塔东南地区寒武系海退背景下微生物丘沉积演化特征

Sedimentary Evolution of Microbial Mound in Cambrian Regression Background,Southeastern Tarim Basin

微生物丘作为新型的碳酸盐岩油气储层,是现今深层油气勘探和研究的热点。以塔东南地区寒武系微生物丘为例,综合利用岩心、测井、分析测试和高精度三维地震数据,在微生物丘微相组成和外部形态分析基础上,探究海退背景下微生物丘的发育期次、叠置样式及其生长发育、迁移演化特征。研究结果表明:1)研究区寒武系处于相对高能的台地边缘相带,有利于微生物丘的生长发育。钻井揭示微生物丘岩性为菌藻类微生物颗粒白云岩(局部硅化),储集空间为晶间(溶)孔、溶蚀孔洞及微缝隙。2)中、上寒武统发育4期微生物丘复合体,依次向广海方向推进叠置,近南北向条带状展布,叠合面积大于1400 km^(2)。3)①期微生物丘形成于早寒武世缓坡台地之上,“三明治”式垂向加积生长,形成较对称的丘状,生长规模大。②期发育在丘前塌积岩之上,古地貌坡度陡,可容纳空间大,水动力强,垂向加积—弱前积生长,形成不对称丘状。随着相对海平面快速下降和生物丘建隆进一步发展,③期丘体楔状前积于②期丘的靠海一侧,侧向叠置分布。④期丘体生长具有继承性,规模变小。寒武纪末期,海侵作用加强,微生物丘停止发育。总体上反映海退背景下研究区由缓坡—镶边型台地—淹没型台地的变化特征。

As a new type of carbonate reservoir,microbial mounds are becoming the focus of deep oil and gas exploration and research.This study of a Cambrian microbial mound in the southeastern Tarim Basin included an analysis of the microfacies composition and external morphology,the developmental stages,stacking patterns and sedimentary evolution against a Cambrian regressive background using high-resolution 3D seismic data from a newly drilled well.The results are as follows:(1)The Cambrian system in the study area is located in a relatively high-energy platform-edge facies belt,which is conducive to the growth and development of microbial mounds.Drilling data revealed that the physical properties in the core of the microbial mound are relatively suitable as a hydrocarbon reservoir.The lithology is bacteria-algal microbe particle dolomite with local silicification,containing intercrystal(dissolution)pores,dissolution pores,and fractures.(2)In the middle and upper Cambrian,four stages of microbial mound body complexes are evident,successively superimposed over a vast sea area of more than 1400 km^(2),occurring in northsouth planar stripes.(3)The first mound body was formed on a gentle slope platform in the Early Cambrian.Largescale“sandwich-like”vertical accretions grew and eventually formed a symmetrical mound shape.The second mound body was developed on steeply sloping pre-mound colluvial rock,and contains a large volume of accommodation space.The hydrodynamic condition is strong.The mound body character is vertical accretion with weak forward growth,forming an asymmetrical mound.With the rapid fall in sea level,three more laterally superposed mound bodies developed,wedged on the seaward side of the original mound body.The last of these shows smaller-scale inherited development.The mound bodies ceased developing as the transgression was strengthened at the end of the Cambrian,reflecting the gradual evolution process of a carbonate platform from a low-angle sloping edge platform to a submerged platform in a regression environment.