Structural Geometric, Kinematic Features And Three-dimentional Restoration Of Tongnanba Anticline

Tongnanba Anticline geographically lies in in the middle of Tongjiang county, Nanjiang county and Bazhong city, around the northeast edge of Sichuan province.It is also tectonically located at the transition zone of the northern Yangtze plate and the South Qinling plate, as well as part of foreland uplift belt of Micang – Daba foreland basin in the northeast Sichuan basin. Its structural formation and evolution is mainly controlled by the tectonic movement of Micangshan and Dabashan. To research on the Tongnanba Anticline is of great scientific significance to reveal the mountain-basin coupling relationship in northeast Sichuan, tectonic superposition and intra – continental deformation effect, and is of practical importance to guide the oil and gas prospecting and exploration in the northeast Sichuan. By using the latest 2D and 3D seismic profiles and drilling data in the northeastern Sichuan basin and combining with the field data and other evidence, this paper have established finely geometric and kinematic model of Tongnanba Anticline by studying its structural features. And through the structural interpretations and analysis of regional long cross sections, as well as the analysis of apatite fission track thermal simulation regionally, the influence on the formation and evolution of Tongnanba Anticline from the Micang Shan and Dabashan is discussed. Lastly, by analyzing and applying the three dimensional structural restoration theory on the Tongnanba Anticline, this paper discusses its structural control on the hydrocarbon accumulation. The study mainly has the following understanding:(1)On the one hand, the NE structure of Tongnanba Anticline is divided into three segment on the plane(i.e., the southwestern, the middle and the northeastern), the structural style transforms from the hybrid of fault bent fold and the breakthrough – fault propagation fold at the southwestern segment to the hybrid of structural wedge and front – fold accommodation fault at the northeastern segment. The NW structure of Tongnanba Anticline, on the other hand, exhibit stratified structural superposition style with upper part superimposed by a series of NW faults and related folds and lower part superimposed by imbricate and duplex structure in the Heichiliang area.(2)Controlled by Triassic Jialingjiang – Leikoupo gypsum – salt detachment layer,Silurian shale detachment layer and Cambrian shale detachment layer, the study area is therefore vertically divided into 4 structural layers(i.e., the upper, the middle, the lower and the deeper) and exhibits multi – detachment deformation system feature with different structural style and intensity in different layer.Among these 4 structural layers, the middle is the most deformed layer with the most shortening rate by developing a seris of subfolds and faults regionally and imbricate and duplex structure locally.(3) The formation and evolution of Tongnanba Anticline is controlled by Micangshan and Dabashan. Specifically, the displacement from Micangshan moves along the pre – Sinian detachment to the interior basin, and is considerably accommodated in the Micangshan region with large – scale thrusting developed and the basement involved in the deformation. Then it continually moves to the Tongnanba area and leads to the folding of Tongnanba area. The displacement from Dabashan, however, mainly moves along the Triassic Jialingjiang – Leikoupo gypsum – salt detachment to the interior basin, and is gradually accommodated by the overlying layers, leading the formation of a series of NW faults and related folds without the basement involved in the deformation.(4)The prototype outline of Tongnanba Anticline was formed during the Xujiahe Formation – the Early Jurassic sedimentary period. In the Late Jurassic, affected by the large – scale thrusting from Micangshan, the NE structure of Tongnanba Anticline was basically developed. During the late Early Cretaceous – the early Late Cretaceous, the piggyback – thrusting propagated from the Micangshan to the southeast and the NE structure of Tongnanba Anticline was therefore basically formed. In the late Cenozoic, affected by the tectonic activity of the Qinghai – Tibet plateau, the NE structure of Tongnanba Anticline was finally formed. On the other hand, a group of NW imbricate structure and faults and related fold were superimposed on the Tongnanba Anticline in the Late Cretaceous and Late Miocene respectively by the thrusting- nappe movement from Dabashan.(5)Through the analysis of three dimensional structure restoration applied on multiple layers by employing Gocad software, the high Dilatation mainly distributes in the Hebachang high, the two flanks of Tongnanba Anticline and Malubeinortheastern slope with belt or sheet morphology, which indicates that these areas are relatively stress concentrated and are where the potential large fracture zones develop. Combined with the of reservoir feature, this paper predicts that the Permian Changxing Formation of the Malubei northeast slope – Heichiliang area is the most favorable oil and gas prospecting zone.