南海北部台西南盆地浅层沉积物中自生碳酸盐岩形成动力学模拟研究

The project proposed to experimental and numerical study the geodynamic formation of authigenic carbonate in the shallow sediments in Taixinan Basin, northern South China Sea. .Plenty of authigenic carbonates has been discovered in Tianxinan Basin during the exploration cruises. They were chemoherm carbonates, chimney-like carbonate and other types of methane seep carbonates. The carbonates archived the information of geodynamic and environmental condition during the formation. Some gravity sediments columns have been recovered in the basin and the columns show one to 13 carbonate-rich layers. Each layer could be caused by one event of methane seepage, and the event corrected to the climatic fluctuations. The data of stable C-and O-isotopes confirm that the carbonates dominated by anaerobic oxidation of biogenic methane (AOM) to dissolved biocarbonate to solid calcium carbonate..Geodynamic formation of authigenic carbonate concerns the source of elements, the transport of chemicals, the geochemical reactions and the deposition and evolution. We have some knowledge of the series reactions, the source of carbon and oxygen and the transport of solid and liquid. However, the knowledge of the reaction rate, the mechanism of the geochemical reaction, and the evolution is still limit which motive the study of this proposal. This application will investigate those unknowns by experimental and numerical simulation..The first step of this proposal will define the time period of carbonate deposition and the environmental conditions, e.g. temperature, sea level and other constraints with local geochemical and mineralogical data. Then perform the experimental study under the determined environmental conditions. The experiments will reveal the process of carbonate forming, determine the deposition rates, estimate the dynamic change of porosity and permeability. With the assistant of the experimental data and the constraints of the field determination, the numerical simulation tool could be refined. Eventually, the proposal will model the deposition and evolution of authigenic carbonate in Taixinan Basin. Further, the methane flux during the time period of the carbonate formation will be estimated. The results will enhance our understanding of local ecosystem and provide information for the exploration of local geo-resource.

海域沉积自生碳酸盐岩保存了形成时期的环境、地质信息，是深部油气、天然气水合物藏的表层指征。全球学者对自生碳酸盐岩形成动力学进行了深入研究,并在地球化学反应路径、运移方式、物质来源等方面取得了重要进展。遗憾的是对于反应机制、时空演化等仍缺乏深刻理解。我国南海北部台西南盆地是甲烷渗漏区，存在大量自生碳酸盐岩，成为自生碳酸盐岩形成动力学研究的理想海域。本项目将依据海域实测资料，开展实验和数值模拟研究。实验模拟古海洋条件下自生碳酸盐岩的形成过程、计算反应速率、揭示环境条件对于反应速率和反应对孔隙介质的影响规律、探讨反应机制；根据实验结果校验数值模拟工具；并以海域和实验数据为边界和约束条件，模拟台西南盆地自生碳酸盐岩的形成过程和时空分布。进而推导碳酸盐岩形成时期的甲烷通量，估算地质历史时期进入大气的甲烷通量。研究结果将为区域海洋生态环境变化提供资料，为地质资源勘探、开采及其环境效应研究提供依据。

海域浅层沉积物中自生碳酸盐岩是有机物氧化沉淀的产物，是重要的碳“汇”，主要由甲烷厌氧氧化成矿作用贡献。浅层沉积物中甲烷主要来源于原位有机质降解、下层水合物分解和深部油气运移，因此自生碳酸盐岩可以作为水合物和油气资源的勘探标识。此外，碳酸盐岩的矿物种类、元素组分、同位素分馏等保存了成岩时的地质环境信息，是古气候古环境研究的重要载体。项目执行期间开展了南海北部陆坡、深海盆地和泥火山浅层沉积物的地球化学调查和模拟。.主要研究内容包括（1）甲烷厌氧氧化生物地球化学过程，包括典型区域的微生物种群分布、活性、环境因素，孔隙水及微生物种群对于甲烷厌氧氧化过程的影响和约束；（2）水岩反应过程，即根据不同甲烷渗漏强弱海域沉积孔隙水及岩石矿物组分，推导自生矿物形成就区间及速率；（3）实验和数值模拟自生碳酸盐岩形成，搭建多相流传质、传热、运移模型，开发相应模拟程序模拟多相流体系的变温、变压、流动、反应（结晶）过程；实验模拟不同对流条件下自生碳酸盐岩形成过程；依据台西南海洋四号沉积区数据数值模拟多层自生碳酸盐岩形成过程。.项目研究明确了南海北部不同甲烷渗漏区的微生物种群分布，发现微生物丰度与孔隙水中铁、硫酸根等浓度之间相关性不显著；揭示沉积柱中甲烷形成和甲烷消耗过程共存，且产、消速率差异不大；发现铁-硫酸盐-硝酸盐共同驱动厌氧甲烷氧化并揭示其微生物参与反应机制。实验显示500 m year-1和100 m year-1流速条件下碳酸盐岩生长1米分别需要12000年和7000年。台西南海洋四号海域当前甲烷渗漏较弱，甲烷氧化速率介于0.1-0.5 mmol m-2 year-1，而且自生碳酸盐岩对于浅部（5-8米）成岩贡献较弱（总自生碳酸盐岩的2 wt.%）。但地质历史时期（4-50ka BP.）有多次甲烷渗漏，并形成多层富碳酸盐岩沉积地层，其中GC-9站位附近7-50ka间至少有三次大规模的渗漏事件。推断高渗漏期甲烷氧化速率接近170 mmol m-2 year-1。项目工作为海域能源环境研究奠定了基础。.