南海北缘东沙隆起中生代以来的埋藏史和热史重建——来自LF35-1-1井(U-Th)/He和Ro的证据

沉积盆地埋藏史和热历史重建是了解盆地成因和油气形成条件的重要依据,而目前利用古温标手段研究珠江口盆地内东沙隆起热史和埋藏史的成果寥寥无几.本研究中,我们基于磷灰石与锆石(U-Th)/He年龄的反演结果给出了更多合理的约束条件,在此基础上对同一套镜质体反射率Ro数据进行了古热流法模拟计算,获得了钻井自中生代以来的地层温度史.反演结果显示LF35-1-1井区在早始新世-早渐新世发生了一期强烈的抬升剥蚀,地层剥蚀量为2000 m左右.拟合获得的埋藏史和热流史显示该井区在早始新世(~55Ma)经历了最高井底古热流(100 mW·m~(-2)),之后热流减小,持续至现今64.3 mW·m~(-2).最高古地温与隆升剥蚀在时间上的耦合体现了抬升事件对地温冷却的影响,但不足以排除基底热流下降因素.本次研究首次将(U-Th)/He技术用于南海北缘深水区盆地的热史研究,获得的热史结果更加符合现有的构造沉积大地构造方面的认识,展示了利用多种古温标手段进行盆地精细热史研究的良好效用.

Mesozoic burial and geothermal history reconstruction of the Dongsha Uplift located in northern margin of the South China Sea:Evidence from (U-Th)/He and vitrinite reflectance Ro data of drillhole LF35-1-1

The burial history and geothermal history reconstruction of the sedimentary basins is an important basis for understanding the genesis and hydrocarbon formation conditions of the basin. So far, there are very few achievements in the study of the burial and geothermal history of the Dongsha Uplift in the Pearl River Mouth Basin. In our research, geothermal history inversions based on apatite and zircon (U-Th)/He data give out more reasonable constraints, by which we perform another round of simulation on the same set of vitrinite reflectance Ro so as to obtain a delicate burial and geothermal history since the Mesozoic. Further thermal inversion reveals an intensive uplift and exhumation from early Eocene to early Oligocene with a denudation amount to about 2 km. The bottom heat flow for the LF35-1-1 well area also experienced a peak value of 100 mW·m^-2 in early Eocene (~55 Ma) and decrease to 64.3 mW·m^-2 at present. The good consistence between the maximum palaeo-geotemperture and the beginning of denudation confirms the effect of uplift on the decrease of geo-temperature, but not enough to rule out factors such as basal heat flow decline. This is the first try to apply (U-Th)/He technology on the thermal history study of the deep water area on the northern margin of the South China Sea. Moreover, the co-simulation method on the basis of multiple geothermometers, whose results are more agreement with the existing achievements in terms of structure and deposition settings, has got well efficiency in the thermal history study.