HYDROCARBON CHARGE AND ACCUMULATION MECHANISMS IN THE DEEPLY BURIED TIGHT RESERVOIRS IN THE KUCHE DEPRESSION,WEST CHINA

The Kuche Depression in the northern Tarim Basin, west China, is one of the most petroliferous provinces in China that sourced by a terrestrial source. Since the discovery of the giant Kela-2 Gas Field in the depression, there has been a great deal of research into the hydrocarbon charge mechanisms that formed such a giant accumulation. Recently a number of giant gas fields have been discovered in the deep reservoirs (6000-8000 m) in the surrounding region including the Dabei-1, Dabei-3, Keshen-1, Keshen-2 and Keshen-5 gas fields. These new discoveries are typical tight reservoirs with extremely low porosity and permeability. Compared with other similar tight reservoirs elsewhere, however, the gas saturation levels in the newly discovered reservoirs are exceedingly high (around 60-90%), comparable to that of conventional reservoirs. Through an integrated petroleum geological and reservoir petrophysical investigation we conclude that these giant accumulations and their high gas saturation in the deep reservoirs are caused by three key enabling factors including (1) abnormally high pressure caused by the foreland basin evolution, (2) the development of extensive fracture networks via thrusting and (3) reservoir wettability alteration resulted from early oil charge. This paper provides a detailed account and analysis on how each of the factors affecting hydrocarbon charge, accumulation and evolution in the deeply buried tight reservoirs that produce those giant discoveries. We proposed a revised hydrocarbon charge and accumulation model by incorporating some key petrophysical and reservoir engineering concepts, which have broad application in exploring tight sandstone reservoirs.