Ordovician source rocks and natural gas potential in the southern Ordos basin marginal paleo-depression

Abstract：Ordovician source rocks are widespread and very thick in the southern Ordos basin marginal paleo-depression. Of these rocks, mudstone and muddy carbonate source rocks are 10-57.6 m and 19.5-285 m thick respectively. They have a high organic abundance with the total organic content (TOC) clustering at 0.15%-0.20%. Organic matter of the source rocks are mainly of type I and type II1. The Ordovician source rocks began to generate gas in the early part of the Late Triassic (which is equivalent to the period of deposition of the Yanchang Formation) and gas generation reached the peak stage at the end of the Late Triassic. Later, the rocks stopped generating gas because the study area began strong uplift from the Jurassic on. There is no gas potential in the mountainous area in the southern part of the Weibie uplift at the southern margin of the Ordos Basin because of poor conditions of gas preservation. However, in the northern part of the Weibie uplift and the southern part of the Yishan slope of the Ordos basin, the burial depth of the Ordovician generally exceeds 3000 m, the time for gas generation was long and there are good conditions for forming a good gas accumulation; so the two area are favourable exploration targets for exploring for the structural gas accumulation and structural-lithologic gas accumulation.