Early Cretaceous Sedimentary Systems And Their Structural Controls In Zhangwu Basin, NE China

Songnan early Cretaceous fault basin group is potential area of oil/gas exploration. Progressiveexploration and development has been carried out in Zhangwu basin, as a typical representative. Drilling and oil testing results show that, Zhangwu basin is full of oil, but only X2 and X8 well area reached industrial oil flow standard. In other parts of the basin, only oil and gas shows, but massive oil and gas reservoirs are not found. The oil and gas producing region of Zhangwu basin is located in the X2-X8 uplift which trends NNW. How the low uplift in the basin generated, how the sedimentary systems lie, and how the basin evolved becomeimportant problems to be solved in the research of Zhangwu basin. To solve these problems, not only can provide a reference for basin in the further exploration and development,also has reference value on the vicinity fault depression basins exploration and development.In this study, we will analysisdepositional systems and structural controls, and we will take the whole Zhangwu basin as the study area and the lower Cretaceous Jiufotang and Shahai formations as target stratas. According to the seismic section interpretation, fault and strat’s appearance and basin tectonic style are identified. With the use of the principle of balanced cross section, tectonic development history sections are protracted to regain the reconstruction of the basin filling process and tectonic evolution, and the basin development mechanism is discussed. Based on the logging curve, lithology logging and core data, whole basin sand groups are diveded throughcycle correlation and hierarchical control under the control of standard layers. The sequence stratigraphic framework was established, and the sequence stratigraphic models are summarized.The plane distribution of sedimentary system is confirmed and the sedimentary facies models are established. On three aspects of formation development, sedimentary system and reservoir forming, the control effects of structural- volcanic action on the basin is studied.1) Structural characteristics and evolution of the basinZhangwu basin is a half graben basin with down faulting at the eastern boundary and overlapping at the western boundary. The fault F1 is the controling fault, which isa listric normal fault steep at the top and gentle at the bottom. Fault F3, F2 are regulation faults which twisted while extending.In the Jiufotang and Shahai formation period, the basin experienced extensional stage and twisting stage. Extensional stage including stretching extensional period(equal to the lower Jiufotang formation) and transtension period(equal to the upper Jiufotang formation), tectonic inversion period(equal the end of Jiufotang formation). Torsional stage includes conversion extensional stage(equal to the lower Shahai formation), reverse period(equal to the upper Shahai formation).The most important low uplift in the basin, X2-X8 uplift, is the result of the stress field caused by the right lateral torsional stress field. The uplift zone is the favorable area of oil and gas in the whole basin.2) Sequence stratigraphic frameworkWells with completeformation are determined standard wells. According to the well logging standard curve and comprehensive logging, oil bearing layers are indentified in standard wells. After that, the standard layer is optimized to establish typical profiles. The key section of the standard well is established, and based on the standard well,comparing with the adjacent wells, and gradually spreading,the formation of the whole area is divided.The sequence interfaces are distinguished and the sequence stratigraphic framework is established, Jiufotang and Shahai formation is divided into 13 third order sequences and four secondary order sequence. 4 secondary order sequencesare equivalent to lower Jiufotang formation, upper Jiufotang formation, lower Shahai formation and upper Shahai formation. 13 third order sequences are seven sequences equivalent to sand sets 7 to 1 of Jiufotang formation,3 sequences equivalent to 5, 4-3, and 2 sand sets of the lower Shahai formation and 3 sequences equivalent to 1-1,1-2, and 1-3 sand sets.According to the above, there are 8 types of 5 kinds of basin sequence models. Including the steep slope kind, which can be divided into 2 types, coarse-fine-coarse symmetry type under normal tensile extensional tectonic control and fine-coarse-fine partial symmetry type under strike slip control. And gentle slope kind can be divided into 2 types, coarse to fine non symmetry type due to the positive tension extensional tectonic control and fine-coarse- fine type under strike slip control. Deep water type is fine mixed of coarse under normal tensile extensional tectonic or strike slip control.Deep groove type isfine-coarse- fine underpositive tension extensional tectonic control. Platform type isfine-coarse- fine under oblique tensional strike slip tectonic control.3) Provenance and depositional systemRocks in Zhangwu basin can be divided into 4 major categories, 15 categories, 20 sub categories and 47 basic types. 4 major categories including terrigenous clastic rock type and volcano lava type and pyroclastic rock type and intrusive rock type(dike rock). 15 categories include conglomerate, sandstone, limestone and 20 sub categories include andesite, basalt, etc. 47 kinds of basic types including the complex components of conglomerate, glutenite etc. The sedimentary facies(facies, sub facies and microfacies) of Zhangwu basin are identified, and 4 facies, 10 sub facies and 20 micro phase. Four phases are lacustrine facies, fan delta facies, delta facies and volcanic facies, and ten sub facies include shallow lake, semi deep lake, underwater fan etc.20 microfacies include turbidity, still water mudstone, underwater channeletc.Depositional systems include the eastern depositional system, the western depositional system, the southwest depositional system andthe northeast depositional system. The main depositional system is the eastern depositional system and the western depositional system. The eastern depositional system is the source of the volcanic matterand fan delta depositional system, the western depositional system is the weathering material source mixed with volcanic matter-braided river delta depositional system.4) Structural controlsWhile basin’stwisting and expanding, tectonic subsidence resulted in the change of relative lake level, and therefore the sequence development was controlled. In particular, large-scale sediment accumulation in the transgressive system tract is a prominent feature of the Zhangwu basin.The volcanic activity associated with tectonic activity provides a large amount of sediment, and makes the eastern and western sources of the basin have a difference. Hydrocarbon source rocks are provided by coals and shale rocks of siltationor swamping coursed bythe development of the syngenetic uplift.X2- X8 low uplift produced by tectonism and on both sides of the source sagconstitute the near source combination, that is conducive to the formation of oil and gas reservoir. The X2- X8 area becomes favorable area.