Research on Paleotemperature in the Ordos Basin
摘要: 本文采用镜质体反射率、包裹体测温、磷灰石裂变经迹等多种古地温研究方法，确定了鄂尔多斯盆地中生代晚期高地温场的存在，古地温梯度高达３．５～４．０℃/１００ｍ，高于现今盆地的平均地温梯度２．８９°Ｃ/１００ｍ。古生界地层最大古地温一般在１５０～２４０°Ｃ之间，最高可达２７０°Ｃ以上，古地温明显高于今地温。高古地温梯度的形成与中生代末期的构造活动密切相关。根据盆地热演化史与油气关系研究可知，在古生代和中生代早期，地温梯度低。推迟了生气时代，有利于有机质的保存。中生代晚期，地温梯度高，是古生界碳酸盐岩及煤系地层的主要生气期及运移期。生气高峰期较晚，盆地内又缺乏断裂，有利于大气田的保存。Abstract: The Ordos basin, situated in the western part of the North China platform, has been classified as an intracrationic basin. The basin is rich in oil and gas resoureces contained three kinds of source rocks. In the exploration for oil and gas an important breakthrouths have been made in recent years. A giant gas field has been discovered in Jingbian county in the central part of the basin. Based on analyses of data of vitrinite reflectance, fluid in clusions and apatite fisson track, this paper has determined the high palaeothermal filed of the Ordos basin in the late mesozoic. The palaeotemperature in the palaeozoic strata is abort 150~240℃., the highest temperature exceeding 270℃,which is higher than the present tempreture. The palaeotemperature gradient (3. 5~-4. 0℃/100m) which is closely related to tectonic activity also exceeds that (2. 89℃/1OOm) of the present. Apatite fission track data show that there is a cooling event which ocurred in 20 to 23 Ma. The uplift scope in North shaanxi slope in the east is obviously bigger than in the Tianhuan syncline. Reseach on relations between the thermal history of the Ordos basin and oil gas fields suggests that low temperature gradients in paleozic and early mesozoic times was favourable for preservation of organic material and the gas generation stage was postponed. The late Mesozoic which is characterized by highertemperatures in the period of main gas generation and migration of paleozoic coal series and carbonates. The late gas generation stage and lack of faults were favourable for preservation of giant gas fields. The central palaeouplift in the basin which has good carbonate reservoirs and is close to gas generation regions should be first chosen as an exploration target. The southern part of the yiming uplift, the northern Part of the weibei uplift, the eastern part of the north shaanxi slope and the western part of Tianhuan syncline are also potential regions of gas migration and the reserve bases for looking for new gas fields.
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