase. The rising mountain front likely creates a barrier that diverts and/or traps the Early to middle Zagros. The main Zagros flexural hinterland detritus (Koshnaw et al. 2017). foreland system begins in Early Miocene time This increasing taper exhumed the wedge-top although older recognized folds in Lurestan started basins, shut down most of the fold and thrust belt at about early-Middle Miocene time (Vergés et al. petroleum systems and destroyed existing accumu- 2019). The subsidence analysis of Zagros deforma- lations. Behind the MFF, the petroleum systems tion is dominated by a progressive flexural wave degrade from active to survival mode (Bordenave (Homke et al. 2004; Pirouz et al. 2017; Vergés et al. 2014). The destruction is most extreme where the 2019). Early to middle Zagros (Middle Miocene to MFF is at the deformation front. Success rates late Miocene–Pliocene boundary) deformation was change from very high in front of the MFF to very dominated by low taper, utilizing numerous high- low behind the MFF. Abundant seeps, exhumed efficiency detachments. The middle Zagros orogenic giant palaeo-accumulations (Goff et al. 2004) and a wedge jumped out to its present-day deformation few successfully reconfigured accumulations from front by c. 5 Ma (see long blue arrow of Fig. 6) deeper petroleum systems behind the MFF (Borde- (Emami et al. 2010). Homke et al. (2004) document nave 2014) are evidence of the former richness of 8.1–7.2 Ma early growth at the present day thrust the exhumed wedge-top zone (Fig. 6). front in Lurestan including folding and uplift above The Zagros orogenic system has consumed at what will become the Mountain Front Flexure least half of its hydrocarbon endowment. It was at (MFF). Koshnaw et al. (2017) observe the same its peak petroleum systems effectiveness during the events in the Iraqi Kurdistan segment of the Zagros early low-taper phase but is still outstanding, even with a c. 8 Ma growth section at the present-day after losing more than half of its wedge-top basins thrust front. to exhumation. The organic endowment and the pre- This low-taper system with a wide wedge-top disposition of the Zagros to be a low-taper fold and zone was the most robust petroleum system of the thrust belt system are major contributors to its suc- Zagros fold and thrust belt. The maximum volume of cessful petroleum systems. source interval was driven to its generative depth during this phase (Bordenave 2014). The thick Southern Pyrenees fold and thrust belt wedge-top basin sediments drove active generation while preserving the fold and thrust belt traps from This analysis specifically addresses the southern exhumation. The surviving portions of the low-taper Pyrenees as reconstructed in the critical taper wedge wedge-top basins in the Dezful and Iraqi Kurdistan models of Meigs & Burbank (1997) and Beaumont embayments of the fold and thrust belt (see map, et al. (2000). This cross-section along the central Fig. 6) have a very high success rate for petroleum part of the South-Central Unit (Grool et al. 2018) accumulations in structural traps (Pitman et al. is analysed because it has multiple sequential steps 2004; Bordenave 2014; English et al. 2015). and is focused on taper evolution (Fig. 7). Variations of the significant themes identified can be extrapo- Late Zagros. The late Zagros stage of deformation lated to the other portions of the southern Pyrenees (c. 5–0 Ma) involves an internal thickening of the to different degrees. The southern Pyrenees is a orogenic wedge, with the basal detachment stepping small, classic, well-documented systematic advance Downloaded from http://sp.lyellcollection.org/ by guest on March 31, 2020

PETROLEUM TECTONIC OF FOLD-THRUST BELTS of a flexural foreland basin system (Muñoz 1992; excellent detachment (Grool et al. 2018) at the base Vergés et al. 1998; Beaumont et al. 2000; Grool of the pre-orogenic section would tend to make this a et al. 2018). Its pre-orogenic sedimentary section short-lived petroleum system. along this cross-section is inferred to have similari- ties to the less deformed Aquitaine Basin on the Middle to late Pyrenees. By middle Pyrenees opposite side of the of the Pyrenean Orogeny (see (30 Ma), the inferred pre-orogenic organic endow- map in Fig. 7). ment was being exhumed, as were any palaeo-accu- mulations. Any accumulations would have had to Aquitaine Basin, early southern Pyrenees analogue. survive the 65 km of deformation front advance The Aquitaine Basin (2562 mmboe; Cooper 2007)is with limited chance for recharge. The taper is gener- a significant preserved petroleum system in the Pyre- ally high at 4–5° (Meigs & Burbank 1997). During nees Orogen (Bourrouilh et al. 1995; Biteau et al. the mid to late Pyrenees (long blue arrow in Fig. 7) 2006). The Aquitaine Basin is a foreland basin in there is a rapid advance of the deformation front, the northern Pyrenees on the opposing upper plate facilitated by syntectonic detachments with a slight of the doubly vergent Pyrenees Orogen (Bourrouilh lowering of the taper (Meigs & Burbank 1997). et al. 1995; Biteau et al. 2006; Rougier et al. 2016; This occurs after consumption of the pre-orogenic Teixell et al. 2018). The Aquitaine Basin and south- source rocks. A small preserved petroleum system ern Pyrenees share a similar rift history (Grool et al. in the southern Pyrenees corresponds to early Eocene 2018). However, the southern Pyrenees rift section is syntectonic source rocks locally deposited during the mostly exhumed (Grool et al. 2018) while the Aqui- early Pyrenees stages of the marine foreland basin. taine Basin is only mildly deformed and exhumed This sourced a small unique productive gas field (Vergés & García-Senz 2001; Biteau et al. 2006). (e.g. Alvarez 1994). The lack of an extensive syntec- The relatively abrupt lateral variation of the rift struc- tonic source, like that in the Sevier Orogeny (below), ture, as observed on the map in Figure 7 from the precludes the development of late robust petroleum western Basque–Cantabrian zone to the easternmost systems. The southern Pyrenees petroleum potential Pyrenees, constrains the variations of the petroleum is thus in preserved remnants of an early petroleum systems (Vergés & García-Senz 2001)(Fig. 7). system or in local anomalies.

fl Pre-Pyrenees. The pre-orogenic architecture in u- Sevier fold and thrust belt enced the petroleum and tectonic evolution of the southern Pyrenees. The thick and narrow geometry The Sevier Orogen (Fig. 1) is a composite fold of the pre-orogenic depositional wedge, with an and thrust belt with an early flexural wave that is excellent Upper Triassic detachment near the base overwhelmed by dynamic subsidence and uplift pro- of a thick stiff sequence (Grool et al. 2018), encour- cesses (DeCelles 2004; Painter & Carrapa 2013; aged the involvement of the entire pre-orogenic Yonkee & Weil 2015). The dynamically driven, wedge early in the orogen. This led to a relatively exceptionally broad and thick foreland sets the high-taper orogenic wedge (Meigs & Burbank Sevier apart from the previously mentioned orogens. 1997). The inferred concentration of organic matter Schematic cross-sections (Fig. 8), based on the at the base of the Mesozoic syn- and post-rift pre- sequential restorations of Constenius et al. (2003), orogenic section focuses the petroleum potential to DeCelles & Coogan (2006) and Yonkee & Weil the earliest phases of deformation (Table 1). Pre-50 (2015) and on the foreland stratigraphy of DeCelles Ma the 5–6 km thickness (Meigs & Burbank 1997; (2004) and Painter & Carrapa (2013), attempt to Vergés et al. 2002) of the sequence was sufficient capture the interplay of the flexural v. dynamic pro- to convert much of the organic matter to hydrocar- cesses. The consumption of the pre-orogenic source bons even before the orogen had begun. Traps are intervals and involvement of significant syntectonic commonly the missing element in late-rift and post- source intervals created a world-class petroleum sys- rift petroleum systems with deep source intervals. tem (Campbell & Ritzma 1979). Exhumation driven by post-orogenic events has left only tar sand rem- Early Pyrenees. The earliest Pyrenees deformation nants of this giant petroleum system (Campbell & around c. 50 Ma (Grool et al. 2018), when traps Ritzma 1979). formed but the deep sources were not yet exhumed, was probably the most robust phase of the southern Early Sevier. Early Sevier (c. 155 to c. 85 Ma) is Pyrenees petroleum system, assuming it was similar characterized by a progression of flexural basins to the Aquitaine Basin petroleum system. Discovery and relatively low-taper system (DeCelles & Mitra of tar sands and/or hydrocarbon fluid inclusions in 1995). Isopachs track this progression of sediment the erosional remnants of this system would indicate accumulation (Burtner & Nigrini 1994; DeCelles a petroleum system that was once working. The loca- 2004; Painter & Carrapa 2013). DeCelles (2004), tion of both a possible source (inferred) and an Painter & Carrapa (2013) and Yonkee & Weil Downloaded from http://sp.lyellcollection.org/ by guest on March 31, 2020

J. KENDALL ET AL.

(2015) show flexural basin progression with maxi- Late Sevier. Late Sevier (c. 70–50 Ma) tectonics mum accumulation at the thrust front. Sequential are characterized by limited advance of the defor- reconstructions in Wyoming (Peyton et al. 2011; mation front, enhanced dominance of dynamic pro- Yonkee & Weil 2015) and central Utah (Constenius cesses and the addition of multiple high-relief et al. 2003; DeCelles & Coogan 2006) interpret early basement-cored Laramide style uplifts in the fore- high-taper systems, subsequently jumping out to a land. These local uplifts, illustrated on the late Sevier low-taper system (long blue arrow of end of early c. 66 Ma cross-section in Figure 8, complicate the Sevier in Fig. 8). The deformation steps forward to subsidence pattern, adding local flexural subsidence the thinning hinge of the thick pre-orogenic deposi- to the long wavelength dynamic subsidence (Yonkee tional section where pre-orogenic detachments & Weil 2015). There was very little sediment accu- diminish. Most of the pre-orogenic source rocks were mulation at the deformation front at this time, as probably consumed during this early phase (Burtner the centre of accommodation shifted to 400 km & Nigrini 1994), which was likely the most robust east of the deformation front. time for fold and thrust belt petroleum systems. Exhumation of the fold and thrust belt continued The surviving fold and thrust belt accumulations to shut down and destroy its petroleum systems are associated with the best-preserved wedge-top (Burtner & Nigrini 1994) while local flexurally- basins in northern Utah and southern Wyoming. enhanced dynamic subsidence of the foreland Pre-orogenic sources probably generated hydrocar- drove some of the syntectonic source intervals to bons during this period (Burtner & Nigrini 1994); hydrocarbon generation (Anna et al. 2007). One however, unlike in the Zagros Orogen, there were challenge for the foreland uplifts is breaching of few foreland traps to catch hydrocarbons migrating the foreland traps as a result of structural relief and out of the Sevier fold and thrust belt during this exhumation (Fig. 8; Fan & Carrapa 2014). phase. The post-orogenic events extended, uplifted, tilted and eroded the fold and thrust belt province (Constenius et al. 2003; DeCelles & Coogan 2006; Middle Sevier. During the middle Sevier (c. 85– Yonkee & Weil 2015) while exhuming 1–2kmof 70 Ma), the deformation front stopped advancing section from the foreland (Lazear et al. 2013). (short blue arrow of middle and late Sevier in Only complicated erosional remnants (e.g. Chidsey Fig. 8) as the orogen built taper (DeCelles & Mitra et al. 2007), containing 3167 mmboe of economic 1995; Constenius et al. 2003; DeCelles & Coogan hydrocarbon accumulations (Cooper 2007) remain 2006), creating a number of structural culminations. of what probably were world-class petroleum sys- Dynamic uplift of the fold and thrust belt and tems (Fig. 8). Some 29 000 mmboe of uneconomic dynamic subsidence of the distal foreland began to heavy oil accumulations (Campbell & Ritzma dominate during this period of taper increase 1979), which occur as small remnants on the flanks (DeCelles 2004; Painter & Carrapa 2013; Yonkee of deeply eroded foreland highs, provide evidence & Weil 2015). The middle Sevier cross-section of major foreland petroleum systems that have (Fig. 8) shows limited sediment accumulation at been exhumed. The situation would be similar if the deformation front, with maximum accumulation 2 km of overburden were eroded from the Zagros shifting 200 km east into the distal foreland. By the system. end of this phase most of the fold and thrust belt petroleum systems were shutting down and being Beni Sub-Andean fold and thrust belt exhumed by the increasing taper (Burtner & Nigrini 1994). Few foreland structures existed to trap the The Beni fold and thrust belt segment of the hydrocarbons generated during the middle Sevier northern Bolivian Sub-Andean (Fig. 1) is poorly phase. The Moxa Arch (Fig. 8, red arrow on map) constrained due to sparse data in a difficult-to-date is an exception. It is interpreted to be a basement- non-marine syntectonic section. It has been included cored Laramide style foreland uplift in a subsiding in this discussion to demonstrate the applicability foreland basin (Becker & Lynds 2012). The Moxa of PTA techniques to fold and thrust belts that Arch is probably too small (c. 20 × 70 km) (Becker have not yet been extensively investigated. In gene- & Lynds 2012) to be driven by geodynamic unload- ral, PTA of an under-constrained interpretation high- ing processes in the subsiding foreland. The Moxa lights the uncertainties and offers potential ways to Arch hosts the most successful surviving foreland add constraints. petroleum accumulations of the Sevier foreland Roeder (1988), Baby et al. (1995, 1997) and and the world’s largest CO2 accumulation. It con- McQuarrie et al. (2008) interpreted the Beni fold tains early traps able to catch hydrocarbons gener- and thrust belt as a thin-skinned system. The analysis ated in the fold and thrust belt, with minor late presented here tracks the organic endowment utiliz- reactivation aiding preservation. This scenario is ing sequential restorations generated by Louterbach similar to the successful Zagros foreland. et al. (2018) involving both thin-skinned thrusts Downloaded from http://sp.lyellcollection.org/ by guest on March 31, 2020

PETROLEUM TECTONIC OF FOLD-THRUST BELTS and high angle basement faults. As discussed in the with its maximum wedge-top section at this time. stratigraphic analysis and in Table 1, all the elements This is the most favourable time for a robust hydro- for petroleum accumulations are present in the Beni carbon system in the Beni fold and thrust belt. The fold and thrust belt; the primary uncertainty is when broad generative areas and simple traps of the early the overburden was applied and removed. Louter- Beni fold and thrust belt are analogous to the Zagros bach et al. (2018) interpreted an early history of a wedge-top basins. The thick wedge top keeps gener- step-out at the deformation front with a thick ation going on and precludes exhumation of the wedge top, followed by internal deformation and traps, but there is a potential risk of over-maturation erosion of the fold and thrust belt. In this context, of source rocks and degradation of reservoir quality. the greatest probability of a robust petroleum system Identifying the processes controlling the possible occurred in the early phase and accumulations would extra thickness is important for distributing this need to have survived the later exhumation phase to potential risk of over-maturity. The sub-economic be preserved. Candamo accumulation, located c. 200 km to the NW in the Sub-Andean zone of the southern Peru- Pre-orogenic Beni. The pre-orogenic Beni (40 Ma) vian Madre de Dios fold and thrust belt, is character- consists of a Paleozoic to Early Cenozoic narrow ized by high maturity gas/condensate with marginal thin pre-orogenic sedimentary wedge (in cross- reservoir quality (McGroder et al. 2015) and may be section view) described in the stratigraphic analysis a survivor of a similar overburden scenario. discussion above and in Table 1. Upper Paleozoic source and detachment intervals are both in the Middle Beni. It is speculated that around 16–8Ma upper half of the pre-orogenic sedimentary wedge the orogenic wedge began building taper by an (Fig. 5), geometry favourable for a robust low-taper increase of the internal deformation, modifying and wedge-top petroleum system. Petroleum system developing new structures in the orogenic wedge modelling by Baby et al. (1995) indicates that deeper while not significantly advancing (short blue arrow Paleozoic source intervals are in a generative phase of middle Beni in Fig. 9). The internally thickening before initiation of the fold and thrust belt deforma- orogenic wedge complicates traps, reducing some tion (green lines in Fig. 9). As in the Zagros and generative areas while potentially over-maturing Sevier PTAs above, any inferred pre-orogenic gener- others. Integration of thermochronological, geo- ative phases could be audited in pre-orogenic traps in chemical, magnetostratigraphy and detrital zircon the modern Beni foreland basin. data would provide tighter constraints on the sequencing of deformation. It is inferred that this Early Beni. The early (40–16 Ma?) evolution of the phase would disrupt potential generative areas Beni involved the step-out of the thrust front to (shortened green lines in Fig. 9). near the edge of the Paleozoic detachment (long blue dashed arrow of early Beni in Fig. 9). The tim- Late Beni. In the late phase (8–0 Ma), the front of the ing is consistent with that proposed by Rak et al. orogenic wedge did not significantly advance (short (2017). This is interpreted as a low-taper wedge blue arrow of late Beni in Fig. 9) but continued build- with 5 to possibly 8 km thick wedge-top deposits ing taper, while the dynamic influence may have (Louterbach et al. 2018). This step-out is analogous reversed from dynamic subsidence to dynamic uplift to the middle Zagros step-out, discussed above, in the fold and thrust belt. This interpreted regional which created its most robust wedge-top petroleum uplift is based on the highly eroded aspect of the systems. There is a clear early Miocene flexural sub- fold and thrust belt and on the projection of thermo- sidence signal with subsidence accelerating through chronology results from the Madre de Dios Basin in time (Louterbach et al. 2018). A comparison to the southern Peru, c. 200 km to the NW (Louterbach Sevier orogenic system implies that there could be et al. 2014, 2018), which show a Plio-Pleistocene additional processes active at this time. The inter- period of broad uplift of both the eastern side of preted very thick wedge top could be due to dynamic the Eastern Cordillera and the Sub-Andean zone. subsidence enhancement. To identify an overprint- The interpreted lack of a foredeep for the late Beni ing dynamic subsidence process, detailed age control fold and thrust belt may indicate a dynamic uplift, and area-wide subsidence analyses showing a pro- similar to the late Sevier (Fig. 9). Taper increases gressing flexural signal, similar to that in the Pyre- with the involvement of a deep basement level nees but with excess regional subsidence, is required. detachment similar to the MFF of the late Zagros Deformation was initiated by the Early Miocene phase discussed above. The late erosion could be dri- when Paleozoic source rocks received additional ven by dynamic uplift and/or the basement related overburden, resulting in a second phase of hydrocar- faults. Higher density thermochronological uplift bon generation (Baby et al. 1995), as indicated by the analysis, as in the Sevier, is required to constrain the green lines in Figure 9. As interpreted, the deforma- dominant driving mechanism and areal extent of the tion front has jumped out to near its furthest extent, exhumation with more certainty. From a petroleum Downloaded from http://sp.lyellcollection.org/ by guest on March 31, 2020

J. KENDALL ET AL. systems context the inferred broad uplift would shut significant horizontal displacement. This is espe- down potential generative areas. Integrated studies cially the case in fold and thrust belts where there like that of the Dukhan Field (Norlund et al. 2009) can be significant shortening of potential source in the Zagros foreland could document this shut- intervals. More sophisticated kinematic and petro- down and identify modified palaeo-accumulations, leum system appraisal (e.g. Burtner & Nigrini further constraining the possible interpretations. 1994; Roure & Sassi 1995; Calderón et al. 2017) A PTA of the dominantly thin-skinned inter- can be enhanced with the integration of a genetic tec- pretations (Roeder 1988; Baby et al. 1995, 1997; tonic context incorporated into PTA. PTA attempts McQuarrie et al. 2008) of this portion of the Beni to be more of a four-dimensional proxy for the evo- fold and thrust belt has different petroleum system lution of petroleum systems in fold and thrust belts, implications. A continuous break-forward sequence potentially providing a more rigorous analysis. with a narrow wedge top would have more limited Tectonic analyses do not commonly fully utilize hydrocarbon opportunities: for example, the frontal the constraints on tectonic evolution that can come trap in a break-forward thin-skinned sequence would from an integrated petroleum system analysis. A be the youngest, with limited to no connections to web search of ‘fold and thrust belt tectonics’ publica- generative areas, while in the analysis presented tions yields over 145 000 publications. Of these only above the frontal traps are among the oldest in the c. 5000 have any mention of ‘petroleum systems’. fold and thrust belt and had access to long-lived gen- When an organic-rich interval has been incorporated erative areas. The broad range of uncertainties in the into a fold and thrust belt the presence and/or Beni fold and thrust belt highlights the importance of remains of petroleum systems can provide powerful well constrained geochronologic, thermochronolog- constraints on the processes involved in the fold ical and integrated geochemical data. belt’s evolution. PTA is another tool that can assist The limited data in the Beni precludes definitive in investigating fold and thrust belt genetics. identification of the dominant processes (flexural v. dynamic) through time, but the PTA puts the Orogen comparison and contrast Beni fold and thrust belt in a global context that enhances our understanding of its tectonic evolution Figure 10 is a side-by-side overview of the interplay and provides perspective for where new data could of tectonic processes with their petroleum system have the most significant impact. impacts through time for each orogen analysed. It Recognizing the uncertainties, the Beni fold and identifies times most favourable for accumulation thrust belt’s pre-orogenic architecture and possible of hydrocarbons (dark green polygons) and the thick syntectonic load indicate an orogenic system times with greater risk for their destruction (red that was more favourable for hydrocarbon accumula- polygons). tions in its early development. The Beni fold and The Zagros is a flexural orogen that has a fortu- thrust belt is under-explored, with only 3 mmboe itous alignment of elements and processes for hydro- of discovered reserves (Cooper 2007). A focus on carbon generation and preservation (Fig. 10). Its defining the dominant tectonic processes is required. multiple stacked sources ensure continuous hydro- In the present day, the Beni appears to be in exhum- carbon generation while stacked detachments foster ing mode with the best chance for survival of petro- a low taper and wide wedge-top basins. A pre- leum accumulations in areas that are exceptions to orogenic, gently deformed foreland catches hydro- the general tectonic trends, similar to the preserved carbons that migrate out of the fold and thrust belt wedge-top basins of the Sevier Orogen. and provides ‘hotels’ to source re-migrating hydro- carbons as early traps are incorporated into the fold and thrust belt. Discussion The Pyrenees is also a flexurally dominated oro- gen, but the early consumption of its source rocks Petroleum-systems evolution of fold and thrust belts means few accumulations survived exhumation in provides a perspective for the genetic analysis of oro- a long lasting, relatively high-taper orogenic wedge. genic systems. The distribution of organic matter and The Sevier is a flexural system that was later its deformation pathway during orogeny determine dominated by dynamic uplift of the fold and thrust the realization of hydrocarbon potential and preser- belt and distal foreland subsidence with foreland vation of petroleum accumulations. Common petro- deformation. The late high-taper phase exhumed leum system basin modelling in the industry is one- much of an earlier more robust lower taper fold and or two-dimensional, stacking stratigraphy or maps thrust belt petroleum system. The addition of syn- based on present day geometries (Magoon & Dow tectonic foreland sources to be matured by dynamic 1994; Al-Hajeri et al. 2009; Peters 2009). This subsidence with contemporaneous traps added a approach is a good proxy in areas of simple vertical second optimum time for hydrocarbon accumula- motion but breaks down completely when there is tions in the Sevier foreland. Post-orogenic uplift Downloaded from http://sp.lyellcollection.org/ by guest on March 31, 2020

PETROLEUM TECTONIC OF FOLD-THRUST BELTS

Zagros S Pyrenees US Sevier Beni Sub Andean 65 55 Syntectonic sources Syntectonic 105 95 85 75 Lower taper Lower

Fig. 10. PTA summary chart. The vertical axis is time stretched to the length of time investigated. Note each orogen has a different time scale. Blue dashed horizontal lines correspond to restored cross-sections in Figures 6–9. The left side of each orogen summary tracks the tectonic processes while the right side tracks their impact on the petroleum systems. The first two columns track the interplay of flexural and dynamic subsidence. Dynamic subsidence is light grey. Dynamic uplift is black. The taper and wedge-top columns track the evolution of the frontal orogenic wedge. The exhumation column (magenta) tracks whether the net result of the orogenic system is dominated by erosion of the wedge top or by the preservation of the petroleum-system elements. The width of the sixth column (light green polygons) tracks the potential for active hydrocarbon generation and is labelled with the dominant source area: pre-orogenic, syntectonic, or foreland. When the source has been completely incorporated into the fold and thrust and subjected to extensive exhumation its generative polygon terminates and it is labelled ‘exhausted’. This is an indication of shortening relative to the source distribution. The traps formed (green lines) and traps destroyed (red lines) columns track the formation of traps and their destruction in the foreland (when present) and in the fold and thrust belt; the width of the line indicates which is dominant. The last column with dark green polygons highlights optimum petroleum system timing in the fold and thrust belt and in the foreland. The Zagros foreland, with pre-existing traps and no exhumation, is always optimum (long dark green polygon). The Zagros fold and thrust belt was optimum during the low-taper phase with wide wedge-top basins (shorter dark green polygon). The southern Pyrenees is inferred to have an early optimal hydrocarbon time (short dark green polygon) long before it went through a long destructive phase. The Sevier fold and thrust belt petroleum system is interpreted to have been optimal before c. 85 Ma (first long dark green polygon) after which it had to survive high-taper and probably eventually dynamically driven exhumation. The Sevier has a second optimum petroleum system (second long dark green polygon) driven by dynamic subsidence in its foreland. The Beni fold and thrust belt has greater uncertainty but the early low-taper phase with possible dynamic enhancement was probably its optimum petroleum system (short dark green polygon). and exhumation have left only remnants of what during its early low-taper flexural phase with were once world-class petroleum systems. dynamic subsidence enhancement. Its late high-taper The Beni segment of the Sub-Andean Orogen is system with possible dynamic uplift shut down and similar to the Sevier, with a pre-orogenic source stressed the petroleum systems. The lack of signifi- that is expected to have had high thermal exposure. cant syntectonic sources and foreland traps like It probably had its most robust petroleum system those in the Sevier and Zagros limits the Beni’s Downloaded from http://sp.lyellcollection.org/ by guest on March 31, 2020

J. KENDALL ET AL. hydrocarbon potential to early accumulations that et al. 2013; Painter & Carrapa 2013; Yonkee & Weil survived later exhumation. 2015). Heller & Liu (2016) see a consistency The broadly deformed forelands of the Zagros between late Sevier subsidence patterns and a geody- and Sevier create a set of traps in addition to the namic inverse convection model for the passing fold and thrust belt traps (Fig. 10). These foreland lower plate. Petroleum system evolution has been traps complete exceptional foreland petroleum sys- significantly impacted by these overprinting deep tems that complement the fold and thrust belt petro- processes. leum systems. The controls on foreland deformation The lower plate Southern Pyrenees (Iberian are unclear but they appear to be in part related to microplate) and Zagros (Arabian plate) do not appear reactivation of inherited zones of weakness. In the to have a major geodynamic overprint. The proto- foreland, as in the fold and thrust belt, the inherited Zagros (c. 70 Ma) foreland’s subtle reactivation of architecture is important. The Zagros and Sevier features indicates some stresses are being transmitted foreland traps capture hydrocarbons escaping from far into the foreland during that time frame. The the fold and thrust belts and record the history of Aquitaine basin, discussed as possibly sharing a the fold and thrust belts, as is seen in the Dukhan rift history with the analysed southern Pyrenees, is Field of the Zagros and the Moxa Arch of the Sevier. on the upper plate side of the Pyrenean orogenic sys- In addition, foreland traps can act as hotels for hydro- tem. A PTA comparison of the southern Pyrenean carbons that can be re-migrated and trapped again and Aquitaine systems would be informative. Zie- when they are incorporated into the fold and thrust gler et al. (2002) identified the Ouachita–Marathon belt, as appears to happen in the Paleozoic petroleum orogenic system of North America as a potential system of the Fars segment of the Zagros. upper plate system with anomalous uplifts and sub- The interplay of flexural and dynamic processes sidence over 1500 km into its foreland. The multiple in the evolving deformation front strongly influences petroleum systems in the Ouachita–Marathon fore- the success and/or failure of hydrocarbon systems. land would lend themselves to an interesting PTA Dynamic subsidence provides an opportunity for effort. PTA offers another avenue for investigation the deposition of more overburden, whereas dyna- of additional orogenic systems. mic uplift induces exhumation of petroleum systems. The Zagros and Pyrenees do not appear to record a strong dynamic overprint. In the late Sevier the Orogenic processes and fold and thrust belt dynamic processes overwhelmed the flexural pro- petroleum systems cesses, stressing the fold and thrust belt petroleum systems, while in the early Beni dynamic processes Flexural processes create and destroy petroleum sys- may have enhanced and/or overmatured the petro- tems as they advance. The most favorable tectonic leum systems. In addition, the late Sevier has strong circumstance for hydrocarbon accumulations is dynamic subsidence far into its foreland, creating a when the frontal orogenic wedge is at a relatively major foreland generative region that the other fold low-taper phase after a rapid advance. In these con- and thrust belt systems addressed lacked. ditions the maximum amount of source interval can be preserved from exhumation in generative condi- Upper plate v. lower plate processes tions while directly adjacent to preserved traps. The present day Dezful and Kirkuk embayments in It is interesting to note that the dynamic influences the Zagros, the early Zagros, the end of the early appear more pronounced on the Sevier and Beni Sevier and the early Beni Sub-Andean are robust Sub-Andean orogenic systems, which are both on low-taper thick wedge-top petroleum systems. In the upper plate of convergent orogenic margins. the Pyrenees, the lowering of taper associated with The Zagros and southern Pyrenees are on the lower a step-out of the deformation front along a syntec- plate and do not appear to have been as significantly tonic detachment at c. 29 Ma had the most expansive influenced by dynamic processes. Ziegler et al. wedge top but the lack of a well-developed syntec- (2002) argued that the lower plate foreland subsi- tonic source interval precluded an extensive petro- dence is exclusively controlled by the topographic leum system. load exerted by the orogenic wedge whereas the A taper-building phase trends toward destruc- upper plate foreland has an additional load exerted tion and exhumation of petroleum systems. The by the pull of the down-going slab. This might be Lurestan and Fars salients of the Zagros, the southern expected given the upper plate is subjected to varied Pyrenees, the late Sevier and the late Beni Sub- influences from the lower plate it is overriding. Andean are high-taper phases that destroy petroleum Upper plate responses to lower plate changes in con- systems. Petroleum system preservation in high- vergence, slab angle and slab coupling have been taper systems is most likely in areas that diverge proposed for the Andes (Ramos & Folguera 2009; from the dominant theme of exhumation. The pre- Horton 2018) and the Sevier (DeCelles 2004; Lazear served accumulations associated with remnants of Downloaded from http://sp.lyellcollection.org/ by guest on March 31, 2020

PETROLEUM TECTONIC OF FOLD-THRUST BELTS an eroding wedge-top cover in the Sevier and in the pre-orogenic sedimentary wedge and source inter- Fars and Lurestan portions of the Zagros are exam- vals. It is less challenged. The Sevier has consumed ples of effective accumulation survival scenarios. more than half of its pre-orogenic sedimentary The exploration potential for the Beni Sub-Andean wedge and source intervals but adds syntectonic fold and thrust belt as analysed is best in areas that source intervals, resulting in more robust petroleum are exceptions to its interpreted strong exhumation systems. The Zagros has consumed only half of its theme. pre-orogenic sedimentary wedge and adds an addi- The pre-orogenic architecture, including the tional syntectonic source interval. The Zagros has shape of the pre-orogenic sedimentary wedge and very robust petroleum systems. its mechanical stratigraphy, heavily influence the In the context of the fold and thrust belts ana- evolution of fold and thrust belts. The fold and thrust lysed above, the presence and distribution of source belt kinematics are regulated by the presence or intervals sets the stage for the evolution of the petro- absence of mechanically weak and strong layers leum systems in the fold and thrust belt. The amount within the orogenic wedge. The Zagros, with a thick of shortening relative to the source intervals is a broad pre-orogenic depositional wedge and multiple major control on how the petroleum potential is real- detachments in both pre-orogenic and syntectonic ized and preserved. The pre-orogenic architecture sections, favours lower taper and a robust wedge-top and mechanical stratigraphy as they impact the basin. The thick narrow Pyrenees pre-orogenic dep- deforming orogenic wedge modulate the petroleum ositional wedge, with a dominant basal detachment, systems, affecting their robustness and preservation. favours a high taper. Both the pre-orogenic detach- The dominance of flexural v. dynamic processes also ments in the thick portion of the Sevier pre-orogenic modulates the petroleum systems. Where and when sedimentary wedge and the syntectonic detachments the flexural and dynamic processes are active above the thin portion, favour an early low taper. impacts the distribution, robustness and preservation However, the termination of the pre-orogenic of the petroleum systems. detachments as the depositional wedge thins favour PTA is an effective way to examine the major a later high taper. The Beni Sub-Andean has a pre- processes of an orogenic system. Tracking the nar- orogenic sedimentary wedge shape similar to the row range of deformation pathways for the genera- Pyrenees but a good detachment in the upper half tion and preservation of hydrocarbon accumulation of the wedge makes it favourable to have an early provides an audit of the dominant processes in an low taper phase. orogen. The Zagros is an organic-rich flexurally The distribution of source rocks sets the frame- dominated orogen, the Pyrenees is an organic-poor work for the evolution of petroleum systems in flexurally dominated orogen, the pre-orogenic and fold and thrust belts. The Zagros has stacked multi- syntectonic sources of the Sevier orogen had optimal ple source rocks, making it the best fold and thrust conditions for petroleum systems in the fold and belt petroleum province in the world. The Pyrenees thrust belt during its early, flexurally dominated is challenged because the best source is at the bottom phase and in the foreland during its later, dynami- of the section. The Sevier has sources in the thick cally dominated phase, and the Beni Sub-Andean portion of the pre-orogenic depositional wedge and is an organically challenged flexural system with the addition of excellent syntectonic source rocks. possible dynamic overprints. The Beni Sub-Andean, with its best-known source in the upper half of the pre-orogenic sedimentary wedge, is less challenged than the Pyrenees. Conclusions PTA Shortening relative to source length PTA is an effective way to investigate the genetic The amount of shortening relative to the length of evolution of a fold and thrust belt that has organic- source intervals is a key control on the realization rich intervals. Tracking the petroleum system evolu- of the petroleum potential of a fold and thrust belt tion is an audit of the dominant processes in an oro- (Table 1). The further back into the orogenic gen. How the pre-orogenic architecture interacts wedge a source interval is incorporated, the more with competing orogenic processes is reflected in likely its petroleum systems are to be consumed. the creation and destruction of petroleum systems. The red arrow in Figure 5 indicates the present-day deformation front of the fold and thrust belt relative Analysed fold and thrust belts to the pre-orogenic architecture and source distribu- tion. The southern Pyrenees has completely con- The nature of the rocks that are incorporated into a sumed its pre-orogenic sedimentary wedge and fold and thrust belt can be tracked in its petroleum source intervals, leaving a challenged petroleum sys- system evolution. The distribution of source rocks tem. The Beni Sub-Andean has consumed most of its and the pre-orogenic architecture, including the Downloaded from http://sp.lyellcollection.org/ by guest on March 31, 2020

J. KENDALL ET AL. shape of the pre-orogenic sedimentary wedge and its Funding This work was partially supported by mechanical stratigraphy, greatly influence the evolu- SUBTETIS (PIE-CSIC-201830E039) and ALPIMED tion of petroleum systems within the analysed fold (PIE-CSIC-201530E082) projects. and thrust belts. The amount of shortening relative to source distribution controls the realization and preservation of the petroleum potential. The Zagros References fold and thrust belt, with its limited relative shorten- ing and generous endowment of organic and detach- ABDOLLAHIE FARD, I.A., BRAATHEN, A., MOKHTARI,M.& ALAVI, S.A. 2006. Interaction of the Zagros fold–thrust ment intervals, had the best set-up for petroleum belt and the Arabian-type, deep-seated folds in the Aba- systems; in contrast, the Pyrenees, with relatively dan Plain and the Dezful Embayment, SW Iran. 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ETOPO1 1 Arc-Minute seen in the Sevier foreland and as inferred for the Global Relief Model: Procedures, Data Sources and Beni, provides additional overburden, whereas dyna- Analysis. NOAA Technical Memorandum NESDIS mic uplift induces exhumation, as seen in the late NGDC-24. National Geophysical Data Center, Sevier and as inferred for the late Beni Sub-Andean NOAA, https://doi.org/10.7289/V5C8276M [last fold and thrust belts. accessed 9 September 2018] The most favourable tectonic circumstance for ANNA, L.O., ROBERTS, N.R. & POTTER, C.J. 2007. Geologic assessment of undiscovered oil and gas in the Paleo- hydrocarbon accumulations in the fold and thrust zoic–tertiary composite total petroleum system of the belts analysed is when the deformation front maxi- Eastern Great Basin, Nevada and Utah. US Geological mizes wedge-top basins. This can happen when the Survey Eastern Great Basin Province Assessment orogenic wedge front steps out to a local relatively Team, Geologic Assessment of Undiscovered Oil and low-taper phase. The maximum amount of source Gas Resources of the Eastern Great Basin Province, interval can be preserved at optimum generative con- Nevada, Utah, Idaho, and Arizona, US Geological Sur- ditions while directly adjacent to preserved traps. vey Digital Data Series DDS–69–L, 1–50, https:// The Dezful and Kirkuk embayments in the Zagros pubs.usgs.gov/dds/dds-069/dds-069-l/REPORTS/ and the early Zagros, the early Sevier and the 69_L_CH_2.pdf AQRAWI, A.A.M. & BADICS, B. 2015. Geochemical charac- inferred early Beni Sub-Andean are local relatively terisation, volumetric assessment and shale-oil/gas low-taper wedge-top phases favouring the formation potential of the Middle Jurassic–Lower Cretaceous and preservation of petroleum systems. Local source rocks of NE Arabian Plate. 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