The Genetic Mechanism and Model of Deep‐Basin Gas Accumulation and Methods for Predicting the Favorable Areas

Abstract  As a kind of abnormal natural gas formed with special mechanism, the deep‐basin gas, accumulated in the lower parts of a basin or syncline and trapped by a tight reservoir, has such characteristics as gas‐water inversion, abnormal pressure, continuous distribution and tremendous reserves. Being a geological product of the evolution of petroliferous basins 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 hydrostatic pressure within tight reservoirs, are favorable for forming deep‐basin gas pools. The range delineated by the above two forces corresponds to that of the deep‐basin gas trap. Within the scope of the deep‐basin gas trap, the balance relationship between the amounts of ingoing and overflowing gases determines the gas‐bearing area of the deep‐basin gas pool. The gas volume in regions with high porosity and high permeability is worth exploring under current technical conditions and it is equivalent to the practical resources (about 10%‐20% of the deep‐basin gas). Based on studies of deep‐basin gas formation 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 Upper Paleozoic He‐8 segment of the Ordos basin.