The Underpressure Gas Reservoir and Gas Migration of Ordovician in Oerhtossu

The Lower Paleozoic Gas field in the central part of Oerhtossu Basin is of underpressure gas reservoir with the source rock and reservoir rock of carbonate complex of Ordovician,and its cap rock is alum clay of Middle Carboniferous. This gas field is located at the relatively low part of the regional monocilne which inclines to west. The pressure coefficient of the top of Ordovician gas reservoir on Shancan-1 Well and Lin-1 Well is about 0. 91. This type of underpressure gas reservoir can be seen in USA, Canada and prior USSR. The characters of underpressure reservoir for Oerhtossu are similar to the underpressure reservoir of Lower Cretaceous of Elmworth and Upper Cretaceous of Medician Hat in Alberta of Canada,and the Clinton sandston of Silurian in Ohio of United States. Their common characters are reservoir being saturated with gas, and the water saturation being lower than 38%,no formation water being produced. However, the reservoir saturated with gas dose not always developed underpressure. In this type of gas reservoir,underpressure developed or not relates to the open degree of gas reservoir and the generation rate of hydrocarbon. The developing of underpressure gas reserevoir of weathering crust in Oerhtossu Basin is not only due to it saturated with gas,so the fomational pressure of overlying strata can't be transmited efficiently, but also caused by it's gas generation rate decreased since Upper Cretaceous. The natureal gas of studied area accumulates abundantly at the top of weathering crust with it's thickness being 30-80m,pores and miarolitics are developed strongly in the weathering crust of Jingbian-Hengshan and then composes a good reservoir. The flow potential calculation shows that simply used the buoyance or overpressure value to indicate the migration direction is unilateral.There is no free water in the reservoir today,and it is similar that buoyance isn't a driven force of gas secondary migration at present. In the end of Jurassic,however, the gas potentials at the section of Shancan-1 to Yu-1 Well were higher at east and west ends and lower at central part,higher at the bottom and becoming lower at the top of Ordovician. So the migration directions of gas were from east and west ends to central and from bottom to top. The writers guess that the buoyance is a driven force of gas migration at that time. At present,there is no obvious potential difference between east and west at this section, and it is similar that there does not occur lateral migration of long distance. The gas migration paths at Shancan-1 and Lin-1 Wells are mainly along the direction of maximum potential gradient, from the bottom to the top of Ordovician. The low gas potential areas indicating migration direction combined with the reservoir of high pores,miarolitics and seams are the richly accumulated areas of gas.