CHARACTERISTICS OF RESERVOIRS IN THE UPPER PALEOZOIC TIGHT SANDSTONE, LINXING BLOCK, ORDOS BASIN

Seen from structure point of view, Linxing block on the east margin of Ordos Basin is situated on Jinxi fold belt. The tectonic is relatively simple, which is a monoclonal, western dip. Drilling and field outcrops of Linxing block reveal formations which include Benxi formation, Taiyuan formation, Shanxi formation, Shihezi formation and Shiqianfeng formation from bottom to top. Among them, the first three strata are the main coal-bearing system.

Based on thin section, cast thin section, cathode luminescent thin sections, scanning electron microscope, high pressure mercury injection and nuclear magnetic resonance, pore characteristics of tight sandstone reservoirs were studied in the Upper Paleozoic of Linxing block. The results show that feldspathic lithic sandstones and lithic sandstones account for a dominant position, but sometimes lithic feldspathic sandstones and lithic quartz sandstones. The reservoir physical properties are poor, with the average reservoir porosity of 6.2% and the average permeability of 0.6mD. Pores of tight reservoirs in Linxing block were divides into six categories namely residual intergranular pores, intergranular dissolution pores, intragranular dissolution pores, cement dissolution pores, intercrystalline pores and microfractures. Triangular, polygonal and other irregular shapes can be observed in pores, the average diameter of which is 0.2mm or so. The throat type is dominated by flakes and its structure is extremaly complex. Further analysis of high pressure mercury injection and nuclear magnetic resonance indicates that the connectivity of pore throat is poor and the heterogeneity of pore throat is strong.