华北早中三叠世盆地恢复与古生界生烃

Reconstruction of Early-Middle Triassic basin in North China and hydrocarbon generation in Palaeozoic

华北东部地区中下三叠统分布零星,面积小,但具有相似的岩性、岩相和韵律特征,反映为大型盆地的沉积背景。在广大范围内,由西北至东南,砂岩颗粒变细、泥质含量增多、沉积厚度变大的方向性明显,反映早中三叠世的沉积面貌和晚二叠世晚期(石千峰期)相似,表明其不但继承了晚二叠世“北聚南散”的古地球动力学背景,而且继承了北西高、南东低的古地理格局,说明海西运动一直延续到中三叠世。应用镜质体反射率(R_o)法、声波时差法及磷灰石裂变径迹法等多种方法恢复了中下三叠统的沉积厚度,编制了原型盆地沉积等厚图。中下三叠统沉积沉降中心位于临清-东濮-太康和两淮地区,沉积中心的中下三叠统厚度达2000～2600 m。伴随中下三叠统的沉积,至中三叠世末,下古生界马家沟组顶面埋深达3080～3690 m,古地温为112.4～130.7℃;R_o 值为0.85%～1.10%,下古生界以海相有机质为主的烃源岩已过了生油高峰,有机质的80%已完成向石油的转化,后期生烃潜力微乎其微;而上古生界以煤系为主的烃源岩,由于腐殖煤生烃高峰比Ⅰ-Ⅱ_1型干酪根生烃高峰滞后,尚处于早期生烃阶段,仍保留有较大的生烃潜力。

Middle-Lower Triassic in eastern part of North China is fragmentarily distributed in small areas;however, they have similar lithologies,fades and rhythmic characteristics,indicating a depositional setting of a large basin.It can be observed in a wide area that the grain size of sandstone is fining,shale content is increasing,and sediment is thickening apparently from NW to SE,indicating that sedimentary features in Early-Middle Triassic were similar with that in late Late Permian(Shiqianfeng age).Middle-Lower Triassic has inherited not only the Late Permian palaeo- geodynamic setting of“convergence in the north and divergence in the south”,but also the palaeogeographie flame- work of high in the northwest and low in the southeast.All these show that Hercynian movement would have lasted till Middle Triassic.Several methods including vitrinite reflectance(R_o),interval transit time and apatite fission track, are used to reconstruct the sedimentary thickness of Middle-Lower Triassic,and to plot the isopach map of the proto- type basin.The sedimentary-subsidence center is located in Linqing-Dongpu-Taikang and Huainan-Huaibei areas, where the Middle-Lower Triassic is as thick as 2000～2600m.At the end of Meddle Triassic,the burial depth of the top of Majiagou Formation in Lower Paleozoic would have reached 3080～3690m,the palaeogeotemperature would be 112.4～130.7℃,and the Ro would be 0.85%～1.10%.The Lower Paleozoic source rocks,mainly con- sisting of marine organic matters,would have passed the peak oil generation stage,with 80% of the organic matters have been converted into oil,thus the hydrocarbon generation potential in later stage would be very low.While the Upper Paleozoic source rocks,mainly consisting of coal formation,were still in the early stage of hydrocarbon genera- tion with relatively good hydrocarbon generating potential,because the peak hydrocarbon generation of humic coal would lag behind the Type Ⅰ-Ⅱ_1 kerogens.