羌塘盆地中生界海相黑色泥页岩特征及页岩气远景区预测

青藏高原羌塘盆地中生界发育上三叠统肖茶卡组(T3x)、中侏罗统布曲组(J2b)和上侏罗统索瓦组(J3s)3套海相黑色泥页岩,形成于有机质比较丰富且易于保存的营养型还原性沉积环境,以台地相为主,其次为盆地相和三角洲相;厚度在41～474m范围内,有机碳(TOC)含量平均值为0.25%～8.34%,有机质类型主要为Ⅱ型干酪根,镜质体反射率(Ro)平均值为1.21%～3.0%,处于高—过成熟生气阶段,脆性矿物以石英、长石为主,含量为65%～70%,页岩孔隙度约2%。与美国主要盆地产气页岩各项指标相比较,羌塘盆地中生界海相黑色泥页岩具有厚度大、有机质丰度低、热演化熟度高、脆性矿物种类多、石英含量较低、长石和粘土矿物含量较高及孔隙度低的特征,具备了形成页岩气基本地质条件,是我国海相页岩气勘探的沉积盆地之一,综合评价预测的页岩气远景区以上三叠统肖茶卡组(T3x)分布范围大,其次是布曲组(J2b)和索瓦组(J3s)。

Characteristics of Mesozoic marine black shales and prediction of shale gas prospective area in Qiangtang Basin,Tibet

Mesozoic marine black shales are mainly distributed in the Qiangtang Basin,Qinghai-Tibet plateau,in the Upper Triassic Xiaochaka Formation(T3x),the Jurassic Buqu Formation(J2b) and the Suowa Formation(J3x).They are formed in the reducing sedimentary environment which is abundant in organics that are easily preserved.The sedimentary facies are mainly platform facies,followed by basinal facies and delta facies.The thickness is 41-474 m;TOC is 0.25%-8.34%;organic matter is typeⅡ;the average value of Ro is 1.21%-3.0%,showing in the gas stage of high-over mature.The brittle minerals are mainly quartz and feldspar,total contents are 65%-70%,and the shale porosity is about 2%.Compared with each index of gas shales in the main basins of American,we can consider that the Mesozoic marine black shales in Qiangtang Basin have the features of large deposition thickness,low organic abundance,high maturity of thermal evolution,numerous species of brittle minerals,low content of quartz,high content of feldspar and clay minerals,and low porosity.It means that the shales have the condition of producing shale gas and the basin is one of the sedimentary basins,in which the marine shale gas could be explored.From the comprehensive evaluation of the shale gas prospective areas,it can be predicted that in the Upper Triassic Xiaochaka Formation shale gas is widely distributed,followed by the Buqu Formation and the Suowa Formation.