土壤蚀变碳酸盐的区域性分布特征及其地球化学意义

摘要

The soil secondary carbonate (∆C) method is a commonly used surface geochemical exploration technique for oil and gas geochemical surveys. However, its application effectiveness is unsatisfactory due to lack of understanding of the carbon source and the mineral morphology of ∆C. In this study, the concentration and carbon isotopic composition of ∆C in surface soils, offshore sediments, and drilling cuttings from different areas were measured to reveal their carbon sources, distribution characteristics and mineral morphology. The results show that the concentration of∆C in different samples is closely related to the depositional environment of these samples. For example, the near-surface soils on land have a relatively higher concentration of∆C, followed by the offshore sediments, and the ∆C content in drilling cuttings is the lowest. While theδ13C values of ∆C in drilling cuttings are generally more negative than those in near-surface soils and offshore sediments. These results further indicate that the carbons formed∆C in different samples may have three sources, including inorganic, organic and mixed sources, and the contributions of different carbon sources are also depended on the depositional environment of samples. The result of X-ray diffraction and phase analysis of some typical samples shows that the CO2 released by pyrolysis is mainly derived from the decomposition of calcite and dolomite in samples, and a small amount provided by some unknown minerals.%土壤蚀变碳酸盐(∆C)是油气化探的一个重要指标，在油气化探中得到广泛的使用，并取得了较好的效果。然而由于对∆C 的碳源及其具体矿物形态相知较少，导致该法在业内存在一定的质疑。本文通过对不同地区地表土壤样品、海底沉积物以及钻井岩屑样品进行对比分析，研究其碳源、分布特征及矿物形态。结果表明，不同化探样品中∆C的含量与所处的地质环境有关，陆地地表土壤样品中∆C的含量普遍较高，其次为海底沉积物样品，钻井岩屑样品中的最低；而钻井岩屑样品(还原环境)中∆C 的δ13C 值比地表土壤样品及海底沉积物样品(近地表氧化环境)都要偏轻；由此推测形成∆C的碳源有无机、有机以及混合源三种，各种来源的 C 对∆C 的贡献因化探样品所处的地质环境的不同而不同； X 射线衍射物相分析结果表明，形成ΔC 的CO2除了由方解石和白云石的分解产生外，还有少量的由未知矿物提供。