A new method of reconstruction of original total organic carbon for sediments in salt water lakes-A case study on modern sediments in Buha River estuary, Lake Qinghai

Abstract

烃源岩中原始有机碳（TOC）总量的恢复是烃源岩评价的重点和难点，以青海湖西北部的布哈河口区沉积物为研究对象，探讨一种全新的利用沉积物中硼元素含量对原始TOC含量进行恢复的方法.布哈河口区沉积物主要为泥、粉砂、细砂及其过渡类型，黏土矿物含量总体较低，沉积物中干酪根类型主要为Ⅱ₁和Ⅱ₂型，有机质大部分来自陆源生物，少量来自水生生物.布哈河口区沉积物中TOC含量介于0.02%~4.29%之间，其中半深湖及前三角洲环境TOC含量总体较高.有机质对部分微量元素有明显的富集作用，沉积物中TOC含量与硼、钪、钴、镍、锌、镓、铷、铯、铅及铀等微量元素存在明显相关性.有机质对微量元素的富集能力和强度受沉积物和水体接触时间控制，因此微量元素与TOC含量的相关性在不同沉积环境存在明显差异：在河流、三角洲平原、前三角洲、浅湖及半深湖沉积区较高，而在滨湖区较低.硼元素对TOC含量变化的反映最灵敏，即便是在间歇暴露的滨湖沉积中硼含量与TOC含量呈明显的正相关性，因此硼可以作为反映烃源岩原始TOC含量的有效指标.根据布哈河口区沉积物中硼含量与TOC含量之间极好的线性相关性，建立了利用烃源岩中的硼含量对原始TOC含量进行定量计算的公式，该公式主要适用于陆相微咸水、半咸水及咸水环境泥质岩类原始TOC总量的恢复.;Reconstruction of original total organic carbon (TOC) in source rocks is the key and difficult point of hydrocarbon source rock evaluation. The paper focuses on the sediments of Buha River estuary in the northwest of Lake Qinghai, and a new method of original TOC reconstruction via born element in sediments was discussed. The sediments of Buha River estuary sediments are mainly made of mud, silt, fine sand and transitional type composition. The sediments are generally have low clay mineral content. The main types of kerogen in the sediments were Ⅱ₁ and Ⅱ₂, and most of organic matter come from terrigenous organisms while a small quantity of organic matter come from aquatic organisms. TOC in Buha River estuary sediments is in the range of 0.02% to 4.29%. TOC in the sediments of semi-deep lake and prodelta is usually higher than that in other environmental sediments. Organic matter has a significant effect on the enrichment of some trace elements. There is a significant positive correlation between the trace element concentration and TOC content in the sediments, the trace elements include boron, cobalt, nickel, zinc, gallium, rubidium, cesium, lead and uranium. The enrichment ability and intensity of organic matter to trace elements are controlled by the contact time of sediment and water, and so the correlation between trace element and TOC has obvious difference in different sedimentary environment. TOC is higher in sediments of fluvial, delta plain, prodelta, shallow lake and semi-deep lake environment, while it is lower in sediments of lakeshore environment. Boron was the most sensitive to the variation of organic carbon content, and the content of boron in the sediments of intermittent exposure environment like lakeshore is also showed a significant positive correlation with TOC. Boron is an effective index that can reflect original organic carbon content of the hydrocarbon source rocks. According to the excellent linear relationship between the boron concentration and TOC in the sediments of Buha River estuary, a formula for the quantitative calculation of the original organic carbon content via the content of boron in the source rock was established. The formula is suitable for the reconstruction of the original total organic carbon in the terrestrial brackish water and salt water of environment.