酒西盆地早白垩世湖生浮游藻类及其与烃源岩发育和热流体活动的关系

The Lower Cretaceous dark mudstone and argillaceous dolomite are the main oil source rocks in the Jiuxi Basin. China's first oil industrial base, Yumen Oilfield, was established on the basis of the early exploration of the layer, and which laid the foundation of the terrestrial oil-generation theory. Recent studies show that the abundant algae laminae of the Xiagou Formation in Qingxi Sag, the main oil-producing region, is closely related to the enrichment of organic matter in hydrocarbon source rocks. In addition, the iron dolomite deposits interbedded with normal mudstone in deep lacustrine source rocks are considered as rare lacustrine exhalative related to the source rocks closely. The applicant found abundant algal fossils, such as Dinoflagellates, Botryococcus etc. in a preliminary study of the Lower Cretaceous source rocks in Jiuxi Basin, and thought that they may be the direct biological origin of algae laminae and high-quality source rocks in the basin. To further confirm this inference, this project will choose typical Cretaceous profiles to carry out systematical source rock evaluation and microfossil analysis, depict geochemical sections and algae combinatorial sequences in detail, and via the establishment of quantitative relationship between fossil abundance and source rock evaluation indexes to reveal the close relationship between algae distribution and source rock development. through the research on hydrothermal characteristic minerals and typochemical elements to discuss the coupling relationship between algae blooming and thermal fluid activities. The research of alga of terrestrial source rocks and their paleoenvironment have important significance in guiding oil exploration and improving the terrestrial oil-generation theory.

下白垩统暗色泥岩和泥质白云岩是酒西盆地石油的主要源岩，该层系的早期勘探建成了我国最早的石油基地——玉门油田，奠定了我国陆相生油理论的基础。近年来的研究表明，主要产油区青西凹陷下沟组丰富的藻纹层与烃源岩有机质的富集密切相关。此外，深湖相烃源岩中与正常泥岩互层的铁白云石沉积，被认为是与烃源岩关系密切的罕见湖相喷流岩。申请者对酒西盆地下白垩统烃源岩初步研究中发现了丰富的沟鞭藻、葡萄藻等藻类化石，认为他们可能是该地区藻纹层及优质源岩形成的直接生源。为了进一步证实这一认识，本项目选择下白垩统典型剖面开展系统的烃源岩综合评价与微化石分析，详细刻画地球化学剖面和藻类组合序列，通过化石丰度与烃源岩评价指标定量关系的建立，揭示藻类分布与烃源岩发育的密切联系。并通过热水成因特征矿物和标型元素研究，探讨藻类勃发与热流体活动之间的耦合关系。湖相成烃藻类及其古环境研究对指导石油勘探和完善陆相生油理论均具有重要意义。

下白垩统赤金堡组和下沟组是酒西盆地的主要烃源岩，迄今他们的时代归属仍存在不同认识。孢粉组合时代讨论认为，赤金堡组形成于早白垩世早期Berriasian-Valanginian，下沟组形成于早白垩世早中期Hauterivian-Barremian。盆地区早白垩世早期植被主要由松柏类针叶林构成，气候较为温暖，早白垩世中晚期植被由松柏类针叶林、蕨类和苏铁类灌木共同构成，较为湿热，并表现出干-湿气候的波动。烃源岩评价和孢粉相分析表明，赤金堡组是半干旱气候下的河湖相沉积，中部和顶部为半深-深湖相，出现大量II1型有机质，可发育好-优质烃源岩。盆地北部区域下沟组为相对温湿气候的河流-滨浅湖沉积，以III型有机质为主，烃源岩不发育。下白垩统纹层状暗色泥岩被认为是酒西盆地的重要烃源岩，而浮游藻类是纹层状源岩的主要生源，但长期以来烃源岩中并未发现可靠的藻类化石。有机壁微化石分析，首次在下沟组发现较丰富的以沟鞭藻为主的藻类组合。纹层微细结构分析显示，暗色层由密集的藻类残骸组成，颗粒AOM在暗色纹层中也很丰富，浅色层以植物碎屑为主。此外，丰富的甲藻甾烷的检出也为沟鞭藻母质来源提供了佐证。沟鞭藻的发现不仅确认了下白垩统烃源岩母质来源，而且为揭示盆地区“小而肥”油藏的形成机制提供了依据。通过鄂尔多斯盆地全取芯钻井系统的热水沉积演化历史研究，探讨了优质烃源岩形成与热流体活动的关系。延长组热水活动强度指标Al/(Al+Fe+Mn)和(Fe+Mn)/Ti指示，盆地区长7-长6期经历了3幕较强的热水活动，其中长7-3期为热水活动高峰，该段烃源岩以Ⅰ型和Ⅱ1型有机质为主，并出现高丰度的藻类体，指示深湖相缺氧沉积环境，强烈的热水活动诱发了藻类的勃发和优质烃源岩沉积。热流体活动不仅使烃源岩TOC含量明显升高，而且使I型有机质丰度显著提高，从而导致湖泊优质烃源岩的形成。