柴达木盆地冷湖地区侏罗系致密砂岩储层特征与控制因素

Characteristics and controlling factors of tight sandstone reservoir of the Jurassic strata in the Lenghu region, Northern Qaidam Basin

关键词:  冷湖地区  致密砂岩  成岩作用  储层物性

Key words:  Lenghu region  tight sandstone  diagenesis  reservoir physical properties

摘要:

冷湖地区侏罗系地层油源丰富，储层致密化程度高，为查明低孔低渗储层成因机制，剖析储层发育控制因素，利用普通薄片、铸体薄片、扫描电镜、全岩X衍射矿物分析、物性分析、核磁共振分析等多种分析测试手段，对研究区侏罗系储层的岩石学特征、孔隙类型、物性与成岩作用特征进行研究。结果显示：（1）研究区侏罗系储层主要为长石岩屑砂岩和岩屑砂岩，孔隙类型以次生溶孔为主，原生孔隙残留少，微孔隙占比高，孔隙度平均为9.96%，渗透率平均为2.26×10^-3 μm²，属于特低孔、超低渗致密储层。（2）储层成岩作用类型以压实作用、胶结作用和溶蚀作用为主，成岩演化主要处于中成岩A期，成岩序列可概括为：机械压实改造－少量早期方解石胶结－长石、岩屑轻微溶蚀－石英I期加大－有机酸流体侵入、长石岩屑强烈溶蚀－黏土矿物广泛出现－石英Ⅱ期加大－长石、岩屑、黏土矿物伊利石化－少量含铁碳酸盐胶结。（3）多种成岩作用综合制约着致密砂岩储层的发展进程。煤系地层富含水生、陆生动植物遗体，沉积后至成岩早期有机质分解产生腐殖酸并形成酸性环境，早期碳酸盐胶结物不甚发育，碎屑颗粒间欠缺方解石胶结物支撑，压实作用导致原生孔隙大幅降低；成岩中期广泛存在的黏土矿物占据孔隙空间，分割大孔隙为无数微孔隙，黏土矿物的胶结作用进一步加剧了储层致密化；有机质热演化过程中释放的有机酸性流体对长石的持续溶蚀形成较多次生孔隙，有效改善了储层物性。本项研究深化了柴北缘致密砂岩储层物性与成岩作用特征的认识，对开展进一步油气勘探具有指导意义。

Abstract:

The Jurassic strata in Lenghu area are rich in oil and gas resources and have a high degree of reservoir densification. In order to find out the genetic mechanism of porous and low permeability reservoirs and to analyze the controlling factors of reservoir development, a variety of experimental methods are used in this work, such as analyses of normal thin-section, cast thin-section and scanning electron microscope, X-ray diffraction of clay minerals and nuclear magnetic resonance. Based on the description of petrology characteristics of reservoir pore structures characterization and physical characteristics, the characteristics of reservoir diagenesis are investigated. The results show the following: (1) The rock types of the Jurassic reservoir rocks in the study area are mainly feldspathic lithic sandstones and lithic sandstones. The main pore types of sandstones are mainly secondary solution pore and the proportion of micro pores are relatively high. The average porosity of sandstone is 9.96% and the average permeability of sandstone is 2.26×10^-3 μm², belonging to special low porosity-ultra-low permeability reservoirs. (2) The types of diagenesis are mainly compaction, cementation and dissolution. The diagenetic evolution mainly takes place during the mesodiagenetic phase A. The diagenetic sequence can be summarized as follows: compaction - a small amount of calcite cement is formed - feldspar and rock debris are slightly corroded - secondary enlargement of quartz - feldspar and rock debris are strongly corroded due to the formation of a large number of organic acids - clay minerals appear widely - secondary enlarged edges appear again in quartz particles - feldspar, rock debris and clay minerals are gradually transformed into illite - a small amount of iron-bearing calcite cement is formed. (3) Various types of diagenesis had once played important roles in the formation of ultra-low permeability tight sandstone reservoirs. The coal measure strata were rich in the remains of aquatic and terrestrial animals and plants. After the sediment was buried, the organic matter can decompose and produce humic acid, and then form an acidic environment. There were little calcite cements in the clastic reservoir because of the acidic water medium condition of the coal measures strata in the early diagenetic stage. The strong compaction greatly reduced the primary porosity. The widespread clay minerals occupy the pore space and divide the macropores into countless micro pores. The cementation of clay minerals further intensifies the reservoir densification, which intensifies the process of reservoir densification. The continuous dissolution of feldspar and rock debris components by organic acid fluids which came from thermal evolution of organic matter played a key role in improving reservoir physical properties. This study can deepen the understanding of diagenetic evolution of tight sandstone reservoir in the northern margin of Qaidam Basin, which has a guiding significance for further oil and gas exploration.