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A Three-Fold Classification of Tight Gas Based Primarily on DynamicRelationship Between Gas Charging History and Reservoir TighteningProcess and its Application

Building upon the foundation of the prior investigations, a three-fold classification of tight gas reservoirs is proposed in this paper which is based primarily on dynamic relationship between gas charging history and reservoir tight- ening process, coupled with tectonic evolution, source-reservoir relationship, migration and charging pattern. The three categories of tight gas are: (1) "pre-existing" basin-centered gas reservoir, in which the reservoir sands experienced early- stage tightening processes, occurring before peak gas generation, expulsion from source rock, and charging of reservoir; (2) "pre-existing subsequent-improved" tight gas reservoir, in which the reservoir sands were also tightened before gas charging and then underwent reservoirs improvement mainly caused by the tectonic activities; and (3) "subsequent- conventional" tight gas reservoir where reserved sands were tightened after the peak of gas generation, expulsion from source rock, and charging of reservoir. This type of tight gas initially formed conventional gas accumulation during gas charging of reservoir, and subsequently modified to tight gas reservoir. All the three categories of tight gas have different geological conditions of gas accumulation and gas accumulation patterns, which can be used as characteristics to classify these tight gas systems, and thus have distinctive control on regional gas distribution. The results of applying this tight gas classification for an actual basin show that correctly distinguishing these three kinds of tight gas reservoirs from each other could contribute greatly to the exploration and development of tight gas reservoirs.