柴达木盆地东部燕山期剥蚀量恢复:来自地球物理和低温热年代学的证据

依据地面地质调查分析了柴达木盆地东部主要不整合面的接触关系和发育特征,并利用古温标(镜质体反射率(R_o)、磷灰石裂变径迹)反演和地震地层趋势延伸法对柴达木盆地东部燕山期剥蚀量进行了恢复。结果表明,研究区东部旺尕秀、牦牛山一带晚燕山期剥蚀量为500~2 300 m,研究区南缘格尔木、大干沟、纳赤台一带剥蚀厚度最高达2 000 m,研究区北缘绿梁山、大柴旦一带增大到平均1 750 m,至柴东欧南凹陷、霍布逊凹陷逐渐减小到平均700 m。该区剥蚀厚度表现出由周缘老山向坳陷中心逐步减小的趋势,并呈现出“东西分块、南北分带”的特征。柴达木盆地东部燕山运动发生的时间为87~62 Ma,该时期研究区整体处于SE-NW向弱挤压的环境,与早白垩世末冈底斯—念青唐古拉地块群和欧亚大陆拼合有关,导致柴达木盆地东部差异隆升,形成现今欧龙布鲁克隆起带的雏形,并进一步控制该区油气的富集,一方面是造成局部地区地层剥蚀严重,油气散失;另一方面是因构造挤压,在覆盖区挤压拗陷,主力烃源岩进一步深埋,该区侏罗系烃源岩进一步成熟生烃,下伏石炭系烃源岩经历二次生烃;同时在挤压环境下形成一系列的近EW向的圈闭,有利油气的调整、聚集。

Rehabilitation status of denuded land in the eastern Qaidam Basin: Geophysical and thermochronological evidences

On the basis of geological field survey of the eastern Qaidam Basin, the contact relationships and development characteristics of unconformities in the basin were analyzed. Combined with geophysical and apatite fission track data and through seismic stratigraphic analysis, the rehabilitation status of denuded land in the eastern Qaidam Basin were evaluated. The results showed that during the period of Late Yanshanian movement, erosion thickness in the eastern Qaidam Basin ranged between 500-2300 m. In the basin’s southern and northern margins erosion thickness ranged between 1100-2000 and 1200-2300 m, respectively, and reduced gradually to 500-1000 m at the basin center. The overall denudation decreased gradually from the edge to the center of the basin, with its western section significantly more denuded than the eastern counterpart. The Yanshanian movement in the eastern Qaidam basin occurred between 87-62 Ma. During this period, the basin as a whole experienced weakly SE-NW compression. which caused the differential uplifting in the eastern part of the Qaidam basin to form the Olongbulukon belt prototype, further controlling oil and gas accumulation in the region. On the one hand, differential uplifting caused severe formation denudation in some areas, leading to loss of oil and gas; on the other hand, structural compression caused further burial of main source rocks in the affected area, where Jurassic source rocks further matured to generate hydrocarbons. The underlying Carboniferous source rock underwent secondary hydrocarbon generation; at the same time, a series of near-EW-trending traps were formed under compression, which favored oil and gas accumulation.