| Crossing conjugate normal faults(CCNFs)are common structural style with "Y" and "X" patterns,developed in regions of crustal tectonic tensile or tension-torsion,and generally comprise two intersecting opposite-dipping sets of faults with the same strike and opposite trend on the plane.These structures are similar to the semi-hourglass(shovel)"Y" pattern and hourglass "X" pattern on the section,also referred to as“hourglass structures”,occur at a range of scales,from regional seismic(kilometers)to outcrop(meters to centimeters)in a variety of tectonic settings.Besides,these faults usually can not generally intersect the surface or basement,and the extensional system of CCNF favors the migration of oil and gas from deep oil sources to shallow regions and the formation of traps.Hydrocarbon exploration practice has proved that crossing conjugate normal faults(especially inherited crossing conjugate normal faults)are closely related to oil and gas.Many oil fields are developed in the horst or flank of graben CCNFs.Moreover,the high-resolution seismic tomography results show that the deep strata of CCNFs usually have obvious deformation inside the conjugate zone,and the deformation pattern directly affects the migration and trapping effect of oil and gas.Therefore,it is very important to understand the CCNFs deformation process and tectonic mechanism.At present,most studies on CCNFs are based on elastic models to explain the slip of faults,the influence on the geological structure on both sides of faults and the surface fold.However,in the elastic models considered in the numerical simulation studies,the deformation is related mainly to the difference in displacement between the hanging wall and foot wall of the blocks on the two sides of the fault plane,it is difficult to accurately describe the geological deformation on both sides of faults.In addition,previous studies did not consider the influence of the non-conjugate zone on the deformation of the strata inside the conjugate zone or only considered the case of complete CCNF.But the actual fault scale is often limited.No matter it is incomplete Y-type or X-type CCNF,the influence of the structural characteristics of the non-conjugate zone(i.e.,the zone adjacent to the conjugate relay zone where faults do not intersect)on the deformation of the internal strata in the conjugated zone(i.e.,grabens in the conjugate relay zone)cannot be ignored.Although the studies of elastic models can qualitatively reflect the general structure of the exploration area,it is far from satisfying the demand for the fine reservoir exploration and development in the later stage.Especially in recently years,the high-resolution seismic tomography data have shown that such faults can form convex-up deformation folds inside the conjugate zone.For example,in the V-shaped graben formed by opposite-dipping faults in the Nanpu Sag region of Bohai Bay basin in China,in addition to the overall subsidence,a convex-up style of deformation occurs between the faults above the line of fault intersection.Some studies have attempted to explain the development of CCNFs and subsequent related deformation in the Nanpu Sag in terms of compressional tectonics,but this hypothesis has not been supported by geological data,as a compressional tectonic environment did not occur during the formation of CCNFs.Considering the above problems,here we use the ANSYS finite element to simulate the internal deformation modes of CCNFs under different conditions.Firstly,by simulating the deformation process of 2-D Y-type and X-type CCNFs in elastic and elastic-plastic media,and comparing the results of the internal deformation difference of conjugate graben and horst;Secondly,we compare and analyze the internal deformation process of 3-D complete and incomplete CCNFs by simulating 3-D Y-type and X-type complete and incomplete CCNFs models;Then,a series of model simulations were performed to examine the effects of horizontal tectonic extension,fault size,and fault depth on the deformation of conjugate relay zones of incomplete CCNFs.At the end,taking a large number of CCNFs in Nanpu sag,Bohai Bay as an example,the relationship between the formation deformation and hydrocarbon migration and accumulation in the Y and X type CCNFs is discussed.The results show that,the three-dimensional elastic-plastic model is more accurate to reflect the tectonic deformation of underground strata,and the deformation of the strata in the conjugate zone of the incomplete CCNFs is completely different from that of the complete CCNFs models;In the extensional tectonic background(especially in the strong tectonic tension),when the existence of the non-conjugate zone under certain conditions(such as greater tectonic tension,burial under certain conditions)will inhibit the relative dislocation of both sides of the fault,resulting in flexural deformation inside the conjugated zone.According to the result,the lateral subsidence of the strata of Y-type incomplete CCNFs form a shovel-like deformation in the lateral with the fault.The model of X-type CCNF predicts a convex-up style of deformation in the conjugate graben region superimposed on overall subsidence under applied horizontal tectonic extension.Meanwhile,the degree of deformation in the inner strata of incomplete CCNF is related to the existence of non-conjugate zones and their influence on conjugate zones above the intersection lines,which varies with the amount of horizontal tectonic extension,fault size,and fault burial depth.At the same time,Our results indicate that incomplete CCNFs can form convex-up deformation,similar to that in the Nanpu Sag area and provide a sound understanding of hydrocarbon migration and accumulation.Besides,when studying the influence of CCNFs on hydrocarbon migration and accumulation,we found that underground oil and gas will generate kinetic energy in the process of spontaneous migration.In essence,the upward migration of the underground natural gas is the interaction effect between the natural geostress and gas fluid.Due to the gaseous fluids which are more compressible than liquid fluids,are easier to accumulate and release energy based on properties of compression,this phenomenon is especially obvious in the spouting gas well in the oil field.Hence,based on the knowledge of previous geophysics and geochemistry studies and the data of existing oil fields,we define the basic concept of geostress energy and estimate the value of geostress energy in this thesis,which provides a basic reference for evaluating the application potential of geostress energy. |
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