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 rocks are favorable conditions for the enrichment and accumulation of the Upper Paleozoic natural gas. They show that the Ordos Basin have the material basis to form deep-basin gas. From the Triassic to the Early Cretaceals, 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 bodies of distributary channels and river-mouth sandbars in the Lower Shihezi-Shanxi Formation, which is distributed in the four large scale fluvial-deltaic sedimentary systems, have best physical properties. The channel sandbodies in these sedimentary systems are favorable places to the enrichment of natural gas. Stable tectonic setting as well as good seeling ability of regional and direct covers forms advantageous conditions to the preservation of the Upper Paleozoic natural gas. The key factor to the accumulation of the Upper Paleozoic natural gas. in the Ordos Basin is the distribution of favorable sand bodies. To strengthen the prediction of distribution for the sandstone reservoir of the Shihezi-Shanxi Formations has important significance to the exploration of the Upper Paleozoic natural gas.