致密砂岩的形成机制及其地质意义——以塔里木盆地英南2井为例

Formation Mechanism and Geological Implication of Tight Sandstones:A Case of Well YN-2 in Tarim Basin

砂质岩的孔隙和喉道被网格状粘土矿物和次生加大矿物充填成微细孔喉状结构时,形成具有较高毛细管压力的致密砂岩。它与地层水发生水锁效应,可大大降低渗透率,成为致密砂岩盖层。当含水饱和度在50%以下时,束缚水饱和度比较低,致密砂岩储层可以产气;当含水饱和度在50%～90%区间时,具有较高的束缚水饱和度,相对渗透率非常低,它既不能产气也不能产水,反映为渗透率瓶颈区(具有盖层性质);当含水饱和度大于90%以上时,致密砂岩储层仅微量产水。塔里木盆地英南2井侏罗系气藏盖层由致密砂岩构成,不含水时的气体渗透率在(0.027～0.081)×10-3μm2,不能构成封堵;当含水饱和度达到60%以上时,相对渗透率几乎为零,构成有效盖层。

If sandstones have higher capillary pressure as their pore and throat are filled with I/S clay in lattices, it will be tight. The water-locked phenomenon will be generated by tiny pore in which filled with I/S clay, which leads to reduce permeability of the tight sandstone and make sandstones become the low-permeability tight-sandstone reservoirs or tight-sandstone updip seal. In low-permeability reservoirs, gas can flow when the water saturation is less than 50% but neither gas nor water can flow when water saturation is in a broad range of 50%～90%, which behaves permeability bottle-necking as cap sealing. The cap pf Jurassic gas reservoir in Well YN-2 in Tarim Basin is composed of tight sandstone with gas permeability of 0.027～0.081×10-3μm2 without water, which cannot be as sealing,but only little water can flow under the condition of water saturation in excess of 60% with nearly permeability of 0×10-3μm2 so it can be as a sealing cap.