Lithologic Trap Prediction Of Paleogene Under The Exploration Degree Area Of Bohai Sea

Bohai sea area is a faulted basin developed in Huabei craton. In the tectonic setting of multiple phase faulted depression, multicycle superimpose and multiple genesis mechanism, the fifteen main chronohorizons including two 1^st-order sequence boundaries, three 2^nd-sequence boundaries and ten 3^rd-order sequence boundaries can be recognized in the Palaeogene of Bohai sea area. The Palaeogene in Bohai sea area can be divided into one 1^st-order sequence, four 2^nd-sequences and fourteen 3^rd-order sequences.The paper builds the models of the sequence framework and sedimentary systems constituent of the different structural units in Bohai sea area in Palaeogene. Bohai faulted depression in Palaeogene can be divided into four kands of 3^rd-order structural units which are faulted steep slope zone, gentle slope zone, uplift zone in the sag and sag-axial zone. The physiognomy shapes and characteristics of the accommodation and base level change in the different structural units are very different, which results in the different sequence models in the different structural units.In Liaodongwan area and Bozhong area, the distribution of 4^th member of Sha He Jie Formation-Kong Dian Formation is limited. The distribution of 3^rd member of Sha He Jie Formation which is controlled by the boundary faults is very wide. The thickness of l^st and 2^nd member of Sha He Jie Formation is steady in the whole area, although it is thin as a whole. There are different deposition characteristics in the different period of Dongying Formation, which forms the different strata distribution characteristic. The depocenter of Palaeogene in Liaodongwan area located in the north of Liaozhong Sag and it moved from south to north of Liaozhong Sag. The depocenter of Palaeogene in Bozhong area with good inheritance located in the west of Bodong uplift.The paper analyses the characteristic of sedimentary system in the area. There are seven types of sedimentary systems including alluvial fan, fan delta, braided delta, fluvial delta, sublacustrine fan, carbonatite platform and lacustrine. In 3^rd member of Sha He Jie Formation, the coarse clastic deposit from the proximal and internal source is developed. The proximal sublacustrine fan is developed in the steep slope. Fan delta is developed in the gentle slope. In 1^st-2^nd member of Sha He Jie formation, the deposition is proximal and internal still and the lake district expands farther. The sedimentary system in the sag area is mainly the semi-deep lacustrine facies,and it is mainly proximal sublacustrine fan in the steep slope existing partly the fan delta, and it is mainly fan delta in gentle slope existing partly the sublacustrine fan. In 3^rd member of Dongying Formation, the low stand system developed the sublacustrine fan and the high stand system developed deep or semi-deep lacustrine facies except for the HST of Liaodong zone that developed braided delta. In 2^nd member of Dongying formation, the fluvial delta in the long axis and braided delta in minor axis are developed in the whole area. In 1^st member of Dongying formation, the topography was gentle in the whole area and the lake was shallow and the sedimentary system was mainly the shallow lacustrine or the delta plain in the most of the areas.It is the complicated and ordered fault systems with different time, different scale, different attitude and different attribute in the Bohai sea area in Palaeogene that make the slope break zone in the basin have obvious characteristic. The slope break zone in Paleogene in Bohai sea area can be divided into 4 types, viz. extensional boundary faulted slope break zone, strike-slip boundary faulted slope break zone, depositional slope break zone and basement antecedent topographic slope break zone according to the geneses, the plane combination styles and the differences in sedimentary facies control. The extensional boundary faulted slope break zone can be divided into single-fault steep slope break zone, step fault slope break zone and transitional structure slope break zone. Although the plane combination styles of the single-fault steep slope break zone are complicated, there are mainly two types, viz. straight style and "wall-corner" style. Step fault terrace slope break zone can be divided into synthetic step fault slope break zone and antithetic step fault zone according to the relation between fault trend and center of subsidence. The classification of transitional structure slope break zone is complex. Transitional structure slope break zone in Bohai sea area can be divided into strike-slope transitional structure slope break zone and transverse uplift transitional structure slope break zone according to the differences in the sandstone control characteristics in the view of the oil and gas exploration. The sand-control characteristics of different slope break zone are different. The slope break zones in continental faulted basin are the main location in which the sandstone is rich. But this doesn't draw a conclusion that the sand body must be found in the slope break. The matching of "hill (provenance system)-canyon (sediment discharge system)-slope (slope break zone)-boundary (base level system)" is the necessary condition of the sandstone enrichment in continental fault basin.There are 6 kinds of lithologic traps including stratigraphic unconformity trap, stratigrapbicoverlapping trap, sandstone updip pinchout trap, sandstone lenticular trap, tectonic-lithologic compounding trap and lithologic-tectonic compounding trap in Bohai sea area, Palaeogene. The paper analyzes the distribution models of lithologic trap in the different tectonic zone in Palaeogene in Bohai sea area resulted from the characteristic constituent styles of the sequence and depositional system tract. According to the couple of the sequence, sedimentary system and tectonic setting, there are 5 blocks in Liaosongwan area such as the eastern steep slop of Liaozhong Sag and 7 blocks in Bozhong area such as the southwestern low uplift of Shijiutuo Uplift, which are favor of lithologic trap development.The high resolution sequence stratigraphy and sedimentary system are analyzed and the lithologic trap are recognized and described in Bozhong 2-1 block in the west of 427 Tectonic Zone and Jinzhou 31-6 block in the center of Liaozhong Sag in the paper. Bozhong 2-1 block developed the tectonic-lithologic trap with a good exploration potentiality besides the structural trap. There are several turbidite sandstone lithologic traps with a good exploration potentiality near Jinzhou 31-6 lithologic trap. The two blocks are the tangible area of the lithologic trap exploration in Bohai sea area.Furthermore, the technique methods of the lithologic trap predication in the low degree of exploration in Bohai sea area in Palaeogene are summarized in the paper. The core contents of the lithologic trap research under the exploration degree are the structure research seeking the development setting of the lithologic trap, the couple research of "provenance system- sediment discharge system-slope break system-base level system" seeking the sand body, the couple research of the sand body and structure seeking the favor zone of the lithologic trap development and the integration analysis of seismic and geologic data for describing subtly the lithologic trap.Sequence stratigraphy, Sedimentary system,Lithologic trap Predication, Under the exploration degree area, Palaeogene, Bohai sea area...