桂中坳陷西北部中泥盆统页岩地球化学特征及沉积环境

Geochemical Features and Sedimentary Conditions of the Middle-Devonian Shale in the Northwestern Central-Guangxi Depression

本文研究桂中坳陷西北部中泥盆统页岩氧化还原条件和古沉积环境。利用该区典型页岩气调查井(桂页1井)岩心,通过X射线荧光光谱分析(XRF)和电感耦合等离子体质谱分析(ICPMS),对中泥盆统岩心(320.35m^938.5m)以1.5m采样间隔进行了微量元素测试。结果显示,纳标组地层V/(V+Ni)、V/Cr和Ni/Co平均值分别大于0.67、4.65和7.71,且富集竹节石、菊石等生物组合,指示其沉积时期海平面相对上升,处于有利于有机质保存的深水陆棚环境;罗富组和塘丁组V/(V+Ni)、V/Cr和Ni/Co分别为0.38~0.65、0.73~4.10和3.70~6.72,指示沉积阶段海平面相对下降,水体变浅,沉积环境为弱氧化的浅水陆棚环境;同时,有机碳含量(TOC)的变化与Ba元素的富集程度相关性较高,表明缺氧环境有利于机质的富集和保存;此外,通过微量元素指标判别,认为桂中坳陷西北部中泥盆统沉积时期,自下而上经历了海平面的相对上升到相对下降的沉积旋回,纳标组时期相对海平面上升至最高,沉积了厚度稳定且有机质含量高的富有机质页岩,可作为本区有利的勘探目标层段。

The objective of this work is to study the redox conditions and ancient sedimentary environments of Middle- Devonian shale in the northwestern Central- Guangxi depression. Based on the cores from a typical shale- gas survey well,microelement analysis was made on samples at 1. 5m interval in this well using the XRF and ICP- MS methods. The results show that V /（ V ＋ Ni）,V / Cr,and Ni / Co of the Nabiao Formation are greater than0. 67,4. 65,and 7. 71,respectively,rich in bio- combinations such as Tentaculites and ammonites,indicating relative rise of sea level,a deep- water shelf setting that was favorable for organic matter preservation. The Luofu and Tangding Formations have V /（ V ＋ Ni） ratio of 0. 38 - 0. 65,V / Cr ratio of0. 73 - 4. 10,and Ni / Co ratio of 3. 70 - 6. 72）,implying relative drop of sea level,shallow water,and weak redox environment. Meanwhile,the TOC values and Ba,V /（ V ＋ Ni）,V / Cr,and Ni / Co are well positively correlated,showing that the anoxic depositional environment was favorable for the organic preservation and transformation. Combined with pyrite and tentaculites development situation,the black shale of NaB iao Formation can be considered to be the potential target of Middle Devonian in the Central- Guangxi depression.