Inclined eastward and consisting of the Hetianhe, Hetianhedong, Tazhong paleouplifts and Bachu paleoslope, the central paleouplift
belt in the Tarim Basin was a large composite paleouplift and paleoslope belt with complicated palaeogeomorphic features during
the Middle to early Late Ordovician. A number of paleostructural geomorphic elements have been identified in the paleouplift
belt and surrounding areas, such as the high uplift belts, the faulted uplift platforms, the marginal slopes and slope break
zones flanking the paleouplift belt, the surrounding shelf slopes or low relief ramps, the shelf slope break zones and deep
basin plains. They exerted great influence on the development of paleogeography of the basin. The marginal slopes and slope
break zones flanking the uplift belt constrained the formation and deposition of the high-energy facies including reefal and
shoal deposits during the Late Ordovician, which comprise the major reservoirs of the Lower Paleozoic in the basin. Toward
the end of the Ordovician, the Tazhong paleouplift hinged westward and became a westward-dipped nose as the southeastern margin
of the basin was strongly compressed and uplifted. The tectono-paleogeomorphic framework of the central northern basin during
the Early Silurian and the Late Devonian to Early Carboniferous changed remarkably in topography from the initial low in east
and high in west to high in northeast and low in southwest. The major paleogeomorphic elements developed in these periods
included the strong eroded uplift high, the uplift marginal slope, the gentle ramp of the depression margin and the depression
belt. The sandstones of the lowstand and the early transgressive systems tracts were deposited along the uplift marginal slopes
and the gentle ramps of the depressions comprise the prolific reservoirs in the basin. The study indicates that the distribution
patterns of the unconformities within the basin are closely related to the paleogeomorphic features and evolution of the paleouplift
belt. From the high uplift belt to the depression, we found the composed unconformity belts at the high uplift, the truncated
and onlap triangular unconformity belts along the uplift marginal slopes, the minor angular unconformity or discontinuity
belts along the transitional zones from the uplift marginal slopes to depression and the conformity belt in the central depression.
The truncated and the onlap triangular unconformity belts are the favorable zones for the formation of stratigraphic trap
reservoirs.
Supported by National Basic Research Program of China (Grant No. 2006CB202302), National Natural Science Foundation of China
(Grant No. 40372056) and Frontier Research Project of Marine Facies 相似文献
Shallow gas reservoirs are distributed widely in Chinese heavy oil-bearing basins. At present, shallow gas resources have opened up giant potentials. The previous researches indicate the intimate genetic relationship between shallow gas and heavy oil. Shallow gas resources are generated from crude oil degraded by anaerobic microscopic organism, it belongs to biogenic gas family of secondary genesis, namely heavy oil degraded gas. Shallow gas resources are usually distributed in the upward position or the vicinity of heavy oil reservoirs. They are mainly of dry gas, which are composed of methane and only tiny C+2 heavy hydrocarbon and relatively higher contents of nitrogen gas. Generally, methane isotopes are light, whose values are between biogenic gas and thermal cracking gas. Ethane isotopes are heavy, which mixed possibly with thermogenic gas. Carbon dioxide bear the characteristics of very heavy carbon isotope, so carbon isotopic fractionation effects are very obvious on the process of microscopic organism degradation crude oil. The heavy oil degraded gas formation, a very complex geological, geochemical and microbiological geochemical process, is the result of a series of reactions of organic matter-microbes and water-hydrocarbon, which is controlled by many factors.
Shallow gas reservoirs are distributed widely in Chinese heavy oil-bearing basins. At present, shallow gas resources have opened up giant potentials. The previous researches indicate the intimate genetic relationship between shallow gas and heavy oil. Shallow gas resources are generated from crude oil degraded by anaerobic microscopic organism, it belongs to biogenic gas family of secondary genesis, namely heavy oil degraded gas. Shallow gas resources are usually distributed in the upward position or the vicinity of heavy oil reservoirs. They are mainly of dry gas, which are composed of methane and only tiny C2+ heavy hydrocarbon and relatively higher contents of nitrogen gas. Generally, methane isotopes are light, whose values are between biogenic gas and thermal cracking gas. Ethane isotopes are heavy, which mixed possibly with thermogenic gas. Carbon dioxide bear the characteristics of very heavy carbon isotope, so carbon isotopic fractionation effects are very obvious on the process of microscopic organism degradation crude oil. The heavy oil degraded gas formation, a very complex geological, geochemical and microbiological geochemical process, is the result of a series of reactions of organic matter-microbes and water-hydrocarbon, which is controlled by many factors. 相似文献