中国南方下寒武系牛蹄塘组沉积相特征与有机质富集机理

Sedimentary facies characteristics and organic matter enrichment mechanism of lower Cambrian Niutitang Formation in South China

为了研究中国南方下寒武统牛蹄塘组沉积相特征,揭示有机质富集机理,指导页岩气实践勘探采用宏观调查与实验分析相结合的手段,详细分析研究区内寒武系牛蹄塘组的岩性、有机质质量分数(w(TOC))、矿物组成及微量元素特征,提出有机质富集影响因素以及沉积相模式,并指出未来勘探方向。研究结果表明:∈1n沉积早期,鄂阳页1、慈页1、常页1井和安页1井所在位置分别发育为台内凹陷、外陆棚、江南斜坡带、华南滞留盆地。伴随着海平面下降,∈1n沉积晚期,沉积相发生演化;研究区早寒武世呈现一个完整的三阶式海盆。1n沉积早期,第1阶为扬子碳酸盐台地,第2阶为深水陆棚−斜坡,第3阶为深水盆地;自扬子碳酸盐台地至深水盆地,w(TOC)逐渐增大,碳酸盐岩矿物质量分数逐渐降低,石英质量分数逐渐升高,而黏土矿物质量分数在第2阶较高。海平面变化导致纵向上∈1n下部页岩发育的w(TOC)较高,而古地理(物源)条件导致横向的有机质在深水陆棚、斜坡和滞留盆地发育较好;∈1n下部页岩富集大量微量元素,且w(TOC)与页岩中多种微量元素呈现较好的正相关性;在深水陆棚和斜坡及其邻近区域,热液活动和上升洋流作用带来了丰富的营养物质,并使水体处于强还原环境,促进页岩有机质丰度进一步提升。深水陆棚、斜坡和深水盆地是有利于有机质形成和保存的沉积相,尤其是深水盆地相最佳,未来需要加强滞留盆地,即湘中地区的牛蹄塘组勘探。

The purpose of this study was to examine the sedimentary facies characteristics of lower Cambrian Niutitang Formation(∈1n)in South China,to reveal the mechanism of organic matter enrichment,and to guide exploration of shale gas.Macro investigation and experimental analyses were used to assess the lithology in detail,total organic matter mass fraction w(TOC),mineral composition,and trace element characteristics of∈1n.The influencing factors of organic matter enrichment were discussed extensively,and a sedimentary facies mode was suggested.In the early stage of∈1n,the locations of Well E’yangye 1,Well Ciye 1,Well Changye 1,and Well Anye 1 respectively develop,platform inner sag,outer shelf,Jiangnan slope belt,and South China detention basin.In the late stage of∈1n,the sedimentary facies evolve with decreasing sea level.The study area presents a complete three-step basin in the Early Cambrian.In the early stage of∈1n,the first step is the Yangtze carbonate platform,the second step is the outer shelf and slope,and the third step is the deep-water basin.From the Yangtze carbonate platform to the deep-water basin,w(TOC)and the mass fraction of quartz gradually increase,the mass fraction of carbonate mineral decreases,and the mass fraction of clay mineral is higher in the second step.The sea level fluctuation results in a higher w(TOC)vertically in the lower∈1n shale,and the paleogeographic(provenance)conditions lead to better horizontal development of organic matter in the outer shelf,slope and detention basin.Trace elements are abundant in the lower∈1n,and w(TOC)is correlated positively with many trace elements.In the outer shelf,slope,and adjacent areas,hydrothermal activity and upwelling current bring nutrient-rich material and promote organic matter enrichment under a strong reducing condition.Deep-shelf,slope and deep-water basin are the best facies for the formation and preservation of organic matter,especially deep-water basin facies.It remains necessary to strengthen the exploration of shale gas in the deep-water basin of∈1n in central Hunan,China.