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Structural Characteristics of Ultra-Deeply Buried Structures in the Northern Longmen Shan Fold-Thrust Belt, Sichuan Basin, China

Abstract

The Longmen Shan Fold-thrust Belt (LFTB) is located in the eastern boundary of the Tibetan Plateau. The early-stage exploration in the Northern LFTB primarily focused on the shallow Cenozoic and Mesozoic traps. However, the early acquired seismic data failed to present high-resolution features of fault geometry and structural deformation of the ultra-deep buried structures beneath the thrust nappes, due to the low signal-noise ratio of seismic survey and complicated structural deformation. The latest data obtained in recent years by CNPC, such as 3D seismic survey, gravity-magnetic-electronic data, and borehole data, highly improved the understanding in the structural characteristics of the northern LFTB. The northern LFTB can be sub-divided into three structural segments, including upper thrust nappes, lower imbricated structures and ultra-deeply buried structures. The frontal faults separated the imbricated structures from the ultra-deeply buried structures, which formed overturned fault-propagation folds above the fault tips. The hanging wall of the frontal faults have been subjected to two-stage intense structural deformation in Late Indosinian and Himalayan periods, respectively. Large-scale ultra-deeply buried upper Paleozoic structures are developed in the footwall of the frontal fault. The ultra-deeply buried structures present stepwise decreasing altitude southwards, and formed duplex structures that are accounted by two detachment layers (i.e., the Cambrian mudstones/shales at bottom and the upper Triassic evaporates atop). The ultra-deeply buried structures are primarily formed during the Himalayan period. Integrated with analyses on hydrocarbon accumulation conditions, this study has provided good hints on petroleum exploration in the northern LFTB. Six prospected wells, with depth exceeding 7 km, have been drilled to reach the frontal zone of the ultra-deeply buried upper Palaeozoic structures in recent years. The six ultra-deep wells got high industrial gas production, with maximum 876 × 103 m3 /D, which solidities confidence in the subsequent exploration in the ultra-deeply buried structures beneath the frontal faults of the imbricated structures.