四川盆地龙马溪组页岩气储层地震—测井层序分析

Seismic-logging sequence analysis of Longmaxi shale gas reservoirs in the Sichuan Basin

四川盆地下志留统龙马溪组为一套黑色页岩沉积,是潜在的页岩气储层发育段。应用层序地层学原理,根据自然伽马和声波测井曲线,结合地震资料,开展了该区龙马溪组层序地层学研究,并预测有利页岩气储层发育段的分布。结果表明,龙马溪组可以划分为5个地震层序和22个测井层序。通过合成记录将测井层序划分结果与地震层序进行对比,并以地震层序框架为约束,开展井间测井层序对比分析,可在该区建立一个3层结构的层序模式,即2个低频超层序、5个中频地震层序和22个高频测井层序。综合分析每口井的伽马测井曲线特征,建立起了该区页岩气储层发育的层序地层模式:伽马曲线表现出升高和降低两种形态,分别解释为海侵体系域和高水位或海退体系域,优质含气页岩主要发育于最大海泛面附近的凝缩层内。根据层序地层学原理,该区有利的页岩气储层主要分布于龙马溪组底部,对应于超层序1或地震层序1的中下段以及测井层序2～5。该研究成果对四川盆地龙马溪组页岩气储层勘探及其他地区页岩气储层的预测具有借鉴作用。

A set of dark shale deposits was found in the Lower Silurian Longmaxi Formation in the Sichuan Basin and was considered as potential shale gas reservoirs. According to the theory of sequence stratigraphy, natural gamma ray and sonic logging data were integrated with seismic data to perform the sequence stratigraphic study and to map the favorable shale gas reservoirs. The Longmaxi Fm can be divided into 5 seismic sequences and 22 logging sequences. The logging sequences were correlated with the seismic se- quences on synthetic profiles and inter-well logging sequence correlation was performed with the seismic sequence framework as con- straint. A three layer sequence model was thus built in this study area, consisting of 2 low frequency super sequences, 5 medium fre- quency seismic sequences and 22 high frequency logging sequences. Additionally, a sequence stratigraphic model of the shale gas res ervoirs there was also established through a comprehensive analysis of each well＇s gamma ray curve. The gamma ray curves show two geometries, i.e. an ascending one and a descending one, which were interpreted as transgressive system tract and retrogressive sys- tem tract respectively. The quality gas shale mainly occurs in the condensed succession near the maximum flooding surface. It is be- lieved that the favorable shale gas reservoirs are mainly distributed at the bottom of Longmaxi Fm and correspond to the middle lower interval of super sequence 1 or seismic sequence 1 and logging sequences 2 - 5. These understandings can be used to guide the exploration of shale gas in the Longmaxi Fm of the Sichuan Basin and mapping of shale gas reservoirs also in other regions.