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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 deformation 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, preservation and destruction of petroleum accumulations. The Zagros, Pyrenees, Sevier and Beni Sub-Andean convergent 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 detachments 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 consumption 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 created 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 worldwide discovered hydrocarbon reserves are within the fold and thrust belts along convergent plate boundaries (Cooper 2007). Oil and gas exploration in fold and thrust belts is risky due to the complexity in tectonic style, structural a...

Apatite (U-Th)/He (AHe) thermochronometric results are integrated with geologic cross sections, structural relationships, and stratigraphic data to reconstruct the growth of the NW Zagros orogenic belt in the Kurdistan region of Iraq. Prolonged exhumation is documented across the belt with deformation advances and retreats from~14 Ma onward. After in-sequence propagation of deformation during middle to late Miocene times, preserved growth strata and AHe data show a deformation retreat by latest Miocene time (~5 Ma). In the NW Zagros, the Phanerozoic succession contains two principal décollements in Lower Triassic and middle Miocene units. The Triassic strata are interpreted as the main décollement for a thinskinned system that was dominant during most of the Cenozoic. By~8-5 Ma, the fold-thrust belt shifted to basement-involved deformation in association with growth of the mountain front flexure and reactivation of frontal structures. The shift from thinskinned to a hybrid thin-and thickskinned mode of shortening may reflect variations in the mechanical behavior of the upper crust and the presence of inherited basement discontinuities. On the basis of two NE-SW balanced cross sections spanning the NW Zagros, the estimated total minimum horizontal shortening is~18.2 km (6%) in the central and~16 km (7%) in the southern sectors of the Kurdistan region of Iraq. These findings suggest that the evolution of the NW Zagros orogenic belt was likely driven by the mechanical stratigraphy of the sedimentary cover, inherited basement discontinuities, and the dynamic and thermomechanical effects of potential slab breakoff and lithospheric mantle delamination events.

Spatiotemporal constraints for Late Cretaceous tectonism across the Colorado Plateau and southern Rocky Mountains (northern Arizona−New Mexico, USA) are interpreted in regards to Laramide orogenic mechanisms. Onset of Laramide arch development is estimated from cooling recorded in representative thermochronologic samples in a three-step process of initial forward models, secondary HeFTy inverse models with informed constraint boxes, and a custom script to statistically estimate timing of rapid cooling from inverse model results. Onset of Laramide basin development is interpreted from increased rates of tectonic subsidence. Onset estimates are compared to published estimates for Laramide timing, and together suggest tectonism commenced ca. 90 Ma in northwestern Arizona and progressed eastward with later onset in north-central New Mexico by ca. 75−70 Ma. The interpreted sweep of onset progressed at a rate of ∼50 km/m.y. and was approximately half the 100−150 km/m.y. rate estimated for Late Cretaceous Farallon-North America convergence during the same timeframe. Previous suggestions that the Laramide tectonic front progressed at a rate similar to convergence via basal traction are not supported by our results. We thereby suggest that (1) a plate margin end load established far field compression and that (2) sequential Laramide-style strain was facilitated by progressive weakening of North American lithosphere from the dehydrating Farallon flat slab. Results are compared to models of sweeping tectonism and magmatism in other parts of the Laramide foreland. Discussions of the utility of the custom script and the potential for stratigraphic constraints to represent only minimum onset estimates are also presented.

The giant Dukhan Field is a mature oil and gas field located onshore in Qatar, approximately 80 km west of Doha (Figure 1). The Dukhan structure is a north-south plunging anticline approximately 70 km long by 8 km wide. The field was discovered in 1939 and first production occurred in 1949, with more than 750 well penetrations drilled since then. Major reservoirs are the Upper Jurassic Arab C and Arab D formations, the focus of this study, which contain oil and oil with associated gas, respectively. Oil and associated gas are also produced from the Middle Jurassic Uwainat formation, and the Permo-Triassic Khuff formation contains non-associated gas. The origin of the Dukhan structure is interpreted to be the shallow expression stemming from subtle reactivation of basement faulting that originated during the initial amalgamation of the Arabian plate in the Late Proterozoic. A genetic model for the evolution of the Dukhan structure was generated that establishes a context for interpretation of the currently poorly imaged faulting at Dukhan, and provides a model of the timing, distribution, and potential reservoir impact of subseismic faulting. In addition, the regional and field scale evaluation of structure integrated with geochemical analysis provides an enhanced approach to interpret structural evolution, charge history, and bitumen distribution for the Jurassic reservoirs. A variety of geochemical analytical techniques have been applied to both rock and fluid samples from Dukhan, including fluid inclusion volatile (FIV) analysis, high-resolution compositional analysis of core samples from bitumen/tar mat intervals, and high-resolution compositional analysis of produced hydrocarbon fluids. These geochemical analyses provide further support to the interpretation of the field's structural evolution. Several key observations related to the geochemical analyses are:•Light hydrocarbon analyses indicate that the Arab C, Arab D, and Uwainat reservoirs share similar sources, but are not in chemical communication. Similar oil composition within each reservoir indicates that there is no lateral compartmentalization.•An early charge of oil in the Arab D was partially displaced later by gas.•Oil-filled inclusions are present well below the estimated original oil-water contacts (OOWCs), and suggest trap modification after oil migration, or the active migration of oil through these strata.•Multiple levels of hard, pore-occluding bitumen occur within the Arab D reservoir and correlate roughly to paleoclosures. Initially, this relationship was taken to suggest that the bitumen distribution might reflect paleo oil-water contacts that could track the Dukhan trap evolution. Subsequent geochemical evaluation suggests instead that bitumen development is most likely the result of gas de-asphalting and most likely relates to the Late Cretaceous and Tertiary charge of light hydrocarbons and gas. These observations provide a strong technical foundation for an improved understanding of field compartmentalization and the controls on original fluid distribution. Accurate models for fluid distribution and structural evolution are critical inputs to 3D reservoir models, and also provide support for future field development decisions. As an example of this utility, these models will assist in interpretations of irreducible and residual water saturation distribution across the field.

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