Sedimentary environment and organic matter accumulation of Wufeng-Longmaxi shales,southwest Yangtze Plate,China:Insights from geochemical and petrological evidence

Upper Ordovician-Lower Silurian Wufeng-Longmaxi Formation is the most developed strata of shale gas in southern China.Due to the complex sedimentary environment adjacent to the Kangdian Uplift,the favorable area for organic-rich shale development is still undetermined.The authors,therefore,focus on the mechanism of accumulation of organic matter and the characterization of the sedimentary environment of the Wufeng-Longmaxi Shales to have a more complete understanding and new discovering of organic matter enrichment and favorable area in the marginal region around Sichuan Basin.Multiple methods were applied in this study,including thin section identification,scanning electron microscopy(SEM)observations and X-ray diffraction(XRD),and elemental analysis on outcrop samples.Five lithofacies have been defined according to the mineralogical and petrological analyses,including mudstone,bioclastic limestone,silty shale,dolomitic shale,and carbonaceous siliceous shale.The paleo-environments have been reconstructed and the organic enrichment mechanism has been identified as a reduced environment and high productivity.The Wufeng period is generally a suboxic environment and the early Longmaxi period is a reducing environment based on geochemical characterization.High dolomite content in the study area is accompanied by high TOC,which may potentially indicate the restricted anoxic environment formed by biological flourishing in shallower water.And for the area close to the Kangdian Uplift,the shale gas generation capability is comparatively favorable.The geochemical parameters implied that new favorable areas for shale gas exploration could be targeted,and more shale gas resources in the mountain-basin transitional zone might be identified in the future.

Sedimentary Environment and Mineralization of the Black Shale Polymetallic Layer in the Early Cambrian,SW China:Constraints from in situ LA-ICP-MS Analysis of Pyrite

A polymetallic layer is usually developed at the bottom of the early Cambrian black shale in Guizhou Province.The mineral that makes up the polymetallic layer is related to the sedimentary facies.To analyze the differentiation mechanism between polymetallic deposits(Ni-Mo and V),the Zhijin Gezhongwu profile located in the outer shelf and the Sansui Haishan V deposit located in the lower slope are selected to study the in situ sulfur isotopes and trace elements of pyrite.The results show that δ^(34)S values of pyrite vary widely from−7.8‰to 28‰in the Gezhongwu profile,while the δ^(34)S values are relatively uniform(from 27.8‰to 38.4‰)in the Haishan profile.The isotopic S composition is consistent with the transition that occurs in the sedimentary phase from the shelf to the deep sea on the transgressive Yangtze platform;this indicates that the δ^(34)SO_(4)^(2−)values in seawater must be differently distributed in depositional environments.The sulfur in the Ni-Mo layer is produced after the mixing of seawater and hydrothermal fluid,while the V layer mainly originates from seawater.Overall,the Ni-Mo and V deposits have been differentiated primarily on the basis of the combined effect of continental weathering and hydrothermal fluid.

Sedimentary Dynamics of the Sands of the Cover Formation in the Batéké Plateaus (Republic of Congo): Paleoenvironmental Implications

The depositional environment of the sands of the cover formation is discussed. This study aims to determine the paleoenvironments of deposition of the sands of the cover formation in the Batéké Plateaus by studying sedimentary dynamics based on the description of lithological facies in the field and granulometric analyses in the laboratory. In the field, six (6) lithostratigraphic logs were surveyed and 42 sand samples were taken for laboratory analysis. In the laboratory, the samples underwent granulometric, sieving and sedimentometry analyses, after washing with running water using a 63 µm sieve. These analyses made it possible to determine the granulometric classes of the samples. The sieving results allowed to determine the granulometric parameters (mean, standard deviation, mode, median, skewness, flattening or kurtosis) using the method of moments with the software “Gradistat V.8”, granulometric parameters with which the granulometric facies, the mode of transport and the deposition environment were determined using the diagrams. Morphoscopy made it possible to determine the form and aspect of the surface of the quartz grains constituting these sands. Granulometric analyses show that these silty-clay or clayey-silty sands are fine sands and rarely medium sands, moderately to well sorted and rarely well sorted. The dominant granulometric facies is hyperbolic (sigmoid), with parabolic facies being rare. The primary mode of transport of these sands is saltation, which dominates rolling. The dispersion of points in the diagrams shows that these sands originate from two depositional environments: aeolian and fluvial. Morphoscopic analysis reveals the presence of clean rounded matt grains (RM), dirty rounded matt grains (RS), shiny blunt grains (EL) and shiny rounded grains (RL). The rounded matt grains exhibit several impact marks. The presence of dirty rounded grains with a ferruginous cement on their surface indicates that these sands have been reworked. These sands have undergone two types of transport, first by wind (aeolian environment) and then by water (fluvial environment).

Sedimentary Environment Analysis of Ancient Sand Ridges from Zk23 Hole in the East China Sea

Abstract： Based on the analysis of core samples from the hole of Zk23 in the East China Sea Continental Shelf and by means of sedimentary stratigraphy, biostratigraphy and chronostratigraphy, the authors consider that the fine-sand deposition in borehole was part of buried ancient estuary sand ridges of the Yangtze River. The deposition history of study area around the hole before and after the glacial period as well as postglacial period is made clear after our research： （1） the estuarine sublayer -undersea delta facies strata was deposited under the fast sea level rise about 15 kaB.P; （2） sand ridges mostly consisting of fine-sand, were formed when the sea level was the fluctuant range of 60 - 80 m of isoba during the deglacial period around 15 - 12 kaB.P; （3） first silty clay and clay silt strata above the sand ridges were deposited during the period when the sea level rose fast from 12 to 7 kaB.P, and then it keeps stable to the present.

Development of Sedimentary Environment in the Northeastern South China Sea Since the Last Glacial Stage

Core D （21°23′02″N, 116°47′13″E, water depth 405 m） was sampled from the upper slope from the northeastern South China Sea （SCS） and applied to analyze the sedimentary environmental change in this sea area since the last glacial stage. The results of grain size analysis, diatom analysis and detrital mineral analysis were well matched. We divided the core D into two layers. The surface sand layer （0 - 2 cm） consisted of residual sediments, which might be originally the sediment in the late Pleistocene and later suffered from being transformed in the post glacial transgression. The lower layer （2 - 130 cm） was quite different from the surface one, which might mainly result from a neritic sedimentary environment in the last glacial stage. Two sedimentary cycles could be detected in the core D： regression during Marine isotope stage （MIS） 4 to transgression during MIS 3 and regression during MIS 2 to transgression during the post glacial.

Holocene ostracoda and sedimentary environment implication in The core NG93－1 from the Great Wall Bay， Antarctica

the holocene ostracode are analyzed from the core sample ng93-1 collectedIn the great wall bay, antarctica during 1992～1993. Totally 11 genera 21 speciesHave been identified in the sediments, of which the most abundant is loxoreticulatumFallax, the second abundant are xestoleberis keguelenensis, xestoleberis spp., Semicytherura spp., australicythere polylyca, etc. Most species of ostracodes are thoseKnown in antarctic area. Base on the study of ostracodes from the core ng93-1, theirSedimentary environment may be of shore-shallow sea.

Study on the Marine Sedimentary Environment Evolution of the Southern Laizhou Bay Under the Impact of Port Projects

The southern Laizhou Bay is mainly composed of silt-sandy coasts with diverse landforms, and its marine hydrodynamic environment is sensitive to human activities. Marine hydrodynamic and sedimentary environments of the study area have changed under the influence of large-scale port projects in recent years. In this paper, the evolution of hydrodynamic environment, deposition rate, and geochemical characteristics were studied based on sediment grain size, element analysis and ^(210)Pb dating of two cores, in order to analyze the influence of Weifang Port on marine environmental evolution, and provide theoretical and practical basis for protecting marine environment in developing marine resources reasonably. Results showed that sediments of the two cores were relatively coarser and mainly composed of silty sand. Sediments above 230 cm in core WF1 and 218 cm in core WF2 were deposited since 1855 when the Yellow River appeared to deposit its sediments within the modern active delta, and the average deposition rate was between 0.3 and 0.5 cm a^(-1). Implement of Weifang Port projects in 1997 and 2007 created great influence on the sedimentary environment evolution in the surrounding waters, and the deposition rate was significantly increased. The average annual deposition rates were 5.1 cm and 3.5 cm in WF1 and WF2 respectively between 1997 and 2007. Content of heavy metals in sediments showed no obvious change in the vertical, indicating that the heavy metals were less affected by human activity and there was no significant accumulation of such metals in the study area.

SEDIMENTARY ENVIRONMENT IN TAIWAN SHOAL

Taiwan Shoal sediments are well sorted and rounded medium-coarse sands eontaining a large amount of shell and beach rock fragments and basalt gravels, and are of deltaic, coastal and eolian origin. Underwater sand waves are formed by the combined tidal currents and are remolded by storm waves.

Shale Lithofacies and Sedimentary Environment of the Third Member,Shahejie Formation,Zhanhua Sag,Eastern China

Researches into shale lithofacies,their sedimentary environments and relationship benefit understanding both of sedimentary cycle division and unconventional hydrocarbon exploration in lacustrine basins.Based on a 100~300-m-thick dark shale,mudstone and limestone encountered in the lower third member of the Eocene Shahejie Formation(Es3l member)in Zhanhua Sag,Bohai Bay Basin,eastern China,routine core analysis,thin sectioning,scanning electron microscopy(SEM),mineralogical and geochemical measurements were used to understand detailed facies characterization and paleoclimate in the member.This Es3l shale sediment includes three sedimentary cycles(C3,C2 and C1),from bottom to top,with complex sedimentary characters and spatial distribution.In terms of the composition,texture,bedding and thickness,six lithofacies are recognized in this succession.Some geochemical parameters,such as trace elements(Sr/Ba,Na/Al,V/Ni,V/(V+Ni),U/Th),carbon and oxygen isotopes(δ^(18)O,δ^(13)C),and total organic carbon content(TOC)indicate that the shales were deposited in a deep to semi-deep lake,with the water column being salty,stratified,enclosed and reductive.During cycles C3 and C2 of the middle-lower sections,the climate was arid,and the water was salty and stratified.Laminated and laminar mudstone-limestone was deposited with moderate organic matter(average TOC 1.8%)and good reservoir quality(average porosity 6.5%),which can be regarded as favorable reservoir.During the C1 cycle,a large amount of organic matter was input from outside the basin and this led to high productivity with a more humid climate.Massive calcareous mudstone was deposited,and this is characterized by high TOC(average 3.6%)and moderate porosity(average 4%),and provides favorable source rocks.

Sedimentary Environments Can Be Changed by Geotechnology (Case Study: A Morphotectonic Idea for Design of Extensive Artificial Bay on the Iranian Plateau)

Iranian Plateau between the Lesser Caucasus-Alborz Mountains on the north and Zagros-Makran Ranges on the south has several inter-mountainous depressions which were filled by Quaternary deposits. Geologic evidence implied that, the last marine conditions in some depressions such as the Dasht-e Kavir, Dasht-e Lut and Jazmourian basins, had been changed to land conditions in middle Miocene. Based on shape and elevation of the Dasht-e Kavir, Dasht-e Lut and Jazmourian plains related to sea level and geomorphology of Iranian plateau, three semi-connective artificial lakes can be constructed upon the mentioned plains by consideration of many geologic and geotechnical parameters. These artificial lakes can feed by pumping of water from Oman Sea and form a triple artificial bay which they must be connected together by two gated straits. Therefore, a possible morphotectonic idea with many advantages has suggested that it can be present as an international geotechnologic design. This design has the important environmental impacts which can be changed desert to lake sedimentary basins.

Grain-size features of aeolian sand on the east coast of Hainan Island and the revealed evolutionary processes of the sedimentary environment

The Mudui stratigraphic section represents the typical records of sedimentation processes of sand dunes and interdune depressions on the east coast of Hainan Island.Based on high-density sampling and optically stimulated luminescence(OSL) dating of the strata of the section,the grain-size composition,grain-size parameters,cumulative distribution probability curve,and grain-size-sensitivity indexes(SC/D) were analyzed.The analyzed results show that the grain-size features of aeolian sand,weakly developed sandy paleosol,two-facies(aeolian and aqueous) deposits,and lagoon deposits are all different.This indicates four evolutionary phases of the sedimentary environment of the east coast of Hainan Island since 38 ka B.P.Phase I:38-22 ka B.P.;phase II:22-17 ka B.P.;phase III:17-10 ka B.P.;phase IV:10 ka B.P.-present.The climate experienced the hot-wet/hot-dry,hot-wet/hot-dry,and warm-wet/hot-wet fluctuations,and the sedimentary environment also underwent lagoon deposition,dune and interdune depression deposition,dune stabilization and soil development,shifting sand deposition,and evolutionary processes.

Geochemical characteristics and sedimentary environment of the Middle Devonian organic-rich shales in the Northwest of Guizhong Depression, Southwest China

In order to figure out the redox conditions and paleo-sedimentary environment of the Middle Devonian shales in the northwest of Guizhong Depression,the trace element analysis was conducted on the Middle Devonian cores(320.35–938.50 m)of the typical shale gas investigation well(GY-1)at a 1.50 m sampling interval through X-ray fluorescence spectroscopy(XRF)and inductively coupled plasma mass spectrometry(ICP-MS).According to the test result,the average values of V/(V+Ni),V/Cr and Ni/Co in Nabiao formation(Fm.)are larger than 0.67,4.65 and 7.71 respectively,and Nabiao Fm.is rich in biological assemblages such as tabasheer,ammonite,etc.These evidences indicate the rising sea level rose relatively in the sedimentation period of Nabiao Fm.and a deepwater shelf environment,which was favorable for the preservation of organic matters.The V/(V+Ni),V/Cr and Ni/Co in Luofu Fm.and Tangting Fm.are 0.38–0.65,0.73–4.10 and 3.70–6.72 respectively,indicating that the sea level dropped relatively in their sedimentation period,during which the water bodies became shallow,and the sedimentary environment was a weak oxidizing shallow water shelf environment.In addition,the variation of TOC has a high correlation with the enrichment degree of Ba element,indicating the favorable conditions for the enrichment and preservation of organic matters under an oxygen-deficient environment.Moreover,according to the identification of trace element indexes,the northwest of Guizhong Depression experienced the sedimentary cycle of relative rise to relative fall of sea level from bottom to top in the Middle Devonian sedimentation period.The relative sea level rose to the highest in the sedimentation period of Nabiao Fm.,in which the organic-rich shales with stable thickness and high organic content were deposited.Hence,the Nabiao Fm.could be regarded as the favorable exploration target interval in this area.

Comparison of the Macrozoobenthic Community and Sedimentary Environment with and Without Horseshoe Crab Presence in the Crocodile Island Intertidal Zone,Xiamen,China

In order to compare the macrozoobenthic community and sedimentary environment with and without the presence of horseshoe crabs,the benthic macrofauna,sediment grain size,chromium(Cr),cobalt(Co),nickel(Ni),polycyclic aromatic hydrocarbon(PAHs),organic carbon and nitrogen were seasonally investigated at site A(with horseshoe crab presence),sites B and C(without horseshoe crab presence)in the Crocodile Island intertidal zone in Xiamen from June 2018 to July 2019.The results showed that most of the community parameters of benthic macrofauna,population parameters of common benthic macrofauna and environmental parameters were significantly different at the sites with horseshoe crab versus non-horseshoe crab sites.A two-way ANOVA test showed that the densities of Ceratonereis erythraeensis and Sigambra hanaokai had significant site variation.Cluster and nonmetric multi-dimensional scaling(NMDS)analysis showed the community composition of benthic macrofauna was significantly different among the three sampling sites over four seasons.The mean sand content at site A(64.32%)was higher than those at site B(36.01%)and site C(18.86%).Conversely,the mean contents of silt,clay,Cr,Co,Ni,organic carbon,organic nitrogen,phenanthrene,and pyrene at site A were lower than those at site B and site C.These observations are consistent with the expected preferences of horseshoe crabs to live in areas with 60%sand content,which is associated with abundant and edible clamworms in the Crocodile Island intertidal zone,Xiamen.

Sedimentary Characteristics and Paleoenvironmental Records of Zabuye Salt Lake,Tibetan Plateau,since 128 ka BP

This paper for the first time reveals high-resolution core records of Zabuye Salt Lake in the interior of the Qinghai-Tibet Plateau. According to 1346 samples taken continuously, relatively accurate 14^C, U-series disequilibrium and ESR ages have been obtained, thus revealing that the lake core ages from 0 to 83.63 m of hole SZK02 are -800 to over 128 ka. In the paper, the lake core sedimentary characteristics （including the lithologies and mineral assemblages） are analyzed in detail and correlated with ostracod assemblages I to XX and sporopollen zones A to I, and on the basis of an integrated analysis of the δ^18O values of authigenic calcium-magnesium carbonate and environmental proxies of minerals, sporopollen and microfossils in the lake core, a correlation has been made of oxygen isotope change between this lake core and the Greenland GISP2 and GRIP and Guliya ice cores, and the climate of Zabuye Salt Lake since 128 ka BP is divided into the last interglacial stage （including substages e, d, c, b and a） of oxygen isotope stage （OIS） 5, early glacial stadial of the last glacial stage of OIS 4, interglacial stadial of the last glacial stage of OIS 3, late glacial stadial of the last glacial stage or Last Glacial Maximum of OIS 2 and postglacial state of OIS 1; in addition, 6 Heinrich （H6-H1） events, Younger Dryas event and 8.2 ka BP cold event have been recognized.

Classifying the sedimentary environments of the Xincun Lagoon,Hainan Island,by system cluster and principal component analyses

An understanding of the sedimentary environment in relation to its controlling factors is of great importance in coastal geomorphology,ecology,tourism and aquaculture studies.We attempt to deal with this issue,using a case study from the Xincun Lagoon,Hainan Island in southern China.For the study,surficial sediment samples were collected,together with hydrodynamic and bathymetric surveys,during August 2013.Numerical simulation was carried out to obtain high-spatial resolution tidal current data.The sediment samples were analyzed to derive mean grain size,sorting coefficient,skewness and kurtosis,together with the sand,silt and clay contents.The modern sedimentary environments were classified using system cluster and principal component analyses.Grain size analysis reveals that the sediments are characterized by extremely slightly sandy silty mud（ESSSM） and slightly silty sand（SSS）,which are distributed in the central lagoon and near-shore shallow water areas,respectively.Mean grain size varies from 0 to 8.0Ф,with an average of 4.6Ф.The silt content is the highest,i.e.,52% on average,with the average contents of sand and clay being 43% and 5%,respectively.There exists a significant correlation between mean size and water depth,suggesting that the surficial sediments become finer with increasing water depth.Cluster analyses reveals two groups of samples.The first group is characterized by mean grain size of more than 5.5Ф,whilst the second group has mean grain size of below 3.5Ф.Further,these groups also have different correlations between mean grain size and the other grain size parameters.In terms of the tidal current,the average values of the root mean square velocity（RMSV） are 7.5 cm/s and 6.9 cm/s on springs and neaps,respectively.For the RMSVs that are higher than 4 cm/s,a significant positive correlation is found between the content of the 63–125 μm fraction and the RMSV,suggesting that the RMSV determines the variability of the very fine sand fraction.Based on system cluster and principal component analyses（PCA）,the modern sedimentary environments are classified into three types according to the grain size parameters,RMSVs and water depth data.The results suggest the importance of grain size parameters and high-spatial resolution hydrodynamic data in differentiating the coastal sedimentary environments.

Characteristics of Rearranged Hopanes of Hydrocarbon Source Rocks in Saline Sedimentary Environment： A Case Study of the Songliao Basin

The Songliao Basin, one of the biggest continental petroliferous basins in eastern China, is a Mesozoic- Cenozoic fault-depressed and fault-subsided basin developed on the Hercynian fold basement. Generally, rearranged hopanes are considered to be formed by clay- mediated acidic catalysis under oxic or suboxic environment, whereas high abundance of rearranged hopanes were found in hydrocarbon source rocks and crude oils that are derived from salty environment in the Songliao Basin. This phenomenon rarely happens all over the world.

New Discoveries of the Influence of Sedimentary Environment on Rearranged Hopanes in Source Rocks

Objective The distribution of rearranged hopanes in hydrocarbon source rocks is influenced by thermal maturity and original source input, and is also controlled by depositional conditions. Through comparison of lacustrine and coal-bearing source rocks, this work attempted to analyze the composition and origin of rearranged hopanes in hydrocarbon source rocks. Taken the source rocks from the Songliao Basin, Ordos Basin and Kuqa Depression as examples, we aimed to investigate the effect of the redox conditions, water salinity and oxygen content of the source-rock depositional environment on the formation of rearranged hopanes to provide theoretical basis for the genesis of rearranged hopanes.

A Study of Sedimentary Environment and Reservoir Quality of the Carboniferous Donghe Sandstonein the Tarim Basin

Very thick, fine-grained quartzose sandstone of the Lower Carboniferous (called the Donghe sandstone) was discovered in Donghe-1 well in the Tarim basin. Highly-productive commercial oil and gas flows were obtained when a well completion test was conducted. This important discovery proved that the quartzose sandstone is a prospecting target with good prospects. After that, other two paying oil and gas flows were found in the Lower Carboniferous in the Tazhong-4 and Tazhong-10 structures (Tazhong means central Tarim), equivalent to the Donghe sandstone. The Tazhong-4 structural oil field is the biggest oil field ever discovered. Therefore it is of guiding importance in oil and gas exploration to deepen the study of sedimentary and reservoir features and the sedimentary environment of the Donghe sandstone and to build a sedimentary model in order to understand the reservoir distribution pattern.

Evolution of deepwater sedimentary environments and its implication for hydrocarbon exploration in Qiongdongnan Basin, northwestern South China Sea

Over the past several years, a number of hydrocarbon reservoirs have been discovered in the deepwater area of Qiongdongnan Basin, northwestern South China Sea. These oil/gas fields demonstrate that the evolution of the deepwater sedimentary environment are controlling the formation and distribution of large-scale clastic reservoirs. Integration between seismic and borehole data were necessary to best clarify the distribution and quality of these deepwater reservoirs. Geochemical and paleobiological evidence from discrete samples was also applied to document specific information regarding the sedimentary environment. Results show that the Qiongdongnan Basin has existed as a thriving marine environment since Oligocene, when several rifting depressions developed throughout the entire Qiongdongnan Basin. Triggered by the faults activities, several distinct provenances supplied the coarse sediments, transporting and depositing them in deep parts of the rifting depressions. A fan delta system then formed nearby the source in the deeper area of these rifting depressions. The sedimentary environment of Qiongdongnan gradiationally became deepwater since early Miocene. Consequently, abundances of sediments were transported from Hainan Island and Southern Uplift, and then sunk into the basin center. The submarine fans revealed by many boreholes in this area verified them as good reservoir. Because the area reached its lowest sea level at late Miocene and the Southern Uplift subsidenced under sea level, not providing any sediment, so that the carbonate mesa and biorhythms characteristic of this area also developed during this period. In the west part of Qiongdongnan Basin, sediments transported from Vietnam increased in response to the Tibetan Uplift. Consequently, a central canyon developed along the center of Qiongdongnan Basin, which has been confirmed by several boreholes as a favorable hydrocarbon reservoir. The clarification of the deepwater sedimentary environment’s evolution is potentially highly beneficial to future hydrocarbon exploration in the deepwater area of Qiongdongnan Basin.

Sedimentary Environments and Correlative Sequence Stratigraphy of Upper Cretaceous—Paleogene Succession in Shiraz Area, Fars, SW IRAN

Sequence stratigraphic evidences have been used in this research to investigate the stratigraphic correlation of cretaceous paleogene succession in Shiraz area, Fars, SW Iran. For this purpose, 4 stratigraphic sections have been chosen and provided. According to petrographic study and field evidences, 4 depositional sequences were identified including the Tarbur, Sachun and Jahrum formations in the east and the Gurpi, Pabadeh and Asmari formations in the west of study area. After identifying the boundaries and systems tract facies, it has been found that the mentioned formations have been deposited among the continuous/discontinuous boundaries in an equivalent manner. In this case, the purple shale (between the Gurpi and Pabdeh formations) could be considered equivalent to the upper part of the Sachun formation. From the point of Paleogeography, the plat-form sedimentary environment covered almost the whole area during the Oligo-Miocene age, so that the Asmari formation would have been deposited, which is marked by sharp contact as disconformity on Jahrum formation in the east of the basin and with continuous contact (C.C) on the Pabdeh formation in the west and southwest.