川东南地区中二叠统茅口组一段眼球状灰岩成因机理

Genetic mechanism of eyeball-shaped limestone in First Member of Middle Permian Maokou Formation in southeast Sichuan Basin

近期,川东南地区多口井在中二叠统茅口组一段(茅一段)眼球状灰岩中见到良好的油气显示,但其成因机理一直颇有争议。基于重点钻井岩心和野外露头观察,通过岩石薄片、扫描电镜和微量元素分析,结合区域地质背景,探讨眼球状灰岩的特征及成因机理。结果表明:川东南地区茅一段眼球状灰岩发育卷曲变形构造、液化底劈、液化脉、热液脉和生物壳体反扣等现象,具有地震震动改造的特征。眼球状灰岩主要由方解石、石英、白云石、滑石、黄铁矿以及有机质组成,滑石以交代生屑或呈星点状、团块状、纤维放射状、岩流产状等形态赋存。微量元素分析结果表明“眼皮”形成于比“眼球”更为缺氧的环境中,沉积时期受火山活动的影响。茅一段眼球状灰岩是在峨眉山大火成岩省地质背景下由地震活动与沉积-成岩综合作用形成的,地震活动会引起较为动荡的瞬时水流,间歇性破坏碳酸盐岩的稳定沉积条件,造成不同组分的混合形态不同,使得沉积物的成分和产状存在差异,从而形成具有特殊沉积构造的眼球状灰岩。

Recently,a number of drilling wells in the southeast Sichuan Basin have good oil and gas display in eyeball shaped limestones in the First Member of the Middle Permian of Maokou Formation(Mao1),but the genetic mechanism of the eyeball-shaped limestone has been controversial.Based on the observation of key drilling cores and field outcrops,as well as thin sections of rocks,scanning electron microscopy and microelement analysis,the characteristics and genetic mechanism of the eyeball-shaped limestones are explored,combined with the regional geological background.The results show that the eyeball-shaped limestones of Mao1 in the southeastern Sichuan Basin develop curl deformation structure,liq⁃uefied diapir,liquefied veins,hydrothermal veins,and biological shell convex-down,which have the characteristics of earthquake vibration modification.In addition,eyeball-shaped limestones are mainly composed of calcite,quartz,dolomite,talc,pyrite and organic matter.Talc exists as metasomatic bioclasts or in the form of star points,agglomerates,fibrous radia⁃tions and rock flowage.The microelement analysis results indicate that the“eyelids”were formed in a more anoxic environ⁃ment than the“eyeballs”,which are both influenced by volcanic activities during the deposition period.The Mao1 eyeballshaped limestones were formed through seismic activities and deposition-diagenesis under the geological background of the Emeishan Large Igneous Province.Seismic activities could cause turbulent instantaneous water flow and intermittently de⁃stroy steady carbonate deposition conditions,which resulted in different mixing patterns of different components,leading to differences in composition and occurrence of deposits,and thus the eyeball-shaped limestones with special sedimentary structures were formed.