| At present in the world scope, tight sandstone gas has become one of the focuses of natural gas exploration. The Turpan-hami basin is a coal-rich basin, whose hydrocarbon source rocks of Shuixigou group among Jurassic coal strata has good condition of gas accumulation. Natural gas resources here have solid foundation of a more than one hundred billion cubic meter scale. Since the prospecting breakthrough of 2007, favorable gas-bearing areas such as Kekeya, Wenjisang, Hongqikan, Aketash have been found, among which the Kekeya block has been put into producing test. Based on the study of regional characteristics of sediment, reservoir and dynamic field, this article summed up two kinds of tight sandstone gas accumulation mechanisms in the Shuixigou group of the Turpan-hami basin, which is a "self-generation and self-storage" type and a “proximal-generation and proximal-storage " type respectively. At the same time, three accumulation patterns were sumed up and explained according to their corresponding regional tectonic units.A variety of evidence in the Turpan-hami basin shows that Jurassic tight sandstone gas and the coal measure strata of the Shuixigou group has genetic relationship, and formed a "self-generation and self-storage" type and a “proximal-generation and proximal-storage " type of tight sandstone gas accumulation. The main genetic type&occurrence form of the tight sandstone gas in the Shuixigou group of the Turpan-hami basin is immature secondary thermogenic free gas. Preliminary studies suggest that the formation and distribution of local high quality and high yield tight sandstone gas reservoir depends on good source-reservoir plane configuration composite, and perfect space-time corresponding of the accumulation material systems and the structural trap formation periods. As a main accumulation mechanism, the " proximal-generation and proximal-storage " type developed well in the north piedmont belt, where tectonics reformations and liftings were strong, and its best prospective areas are structural upper zones and trap fold center parts near the depression center edge sandbodies, which have good physical properties and correspond wonderfully with coal seam thickness; whereas the " self-generation and self-storage " type is widely distributed in the entire area, especially in those sedimentary unstable splay areas, it is a kind of supplementary accumulation mechanism; in addition, there are still composite accumulation mechanism of the foregoing two developed in the basin.The widely-developed hydrocarbon source rocks and braided river delta sandstone directly contacting with it in Shuixigou Group of Turpan-hami basin provided favorable material foundation for the formation of tight sandstone gas reservoir; while the suitable evolution and distribution features of temperature and pressure field, together with tectonic stress field evolution that closely related to the development of structural traps and faults, provided a suitable dynamic field environment for it. Through comprehensive study on the geochemical characteristics of source rocks and gas, combined with the research on such geological factors of the reservoir as its petrology, diagenesis evolution, as well as the fluid inclusions within it, all showed that the main source of the tight sandstone gas accumulation in Shuixigou Group was the coal strata within it, which began generating gas as far back as late Jurassic-Early Cretaceous when its Ro were still in low-mature stage of about 0.6-0.8%, then it reached a gas-generation peak when its Ro exceeded 1.1%. Under the effect of rapid deep burial and diagenesis compaction, densification time of the reservoir was calculated to be in the Late Jurassic. This relationship between source rock and reservoir such as "pre-densified" or "late-gas-generation" was conducive to the wide development of tight sandstone gas accumulations in the “pre-formed type”, which aggregated vertically near the gas generation center of Turpan-hami basin.The typical continuous filling " pre-formed " gas reservoir developed in deep depression area, it was a kind of tight sandstone gas reservoir formation controlled by “reservoir dessert”and had some characteristics that similar to " deep basin gas traps" or " continuous gas reservoir ". Affected by the later Himalaya movement, a type of "pre-forming modified " tight sandstone gas reservoir further developed in the steep slope zone of Northern Piedmont and at the updip segment in the southern gentle slope zone, while the inherited “post-formed” tight sandstone gas reservoir developed at the descending segment in the southern slope zone( at high-in-sag position). |
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