摘要
本论文在综合利用地质、地球物理及地球化学资料的基础上,对鄂尔多斯盆地东南缘地区构造-热演化史进行了研究。应用镜质体反射率、裂变径迹等分析方法,使用Basinmod、AFTsolve等软件,结合鄂尔多斯盆地的构造和埋藏史,对盆地东南缘上三叠统的热演化历史进行了恢复,并对热演化史的差异性进行了分析,最后对热演化史与油气关系进行了讨论。
鄂尔多斯盆地与华北盆地在早中三叠世存在着大型统一的沉积盆地。晚三叠世盆地在挤压背景下,克拉通东缘隆升作用增强,华北大盆地不断向鄂尔多斯地块收缩,秦岭造山带闭合,产生强烈的岩浆活动及挤压作用,在周至柳叶河剖面及南召太山庙剖面见到晚三叠世砾岩,表明秦岭商丹断裂带是盆地的南界。晚三叠世鄂尔多斯大型坳陷型盆地具有南深北浅不对称的结构,具有前陆盆地性质。应用镜质体反射率法和声波时差法对研究区东南缘剥蚀厚度进行恢复,总体规律为：西部剥蚀量小,东部剥蚀量大。
鄂尔多斯盆地东南缘三叠系延长统热演化程度较高,Ro值在0.65%-1.2%之间。晚三叠世盆地南缘的洛南景村剖面由于受秦岭中生代陆内造山作用的影响,三叠系延长统Ro值高达3.71%,演化程度最高。因此鄂尔多斯盆地东南与原晚三叠世大盆地南缘洛南景村剖面属于不同的热演化史类型。鄂尔多斯盆地东南区主要属于埋藏增温型,而南缘洛南景村剖面属于受构造岩浆活动控制类型。
运用Basinmod和AFTsolve软件,结合镜质体反射率法和裂变径迹方法,模拟出了韩城薛峰川剖面、洛南地区的热演化历史。模拟结果表明鄂尔多斯盆地东南缘在中生代早期古地温梯度低；中生代晚期(100Ma左右)埋深达到最大,地温梯度增高,最高达到3.6-4.4℃／100m之间；晚白垩世以来地温梯度逐渐降低。原晚三叠世大盆地南缘洛南景村剖面中生代晚期约100Ma受构造岩浆侵入的影响,产生地热异常,达到最高热演化程度。因此在鄂尔多斯盆地东南缘中生代晚期存在—期约100Ma的重要构造热事件,洛南景村剖面更为强烈。
盆地东南缘地区从晚三叠世至早白垩世末,延长组烃源岩持续埋深,受热温度增加,热演化程度不断加强；在早白垩世晚期达到最大埋深,烃源岩达到最高热演化程度,早晚白垩世以来持续抬升,地层遭受剥蚀,地温降低,烃源岩的生烃作用明显降低并趋于停止。
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.
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