塔里木盆地西北缘震旦系微生物白云岩地球化学、年代学特征及其地质意义

Geochemistry and geochronology characteristics and their geological significance of microbial dolomite in Upper Sinian,NW Tarim Basin

震旦系奇格布拉克组微生物白云岩是塔里木盆地寒武系盐下超深层重要的勘探领域之一,但目前该领域古沉积环境及孔隙形成的成岩环境认识仍然不足。本文以阿克苏地区西沟剖面为主要研究对象,精细描述了上震旦统奇格布拉克组,系统分析了175块岩石薄片,并优选样品开展了白云石有序度、碳氧锶同位素、微量元素和激光U-Pb同位素年龄测试分析,得出以下认识:(1)明确了奇格布拉克组主要发育叠层石、凝块石、泡沫绵层石三种微生物岩和鲕粒、粘结颗粒两种与微生物作用相关的颗粒岩,自下而上的相序组合构成晚震旦世碳酸盐岩缓坡背景下的内缓坡潮坪-微生物丘滩沉积;(2)根据碳氧同位素计算的古盐度(Z)值和古温度(T)值分别指示古海水具有较高的盐度和相对温暖的温度;(3)较高的Sr含量和低V、Ni、Ba、Co含量指示相对干旱的气候背景,低ΣREE、Mo、Cu、Co、Ba含量及相对高的Pb含量及87Sr/86Sr值指示晚震旦世古水深整体浅、离岸近,而相对低的V/(V+Ni)值、Ce弱负异常和低Cu、Zn含量则指示了弱氧化海水的特征;(4)5个微生物白云岩的U-Pb同位素年龄(531±29Ma-619±38Ma)和9个与之对应的不同产状碳酸盐胶结物的年龄(516±20Ma-572±25Ma和174.3±4.7Ma)以及δ13C、δ18O值特征综合指示微生物白云岩储层的孔隙主要为原生孔和准同生期大气水溶蚀孔,经历震旦纪末表生岩溶作用的改造及燕山期埋藏热液胶结作用的破坏。以上研究成果对该领域的岩相古地理研究和储层成因认识具有重要意义。

Upper Sinian Qigeblak Formation microbial dolomite is an important potential exploration field of Cambrian pre-salt ultra-deep strata in Tarim Basin,NW China.Based on detailed geological observation,175 thin sections identification,multi-parameters geochemical data analysis(order degree,trace elements,REEs,carbon-oxygen and strontium isotope)and 14 samples'LA-MC-ICP-MS U-Pb dating on the Xigou profile in Aksu area(NW Tarim Basin),it was found that:(1)stromatolite dolomite,thrombolite dolomite,foamy microbial dolomite,oolite dolomite and grain dolomite with microbial bonding structure are the main lithofacies of the Qigeblak Formation;the characteristics of lithofacies assemblage from bottom to up indicate that the sediment sequence is from inner ramp tidal flat to inner ramp microbial mound/shoal;(2)the Z value and T value calculated from the carbon and oxygen isotopes indicate that the ancient seawater had high salinity and relatively warm temperature;(3)high Sr content and low V,Ni,Ba,Co content indicate relatively arid climatic background,while lowΣREE,Mo,Cu,Co,Ba content and relatively high Pb content and 87Sr/86S values indicate the sea water depth is shallow,i.e.,Aksu area is close to the paleo-land;relatively low V/(V+Ni)values,weak Ce negative anomaly and low Cu and Zn contents indicate the characteristics of weakly oxidized seawater;(4)the U-Pb isotope ages of the 5 microbial dolomite samples and 9 carbonate cements,andδ13C andδ18O characteristics comprehensively indicated that the pores in the microbe dolomite were mainly primary pores and penecontemporaneous pores;some pores underwent karstification at the end of Sinian Period and cementation in Yanshanian Period.These research results are of great significance to the lithofacies paleogeography mapping and reservoir genesis analysis of Cambrian pre-salt ultra-deep field.