湘鄂西地区下寒武统牛蹄塘组页岩气储层特征

Shale gas reservoir characteristics of the Lower Cambrian Niutitang Formation in the western Hunan and Hubei areas

湘鄂西地区下寒武统发育一套有利的页岩气储层。选择不同沉积-构造单元内的典型页岩气井,通过有机地化测试、矿物X衍射、场发射扫描电镜及核磁共振等技术方法,对牛蹄塘组页岩气储层特征进行了对比分析。研究表明,页岩矿物组成受沉积相控制明显,浅水陆棚区页岩矿物组成表现出“低硅高钙”特征,盆地相区页岩矿物组成表现出“高硅低钙”特征;随着沉积水体的加深,页岩岩相类型由灰质页岩和混合质页岩向硅质页岩转变,储层的脆性指数也相应增大,岩石可压性更好。有机碳含量与孔隙度之间存在较好的正相关性,尤其是在有机碳含量大于2.0%的富有机质页岩段,两者相关性明显增强,表明页岩中有机质的富集对孔隙发育具有重要意义。研究区页岩主要发育纳米级孔隙,具有相对较低成熟度和埋深的宜昌地区页岩有机质孔隙的孔径较大、面孔率较高。宜页1井页岩纳米级孔隙以微孔(0~10 nm)和小孔(10~100 nm)为主,张地1井页岩纳米级孔隙以小孔为主,页岩孔隙体积主要来源于直径大于10 nm的孔隙;纵向上靠近有机质富集层段,页岩孔隙度增大的同时也会导致孔隙结构发生一定程度改善。

The Lower Cambrian black shale is a set of favorable shale gas reservoirs. In the selection of typical shale gas wells in different sedimentary-structural units, comparative analysis of shale gas reservoir characteristics of Niutitang Formation have been done by means of organic geochemistry testing, mineral X-ray diffraction, field emission scanning electron microscopy and nuclear magnetic resonance. The research shows that the mineral composition of shale is obviously controlled by sedimentary facies, and is characterized by “low silicon and high calcium” in shallow shelf region, and by “high silicon and low calcium” in the basin facies area. With the increase of sedimentary water from Yichang to Zhangjiajie, the lithofacies types of shale changed from calcareous shale and mixed shale to siliceous shale. Correspondingly, the brittleness index of shale reservoir increases and the rock compressibility improved. There is a good positive correlation between organic carbon content and porosity, especially in the organic-rich shale with organic carbon content more than 2.0%, which indicates that the enrichment of organic matter in shale has an important contribution to the porosity. In addition, nanopores are well developed in shale reservoir, and the Niutitang Formation shale in Yichang area with relatively low maturity and burial depth has larger pore size and higher surface porosity. The nano-pores of shale is dominated by micropores(0~10 nm) and small pores(10~100 nm) in well YY1, while the small pores are abundant in well ZD1, and the pore volume of shale is mainly provided by small pores less than 100 nm. In the longitudinal direction, the pore structure of shale will improve near the organic enrichment layer with the increase of porosity.