据磷灰石裂变径迹和镜质体反射率重建苏丹Muglad盆地北部的构造-热演化历史

Tectonic-thermal evolution history reconstruction of the northern Muglad Basin in Sudan based on apatite fission track analysis and vitrinite reflectance data

Muglad盆地是中非裂谷系在前寒武系结晶基底上发育的大型裂谷盆地,经历了早白垩世、晚白垩世和古近纪3期裂谷发育阶段。通过选取盆地北部不同构造位置3口探井不同层位的岩屑样品,开展磷灰石裂变径迹和镜质体反射率测试,进行构造-热演化历史分析。研究结果表明,盆地存在两期冷却事件,但不同构造单元的演化史有较大的差异。第1期冷却事件发生在晚白垩世早期,对应区域上的"圣通期挤压事件",主要在Tomat隆起表现明显;第2裂谷期末期是盆地区域冷却事件(古新世末期—始新世),造成Nugara坳陷中央凸起形成和Tomat隆起进一步发育,也控制了盆地内油气藏的形成和分布。盆地第1裂谷期和第2裂谷期的古地温梯度只有2.00~2.40℃/hm,第3裂谷期盆地深部地幔隆起发育、岩浆活动增强、地温梯度升高,指示了盆地由白垩纪期间的"被动裂谷"演变为古近纪的"主动裂谷"。第3裂谷期末期的剥蚀作用不明显,对油气的生成和聚集影响不大或没有影响。

The Muglad Basin is a large rift basin developed on the Precambrian crystalline basement in the Central African Rift System, and has undergone three rifting development stages, namely Early Cretaceous, Late Cretaceous and Paleogene. In this paper, the cutting samples from three exploration wells of different structural units in the northern part of the basin were selected to carry out the apatite fission track(AFT)and vitrinite reflectance(Ro)analyses, so as to conduct the reconstruction of tectonic-thermal evolution history. The results reveal that there were two cooling events and the thermal evolution history at different structural units had great difference. The first cooling event occurred in the early Late Cretaceous, corresponding to the "Santonian compressive event", mainly manifest in the Tomat uplift. The second cooling event was a basin-wide cooling event(the end Paleocene to Eocene), resulting in the formation of the Central Rise of the Nugara subbasin and the further development of the Tomat uplift, and controlling the occurrence of oil and gas fields in the basin. AFT and Ro analyses also revealed that the paleo-geothermal gradient was only 2.00-2.40 ℃/hm during the Cretaceous. The increase of paleothermal gradient, occurrence of volcanic rocks, due to the development of positive gravity response of the elevated Moho over the Muglad, indicate that the basin type changed from a "passive" rift to "active" rift. The denudation at the end of the third rifting development stage was not obvious and had little effect on the generation and accumulation of oil and gas.