高丰度低演化程度湖相页岩储层特征——以柴达木盆地北缘中侏罗统为例

Reservoir Characteristics of High Abundance and Low Thermal Stage Lacustrine Shale:An Example from the Middle Jurassic Shale in the Northern Qaidam Basin

页岩储层特征对于页岩气的赋存状态、含气量和后期开发均具有重要意义。湖相页岩多处于生油窗阶段,其储层特征与海相页岩区别较大。以柴达木盆地北缘中侏罗统页岩为研究对象,利用岩芯观察、有机地球化学分析、XRD矿物成分分析、低温氮气吸附、FE-SEM以及甲烷等温吸附等手段分析页岩储层的岩石类型、地化、矿物组成、孔隙结构和含气性特征。结果表明,与海相页岩相比,柴达木盆地北缘中侏罗统页岩具有"两高两低两种类型"的特点,即有机碳含量高、黏土矿物含量高,热演化程度低、脆性矿物含量低,有机质类型以Ⅱ、Ⅲ型为主。较低的热演化程度导致样品有机质孔隙发育较少;有机酸的溶蚀作用及矿物的转化作用是次生孔隙形成的主要因素。与海相页岩不同的是,高丰度低演化程度导致该演化阶段以产液态烃为主,且样品中可溶有机质含量较高,它对于样品的孔隙结构和甲烷赋存状态都有影响,表现为随比表面积增大,样品等温吸附实验结果呈现下降的趋势。对于高丰度低演化程度湖相页岩储层特征的研究,不仅为柴达木盆地北缘中侏罗统页岩气的勘探开发提供地质依据,同时也完善了页岩气地质理论。

Characteristic of shale reservoir is of great importance to the occurrences of shale gas,gas content and the exploitation.The lacustrine shale in the oil window is quite different from marine shale.This study takes Lacustrine Shale of Middle Jurassic,North Qaidam Basin as an example to study rock type,geochemical and mineralogical compositions,porosity structure and gas-bearing features using cores observation,organic geochemical analysis,XRD mineral testing and low temperature nitrogen adsorption,FE-SEM,methane isothermal adsorption,FE-SEM and methane isothermal adsorption.The results show that,compared with marine shale,shale in the study area is characterized by ＂ two high,two low,two types＂,i.e.high organic matter and clay mineral contents,low thermal maturity and brittle mineral contents.Organic matter types are mainly type Ⅱ and type Ⅲ.Low thermal maturity leads to low development of organic pores in the samples;dissolution and mineral transformation of organic acid are the main factors that contribute to secondary pores.Different from marine shale,high organic matter contents and low thermal maturity lead to liquid hydrocarbons and high soluble organic matter contents,which influences the pore structure and the occurrences of shale gas of the samples.The experiment shows a decreasing trend of isothermal adsorption with increasing surface area.The study of the characteristics of high abundance and low thermal stage lacustrine shale not only provides geological evidence for shale gas exploration and development but improves the geological theories of shale gas also.