Double carbonate breakdown reactions at high pressures: an experimental study in the system CaO–MgO–FeO–MnO–CO2

The pressure–temperature conditions of the reactions of the double carbonates CaM(CO₃)₂, where M = Mg (dolomite), Fe (ankerite) and Mn (kutnohorite), to MCO₃ plus CaCO₃ (aragonite) have been investigated at 5–8 GPa, 600–1,100°C, using multi-anvil apparatus. The reaction dolomite = magnesite + aragonite is in good agreement with the results of Sato and Katsura (Earth Planet Sci 184:529–534, 2001), but in poor agreement with the results of Luth (Contrib Mineral Petrol 141:222–232, 2001). The dolomite is partially disordered at 620°C, and fully disordered at 1,100°C. All ankerite and kutnohorite samples, including the synthetic starting materials, are disordered. The P–T slopes of the three reactions increase in the order M = Mg, Fe, Mn. The shallower slope for the reaction involving magnesite is due partly to its having a higher compressibility than expected from unit-cell volume considerations. At low pressures there is a preference for partitioning into the double carbonate of Mg > Fe > Mn. At high pressures the partitioning preference is reversed. Using the measured reaction positions, the P–T conditions at which dolomite solid solutions will break down on increasing P and T in subduction zones can be estimated.     

The origin of dolomite associated with salt diapirs in central Tunisia: Preliminary investigations of field relationships and geochemistry

Black and white dolomite crystals (mm to cm width) of different isotopic composition are associated with Triassic diapirism in central Tunisia, as well as with evaporite minerals and clays. The white dolomites occur mostly in the Jabal Hadifa diapir near the contact with Cretaceous limestones, whereas the smaller black dolomites occur in the Jabal Hamra diapir. The former dolomite has a narrow range of δ¹⁸O and δ¹³C values (− 3.83‰ to − 6.60‰ VPDB for δ¹⁸O; − 2.11‰ to − 2.83‰ VPDB for δ¹³C), whereas the latter dolomite has a wider range and more depleted values (− 4.92‰ to − 9.97‰ for δ¹⁸O; − 0.55‰ to − 6.08‰ for δ¹³C). However, the ⁸⁷Sr / ⁸⁶Sr ratios of most of the samples are near Triassic seawater values. Dolomite formation is due to at least two different fluids. The main fluid originated from deeper hydrothermal or basinal sources related to the Triassic saliferous rocks and ascended through faults during the diapiric intrusion. The second, less important fluid source is related to meteoric water originating from Cretaceous rocks.     

Dolomite formation in breccias at the Musandam Platform border, Northern Oman Mountains, United Arab Emirates

The presence of dolomite breccia patches along Wadi Batha Mahani suggests large-scale fluid flow causing dolomite formation. The controls on dolomitization have been studied, using petrography and geochemistry. Dolomitization was mainly controlled by brecciation and the nearby Hagab thrust. Breccias formed as subaerial scree deposits, with clay infill from dissolved platform limestones, during Early Cretaceous emergence. Cathodoluminescence of the dolostones indicates dolomitization took place in two phases. First, fine-crystalline planar-s dolomite replaced the breccias. Later, these dolomites were recrystallized by larger non-planar dolomites. The stable isotope trend towards depleted values (δ¹⁸O: − 2.7‰ to − 10.2‰ VPDB and δ¹³C: − 0.6‰ to − 8.9‰ VPDB), caused by mixing dolomite types during sampling, indicates type 2 dolomites were formed by hot fluids. Microthermometry of quartz cements and karst veins, post-dating dolomites, also yielded high temperatures. Hot formation waters which ascended along the Hagab thrust are invoked to explain type 2 dolomitization, silicification and hydrothermal karstification.     

镁同位素在“白云岩问题”研究中的应用及进展

困扰几代地质学家的"白云岩问题"不仅是重要的基础地质问题,而且与油气勘探开发实践密切相关。传统岩石学、地球化学研究手段在白云岩成因研究中起了重要作用,但均不能对其形成过程及Mg离子来源提供很好的约束。作为一种新兴的非传统稳定同位素,镁(Mg)同位素已经在白云岩问题研究领域受到了广泛关注。随着Mg同位素地球化学体系研究的不断深入,目前已经积累了大量不同时代、不同类型白云岩的Mg同位素数据。此外,通过理论计算、模拟实验与实际观测等方法,对白云岩(石)形成过程中的Mg同位素分馏研究已取得较大进展,为Mg同位素在白云岩问题研究中的应用奠定了坚实的理论基础。理论计算、实验模拟和实际观测的Mg同位素分馏存在较大差异,造成这种差异的原因一方面可能是模拟或实际观测的Mg同位素分馏未达到平衡,另一方面可能是实际白云岩的形成过程与理论过程不同。由于直接根据Mg同位素值判断白云岩成因存在较大难度,基于典型的白云石化地质模式,最新的研究建立了两个Mg同位素地球化学数值模型(DAR模型、AF模型),用于模拟不同白云石化过程中的Mg同位素变化,可对白云石化过程及Mg离子来源进行定量-半定量研究。其中DAR模型用于模拟海水白云石化过程的Mg同位素地球化学特征,而AF模型用于模拟流动水体白云石化过程的Mg同位素地球化学特征。通过这两个模型可以示踪白云岩的Mg离子来源并区分不同白云石化过程。尽管目前已建立的Mg同位素地球化学模型总体框架正确,但在适用范围和参数设置等方面存在一定不足,仍需要在后期研究中不断完善。     

Seawater normalized REE patterns of dolomites in Geshan and Panlongdong sections,China: Implications for tracing dolomitization and diagenetic fluids

In conventional studies of tracing dolomitization and diagenetic fluids, REEs of dolomites were widely used as been normalized by PAAS, NASC or chondrite. However, most dolomites are formed in seawater or seawater-derived fluids. Thus, we conduct a new attempt to normalize the REEs of dolomite using seawater standard, based on case studies on 36 Triassic limestone–dolomite samples from the Geshan section of southeast China and 26 Permian–Triassic dolomite samples from the Panlongdong section of northeastern Sichuan Basin, southwest China.The Geshan seawater-normalized (SN) REE patterns are characterized by notable positive Ce_SN (average Ce_SN/Ce* = 6.823, SD = 0.192) and negative Pr_SN anomalies (average Pr_SN/Pr* = 0.310, SD = 0.010), and slightly negative Gd_SN anomalis (average Gd_SN/Gd* = 0.864, SD = 0.053), with no obvious Eu_SN anomaly (average Eu_SN/Eu* = 1.036, SD = 0.094). The signatures of REE patterns barely changed during the dolomitization process. For the REE compositions of the Panlongdong dolomite, it can be found that (1) the recrystallization process can result in varied total REE concentrations (between 7.16 ppm and 37.87 ppm), but do not alter the REE patterns, including consistent positive Ce_SN anomalies (average = 4.074, SD = 0.27) and LREE enrichment (average Nd_SN/Yb_SN = 3.164, SD = 0.787); (2) meteoric incursion can reverse Ce anomaly, from the strong positive Ce anomalies (Ce_SN/Ce* = 5.059) to slightly positive (Ce_SN/Ce* = 2.459) or even negative Ce anomalies; and (3) hydrothermal fluid altered REE pattern is complicated by fluctuated distribution curve, negative Ce anomaly and positive Eu anomaly (Eu_SN/Eu* = 1.862). These results suggest that the seawater normalized REE patterns of dolomite can serve as an index to study the source of the dolomitization fluids and distinguish complex diagenetic processes, providing a complement to previous works.     

川东北飞仙关组甲烷为主的TSR及其同位素分馏作用

川东北开江-梁平陆棚东北侧飞仙关组多孔鲕粒白云岩中发生了以甲烷为主的热化学硫酸盐还原作用(TSR),产生高达20%的H₂S;而西南侧鲕粒灰岩以低孔、低H₂S天然气为特征。东北侧白云岩主要发育白云石粒间溶孔或粒间扩大溶孔,这些溶孔可与方解石(δ¹³C=-10‰~-19‰)、储层沥青、元素硫、黄铁矿和石英紧密共生,可分布于片状储层沥青与白云石晶体之间,说明白云石溶解作用发生在沥青形成以后。白云石的溶解作用导致现今天然气以无机CO₂为主,δ¹³C_CO2主要介于-2‰~+2‰之间。这种溶解作用是在酸性条件下,硬石膏或天青石参与下发生的,可能先产生MgSO₄配对离子,而后MgSO₄又与甲烷反应产生H₂S,净增大了孔隙。研究还发现,普光气田及以东天然气的来源不同于河坝和元坝天然气;对普光气田及以东天然气分析显示,甲烷δ¹³C值与残余烃含量 之间存在对数相关关系。这表明TSR过程中,甲烷同位素分馏作用遵从封闭体系下瑞利分馏原理。据此计算显示,渡4井约有15%甲烷被氧化了。     

塔中古城墟隆起下古生界碳酸盐岩沉积与储层特征

古城墟隆起作为塔里木盆地中央隆起带的一部分,近年来,油气显示丰富,展现了良好的油气勘探前景.但由于长期以来勘探程度低、钻井少等因素影响,碳酸盐岩沉积与储层研究十分薄弱,已成为制约该区油气勘探突破的瓶颈问题.为此,本文以古城墟隆起与卡塔克隆起构造、沉积演化差异对比分析为基础,结合单并相分析及沉积相模式的建立,刻画了晚寒武...     

Systematic sequence-scale controls on carbonate cementation in a siliciclastic sedimentary basin: Examples from Upper Cretaceous shallow marine deposits of Utah and Coloradao,USA

Whilst the relationship between stratigraphic development and carbonate cementation within siliciclastic succession has been documented through a number case studies, these studies have been generally restricted to observations upon individual sequences and/or limited sub-surface data. In this paper, long-term (5 million years), large-scale (>200 km) stratigraphic controls on carbonate cementation patterns are documented from the Upper Cretaceous Panther Tongue Member, Blackhawk Formation and Castlegate Sandstone exposed in the Book Cliffs in Utah and Colorado, USA. Together, these comprise eight progradational wedges of sandstones, which interfinger with the Mancos Shale, deposited within the Western Interior Seaway foreland basin. Petrographic analyses of ferroan dolomite cement bodies within these sandstone wedges show that the ferroan dolomite cements are all early, relative to burial diagenesis within the host sandstones. Stable isotope analyses indicates that a significant meteoric component was present in precipitating fluids and this is consistent with the observation that cements, are always present down-dip of sequence boundaries and/or leached whitecaps beneath coals. In addition, the lateral distribution of cement bodies increases consistently up-succession from less than 5 km in extent in the older sequences, to 30 km in extent in the youngest sequences. These changes in distribution are in response to the increased progradation and increased and more aerially extensive sequence-boundary development in younger sequences. The implications of these data are that whilst localized spatial patterns of diagenesis, and in particular carbonate cementation, are predictable and controlled by the nature and presence of individual stratal surfaces, systematic diagenetic alteration patterns are also present at the sedimentary basin scale and controlled by the nature of larger-scale stratigraphic development and basin evolution. This evolution may be driven by eustatic shifts, or through tectonic or climatic driven base-level shifts. These observations allow an improved insight into the basin-scale processes that control the macroscopic diagenetic properties of sedimentary successions and sub-surface hydrocarbon reservoirs.