Post-Sedimentary Structural Deformation Characteristics Of Major Lithologic Assemblages Of Mesozoic And Upper Paleozoic In Ordos Basin

The post-sedimentary tectonic deformation of lithologic assemblages affects the formation and evolution of unconventional and conventional hydrocarbons.Previous studies focused on the deformation characteristics of single lithology in lithologic assemblages.The study of structural deformation of lithologic assemblages is not deep enough,which restricts the effective exploration of unconventional hydrocarbon resources.In this paper,the characteristics of post-sedimentary tectonic deformation of the main lithologic assemblages of Mesozoic and Upper Paleozoic in Ordos Basin are preliminarily studied based on core,thin section,sonic time difference curve and compression and rebound fracture experiments.Major conclusions are as follows:(1)Major types of lithologic assemblagesAccording to the composition and grain size classification of sedimentary rocks,the main lithologic assemblages of Mesozoic and Upper Paleozoic can be divided into two types: sandstone-fine-grained rock assemblages and fine-grained rock assemblages.(2)Structural Deformation Characteristic Reflected by Interval Transit Time CurveThrough the analysis of Interval transit time curves of 110 wells in Ordos Basin,it is found that the sonic time difference decreases greatly in shallow part with the increase of burial depth,reflecting that the rock in this section is mainly plastic deformation,while the rock in deep part is mainly elastic deformation or fracture.At present,the depth of the boundary between the two sections is different in different regions of the basin.During the maximum subsidence period of the basin at the end of the Early Cretaceous,the boundary depth between the two segments was about 1700 m.(3)Structural Deformation Characteristics of Major Lithologic Assemblages in Cores and SlicesThrough the observation of cores and thin sections,the fracture of sandstone-fine-grained rock combination mainly develops at the lithologic interface and in the finer-grained rock;the fracture of fine-grained rock combination mainly develops at the lithologic interface.(4)Comparison of stress-strain curves of different rock typesBy comparing the stress-strain curves of different types of rocks,it is found that under the same stress conditions,the strain of different types of rocks is different,among which the coal strain is the largest;under the same stress conditions,the larger the porosity of quartz clastic rocks is,the larger the strain is;under the same stress conditions,the larger the grain size of quartz clastic rocks is,the larger the strain is.(5)General characteristics of tectonic deformation after sedimentation of major lithologic assemblagesWith the increase of burial depth,the post-sedimentary tectonic deformation of the main lithologic assemblages in the Ordos Basin can be roughly divided into two zones: shallow plastic deformation zone and deep elastoplastic and fracture deformation zone.During the maximum subsidence period of the whole basin at the end of the Early Cretaceous,the depth of the plastic deformation zone was about 0-1700 m,and the elastic-plastic and fracture deformation zone was about below 1700 m.At present,the depth of the boundary between plastic deformation zone and elastic-plastic deformation zone is different in different regions of the basin.