Abstract
Tight sandstone gas is on the first position of unconventional natural gas sources, which can be developed under today’s technical conditions. In recent years, tight sandstone gas reservoirs have been found in several wells in the Linxing area, eastern margin of Ordos Basin, China. In this article, a variety of methods, including cast thin sections, X-ray diffraction analysis, scanning electron microscope, and drill core data were used to study the petrological characteristics and their influences on tight sandstone reservoir in coal-bearing strata of the Linxing area. Based on the analysis of thin section, it can be concluded that the sandstone reservoir is essentially constituted of lithic sandstone as well as lithic arkose and feldspathic litharenite. Cement types are complicated, including carbonate minerals, clay minerals, and quartz overgrowth. Illite, kaolinite, chlorite, illite–smectite mixed layer, and chlorite–smectite mixed layer are found in clay minerals. Compared with other clay minerals, illite is in the dominant position. Pores can be divided into residual intergranular pore, intragranular dissolution pore, intergranular dissolution pore, cement dissolution pore, intercrystalline pore, and microcrack in sandstone reservoir of the Linxing area. Quartz has an average content of 68% with the feature of low compositional maturity and plays a major role in increasing porosity due to dissolution and protecting of quartz. Feldspar dissolution plays a role in decreasing porosity because the by-product materials of feldspar dissolution remain in original place, instead of being transported to other areas. Dissolution pores are 2–20 µm and may be filled with kaolinite, illite, or halite. It is worth mentioning that grain-coating chlorite may be of sufficient thickness to protect reservoirs along with the increasing content of chlorite, which is testified by the crossplot between the chlorite and porosity when the absolute content of chlorite is less than 1.5%.
Highlights
IntroductionThe genesis of tight sandstones has been discussed in many aspects, such as tectonism, sedimentary process, and diagenesis (Shrivastava and Lawatia, 2011). Morad et al (2010) and Nguyen et al (2013) believed that the influence of deposition on tight sandstones was reflected by the distribution of pore and throat in different composition and size of sandstones. Mansurbeg et al (2012) and Walderhaug et al (2012) investigated the effect of compaction on decreasing the porosity of sandstones. Ajdukiewicz and Lander (2010) and McKinley et al (2011) believed that the radius of sandstone pore and throat may be decreased with the pores transforming into intercrystal pores and argued that cements may block up the throat, reducing the connectivity of pores. Harris (2006) and Taylor et al (2010) introduced the dual role of the tectonization about the reservoir physical property, including the unfavorable tectonic compression and beneficial fractures to increase the reservoir porosity
Based on the analysis of the thin sections, it can be seen that sandstone reservoir is essentially constituted of lithic sandstone as well as lithic arkose and feldspathic litharenite in the coal-bearing strata of the Linxing area
Seen from scanning electron microscope (SEM) images, quartz dissolution can be observed in rock samples of the Benxi, Taiyuan, and Shanxi Formations from the eastern margin of Ordos Basin
Summary
The genesis of tight sandstones has been discussed in many aspects, such as tectonism, sedimentary process, and diagenesis (Shrivastava and Lawatia, 2011). Morad et al (2010) and Nguyen et al (2013) believed that the influence of deposition on tight sandstones was reflected by the distribution of pore and throat in different composition and size of sandstones. Mansurbeg et al (2012) and Walderhaug et al (2012) investigated the effect of compaction on decreasing the porosity of sandstones. Ajdukiewicz and Lander (2010) and McKinley et al (2011) believed that the radius of sandstone pore and throat may be decreased with the pores transforming into intercrystal pores and argued that cements may block up the throat, reducing the connectivity of pores. Harris (2006) and Taylor et al (2010) introduced the dual role of the tectonization about the reservoir physical property, including the unfavorable tectonic compression and beneficial fractures to increase the reservoir porosity. Mansurbeg et al (2012) and Walderhaug et al (2012) investigated the effect of compaction on decreasing the porosity of sandstones. Types and contents of minerals and rocks have a significant influence on the densification of tight sandstone reservoir (Li et al, 2016; Ma et al, 2016). Few studies have paid sufficient attention to the influence of mineral rock composition, content, structure, and second changes on reservoir densification (Zhong et al, 2012). Tight sandstones are composed of rock skeleton, pores, and fluid, so the discussion of petrological characteristics is to explore the nature of reservoir densification. The discussion of the studied coal-bearing strata is of great significance to understand tight sandstone reservoir and explore the potential tight gas reservoir
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