| 摘要: |
| 为查明康滇古陆西侧龙马溪组黑色岩系的地质成因及沉积环境,指导区域页岩气勘探开发,对宁蒗盆地东南缘李子河地区龙马溪组岩层开展了详细的元素地球化学研究。结果表明,研究区硅质含量较高(SiO2值为64.78%~96.89%),Al2O3与TiO2呈正相关,SiO2/A12O3与A12O3呈负相关,这种相关关系表明该区硅质岩系为非纯硅质岩,含有较高比例的陆源泥质沉积物。Al/(Al+Fe+Mn)值(0.53~0.75)、Fe/Ti值(6.38~14.96)、(Fe+Mn)/Ti值(6.40~18.20)、Si/(Si+Al+Fe)值(0.82~0.99)及Al-Fe-Mn三角图解综合显示出生物沉积硅质岩的特点。MnO2/TiO2值(0.005~1.20)、δCe值(0.46~0.92)、相关构造判别图解以及稀土元素分布形式特点均说明源区形成于接近大陆边缘的深水-半深水沉积环境。沉积期水体盐度指标m值(4.96~36.88)、CaO/(Fe+CaO)值(0.017~0.83)以及氧化还原指标δCe负异常、δEu正异常、Ce/La值(1.04~2.09)等元素特征指示其沉积于还原条件下低中等盐度海水环境,这种环境下适合有机质的富集和保存。高有机碳(TOC)与高硅质含量显示弱的负相关,说明生物成因硅质岩是造成高TOC的主要原因,而陆源物质硅对有机质进行稀释和破坏反而降低了TOC含量。受上述成因构造及沉积环境的影响,在宁蒗盆地李子河地区龙马溪组形成一套品质较好的富有机质烃源岩储层,具有一定的页岩气地质评价意义。 |
| 关键词: 龙马溪组 黑色页岩 地球化学特征 地质意义 |
| DOI： |
| 附件 |
| 投稿时间：2016-09-12修订日期：2016-10-18 |
| 基金项目:受国家重大科技专项课题《页岩气勘查评价技术试验与应用推广》支持,课题编号2016ZX05034004 |
|
| Black shales from the Longmaxi Formation in western Xikang-Yunnan ancient land:Geochemistry and geological implications |
| ZHANG Qian, WANG Jian, YU Qian, WANG Xiao-fei, ZHAO An-kun, ZHANG Hai-quan, WANG Zheng-he |
(1. Chengdu University of Technology, Chengdu 610059, Sichuan, China;
2. Key Laboratory of Sedimentary Basins and Oil and Gas Resources, Ministry of Land and Resources, Chengdu 610081, Sichuan, China;
3. Chengdu Center, China Geological Survey, Chengdu 610081, Sichuan, China) |
| Abstract: |
| The detailed element geochemical studies are conducted to ascertain the geological genesis and sedimentary environments of the black shales from the Longmaxi Formation in the southeastern Ninglang Basin on western Xikang-Yunnan ancient land, and to provide helpful information for the shale gas exploration in this area. The black shales have higher SiO2 contents ranging between 64.78% and 96.89%, positive correlation of Al2O3 and TiO2 contents and negative correlation of SiO2/Al2O3 ratios and Al2O3 contents, suggesting that the siliceous rocks in this area are not pure siliceous rocks but higher proportion of terrigenous argillaceous deposits. The Al/(Al + Fe + Mn)(0.53-0.75), Fe/Ti (6.38-14.96), (Fe + Mn)/Ti (6.40-18.20), Si/(Si + Al + Fe)(0.82-0.99)ratios and Al-Fe-Mn triangular diagram indicate that the siliceous rocks mentioned above are biogenetic. The MnO2/TiO2 ratios (0.005-1.20), δCe values (0.46-0.92), tectonic discrimination diagram and REE distribution patterns also indicate the continental-margin abyssal-bathyal sedimentary environments of the provenances. The salinity index of the sea water (m=4.96-36.88), CaO/(Fe + CaO)ratios (0.017-0.83), negative δCe anomalies and positive δEu anomalies, and Ce/La ratios (1.04-2.09)indicate the anoxic and low-to moderate-salinity marine sedimentary environments. There are slightly negative correlation between high organic carbon and high SiO2 contents, indicating that the biogenetic siliceous roks should be responsible for the high organic carbon contents. The organic-rich source rocks from the Longmaxi Formation in the Lizihe region are interpreted to be favourable for shale gas exploration due to high organic carbon contents, greater shale thickness and abundant surface oil and gas showings. |
| Key words: Longmaxi Formation black shale geochemical signature geological implication |
