Silty Mudstone Research Articles

Evolution and migration of shale facies and their control on shale gas: A case study from the Wufeng-Longmaxi Formations in the Sichuan Basin and its surroundings

The Fuling gas field, the largest commercial discovery of shale gas in China, has been producing from the organic-rich shale in the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation in the Sichuan Basin, China. A refined shale gas E&P and sweet spots identification require studies to be carried out not only at the sedimentary facies scale but also at the petrographic scale. This paper focuses on the characteristics of organic-rich shales. Based on data from outcrops, drilling cores, X-ray diffraction, thin sections, and scanning electric microscope, a systematic analysis of wells and well-correlation sections indicates that shales in the study area underwent two progressive and regressive cycles. In addition, differences in lithofacies and their organic or inorganic components are significant either in the vertical or in the horizontal direction, which generally controls the material basis of shale gas. The early-stage siliceous shale distributes in the east, whereas the late-stage carbonate-bearing shale extends to the west. In the southeastern Sichuan Basin, the siliceous composition decreases, whereas the clay minerals increase upward. In the central Sichuan uplift, there is a higher carbonate content. And with our new lithofacies maps, distribution of the sweet spots was identified. The northeastern Sichuan Basin has the siliceous shale in the Wufeng Formation and the silicon-bearing shale in the Longmaxi Formation, making it a play area with the greatest gas potential. The southern Sichuan Basin occurs as a favorable lithofacies area with the stacked siliceous shale in the Wufeng Formation and the calcareous shale in the Longmaxi Formation. The relatively favorable lithofacies area covers the regions extending from the periphery of southern Sichuan Basin toward the central Sichuan uplift and the northern Guizhou uplift, featuring stacked carbonate-bearing shale and clay mineral-bearing shale in the Longmaxi Formation. The shallow-marine areas close to the uplifts are dominated by mudstone and silty mudstone, thereby possessing poor material basis.

Petrological characteristics and shale oil enrichment of lacustrine fine-grained sedimentary system: A case study of organic-rich shale in first member of Cretaceous Qingshankou Formation in Gulong Sag, Songliao Basin, NE China

Abstract Taking organic-rich shale in the first member of Cretaceous Qingshankou Formation in the Gulong Sag, northern Songliao Basin as an example, this study examined the lithofacies classification, petrological characteristics, pore size distribution and their implications on the enrichment of shale oil of lacustrine detrital fine-grained shale. The spatial variation of lithofacies, controlled by Milankovitch cycle and influenced by sediment provenance, has an obvious sequence. The fine-grained sedimentary rocks of studied section could be classified into seven lithofacies according to a three-step classification criterion that consists of total organic carbon (TOC), sedimentary structure and mineral composition. Among them, the laminated siliceous mudstone lithofacies with moderate TOC has high hydrocarbon generation potential and abundant reservoir space, making it the most favorable lithofacies for the enrichment of matrix shale oil. Under the background of abnormally high pressure, the laminated siliceous mudstone lithofacies with moderate TOC deposited between the top of SSC2 and the bottom of SSC3 is stable in lateral distribution in the delta-lacustrine transition zone, with continuous thickness greater than 30 m. The massive siliceous mudstone lithofacies with high and moderate TOC developed in the middle of the these two cycles can act as the roof and floor seal for shale oil, therefore, the study area has good conditions for forming matrix type shale oil reservoirs.

Gas and liquid permeability measurements in Wolfcamp samples

Argon gas and liquid (dodecane) permeability measurements in two mixed quality siliceous mudstone samples within the Wolfcamp formation demonstrate that it is possible to close multiple bedding parallel open artificial micro-fractures and obtain representative matrix permeability by applying two confining stress cycles at a constant pore pressure under effective stresses ranging from 6.9 MPa to 59.7 MPa. The clearly micro-fractured sample exhibited a three-order decrease in permeability from 4.4 × 10−17 m2 to 2.1 × 10−20 m2. In contrast, the relatively intact sample exhibited initial liquid permeability of 1.6 × 10−19 m2 that declined gradually with stress to 3.0 × 10−20 m2. In this study, we developed a new permeability testing protocol and analytical approaches to interpret the evolution of micro-fractures and estimate the matrix permeability based on initial pulse-decay gas permeability measurements. The insights gained are useful to identify pore-scale controls on fluid flow behavior using SEM petrography and pore-space characterization and validate fundamental fluid flow mechanisms examined using pore-scale numerical models.

Characterization of the salinity conditions of Cenomanian formations based on sedimentological and ichnological data of selected cores from Ivoirian offshore basin (septentrional Gulf of Guinea)

Detailed sedimentological and ichnological analysis of core material from selected wells were performed in order to fully characterize the range of facies encountered within Cenomanian formations of Ivorian offshore basin.Results indicate that the cored successions comprise four major groups of lithofacies: Pebbly (CS, PS), clean sandstone (S), muddy sandstone (S*M) and mudstone (HM, ML, MJ & M). These lithofacies groups were further sub-divided into component facies based mainly on sedimentary structures. The pebbly group comprises conglomerate sandy and pebbly sandstone. The clean sandstone group includes seven facies: massive sandstone, laminated sandstone, cross-bedded sandstone, bioturbated sandstone, deformed sandstone, wavy-bedded sandstone and rippled sandstone. The muddy sandstone group comprises bioturbated muddy sandstone, while the mudstone group comprises four main facies, namely wavy-bedded heterolithic/sandy mudstone, calcareous mudstone, silty mudstone and pure mudstone.A range of ichnofossils were identified in the cores including: Planolites isp, Palaeophycus isp, Lockeia isp, Phycosiphon isp, Thalassinoides isp, Rhizocorallium isp, Cylindrichnus isp, Chondrites isp and Teichichnus isp. Among these, the Phycosiphon isp Chondrites isp and Rhizocorallium isp reveal fully marine salinities in some intervals of these cores, although Chondrites isp usually used as proxy for disoxia and Rhizocorallium isp were sometimes associated with Glossifungites isp ichnofossils. Integration of the sedimentary data (lithofacies) and trace fossil ichnofossils data revealed several depositional environments ranging from coastal/deltaic to shallow marine settings within the Cenomanian cores.

Organic matter accumulation in the Upper Devonian Duvernay Formation, Western Canada Sedimentary Basin, from sequence stratigraphic analysis and geochemical proxies

We present a model for organic carbon accumulation in the Upper Devonian Duvernay Formation of the Western Canada Sedimentary Basin, relating total organic carbon (TOC) concentrations to a high resolution geochemical and organic petrologic database and comparing these to a core-based sequence stratigraphic interpretation. Many previous source rock studies have suggested that organic matter is enriched in transgressive systems tracts or at maximum flooding surfaces, an observation that is commonly attributed to an assumed relationship between water depth and oxygen levels. Our data set enables us to associate total organic matter content and organic assemblages with particular systems tracts and to examine how the organic assemblages change across the basin, tested against proxies for redox conditions and bioproductivity to relate relative sea level to specific mechanisms for organic accumulation.The Duvernay Formation comprises three third-order depositional sequences, superimposed on a second-order late transgressive systems tract and early highstand systems tract. Sequences are composed almost entirely of transgressive systems tracts and highstand systems tracts, with a lowstand systems tract only at the base of the upper cycle that marks the second-order sea level turnaround. Depositional facies generally vary from bioturbated carbonate-rich siltstones to siliceous mudstones from the platform margins to basin center. Organic petrologic analysis records predominantly amorphous organic matter and solid bitumen, with much less abundant vitrinite and inertinite. Organic matter type indicated by Rock-Eval analysis shows that in most cases, high total organic carbon (TOC) content is associated with better quality organic matter (high hydrogen index) and vice versa, indicating more reducing conditions or higher bioproductivity were responsibility for organic enrichment. However, near carbonate reefs, hydrogen index is uncorrelated with TOC, indicating that here, dilution by carbonate minerals was the primary control on organic enrichment. Across most of the basin, highest TOCs are recorded just above the second-order maximum flooding surface. Secondary peaks in TOC are recorded above the maximum flooding surface in the upper third-order cycle and in the transgressive systems tract of lower third-order cycle. Low TOC values are recorded in the lowstand systems tract and in the middle and upper third-order transgressive systems tracts almost up to the maximum flooding surfaces.Throughout much of the basin, proxies for relative sea level, restriction of water masses (Mo/TOC), redox conditions (Mo/Al and S/Fe) and bioproductivity (biogenic silica) coincide. These relationships demonstrate that organic carbon accumulation resulted from influxes of nutrient-rich upwelled water during high sea level; thus, TOC values are highest in the upper part of transgressive systems tracts and lower highstand systems tracts. Anoxia typically developed as a result of bioproductivity and enhanced organic matter accumulation but was not itself a trigger for organic sedimentation. Near carbonate reefs, however, varying sea level regulated carbonate sedimentation rate and organic matter dilution, and bioproductivity had less impact on organic matter concentrations; thus, TOC was high when carbonate deposition was low during transgressions, which forced carbonate reefs to backstep, limiting carbonate sedimentation in deeper water area.

Paleo-environmental reconstructions of the Upper Cretaceous-Paleogene successions, Safaga, Egypt

The Late Cretaceous-Early Eocene sedimentary sequences in the Central Eastern Desert of Egypt were studied in three shallow boreholes (Wassief, Um El-Huaitat, and Mohamed Rabah). This study focuses on an integrated investigation of biostratigraphy, lithostratigraphy and some geochemical parameters, to provide important clues to the relative sea-level changes and to interpret the prevailed depositional conditions.Micropaleontological study on planktonic foraminifera allows to identify eight zones, ranging in age from Late Cretaceous to Early Eocene. Three hiatal surfaces have been recognized as evidenced by missing formaniniferal zones: The first spans the Late Maastrichtian-Danian regional unconformity. The second one is recorded in the Late Paleocene. The third one is located at the Late Paleocene-Early Eocene (Thanetian-Ypresian) boundary. The environments of the study foraminiferal parameters range from the non-marine to outer neritic-upper bathyal.Petrographically, thirteen Microfacies Types (MFT) have been recorded. Three MFTs constitute the siliciclastic facies; MFT-1; silty to sandy mudstone, MFT-2; organic-rich silty mudstone, and MFT-3; fine-grained sandstone/glauconite intercalations. Two MFTs represent a phosphorite facies which are MFT-4; phosclast packstone to phosclast grainstone and MFT-5; phosooid packstone to phosooid grainstone. Eight MFTs characterize the carbonate facies; MFT-6; non-fossiliferous lime-mudstone, MFT-7; phosphatic foraminiferal pack/grainstone, MFT-8; bioclastic foraminiferal packstone, MFT-9; bioclastic foraminiferal wackestone, MFT-10; glauconitic phosphatic foraminiferal Wack-Packstone, MFT-11; non-fossiliferous dolomitic mudstone, MFT-12; dolomitic glauconitic lime-mudstone, and MFT-13; dolomitic bioclastic wackestone. The depositional environments of the study microfacies range from the terrestrial to open marine outer neritic-upper bathyal.Geochemical parameters have been used to improve the litho- and biostratigraphic framework: the major oxides, TOC wt.% and the Chondrite-normalized REE. Based on the vertical distribution of CaO/P2O5, TiO2/CaO and cerium (Ce)-anomaly, changes in paleo-bottom water redox conditions were reconstructed and correlated with relative sea level changes. Prior to the major transgression during deposition of the Duwi Formation under reducing marine conditions, the prevailing oxic to sub-oxic bottom water terrestrial conditions during deposition of the Quseir Formation.

Mineralogical and morphological characterization of Older Cambay Shale from North Cambay Basin, India: Implication for shale oil/gas development

Abstract Effective development of shale oil/gas reservoirs depends on the success of hydraulic fracturing. Mineralogy and morphology play a very important role in establishing brittleness and therefore controlling its hydraulic fracturing characteristics. In this paper, mineralogical and morphological characteristics of the Older Cambay Shale and the top Olpad Formation from the northern Cambay Basin, India have been characterized using well logs, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) coupled with energy dispersive X-ray spectroscopy (EDS or EDX) and atomic force microscopy (AFM) with the objective of identifying the fracturing potential of the units. Presence of mixed argillaceous mudstone, silica-rich argillaceous mudstone to argillaceous siliceous mudstone in the studied shale samples shows that the area under investigation is compositionally similar to the high clay facies of Marcellus and Barnett shale systems having low fracability. The studied formation is rich in kaolinite and chlorite, which aggregately contribute half of mineral composition ranging from 43 to 56%. Quartz, feldspar and mica combined to add approximately 30–43%, while siderite and magnesite varied between 8 and 12%. The interpreted results of mineralogical brittleness are supported by log based geomechanical properties. FESEM images at micro to nanoscale confirm the presence of various minerals with distinct crystal structure, along with inter-and-intra particle pores in the matrix. AFM scan at the nano scale shows mesopores ranging from 12 to 19 nm, with an average pore diameter of 17.57 nm and areal coverage of 10.9%. The log derived mechanical properties correlate with the mineralogy-based brittleness index (BI) of the studied shales indicating that the formations lie in the low to medium range of fracability. High clay content and low to moderate BI are likely to complicate the fracturing process of the potential resource rich area. Outcome of the present investigation demonstrates the limited shale oil/gas potential in the study area; however an extensive exploration program may unlock some sweet spots in the basin with distinctly different characteristics.

MESOZOIC TETHYAN RADIOLARITE AGE DETERMINATION, ZAGROS SUTURE ZONE, KURDISTAN, NE IRAQ

A regional belt of Mesozoic Tethyan radiolarites extends along the northeastern Arabian plate's margin. Outcrops of this radiolarite were documented from Hawasina series in Oman, and northeastwards to Pichakun Series of southern Iran, to Kermanshah of northwest Iran passing onto the Qulqula Group of northeast Iraq up until the Kocali Series of Turkey. The age of most of these radiolarite suites was already constrained. It is the purpose of this work to investigate the age of the radiolarite Suites in northeast Iraq using Radiolarian microfossils chronostratigraphy. Patches of the Mesozoic radiolarite suites are exposed at the Zagros suture zone in Kurdistan Region of northeast Iraq and known as the Qulqula Group. It consists of complexly deformed and thick slices of pelagic radiolarian chert and siliceous limestone, radiolarian red claystone and siliceous mudstone, and reddish-green mudstone and limestone. These units were sampled from three areas (Naurabab, Penjween, and Hawraman) located to the south and east of Sulaimani city in Kurdistan, for radiolarian micropaleontological analysis. Radiolaria microfossils identification and age determination is done at specialized laboratories. Results of the radiolarian age constraint show that these sediments range from Bajocian to the end of the Cenomanian.

EXTENDED ABSTRACT: RE‐OS GEOCHRONOLOGY AND CHEMOSTRATIGRAPHY OF THE MAIKOP SERIES SOURCE ROCKS OF EASTERN AZERBAIJAN

The Oligocene – Miocene Maikop Series is a world‐class source interval and is the most important source rock for the petroleum systems of the South Caspian Basin and surrounding region. The Maikop is a thick (up to 3 km) succession of silty mudstones with intervals of high organic carbon content. However correlation of organic‐rich intervals within the South Caspian Basin and beyond to basins of the Eastern Paratethys is problematic. Current age constraints are based primarily on microfaunal assemblages, but the assemblages are lacking in precision and are sparsely dispersed.This study provides a numerical age datum for Maikopian strata that may be used to improve correlation of organic‐rich intervals both locally and regionally. The age date obtained was produced via Re‐Os isotope geochronology. Of the five sample suites analyzed, one Re‐Os data‐set yielded an isochron of 17.2 ± 3.2 Ma. Re‐Os data sets that failed to produce statistically meaningful isochrons provide qualitative age constraints by comparing calculated initial 187Os/188Os ratios from these samples to187Os/188Os ratios in seawater through time.

Effects of Confining Stress on the Semipermeability of Siliceous Mudstones: Implications for Identifying Geologic Membrane Behaviors of Argillaceous Formations

AbstractThe semipermeability arising from filtration effects on ionic species at the pore spaces in clay minerals has been recognized as a fundamental transport property of argillites. However, little is known about the dependence of the semipermeability on the confining stress that controls the pore sizes. Thus, we investigated the evolution of the semipermeability during cyclic loading by performing chemical osmosis experiments on siliceous mudstones. Results show that the semipermeability irreversibly increases with increasing confining stress, but once the stress exceeds the past maximum effective burial stress, it reversibly changes in accordance with the change in the stress. The net increase in the semipermeability from the first loading cycle to the last unloading cycle implies the irreversible shrinking of pores that should have expanded owing to stress relief before testing. The reversible change in the semipermeability probably reflects the reversible pore deformation due to the elasticity of argillites in the overconsolidated state.

Carbonate platform crisis in the Carnian (Late Triassic) of Hanwang (Sichuan Basin, South China): Insights from conodonts and stable isotope data

During the Carnian, the Hanwang area in the northwestern Sichuan Basin (South China) was characterized by shallow water carbonate sedimentation that underwent an abrupt demise associated to a sudden input of terrigenous sediments. This major facies change was considered to be the expression of the onset of the Carnian Humid Episode, a most remarkable environmental crisis in Late Triassic that is well recognized in northwestern Tethys margins and coincides with a major global perturbation of the carbon stable isotope record. However, the lack of detailed biostratigraphic constraints have so far prevented a precise dating of the carbonate platform demise in western Sichuan Basin.In this work, the Qingyan Gou (HWQ) section, cropping out in Hanwang, was investigated for its facies and microfacies, carbonate carbon and oxygen isotopes on brachiopods, microbial grains and bulk matrix. Facies analysis shows a marine transgression from inner ramp oolitic shoal to middle ramp siliceous sponge mound, then overlain by shale and calcareous siltstone with interbedded silty mudstones. Refined biostratigraphic data from HWQ and Guanyin Ya (HWG) sections imply that the demise of sponge mounds occur in the late Tuvalian or later.A negative carbon isotope perturbation was found in the bulk matrix immediately below the sponge mounds demise, but it was not found on the isotope record from brachiopods. This suggests that the negative shift in the bulk carbonate was probably related to diagenesis. Given the late Tuvalian (last substage of Carnian) age attributed to the demise of the sponge mounds and the absence of a carbon isotopic excursion, we infer that the carbonate platform crisis and strong terrigenous input in Hanwang cannot be related to the onset of the Carnian Humid Episode. These the demise of the carbonate platform and the facies deepening trend could be rather due to the interplay between accelerating subsidence rates, environmental chants and enhanced siliciclastic input related to the formation of a foreland basin during Indosinian orogenesis.

Evaluating test conditions for in situ tracer migration test in fractured siliceous mudstone involving groundwater with dissolved gas

Evaluating test conditions for in situ tracer migration test in fractured siliceous mudstone involving groundwater with dissolved gas

Pedogenic record of climate change across the Pennsylvanian-Permian boundary in red-bed strata of the Cutler Group, northern New Mexico, USA

The Upper Pennsylvanian to lowermost Permian El Cobre Canyon and Arroyo del Agua formations are well exposed in a 250+ m section in the Chama Basin in northern New Mexico. This is one of the few known successions of fossiliferous red-bed siliciclastic sediments encompassing the Pennsylvanian-Permian boundary in the American Southwest and thus may inform questions regarding changing paleoclimate in tropical Pangea across the Pennsylvanian-Permian boundary. The section consists mainly of silty mudstones and sandstones with sheet-like to ribbon geometry and very minor limestones and records an upward transition from mainly braided stream deposition in the El Cobre Canyon Formation to anastomosing stream deposition in the Arroyo del Agua Formation.We recognized multiple paleosol types within these strata, including calcic Protosols, calcic Argillisols, argillic Calcisols and Calcisols, all generally consistent with the interpretation of a seasonal sub-humid to semi-arid climate. The calcretes in these paleosols occur primarily as truncated profiles. Although Calcisols occur in both formations, we note that Protosols are more common in the El Cobre Canyon Formation, and Argillisols dominate in the Arroyo del Agua Formation. Also, we find that the calcretes in the Arroyo del Agua Formation are generally thicker than those in the underlying El Cobre Canyon Formation. The difference in calcrete maturity could be explained by changes in source-area relief and sedimentation rate. However, stable isotope analyses of pedogenic carbonate indicate that a significant enrichment of δ18O occurs near the transition between the two formations, which approximates the Virgilian-Wolfcampian (equals Gzhelian-Asselian) boundary. Although the δ13C data suggest that pCO2 remained stable during the interval of deposition, the δ18O data are consistent with the interpretation of climate aridification across the Pennsylvanian-Permian transition, potentially related to the onset of Gondwanan glaciation in the earliest Permian.

Characteristics and favorable area prediction of Ordovician buried-hill carbonate reservoirs in the Bozhong 21–2 tectonic belt, Bohai Bay Basin, China

ABSTRACTBased on core data, thin section and logging, this study investigates the basic characteristics and occurrence regularities of the Ordovician buried-hill carbonate reservoir in the Bozhong 21-2 tectonic belt. Results show that the reservoir lithology is primarily limestone, followed by dolomite, dolomitic limestone or limey dolomite, and silty mudstone. The reservoir spaces are primarily composed of secondary porosity. These reservoirs can be divided vertically into weathered-crust karst reservoirs and inner dissolution-type reservoirs. The distribution of reservoirs is controlled chiefly by karstification. The high karst units in the study area are favorable for the development of the two reservoir types.

Oligocene syndepositional lacustrine dolomite: A study from the southern Junggar Basin, NW China

Despite the continuously growing understanding of dolomite over the years, the dolomite problem remains unclear. Lacustrine dolomite found in the Anjihaihe Formation (E3a) on the margin of the southern Junggar Basin provides an opportunity to look into the dolomite formation process in a lacustrine environment. The Anjihaihe dolomite occurs as 10- to 20-cm-thick white-gray layers interbedded with lacustrine dark mudstone and silty mudstone. Based on scanning electron microscopy, X-ray diffraction, major and trace elements, and carbon and oxygen isotopes, as well as magnesium isotope analyses, the Anjihaihe dolomite is a syndepositional low-order dolomicrite formed under a reducing environment in a saline lake. Unlike the usual bacterial sulfate reduction and evaporation background of lacustrine dolomite, no bacteria or evaporite evidence is found in this study. Rather, increased Mg concentrations and carboxyl group catalysis may be the main factors that facilitate the dolomitization process. The results of a one-dimensional diffusion-advection-reaction numerical model of the Mg isotopes indicate that the dolomitization time, degree of dolomitization and diffusion in thin dolomite layers have little effect on the Mg isotopes in the Anjihaihe dolomicrite. Under the assumptions that seawater δ26Mg equals −0.82‰ and the fractionation factor is 1.75‰ at 25 °C, seawater as the Mg source for the Anjihaihe dolomicrite would be a plausible explanation and fits well with our other results. This explanation suggests that the contemporaneous adjacent proto-Paratethys might have provided the magnesium ions by episodic transgression and had a paleoenvironmental influence on the northern Tianshan area during the Oligocene.

Paleogeography and shale development characteristics of the Late Permian Longtan Formation in southeastern Sichuan Basin, China

Abstract The Late Permian Longtan Formation is extensively developed within the southeastern Sichuan basin which lies on the western margin of the Yangtze Plate in southwestern China. The stratigraphy and sequence boundaries of the Longtan Formation were interpreted from a combined study of field data, well log data and various data analysis in order to determine the extent and thickness of shale sequences. This interpretation defined the broad sequence stratigraphic framework and separated the Longtan Formation into three 3rd-order sequences. In addition, the formation was divided into five sedimentary facies using sedimentary, paleontological, geophysical and other data. The facies identified were fluvial, low energy shore, tidal flat, melanged accumulated shelf and platform basin. Three paleogeographic maps were created based on the 3rd-order sequences: SQ1, SQ2 and SQ3. The study shows that SQ1- SQ3 sedimentary environment of the study area, which extends from the southwest to northeast, was developed followed by fluvial, shore, tide, shallow water melanged accumulation shelf, deep water melanged accumulation shelf and platform basin. The mud shale rock types of the Longtan Formation that were identified are carbonaceous mud shale, silty mudstone and lime mudstone. According to the divided standard of mud shale effective thickness, there are three mud shale assemblages present in the sequences SQ1, SQ2, SQ3, namely, mud shale with sandstone, mud shale with limestone, and mud shale. SQ1 has the thickest mud shale identified on well logs. Mud shale with sandstone was well developed in the southwest of the study area and was interpreted to have been deposited in a shore or tidal flat sedimentary environment. This rock assemblage has a large effective thickness and extensive lateral distribution. In contrast, the assemblage of mud shale with limestone was mainly developed in the northeast of the study area and was interpreted to have formed in a melanged accumulation shelf sedimentary environment. The effective thickness of the mud shale with limestone is small and the distribution is limited. There is developed mud shale in above two type's sedimentary environment, but the distribution is also limited. The Longtan Formation's mud shale reached stage B diagenesis and is principally composed of quartz and clay minerals. The average quartz content is 43.40% and the average clay mineral content, which is mainly composed of illite, mixed illite/smectite, chlorite and kaolinite, is 39.58%. The sedimentary environment has controlled the distribution of minerals within the sequences. The clay mineral content gradually decreases as the sedimentary environment changes from shore-tidal flat in the southwest to deep water melanged accumulation shelf in the northeast. Therefore this comprehensive analysis shows that the southwest of the study area is the key area for shale gas exploration in the Longtan Formation.

Sedimentology and geochemistry of a human‐induced tufa deposit: Implications for palaeoclimatic research

AbstractGeochemical variations across laminated tufas and travertines may reflect the growth style of the carbonate build‐up and not just climate‐related changes. This work presents the study of a carbonate deposit, formed on a ravine wall in Gran Canaria Island (Spain), from a broken pipe system used for irrigation of banana plantations. The deposit is a few tens of metres long and has a stepped morphology formed by successive cascade‐barriers and pools. The main facies are framestones of coated stems, laminated bindstones, phytoclastic wackestones and silty mudstones, all of which display micritic or coarse crystalline textures. Lamination from a framestone with coarse crystalline texture was studied from a petrological–sedimentological and geochemical perspective, and water palaeotemperatures were calculated. Lamination displaying five orders of magnitude, from daily to annual or higher, was controlled by the discontinuous supply of water. Lamination consists of crystalline laminae–discontinuity couplets at all observed scales. Estimated mean precipitation rates are 0·7 mm year−1, but discontinuity of sedimentation at all lamination orders may have involved greater precipitation rates. Whereas elemental geochemistry suggests variable conditions not far from chemical equilibrium, stable isotopes suggest that calcite precipitated under disequilibrium conditions. However, the small dimensions of the deposit and the relatively high flow velocities allowed lack of δ13C and δ18O isotope fractionation in CO2‐ nor in ‐calcite, leading to independent temperature calculations, both with mean values of 25°C. Isotopic trends found throughout lamination cannot be explained by strong changes in water temperature nor in δ13CDIC or δ18Ow. The correction made to eliminate these isotopic trends yielded narrower temperature ranges. This paper discusses the accuracy of temperature estimations despite the difficulties coming from disequilibrium and how isotopic trends through time could be explained by the growth of the deposit and not by climate‐related changes.

Characteristics of Ductile Shear Zone and Its Ore-controlling Effect in Dongtian Polymetallic Mining Area

The genesis of the Red Pacific polymetallic deposit in Yanbian area has long been regarded as a volcanic hydrothermal deposit or a volcanic jet deposit. In the field investigation and comprehensive study, authors found that the so-called 'metamorphic volcanic rocks and volcanic clastic rocks' is actually a set of highly deformed terrigenous clastic rocks. The main rock types are metamorphic conglomerate, sericite schist, sericite to form a structural deformation belt having a width of 50-400 m. The deformed conglomerate rock is the conglomerate of the valley flag, and the latter two are the former siltstone, the silty mudstone and the tectonic deformation zone are basically the same as those of the strata and ore body. The red and Taiping polymetallic ore bodies are produced in the Miaopun Formation in the deformation zone. The ore and the surrounding rock also have certain deformation structures, such as pleated structure, shear page and so on. Therefore, the cause of this deposit is ductile shear zone type, layer structure ore control.

Exploring the end‐Ordovician extinctions in Hirnantian near‐shore carbonate rocks of northern Guizhou, SW China: A refined stratigraphy and regional correlation

Richly fossiliferous Hirnantian shelly strata of near‐shore facies in northern Guizhou, South China, known as the Kuanyinchiao Formation, superbly record glacioeustatic sea‐level fluctuations and benthic faunal turnover. Recent studies of the temporal and spatial distribution of these carbonates and shelly fossils permit a critical stratigraphic revision and establishment of a robust regional stratigraphic correlation. The formation is revised to include three informal subdivisions, that is, units A, B, and C, in ascending order. Unit B of the formation typically is dominated by peloidal or oolitic grainstones, and unit C is composed of skeletal wackestone and calcareous mudstone, both units sharing distinctive coral and brachiopod faunas. This contrasts sharply with unit A of the formation, consisting of mudstone, silty mudstone, or calcareous mudstone, that yields the cool‐water Hirnantia fauna and associated coral fauna. In view of the presence of carbonate ooids and peloids, rugose corals, and a distinctive brachiopod assemblage, all indicative of warm‐water conditions, unit B, as well as unit C yielding the same shelly fauna, is interpreted as representing postglacial sedimentation immediately following the major Hirnantian glaciation, thus marking a significant climatic shift. Similar warm‐water carbonate rocks have been recognized in a number of regions along the margin of the Qianzhong Oldland, including Bijie, Renhuai, Tongzi, and Fenggang of northern Guizhou. Such a vast distribution area of these rocks indicates that postglacial carbonates are more widespread on the Yangtze Platform of South China than previously thought, providing a rare window into rocks and fossils of the survival interval immediately following the extinction event associated with the Hirnantian glacial episodes.

Oceanic redox conditions through the late Pliensbachian to early Toarcian on the northwestern Panthalassa margin: Insights from pyrite and geochemical data

The early Toarcian oceanic anoxic event (T-OAE; ~183Ma) was a significant palaeoenvironmental perturbation associated with marked changes in oceanic redox conditions. However, the precise redox conditions and redox history of various water masses during the T-OAE, especially those from outside the Boreal and Tethyan realms, are unclear. To address this issue, we present pyrite framboid data from an upper Pliensbachian to lower Toarcian succession deposited on the NW Panthalassa margin in a shallow-water setting (Sakuraguchi-dani section, Toyora area, SW Japan). Available data on lithofacies and redox-sensitive trace elements from the same succession suggest that dysoxic bottom-water conditions generally prevailed, with intermittent short-term oxygenation events. Size-distribution analysis of pyrite framboids reveals that framboid size populations from the silty mudstones during the T-OAE were characterized by small mean diameters and standard deviations. This suggests that euxinic conditions at least intermittently occurred during the T-OAE interval. Most likely, this water-column euxinia was associated with the expansion of an oxygen minimum zone linked to increased primary productivity. This interpretation is consistent with a previously reported increase in fluvial discharge and thus nutrient flux caused by a strengthening of the hydrological cycle.