Depositional sequence architecture and filling response model of the Cretaceous in the Kuqa depression, the Tarim basin

The Cretaceous system of the Kuqa depression is a regional scale (second order) depositional sequence defined by parallel unconformities or minor angular unconformities. It can be divided into four third-order sequence sets, eleven third-order sequences and tens of fourth- and fifth-order sequences. It consists generally of a regional depositional cycle from transgression to regression and is composed of three sets of facies associations: alluvial-fluvial, braided river-deltaic and lacustrine-deltaic facies associations. They represent the lowstand, transgressive and highstand facies tracts within the second-order sequence. The tectonic subsidence curve reconstructed by backstripping technique revealed that the Cretaceous Kuqa depression underwent a subsidence history from early accelerated subsidence, middle rapid subsidence and final slower subsidence phases during the Cretaceous time, with the correspondent tectonic subsidence rates being 30-35 m/Ma, 40-45 m/Ma and 5-10 m/Ma obtained from northern foredeep. This is likely attributed to the foreland dynamic process from early thrust flexural subsidence to late stress relaxation and erosion rebound uplift. The entire sedimentary history and the development of the three facies tracts are a response to the basin subsidence process. The slower subsidence foreland gentle slope was a favorable setting for the formation of braided fluvial deltaic systems during the late period of the Cretaceous, which comprise the important sandstone reservoirs in the depression. Sediment records of impermanent marine transgression were discovered in the Cretaceous and the major marine horizons are correctable to the highstands of the global sea level during the period.     

Evaluation of hydrogeochemical characteristics and the impact of weathering in seepage water collected within the sedimentary formation

A study was conducted by collecting eight seepage water samples that drain through the sedimentary rocks,mainly sandstone and shale,to evaluate the hydrogeochemical characteristics.The collected samples were analyzed for physico-chemical parameters using standard procedures.Three water types were identified in the Piper plot and the hydrogeochemical evolution starts from a CaCl facies(type 1) via mixed Ca-Mg-Cl and Ca-Na-HCO_3 facies(type 2) to Na-Cl facies(type 3).Increasing trend of electrical conductivity(EC) values were observed from type 1 water to type 3 water.Lower ionic concentrations with an average EC value of 35.7(is/cm in Ca-Cl facies indicate the recharge water by monsoonal rainfall,and ion exchange/weathering process is reflected in the mixing zone.Higher ionic concentration with an average EC value of 399(is/cm is noted in Na-Cl facies,which indicates the ion exchange during water-rock interaction.Higher log PCO_2 values are also found in this facies,revealing the longer residence time of seepage water in the rock matrix,which release more ions into the water.The relative mobility of elements during weathering suggest that the order of mobility in both sandstone and shale is Na Ca Mg K.It was observed that the hydrogeochemistry of seepage water is mainly controlled by the bedrock geology.     

Sandstone diagenesis of the Lower Cretaceous Sindong Group, Gyeongsang Basin, southeastern Korea: Implications for compositional and paleoenvironmental controls

Abstract The Gyeongsang Basin is a non‐marine sedimentary basin formed by extensional tectonism during the Early Cretaceous in the southeastern Korean Peninsula. The sediment fill starts with the Sindong Group distributed along the western margin of the basin. It consists of three lithostratigraphic units: the Nakdong (alluvial fan), Hasandong (fluvial) and Jinju (lacustrine) formations with decreasing age. Sindong Group sandstones are classified into four petrofacies (PF) based on their detrital composition: PF‐A consists of the lower Nakdong Formation with average Q₇₃F₁₂R₁₅; PF‐B the upper Nakdong and lower Hasandong formations with Q₆₆F₁₅R₁₈; PF‐C the middle Hasandong to middle Jinju formations with Q₄₉F₂₉R₂₂; and PF‐D the upper Jinju Formation with Q₂₆F₃₄R₄₁. The variations of detrital composition influenced the diagenetic mineral assemblage in the Sindong Group sandstones. Illite and dolomite/ankerite are important diagenetic minerals in PF‐A and PF‐B, whereas calcite and chlorite are dominant diagenetic minerals in PF‐C and PF‐D. Most of the diagenetic minerals can be divided into early and late diagenetic stages of formation. Early diagenetic calcites occur mostly in PF‐C, probably controlled by arid to semiarid climatic conditions during the sandstone deposition, no early calcite being found in PF‐A and PF‐B. Late‐stage calcites are present in all Sindong Group sandstones. The calcium ions may have been derived from shale diagenesis and dissolution of early stage calcites in the Hasandong and Jinju sandstones. Illite, the only diagenetic clay mineral in PF‐A and lower PF‐B, is inferred to be a product of kaolinite transformation during deep burial, and the former presence of kaolinite is inferred from the humid paleoclimatic conditions during the deposition of the Nakdong Formation. Chlorites in PF‐C and PF‐D are interpreted to be the products of transformation of smectitic clay or of precipitation from alkaline pore water under arid to semiarid climatic conditions. The occurrence of late‐stage diagenetic minerals largely depended on the distribution of early diagenetic minerals, which was controlled initially by the sediment composition and paleoclimate.     

Unifying NAPL Drawdown and Transmissivity Testing in Unconfined,Confined, Perched,and Fractured Settings using the Z-Factor and MH Principles

Light nonaqueous phase liquid (LNAPL) flow in in fractured rock is governed by the same physics as porous media, but LNAPL discharge to a well from fractured rock is subject to the unique geometry of the fractures within the rock and the degree of interconnectivity between the factures. Previous conceptualization and definition of drawdown of nonaqueous phase liquids (NAPL) has employed a single drawdown value to represent the entire vertical interval of mobile NAPL. Application of the single drawdown model may result in erroneous calculation of NAPL transmissivity in fractured rock settings. This work illustrates how drawdown in multiphase systems can be variable over the vertical interval of mobile NAPL. In settings with discrete fracture networks, it is clear that consistently applying a single drawdown value will not accurately represent the pressure gradients. This work presents the multiphase head (MH) model, which is proposed as a comprehensive methodology for evaluating NAPL drawdown in fractured rock, and unconsolidated porous media. The MH model utilizes fluid statics and physical principles to accurately represent pressure differences in the formation and convert those into NAPL drawdown for discrete elevations. This first principles approach to describing how drawdown varies with NAPL-production zone elevations and fluid levels, resulting in a more accurate representation of discharge vs. fluid elevation behavior. Application of the MH model to various scenarios has identified that dissimilar scenarios can represent similar behavior during recovery from a NAPL removal event or baildown test. The resulting understanding improves the selection of representative portions of baildown test data to use in NAPL transmissivity analysis. Proper conceptualization of drawdown in bedrock identifies an alternate analysis method, the Z-factor, to estimate NAPL transmissivity. The resulting drawdown calculations and transmissivity analysis method result in a comprehensive approach to calculating NAPL transmissivity in both bedrock and unconsolidated porous media.     

徐州大北望地区馒头组化石碳酸盐岩微相分析

江苏省徐州市大北望剖面早寒武世馒头组的碳酸盐岩地层发育较好,沉积类型多样,成岩特征明显,是研究碳酸盐岩沉积及成岩作用的理想剖面之一。文章采用微相分析的方法 ,对该剖面碳酸盐岩的生物组合、颗粒成分、基质类型及其结构等微相特征做了研究。根据颗粒类型、沉积结构和化石,本区的碳酸盐岩可以被分为3个大类和10种化石碳酸盐岩微相类型:泥晶颗粒灰岩(微相1至微相5),颗粒泥晶灰岩(微相6至微相8)和泥晶灰岩(微相9和微相10)。每一种微相对应特定的沉积环境及演化特征。综合分析表明,研究区下寒武统形成于泻湖、局限台地、开阔台地和台地边缘的浅滩环境,据此建立了沉积相模式。在徐州大北望地区进行的关于碳酸盐岩微相的研究表明早寒武世晚期的馒头组继承了早寒武世中期的特点,且海侵持续扩大的持续沉降活动。     

Modelling the effect of evaporation on the salinity-Sr/Sr relationship in modern and ancient marginal-marine systems: the Mediterranean Messinian Salinity Crisis

Palaeosalinity reconstructions in marginal marine environments based on Sr isotope data assume simple two end-member mixing of river and ocean water. An improved model that incorporates the effect of evaporation is used to demonstrate that this approach is not valid in evaporation-dominated systems. However, Sr isotope records combined with faunal and lithological constraints on salinity can be used to assess the palaeohydrological conditions of ancient marginal marine systems. This new method is used to interpret deviations from the global seawater Sr isotope curve, observed in Mediterranean successions during, and up to 3 Myr prior to, Messinian evaporite deposition. Results suggest that no absolute change in the net evaporation flux is required to explain the three- to four-fold rise in salinity of Mediterranean water at the carbonate-evaporite transition. Modelled Sr isotope records indicate that this increase in salinity was synchronous with an increase in the dominance of Atlantic water inflow in the hydrological budget of the Mediterranean. The Messinian Salinity Crisis may therefore have been the response to a marine transgression.     

Coexisting different types of zoned garnet in kyanite‐quartz eclogites from the Sanbagawa metamorphic belt: Evidence of deformation‐induced lithological mixing during prograde metamorphism

Garnet grains in Sanbagawa quartz eclogites from the Besshi region, central Shikoku commonly show a zoning pattern consisting of core and mantle/rim that formed during two prograde stages of eclogite and subsequent epidote–amphibolite facies metamorphism, respectively. Garnet grains in the quartz eclogites are grouped into four types (I, II, III, and IV) according to the compositional trends of their cores. Type I garnet is most common and sometimes coexists with other types of garnet in a thin section. Type I core formed with epidote and kyanite during the prograde eclogite facies stage. The inner cores of types II and III crystallized within different whole‐rock compositions of epidote‐free and kyanite‐bearing eclogite and epidote‐ and kyanite‐free eclogite at the earlier prograde stage, respectively. The inner core of type IV probably formed during the pre‐eclogite facies stage. The inner cores of types II, III, and IV, which formed under different P–T conditions of prograde metamorphism and/or whole‐rock compositions, were juxtaposed with the core of type I, probably due to tectonic mixing of rocks at various points during the prograde eclogite facies stage. After these processes, they have shared the following same growth history: (i) successive crystal growth during the later stage of prograde eclogite facies metamorphism that formed the margin of the type I core and the outer cores of types II, III, and IV; (ii) partial resorption of the core during exhumation and hydration stage; and (iii) subsequent formation of mantle zones during prograde metamorphism of the epidote–amphibolite facies. The prograde metamorphic reactions may not have progressed under an isochemical condition in some Sanbagawa metamorphic rocks, at least at the hand specimen scale. This interpretation suggests that, in some cases, material interaction promoted by mechanical mixing and fluid‐assisted diffusive mass transfer probably influences mineral reactions and paragenesis of high‐pressure metamorphic rocks.     

Continental shelf sediments of northern Portugal

Recent research provides the first significant information about sedimentation on the continental shelf of northern Portugal. This paper presents data and interpretations obtained from textural and compositional studies of surface sediment. The shelf primarily contains sand-sized particles of lithogenic quartz (<80m water depth) and biogenic carbonate (80m). A nearshore deposit (<30m) is actively forming from fine sand composed of immature quartz particles (subangular, hyaline) which are transported southward by prevailing waves and currents from fluvial sources in the northern part of the study area. Near the northern rivers (e.g., Minho, Lima, Porto rivers), mica also is found nearshore in high concentrations (20 %). The mid-shelf region (30 to 80 m) is dominated by coarse sand and gravel composed of mature quartz particles (rounded, frosted, iron-stained), which probably were excavated during the Holocene transgression from terrestrial deposits and redistributed by littoral processes. During low stands of sea level, lithogenic sediment was carried off the shelf through submarine canyons, resulting in reduced contribution of lithogenic particles to the outer shelf region. This has caused biogenic carbonate to dominate the sediment there. Grain size decreases across the outer shelf from medium to very fine sand, as the predominant carbonate material changes from mollusk fragments to foraminifera tests. Glauconite has formed in many of the foraminifera tests, and glauconite concentrations are high (20%) in some localities near the shelf break.     

Wavelet transform-based seismic facies classification and modelling: application to a geothermal target horizon in the NE German Basin

At the geothermal test site near Groß Schönebeck (NE German Basin), a new 3D seismic reflection survey was conducted to study geothermal target layers at around 4 km depth and 150°C. We present a workflow for seismic facies classification and modelling which is applied to a prospective sandstone horizon within the Rotliegend formation. Signal attributes are calculated along the horizon using the continuous Morlet wavelet transform. We use a short mother wavelet to allow for the temporal resolution of the relatively short reflection signals to be analysed. Time-frequency domain data patterns form the input of a neural network clustering using self-organizing maps. Neural model patterns are adopted during iterative learning to simulate the information inherent in the input data. After training we determine a gradient function across the self-organizing maps and apply an image processing technique called watershed segmentation. The result is a pattern clustering based on similarities in wavelet transform characteristics. Three different types of wavelet transform patterns were found for the sandstone horizon. We apply seismic waveform modelling to improve the understanding of the classification results. The modelling tests indicate that thickness variations have a much stronger influence on the wavelet transform response of the sandstone horizon compared with reasonable variations of seismic attenuation. In our interpretation, the assumed thickness variations could be a result of variable paleo-topography during deposition of predominantly fluvial sediments. A distinct seismic facies distribution is interpreted as a system of thicker paleo-channels deposited within a deepened landscape. The results provide constraints for the ongoing development of the geothermal test site.     

成岩过程对五峰-龙马溪组页岩地震岩石物理特征的影响

五峰-龙马溪组页岩是目前国内页岩气勘探的首选层位,而其地震岩石物理特征是利用地震方法进行"甜点"预测的重要基础之一,但对五峰-龙马溪组页岩地震弹性特征变化规律的研究并未考虑沉积、成岩过程的影响,致使相应的规律性认识缺乏地质意义.在对五峰-龙马溪组页岩样品系统声学测量基础上,分析了页岩样品地震弹性性质的变化规律.利用X射线衍射分析、扫描电镜（SEM）、阴极发光（CL）与能谱分析确定了五峰-龙马溪组页岩在不同沉积环境下的成岩过程,并讨论了成岩过程与地震弹性性质变化规律的因果关系.研究结果表明,页岩中有机质（TOC）受高热演化程度的影响,其密度通常高于1.4 g·cm^-3,并接近于有机碳密度上限1.6 g·cm^-3（石墨密度）.五峰-龙马溪组页岩地震弹性性质变化规律整体受沉积环境控制,沉积环境的差异形成不同的成岩过程,致使地震弹性特征也表现出不同的变化规律.表现在五峰-龙马溪页岩样品动态岩石物理特征主要受岩石结构控制（支撑颗粒弹性性质）,而孔隙度、TOC含量以及孔隙形状则为对地震弹性特征影响的次一级因素.五峰-龙马溪组页岩上段为浅水陆棚相,机械压实与化学压实（硅质胶结）为先后两个过程,造成样品表现出高的速度-孔隙度变化率、高速度比（泊松比）、高各向异性以及低TOC含量的特征.五峰-龙马溪组页岩下段为深水陆棚相,机械压实过程中同时伴有生物成因的硅质胶结,造成岩石样品表现出较高TOC含量与孔隙度、各向异性较弱以及较小的速度-孔隙度变化率.研究结果可为五峰-龙马溪页气储层的测井解释和地震"甜点"预测提供依据.     

Numerical modeling of sediment flushing from Lewis and Clark Lake

Lewis and Clark Lake is located on the main stream of the Missouri River. The reservoir is formed behind Gavins Point dam near Yankton, South Dakota, U.S.A. The Lewis and Clark Lake reach extends about 40 km from the Gavins Point dam. The reservoir delta has been growing since the closure of Gavins Point dam in 1955 and has resulted in a 21% reduction of storage within the maximum pool of the reservoir. Among several sediment management methods, drawdown flushing has been recommended as a possible management technique. The engineering viability of removing sediments deposited in the lake should be examined by numerical modeling before implementing a drawdown flushing. GSTARS4 was used for this study and calibrated by using measured data from 1975 to 1995. Channel cross-section changes and amount of flushed sediment were predicted with four hypothetical flow scenarios. The flushing efficiencies of all scenarios were estimated by comparing the ratios between water consumption and flushed sediment during flushing.     

致密砂岩气藏裂隙-孔隙型弹性岩石物理模板研究:以川西坳陷A区为例

相对于常规砂岩,致密砂岩在岩石物理性质、力学性质等方面具有明显差异,并呈现出很强的非均质性.岩石物理模型能将储层参数与地震特性信息联系起来,因此可以作为致密砂岩储层参数与地震特性信息转换的桥梁.常规的岩石物理模型通常只考虑单一因素(例如非均匀性,单一孔隙,单一尺度等),建立的岩石物理模板并不适用于致密砂岩.本文针对高饱和气、微裂隙发育、非均质性强的致密砂岩储层,利用Voigt-Reuss-Hill模型计算混合矿物的弹性模量,采用微分等效介质(DEM)模型描述含裂隙、孔隙岩石的骨架弹性模量,基于Biot-Rayleigh波动方程构建了岩石物理弹性模板,给出了致密砂岩储层弹性参数与物性的关系.基于测井数据和实验数据对岩石物理弹性模板进行校正,并将校正后的岩石物理弹性模板结合叠前地震资料应用于川西地区储层孔隙度与裂隙含量预测.结果显示,反演裂隙含量、孔隙度与储层试气报告、测井孔隙度基本吻合,表明该模板能够较合理地应用于致密砂岩储层孔隙度及裂隙含量解释中.     

Research on diagenesis of the sandstone-type uranium deposits in Dongsheng area,Ordos Basin

Synthetic methods of thin section petrography, scanning electron microscope, electron microprobe, energy spectrum analysis, cathodoluminescence, isotopic analysis and temperature measuring for fluid inclusions were used in analyzing sandstone samples collected from the Zhiluo Formation in order to fully understand the diagenesis evolution and the mineralizing response as well as the genesis of the uranium-bearing sandstone in Dongsheng area. The result shows that (1) the sandstone include lithic silicarenite, feldspathic litharenite and litharenite; (2) the authigenic minerals include clay minerals, carbonate minerals, siliceous and ferric minerals; (3) the physical property of sandstone is obviously controlled by diagenesis; and (4) the sandstone with favorable physical property is propitious to migration and storage of ore-forming fluid, and finally, forming the ore deposit. The sandstone of the Zhiluo Formation had undergone the early diagenetic stage (periods A and B) and the epidiagenetic stage. The evolution of diagenetic environment is in the order of acidic oxidation, alkalescent deoxidization, acidity to transitional environment of oxidation-deoxidization and acidity-alkalescence. The uranium exists in forms of pre-enrichment uranyl ion, active uranyl ion, dispersive adsorptive uranium and uranium mineral, respectively. In addition, the authors also hold that the formation of the sandstone-type uranium is not only related to the oxidation-deoxidization environment, but also closely related to the acidic-alkaline transitional environment, which are propitious to uranium mineralization in sandstone.     

Research on diagenesis of the sandstone-type uranium deposits in Dongsheng area, Ordos Basin

Synthetic methods of thin section petrography, scanning electron microscope, electron microprobe, energy spectrum analysis, cathodoluminescence, isotopic analysis and temperature measuring for fluid inclusions were used in analyzing sandstone samples collected from the Zhiluo Formation in order to fully understand the diagenesis evolution and the mineralizing response as well as the genesis of the uranium-bearing sandstone in Dongsheng area. The result shows that (1) the sandstone include lithic silicarenite, feldspathic litharenite and litharenite; (2) the authigenic minerals include clay minerals, carbonate minerals, siliceous and ferric minerals; (3) the physical property of sandstone is obviously controlled by diagenesis; and (4) the sandstone with favorable physical property is propitious to migration and storage of ore-forming fluid, and finally, forming the ore deposit. The sandstone of the Zhiluo Formation had undergone the early diagenetic stage (periods A and B) and the epidiagenetic stage. The evolution of diagenetic environment is in the order of acidic oxidation, alkalescent deoxidization, acidity to transitional environment of oxidation-deoxidization and acidity-alkalescence. The uranium exists in forms of pre-enrichment uranyl ion, active uranyl ion, dispersive adsorptive uranium and uranium mineral, respectively. In addition, the authors also hold that the formation of the sandstone-type uranium is not only related to the oxidation-deoxidization environment, but also closely related to the acidic-alkaline transitional environment, which are propitious to uranium mineralization in sandstone.     

Depositional model of Permian Luodianian volcanic island and its impact on the distribution of fusulinid assemblage in southern Qinghai, Northwest China

Pan-riftizational tectonic activity reached climax at Luodianian (Permian) in the East Tethyan Domain, Qinghai-Tibet Plateau. Because of eruptive volcanics and influence of terrigenous materials, a complex volcanic-sedimentary landform formed on the sea floor in southern Qinghai. Four sedimentary facies types were recognized based on detailed field mapping. Spatially, platform facies volcanic-limestone type was located at the center belt approximately trending NWW, surrounded by shallow water slope facies tuff/tuffite type at the two flanks and deep water slope facies breccia/calcirudite at the most outside. The depression facies sandstone-mudstone type, which comprised mainly mudstone, de-posited between volcanic islands (platform facies volcanic-limestone type). Based on the field map-ping and stratigraphic section data, seven rift-related sedimentary facies were recognized and a depo-sitional model for volcanic island was proposed. It is revealed that some volcanic island chain formed quickly and intermittently in the Qamdo Block during violent eruption, and small carbonate reef, shoal, platform occurred above or on edge of volcanic island, and some slope sedimentary facies surrounded volcano island chain during dormant period of volcanic activities. Three types of fusulinid assemblages were distinguished in the carbonate rocks, which deposited in varied positions of a palaeo-volcanic island: (1) Misellina-Schwagerina assemblage occurred above or on edge of volcanic island, (2) Para-fusulina assemblage was located at restricted depression facies among volcanic islands or carbonate platform, and (3) the reworked Pseudofusulina-Schwagerina assemblage occurred at slope facies near margin of volcanic island, which originally deposited in the shallow-water carbonate platform, then collapsed along the volcanic island margin with fusulinid-bearing grain-supported carbonate con-glomerate or calcirudite, and finally re-deposited on the deeper slope. The sedimentary sequence re-sulting from calm shallow water was deposited at the interior of the Qamdo Block from the Devonian to early Early Permian. At the beginning of the peak period of activity of pan-riftzation (Luodianian), al-ternate volcanic island and shallow marine environment within continent crust came into being. Uni-form and stable shallow-water carbonate platform was formed during the Xiangboan. This suggested that the activity of rift basin was evidently weakened. Subsequently the instability of the basin appre-ciably increased with the occurrence of basalt in late Kuhfengian. At last the whole Qamdo Block turned into the closure period of rift during the Late Permian.     

A test of the reliability of Rb-Sr dates for selected glauconite morphologies of the upper Cretaceous (Navesink Formation) of New Jersey

Glauconite samples from the Navesink Formation of New Jersey were separated into different morphological groups. Each morphological type was dated using the Rb-Sr method in an attempt to test the hypothesis that the mamillary and the lobate glauconite morphologies are more suitable than other morphologies for radiometric dating. Isochron dates of unaltered dark-green lobate glauconites were found to be reliable indicators of the true age of the sedimentary rock. Weathering processes may drastically affect the⁸⁷Rb/⁸⁷Sr ratios of glauconite pellets by removal of Rb from the glauconite. This results in anomalously high apparent ages. Light-green glauconites were shown to have suffered extensive Rb loss and are hence inappropriate for dating. A slightly altered dark-green lobate glauconite appears to have suffered some Rb loss. X-ray diffraction criteria were not always sufficient in indicating whether glauconite samples have remained closed systems with respect to⁸⁷Rb and⁸⁷Sr.     

Molecular characterization of sulfur compounds in some special sulfur-rich Chinese crude oils by FT-ICR MS

Routine GC/MS analysis may apply to the volatilized Low-Molecular-Weight compounds in saturate and aromatic hydrocarbon fractions;thus,relative studies using this technique inevitably bring about some limitations on distribution of miscellaneous sulfur atom.In this article,Fourier Transform Ion Cyclotron Resonance Mass Spectrometry(FT-ICR MS)with high resolution is employed to investigate the distribution of organic sulfur compounds(OSCs)in the crude oil typically derived from the Eogene carbonate-evaporite sediments with further chemical compositional characterization in molecular level by miscellaneous atomic type,carbon number,and double bond equivalent(DBE).A variety of miscellaneous atomic types with S1,S2,S3,OS,OS2,O2S,O2S2,NS,and NOS etc.(S1 means those OSCs with one sulfur atom in a molecule)were identified in OSCs in these oil samples.High levels of alkyl thioether series compounds with one ring structure were presented mainly in the crude oil in the Jianghan Basin whereas high amounts of benzothiophene,dibenzothiophene etc.compounds with higher values in DBE and carbon number range occurred in the sulfur-rich heavy oil in the Jinxian Sag.Although carbonate-evaporite sediments deposited in the saline lacustrine facies in the Eogene basin both occurred in the Jinxian Sag and Jianghan Basin,obviously,they possess different chemical diagenetic pathway of sulfur under various microbial reactions,leading to diverse distributional characteristics on biomarkers,OSCs,and even different hydrocarbon generation mechanism of immature crude oil.     

Research on diagenesis of the sandstone-type uranium deposits in Dongsheng area,Ordos Basin

Synthetic methods of thin section petrography, scanning electron microscope, electron microprobe, energy spectrum analysis, cathodoluminescence, isotopic analysis and temperature measuring for fluid inclusions were used in analyzing sandstone samples collected from the Zhiluo Formation in order to fully understand the diagenesis evolution and the mineralizing response as well as the genesis of the uranium-bearing sandstone in Dongsheng area. The result shows that (1) the sandstone include lithic silicarenite, feldspathic litharenite and litharenite; (2) the authigenic minerals include clay minerals, carbonate minerals, siliceous and ferric minerals; (3) the physical property of sandstone is obviously controlled by diagenesis; and (4) the sandstone with favorable physical property is propitious to migration and storage of ore-forming fluid, and finally, forming the ore deposit. The sandstone of the Zhiluo Formation had undergone the early diagenetic stage (periods A and B) and the epidiagenetic stage. The evolution of diagenetic environment is in the order of acidic oxidation, alkalescent deoxidization, acidity to transitional environment of oxidation-deoxidization and acidity-alkalescence. The uranium exists in forms of pre-enrichment uranyl ion, active uranyl ion, dispersive adsorptive uranium and uranium mineral, respectively. In addition, the authors also hold that the formation of the sandstone-type uranium is not only related to the oxidation-deoxidization environment, but also closely related to the acidic-alkaline transitional environment, which are propitious to uranium mineralization in sandstone.

     

Sedimentology of the Krakatau 1883 submarine pyroclastic deposits

The majority of tephra generated during the paroxysmal 1883 eruption of Krakatau volcano, Indonesia, was deposited in the sea within a 15-km radius of the caldera. Two syneruptive pyroclastic facies have been recovered in SCUBA cores which sampled the 1883 subaqueous pyroclastic deposit. The most commonly recovered facies is a massive textured, poorly sorted mixture of pumice and lithic lapilli-to-block-sized fragments set in a silty to sandy ash matrix. This facies is indistinguishable from the 1883 subaerial pyroclastic flow deposits preserved on the Krakatau islands on the basis of grain size and component abundances. A less common facies consists of well-sorted, planarlaminated to low-angle cross-bedded, vitric-enriched silty ash. Entrance of subaerial pyroclastic flows into the sea resulted in subaqueous deposition of the massive facies primarily by deceleration and sinking of highly concentrated, deflated components of pyroclastic flows as they traveled over water. The basal component of the deposit suggests no mixing with seawater as inferred from retention of the fine ash fraction, high temperature of emplacement, and lack of traction structures, and no significant hydraulic sorting of components. The laminated facies was most likely deposited from low-concentration pyroclastic density currents generated by shear along the boundary between the submarine pyroclastic flows and seawater. The Krakatau deposits are the first well-documented example of true submarine pyroclastic flow deposition from a modern eruption, and thus constitute an important analog for the interpretation of ancient sequences where subaqueous deposition has been inferred based on the facies characteristics of encapsulating sedimentary sequences.     

Submarine slope–fan sedimentation in an ancient forearc related to contemporaneous magmatism: The Upper Cretaceous Izumi Group,southwestern Japan

The Izumi Group in southwestern Japan is considered to represent deposits in a forearc basin along an active volcanic arc during the late Late Cretaceous. The group consists mainly of felsic volcanic and plutonic detritus, and overlies a Lower to Upper Cretaceous plutono‐metamorphic complex (the Ryoke complex). In order to reconstruct the depositional environments and constrain the age of deposition, sedimentary facies and U–Pb dating of zircon grains in tuff were studied for a drilled core obtained from the basal part of the Izumi Group. On the basis of the lithofacies associations, the core was subdivided into six units from base to top, as follows: mudstone‐dominated unit nonconformably deposited on the Ryoke granodiorite; tuffaceous mudstone‐dominated unit; tuff unit; tuffaceous sandstone–mudstone unit; sandstone–mudstone unit; and sandstone‐dominated unit. This succession suggests that the depositional system changed from non‐volcanic muddy slope or basin floor, to volcaniclastic sandy submarine fan. Based on a review of published radiometric age data of the surrounding region of the Ryoke complex and the Sanyo Belt which was an active volcanic front during deposition of the Izumi Group, the U–Pb age (82.7 ±0.5 Ma) of zircon grains in the tuff unit corresponds to those of felsic volcanic and pyroclastic rocks in the Sanyo Belt.