黔北凤冈地区海相牛蹄塘组页岩气成藏地质特征及评价

Geological features and evaluation of Niutitang Formation shale gas in Fenggang block,northern Guizhou

依据岩石学、有机地球化学、储集性、含气性特征等页岩基础地质要素和构造样式及埋深、断层及裂缝、顶底板盖层与自封闭性条件、地层压力等保存条件对黔北凤冈地区海相牛蹄塘组页岩成藏地质特征进行系统研究。结果表明,研究区下寒武统牛蹄塘组页岩具有高的TOC含量、RO值、脆性矿物含量及甲烷吸附能力;页岩孔隙以原生孔隙为主,包括泥粒孔、顺层缝隙、层间缝隙等,对储层孔渗物性有明显影响;页岩总含气量与孔隙度、比表面积和TOC值呈显著的正相关关系,与黏土矿物含量成反比关系,页岩储层孔隙度和比表面积对含气量的贡献大于黏土矿物含量。从保存条件看,研究区宽缓状背斜及其所夹宽缓的鞍状构造带,地层产状较缓,埋藏深度适中,具有良好的顶、底板盖层与自封闭条件,有利于页岩气的保存,但若宽缓背斜核部发育通天断裂、走滑断层和高角度裂缝则不利于页岩气的保存,此外地层压力对保存条件并不起决定性作用,低压和常压地层也存在工业价值的页岩气聚集。在此基础上,提出了黔北凤冈地区海相牛蹄塘组页岩气成藏的评价指标。

On the basis of characteristics of shale reservoir formation,such as lithology and mineral composition,organic geochemistry,reservoir characteristics,gas-bearing characteristics,combined with structural style and burial depth,faults and fractures,top and floor cap rocks,reservoir pressure,a comprehensive characterization of the Niutitang shale is performed.The results indicate the Niutitang shale has high TOC,high RO,brittle mineral contents and methane sorption capacity.The main pores of shale minerals include mud particle pores,bedding cracks,interlayer cracks,which are large in number and mainly primary,and have a significant impact on the porosity and permeability of the reservoir.The total gas content of shale has a significant positive correlation with porosity,specific surface area and TOC,and is inversely proportional to clay mineral content.So the contribution of porosity and specific surface area to gas content of shale is greater than that of clay mineral content.From the perspective of preservation conditions,the gentle anticline and saddle structural strata sandwiched by the anticline in the study area is relatively slow,the burial depth of Niutitang Formation is moderate,and it has good roof and floor caprock,which is conducive to the preservation of shale gas.But poor sealing,strike slip and high angle fracture fault are not conducive to preservation,which will destroy the preservation of shale.In addition,reservoir pressure does not play a decisive role in preservation conditions,and there is also industrial value of shale gas accumulation in low-pressure and atmospheric pressure formations.Finally,a valuation index system of preservation conditions is put forward and the comprehensive favorable areas for shale accumulation in the study area are optimized.