摘要
为了探索中国南方海相地层牛蹄塘组原始储层中页岩气赋存状态，笔者采用了液氮低温吸附/脱附实验和甲烷等温吸附实验等手段，分别表征了页岩对吸附气和游离气的储集能力；同时结合典型井的含气性特点，研究页岩气的赋存机理，识别和优选出了研究区储层的“甜点段”。结果表明，牛蹄塘组发育1~2层富有机质页岩（有机碳含量>2.0%），岩性以黑色硅质页岩为主；储层微观主要发育有机质孔（两端开口的圆柱孔），粘土矿物层间孔（四面开口的狭窄平板孔）以及粒间孔和晶间孔（四面开口的锥形平板孔），而最重要的孔隙类型是有机质孔。牛蹄塘组含气性普遍较差，但慈页1井残余气体积分数高达69.7%。理论计算和现场实验结果均表明，牛蹄塘组页岩储层赋存甲烷特征变化随发生改变，由于有机质孔的孔径较小（一般仅为几十纳米以下），导致原始储层中赋存吸附气能力较强，而赋存游离气能力较弱。随着有机质质量分数的增加，岩石的弹性模量逐渐降低，但泊松比却逐渐增加。牛蹄塘组“甜点段”具有“甜”和“脆”的特点，即厚度为20~40 m，含量为4%~8%，石英质量分数超过40%，粘土矿物质量分数30%~40%，碳酸盐岩质量分数低于20%，在（10~17）×10 cm/g，在4~6 cm/g，游离气体积分数超过40%，弹性模量为25~35 GPa，泊松比为0.20~0.25，整体上，牛蹄塘组“甜点段”主要发育在深水陆棚泥质相带，且游离气富集能力强的层段。
Abstract
Low-temperature adsorption/desorption of liquid nitrogen and isothermal adsorption of methane were used to characterize the shale reservoir capacity for adsorbed gas and free gas so as to describe the occurrence of shale gas in the original reservoirs of the Niutitang Formation in southern China.Combined with the gas-bearing properties of typical wells, the occurrence mechanism of the shale gas was studied and the "sweet spots" were identified and optimized in the study area.The results show that 1 to 2 layers of organic-rich shale dominated by black siliceous shale are developed in the Niutitang Formation (>2.0%).Organic-matter pores (cylindrical pores with both ends open), interlayer pores of clay mineral (narrow parallel-plate pores with four sides open) and intergranular and intercrystalline pores (conical parallel-plate pores with four sides open) are developed with the organic-matter pores being the most important.The Niutitang Formation is generally poor in gas-bearing properties, except for one well (Well Ciye 1) measured with a residual gas volume fraction of as high as 69.7%.Both theoretical calculation and field experiments have shown that the occurrence of methane in Niutitang Formation shale reservoirs vary with .Due to the relatively small organic-matter pore size (usually smaller than tens of nanometers), the original reservoirs in the formation store more adsorbed gas than free gas.With the increase of organic matter mass fraction, the elastic modulus of rocks decreases and the Poisson's ratio increases gradually.The "sweet spots" in the Niutitang Formation are characterized by "sweetness" and "brittleness", i.e.they have greater thickness (20-40 m) and high values between 4% and 8%, high quartz mass fraction (>40%) and clay mineral mass fraction (30%-40%), low carbonate rock mass fraction (< 20%), a of (10-17)×10 cm/g, a of 4-6 cm/g, a free gas volume fraction of more than 40%, an elasticity modulus of 25~35 GPa and a Poisson's ratio of 0.20~0.25.In short, the "sweet spots" in the Niutitang Formation are mainly developed in deep shelf argillaceous facies with high free gas storage capacity.
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