The Biyang depression is a down-faulted lake basin filled with Tertiary continental deposits． It is one of the basins containing the largest petroleum reserves per unit area in China．The Hetaoyuan Formation sandstones are the main reservoir rocks，and the oil shales and mudstones of this age are the main hydrocarbon source rocks in the basin． Petrographic study shows that the Hetaoyuan Formation sandstones have highly variable mineral composition。including fine—grained to coarse—grained，poorly—sorted to well—sorted quart— ZO—feldspathie sandstone，arkose，lithic arkose and feldapathic litharenite．The diagenesis of the Hetaoyuan Formation sandstones varies greatly from place to place in the basin．Diagenetic min— erals include relatively early—formed grain—coating chlorite，quartz and feldspar overgrowths，and poikilitic carbonate cements and relatively late—formed pore—filling barite，pyrite，kaolinite，smec— rite and euhedral calcite crystals．The porosity in the Hetaoyuan Formation sandstones is also quite variable，ranging from 0 to 29％；most of the pores are secondary in origin．Porosity reduc— tion is due to compaction and authigenic mineral cementation，while enhancement of porosity re— suIted from the dissolution of both carbonate cements and unstable framework grains．Sandstones with secondary pores are main hydrocarbon reservoir rocks．