| This study considers basin dynamics, tectonic geology, and the integration analysis of structure-sequence-sedimentary research methods as a guide. Based on the refined well-seismic calibration and structural seismic geological interpretation, using the recent 3D seismic data and abundant of drilling and logging data, the structures of Dongying sag in Paleogene were studied in details. The structural transformation interface and its geological characteristics of Dongying sag in Paleogene were figured out. Structure types in different evolution stages and the rules of fracture activities were also been summarized, the stress mechanism and the formation mode of structural transformation were discussed. The thorough analysis about transtensional structure style, the faults activity rule and its evolution mechanism would provide an important evidence for the study of transtension function mode in Bohai Bay basin. The transtensional effects in Dongying sag developed during the period of the formatting of the main hydrocarbon source rock segments and the key sequences. Therefore, this study would be very helpful for oil and gas exploration in this area.Principal understandings from this study are as follows:1. The isochronous stratigraphic framework in Paleogene in Dongying sag was established and the interface features of each secondary structural sequence were clarified as well. Most of all, the structural transformation interface T6x (from extension to transtension) was pointed out and its geological characteristics were analyzed.According to the agreement principle among stratigraphic structure, logging response, and seismic response, the main structural sequence interfaces of Dongying sag were defined. The first sequence interface includes two primary sequence interfaces, Cenozoic bottom interface Tr and the Paleogene top interface T1, respectively. The secondary sequence interfaces includes 3 sequence interfaces, the bottom interface T8s of Es4, the bottom interface T6x of Es3, and the bottom interface T2 of Esl. Among them, as the seismic unconformity characteristics were clearly showed and easily been recognized in logging, the bottom interface Tr and the top interface T1 in Paleogene, and the bottom interface T2 in Es1 were consistent with the that of previous studies. The bottom interface T8s is a newly calibrated and interpreted interface in this study, with the characteristics of strong amplitude on seismic and good continuity in the interior basin. The strata up and down the interface have a strong inheritance development characteristic.In addition, as a significant interface of Dongying sag in Paleogene, T6x was considered to be the bottom interface of Es3 in this study. The main reasons are as follows:(1) In lithology, the interface lies in a large set of oil shale at the bottom of the basin; the northern steep slope belt is situated on top of large sets of sandy conglomerates formed during Es4; the gentle slope belt is situated on top of beach-bar sand and the square king Wang Qianshan area on top of the limestone. Therefore, it should be an abrupt changing interface in lithology. (2) In well logging, the interface is situated at the bottom of significant abnormal R, while the bases values of GR and SP change as well. Thus, it should be a logging response abrupt interface. (3) In seismic analysis, it lies at the bottom of a group of seismic reflection wave in good continuity and strong amplitude. Above the interface, the structure of the seismic reflection layer of good continuity is clear. Nevertheless, under the interface, the seismic reflecting layer structure is not easy to identify, and the continuity of reflecting layer is poor, which shows that it is a sedimentary discontinuity interface. (4) In basin structure, the structural patterns of the basin above and under the interface have undergone a fundamental changing, and the center of subsidence has at the same time migrated from the east to the west. (5) Besides, this study further defined the contact relationship between interface T6x and T6, which coexisted in the internal part of the sag. However, towards the edge of the salient or buried hill zone, interface T6 would overlap to T6x. The developing time of this interface T6x agreed with that of the changing of subduction direction of Pacific plate. In conclusion, it is considered that the interface T6x is a regional structural transformational interface (or structural changing interface) during the fault depression period of Dongying sag, which plays an important controlling role in the basin evolution, configuration of sedimentary system and oil or gas accumulation.2. The structural transformation process of Dongying sag was studied systematically. Two different structural types (the former one of extension and the later one of transtension) were identified. Besides, the plane distribution regularities of structures and the structural deformation features were analyzed as well. Thereafter, the transtensional structure system were confirmed during late Eocene in Dongying sag.It is considered that the extensional structure mainly developed during the Paleogene period of Ek-Es4 in Dongying sag with refined structural interpretation of seismic sections, in which a great number of extensional structural types were identified. These extensional structures mainly include a series of structural assemblages, such as plate-type normal fault-rotating half graben structures, shovel-like normal fault-rolling half graben structures, ramp-flat faults-compound half graben structures, and also include the synclastic (or reversed) warped fault block, graben horst fault block, fault block buried hills, and bidirectional reverse drag in extensional structural system. Meanwhile, the transtensional structure types developed in Es3 during Dongying Formation sedimentary period were also been identified. In seismic section, it is figured out that the transtension structure in the study area is mainly characterized by negative flower shaped structure, narrow syncline, and transformational fold, and characterized in the plane by large en-echelon fault combined with broom-like fault, which controlled the distribution of the subsidence center. The transtensional faults appear in pairs and are a couple forces in dynamics. The forming subsidence centers show a rhombic shape type. At last, it is established that the rhombic distribution of transtensional structure system in the subsidence center were controlled by three rows of nearly paralleled NNE Y-shear (especially the Gaoqing and Chunhua shear zone) and NE R-shear (especially Lijin and the central shear zone) in Es3 during Dongying Formation sedimentary period.3. Quantitative analysis about Cenozoic faults activity of main fractures together with its temporal-spacial distribution regularities in Dongying sag and the restoration of the Mstory of basin subsidence were both carried out progressively, which clearly described the evolution process changing from extensional structure to transtensional ones during the Paleogene period in Dongying sag.With the ancient fall and its calculation methods, syn-sedimentary faults activities developed in Paleogene in Dongying sag were analyzed and calculated. The results strongly indicated that these faults were characterized by the features of episodic activities and periodicity activities. Among them, the vast majority of Binnan fault, Shicun fault and Chennan fault were dominated during Ek-Es4 activity period, represented the extensional structure activities. However, the faults in Gaoqing, Bindong, Lijin, Shengbei, and the central shear zone along with the faults in Chen Guanzhuang-Wang Jiagang were mainly controlled during the Es3-Ed activity period, and many of them were newly coming out faults, characterized by the transtensional structural activities. Moreover, some faults possessed inheriting activities, but still varied a lot in different periods of fault activities. Such regular activities in different period of fracture reflected the stress field evolution of the basin structure. In addition, from the restoration of the subsidence history about key seismic section and interpretation of the balanced cross section in SN direction and NW-SE direction in basin’s central part, it clearly showed that although the central basin subsidence rate in Es3-Es2 sedimentary period was the same or slower than that in Ek-Es4 period, horizontal extension rate was larger than that of Ek or Es4 sedimentary period. All above indicated that horizontal shift in Es3-Es2 period was greatly strengthened, which was mainly caused by strike-slip activity according to the horizontal shift direction. Thus, the corroboration indicated that the structural evolution during this period in Dongying sag had been strongly influenced by the transtensional structural activities.4. The dynamic mechanism of transtensional structural systems in Paleogene in Dongying sag of Bohai Bay Basin was discussed, and the stress patterns that changed from extensional type to transtensional one in Dongying sag had also been preliminarily established.During Ek-Es4 period (65～43.5Ma), it showed retrogressive subduction characteristics of the Pacific plate towards the Eurasian continent for sudden decreasing of the subduction speed, which resulted in the extensional thinning of lithosphere in the direction (NNW-SSE) of subduction and retreat of the Pacific plate, and caused a typical NNW-SSE extension in this period in Bohai Bay Basin. Besides, a series of NE trending basins developed in this period. Therefore, in this period, Dongying sag also had the feature of NNW-SSE trending extension. Up to Es3-Es2 period, the subduction direction of the Pacific plate changed from NNW to NWW, which correspondingly caused the dextral strike-slip activity occurred in Tanlu fault zone. Consequently, the stress field of the whole Bohai Bay Basin should be dominated by the dextral strike-slip faults activities. Furthermore, Tanlu fault zone was dispersively distributed in Dongying sag and its eastern area, where several branches of strike-slip faults developed. In this study,4 NNE strike-slip branches and 2 NE R-shear zones were identified, where in the most significant control effects on the basin came from Gaoqing structural zones (Y-shear), Lijin structural zone (Y-shear) and central R. shear zone. Under the double influences of the dextral strike-slip effect and regional NW-SE extension effect, the basin was in the transtensional structural stress systems, resulting in the most significant subsidence of the basin in stress concentrated area. The subsidence center of Dongying sag in this period developed along Gaoqing structural belts and Lijin structural belts, and formed a force couple with Chunhua structural belts and the central structural belt, which togetherly controlled the rhombic subsidence center of the basin. Nevertheless, the transtensional structural belts (in Wangjiagang) developed in the southeast of Niuzhuang subsag, but no corresponding force couple existed in the northwest. So the formed triangle-like basin shape lacked an angle of the rhombus. Up until Esl-Dongying Formation period, the subduction acceleration of the Pacific plate leaded to an obvious enhancement of dextral strike-slip activities in Tanlu fault zone in the east of the basin and Lanliao fault zone in the west. The subdcution increase together with the extensional stress field in this period leaded to the formation of an oblique strike-slip pull-apart basin in Bohai Bay Basin between the two major strike-slip zones, while the nearly SN tensile stress field was formed in the central part of the basin, which was why a series of near EW synsedimentary faults developed in this period in Dongying sag, especially the Xinzhen structural faults and Pingnan faults controlled the distribution of a subsidence center. |
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