| Qianbei is an old oil and gas exploration area which is north of Qianjiang depression in Jianghan Basin. Qianbei fault zone is a boundary normal fault which developed in sedimentary period of Qianjiang Formation. It includes four secondary tectonic units distributed in the upthrown side of Qianbei fault, such as Jingmen sag, Lexiangguan horst, Hanshui graben, and Yonglonghe uplift. There develops buried hills like Changshi anticline, the north of Zhongtan fault zone, and Zhongshi and Yanhe oil fields.The study area has been explored for 50 years. More than 350 exploratory wells and 128 development wells has been completed successively, and four sets of oil and gas formation were found, namely Miocene Guanghuasi Formation (N1g), upper Eocene Qianjiang Formation (E23q), middle Eocene Jingsha Formation (E22j), lower Eocene Xingouzui Formation (E21x). The resources according to Lever 3 evaluation standard amounts to 39.76 million tons, the residual resource amounts to 20 million tons, which proves high potential of oil and gas resource. Previous research has shown that the reservoirs, different sizes of faults, and distribution of numerous structure, lithology and complex traps especially develop near the source, in which has a good exploration prospect.Subjects such as structural geology, petroliferous basin analysis, seismic stratigraphy and geophysics sever as the theoretical guidance in the study. The research began from the pattern of basin structure, formation development, regional tectonic setting and other aspects by multidisciplinary methods and the comprehensive application of geological, well logging and seismic data. Based on the full understanding of predecessors’research results and fine horizon calibration,3D seismic data with high precision of Qianbei area has been carefully tracked and interpreted. The spatial distribution of faults was calibrated in great detail in order to fully dissect the tectonic development of Qianbei fault zone. On the basis of the tectonic dynamics background of Jianghan basin, the fracture characteristics and their development were deeply analyzed in study area and its peripheral zones by the restoration of erosion strata and simulation technology of profile structure. The textural and structural characteristics, the structural styles, the characteristics of the faults developed in the basin were systematically studied. And the structural evolution stages of Qianbei fault zone were discussed following closed behind.After the above analysis, the main results and understanding of this study are as follows:(1) By the use of geological, well logging and seismic data and the combination of the unconformity surfaces’features on the seismic section, horizons were calibrated according to the seismic synthetic record. Seven horizons including T1, T3, T5, T6, T6’, T7, T11 had been explained by their seismic reflection interfaces.(2) The big Qianbei fracture expresses a three-step structure of low-middle-high in vertical with a NNE strike and southeast dip. Qianbei fault, as the boundary between low-middle and high step, is also the north boundary of most Qianjiang Formation in study area. The formation has similar morphology and some tectonic inheritance. The structural types overall can be divided one bulge, two concaves and two slopes laterally, namely west slope of Qianjiang depression, bulge of Zhongsi fault-nose, two secondary concaves in north of Banghu syncline, and west slope of Tangkou bulge, while the tectonic framework of the upthrown side is characterized by one slope and one uplift.(3) The Qianbei fault in the east of work area is a normal fault which is the most important in this region. Its fault plane is complete and with ups and downs. The fault extends from east to west and finally to the east of Liang 1 and Liang 2 in northwest region. Some secondary faults in north east direction develop in the west.(4) The trap types are mainly fault-blacks and fault-noses which are controlled by extensional faults in tension environment during the Himalayan period. Concomitant with the Qianbei fault, Local traps distribute in groups and in zones. Some of them controlled by the extensional fault mainly develop on both sides of the fault.(5) The plane distribution of faults in the area has intense direction. There are mainly two groups of faults in NE and NEE. The plane combination of faults is complex, such as nearly parallel to the parallel type, the skew combination of main fracture and secondary faults or faults at the same level, and horsetail ones. The same complex diversification in vertical of faults express a combination of parallel ladder in sequence or reverse, another combination of forward and reverse of Y type, X type, inverted-Y shape, and λ shape, and additional combination of horst and graben, etc. Faults in study area are mainly distributed at the junction of the front downthrown side of Qianbei fault and Jingmen sag in the west. The activity phases of faults aroused mainly from the sedimentary period of Jingsha Formation to the fourth member of Qianjiang Formation and Jinghezhen Formation.(6) According to the seismic, drilling and logging data, Qianjiang depression has experienced five tectonic cycles of uplift and subsidence since Cretaceous and developed five large-scale unconformities. The evolution of Qianbei fault zone can be divided into five movement episodes.(7) The structural evolution of Qianbei area was studied by the balanced cross-section technique. The strata from Cretaceous to Paleogene in this area are divided into three tectonic cycle, and other evolutionary stages, such as fault depression, depression, strong fault depression, depression, uplift and denudation, depression. It was found that the various structures mainly developed during the sedimentary period of Jingsha Formation and the first member of Qianjiang Formation of Paleogene. They finally become unchangeable by the end of the sedimentary period of Jinghezhen Formation. The present complex tectonic framework has experienced the process of strong tension-weak tension-uplift. |
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