Abstract:

Usually there are three kinds of fluid inclusions in Tertiary sedimentary rocks in Qaidam Basin:primary,secondary and hereditary.Among them the most important information for oil and gas exploration are provided by primary fluid inclusions formed in diagenetic stage.The pressure measurement of the inclusions indicates that there is an abnormal high pressure body in the lower Ganchaigou Formation in the south part of the western Qaidam Basin.Oil pools of the red beds just preserved in this body.The reservoir voids are mainly residual primary pores and non-fractured secondary pores in sandstone.So many pores could be preserved because the abnormal high pressure of pore fluids keep quartz and feldspar from pressure-solution and the impermeable beds block the outside siliceous solution source.So the quartz and feldspar can not overgrow.Oil composition show that source rocks are mainly lateral lacustrine synchronous deposits of the red beds,while oil from overlying and underlying source rocks can not enter the trap in large quantity.According to the temperature determination of fluid inclutions,the oil generated and entered the trap when paleotemperature was 78℃.It is 26℃ lower than the threshold temperature calculated formerly.If other conditions remained constant,the quantity of oil generated in western Qaidam Basin should be much more than that calculated.The time of generation,migration and accumulation of oil and gas might start approximately from the Late Miocene and continued to the Late Pliocene.To compare the sedimentary characters of the southern part with that of the northern part of the western Qaidam Basin,one can see that the formation of abnormal high pressure is mainly due to easy closing of the small deltaic sandstone bodies and the diluvial sandstones and conglomerates within lacustrine mudstones,thus the pressure could be accumulated by rapid deposition of overlying sediments and differential compaction.Accordingly,the most favourable areas for oil and gas prospecting in red beds in western Qaidam Basin must be paleolacustrine area,and the west and south sides of the area.As for the east and north parts,it is better to pay attention to the areas where secondary pores are well developed and/or discontinued sandstone beds are existed.