南海珠江口盆地白云凹陷现今地温场与新生代构造—热演化特征

Present-day temperature field and the Cenozoic tectono-thermal evolution in Baiyun Sag of the Pearl River Mouth Basin,South China Sea

位于南海北缘的珠江口盆地深水区作为我国海洋油气勘探的重点区域,具有良好的油气勘探前景。本文利用钻井地热测量数据分析了珠江口盆地深水区现今地温场及岩石圈热结构特征,通过古温标反演和拉张盆地模型正演相结合的方法定量揭示了白云凹陷新生代以来的热史,总结了不同地热地质条件对油气生成的影响。研究结果表明:珠江口盆地大地热流分布特征具有北低南高的特点,同时具有“热幔冷壳”的特征。珠江口盆地深水区始新世以来经历了两期拉张过程,第一期(47.8~33.9 Ma)拉张自始新世发生,拉张强度较大,凹陷中心基底热流快速上升至~82 mW/m^2;第二期(23~13.8 Ma)拉张发生于中新世,此次拉张在白云凹陷南部更强烈,白云凹陷主体在13.8 Ma达到最高古基底热流,此后进入热沉降阶段,基底热流值一直缓慢下降。磷灰石裂变径迹、磷灰石(U-Th)/He及锆石(U-Th)/He联合反演给出了最高古地温在13.8 Ma附近达到,后期温度基本稳定。

The deep water area of the Pearl River Mouth Basin,located at the northern margin of the South China Sea,is a key area for offshore oil and gas exploration in South China Sea,and has a good prospect for oil and gas exploration.In this paper,the characteristics of the present geothermal field and lithospheric thermal structure in the deep water area of the Pearl River Mouth Basin are analyzed by means of the borehole geothermal measurements.The results show that the distribution of terrestrial heat flow in the Pearl River Mouth Basin is low in the north and high in the south.Since the Eocene,the Baiyun Sag in the deep-water area of the Pearl River Mouth Basin has experienced two stages of extensions.The first stage(47.8~33.9 Ma)rifting occurred in the Eocene,with a large extension intensity,and the basal heat flow in the depression rose rapidly to^82 mW/m^2.The second phase(23~13.8 Ma)occurred in the Miocene,which was more intense in the south of the Baiyun Sag.The main body of the Baiyun Sag reached the highest paleo-heat flow at 13.8 Ma,and then entered the thermal subsidence stage,and the basement heat flow decreased slowly.The joint inversion of apatite fission track,apatite(U-Th)/He,and zircon(U-Th)/He show that the maximum paleo-temperature reached around 13.8 Ma,and then the temperature was basically stable.