A molecular and carbon isotope biogeochemical study of biomarkers and kerogen pyrolysates of the Kimmeridge Clay Facies: palaeoenvironmental implications

Structures and carbon isotopic compositions of biomarkers and kerogen pyrolysis products of a dolomite, a bituminous shale and an oil shale of the Kimmeridge Clay Formation (KCF) in Dorset were studied in order to gain insight into (i) the type and extent of water column anoxia and (ii) changes in the concentration and isotopic composition of dissolved inorganic carbon (DIC) in the palaeowater column. The samples studied fit into the curve of increasing δ¹³C of the kerogen (δ¹³C_TOC) with increasing TOC, reported by Huc et al. (1992). Their hypothesis, that the positive correlation between TOC and δ¹³C_TOC is the result of differing degrees of organic matter (OM) mineralisation in the water column, was tested by measuring the δ¹³C values of primary production markers. These δ¹³C values were found to differ on average by only 1‰ among the samples, implying that differences in the extent of OM mineralisation cannot fully account for the 3‰ difference in δ¹³C_TOC. The extractable OM in the oil shale differs from that in the other sediments due to both differences in maturity, and differences in the planktonic community. These differences, however, are not likely to have significantly influenced δ¹³C_TOC either. All three sediments contain abundant derivatives of isorenieratene, indicating that periodically euxinia was extending into the photic zone. The sediments are rich in organic sulfur, as revealed by the abundant sulfur compounds in the pyrolysates. The prominence of C₁-C₃ alkylated thiophenes over n-alkanes and n-alkenes is most pronounced in the pyrolysate of the sediment richest in TOC. This suggests that sulfurisation of OM may have played an important role in determining the TOC-δ¹³C_TOC relationship reported by Huc et al. (1992).     

Diagenetic effects on the well yield of dolomite aquifers in Slovenia

Diagenetic effects upon the hydraulic parameters of dolomite aquifers were examined. Results indicate that dolomites affected only by late (burial) diagenesis are economically more interesting to exploit than those formed first by early and then also affected by late diagenesis, as the values of most studied hydraulic parameters (transmissivity, specific capacity, linearized specific capacity and maximum yields) are higher in the former group. Data were collected from the hydrogeological reports of 298 wells in dolomites from Permian to Cretaceous age in Slovenia, classified in a relational hydrogeological database and analyzed by statistical methods. The studied parameters are hydraulic conductivity K, transmissivity T, specific capacity q (=Q/s), linearized specific capacity Ql (=q/d) and maximum yields Q _max. Values of the studied parameters are distributed log-normally according to most normality tests and span several orders of magnitude. Statistical tests show significant differences between the “late” and “early + late” groups for parameters q, Ql and T, and non-significant differences for K and Q _max.     

Exhumed hydrocarbon-seep authigenic carbonates from Zakynthos Island (Greece): Concretions not archaeological remains

In Zakynthos Island (Greece), authigenic cementation of marine sediment has formed pipe-like, disc and doughnut-shaped concretions. The concretions are mostly composed of authigenic ferroan dolomite accompanied by pyrite. Samples with >80% dolomite, have stable isotope compositions in two groups. The more indurated concretions have δ¹⁸O around +4‰ and δ¹³C values between −8 and −29‰ indicating dolomite forming from anaerobic oxidation of thermogenic methane (hydrocarbon seep), in the sulphate-methane transition zone. The outer surfaces of some concretions, and the less-cemented concretions, typically have slightly heavier isotopic compositions and may indicate that concretion growth progressed from the outer margin in the ambient microbially-modified marine pore fluids, inward toward the central conduit where the isotopic compositions were more heavily influenced by the seep fluid. Sr isotope data suggest the concretions are fossil features, possibly of Pliocene age and represent an exhumed hydrocarbon seep plumbing system. Exposure on the modern seabed in the shallow subtidal zone has caused confusion, as concretion morphology resembles archaeological stonework of the Hellenic period.     

印度南部元古代Cudappah盆地白云岩中的菱镁矿成矿作用及其构造-气候指示意义

Cudappah盆地是位于印度南部的一个元古代盆地，盆地中含有许多矿床，包括菱镁矿和滑石矿。该盆地发育于Chat活动带东部边缘，并发育沙质、粘土质和碳酸盐岩的多期重复沉积旋回。盆地可划分为四个次级盆地，即Papaghni、Nallamalai、Srisailam和Kurnool次级盆地。晶质菱镁矿床赋存于Papashni次级盆地的Vempalli组地层中。每个次级盆地很可能都沿着一系列裂谷中的断块发育，并且这些裂谷形成于中元古代的热事件。每个沉积旋回中发育有相似的沉积岩套表明，在盆地演化过程中具有相同的构造和气候环境。根据盆地中矿床(重晶石矿、菱镁矿和滑石矿)的产出，我们对每个次级盆地均提出了一个多期重复沉积旋回的演化模式，该旋回包括了从角砾岩到白云岩的变化。     

Multiple-scale facies and reservoir quality variations within a dolomite body – Outcrop analog study from the Middle Triassic,SW German Basin

This study focuses on a porous dolomite body in the proximal part of an epeiric carbonate ramp (Triassic Muschelkalk, South German Basin), analogous to hydrocarbon-bearing dolomite reservoirs in the Middle East (e.g. Khuff and Arab Formations).     

川东北飞仙关组白云岩的主要类型、地球化学特征和白云化机制

四川盆地东北部蕴藏有丰富的烃类资源,三叠系飞仙关组的白云岩是最主要的储层和产层,这使得飞仙关组白云岩成为人们近年来高度关注的对象.按照结构,我们可以将飞仙关组白云岩分为三种端元类型:微晶白云岩、具原始结构的粒屑白云岩和结晶白云岩,在一个向上变浅的剖面旋回中,这三种白云岩分别位于旋回的顶部、上部和中下部.微晶白云岩和具原始结构的粒屑白云岩具有较高的锰含量和较低的包裹体均一化温度,白云化流体的δ~(18O) 值(从白云石的包裹体均一化温度和δ~(18O) 值反演,以下同)介于大气水和海水之间,反映这类白云岩形成于有大气水参与的相对开放的沉积-成岩环境中,蒸发泵作用(对于微晶白云岩)和混合水作用(对于具原始结构的粒屑白云岩)可能是其主要的形成机制;结晶白云岩(表现为好储层)具有很低的锰含量和较高的包裹体均一化温度,白云化流体的δ~(18O) 值显著高于早三叠世海水,表明这类白云岩形成于相对封闭的埋藏成岩环境,白云化流体具较高的温度和盐度,但到目前为止,还没有一个已有模式可以较好地解释这类白云岩的成因.     

Bed‐scale spatial patterns in dolomite abundance: Part II. Effect of varied fluid chemistry,flow rate,precursor mineralogy,temperature, textural heterogeneity,nucleation density and bed geometry

Reaction‐transport modelling shows that a lateral, metre‐scale pattern in dolomite abundance can form during replacive dolomitization of calcite and aragonite precursors by Mississippian, Triassic and modern seawaters advecting at >20 cm year^?1 in low temperature (≤60°C) hydrogeological systems. The modelled pattern develops best in beds >1 m thick with a relatively uniform nucleate density and a low variance in reactive surface area (for example, grainstones). These conditions suggest that a lateral pattern in dolomite abundance should be expected in dolomites formed by any shallow hydrodynamic system that advects dolomitizing fluid horizontally, including the down‐gradient portion of a reflux system, the oceanward reaches of geothermal (Kohout) convection systems, and the seaward reaches of a seawater entrainment zone below a freshwater aquifer. However, even in those systems, a pattern will not form in all dolomites. Pattern will be muted in thinner (≤ ca 50 cm thick) beds and non‐emergent where the precursor had a high variance in reactive surface area (for example, a skeletal wackestone) and/or large variation in nucleate density. Pattern also did not form at temperatures above ca 60°C, which implies that pattern should not be expected in dolomites formed at intermediate to deep burial depths. As the patterns are horizontal, they also should not be expected where dolomitizing fluids moved vertically (for example, reflux immediately below a brine source). Pattern metrics (short‐range correlation length, and wavelength and amplitude of the longer cyclic component) vary with flow rate, fluid chemistry, bed thickness, porosity, grain size, temperature and the flow complexity (one‐dimensional, two‐dimensional or three‐dimensional) within the bed. However, no modelled pattern produced metrics larger than those documented on dolomite outcrops. The results thus constrain the length scales of porosity and permeability variance to include in petrophysical models of dolomite reservoirs, as well as the geological scenarios in which to consider metre‐scale lateral petrophysical variability.     

The Tashisayi nephrite deposit from South Altyn Tagh,Xinjiang,northwest China

The Tashisayi nephrite deposit is located in South Altyn Tagh.in Qiemo County,Xinjiang Province,northwest China.It is a recent discovery in the vast,well-known Kunlun-Altyn nephrite belt distributed along the south of the Tarim Basin,producing more than half of the nephrite from the whole belt in 2017.Field investigations revealed that it is a dolomitic marble-related(D-type)nephrite deposit,but little is known about its age of formation and relationships between the granites and marble.Here we report field investigations,petrography of the neph rite,as well as petrography,geochemistry,geochronology of the zoisite-quartz altered intrusive rock and adjacent granites.An A-type granite is identified with a SHRIMP U-Pb zircon age of 926± 7 Ma,suggesting it was emplaced in an extensional tectonic environment at that time.The altered intrusive rock has a cluster of U-Pb zircon age of 433± 10 Ma.with similar trace element features to the A-type granite,suggesting it was formed in an extensional regime at this later time.Nephrite formed because of the metasomatism of dolomite marble by hydrothermal fluids.It is inferred that Ca~(2+) was released from the dolomitic marble by metasomatism forming Ca-rich fluids,which caused alteration of both the intrusive rocks(6.00-8.22 wt.% CaO)and granite(1.76-3.68 wt.% CaO)near the nephrite ore bodies.It is also inferred that Fe2+ from the granite migrated towards the dolomite marble.The fluids gave rise to the formation of Ca-minerals.such as zoisite,in the nephrite and altered intrusive rock,and epidote in the granite.Based on the contact relationships.similarity in hydrothermal processes,and consumption of Ca~(2+),the Tashisayi nephrite is considered to have formed at the same time as the alteration of the intrusive rocks,i.e.～433 Ma.The geochronological similarity(～926 Ma.433 Ma)of South Altyn and North Qaidam may suggest that tectonically they belong to one single complex in the past,which was offset by the Altyn Tagh fault(ATF).The similar formation ages of the nephrites from Altyn Tagh(433 Ma)and the previously studied areas of West Kunlun(378-441 Ma)and East Kunlun(416 Ma)indicate that these nephrites formed during the closure of Proto-Tethys and in the accompanving post-collisional.extensional environment.     

白云石镁水泥材料的初步研究

以活性较高的苛性白云石粉和氯化镁溶液制备白云石镁水泥材料。研究氧化镁、氯化镁和水不同配比条件下对白云石镁水泥材料的抗压强度、相组成以及形貌的影响。在所研究的配比条件下,以氧化镁、氯化镁和水摩尔比为3.38∶1.00∶16.86和3.24∶1.00∶16.26配比的镁水泥样块强度较高,结构致密,水化物胶凝相稳定;氯化镁加入量较低,水的加入量较高会使镁水泥试块强度不高;氯化镁加入量较高,水的加入量较低会使镁水泥试块吸潮返卤现象严重,导致强度逐渐下降。     

Microbialites of the Neoproterozoic Beck Spring Dolomite,Southern California

Ancient microbialites reflect interactions between microbial communities and environmental conditions. However, evaluating the relative roles of microbial community processes and environmental influences on microbialite morphology and internal fabric in the rock record can be challenging. The Neoproterozoic Beck Spring Dolomite preserves diverse microbialites, and thus provides an opportunity to explore the factors that influenced microbialite development locally. Stromatolitic, thrombolitic and composite microbialites are abundant in subtidal to upper intertidal carbonates in the Beck Spring Dolomite. Thrombolitic and composite microbialites have not been recognized previously in this unit, but compose much of the newly defined thrombolitic member. Stratigraphic relationships demonstrate that these three types of microbialites formed in close spatial and temporal association in subtidal to intertidal environments. The relative proportions and distributions of stromatolitic and thrombolitic microbialites vary with depositional environment; stromatolitic microbialites dominate in deeper intertidal to subtidal facies, whereas thrombolitic textures are more abundant in upper intertidal facies. Composite microbialites, composed of intermingled clotted and laminated textures, formed in all environments but are most abundant in intertidal facies. The broad environmental distribution of stromatolitic, thrombolitic and composite microbialites and the intermingling of textures suggest that laminated and clotted textures reflect diverse microbial community morphologies rather than environmental variations. Furthermore, the ca 750 Ma age of thrombolitic microbialites in the Beck Spring Dolomite requires that they formed without the influence of calcimicrobes or metazoans colonizing and grazing the microbial mat surface. Thus, these thrombolites provide further evidence that the biostratigraphic distribution of thrombolites cannot be uniquely attributed to evolution of calcifying and grazing organisms in the earliest Cambrian, and that older microbial communities were capable of producing clotted textures.