MAJOR FACTORS CONTROLLING THE ENRICHMENT OF THE UPPER PALEOZOIC NATURAL GAS IN THE ORDOS BASIN

The Upper Paleozoic gas source rocks in the Ordos Basin have the sustained ability to provide gas from the Triassic to the present. They still have considerable gas-expulsion amount after the Early Cretaceous. The sustained gas-provision ability and the large gas-expulsion amount of the Upper Paleozoic gas source nds are favorable conditions for the enrichment and accumulation of the Upper Paloozoic natural gas. They show that the Ordos Basin have the material basis to form deep-basin gas. From the Triassic to the Early Cretaceas, the Upper Paleozoic natural gas in the Ordos Basin migrated northward in a large scale; after the Early Cretaceous, the natural gas migrates and accumulates nearby. This create advantageous gas - source conditions for bearing gas in the large areas of the basin. The sand haies ofdistributary channels and rirermouth sandhars in the lerrShhai - Shansi ffermation, which is distribund in the four large scale fluvial - detaic sndimentary sys-tems, haw hat Phyical protyrties. The channel tandhaies in theSe sndimentary systems are farorable places tothe enrichment of natural gas. Stable tectonic setting as usll as gnd seeling ability of rngional and direct cov-ers f0rms admntage0us cv7ofitions to the presermtion of the UPtyr Palcozoic natural gas. The key factor to theaccumulation of the UANr Paleocoic natural gas. in the Ordos Basin is the distribution offaoorable sand ha-ies. To strengrhen the prediction of distribution for the sandstone resercoir of the Shihezi - Shansi Formationshas important signifcance to the exPloration of the UPPr Paleowic natural gas.