海陆过渡相与海相页岩气储层特征差异性分析——以南川地区龙潭组与龙马溪组为例

The differences in reservoir property between marine and transitional facies shale gas:a case study of the Longtan Formation and the Longmaxi Formation in Nanchuan,Sichuan

南川地区发育海陆过渡相、海相2种沉积类型的页岩层系,以龙马溪组为代表的海相页岩气实现了商业开发,而对以龙潭组为代表的海陆过渡相页岩气的认识程度较低。利用野外露头、钻井岩芯、薄片及氩离子抛光扫描电镜等实验数据和地层微电阻率扫描成像(FMI)测井等资料,从沉积相、岩性组合、储集条件、矿物组分、可压性及含气性等方面,开展这2套页岩储层特征的差异性分析。结果表明,2套储层在以上方面都存在较大差异:①龙潭组岩性复杂,发育粉砂质页岩、炭质页岩、灰质页岩、灰岩及煤层,而龙马溪组主要以硅质页岩为主;②受岩性、有机质类型及热演化程度影响,龙潭组储集空间主要以微裂缝(粒缘缝和收缩缝)和无机孔为主,而龙马溪组主要以有机质孔和微裂缝为主;③龙潭组页岩黏土矿物含量高(47.6%),脆性矿物含量低(35.8%),杨氏模量低(32.3 GPa),泊松比高(0.3),塑性较强,可压性差于龙马溪组页岩;④龙潭组页岩含气量平均为2.2 m^(3)/t,吸附气占比54.5%,而龙马溪组页岩含气量平均为5.2 m^(3)/t,吸附气占比43.3%。通过2套页岩层系储层差异性分析,明确了龙潭组页岩储层具有岩性复杂、黏土矿物含量高、可压性差、吸附气占比高的特征。龙潭组谭一段页岩连续厚度大、含气量高、储集物性好,从整体性评价来看,是龙潭组的“甜点”段;但其黏土矿物含量也高,且地质“甜点”与工程“甜点”的耦合性较龙马溪组页岩差,建议优化压裂工艺,提高缝网改造体积和单井产量,实现勘探突破。

The shale strata in Nanchuan area,Sichuan,SW China,include sedimentary types of transitional facies and marine facies.The gas in marine shale represented by the Longmaxi Formation has been commercially developed,while that in the transitional facies shale represented by the Longtan Formation remain poorly understood.Based on data from field outcrops,drilling cores,thin sections,argon ion polishing scanning electron microscope,and Formation MicroScanner Image(FMI)imaging logging data,the differences in the characteristics of the two shale reservoirs were analyzed in terms of sedimentary facies,lithological combinations,reservoir conditions,mineral components,compressibility,and gas content.Results show that there are great differences in those aspects.①The lithology of Longtan Formation is complex,including silty shale,carbonaceous shale,gray shale,limestone and coal seam,while the Longmaxi Formation is dominated by siliceous shale.②Influenced by lithology,organic matter type,and thermal evolution degree,the reservoir space of Longtan Formation is dominated by micro-fractures(grain margin fractures and shrinkage fractures)and inorganic pores,while that of Longmaxi Formation is dominated by organic pores and micro-fractures.③The shale of Longtan Formation has high clay mineral content(47.6%),low brittle mineral content(35.8%),low Young's modulus(32.3 GPa),high Poisson ratio(0.3),strong plasticity,and lower compressibility than those of Longmaxi Formation.④The average gas content in Longtan Formation shale is 2.2 m^(3)/t,in which 54.5%is adsorption gas;and the average gas content of Longmaxi shale is 5.2 m^(3)/t,in which 43.3%is adsorption gas.Through the difference analysis of the two types of shale reservoirs,the characteristics of Longtan shale reservoir are defined,which are characterized by complex lithology,high clay mineral content,poor compressibility and high adsorption gas proportion.Therefore,the shale of the first member of the Longtan Formation has large continuous thickness,high gas content,good reservoir physical properties.In overall,it should be the"dessert"section of Longtan Formation.However,due to high clay mineral content,the coupling between geological"sweet spot"and engineering"sweet spot"is worse than that of Longmaxi shale.It is recommended to improve the fracturing technology,increase the volume of fracture network transformation and the output of a single well,so as to achieve exploration breakthroughs.