含油气盆地蒸发盐矿物成因类型及其地质意义*

中国是一个多盐湖国家,然而盐湖研究主要集中于分析湖水化学性质、盐类物质来源和盐矿资源开发等,对盐类矿物沉积特征和埋藏成岩改造研究较少,造成从蒸发岩角度去理解古代盐湖盆地的油气富集规律较为困难。在广泛阅读国内外大型盐湖文献的基础上,笔者介绍了盐湖分类方案和蒸发岩中盐类矿物的主要成因类型,并总结了中国陆相含油气盆地中常见的硫酸盐、氯化物、含钠碳酸盐和硼酸盐的沉积—成岩过程及其古环境和古气候意义。同时,尝试利用盐湖沉积最新研究成果去探讨中国含油气盆地蒸发岩研究中存在争议或值得关注的问题,得出: (1)深部热液可为湖泊输送大量元素离子,但要在湖泊环境下富集大量蒸发岩,则(半)干旱气候和蒸发浓缩作用是前提条件;(2)易溶蒸发岩(如石盐)在沉积中心单层厚度大,而在斜坡—边缘区缺失,这是季节性气温变化和温跃层浮动引发“中心聚集效应”的结果;(3)温度可影响蒸发岩中盐类矿物溶解度、晶体结构形态和发育深度,而部分无水盐类矿物在常温常压下却无法结晶,这一现象可用来指示古地温和地层埋藏史;(4)碳酸盐型盐湖中的Na-碳酸盐种类可指示大气CO₂浓度和古温度。     

渤海湾盆地东营凹陷示烃成岩作用及其演化特征*

富含有机酸的烃类流体在由泥质烃源岩进入储集层后将发生一系列的示烃成岩反应。研究发现有6类成岩现象与烃类活动密切相关,即酸性烃类流体的溶蚀作用、蒙脱石脱水与伊利石化、碳酸盐矿物的铁离子浸染、沥青—铁核的轨迹、自生高岭石的形成以及耗水、底积碳酸盐胶结层的隔层分布。这些特殊的成岩现象是含油气盆地内特有的标识,对指示油气初次侵入程度、运移的方向与轨迹、工业聚集度以及最终成藏场所的确定具有重要的参考价值。基于所发现的示烃成岩矿物而建立的“含油气盆地碎屑岩成岩演化规律”,对胜利油区复杂油气藏勘探起到了指导作用,也可为今后含油气区储集层的有效性确定提供借鉴。     

中-新生代塔里木与中亚盐湖链蒸发岩沉积及其控制因素初探

全球性的构造运动末期一般伴着海退和干旱的气候环境,而蒸发岩沉积与大地构造条件紧密相关。中亚卡拉库姆盆地、阿富汗_塔吉克盆地、塔里木盆地自晚侏罗世至中新世以来至少发生了3次大规模海侵_海退旋回,每次海侵_海退均受特提斯构造事件控制。卡拉库姆盆地、阿富汗_塔吉克盆地为晚侏罗世—早白垩世蒸发岩沉积、塔里木盆地的莎车次级盆地为晚白垩世—古新世蒸发岩沉积,库车次级盆地为古新世—中新世蒸发岩沉积,形成特提斯构造域自西向东蒸发岩沉积时代逐渐变新的盐湖链。中亚及塔里木盐湖链在海侵_海退旋回的控制下,至少发生5次大规模的蒸发岩沉积,发育2种不同的蒸发岩沉积序列,分别对应3次海退期蒸发岩沉积序列及2次断续海侵期蒸发岩沉积序列,具体为晚侏罗世晚期(海退期)卡拉库姆盆地、阿富汗_塔吉克盆地蒸发岩沉积;早白垩世—晚白垩世早期(断续海侵期)阿富汗_塔吉克盆地蒸发岩沉积;晚白垩世晚期(海退期)莎车次级盆地蒸发岩沉积;古新世—中新世(断续海侵期)莎车次级盆地、库车次级盆地蒸发岩沉积;中新世晚期—上新世早期(海退期)库车次级盆地蒸发岩沉积。塔里木与中亚古盐湖发育受控于特提斯构造事件及海侵_海退旋回,而海侵_海退旋回又控制2种不同的蒸发岩沉积序列。蒸发岩沉积序列、古盐湖演化阶段、蒸发岩物质来源、沉积环境决定了盐类矿物沉积类型(单一化学岩型、陆缘碎屑岩_化学岩型),卡拉库姆盆地、阿富汗_塔吉克盆地盐类矿物与塔里木盆地相比,种类简单,反映了盆地化学岩与陆源碎屑_化学岩沉积的区别及后期构造运动对盐类矿物种类的主控作用。     

库车前陆盆地古近系蒸发岩岩石学、矿物学与成钾环境分析

库车盆地内古近系—新近系发育巨厚的蒸发岩沉积, 尤其是盆地的中部和西部盐层发育好, 厚度大, 局部盐层己暴露地表, 是找寻各种盐类矿床及钾盐的有利地区。DZK01孔是近年来在库车盆地实施的一口以找钾为目的的科研探井。本文主要通过对钻孔古近系蒸发岩岩芯样品进行岩石学及矿物学特征研究, 进而对盆地成钾环境进行分析。DZK01孔古近系蒸发岩以含泥砾石盐岩为主, 含泥砾石盐岩为构造成因, 是古盐湖高度浓缩的产物。盆地古近系蒸发岩以沉积石盐为主, 钻孔中含钾矿物为钾石膏, 钾石盐和光卤石, 同时含钾矿物的发现证明了库车盆地古盐湖可能达到钾盐析出阶段。通过样品分析可知, 库车盆地古近纪时期古盐湖沉积环境为封闭的浅湖沉积环境, 盐湖发育过程中有五次较大规模的淡化事件, 盐类物质得到很好的聚集, 具有良好的成钾环境。     

上扬子区(四川盆地)寒武系的含盐性与地质背景

据记载,分布于上扬子区寒武系的石膏岩仅有零星资料。如今,大量蒸发岩如溶蚀角砾岩、硬石膏、石盐岩及富钾卤水等已被发现。沉积相经历了从盆地相、台地相到蒸发岩相的演化过程。在中、下寒武统中找到海退沉积序列。古构造-古地理背景是蒸发岩沉积的重要控矿条件之一。周边板块运动及塑性基底的褶皱,导致台缘隆起及台内坳陷的形成。岩相古地理图展示从碳酸盐、硫酸盐到氯化物盐类的“泪滴式”沉积相带。蒸发岩沉积于台缘隆起及礁生长进入堰塞潟湖至盐湖阶段。在稳定坳陷区沉积盆地中蒸发岩系发育;石盐岩较厚,其溴氯比值为0.2—0.4;古气候炎热、干燥;在蒸发岩体周围有沉积(封层)水及溶滤水,其含钾(K~+)量为0.10—4.76g/L;水文地球化学比值(如钾氯比值、钾盐比值及钾溴比值等)指示含钾异常;盐湖浓缩卤水达到氯化物盐类沉积阶段及相应的构造封闭程度;含盐地质标志广泛分布等等。鉴于上述,估计该区可能具备钾盐成矿条件。然而目的层埋藏太深,因此笔者建议,应在浅埋部位进行有效的成矿预测,为普查指出远景区。     

蒸发背景沉积序列精细刻画及沉积学解译:以鄂尔多斯盆地中部中奥陶统马五6亚段为例*

云膏共生组合是受化学沉积分异作用控制而形成的岩石组合,对其沉积序列精细刻画,有利于揭示诸多蒸发环境下的沉积学信息。依据鄂尔多斯盆地中部奥陶系马家沟组五段6(简称“马五₆亚段”)亚段钻井、岩心资料,开展了岩石宏观和微观观察、典型沉积序列刻画等工作。研究结果表明: (1)鄂尔多斯盆地中部马五₆亚段普遍发育碳酸盐岩与蒸发岩共生组合,主要由10种岩石类型构成;(2)主要发育5种沉积序列,下云上膏的岩性组合和序列顶部多发育暴露面等特征表明单个序列具有向上变浅、变咸的演化特征,是蒸发台地叠合海水渐次补给作用的结果;(3)在局限—蒸发台地环境下,主要发育潟湖/滩间海、颗粒滩、灰泥丘、台坪4种沉积亚相及11种沉积微相,与经典的蒸发潮坪序列在岩性组合与沉积构造方面存在明显的差异;(4)马五₆亚段沉积期发育2次海平面升降,沉积环境可分别对应于云(膏)质潟湖—灰泥丘/颗粒滩—膏质潟湖—蒸干膏质潟湖和灰泥丘/颗粒滩—膏质潟湖—蒸干膏质潟湖—台坪。该研究结果可为鄂尔多斯盆地马五₆亚段沉积环境与海平面变化研究提供系统的岩石学证据。     

新疆莎车盆地上白垩统-古近系蒸发岩沉积及成钾前景

新疆莎车盆地发育大范围的蒸发岩。蒸发岩沉积与海侵-海退密切相关,自晚白垩世—渐新世以来,莎车盆地至少有5次小规模海侵-海退旋回,除阿尔塔什组石膏岩为断续海侵期沉积外,其余基本为海退期沉积。莎车盆地主要的蒸发岩沉积层位为吐依洛克组上段及阿尔塔什组。野外调查显示,吐依洛克组石盐岩露头主要沿西昆仑山前呈长条状分布,基本呈透镜体,阿尔塔什组石膏岩露头则见于盆地大部分地区,包括西昆仑山前、南天山山前及麦盖提斜坡,横向上持续稳定。野外调查及室内分析显示,盐类矿物主要为石盐、石膏、硬石膏,少量杂卤石、钙芒硝及钾石膏。盆地中石盐岩透镜体在横向上的不连续性,可能反应了吐依洛克组沉积晚期西昆仑山前存在多个次级古盐湖凹地,在干旱条件下浓缩成盐,而次级古盐湖在演化过程中大范围巨厚石膏岩的缺失,可能与当时的海退时间极短有关。依据莎车盆地蒸发岩沉积特征、盐类矿物组合、古盐湖演化环境,推测盆地有利的成钾层位为吐依洛克组上段,在乌帕尔一带发现成钾显示,可能为有利的成钾区域;而盆地小范围、厚度不大的石盐沉积及埋藏深度大为不利的找钾因素。     

准噶尔盆地沙湾凹陷西斜坡二叠系佳木河组物源区构造背景分析*

准噶尔盆地沙湾凹陷西斜坡二叠系佳木河组物源体系一直备受争议,导致沉积体系“众说纷纭”。利用聚类分析、多维尺度分析及主成分分析等方法,对重矿物含量、微量元素进行分析,明确了沙湾凹陷西斜坡二叠系佳木河组砂砾岩碎屑颗粒富石英(35.41%)、长石(43.2%),贫岩屑(18.71%)。稀土元素总量(ΣREE)平均值139×10^-6接近大陆上地壳(UCC)的平均值148.14×10^-6,Hf-La/Th微量元素物源判别图解和La/Sc-Co/Th值表明佳木河组属于酸性岛弧区。稀土配分模式表现为右倾分布模式,整体上具有“轻稀土元素富集、重稀土元素亏损”的特征,指示源岩以长英质火山岩为主,即“一源”。在同一物源下,又可以分为4个物源区:新光1井—中佳6井物源区,车排10井—中佳5井—中佳4井—中佳3井物源区,车排7井—车排005井体系物源区,车排5井物源区,即“四区”。构造背景判别图解揭示了佳木河组形成于大陆岛弧环境。该研究将有助于对沙湾凹陷西斜坡的沉积体系及古水系的准确恢复和远景区油气储集层的准确预测。     

济阳坳陷富林洼陷中生界复杂岩性风化壳储集层成岩作用*

风化壳储集层成因及其差异性特征分析是当今储层地质学研究热点。渤海湾盆地济阳坳陷沾化凹陷富林洼陷中生界油气勘探遇到瓶颈,目前探井仅钻揭白垩系顶部风化壳,且风化壳储集层岩性复杂、非均质性极强,明确风化壳储集层成岩改造、储集空间类型和组合关系,对本区乃至济阳坳陷中生界潜山油气勘探至关重要。通过岩心和薄片观察,结合录井、测井等资料,对富林洼陷白垩系西洼组复杂岩性风化壳储集层开展研究。结果表明: 风化壳岩性主要为安山岩和凝灰岩,其次为脉岩、隐爆角砾岩和砂砾岩。由于不同构造单元在复杂埋藏过程中经历了“洼—隆—隆”或“洼—隆—洼”的演化过程,不同岩性储集层的成岩序列和孔隙演化差异明显。火成岩和碎屑岩原生孔隙类型和成因明显不同,但现今储集空间均以次生孔隙特别是溶蚀孔为主,被溶蚀物质包括斑晶、基质以及火山岩屑等。溶蚀作用的主导流体多变,继抬升暴露过程中富含CO₂的大气淡水之后,有机质成熟时排出的有机酸对储集层改善有积极作用。有利风化壳储集层的形成受到岩性、埋藏演化过程和成岩流体的共同控制。整体上,富林洼陷白垩系西洼组储集层质量由好到次的顺序为: 凝灰质砂砾岩,火山角砾岩,含残余气孔安山岩,凝灰岩,次火山岩;不同岩性风化壳储集层在复杂埋藏过程中,经历了不同的成岩改造。     

北非地区中生代盆地区域沉积中心发育机制新认识及油气差异富集效应

北非地区为世界上油气富集地区之一,区内油气分布表现出极大的不均匀性,以往研究对这一油气差异性富集控制因素的探讨较为薄弱。本研究重点从中生代期间发育的多个区域沉积中心的演化和形成机制的角度,探讨这一科学问题。对已有的基础地质和油气勘探资料的综合再分析表明,北非地区冈瓦纳大陆北缘发育维德迈尔—佩拉杰、苏尔特、东地中海三个彼此孤立存在的中生代沉积中心,这些沉积中心在空间上处于阿拉拉隆起、苏尔特隆起、黎凡特隆起三个海西运动中形成的NE向古隆起之上,具有“古隆起塌陷反转”的形成机理;沉积中心均靠近新特提斯洋边缘,总体呈现受海西运动形成的古隆起和新特提斯洋开启背景下的伸展作用联合控制。三个中生代沉积中心为中生代优质烃源岩发育区和油气富集区;受海西期塑造的古构造、海西构造剥蚀对砂岩储层的控制以及中生代烃源岩发育等有利因素所控,这些塌陷形成的中生代沉积中心及围区成为最为重要的油气富集区带。中生代盆地的这一形成过程为该区油气差异富集的重要控制因素。     

古盐湖卤水温度对钾盐沉积的控制作用探讨

古盐湖卤水的温度对钾盐沉积的控制作用的定量研究是钾盐成矿机理分析的重点和难点。本文分析和测试陕北盐盆奥陶系马家沟组、四川盆地三叠系嘉陵江组、云南兰坪-思茅盆地白垩系及老挝沙空那空盆地白垩系等八个含盐系的石盐岩中的流体包裹体,并利用均一温度计算了古盐湖的蒸发速率。若以老挝白垩纪时盐湖的蒸发速率为标准值100,陕北奥陶纪、四川三叠纪、云南白垩纪的蒸发速率标准值分别为54、68和90,而目前在老挝和云南白垩系都找到了一定规模的钾盐矿,因此高温(气温及水温)是盐湖成钾的有利条件,在卤水演化成钾的过程中可以起到重要的"催化"作用。     

中国中新生代咸化湖盆烃源岩沉积的问题及相关进展

我国东部断陷湖盆和西部坳陷湖盆第三系均有蒸发岩与烃源岩共生现象。前者水体深、咸化范围小，在氯化盐和碳酸盐沉积环境中形成了优质烃源岩，后者水体浅、咸化范围大，在氯化盐和硫酸盐沉积环境中发育了优质烃源岩，作者认为两种湖盆出现的水体分层是有机质堆积和保存的重要条件。济阳坳陷和柴达木盆地为两类咸化湖盆的典型代表，对它们的研究可以深化蒸发岩-烃源岩共生区油气地质的认识、促进勘探发展。     

Evaporites and siliciclastics of the Permian Nippewalla Group of Kansas, USA: a case for non-marine deposition in saline lakes and saline pans

The mid‐Permian Nippewalla Group of Kansas consists of bedded evaporites, red‐bed siliciclastics and grey siliciclastics deposited in a non‐marine environment. Lithologies and sedimentary features indicate lacustrine and aeolian deposition, subaerial exposure and palaeosol formation. Grey siliciclastic mudstones characterized by planar and convolute laminations, ostracods, peloids and plant material represent a freshwater‐brackish perennial lake facies. Bedded anhydrites containing gypsum‐crystal pseudomorphs, clastic anhydrite grains and grey mud drapes and partings suggest deposition in saline lakes. Bedded halites consist of chevron and cumulate crystals, dissolution surfaces and pipes and mudcracked microcrystalline salt crusts, which were deposited in saline pans dominated by flooding, evaporative concentration and desiccation. Chaotic halite, composed of red‐bed mudstone and siltstone with displacive halite crystals, formed in saline mudflats. Red‐bed mudstone and siltstone with little or no displacive halite, but with abundant cracking, root and plant features, suggest deposition in a dry mudflat. Red‐bed sandstone, composed of well‐sorted, well‐rounded quartz grains cemented with halite, indicate aeolian and rare shallow‐water deposition. Most deposition took place in halite‐dominated ephemeral saline lakes surrounded by saline and dry mudflats, sandflats and sand dunes. Evaporation, desiccation, flooding and wind played significant roles in this environment. The Nippewalla Group siliciclastics and evaporites represent an evolution from a perennial lacustrine system to a non‐marine, acidic saline pan system in the mid‐continent of North America. The problem of distinguishing between ancient marine and non‐marine evaporites, as well as recognizing those evaporites deposited in acid settings, with detailed field, core and petrographical study of both evaporite deposits and associated sedimentary rocks has successfully been addressed. In addition, interpretations of mid‐Permian palaeoclimate data in the form of short‐term air temperature proxies within longer‐term wet–dry trends have been made. These data provide a new palaeogeographic and palaeoclimatic model for the mid‐Permian of western Pangaea.     

内陆湖盆"深水成盐"形成条件和识别标志:以东濮凹陷与现代盐湖为例

位于我国东部的许多新生代盆地,在地质历史时期发育了巨厚的岩盐沉积。尽管这些盆地的成盐模式研究已经历了40多年,但由于缺乏“深水成盐”的现代沉积模式,到底是“深盆-深水”还是“深盆-浅水”成盐,一直难以定论。为了理解“深水成盐”的控制因素,很多现代盐湖开展了水文学和水化学调查,然而这些湖盆的矿物组合各不相同,“深水成盐”的形成条件和控制因素尚不清楚。本文对死海、佛瑞湖等“深水成盐”的现代盐湖以及我国东濮凹陷沙三段的成盐特征进行了解剖,从盐度、水深、湖平面波动以及卤水分层等角度探讨了“深水成盐”的形成条件,对比分析了“深水成盐”的相对位置以及沉积物特征,总结了“深水成盐”的识别标志。研究表明,盐岩层系在岩心和测井资料上显示出多尺度的旋回性。“深水成盐”为水深较大的洼陷中心成盐,与盆缘的“浅水成盐”同盆共存。沉积相对岩盐层系的结构和组成有明显的控制作用,其中洼陷带的“深水成盐”,主要在洼陷带的卤水-湖底沉积物界面附近,析出和增生粗晶的盐类矿物,常与富含有机质和黄铁矿的暗色泥岩共存;而湖滨附近的“浅水成盐”,单层厚度薄,结晶粒度细小,通常含有较多的陆源碎屑。死海为代表的现代盐湖以及东濮凹陷等古代成盐盆地的沉积特征表明,“深水成盐”发生于湖平面的下降期,而且卤水剖面的厚度对蒸发岩的形成分布有明显的控制作用,同时具有“深水”和“深盆”特征的内陆盐湖更容易形成单层厚度大、横向稳定的岩盐沉积。本研究有助于改变人们以往对内陆湖盆成盐机理的认知,尤其是现代盐湖的卤水分层析盐特征,对解读地质历史中的其他成盐事件具有重要启示。基于现代沉积实例的“深水成盐”识别标志,可以为古代岩盐沉积模式的建立提供限定条件。     

Evidence of multiple evaporite recycling processes in a salt‐tectonic context,Sivas Basin,Turkey

The isotopic composition of evaporites can shed light on their environment of precipitation and their subsequent recycling processes. In this study, we performed Sr, O and S isotopic analyses on evaporitic sulphates in the halokinetic Sivas Basin. The main objectives were to decipher the age and origin of the evaporites responsible for the salt tectonics, and to test whether diapir dissolution acts as the source of younger evaporitic layers in continental mini‐basins. The Sr isotopes demonstrate that the first evaporites precipitated from seawater during the Middle–Late Eocene. The similar isotopic values measured in the halokinetic domain confirm that the Eocene evaporites triggered the salt tectonics and were continuously recycled in Oligo‐Miocene mini‐basins as lacustrine to sabkha evaporites. Modern halite precipitates suggest that the dissolution and recycling of diapiric halite is ongoing. This study demonstrates the efficiency of isotopic analyses in constraining evaporite recycling processes in continental halokinetic domains.     

Criteria for the recognition of salt-pan evaporites

Layered evaporites can accumulate in: (1) ephemeral saline pans, (2) shallow perennial lagoons or lakes, and (3) deep perennial basins. Criteria for recognizing evaporites deposited in these settings have yet to be explicitly formulated. The characteristics of the ephemeral saline pan setting have been determined by examining eight. Holocene halite-dominated pans (salt pans) and their deposits (marine and non-marine) from the U.S., Mexico, Egypt and Bolivia. These salt pans are typified by alternating periods of flooding, resulting in a temporary brackish lake, evaporative concentration, when the lake becomes saline, and desiccation, which produces a dry pan fed only by groundwater. The resulting deposits consist of alternating layers (millimetres to decimetres) of halite and mud. The layers of halite are characterized by: (1) vertical and horizontal cavities, rounded crystal edges and horizontal truncation surfaces, due to dissolution during flooding; (2) vertical ‘chevrons’ and ‘cornets’ grown syntaxially on the bottom during the saline lake stage; (3) halite cements (overgrowths and euhedral cavity linings) and disruption of layering into metre-scale polygons, produced during the desiccation stage. The muddy interbeds are characterized by displacive growth of halite during the desiccation stage. Immediately below the surface of the pan the halite layers are ‘matured’ by repeated episodes of dissolution and diagenetic crystal growth. This results in porous crusts with patches of ‘chevron’ and ‘cornet’ crystals truncated by dissolution, clear diagenetic halite cement, and internal sediment. These layers of ‘mature’ halite closely resemble the patchy cloudy and clear textures of ancient halite deposits. Holocene salt-pans are known to cover thousands of square kilometres and cap halite deposits hundreds of metres thick, so they are realistic models for ancient evaporites in scale, e.g. Permian Salado Formation of New Mexico-Texas, which preserves many primary salt-pan features.     

Halite cementation and carbonate diagenesis of intra-salt reservoirs from the Late Neoproterozoic to Early Cambrian Ara Group (South Oman Salt Basin)

Late Neoproterozoic to Early Cambrian carbonates of the Ara Group form important intra‐salt ‘stringer’ reservoirs in the South Oman Salt Basin. Differential loading of thick continental clastics above the six carbonate to evaporite cycles of the Ara Group led to the formation of salt diapirs, encasing a predominantly self‐charging hydrocarbon system within partly highly overpressured carbonate bodies (‘stringers’). These carbonates underwent a complex diagenetic evolution, with one stage of halite cementation in a shallow (early) and another in a deep (late) burial environment. Early and late halite cements are defined by their microstructural relationship with solid bitumen. The early phase of halite cementation is post‐dated by solid reservoir bitumen. This phase is most pervasive towards the top of carbonate stringers, where it plugs nearly all available porosity in facies with initially favourable poroperm characteristics. Bromine geochemistry revealed significantly higher bromine contents (up to 280 p.p.m.) in the early halite compared with the late halite (173 p.p.m.). The distribution patterns and the (high) bromine contents of early halite are consistent with precipitation caused by seepage reflux of highly saturated brines during deposition of the overlying rock salt interval. Later in burial history, relatively small quantities of early halite were dissolved locally and re‐precipitated as indicated by inclusions of streaky solid bitumen within the late halite cements. Late halite cement also seals fractures which show evidence for repeated reopening. Initially, these fractures formed during a period of hydrothermal activity and were later reopened by a crack‐seal mechanism caused by high fluid overpressures. Porosity plugging by early halite cements affects the poroperm characteristics of the Ara carbonates much more than the volumetrically less important late halite cement. The formation mechanisms and distribution patterns of halite cementation processes in the South Oman Salt Basin can be generalized to other petroliferous evaporite basins.     

Criteria for the recognition of acid-precipitated halite

Modern acid and neutral saline lakes in Western Australia are an excellent natural laboratory for testing how pH affects halite, and for developing criteria for distinguishing past acid saline waters from past neutral saline waters in the rock record. This study characterizes and compares physical, chemical and biological features in halite precipitated from acid (pH 1·7 to 4·2) and neutral (pH 6·8 to 7·3) saline lakes in southern Western Australia. Supplemental data include synthetic halite grown from acid and neutral saline solutions, as well as halite deposited in Permian acid lakes. Although physical processes of halite growth are not affected by pH, there are differences in the colour, accessory minerals, fluid inclusions and microfossils between acid and neutral halites. Acid lake halite commonly is yellow or orange in colour; neutral lake halites examined in this study are always snow white. Acid halites tend to contain abundant sulphate and iron oxide minerals, both as solid inclusions and as solids within fluid inclusions; neutral halites contain little, if any, sulphates and no iron oxides. Acid fluid inclusion freezing/melting behaviours include characteristics that differ from neutral fluid inclusion behaviours, such as lower eutectic temperatures, higher and wider temperature range of hydrohalite rims with a definable fuzzy border and more complex metastable phases. Acid halite contains 'hairy blobs', clusters of bacterial/archaeal/fungal remains and sulphate crystals, which are not found in halite from neutral lakes. This distinct assemblage of features characteristic of modern acid lake halites may serve as informal criteria for the recognition of past acid lake evaporites in the rock record.