基底断裂在四川盆地须家河组天然气成藏中的作用

Role of basement faults in gas accumulation of Xujiahe Formation,Sichuan Basin

由于储集层致密、普遍低渗,以及构造平缓、断层和裂缝不发育等因素,四川盆地须家河组呈现大面积含气、储量丰度低、产量低、以岩性气藏为主的特征,寻找勘探"甜点"是须家河组天然气勘探的关键。四川盆地基底非均质性导致北东向和北西向两组基底断裂在中、新生代活动,控制了二级构造带的展布,而且其后期"隐性"活动在须家河组砂体中还产生了微裂隙和小断层,为天然气运移提供了通道,并改善了储集层的物性条件,对致密岩性大面积含气背景下局部富集带的形成起到了重要作用。同时,基底断裂发育区出现的异常高地温,加速了有机质的生烃演化,因此基底断裂发育带是天然气成藏最有利的地区,是今后四川盆地天然气勘探的主攻方向。

The Xujiahe Formation is characterized by large gas bearing area, low reserve abundance, low yield, and dominantly lithologic gas pools because sandstone reservoirs are tight and low permeable, structures have gentle relief, and faults and fractures are undeveloped. Searching sweet-spots is the key step in gas exploration. The basement heterogeneity in the Sichuan basin has led to the activity of basement faults in NE and NW directions since Meso-Cenozoic, which influences the evolution of sub-structure zones and causes the occurrence of fractures and minor faults in Xujiahe Formation reservoirs. These fractures and minor faults improved the reservoir quality, provided path for gas migration, and contributed to the formation of local gas rich trends. Meanwhile, later activity of basement faults caused abnormally high geothermal field, accelerating the thermal evolution of source rock. The area of basement faults has the most favorable conditions for gas accumulation and is the main target for gas exploration.