Abstract
Deep-seated faults and folds of foreland basin systems have become important exploration targets in the recent years because they are crucial in controlling fluid migration and hydrocarbon accumulation. In this study, we analyzed the characteristics and formation history of these structures in the northwestern Sichuan Basin using recently acquired two-dimensional (2D) and three-dimensional (3D) seismic data. The seismic interpretation revealed that the thrust sheets, tectonic wedges, and foredeep were well developed in the northwestern Sichuan Basin from the mountain to the basin. Forward thrusts, fault-bend folds, and wedges are the main types of structures in the thrust sheets and tectonic wedges. The deep-seated faults and folds were easily recognized in the high-resolution 3D seismic data. The imbricate thrust faults that merged into detachment layers of the Lower Cambrian are the main types of structures in the foredeep, and they show a prominent strike-slip influence in the horizontal direction. The formation of these structures in the foredeep in the northwestern Sichuan Basin mainly endured two stages of thrusting, including those during the Middle-to-Late Triassic and Cenozoic. Based on the tectonic evolution and seismic data, we infer that these deep-seated faults and folds in the foredeep may have formed earlier than the northern Longmen Shan fold-and-thrust belts and they may have been initially active in the late of Early Triassic and reactive during the Cenozoic. Furthermore, evaporites in the Lower and Middle Triassic were crucial in forming these structures. The petroleum exploration data suggested that the deep-seated faults can facilitate hydrocarbon accumulation. The thrust faults in the foredeep were more likely to act as migration pathways for fluids instead of sealing barriers along the horizontal direction. The interconnected reservoirs of deep-seated folds possess a great potential to allow large-scale hydrocarbon accumulation. Our study provides a good example for evaluating the hydrocarbon exploration potential in the deeply buried area in the sedimentary basin.
Highlights
Fold-and-thrust belts are widely distributed in continental collision zones [1,2,3] and subduction belts [4,5,6,7] around the world, and they form the most important units of a foreland basin system
Several studies on the geometry [4,5,6,7], kinematics [2, 8], and formation mechanism [9,10,11,12] of foldand-thrust belts have been conducted via different methods, such as seismic interpretation [13], numerical as well as physical modeling [14,15,16,17,18], and tectonic thermochronology [19, Geofluids
Owing to the restrictions in petroleum exploration, most hydrocarbons have been found in the thrust wedges or hanging walls of thrust faults, such as in the Zagros and North American fold-and-thrust belts [25, 26]
Summary
Fold-and-thrust belts are widely distributed in continental collision zones [1,2,3] and subduction belts [4,5,6,7] around the world, and they form the most important units of a foreland basin system. The Longmen Shan fold-and-thrust belt (LSB) along the eastern margin of the Tibetan Plateau defines the west boundary of the Sichuan Basin, which is one of the largest active fold-and-thrust belts in western China with intricate structures and sedimentary sequences as well as a complex petroleum system. The LSB can be defined as an area developing series of parallel NW-dipping thrust faults and forming extremely steep topographic gradient between the Tibetan Plateau and the Sichuan Basin. In this study, combined with the oil production data, we used recently acquired high-resolution seismic and well data to characterize deep-seated thrust faults and related folds and discuss the influence of the formation and activities of these structures on the accumulation and migration of hydrocarbon fluids
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