The Meso-Cenozoic Tectonic Event And Its Gas Accumulation Effect In The Northeast Of Ordos Basin

Based on the systemic field investigation and sampling, several methods, such as heavy mineral association, petrologic geochemistry and tectonic thermal geochronology, have been used to determine the main tectonic events of Meso- to Neo-Cenozoic, its attribution and peak chronological sequence, and periodic uplifts in the northeast part of Ordos basin. Lastly, I also discuss the probably stages of early reservoir- forming and modified reservoir-forming, as well as the relationship between regional tectonic events and gas reservoir-forming in Upper Paleozoic.(1)Detritus source of Paleozoic- Jurassic strata should mostly come from the Ying mountain Uplift in northern of basin, whereas having no relationship with the Lvliang mountain Uplift, he evolution of source's environment indicates that at least two tectonic uplilfting events have happened at the research area from Paleozoic to Jurassic. The first one occurred near the end of late Indosinian, the another one occurred at the early Yanshanian stage.(2) The analyses of tectonic thermal geochronology suggest that at least 4 stages tectonic uplifting events have happened in the northeastern of Ordos basin from Meso- to Neo-Cenozoic, as follows: late Indosinian, middle Yanshanian, Late Yanshanian and middle-late Himalayan.(3)The tectonic uplifting process of northeast part of Ordos Basin during Meso-Cenozoic can be divided into two stages. The 1st stage means a slowly uplifting-cooling process occurring during the end of Cretaceous to Neogene, the 2nd stage means a quick process during Neogene. A distinction caused by tectonic uplift occurred in the northeastern of Ordos basin during late Mesozoic to Cenozoic is presumed. It is shown that the northern of study area uplifted earlier than the southern, while the eastern margin uplifted earlierthan the inner of the basin.(4)The relation between tectonic event and the formation and accumulation of natural gas of Upper-Palaeozoic strata in northeastern of basin suggest that: there is a close relationship between the middle Yanshanian tectonic event and largest gas generating process of coal series source rock of Upper-Palaeoproterozoic, and the key time for gas generating and protogene gas reservoir accumulation are about 140-100Ma. The natural gas adjustment,dissipation and secondary accumulation is controlled by the strong uplifting tectonic event with age about 20Ma of slop belt in northeastern of basin.(5) In northeastern of basin, together with the structural framework of slope belt and the dynamical information of well, we can conclude that the gas reservoir in Taiyuan-early Shihezi formation dominantly belong to protogene depositional gas reservoir under the normal pressure, and the gas favourable reservoir belt was controled by framwork of main sandbody; But in upper Shihezi Formation to Shiqianfeng Formation the gas reservoir show the feature of secondary Tectonic-depositional gas reservoir under the extremely low pressure, and the gas favourable reservoir belt mainly locate in a zone which the low amplitude structure and main sandbody superposed in the slope belt.