The Genetic Mechanism and Model of Deep-Basin Gas Accumulation and Methods for Predicting the Favora

As a kind of abnormal natural gas formed with special mechanism, the deep-basin gas, accumulated in thelower parts of a basin or syncline and trapped by a tight reservoir, has such characteristics as gas-water inversion, abnormalpressure, continuous distribution and tremendous reserves. Being a geological product of the evolution of petroliferousbasins by the end of the middle-late stages, the formation of a deep-basin gas accumulation must meet four conditions, i.e.,continuous and sufficient gas supply, tight reservoirs in continuous distribution, good sealing caps and stable structures.The areas, where the expansion force of natural gas is smaller than the sum of the capillary force and the hydrostaticpressure within tight reservoirs, are favorable for forming deep-basin gas pools. The range delineated by the above twoforces corresponds to that of the deep-basin gas trap. Within the scope of the deep-basin gas trap, the balance relationshipbetween the amounts of ingoing and overflowing gases determines the gas-bearing area of the deep-basin gas pool. The gasvolume in regions with high porosity and high permeability is worth exploring under current technical conditions and it isequivalent to the practical resources (about 10%-20% of the deep-basin gas). Based on studies of deep-basin gasformation conditions, the theory of force balance and the equation of material balance, the favorable areas and gas-containing ranges, as well as possible gas-rich regions are preliminarily predicted in the deep-basin gas pools in the UpperPaleozoic He-8 segment of the Ordos basin.