Preservation Mechanism And Evaluation Method Of Marine Shale Gas With Complex Tectonic Background

Shale of Lower Cambrian and Upper Ordovician-Lower Silurian are research objective intervals for marine shale gas in southern China.And the shale of these two layer series are generally characterized by high TOC content and big thickness of rich-organic shale,which show a good material basis.However,for the exploration of shale gas during the Lower Cambrian and Upper Ordovician-Lower Silurian,due to complicated tectonic evolution and thermal evolution,there are a number of low production wells and even failed wells except high productivity wells.So,the study on preservation conditions of the shale gas is the key to the study on shale gas accumulation.A good material basis is the premise of shale gas preservation,and the accumulation of organic matter is controlled by the tectonic background at the time of sedimentation.In the Early Cambrian,south China was under the background of plate tension.While,in the Late Ordovician and Early Silurian,it was under the background of plate extrusion.In addition,it was surrounded by the ancient land,and was not well connected with the sea.Based on the analysis of Mo element and TOC content,this dissertation showed that the Lower Cambrian shale in the Yangtze region was deposited in the weak-moderate closed water environment,while the upper Ordovician-Lower Silurian shale was deposited in the strong closed water environment.In addition,Si element and Al element were used to accurately calculate the content of excess silicon in typical well shale.Through the analysis of Al-Fe-Mn element,it was determined that the excess silicon in Lower Cambrian shale was caused by hydrothermal activity.On the one hand,hydrothermal activity can enhance the reducibility of water bottom,and on the other hand,promote the improvement of biological productivity,thus enriching the sedimentary organic matter.Similarly,through the analysis of Al-Fe-Mn,it could be concluded that the excess silicon in the shale of the Upper Ordovician and Lower Silurian was biological origin,which could be used as an indicator of biological productivity.The strong sealing property of water and active volcanic activity led to layered water.The upper layer had high biological productivity,and the lower layer had strong reducing property,which was conducive to the preservation of sedimentary organic matter.The vertical sealing of shale and its roof and floor is an important factor controlling the preservation of shale gas.According to the porosity,overburden permeability,breakthrough pressure,scanning electron microscope and gas adsorption experiment,it can be concluded that due to the difference in physical properties,the roof and floor have a vertical sealing effect on the organic-rich shale and due to the difference in connectivity and Klinken-berg effect,self-sealing is formed in shale.The vertical permeability of organic-rich shale was affected by the overburden pressure and the adsorption gas quantity.With the increase of burial depth,the overburden pressure increased,while with the increase of TOC content and thickness,the adsorbed gas quantity increased,both of which can lead to the decline of permeability of organic-rich shale and the enhancement of self-sealing ability.It can be seen from the experiment of shale gas composition that most of the gas in shale was nitrogen due to the destruction of sealing.Detachment layer,thrust fault,thermal evolution of organic matter and volcanic activity developed in Jurassic period were the reasons for high nitrogen and low hydrocarbon in shale gas.Based on carbon isotope and FIB-HIM experiment,it can be concluded that the sealing of shale and its roof and floor had an effect on the amount of cracking gas formed by liquid hydrocarbon and the degree of reverse carbon isotope in shale gas.The better sealing,the less liquid hydrocarbon would be expelled from the shale.As a result,the gas formed by cracking of liquid hydrocarbon would be more,and carbon isotopic reverse would be more intense,and the tar pitch pores in organic pores would be more.The opposite is true.Due to the complex tectonic movement of marine shale in southern China,the study of shale gas enrichment under different tectonic styles has become an important issue to be solved urgently.For negative tectonic style,the parallel and vertical direction of level of shale permeability experiment showed that the gas in shale formation mainly migrated along the direction parallel to the level.Through the instance,10 ° and 20 ° were used to divide the preservation condition.According to the overburden pressure permeability experiment,the permeability of shale gradually decreased with the increase of overburden pressure.While according to the permeability experiments with and without methane adsorption,the adsorbed gas would block the seepage channel and thus led to the decrease of permeability.Under the dual role of the overlying pressure and gas adsorption,shale permeability in parallel to the direction of the level was low with the overburden pressure which was greater than 10?15MPa,and increased suddenly when the overburden pressure dropped to 10?15MPa,leading to the loss of much shale gas in the form of lateral migration,which meant that when there was only negative tectonic style,the drilling depth in order to find high production well should be deeper than 1019?1529m at least.For positive tectonic style,the greater the angle of limbs and the less cracks,the better shale gas preservation condition is.Through the instance,70 ° and 120 ° were used to divide the deformation intensity.During the stratum uplifted as a whole,in a certain level stress field,large-scale concentrated fracture would be produced at a certain depth,thus leading to a large amount of shale gas loss.When both negative and positive tectonic styles existed at the same time,if the positive tectonic style was buried deeper than the fracture depth,the positive tectonic structure would be good and the free gas content would be high,which would be a favorable exploration area.While,if the positive structure was buried shallower than the fracture depth,the negative structure would be a favorable exploration area,and the closer to the core,the better the gas bearing property.The gas has the characteristics of diffusion.The earlier the uplift time is,the earlier the shale gas stops.And without the supplement of hydrocarbon generation,the lower the gas content of shale at present is.Though the instances,the lifting time of 90 Ma and 130 Ma were used to divide the time of lifting.Finally,the dissertation summarized the evaluation of the preservation condition of marine shale gas with complex tectonic background from four aspects,including material basis,uplift time,shale roof and floor and self-sealing,and tectonic styles,and verified the accuracy of the evaluation method with actual exploration blocks.