陆源碎屑-碳酸盐混合沉积及其早期成岩的化学动力学研究

Because of lacking basic theory, and constrained by the conventional study methods of siliciclastic and carbonate rock, the research progress of mixed sediment is slow. There were a lot of papers have reported some cases of mixed siliciclastic-carbonate sediment, which almost focus on the phenomenon and geological controlling factors, but little attention paid on the origin mechanism. The mixed carbonate rock reservoir of Carboniferous was found at Bachu-Makit area in Tarim Basin, which deposit in shoal with good physical properties. Through the study before, the early dolomitization and weak cementation was the key of formation of reservoir. However, the conventional theory can’t give a reasonable explanation for the origin of the type of reservoir, showing that the mechanism of egodiagenesis events were special. So that it’s necessary to study it. The mixed sedimentation is not only controlled by dynamics, but also by the chemistry or bio-chemistry. This project is going to combine the study of mineralogy and chemistry of modern mixed siliciclastic-carbonate sediment, simulation experiment and mixed carbonate rock, and finally make clear two questions. (1)The first question is what chemical constraint of mixed sedimentation is, (2)and the other question is what the basic rock-water reactions in the egodiagensis of mixed sediment are, as well as the precipitation-dissolution and transformation of mineral. This project will make a breakthrough in the theory of chemical dynamics of mixed sediment, and can guide the exploration of mixed rock reservoir, which have great scientific significance and realistic value.

由于缺乏基础理论支撑，又受到常规碎屑岩和碳酸盐岩研究方法的束缚，混合沉积的研究一直进展缓慢。虽然近年来国内外报道了大量混合沉积实例，但大多仅关注现象和地质控制因素，缺乏机理和成因研究。塔里木盆地巴麦地区石炭系发育储集性较好的滩相混积型碳酸盐岩储层，早期白云石化和弱胶结是储层形成的关键，但利用常规方法难以对其成因进行合理解释，表明混积背景下的早期成岩事件具有其特殊性，亟需开展相关研究。混合沉积不仅受动力学作用控制，还受化学和生物化学作用的控制。本项目拟通过现代混合沉积物、模拟混积实验和混积型碳酸盐岩的矿物学和化学特征相结合研究，最终弄清两个基本理论问题：1.混合沉积过程的化学制约问题；2.混合沉积物早期成岩的基本水岩反应类型和矿物沉淀-溶解和转化问题。该项目的研究成果是对混合沉积化学动力学理论的突破，能指导这类油气储层的勘探开发，因而具有重要的科学意义和现实价值。

本项目利用现代沉积物、模拟水岩反应实验和古代混积岩研究，开展了陆源碎屑-碳酸盐混合沉积体系中成岩特征的研究。.现代混合沉积物取样分析表明陆源碎屑的分布受到沿岸流的影响。方解石和白云石与钾长石的分布之间均具有相关性。通过模拟水岩反应，在不同比例的陆源碎屑和碳酸盐矿物的实验体系中，陆源碎屑:碳酸盐组分为1:9的弱碱性体系中随时间增加所释放的离子浓度增加比其他比例和在酸性体系更加明显，并且K和Ca、Si和Mg的化学行为具有一致性。.本研究通过主微量元素的含量与陆源碎屑矿物所占比例之间的相关性，将混合沉积的强度划分为I-IV级。并将I、II级混积归为相缘混合的产物，III-IV级混积归为间断混合的产物。从而初步建立了混合沉积的化学识别标志。.通过对塔里木盆地石炭系混积型碳酸盐岩的研究明确了在混合沉积背景下存在早期白云石化、弱胶结、石英次生加大和自生高岭石等特殊的成岩作用。这些特殊的成岩作用与混合沉积过程本身有关。在海平面下降阶段，碳酸盐浅滩中混入少量陆源碎屑，导致淡水的少量输入。低有序度的白云石化发生在低温近地表环境。淡水的渗入导致了早期的弱胶结。而钾长石等不稳定的陆源碎屑遭受淡水淋滤发生溶蚀，产生较多自生高岭石副产物。而水岩反应过程中硅质释放时形成的局部高镁的环境使石英表面沉淀了环边状白云石。.通过对莺歌海盆地碎屑岩中碳酸盐胶结物的分析，碳酸盐矿物的形成主要受成岩流体性质影响，包括流体中钙离子来源，铁离子含量，Mg/Ca比值，二氧化碳分压、氧化还原条件的影响。.本项目的研究初步解决了陆源碎屑与碳酸盐混合沉积的海相体系中所涉及到的成岩作用类型及其制约因素。研究成果从化学成因角度对于混合沉积中关键元素迁移、矿物溶蚀-沉淀机制等方面进行解释，从理论角度推进地史时期混合沉积物成岩过程的认识，也能补充研究碎屑岩和碳酸盐岩在海相环境中的成岩过程。