巴西桑托斯盆地早白垩世微生物碳酸盐岩沉积环境与成因

Sedimentary environment and genesis of the Early Cretaceous microbial carbonates in Santos Basin,Brazil

巴西桑托斯盆地深水盐下已经成为近年来全球油气勘探最重要领域之一,储层主要为早白垩世阿普特阶Barra Velha(BVE)组微生物碳酸盐岩,但其沉积环境演化和成因尚不明确。通过系统观察岩心102m、井壁取心533块、薄片917块,本文描述了早白垩世BVE组微生物碳酸盐岩特征。优选不同深度样品42个,开展了锶同位素、碳氧稳定同位素、主量和微量元素测试分析。结合岩性特征和测试分析结果,得出以下认识:(1)BVE组主要发育叠层石灰岩、球粒微生物岩和层纹岩三类微生物岩,其分布在纵向上呈现规律性;(2)BVE组沉积时期总体表现为干旱蒸发、高盐度的半封闭-封闭湖盆环境,不同沉积期湖盆封闭性和气候环境变化导致了BVE组上、中、下段微生物岩岩石类型的变化;(3)建立了受古气候控制的微生物岩成因演化模式。BVE组沉积初期主要为半干旱蒸发、高盐度的半封闭-封闭较深水还原环境,有利于微生物粘结作用,导致层纹岩大量沉积;至沉积中期转变为湿润、盐度相对降低、有水体和少量陆源碎屑注入的半封闭环境,导致微生物作用减弱,从而主要以球粒微生物岩沉积为主;至沉积末期变为极度干旱蒸发、超高盐度的封闭浅水还原环境,有利于厌氧嗜盐类微生物大量繁殖,从而使得叠层石灰岩广泛发育。

The pre-salt microbial carbonate has become one of the most important targets for global oil and gas exploration in the deep water Santos Basin in recent years.The pre-salt microbial carbonate was mainly deposited in the Barra Velha Formation(BVE)of the Early Cretaceous(Aptian).However,its genesis and sedimentary evolution need to be further explored.In this study,the lithologic characteristics of microbial carbonate were described by systematically observing 102m cores,533 sidewall cores and 917 thin sections from the BVE Formation.A total of 42 core samples with different depths were selected and tested for strontium isotope,stable carbon and oxygen isotope,major and trace element.Integrated the lithologic characteristics with geochemical tested results,the following conclusions can be drawn:(1)The lithofacies of the BVE Formation mainly includes three types of microbial carbonate of stromatolite,spherulite and laminate,which are distributed regularly in the upper,middle and lower members of the BVE Formation.(2)The general sedimentary environment of the BVE Formation was characterized by a semi-closed and closed lacustrine environment with high salinity in an arid and evaporative paleoclimate.The variation of the lacustrine connectivity and paleoclimate led to formation of different lithofacies of microbial carbonate in the BVE Formation.(3)A sedimentary evolution model of microbial carbonates controlled by paleoclimate is established.In the early sedimentary stage of the BVE Formation,sedimentary environment was mainly a semi-closed to closed deep water lacustrine reducing environment with a characteristic of high salinity in the paleoclimate of semi-drought and evaporation,which was favorable for microbial bonding and a large amount of laminate deposition.In the middle stage,the sedimentary environment changed to a semi-closed lacustrine deposition with relatively low salinity in a relative humid paleoclimate accompanied with water and a small amount of terrigenous detritus injection,which resulted in the weakening of microbial effects and the deposition of spherulite.In the later stage,the sedimentary environment became a closed shallow water reducing lacustrine deposition with ultra-high salinity in extreme arid and evaporative paleoclimate,which was conducive to the proliferation of anaerobic halophilic microorganisms and the extensive deposition of stromatolite.