碳酸盐沉积物的成岩作用

化学沉淀碳酸盐矿物在沉积后很容易受到各种作用的影响,其中最重要的是其在成岩阶段所经历的成岩作用.碳酸盐沉积物在成岩过程中主要受大气降水、海水和埋藏过程中孔隙流体的控制,经历一系列压实、溶解、矿物的多相转变、重结晶、胶结等成岩作用,逐渐转变为固结的岩石.在成岩过程中,由于孔隙流体与沉积流体之间的异同以及温度的变化,碳酸盐沉积物的原始矿物成分、地球化学特征可能会很好的保存下来,但在许多情况下,也可能会改变,从而使我们无法准确反演碳酸盐沉积物在沉积时水体的特征.因此,我们在应用碳酸盐岩重建相关古环境和古气候变化的时候,必须要通过有效的方法来对碳酸盐岩是否受到成岩作用的影响进行鉴定.     

地下孔隙率和渗透率在空间和时间上的变化及影响因素

地下岩石孔隙率和渗透率在空间和时间上的变化受众多因素控制,总的说来,包括沉积与成岩两个最为主要的因素。沉积盆地的性质和沉积环境控制了沉积物的组成、岩石的结构和原生孔隙。沉积作用所经历的时间相对较短而进程较快;成岩作用所经历的时间相对较长而进程较慢。从对孔隙率和渗透率的控制作用来说,成岩作用的研究难度相对较大。近年来,一些传统的形成地下岩石次生孔隙的机制受到挑战,这些传统机制中最为主要的是有机酸对铝硅酸溶解形成次生孔隙,但这会造成介质pH值的升高,碳酸盐矿物和高岭石的沉淀。地下岩石中碳酸水溶解产生的次生孔隙也是有限的。新的机制如大气淡水的溶解作用,深部冷却地下水的溶解作用,硅酸盐的水解都得到了人们的普遍接受,这对于次生孔隙成因的解释及地下岩石孔隙率和渗透率的预测十分重要。对于碳酸盐岩来说,与不整合面附近的古喀斯特有关的油气藏得到了更多的关注。深埋藏过程中碳酸盐岩成岩作用的研究中,温度和压力的影响对不同碳酸盐矿物溶解及沉淀作用的差异性控制了地下碳酸盐岩孔隙率和渗透率分布.     

碳酸盐岩层系中非碳酸盐矿物对层序界面及成岩流体环境的指示——以塔河深层TS2井为例

碳酸盐岩层系中以碳酸盐岩矿物为主,其中所含有的少量非碳酸盐岩矿物与沉积过程、成岩过程或陆源碎屑物有关,非碳酸盐岩矿物类型与含量变化为判别沉积与成岩流体环境等提供了相对客观的依据。本文以塔里木盆地塔河地区深层TS2井上寒武统-中下奥陶统溶洞段及关键界面处的27件岩屑样品的系统矿物分析为例,探讨非碳酸盐矿物对层序界面成岩流体环境的指示意义。结果表明,该井下奥陶统鹰山组下段内部界面、上寒武统丘里塔格组顶面处的赤铁矿含量明显较高,表明这两个界面为沉积间断或不整合面;上寒武统丘里塔格组溶洞段砂泥岩充填物与上寒武统顶不整合面上下的矿物组合较一致,表明塔河深层上寒武统白云岩可能经历了大气淡水的成岩流体改造。     

成岩温度的判别标志

“沉积物沉积之后,在没有受到高压和岩浆热力影响时所经历的一切物理的和化学的变化”,Walther(1894)称之为成岩作用。这一定义包括了我们所谓的准同生作用、成岩作用和表生作用等阶段,即广义的成岩作用。成岩作用的主要控制因素是主岩成分、温度、压力和孔隙流体性质。其中温度是最关键的因素,人们也主要是根据温度来划分成岩作用和后生作用的界限。一般把浊沸石稳定的上     

川东三叠系飞仙关组碳酸盐岩的阴极发光特征与成岩作用

四川盆地东部三叠系飞仙关组是近年来我国发现的重要天然气储层, 高孔隙度、高渗透率的碳酸盐储层都分布于白云岩地层中, 因而碳酸盐的成岩作用, 尤其是白云岩化作用和白云岩的成因为石油地质学家和沉积学家高度关注.对四川盆地东部罗家寨构造三叠系飞仙关组42个碳酸盐岩样品进行了阴极发光分析, 结合与之有关的Mn、Fe、Mg元素分析和岩石学研究, 讨论了包括白云岩化作用在内的碳酸盐岩成岩过程中可能的成岩流体性质及来源.四川盆地东部三叠系飞仙关组碳酸盐岩普遍具有很弱的阴极发光性, 这与其很低的Mn、Fe含量有关, 说明沉积期后非海相流体对飞仙关组碳酸盐岩的影响非常有限, 海源流体在成岩过程中发挥了主导作用; 不同石灰岩类型和不同白云岩类型仍然具有不同的阴极发光性, 成岩组分含量越高的碳酸盐岩, 或者说与沉积期后流体(主要是孔隙流体) 关系越密切的碳酸盐岩的阴极发光强度越低, 说明随着埋藏成岩作用的进行, 四川盆地东部三叠系碳酸盐岩孔隙流体受海源流体的影响是逐渐增强的; 阴极发光分析结果表明, 作为四川盆地东部主要储集岩的结晶白云岩形成机制与埋藏过程中的深循环流体有关, 这种深循环流体没有或很少穿越铝硅酸盐地层, 但穿越了三叠系内部的某些海相地层, 这些海相地层可能是广泛存在于四川盆地三叠系的蒸发盐地层, 由蒸发盐成岩过程提供的海源流体参与了结晶白云岩的白云岩化作用.      

火山作用对碳酸盐岩沉积及成岩的影响

火山作用是熔岩（岩浆）、火山碎屑和火山气体通过火山口喷出到地球表面的现象。火山爆发带来大量火山碎屑物质和气体进入到大气、海洋和陆地环境系统中,这将对其中碳酸盐沉积物的成分、结构、沉积构造、相序及沉积演化过程产生重要影响。火山物质含量的增加不仅会干扰碳酸盐沉积,还能够改变其物理化学性质等,火山沉积物在空间上也遵循粒度分异的规律。在组合方式上往往出现狭义和广义的混合沉积,记录了火山活动的演化。碳酸盐岩主要形成于海洋、湖泊等环境,火山作用可以通过控制基底形态、热液喷涌、不同性质火山碎屑的输入和分布、台地的差异沉降/隆起等,影响生物、沉积水体性质等古环境参数。然而就生物而言,这种影响通常具有破坏性和建设性作用。例如火山灰的埋藏及释放有毒物质会导致生物死亡,而后期火山灰溶解又会释放PO3-、Fe3+、Mn2+等营养离子利于生物生长。就成岩作用及流体性质而言,不同类型的火山物质含不同元素、矿物等,导致流体性质的变化,进而影响成岩作用类型等,如火山灰可以提供Mg2+促进白云石化作用,有利于孔隙的形成,对碳酸盐岩储层的形成有积极作用。因此,研究火山作用对碳酸盐岩沉积、成岩的影响,对油气地质勘探有现实意义。     

碎屑储集岩成岩演化过程中流体—岩石相互作用特征——以塔里木盆地西南坳陷地区为例

塔西南坳陷不同时代碎屑储集岩成岩环境经历了由酸性向碱性演变的过程,成岩环境地球化学性质的这种转变直接控制了发生在碎屑岩成岩体系中的流体—岩石相互作用特征。酸性成岩环境形成于烃源岩—储集岩系统中有机—无机反应最活跃时期,主要分布在早成岩阶段A、B期和晚成岩阶段A期,当古地温小于90℃,Ro值在<0.5%～1.3%之间,酸性的孔隙介质与骨架颗粒之间主要发生蚀变、溶解和氧化硅沉淀作用。当古地温达到90℃以上,Ro值在1.3%～>2%时,有机质脱羧基停止,有机酸发生分解,CO2来源减小,使孔隙流体性质由酸性向碱性变化,成岩环境呈碱性,这时晚期含铁碳酸盐矿物交代作用最为活跃,同时伴随自生伊利石和绿泥石沉淀、陆源伊利石重结晶成绢云母、高岭石向伊利石或绿泥石转变等作用。成岩环境在其演化过程中,由于孔隙流体性质的转变,破坏了早期成岩环境的物理化学平衡状态,使旧的成岩反应停止,新的成岩反应开始,从而形成多种矿物蚀变→交代→溶解→沉淀过程,导致碎屑储集岩结构和孔隙组合特征上的差异,流体-岩石相互作用的强度和范围决定了碎屑储集岩的储集性能及分布特征。     

海水化学演化对生物矿化的影响综述

显生宙非骨屑碳酸盐矿物经历了文石海和方解石海的交替,主要造礁生物和沉积物生产者的骨骼矿物与非骨屑碳酸盐矿物具有同步变化的趋势。这种长期的变化趋势可以用海水化学Mg/Ca摩尔比的变化来解释。流体包裹体、同位素和微量元素等证据也证实了海水化学在地质历史中经历过剧烈的变化。虽然生物诱导矿化和生物控制矿化的相对重要性一直存在争议,但古生物地层记录和人工海水养殖实验结果都表明,海水化学演化对生物矿化有重要的影响,体现在造礁生物群落的兴衰、生物起源时对骨骼矿物类型的选择以及微生物碳酸盐岩在地质历史中的分布等。这些为研究前寒武纪海水化学演化、古气候和古环境的重建、同位素地层对比以及碳酸盐的沉积和成岩等问题提供了新的思路。     

碎屑储集岩成岩演化过程中流体-岩石相互作用特征--以塔里木盆地西南坳陷地区为例

塔西南坳陷不同时代碎屑储集岩成岩环境经历了由酸性向碱性演变的过程,成岩环境地球化学性质的这种转变直接控制了发生在碎屑岩成岩体系中的流体-岩石相互作用特征。酸性成岩环境形成于烃源岩-储集岩系统中有机-无机反应最活跃时期,主要分布在早成岩阶段A、B期和晚成岩阶段A期,当古地温小于90℃,Ro值在<0.5%～1.3%之间,酸性的孔隙介质与骨架颗粒之间主要发生蚀变、溶解和氧化硅沉淀作用。当古地温达到90℃以上,Ro值在1.3%～>2%时,有机质脱羧基停止,有机酸发生分解,CO₂来源减小,使孔隙流体性质由酸性向碱性变化,成岩环境呈碱性,这时晚期含铁碳酸盐矿物交代作用最为活跃,同时伴随自生伊利石和绿泥石沉淀、陆源伊利石重结晶成绢云母、高岭石向伊利石或绿泥石转变等作用。成岩环境在其演化过程中,由于孔隙流体性质的转变,破坏了早期成岩环境的物理化学平衡状态,使旧的成岩反应停止,新的成岩反应开始,从而形成多种矿物蚀变→交代→溶解→沉淀过程,导致碎屑储集岩结构和孔隙组合特征上的差异,流体-岩石相互作用的强度和范围决定了碎屑储集岩的储集性能及分布特征。     

辽河盆地西部凹陷沙河街组砂岩碳酸盐胶结物特征

碳酸盐胶结物中氧碳同位素组成研究是分析成岩过程中流体-岩石相互作用的重要技术方法.综合运用岩石学、矿物学和地球化学方法，对辽河盆地西部凹陷沙河街组砂岩中碳酸盐胶结物的化学组成和碳酸盐胶结物及成岩流体同位素组成特征进行系统分析.研究表明，研究区碳酸盐岩主要为方解石和白云石，胶结物主要类型为嵌晶式胶结、孔隙式胶结、斑块状胶结和星点状胶结.碳、氧稳定同位素组成能有效地反映成岩-成矿流体及其他物质的来源，碳酸盐胶结物与现今浅层地下水氧同位素组成差异巨大而与变质水同位素组成具有相似性，反映了盆地演化过程中活动热流体对成岩作用的影响.包裹体的氢、氧同位素组成可表征成矿溶液的演化特征，砂岩碳酸盐胶结物包裹体更富集氢的轻同位素和氧的重同位素，表明发生了明显的“氧-18漂移”.碳酸盐胶结的成矿溶液表现出“受热雨水”的同位素组成特征，反映了深源活动热流体对成岩作用的影响.      

Carbon and oxygen isotope compositions of the carbonate facies in the Vindhyan Supergroup, central India

The Vindhyan sedimentary succession in central India spans a wide time bracket from the Paleoproterozoic to the Neoproterozoic period. Chronostratigraphic significance of stable carbon and oxygen isotope ratios of the carbonate phase in Vindhyan sediments has been discussed in some recent studies. However, the subtle controls of facies variation, depositional setting and post-depositional diagenesis on stable isotope compositions are not yet clearly understood. The Vindhyan Supergroup hosts four carbonate units, exhibiting a wide variability in depositional processes and paleogeography. A detailed facies-specific carbon and oxygen isotope study of the carbonate units was undertaken by us to investigate the effect of these processes and to identify the least altered isotope values. It is seen that both carbon and oxygen isotope compositions have been affected by early meteoric water diagenesis. The effect of diagenetic alteration is, however, more pronounced in case of oxygen isotopes than carbon isotopes. Stable isotope compositions remained insensitive to facies only when sediments accumulated in a shallow shelf setting without being exposed. Major alteration of original isotope ratios was observed in case of shallow marine carbonates, which became exposed to meteoric fluids during early diagenetic stage. Duration of exposure possibly determined the magnitude of alteration and shift from the original values. Moreover, dolomitization is found to be accompanied by appreciable alteration of isotope compositions in some of the carbonates. The present study suggests that variations in sediment depositional settings, in particular the possibility of subaerial exposure, need to be considered while extracting chronostratigraphic significance from δ¹³C data.     

海相碳酸盐矿物的阴极发光性与其成岩蚀变的关系

引言众所周知,原始的海相碳酸盐矿物为文石、高镁方解石和低镁方解石,但它们对成岩蚀变极度敏感,在大气淡水成岩环境中,文石和高镁方解石都会转变为成岩低镁方解石,同时,原始的低镁方解石也会有不同程度的蚀变。在进行古代碳酸盐岩的沉积环境和古海洋学研究时,往往需要测定岩石中的微量元素及氧、碳、锶等同位素组成,用以判断古海水     

Diagenetic evolution vis-a-vis reservoir characteristics of Dhosa sandstones,Ler dome,Kachchh, western India

The detrital mineralogy as well as diagenetic characters of the Dhosa Sandstone Member of Chari Formation exposed at the Lerdome, south of Bhuj was studied. In order to assess the potential of the Dhosa Sandstone as a reservoir, it is substantial to understand the diagenetic processes that are controlled largely by post-depositional cementation and compaction in addition to framework composition and original depositional textures. The petrologic analysis of 33 thin sections was carried out to discern primary composition and diagenetic features including primary and secondary porosity patterns. Monocrystalline quartz dominates the detrital mineralogy followed by polycrystalline quartz. Among the polycrystalline variety recrystallized metamorphic quartz surpasses stretched metamorphic quartz in terms of abundance. Feldspars comprise microcline and plagioclase where the former is dominant over the latter. Orthoclase too comprises a very small percentage. Mica, chert, rock fragments, and heavies form the remaining detrital constituent in descending order of their constituent percentage. The diagenetic precipitates are mainly carbonate (8.30%) and iron (7.80%) followed by clay (0.66%) and silica (0.88%) that are minor constituent of the total cementing material. The main paragenetic events identified are early cementation, mechanical compaction, late cementation, dissolution, and authigenesis of clays. The overall reservoir quality seems to be controlled by compaction and authigenic carbonate cementation. The minus cement porosity average 29.4%. The porosity loss due to compaction is 21.92% and by cementation is 29.71%. The loss of original porosity was due to early cementation followed by moderate mechanical compaction during shallow burial. Preservation of available miniscule primary porosity was ascribed to dissolution of carbonates and quartz overgrowth which resisted chemical compaction during deep burial. The studied sandstones may have low reservoir quality owing to existing porosity of less than 9%. More carbonate dissolution and its transformation in dolomite in sub-surface condition and macro-fracture porosity may result in enhanced secondary porosity and good diagenetic traps.     

Authigenic albite in carbonate rocks – a tracer for deep-burial brine migration?

Euhedral, post-depositional albite from the Eastern and Western Alps, the western Carpathians and some Greek islands was examined petrographically and geochemically to gain insights into the nature of feldspar reactions in carbonate rocks. This study focuses on coarsely crystalline, homogeneously nucleated albite in order to avoid problems related to the presence of inseparable detrital material in fine-grained albite varieties. All albite samples show a very restricted compositional variability and are typically ≥ 99 mol% Ab component. Unit-cell parameters determined by Rietveld analysis are slightly more variable than previously accepted, but confirm high Al–Si ordering characteristic of low albite. The oxygen isotopic composition of albite ranges from + 19·4‰ to + 28·3‰ VSMOW. There is no direct relationship between the δ¹⁸O value and the inferred temperature of albite formation, nor is there one with stoichiometry. The coarse crystal size (up to several millimetres in diameter), petrographic evidence showing albite cross-cutting stylolites, greater abundance of albite in carbonate rocks subject to high-grade diagenetic or weak metamorphic overprinting and available fluid inclusion data suggest that albite precipitation is favoured at higher temperatures in carbonates than in sandstones. Pore fluids were invariably brines, as suggested by the inferred high positive δ¹⁸O_fluid values, the common association of albite-bearing carbonates and evaporites and reports of saline fluid inclusions in albite. The presence of authigenic albite may thus be a useful tracer of palaeobrine–carbonate reactions, particularly in deep-burial and incipient metamorphic settings.     

Silicification of sulphate evaporites and their carbonate replacements in Eocene marine sediments, Tunisia: two diagenetic trends

Samples of chert nodules, diagenetic carbonates and evaporites (gypsum/anhydrite) collected from the gypsiferous limestones of the Kef Eddour Member (Ypressian‐Priabonian) near Metlaoui and Sehib (Tunisia) show selective silicification with great variety in the silicified by‐products. Based on δ¹³C values, which support an organic origin for the carbon, carbonates replaced evaporites microbially through bacterial sulphate reduction. Observations and results suggest two scenarios for chert formation that are related to the rate and timing of diagenetic carbonate replacement of the evaporites (anhydrite/gypsum). In the absence of early diagenetic carbonate phases, silica with δ¹⁸O values from +25 to +28·6‰ [standard mean ocean water (SMOW)] replaced the outer parts of anhydrite nodules at pH < 9. In contrast, pore‐fluid pH values > 9 in the innermost parts of the anhydrite nodules prevented silica precipitation. The record of this chemical barrier is preserved in the microquartz rims and geode features that formed in the inner parts of the nodules after dissolution of the anhydrite nucleus. The microbial diagenetic replacement of evaporites (bacterial sulphate reduction) by carbonates (calcite, aragonite and dolomite) favoured silica replacement of carbonates rather than evaporites. Silica, with δ¹⁸O signature of +21 to +26‰ (SMOW), replaced carbonates on a volume‐for‐volume basis, yielding a more siliceous groundmass, and accounting for 90–95% of the nodules. The relatively higher δ¹⁸O values of quartz replacing anhydrite can be explained by a diagenetic fluid in equilibrium with mixed (meteoric/marine) to marine water. The lower δ¹⁸O values of the quartz that replaced the diagenetic carbonates are ascribed to flushing by meteoric water in a later diagenetic stage. The silica supply for chert formation could be derived from the reworked bio‐siliceous deposits (diatomites) to the west of the basin [vestiges of an opal‐CT precursor undetectable by X‐ray diffraction (XRD) were revealed by δ²⁹Si magic‐angle‐spinning nuclear magnetic resonance investigations], diagenesis of the extraformational and overlying clay‐rich beds (the host limestones are clay‐poor as shown by XRD measurements), and minor volcanogenic and hydrothermal contributions during early diagenetic stages.     

Petrography and geochemistry of Paleocene–Eocene limestones in the Ching-dar syncline, eastern Iran

The Ching-dar syncline is located to the west of the city of Birjand, in the east of han. The ca. 500 m thick studied section at the eastern flank of the syncline contains a sequence of almost continuous shallow- marine limestones that exhibit no major sedimentary breaks or evidence for volcanic activity. Skeletal grains consist of large benthic foraminifera and green algae whereas non-skeletal grains are mostly peloids and intraclasts. They were deposited on a shallow-marine carbonate ramp. The limestones have undergone extensive diagenetic processes with varying intensities, the most important of which are micritization, cementation, compaction （chemical and mechanical）, internal filling and stylolitization. Chemical analysis of the limestone samples revealed high calcium and low magnesium content. Major and minor element values were used to determine the original carbonate mineralogy of these lime- stones. Petrographic evidence and elemental values indicate that calcite was the original carbonate mineral in the limestones of the Ching-dar syncline. The elemental composition of the Ching-dar car- bonates also demonstrates that they have stabilized in a meteoric phreatic environment. Variation of Sr/ Ca vs. Mn values suggests that diagenetic alteration occurred in an open geochemical system.     

Authigenic quartz and albite in Devonian limestones: origin and significance

Euhedral quartz and albite crystals are common in Devonian (Givetian-Frasnian) shallow-marine shelf carbonates from the Belgian Ardennes. Several features such as morphology, the presence of carbonate inclusions, inversion temperatures and occurrence in the insoluble residues of stylolitic surfaces indicate that these crystals have developed authigenically. Oxygen isotope ratios point to an intermediate deep burial realm of origin at temperatures of 60–90°C. The predominance of illite and the almost total absence of smectite clay minerals is interpreted as an indication that illitization produced the silica needed for authigenesis. The mineral composition of inclusions indicates that the carbonate host rock must have consisted of low-Mg calcite already at the time of authigenesis. These inclusions represent an earlier diagenetic stage than the present carbonate rock, since they were protected from further diagenetic alteration by the surrounding quartz. The calcite inclusions display a higher Sr/Ca and Mg/Ca ratio than the carbonate host-rock. Because neomorphic diagenesis of the carbonate continued after silicate authigenesis, the contents of Mg and Sr in the calcite of the host carbonate are even lower. The authigenic feldspar mineralogy seems to be determined by the composition of the host-sediment.     

An overview of the geology of the Transvaal Supergroup dolomites (South Africa)

 In the Neoarchaean intracratonic basin of the Kaapvaal craton, between approximately 2640 Ma and 2516 Ma, two successive stromatolitic carbonate platforms developed. Deposition started with the Schmidtsdrif Subgroup, which is probably oldest in the southwestern part of the basin, and which contains stromatolitic carbonates, siliciclastic sediments and minor lava flows. Subsequently, the Nauga formation carbonates were deposited on peritidal flats located to the southwest and were drowned during a transgression of the Transvaal Supergroup epeiric sea, around 2550 Ma ago. This transgression led to the development of a carbonate platform in the areas of the preserved Transvaal and Griqualand West basins, which persisted for 30–50 Ma. During this time, shales were deposited over the Nauga Formation carbonates in the southwestern portion of the epeiric sea. A subsequent period of basin subsidence led to drowning of the stromatolitic platform and to sedimentation of chemical, iron-rich silica precipitates of the banded iron formations (BIF) over the entire basin. Carbonate precipitation in the Archaean was largely due to chemical and lesser biogenic processes, with stromatolites and ocean water composition playing an important role. The stromatolitic carbonates in the preserved Griqualand West and Transvaal basins are subdivided into several formations, based on the depositional facies, reflected by stromatolite morphology, and on intraformational unconformities; interbedded tuffs and available radiometric age data do not yet permit detailed correlation of units from the two basins. Thorough dolomitisation of most formations took place at different post-depositional stages, but mainly during early diagenesis. Partial silicification was the result of diagenetic and weathering processes. Karstification of the carbonate rocks was related to periods of exposure to subaerial conditions and to percolation of groundwater. Such periods occurred locally at the time of carbonate and BIF deposition. Main karstification, however, probably took place during an erosional period between approximately 2430 Ma and 2320 Ma. Received: 15 September 1996 · Accepted: 12 May 1998