Research On Tectonic-thermal Evolution History Of Upper Triassic In Southeast Margin Of The Ordos Basin

Using the geological, geophysical and geochemical information, tectonic-thermal evolution history in the southeast margin of the Ordos basin is studied in this paper. According to the tectonic and burial history of Ordos basin, combining vitrinite reflectance and fission track analysis,the thermal evolutional history of the upper triassic in the southeastern region of the basin is recovered by the tools of Basinmod and AFTsolve. Comparative analysis the difference of thermal evolution history and discuss the relationship between the region thermal evolutional history and oil-gas.In early Triassic, Ordos basin and North China basin were a large unified sedimentary basin.Under the compression action in the late Triassic, the eastern margins of Craton uplift, the Huabei basin constantly contract to Ordos block, Qinling orogen was closed and produced strong magmatic activity and orogenic crumpling. Late Triassic conglomerate is found in liuyehe section and Taishanmiao section,which shows that ShangDan fault is the south boundary of Huabei basin. The Structures of late Triassic Ordos large downwarped basin is Asymmetric, the south is deep and the north is shallow.The basin has the characteristics of foreland basins.The erosion thickness of the formation in the southeastern region of the basin is recovered by vitrinite reflectance method and interval transit time Method. The analysis indicates that the general regulation is the western denuded small and eastern denuded.The thermal evolution of organic in Southeastern margin of Ordos Basin is high,and Ro ranges from 0.65%to 1.2%. in late Triassic, the luonanjingcun section at primeval baisin's south edge was influenced by intracontinental orogenesis,the Ro of upper Triassic Yanchang group,which is the higest degree of thermal evolution can reach 3.71%.These two region belong to different type of thermal evolution. Southeastern margin of Ordos Basin is Buried heating type,and the luonanjingcun section is the type control by tectonic-magmatic activities. Combining vitrinite reflectance method and fission track method, the thermal evolution history of Hancheng and Luonan in Southeast margin Ordos Basin has been modeled by the software of basinmod and AFTsolve.The modeling results indicate that the paleo-geothermal gradient was low during the Mesozoic; And it once reached 3.6℃/100m to 4.4℃/100m when the buried depth reached maximum in l00Ma during Late Mesozoic; It has became gradually lower since late cretaceous.in late Triassic, the luonanjingcun section at primeval baisin's south edge generate geothermal anomaly because it influenced by tectonic magmatic intrusion in late Mesozoic,and reached highest thermal evolution degree,therefore in the Southeast Margin of the Ordos Basin had a important structure-heat event at late Mesozoic, especially in Luonanjingcun section.The temperature increased when the hydrocarbon source rocks buried deep of Yanchang formation in southeastern region from late Triassic-Early Cretaceous.In this case, thermal evolution has been constantly strengthened. In Early Cretaceous, hydrocarbon generation was strongest for the deepest burial depth. With the continue upliftting from the early cretaceous late, strata was being eroded and reached the maximum. At that time,the temperature cut down and hydrocarbon generation of hydrocarbon source rocks was significantly reduced and tend to stop.