| It is the true that China has abundant shale gas resources.By the end of 2018,the daily output of shale gas in Weiyuan area reached 5.1 million cubic meters,and the annual-output reached 1.825 billion square meters.Shale gas has become an important field for unconventional oil and gas exploration.It has important theoretical and practical significance to carry out research on shale gas accumulation conditions and enrichment law.The exploration and development process of shale gas in Weiyuan area is short with poor production.The research method and technique are often based on conventional oil and gas exploration.It has some characters of special reservoir conditions,high-quality shale distribution changing larger,pore space and complex reservoir system.The research is more difficult more than conventional oil and gas exploration.There is a lack of systematic exploration theory and effective techniques for the combination of seismic geology of shale gas in studied area.In this paper,the studied area of the Lower Silurian Longmaxi Formation in the Weiyuan area is less than 4000m in depth.The geology,well logging,seismic,drilling and experimental test are used to analyze the structure of the Longmaxi Formation in the studied area.The buied depth,sedimentatary envorment,organic matter,gas content,the thickness of high-quality shale,mechanical properties,brittle mineral composition,fracture character,reservoir control factors has been studied.Through evaluating the key parameters of shale gas reservoirs,conducting a comprehensive geological evaluation of the area where the depth of the Longmaxi Formation is less than 4000m,establishing a comprehensive evaluation index system for shale reservoirs in the studied area,classifing and evaluating favorable explore area.Combining the hydrocarbon accumulation and production state,it is defined the favorable target for shale gas exploration.This paper has mainly achieved results as below:(1)According to the characteristics of lithology and its combination,logging curve,combined with the characteristics of drilled strata,and according to the new stratigraphic stratification standard,the stratigraphic division plan of Longmaxi Formation in the study area is established.It is considered that the top boundary of bioclastic argillaceous limestone in Guanyinqiao section is taken as the bottom boundary of Longmaxi Formation and the siliceous shale is uniformly classified into Ordovician.Presenting in the logging curve,the GR is rising with resistance reducing.It is reasonable to divide into Long-1 and Long-2,as the lithology,electrical properties,and paleontology of the Longmaxi Formation shale are obviously different.The stratum distribution is relatively stable and the stratum thickness and burial depth gradually increase from northwest to southeast.(2)The mineral composition of the Longmaxi Formation in the study area is mainly quartz,feldspar,calcite and dolomite,pyrite,apatite.The average content of quartz is 45.5%,carbonate minerals is 16.7%,and clay minerals is 28.4%,high content of brittle minerals in Long-1 section;illite accounted for 53%of average clay minerals,19.1%of chlorite,no or slightly of kaolinite;rock mechanical parameters(Young's modulus,Poisson's ratio)and brittleness index both indicate that the Long-1section has better brittleness and fracturability,which is conducive to fracture reformed.(3)The shale reservoir of Longmaxi Formation in the study area has the characteristics of low porosity and ultra-low permeability and the correlation is not strong.It belongs to fracture pore type reservoir,the reservoir space type is mainly clay mineral interlayer pore,organic matter pore and structural fracture.The average porosity of the Long-1 section is 3-6%and the average permeability is 273n D,which belongs to the ultra-low permeability reservoir.The gas-content of the Long-1 section is generally 2.0-3.5m~3/t,which has a tendency to increase from northeast to southwest.Based on the analysis of physical properties,cores,and some scanning electron microscope data,the physical conditions and gas-content properties of Long-1 are better than that of Upper Long-2 section.(4)Improved the comprehensive prediction and evaluation technology of shale reservoir.Based on pre-stack inversion of seismic data,the thickness of high-quality shale is predicted between 20-70m.Pre-stack elastic parameters and brittleness index has been established in the study area to predict the brittleness index,with its plane and profile distribution characteristics of high-quality shale.The parameters such as lame constant and shear modulus obtained from inversion of geological,logging drilling data and seismic elastic parameters are used to build the geology and mechanical with computational mathematical model are used to simulate the stress field by using the three-dimensional finite difference simulation method.The relationship between the formation thickness,fault,stress field and fracture development is studied.Based on the numerical simulation technology of stress field,the stress field in the 3D area of seismic data is predicted.The direction and scale of fracture development are defined and the difference between the maximum and minimum stress in the horizontal direction is not obvious,which is good for the reconstruction of volume fracturing.(5)The TOC of Longmaxi Formation in the study area is relatively high,the shale organic matter type is mainly I-II1 and the components are mainly sapropel and asphalt group.The content of sapropel group is 72-90%and the content of asphalt group is10-22%,without crustaceous group with a small amount of vitrinite and inertinite.The vitrinite reflectance of Longmaxi Formation in Weiyuan area is about1.64-2.15%,which is in the stage of high mature,mainly in the stage of over-mature.The vertical change shows the characteristics of gradually increasing from the top to the bottom and increasing from Northwest to Southeast on plane.According to single well analysis and geological analogy,the relationship between TOC and Vp/Vs(P/S ratio)has been found in the study area,and a new relationship between TOC and seismic parameters is formed based on polynomial fitting.The distribution of TOC is predicted by inversion of pre-stack elastic parameters between 2-4.1%,and the brittless is between 30-60.(6)Through post-stack coherence technology,curvature attribute,pre-stack AVAZ,FVAZ technology to predict the fracture development zone,and reliability analysis of the fracture development zone,further clarifing the effective fracture development distribution law,solving the limitation of single means to predict the fracture,and improving the accuracy of comprehensive fracture prediction.According to the influence of shale gas roof and floor conditions,self sealing,seepage direction and structural movement on shale gas,the preservation conditions are evaluated and the distribution law of pressure coefficient is predicted.The overpressure of Longmaxi Formation in the study area is favorable for shale gas preservation,and it is difficult to form abnormal high pressure in the exposed area,shallow buried and strong reconstruction area;the shale gas enrichment area has a pressure coefficient more than1.2.(7)Shale gas in the study area is affected by sedimentary environment,lithofacies,structural-movement and preservation conditions as well as combination of organic pores and fractures,brittleness and good fractered.Based on geological understanding,comprehensive parameters of geophysical methods are used to quantitatively determine the specific distribution characteristics of various parameters,especially the parameters of high-quality shale thickness,organic carbon content,and brittleness index,combined with stress field distribution and storage conditions,Compared with the shale gas enrichment conditions in the neighbouring area.It is proposed that the TOC of the Longmaxi Formation shale in the study area is 2%,gas porosity?3%,high-quality shale thickness?40m,gas content is?2m~3/t,brittleness index?40,pressure coefficients?1.2,located in gentle structural regions,and below third-order fault distances are necessary conditions for shale gas enrichment.Based on the consideration of the geological characteristics of shale in the study area,considering the comprehensive factors of high-quality shale thickness,TOC distribution,brittleness index,pressure coefficient,and storage conditions,the favorable area of shale gas enrichment in the Lower Silurian Longmaxi Formation is predicted.There are two favorable areas,which are?most favorable and?favorable area. |
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