Restraining And Releasing Effect Of Strike-slip Transition Structure And Its Control On Hydrocarbon Accumulation In The East Bohai Sea

The East Bohai Sea is the most favorable oil and gas accumulation area in Cenozoic Bohai Bay Basin,through which the "Tancheng-Lujiang" Fault Zone cross,leading to development of various strike-slip transition structures."Restraining and releasing effect of strike-slip transition structure and its control on hydrocarbon accumulation" has become a hot topic in researching the area.In this paper,based on the detailed description and classification of strikeslip transition structure in the studied area,we establish the quantitative methods to analyze restraining and releasing effect of strike-slip transition structure and reveal the correlation between restraining and releasing effect and petroleum geological conditions,such as the source rock,trap,gas and oil migration,and so on.As a result,the reservoir control of restraining and releasing strike-slip transition structure is revealed.The main achievements and conclusions are as follows:(1)Based on the detailed characterization and identification of the strike-slip fault population,the types of strike-slip transition structures are classified and the development of various types of strike-slip transition structures are clarified.According to their location,the strike-slip transition structures can be divided into three categories(5 types),which can be subdivided into restraining and releasing type on basis of local stress field derived from strikeslip transition.The bending-type transition structures develop at the bends of strike-slip fault.The development of the strike-slip transition structure between faults is controlled by the combination and lateral connection between the main strike-slip faults,so the transition structures between faults can be divided into three types: overlapping,duplex and conjugate type.The fault-tip transition structure mainly develop at the end of the main strike-slip fault,showing as a broom-like transition structure.For the restraining structures,the main fault is characterized with a steep dip close to the perpendicular,and the secondary fault is poorly developed or not developed.And the reverse faulting may occur,and the inner strata are easy to bend to form the uplift area.The main strike-slip faults of releasing structure show relatively slow dip and are mostly listric.The secondary normal faults are well developed,which control the subsidence of the strata to form sags.Controlled by the development of the Tanlu fault zone,the strike-slip transition structures are more developed in the eastern part of the study area than in the western part.(2)Based on the analysis of the stress,the calculation method of the strength coefficient of restraining and releasing effect in the strike-slip transition structure is established.As a consequence,the quantitative characterization of restraining and releasing effect in the strikeslip transition structure is realized.The ratio of derived compressive(or extensional)force perpendicular to faults(or fold)and the main strike-slip stress is defined as the restraining and releasing strength coefficient,which is cosine of rotation angle meaning strike-slip transition structure line turn counterclockwise to the regional strike-slip direction.When the coefficient is positive(or negative),indicating the restraining(or releasing)effect.The greater the absolute value is,the greater the intensity of releasing or restraining will be.Using this method,the restraining and releasing strength coefficients of the main strike-slip transition structures in the study area are calculated,and the restraining and releasing effects in the strike-slip transition structures are quantitatively characterized.(3)Based on the analysis of correlation between restraining and releasing strength coefficient and petroleum geological parameters,such as source rock,trap,physical properties of pore and permeability,formation pressure and hydrocarbon column height,the reservoir controlling effect is revealed,then the hydrocarbon accumulation pattern of different strike-slip transition structures is established.The releasing effect promotes the tectonic subsidence of the sags,which is beneficial to the development of source rock.The greater the absolute value of the pressure-relief intensity coefficient is,the greater the thickness of source rock and the greater the total organic carbon will be.The restraining effect leads to the increase of trap amplitude,and the larger restraining intensity coefficient is,the larger the trap amplitude will be.The restraining effect will lead to the deterioration of the porosity and permeability of the rock,which is beneficial to seal the hydrocarbon.On the contrary,the releasing effect is beneficial for hydrocarbon to migrate.The restraining transition structures are characterized with hydrocarbon accumulation in hanging wall and multi-layer,while the releasing transition structures are characterized of hydrocarbon accumulation in footwall and shallower layers.