The Carboniferous-Permian System of the upper Paleozoic in Ordos Basin is a set of sedimentary formation with wide-coverage coal series. The gas-bearing reservoir is a set of compact sandstone with low porosity and permeability, of which the average porosity is less than 8%; and the permeability is less than 0.42×10–3mm2. The hydrocarbon source rock of coal series is in the period of high maturity-over mature wet gas-lot of dry gas generation, the gas generation amount coming to 539.83×1012m3.
Through the research of the gas-water relations and the pressure features as well as exploration experiences, we determine that there exists the obvious phenomenon of gas-water inversion in the Carboniferous-Permian System of the upper Paleozoic, in the section of P2h8–P1S2, from south to north, develops large areas of deep basin gas pool. Yet, after the Yanshanian Movement, with the rising of east part of the Ordos Basin, the deep basin gas pool lies in a reconstructive state now. Meanwhile, the speed of the hydrocarbon generation out of the source rock slowed down greatly. Although there is the supply of the desorption gas from coal-bearing rock, the amount is likely less than the lost, which made the early boundary of gas-water shrink back toward south. It is predicted that the distribution range of the deep basin gas in Ordos Basin will get to 12×104km2, and the prospect total resource amount exceeds 42.27×1012m3, and the current resource amount exceeds 4.27×1012m3.
Deep basin gas pool refers to the gas accumulation developed in the downdip of structure in basin, and gas source rock connects with compact reservoirs. Generally, it has the following features:
Lie in center or deeper location of a basin;
Reservoirs are compact;
Gas-water inversion;
Mainly develop from the coal series formation;
Large scale reserves etc.
Deep basin gas pool is a kind of unconventional natural gas pool, which has broadened the thought for exploration and the fields of gas exploration.
Ordos Basin lies in the middle of China, of which geological structure characteristic is noted for the steady and the uniformity. Its integrated ups and downs, wide and gentle slope, weak anticline, horizontal formation and contiguity conformity are well known all of world. Hardly are there fractures and folds that will bring complex geological situation. The structure is favorable for oil/gas generation and preservation, which is one of the basic conditions of forming deep basin gas. On the broad gentle slope in Shanbei, the dip of formation in Carboniferous-Permian Period is low (<1o), and the average slope descending rate is only1/100. There isn't
