Enrichment Conditions Analysis And Favourable Target Prediction Of Shale Gas

The Pengshui area is located in a fold area outside the Sichuan Basin.The Wufeng-Longmaxi Formation is an important section of shale development in the area.Shale gas resources are large and have great exploration potentialin this area.However,as exploration continues,some Problems that restrict further exploration are gradually emerging.First,the impact of tectonic activity stage,intensity,and burial hydrocarbon generation process on preservation conditions is unclear.Second,study on the reservoir properties,reservoir space type,pore structure,pore size and the variation of reservoir surface characterization parameters,such as specific surface area of Wufeng-Longmaxi Formation is rare.Third,the shale gas geophysical “sweet spot” evaluation in Pengshui area has not yet formed a technical system.So,two major scientific issues were put forward,one is the shale gas enrichment conditions of the Wufeng-Longmaxi Formation,and the other is the formation of a technical method for the prediction of “sweet spot”.In response to these key scientific issues,in this study,analysis on enrichment condition of shale gas and prediction of its favorable targets of Wufeng-Longmaxi Formation in Pengshui area were carried out,based on the existing exploration and development data,theories of reservoir sedimentology,geophysics,petroleum geology,and geochemistry.Firstly,lithology data,cores,well logging data,and analysis data were used to analyze the lithologic characteristics,lithofacies,and biological characteristics of the study area to determine the type of sedimentary facies and clarify the space distribution of organic-rich shales under different sedimentary facies backgrounds.Combined with geochemical indicators such as organic matter abundance,organic matter type,organic matter maturity,etc.,to determine the criteria for a good material basis.Second,mineral composition,reservoir space type,pore structure,the adsorption characteristics of rock gas were analyzed,and the favorable reservoir characteristics were identified based on the analysis data.The evolution of the tectonic movement period in the area was studied and the tectonic evolution characteristics of the study area were clearly definedbased on the fine structural interpretation.On this basis,PetroMod simulation software was used to study the history of shale sedimentary burial and thermal evolution and hydrocarbon generation.The burial and thermal evolution stages were divided,and the key period of enrichment was determined.The relationship of “tectonic-burial-hydrocarbon generation history” was analyzed.Its impact on the preservation conditions was analyzed,combined with structural style,top and bottom conditions,fault cracking system,etc.Based on the above comprehensive analysis of material basis,reservoir conditions,and preservation conditions,the assessment criteria for Pengshui shale gas enrichment area were determined.Finally,the geo-physical method is used to make a plane prediction of the optimal evaluation parameters,and the favorable target is delineated by using the evaluation criteria of the enrichment zone to complete the favorable target prediction.Through the research of the above content,we have obtained the following main achievements:Using the three-terminal element method to identify the lithology of the Wufeng-Longmaxi Formation,five types of lithofacies were identified: the siliceous shale rich(CM-1),the silicon-rich/mud mixed shale(M-2)Rich siliceous shale(S-3),siliceous shale(S-2),and siliceous rock(S);combined with lithofacies marks and biomarkers to perform the sedimentary facies of the Wufeng-Longmaxi Formation,it shows that the lower member of the Wufeng Formation-Longmaxi Formation is a deep-water shelf,while the middle and upper member are shallow-water shelf.The shale deposition thickness distribution characteristics were analyzed,and it shows that the deposition center was in the Minjiang-Qiliao line.The content of organic carbon in the Wufeng-Longmaxi formation is between 1.3% and 7.73%,and the average content of organic carbon in the lower member of Longmaxi Formation is more than 2%,which is significantly higher than that of the upper two members.The organic carbon content was positively correlated with the gas content and porosity.The organic matter type was mainly type I kerogen.The maturity of organic matter was between 2% and 3%,both of which were over mature stages.Four types of reservoir space types in the Wufeng-Longmaxi Formation in Pengshui were determined based on scanning electron microscopy observations,there are organic pores,brittle mineral pores,clay mineral pores,and microfractures,in which the organic pores are the main reservoir types.Through the analysis of X-whole rock diffraction data,it is believed that the Wufeng-Longmaxi Formation has a complex mineral composition,which mainly includes quartz,potash feldspar,plagioclase,dolomite,calcite,pyrite,clay minerals,and has strong heterogeneity.The content of brittle minerals is high and it has good fractureability.The vertical brittle mineral content decreases from bottom to top;the clay minerals mainly include illite,yamite,chlorite,and a small amount of kaolinite,indicating that the target interval is in the late diagenetic stage.The quantitative characterization of pore structure by cryogenic liquid nitrogen adsorption shows that there are mainly three types of pore structure in the Wufeng-Longmaxi Formation.Pore volume is provided bymicropores,mesopores and macropores in the first type,the pore volume is mainly provided by micropores and mesopores in the second type,and pore volume is not substantially provided by micropores in the third type.The dissolution of diagenesis,maturation of organic matter,metasomatism and transformation of clay minerals are constructive diagenesis,which is beneficial to the formation of reservoirs,while cementation and compaction are not conducive to the formation of pores in shale reservoirs.The organic pores are new secondary pores formed by the kerogen in the mudstone transformed into liquid or gaseous hydrocarbons.Through experimental analysis,it is believed that the formation and development of such pores are heterogeneous and influenced by the degree of thermal evolution;clay minerals have a catalytic effect on the evolution of organic matter.It is believed that the key to affecting the gas content and productivity of the shale gas wells in the study area is the tectonic events that occurred during the J3-K1 period,based on study of the tectonic movements and burial and hydrocarbon generation history in the study area,and the well Pengye 1 began to rise rapidly at K1 period,the hydrocarbon generation is terminated and the seal of the cap layer is weakened,the preservation conditions are deteriorated,and the oil and gas are dissipated on a large scale at the same time.However,the well Longjing 1 is relatively uplifted later with respect to the Peng Page 1 well,and the lifting speed is slow,and the damage to the preservation conditions is small.The current configuration styles,faults,and fractures have a great influence on the preservation conditions.Generally,they have an anticline and a broad structural style background.The faults are not developed,the seal is good,and they are far from the outcrop area and stratum denudation areas.Structure styles with low density and other characteristics are more advantageous for preservation.It was considered that the target interval logging curve has a distinct "triple nature" based on the analysis of logging curve characteristics and rock physical parameters.The organic carbon content increases with the reduction of the P-wave velocity ratio and the density,and the ratio(?/?)and density of Lame impedance and shear resistance decrease as the porosity increases.High-brittle shale has the characteristics of high Young's modulus and low Poisson's ratio.Analysis of the characteristics of rock physical parameters shows that the longitudinal and transverse wave velocity and density decrease with increasing total organic carbon content,and the Poisson's ratio with TOC.The value increases and the longitudinal and transverse wave velocity and density decrease as the porosity increases.Based on the analysis of the material basis,reservoir conditions,and preservation conditions,an evaluation plan for the shale gas enrichment zone in the region was formulated.Seismic inversion and attribute analysis methods were used to predict the horizontal distribution of shale thickness and gas enrichment parameters in the target zone of the area.Overall,the porosity,organic carbon content,and brittleness index of the lower member of the Longmaxi Formation were all betterthan the upper members.A multi-parameter superimposed display method was used to delineate favorable shale gas enriched zones.The results are consistent with the current situation of exploration in the Pengshui area.