南海北部深水区白云凹陷构造-热演化历史研究

It is known to all that deepwater area of passive continental margin is a hot spot for petroleum exploration at present. Owing to a favorable perspective of oil and gas resources in this area, the Baiyun sag has been regarded as the preferred target of deepwater petroleum exploration in China. Located in the junction of the Eurasia Plate, the Pacific Plate and the Australian plate, and the deepwater area of ocean-land transition zone on northern slope of the South China Sea, Baiyun Sag has experienced complex multi-tectono-thermal history since the late Mesozoic. However, there is no systematic study on the tectonic-thermal evolution of the study area because the lack of basic data due to the difficulty of exploration in the offshore deepwater areas. So far, there are many issues on the tectono-thermal history reconstruction in the study area, such as the unclearly understanding of the present geothermal field and lithospheric thermal structure, the controversy in the suitability numerical model for the thermal history forward simulation, and the missing of the constraints on the thermal history from the inversion results of thermal indicators. All this leads to different understanding of time of main hydrocarbon generation and end of hydrocarbon generation, thereby affecting the correct understanding of the law of oil and gas accumulation, which has seriously restricted the oil and gas exploration in the region. Therefore, this project intends to improve and establish a suitable numerical model for the thermal history calculation on basis of the previous models; construct an effective paleo-thermometer profile based on the systemic tests of different types of thermal indicators; and further reconstruct the tectono-thermal history since the Late Mesozoic of the Baiyun sag by combinating constraints from both numerical forward modeling and thermal indicator reversion. The results can not only provide thermal history for a correct understanding of the hydrocarbon accumulation and evolution of the Baiyun sag, but also provide geothermal evidence for us to understand the dynamics since the late Mesozoic at northern margin of the South China Sea.

被动大陆边缘深水区是目前世界油气勘探的热点区，南海北部深水区白云凹陷因具有良好的油气勘探远景而成为我国深水油气勘探的重点区域。白云凹陷自晚中生代以来经历了多期复杂构造-热历史过程，然而由于海上深水区勘探难度大，基础资料稀少，研究区缺乏系统的构造-热演化研究。存在现今地温场与岩石圈热结构认识不清，热史数值模拟正演模型存在争议，热史演化缺少古温标反演约束等问题，难以清晰地认识该凹陷的构造-热演化历史，导致对该凹陷烃源岩主生烃期和生烃结束的地质时间认识不一，进而严重制约其油气勘探。因此，该项目拟在前人模型基础上，改进并建立适合研究区的热史正演数值模型；系统测试不同类型的古温标，建立有效古温标剖面；正反演联合约束其晚中生代以来的构造-热演化史。研究成果不仅可以为正确认识白云凹陷油气成藏演化规律提供热史依据，还可为南海北缘的盆地动力学提供地热学证据。

随着我国对能源的迫切需要，现今南海的油气勘探已向深水推进。为了减少勘探开发的风险与成本，与盆地油气成熟成藏密切相关的盆地构造-热演化过程等成为亟待解决的重要基础科学问题。本项目在系统测试搜集基础地热参数的基础上，首先研究了白云凹陷现今温度场。然后从岩石圈尺度，开发多期有限拉张模型，正演计算了白云凹陷基底的热流演化历史；从盆地尺度，基于磷灰石裂变径迹数据，约束了凹陷的地层温度史。在此基础上进一步分析热演化史的动力学机制，以期为研究区烃源岩热演化与南海构造演化等提供地热学方面的证据。研究中获得了如下关键数据：(1)4口井80个岩石热导率和5口井75个生热率，新增了研究区热物理基础参数；(2)校正了14口井的BHT数据，计算了其地温梯度与大地热流值，填补了南海深水区地温梯度、热流数据空白；(3)根据收集的地震测线，建立了55口人工井，为后期计算构造沉降与基底热流奠定了基础；(4)测试了9口井16个镜质体反射率Ro、6口井24个磷灰石裂变径迹、24个锆石(U-Th)/He数据，为热史反演提供了必要的古温标数据。通过系统研究，取得了以下认识和结论：白云凹陷的地温梯度介于35.6-60.5℃/km，平均值46.2℃/km，大地热流介于62.2-99.8mW/m2，平均79.0mW/m2。研究区49Ma以来分别于49-33.9Ma与33.9-23Ma期间经历了两期持续裂陷拉张，对应两期基底热流升高。第一期拉张结束后，基底热流升高至~57.7-86.2mW/m2，第二期拉张结束后，基底热流升高至~66.7-97.3mW/m2。自23Ma以后，研究区进入裂后热沉降期，基底热流降低。由于经历多期拉张，白云凹陷的现今热岩石圈厚度薄，介于48-64km，平均54km。凹陷地层中始新世以来经历了四次升温三次冷却，前两次冷却可能分别源于珠琼二幕运动与南海运动造成的沉积物抬升剥蚀，第三次冷却事件主要受东沙运动影响。