上扬子地区早寒武世优质烃源岩的形成机理

During the Precambrian/Cambrian transition, a succession of organic-rich sediments was widely developed in the Yangtze area. They may have recorded the interactions among the various earth systems during this transition period, and are the important marine source rocks and unconventional natural gas layers in the Yangtze area, which have great value for scientific research and industrial application. At present, newly-discovered taphrogenic troughs changed the traditional views about palaeostructure pattern of the upper Yangtze area during the early Cambrian. Thus, geographic pattern of the early Cambrian Ocean and formation mechanism of high-quality source rocks need to be reappraisal. Based on the approaches of sedimentology, petrology and geochemistry, this project will reconstruct the geographic pattern of the upper Yangtze area during the early Cambrian by using abundant data from outcrops and wells, as well as regional seismic data. Spatiotemporal fluctuations of redox condition and paleaoproductivity in the upper Yangtze area during the early Cambrian and their controlling factors will be studied by the means of pyrite morphology, total organic carbon, total sulfur, carbon isotopes, as well as major, trace and rare earth elements. Taking detrital input, hydrothermal exhalation, aeolian input, paleoclimate and other factors into consideration, this project will conduct a quantitative study on controlling factors of organic matter accumulation. Combined with geological setting of study area (including tectonics, paleoclimate, special episodes), the formation mechanism and its controlling factors of the early Cambrian high-quality source rocks in the upper Yangtze area will be concluded, and the model for high-quality source rock formation will be established. The results of this project will provide scientific basis for further oil and gas exploration.

前寒武纪/寒武纪转折期，扬子地区广泛发育一套富有机质沉积物，它们记录了该时期地球各圈层之间的相互作用，同时也是扬子地区重要的海相烃源岩层和非常规天然气产层，具有重要的科学和实际研究意义。目前，新发现的古裂陷改变了上扬子地区早寒武世古构造的传统认识，早寒武世古地理格局及其优质烃源岩的形成机理亟待重新审视。本项目以沉积学、岩石学和地球化学等为基础，利用大量野外、钻井资料和区域地震资料重建上扬子地区早寒武世的古地理格局；利用黄铁矿形态、总有机碳、总硫、碳同位素、主量、微量和稀土元素等方法，研究上扬子地区早寒武世的氧化还原状态和古生产力的时空波动规律及其控制因素；考虑陆源输入、热液喷发、风成输入和古气候等因素，定量化研究有机质富集的控制因素；综合研究区古构造、古气候、特殊事件等地质背景，总结上扬子地区早寒武世优质烃源岩的形成机理及控制因素，建立优质烃源岩的形成模式，为下一步的油气勘探提供科学依据。

早寒武世，上扬子地区广泛发育一套富有机质沉积物，它们记录了该时期地球各圈层之间的相互作用，同时也是扬子地区重要的海相烃源岩层和非常规天然气产层。本项研究通过沉积学、岩石学和地球化学等方法，并结合区域地质背景和地震资料，对上扬子地区下寒武统烃源岩的分布以及形成机理进行了细致的研究，得出以下几点结论：（1）上扬子地区发育“绵阳—长宁古裂陷”，其在剖面结构上具有“东陡西缓”的特征，在平面上可以分为三段，即北段、中段和南段，具有“南北宽、中段窄”的特征，在发育时间上具有“北早南晚”的发育特征。由于“绵阳-长宁古裂陷”属于碳酸盐岩台地内部分割出来的裂陷盆地，也被称之为“台内盆地”。因此，扬子板块早寒武世古地理格局可以自西向东划分为碳酸盐岩台地、台内盆地、过渡带、斜坡和深水盆地。（2）区域地层对比揭示，下寒武统在上扬子地区分布并不完整，其在台内盆地相区保存相对较好。其中，麦地坪组岩性较为复杂，包含硅质白云岩、含磷白云岩、硅质岩、磷块岩和黑色页岩等，沉积年龄被限定在541~526.5 Ma，即幸运阶—第二阶早期；上覆筇竹寺组岩性相对单一，主要由黑色页岩、粉砂质页岩和粉砂岩组成，沉积年龄被限定在526.5~515 Ma，即第二阶中期—第三阶中期。（3）上扬子地区早寒武世氧化还原条件主体以缺氧环境为主，于海侵初期伴随有多幕次硫化水体环境，于海退阶段伴随有多幕次氧化水体环境，古海洋的氧化还原条件的波动可能与海平面的升降直接相关。（4）扬子地区下寒武统富有机质烃源岩一般容易发育在地貌低地区域，如斜坡—深水盆地和台内盆地相区。下寒武统烃源岩有机质富集的控制因素随着沉积相变化而变化：台内盆地相区中有机质的富集很大程度上受控于还原的底水环境；碳酸盐岩台地相区中有机质的富集同时受古生产力和底水环境所控制，较强的古生产力和缺氧的底水环境有利于有机质的富集；斜坡—深水盆地相区中有机质的富集主要受较强的古生产力控制，且古生产力可能受控于沿岸上升流的活动。