Cenozoic Strike-slip Associated Structures Of Tanlu Fault And Their Reservoir-controlling Effect In Bohai Sea Area

Tanlu Fault Zone(TFZ)passes throughout the east part of Bohai Sea area(BSA).Its Cenozoic development and evolution was controlled by the combined action of upwelling of deep mantle and the subduction or collision of peripheral plates with the Eurasian Plate,which resulted in the strong dextral transtension of TFZ and resulting development of various transtensional strike-slip associated structures(SAS)between and by the branches of TFZ.These transtensional SAS have obvious differences with the typical ones beside strike-slip principal displacement zone(PDZ).Hydrocarbon exploration practices show most of proven reserves of Bohai Oilfield locate on SAS of TFZ.Thus,the formation and development of SAS is a restraining factor on hydrocarbon accumulation of BSA.Now only few preliminary and unsystematic researches have been done on SAS of TFZ in BSA,which somehow restrains the hydrocarbon exploration practices of Bohai Oilfield.With the purpose of enriching and improving the transtenional SAS theory and providing guides to hydrocarbon exploration of Bohai oilfield,this paper centered on SAS and their controlling effect on hydrocarbon accumulation,chose TFZ in BSA as studying object to analyze its structural features,then carried out researches on genetic types,distribution law of SAS of TFZ and their links with hydrocarbon accumulation.The results and insights showed below.TFZ in BSA is composed of at least 18 branches,which display the superimposed effect of extension and strike slip of different strength and can be divided into four types,strike-slip-extensional fault and three types of extensional-strike-slip fault.Above branches are not continuous,their tails and interior forms would be straight or releasing/restraining(single or double)bend types.Different branches could form extensional or contractional stepover and conjugate with NWW-trending branches of NWW-trending Zhang-peng Fault Zone and Qin-lv Fault Zone.It is above features of TFZ that provide the bases for the formation of various SAS.Under the influence of the form,stepover and assemblage of PDZs,the local stress derived by Cenozoic dextral strike slip of TFZ in BSA is different along TFZ,And according to the differences of development location,local stress properties and specific structural styles of SAS,tail SAS contains extensional horsetail splay and contractional wedge-shaped structures.The types of stepover SAS are determined by the stepover type of PDZs composed of stepover,extensional strike-slip duplexs and strike-slip transfer faults mainly formed in right-stepping stepover,while contactional push-ups and strike-slip relay ramps formed in left-stepping stepover.The types of bend SAS are closely related to bend forms,beside right double bend,extensional synclines or strike-slip duplexs would be formed,while contractional anticlines or transpressional diapir formed by left double bend.Conjugate superimposed stress zone SAS could be sub-classified into contractional anticline and extensional fault sag.As far as the distribution feature of SAS is concerned,only a few tail SAS are developed,which distributed scatteredly.Stepover SAS covered the widest range of area among branches of TFZ and distributed along its east branches from Laizhouwan Sag to Liaodongwan Depression,the most common stepover SAS is extensional strike-slip duplex.The number of bend SAS is the largest among all SAS.They mainly distribute along the bends of branches of TFZ,and most common bend SAS is contractional anticline.The development of conjugate superimposed stress zone SAS is closely related to the sinistral strike slip of NWW-trending Zhang-peng Fault Zone and Qin-lv Fault Zone,they only developed in Bodong and Bonan area.The dissection of typical hydrocarbon reservoirs showed that different types of SAS have different reservoir-forming conditions,and could establish various reservoir-forming patterns.In extensional SAS,such as extensional strike-slip duplex and horsetail splay,mainly fault-block or fault-nose traps are developed,the lateral sealing of source fault is poor,while its vertical channeling is good,so hydrocarbon mainly accumulates in shallow reservoirs or no oil accumulates.In contractional SAS,for instance,push-up,contractional anticline and wedge-shaped structure,mainly anticline or contractional fault-block traps could be found,the lateral sealing of source fault is good,while its vertical channeling is poor,so hydrocarbon commonly accumulates in deep reservoirs.In contractional-extensional composite SAS,various traps can form,such as fault-block,fault-nose and fault-anticline traps,the derived secondary faults developed in extensional SAS mainly serve as vertical channel paths,while the PDZ beside contractional SAS blocks the lateral and vertical channeling of oil and gas,so the hydrocarbon can accumulate in both deep and shallow reservoirs.Thus,contractional or contractional-extensional composite SAS is in favor of hydrocarbon accumulation and the latter has better reservoir-forming conditions.