湘中涟源凹陷下石炭统天鹅坪组页岩气成藏条件及主控因素

Accumulation conditions and main factors of the Lower Carboniferous Tianeping Formation in Lianyuan Sag in the middle of Hunan Province

下石炭统天鹅坪组页岩是湘中地区页岩气勘探的新层系。基于野外地质调查、页岩气钻井的样品采集和分析测试等对湘中涟源凹陷石炭系天鹅坪组泥页岩的发育特征、有机地球化学特征、储层特征和含气性等页岩气成藏地质条件进行了分析。结果表明:涟源凹陷石炭系天鹅坪组有机质丰度较高,w(TOC)分布在0.78%~2.84%之间,有机质类型以Ⅱ1型为主,有机质成熟度R o分布在1.48%~2.09%之间,处于高成熟演化阶段;泥页岩矿物成分以石英和黏土矿物为主,黏土矿物主要由伊蒙混层、伊利石和绿泥石组成;泥页岩有机质孔较发育,孔隙度平均为1.97%,渗透率平均为0.847×10^(-3)μm^(2),但大多小于0.1×10^(-3)μm^(2);湘新地4井天鹅坪组泥页岩含气量分布在1.64~4.29 m^(3)/t,平均2.69 m^(3)/t(不含残余气)。结合区域构造—沉积演化背景的分析表明,涟源凹陷下石炭统天鹅坪组页岩气成藏条件的主控因素为沉积环境、构造保存条件,其中深水陆棚相是页岩气有利相带,复杂构造背景下由对冲式构造样式形成的向斜对页岩气保存条件最为有利,是研究区页岩气勘探的“甜点区”。研究成果可以为该区后续油气勘探提供参考。

Lower Carboniferous Tianeping Formation shale is a new shale gas exploration bed in Linayuan Sag of Xiangzhong Depression.Based on the filed geological investigation,shale gas drilling wells,sample collection and laboratory analysis,the shale gas accumulation conditions of Carboniferous Tianeping Formation in the sag were discussed in this paper,such as distribution characteristics,geochemical characteristics,reservoir characteristics and gas content.The results show that the total organic content of the shale is high and distributed from 0.78%to 2.84%.The organic matter types are mainly typeⅡ1.The thermal evolution degree of the shale is mainly in high mature with the range of 1.48%-2.09%.The shale composition is mainly quartz and clay minerals which are mainly composed of illite/smectite,illite,and chlorite.The pore of organic matter is more developed in mud shale.The porosity and permeability of shale are 2.43%and 0.285×10^(-3)μm^(2) in average respectively,but the permeability of most of the samples is less than 0.1×10^(-3)μm^(2).The gas content of shale of Tianeping Formation in Well Xiangxindi 4 ranges from 1.64 m^(3)/t to 4.29 m^(3)/t,with an average of 2.69 m^(3)/t(without residual gas).Combining with the regional sedimentary-tectonic background,we suggest the sedimentary environment and structural preservation conditions are the main controlling factors of shale gas accumulation conditions of Lower Carboniferous Tianeping Formation in Lianyuan Sag.The basin facies are the favorable facies belt for shale gas,and the syncline formed by the hedge structural style under the complex structural background is the most favorable for the preservation conditions of shale gas,which is the sweet spot area for shale gas exploration in the study area.The research results can provide reference for subsequent oil and gas exploration in this area.