PETROLOGY CHARACTERISTICS AND THEIR INFLUENCES ON TIGHT SANDSTONE RESERVOIR IN COAL MEASURES OF LINXING AREA

It is estimated that tight sandstone gas recoverable reserves, which can be developed under today's technical conditions, is in the first place of unconventional gas resources. Simultaneously, tight sandstone gas is widely distributed and its output increases with years in China. A variety of technical means including common thin section, cast thin section, grain size analysis, cathode luminescent thin sections, scanning electron microscope and X-diffraction analysis were used to study rock properties and their influences on tight reservoir in coal measure strata of Linxing area.

The results show that the main rock types are predominantly feldspathic lithic sandstones and lithic sandstones, but sometimes lithic feldspathic sandstones and lithic quartz sandstones. The components of debris are complex due to sedimentary rock debris, magmatic rock debris and metamorphic rock debris. The composition of interstitial material is mainly composed of mud, and ankerite, calcite and siderite are also involved. Illite and illite-smectite mixed-layer are common clay minerals in tight sandstone reservoirs.

The influences of petrological characteristics on the reservoir physical property are embodied in the following four aspects. Firstly, low levels of quartz lead to low compressive strength of the rock, and strong compaction results in a reduction in porosity. Moreover, the phenomenon of quartz overgrowth which can reduce the physical properties is widespread. Secondly, feldspar is the material basis of secondary pores, and a large number of feldspar dissolution improve the reservoir properties. Thirdly, mudstone, slate, phyllite and other plastic debris reduce the degree of compressive strength. Finally, on account of the filling effect of clay minerals, the porosity and permeability of the reservoirs decrease with the increase of clay mineral content. In addition, the swelling of the clay minerals further reduces the physical properties of the reservoir.