四川盆地涪陵地区五峰组观音桥段成因分析及其对页岩气开发的意义

Significance of shale gas genesis to the development of Guanyinqiao Member,Wufeng Formation,Fuling area,Sichuan Basin

四川盆地涪陵地区五峰组顶部普遍发育观音桥段地层,目前关于观音桥段地层的成因,存在2个主流观点,即浅水成因说和深水成因说。结合沉积速率、生态学等多因素分析,证实涪陵地区五峰组观音桥段为深水异地沉积,底流沉积作用是观音桥段地层发育的主要原因,同时过强的底流沉积作用也是导致部分地区观音桥段地层缺失的主要原因。此外,从沉积作用、工程工艺改造等角度出发探讨了观音桥段地层对于涪陵地区页岩气开发评价工作的影响。研究认为,观音桥段地层的平面展布特征反映了涪陵地区五峰—龙马溪组含气页岩段沉积时期底流作用的活跃程度、微古地貌及距离物源区远近3个地质要素在平面上的差异,这些差异对于五峰—龙马溪组含气页岩段及其上覆顶板龙二段地层的沉积有着重要的影响,因此观音桥段的发育特征可作为中国南方五峰—龙马溪组页岩气层勘探开发选区的一个重要评价指标;此外,观音桥段地层工程工艺改造难度较大,且位于主力气层段内部,在优选开发井水平段穿行层位时应给与充分考虑。

The widely developed Guanyinqiao Member at the top of the Wufeng Formation,Fuling area shows horizontally variable lithology and thickness.There are two main views about the causes of this variability:deep water genesis theory and shallow water genesis theory.This study combined various factors,including sedimentation rate and ecological characteristics,to indicate that the Wufeng Formation was a typical deep water allochthonous deposit.Underflow deposition was the main reason why the underlying Guanyinqiao Member was well developed.Meanwhile,strong underflow sedimentation was the main cause for the lack of Guanyinqiao Member in several areas.Additionally,this work investigated the effect of Guanyinqiao Member on the evaluation of shale gas development from the sedimentary and engineering technological transformation perspectives.The horizontal distribution characteristics of Guanyinqiao Member showed the horizontal differences of three geological elements,including the active degree of underflow deposition,the micro palaeogeomorphology and the distance to source area of gas-bearing shale in the Wufeng-Longmaxi formations,Fuling area.These differences had important effects on the deposition of gas-bearing shale of the Wufeng-Longmaxi formations and the overlying Long2 Member.Therefore,the formation characteristics of Guanyinqiao Member can be used as an important evaluation index for gas-bearing shale strata exploration and development in the Wufeng-Longmaxi formations,South China.Besides,the engineering technological transformation of Guanyinqiao Member is challenging.It is located in the main zone,and should be given full consideration when optimizing the horizontal section of development well.