上扬子地区龙马溪组黑色富有机质页岩的浅水超覆沉积模式

Shallow-water onlap sedimentary mode of black organic-rich shale in the Longmaxi Formation,Upper Yangtze area

细粒沉积作用决定着页岩储集层的物质基础,对优质页岩的形成具有重要作用。文中以上扬子地区龙马溪组黑色页岩为研究对象,通过岩心、露头、薄片、地震资料等分析,对该套黑色页岩的沉积发育模式进行研究。结果表明:(1)研究区龙马溪组黑色富有机质页岩段发育粉砂岩与页岩互层,见生物扰动、交错纹层、波状纹层、粒序纹层、泥砾定向排列、底部侵蚀面等沉积构造,具明显的浅水沉积特征;(2)黑色页岩内部发育多期受波浪作用影响而形成的页岩—粉砂质页岩—粉砂岩反粒序沉积旋回,显示出波浪作用对龙马溪组黑色页岩的形成具有重要影响;(3)风暴作用对于龙马溪组黑色页岩的形成具有一定的影响,表现为在不规则侵蚀面之上形成粉砂岩—黑色页岩正粒序沉积旋回;(4)从地震剖面以及连井剖面对比可以看出,随着海平面的不断上升,页岩总体上具有向古隆起边缘不断超覆迁移的特征。推测上扬子地区龙马溪组黑色页岩形成于水体较浅且受限的沉积环境,易受波浪以及风暴作用的影响,具有随着海平面上升而不断超覆迁移的特征。研究区龙马溪组富有机质黑色页岩的浅水超覆模式对于上扬子地区富有机质页岩的勘探具有重要的指导意义。

Fine-grained sedimentation determines the material basis of shale and plays an important role in the formation of high-quality shale.Based on the comprehensive analysis of drilling cores,outcrops,thin sections and seismic data,the sedimentary mode of the black shale from the Longmaxi Formation in the Upper Yangtze area was studied.The results show that(1)siltstone interbedded with shale,bioturbation,cross bedding,wavy laminae,graded laminae,orientation of mud gravel and basal erosion occurred in the black organic-rich shale of the Longmaxi Formation,which reveals obvious shallow water sedimentary characteristics.(2)The reverse graded deposition sequence of shale-silty shale-siltstone under wave action is developed in the black shale,reflecting that wave action has an important influence on the formation of black shale in the Longmaxi Formation.(3)The storm has a certain impact on the formation of black shale in the Longmaxi Formation,which shows the normal graded deposition sequence of siltstone to black shale formed on the irregular erosion surface.(4)A comparison of the seismic profile and regional stratigraphic well profile shows that the black shale of the Longmaxi Formation presents obvious onlap to the margin of the paleo-uplift with the rise in sea level.These results reflect that the black shale of the Longmaxi Formation in the Upper Yangtze area formed in limited shallow water,which is vulnerable to waves and storms.The continuous onlap of shale occurred with the rise in sea level.The shallow-water onlap sedimentary mode of the organic-rich shale in the Longmaxi Formation has significant guidance for the exploration of organic-rich shale in the Upper Yangtze area.