| Concealed fault zones are a type of weak deformation fault zone that form in sedimentary covers and have expressive forms of faults that show in the early-middle stage of their formation and evolution processes.Without discernible characteristics,they are difficult to identify and are easily neglected.Concealed fault zone is a tectonic phenomenon commonly arising in the cover of sedimentary basin,which is usually displayed by regular secondary faults,reserviors,plays,facies,depressions,buried hills.It is supposed to be closely related to the hydrocarbon accumulation and distribution.According to the geneses,concealed fault zones can be classified into four types: strike slip of basal faults,differential uplift and subsidence of basin basements,rotational shearing of terranes(buried hill)and the cover slip.On based of the analysis of concealed fault zones evolution and formation mechanism,Bohai Bay basin is chosen as the survey,concealed fault zones induced by strike slip basement fault are studied as subject,in addition,basal gravity and magnetic data,structural graph,seismic cross-section,coherence slice,sedimentary facies and reserviors distribution,above of all,different concealed fault zones in geneses and scales are identificated in Qikou,Bamianhe,Wangjiakgang,Bodong,Penglai areas.Except for Bodong Concealed Fault Zones formed by the distributed strike slip stress field,the rest of concealed fault zones are induced by basal fault strike slip.In the plane,concealed fault zones are characterized by the string and belt-shape structure,such as the en-echelon,string and belt small faults,discontinuous faults distributed by certain direction and a series of sedimentary fans or oil plays.In the section,concealed fault zones are characterized by the half flower structure,unobvious flower structure and anticline-shape flower,and their formation mechanism consist of basement fault strike-slip(parallel strike-slip,discrete strike-slip and oblique slip)and block rotation.Studies suggest that the plane structural styles of Bohai Bay basin have the weak deformation-weak en echelon,medium deformation-intense en echelon,moderate and intense deformation-intermittent band,intense deformation-continuous band.The structural styles in section of concealed fault zones in Bohai Bay basin have a through three-layer style,the style of only lower cover exist and the through cover style.And the planar pattern has a ribbon,a parallel strip,a cross-shaped and a grid.According to the formation mechanism of concealed fault zones and the characteristics of cover fracture,analogue modelling verified the formation process of basal strike slip concealed fault zones,and used qualitative and quantitative methods for the distribution of fractures,the density of fractures,fractures size,pull-apart structure and uplift in the sedimentary cover during concealed fault zones evolution.Meanwhile,the experiment studied the difference of a through-going main fault zone gradually developing and becoming overt from its concealed status.Three groups of experiments were designed which are single basal fault transtension,distributed strike slip,cover thickness and shear strength.Experiment 1: the transtension angle of concealed fault in Bamianhe and Wangjiagang area was obtained,regional tectonic stress field may be inferred by the angle between associated fault and main fault.Experiment 2:the cover fractures differences between single basal fault transtension and distributed strike slip were summaried,it confirmed that the mechanism of concealed fault zones in Bodong area is distributed twist rather than a single basal activity.Experiment 3: In condition of the constant clay thickness,the greater shear strength,the wider concealed fault zones,the more R' shear,the space among R shears is lager and the number of R shears is less.In addition,In condition of the constant shear strength,with the clay thickness increasing,the wide of concealed fault becomes large,the number of R shears decreases in proportion.According to analogue modelling and examples in nature,concealed fault zones in the type of basal strike slip may be divided into 5 stages:(1)weak en echelon concealed fracture developmental stage(induced fracture stage);(2)strong en echelon concealed fracture developmental stage(localized fault stage);(3)partial overt main fault developmental stage(main fault stage);(4)overt main fault developmental stage(main slip stage)(5)transtensional main fault developmental stage(slide and fracture stage).The experimental analysis shows that the length of R shear is affected by the thickness of the cover,and is related with slip displacement.The parameters of R shear length,basal strike slip displacement,and transtension angle are used for multiple regression analysis.Take the Slippage of basal trend/Experimental cap thickness(DNBD)as the independent variable X1?Lateral slippage/Experimental cap thickness as the independent variable X2(X2= X1*tan?)?the length of Echelon stitch/Experimental cap thickness as the dependent variable y,and carried out with SPSS,we conducted Multivariate Quadratic Function Fitting.According to the actual formation thickness of respective period of Paleogene in Bamianhe and Wangjiagang,the experimental estimation of transtensional angle,the R shear length of structure diagrams in Paleogene period,the DNBD(Normal basement displacement)strike-slip amount basement correction of concealed fault zones in Bamianhe and Wangjiagang in different periods were calculated,besides,the intensity of tectonic activity of the concealed fault zones were quantitative analyzed and finally got the evolutionary stage?s DVC(Critical value)of the concealed fault zones in the south slope of Dongying Sag in Paleogene.At last,as for concealed fault zones controlling oil and gas,with a natural example,four patterns of oil and gas accumulation in Bohai Bay basin are established and divided into en echelon-shape fault block trap,ribbon-shape fault block trap,parallel composite trap and grid-shape composite trap.According to Qikou concealed fault zones different activity intensities and activity ways in the southern,central and northern of Huanhua Depression,three vertical distribution pattern of oil and gas are established:(1)sandwich-type oil and gas accumulation pattern,which is characterized by the favorable condition of trap,migration and driving force,the multilayer distributed sweet point.In section,this pattern accompanies multiple oil and gas accumulation zone,the effect that new recent tectonic activity on migration and accumulation of oil and gas should be paid attenation,especially the lateral preservation condition;(2)The pie-type oil and gas distribution pattern: oil source fault connected Neogene strata does not grow.The Neogene has good sealing conditions,so oil and gas migrate into Eogene strata.During the exploration,Eogene strata should be paid attention,and Neogene strata do not be ignored.(3)The pizza-type oil and gas distribution pattern,which is characterized by the favorable conditions of hydrocarbon source rock and reservoir,and the vertical migration.oil and gas migrate into Neogene strata.During the exploration,Neogene strata should be paid attention,and Eogene strata do not be ignored.Based on the above research,with fault controlling reservoir physical simulation experiment,four types of accumulation pattern about different evolutional stage of concealed fault zone in Bohai Bay basin are established: Weak deformation-isolated dispersion-single channel accumulate into reservoirs,moderately intense deformation-en echelon Beaded-single channel accumulate into reservoirs,intense deformation-intermittent band-local dual channel accumulate into reservoirs,intense deformation-continuous strip-full channel accumulate into reservoirs.The area near the string reservoir along concealed fault zones and the intersection of the grid-shape concealed fault zones are favorable traps.All these offer us a new oil-gas exploration theory and point out that concealed fault zones are a new direction of oil and gas exploration in future. |
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