The natural gas exploration blocks of Yanchang Petroleum Group(Yanchang exploration blocks)are located in the southeast of Yishan slope of Ordos Basin.The basin has long been believed to be rich in oil in its south part and abundant in gas in its north-a mindset that resulted in pessimistic evaluation data of gas potential in the Upper Paleozoic exploration blocks of the Group.The first real exploratory well for gas was drilled in the area in 2003.Geological data gathered since then have revealed that the blocks sit in the eastern part of the northern Shanbei slope

which is rich in both oil and gas.The reservoirs are dominated by underwater distributary channels and barrier islands

source rocks are widely distributed

and mudstone seals are thick and stable.The gas resource was estimated to be over 750 bcm.Theoretical and technological innovation obtained during exploration in the blocks by the Group had led to the following achievements:(1)A new understanding of gas accumulation is proposed

i.e.gas accumulations are controlled by combination of reservoirs with paleo-fluid migration and accumulation space;(2)A new foam fracturing system(VES-CO

2

) was deve-loped and tested for the Upper Paleozoic tight gas reservoirs.A proven gas in place of more than 300 bcm and annual gas production capacity of 720 million cm

3

were obtained. What's more

the Carboniferous Benxi Formation in the basin was found to be able to produce gas at an absolute open flow of over million cubic meters.