长宁地区龙马溪组页岩储层低电阻率成因

Causes of low resistivity of Longmaxi Formation shale reservoirs in Changning area

在长宁地区龙马溪组相继钻遇低电阻率井,这些井含气性出现不同程度的破坏。基于低电阻率页岩储层特征、地震、构造及断层期次、测井、岩心实验等,对长宁地区页岩储层低电阻率成因和主要影响因素进行了研究。结果表明:长宁地区低电阻率页岩储层产气特征为电阻率大于5Ω·m,全烃含量大于2%,孔隙度大于4.2%,含气量大于2.0 m^(3)/t,含气饱和度大于60%。龙马溪组页岩储层低电阻率成因与断层有关,断层挤压破坏了储层含气性,表征为电阻率相对较低,断层下盘对电阻率的影响大于上盘,断层下盘页岩储层易出现低电阻率,断层上盘电阻率正常。龙马溪组页岩储层低电阻率成因与断层形成的期次有关,建武向斜所有低电阻率井均受加里东期形成的断层影响,下盘井部分天然气运移至上盘,地层水进入断层产生的裂缝中,水在近距离向构造低部位聚集,造成构造低部位的开发井呈低电阻特征,产量受损,而上盘天然气富集,高产井众多。天宫堂背斜、双龙-罗场向斜、建武向斜以西所有低电阻率井均受喜马拉雅中晚期形成的北东向的断层影响。长宁地区部分井存在低电阻率原因是断层附近的页岩储层含水饱和度增高引起电阻率的降低。

Low resistivity wells were drilled successively in Longmaxi Formation of Changning area,and the gas-bearing properties of these wells were damaged to varying degrees.Based on the characteristics of low-resistivity shale reservoirs,seismic survey,structural and fault stages,logging,and core experiments,the research was performed on the causes and main influencing factors of low-resistivity shale gas reservoirs in Changning area.The results reveal that the daily gas pro-duction of low-resistivity shale reservoirs in Changning area show resistivity>5Ω·m,total hydrocarbon display>2%,po-rosity>4.2%,gas content>2.0 m^(3)/t,and gas saturation>60%.The low resistivity wells in Longmaxi Formation shale reser-voirs of Changning area are mainly affected by faults.The compression by faults destroys the gas-bearing properties of res-ervoirs,which is characterized by relatively low resistivity,and the footwall exerts a greater influence on resistivity than the hanging wall;the shale reservoirs in the footwall are prone to low resistivity,while those in the hanging wall tend to be nor-mal.In addition,the low resistivity wells in the study area are mainly related to the faulting stages.All the low resistivity wells in Jianwu syncline are controlled by the faults during the Caledonian.The natural gas in the footwall wells partially migrates to the hanging wall,and formation water enters the fractures induced by faults.Water accumulates in the structural low in a short distance,resulting in the low resistivity wells with a lower output.However,the natural gas in the hanging wall is enriched,leading to many high-yield wells.The low resistivity wells in Tiangongtang anticline and Shuanglong-Luo-chang syncline as well as to the west of Jianwu syncline are controlled by NE-trending faults in the middle and late Himala-yan.Moreover,some low resistivity wells in Changning area are due to the decrease in resistivity caused by the higher water saturation of shale reservoirs near the fault.