鄂西利川见天坝长兴组生物礁内部构成及成礁模式

Reef Composition and Their Forming Models of Changxing Formation in Jiantianba Section of Lichuan,Western Hubei

鄂西利川见天坝位于川东碳酸盐岩台地与鄂西海槽之间的台地边缘相带,晚二叠世长兴期该区水体逐渐由深变浅,发育了一套规模巨大的加积—进积型台地边缘生物礁沉积。通过野外露头剖面精细地质写实研究,对该区生物礁内部构成特征进行了深入解剖,探讨了该区生物礁发育模式。研究表明利川见天坝生物礁礁体位于长兴组层序1的高位体系域,其内部由4个准层序组构成,PSS1中下部由一套斜坡相深灰色薄层状泥晶灰岩和含生屑泥晶灰岩沉积组成,该准层序组顶部发育一套灰色中层状生屑灰岩滩相沉积,见天坝生物礁就是该生屑滩基础上发展起来的。准层序组PSS2属台地边缘生物丘沉积,以灰色块状藻粘结岩、藻粘结—障积岩沉积为主。PSS3为该区生物礁主体部分,为一套灰色块状障积—骨架岩和骨架岩沉积,在礁前斜坡带发育有垮塌灰岩角砾岩沉积。PSS4由浅灰色中厚层状生物碎屑含云灰岩、残余生屑白云岩组成,属台地边缘滩和局限台地沉积。随着相对海平面不断下降,水动力条件逐渐增强,利川见天坝生物礁经历了生物礁奠基、初期繁盛、最大繁盛和生物礁衰亡等四个演化阶段。礁体储层主要分布在高位体系域顶部,其形成环境为台地边缘生物丘、台地边缘生物礁和礁顶局限台地白云岩沉积,该区优质储层与礁体形成沉积条件密切相关。

Jiantianba of Lichuan,western Hubei,is located in platform margin facies between carbonate platform facies in the eastern Sichuan and Western Hubei Trough.In the study area,the water gradually shallowed from the bottom to up in the stage of Changxing Formation of Late Permian,and developed a huge scale platform marginal reef deposition in aggradation-progradation.Based on the multidisciplinary analysis of measured profiles,the authors discussed the reef composition and forming model of Changxing Formation reef in Jiantianba section.The study demonstrated that the study area developed two third-order sequences in Changxing depositional stage of late Permian and the body of reef in the Jiantianba of Lichuan was located in HST of sequence 1of Changxing Formation.And its internal constituted by the four parasequence sets.Pss1 mainly composed by dark grey lamellar or lamellar-middle bedded cryptite,clastizoic cryptite,middle bedded bioclastic cryptite,the internal could be further divided into three parasequences namely PS2,PS3 and PS4.PS2 and PS3 were located in PSS1 lower-middle part which are composed by a set of slope facies of lamellar dark gray mudstone deposit,and PS4 was located at the top of PSS1 and developed a set of gray middle lamellar bioclastic limestone with the shoal facies deposition and the reef of Jiantianba was developed on the basis of the above bioclastic bank.PSS2 was a platform edge bioherm deposition,developed a set of grey massive algae bonding,algal bonding-baffle and baffle bioconstructed limestone,this parasequence set internal developed three parasequences,and PS5 mainly was algae bondstone,the lower part of PS6 and PS7 mainly were algae bondstone,the upper part of PS6 and PS7 was algae bond-bafflestone,which reflects the water shallow gradually,hydrodynamic stronger and bioherm prosperous gradually.PSS3 is mainly composed by a set of gray massive baffle-framework reef limestone,sponge organic framework reef and partly limestone breccia,the internal can be further divided into three parasequences namely PS8 PS9 and PS10;With relative sea level further drops,frame builders extremely flouring and mainly are Sphinctozoa,Inozoa and little Sclerospongiae,Hydrozoa and Bryozoan,this period was flourishing period of reefs,this parasequence set constitutes the main deposition of platform margin reefs.PSS4 constituted by a medium-bedded and light gray bioclastic limestone with dolomite and remains of bioclastic dolomite,and belonged to platform edge and restricted platform deposition.As the relative sea level fell gradually and hydrodynamic force strengthening progressively,the reef in the Jiantianba of Lichuan experienced four stages of evolution,including a foundation,the initial flourishing,prosperous,and the largest decline of organic reef.Reefs in Jiantianba of Lichuan reservoir system mainly distributed at the top of the HST,the environment for the formation include platform edge bioherm,platform edge reef and restricted platform dolomite deposition at the top of reef,high quality reservoir is closely related to the reef composition and deposition conditions.