The Early Cambrian Niutitang Formation is characterized by wide distribution of black shales on Yangtze Block, South China. Here we have reported the pyrite concretions in the bottom of the Niutitang Formation deposited in the slope-basin environment of Yangtze Block. The pyrite concretion was mainly composed of pyrite associated with hydrothermal minerals (barite, hyalophane, tetrahedrite), followed by quartz and organic matter. Trace elements Mo and U displayed significant enrichment (enrichment factors > 10), indicating the euxinic bottom water condition. Cu, Ni, and excess Ba concentrations were relatively high, denoting high primary productivity. In-situ sulfur isotope compositions of pyrite concretions δ 34 S py ) showed little variations (13.2‰–19.4‰) and small fractionations compared to coeval seawater δ 34 Sso 4 . Petrological and geochemical analyses indicated the pyrite concretions were formed in the sediment-water interface during the early diagenesis, with H 2 S diffusing from the euxinic water, and influenced by hydrothermal activity leading to the coexistence of barite, hyalophane, and tetrahedrite. These results imply euxinic bottom water featured by high primary productivity and increasing riverine flux of sulfate from chemical weathering during the Early Cambrian.

湘鄂西地区下寒武统牛蹄塘组是我国南方页岩气勘探的重要目标.通过鄂西古城村和湘西岩屋潭 2 个剖面的下寒武统牛蹄塘组黑色页岩层系的岩石学特征与地球化学分析,开展了黑色页岩沉积的物源和构造背景研究,并探讨了其沉积环境.研究结果表明:①鄂西古城村和湘西岩屋潭2个剖面的牛蹄塘组黑色页岩沉积差异明显,古城村剖面牛蹄塘组页岩主要为泥质页岩,厚度小,底部发育少量硅质页岩;岩屋潭剖面牛蹄塘组页岩则以硅质页岩为主,厚度大,底部富集硅磷质结核及镍、钴、钒等多种金属.岩屋潭剖面的石英含量远高于古城村剖面,并且黄铁矿也更富集.②研究区 2 个剖面的黑色岩系沉积环境、物源背景与构造背景均存在明显差异.古城村剖面物源以长英质火成岩和石英质沉积岩为主,沉积于被动大陆边缘大陆架上的弱还原-弱氧化环境;岩屋潭剖面物源以长英质火成岩、石英质沉积岩和大陆拉斑玄武岩为主,并伴随热液物源的输入,沉积于被动大陆边缘大陆坡上的还原环境.③早寒武世,鄂西鹤峰古城村的沉积以黑色炭质页岩和灰岩为主,且有机质与硅质含量均较低,水体相对较浅,受海平面波动影响明显,为陆棚环境;湘西的沉积厚度大,有机质含量高,且由于靠近深大断裂,热液硅来源丰富,为大陆坡环境.

This study aims to investigate paleoenvironments, hydrothermal activity, and seawater restriction of the lower Cambrian Shuijingtuo Formation in the Three Gorges area and clarify the dominating factors on organic matter (OM) accumulation. In the studied SL well in western Hubei Province, the Shuijingtuo Formation was divided into three intervals based on geological and geochemical characteristics. Total organic carbon (TOC) contents were variable, averaging 0.69% in Unit 1, 5.40% in Unit 2, and 1.12% in Unit 3. Enrichment factors (EF) of Cu and Ni indicated higher paleoproductivity in Unit 2. Unit 1 and the lower part of Unit 2 were affected by hydrothermal activity, whereas hydrothermal events receded in Unit 3. Hydrothermal activities could provide nutrients and may also facilitate the reducing water condition. Redox proxies (Corg/P, EF(Mo)-EF(U)) suggested that Unit 1, Unit 2, and Unit 3 were deposited in oxic-suboxic, euxinic, and suboxic environments, respectively. Upwelling/restriction proxies (EF(Co)*EF(Mn), Co (ppm)*Mn (%), Mo/TOC, and Cd/Mo) implied that Unit 1, Unit 2, and Unit 3 were deposited in upwelling, semi-restricted, strong restricted conditions, respectively. In summary, an anoxic environment, semi-restricted water conditions, and hydrothermal events are beneficial for OM enrichment during the early Cambrian.

During the Ordovician–Silurian transition, the Yangtze Block in South China experienced intense tectonic movements and climate change and was covered by the widely distributed black shales of the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation, which recorded key information about these major geological events. By geochemical and petrological analyses of three profiles located in the Middle Yangtze Region, at least four long-term sedimentary cycles can be identified in the Upper Ordovician–Lower Silurian black shale. These cycles are mainly controlled by paleoclimate changes and tectonic activity. The Late Ordovician tectonic subsidence related to the Kwangsian Orogeny promoted a gradual sea-level rise and the deposition of the Wufeng black shale in the Middle Yangtze Region, while a significant sea-level fall related to the Hirnantian intensifying glaciation terminated this process. Subsequently, post-Hirnantian warming climate and glacier ablation triggered a significant sea-level rise, reduced the water restriction, increased the productivity, and intensified the anoxic environment, promoting the deposition of black shale in the second sedimentary cycle. However, after the short period of climate warming, global climate cooling occurred again with a regional tectonic uplift in the study area. This led to a gradual sea-level fall, reduced anoxic environment, and increased water restriction during the Rhuddanian, as well as the deposition of black shale with a decreased amount of organic matter. Moreover, intensified glaciation occurred during the Rhuddanian–Aeronian transition significantly accelerated the falling speed of the relative sea level, resulting in a stratigraphic gap and a small thickness of the Silurian black shale in the Middle Yangtze Region. Afterward, the study area experienced a significant climate warming event during the Early Aeronian, which led to a large-scale glacier ablation and a short period of sea-level rise. This promoted the deposition of the Aeronian black shale in the third sedimentary cycle. Nevertheless, the intensified weathering condition related to a continuously warming climate and tectonic uplift led to sufficient supply of terrigenous deposits, sustained sea-level fall, and finally terminated the development of Ordovician–Silurian black shale in the Yangtze region. Thus, these cyclic variations of the black shale not only recorded the shift of the Ordovician–Silurian tectonic movement (from Ordovician subsidence to Silurian uplift) in the Middle Yangtze area and the Hirnantian glaciation, but also provide strong evidence for the global intensified glaciation during the Rhuddanian–Aeronian transition and the subsequent climate-warming event. In addition, this study also provides new perspectives for the understanding of global sea-level change during the Ordovician–Silurian transition.

As one of the main reservoirs types in the Sichuan Basin, the granular shoal reservoirs of the Feixianguan Formation have great potential for oil and gas exploration. Therefore, it is very important to study the sedimentary evolution characteristics and the distribution of shoal deposits of the Feixianguan Formation. In this paper, the sequence stratigraphy, sedimentary characteristics and distribution characteristics of the granular shoal deposits of the southwestern section of the Kaijiang–Liangping Trough in the Sichuan Basin were investigated, through core observations, thin section analysis, and field profiles measurement and sampling, combined with well-logging data and interpretations of two and three-dimensional seismic data of the local area. According to the identification marks, three sequence boundaries (SB1, SB2, and SB3) and two third-order sequences (SQ1 and SQ2) were identified, and a transgressive systems tract (TST) and highstand systems tract (HST) were distinguished in each third-order sequence. The analysis of the Feixianguan Formation sedimentary facies indicated that this formation developed in a carbonate platform setting. In the depositional period of SQ1, the study area inherited the sedimentary pattern of the Changxing Formation and developed carbonate platform depositions. During the SQ1-HST stage, the platform margin shoals and the intra-platform shoals in the study area were relatively well developed. During the SQ2-TST period, oolitic shoals were developed in the region, but their thicknesses were smaller than those of SQ1-HST. In addition, the distribution pattern of sedimentary facies was similar to that of SQ1-HST. The platform marginal zone was relatively narrow at that time, whereas the open platform was more widely distributed, and the distribution areas of platform margin shoals and intra-platform oolitic shoals were relatively limited. During the depositional period of SQ2-TST, the sea level dropped to the lowest level of the studied interval, and limited and evaporation platform facies developed in the study area. Based on previous studies, the influence of different factors on the deposition of the shoals in the Feixianguan Formation were evaluated. The results showed that under the influence of sea level, basement faults, and hydrodynamic conditions, the platform margin area in the study area was more favorable to the development of granular shoals, and the growth model of the granular shoals was “accretion before migration”.

The lower Cambrian Niutitang shales, as one of target intervals with the greatest potential for shale gas exploration and development, have attracted much attention. Nevertheless, the organic matter enrichment mechanisms of the lower Cambrian Niutitang shales need further study, especially in the hydrothermal active zone. In this study, samples from ND1 well in western Hubei Province, middle Yangtze region, South China, were investigated for the controlling factors of organic matter accumulation of Lower Cambrian Niutitang shales by detailed petrographic, mineralogic, and geochemical proxies. The results show that hydrothermal activity and sea level fluctuation controlled the redox conditions and paleoproductivity of seawater and ultimately controlled the organic matter accumulation of Niutitang formation. In the Niu-1 member, the intense hydrothermal events lead to a suboxic to anoxic environment, which is conducive to the organic matter preservation. However, low sea level strengthens the restriction of water mass and reduced nutrient upwelling into the shelf, leading to decreased marine primary productivity, which was ultimately responsible for depleted organic matter accumulation in the Niu-1 member. In the Niu-2 member, the anoxic-euxinic environment and high paleoproductivity, driven by continuous hydrothermal activity and rising sea level, were the main factors controlling the enrichment of organic matter. In the Niu-3 member, the dysoxic to oxic condition plus low primary productivity, caused by the disappearance of hydrothermal activities and sea-level fall, resulted in the unfavorable organic matter accumulation. The results of this paper enrich the model of organic matter enrichment in the lower Cambrian black shale in the middle Yangtze region.

Investigating the impacts of rock composition on pore structure is of great significance to understand shale gas occurrence and gas accumulation mechanism. Shale samples from over-mature Niutitang formation of Lower Cambrian in south China were measured by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), low pressure N2 and CO2 adsorption to elucidate the controls of distinct mineral composition on pore development. Two distinct lithofacies, namely siliceous shale and argillaceous shale, were ascertained based on their mineral composition. Due to the variability of mineral composition in different lithofacies, pore structure characteristics are not uniform. Pores in siliceous shales are dominated by interparticle pores and organic matter (OM) pores, among which the interparticle pores are mainly developed between authigenic quartz. Furthermore, most of these interparticle pores and cleavage-sheet intraparticle pores within clay minerals are usually filled by amorphous organic matter that is host to OM pores. Due to the lack of rigid minerals, argillaceous shale was cemented densely, resulting in few interparticle pores, while cleavage-sheet intraparticle pores within clay minerals are common. Comparing siliceous shales with argillaceous shales, specific surface areas and pore volumes are higher on the former than on the latter. The content of total organic carbon (TOC) and authigenic quartz have a great influence on micropore structures, but less on mesopore structure for siliceous shales. The rigid framework structure formed by authigenic quartz is believed to be able to prevent primary interparticle pores from mechanical compaction and facilitate the formation of organic matter-associated pores. In terms of argillaceous shales, due to the lack of authigenic quartz, interparticle pores were rarely developed and its pore structure is mainly controlled by illite content.

Shales from the Lower Cambrian Niutitang Formation of Yangtze Platform have been widely investigated due to its shale gas potential. To better illustrate the pore structure and fractal characteristics of shale, a series of experiments were conducted on outcrop samples from the Lower Cambrian Niutitang Formation on Yangtze Platform, including X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and low-temperature nitrogen adsorption. Frenkel-Halsey-Hill (FHH) model was adopted to calculate the fractal dimensions. Furthermore, the relationships between fractal dimensions and pore structure parameters and mineral composition are discussed. FE-SEM observation results show that interparticle pores are most developed in shale, followed by intraparticle pores. This study identified the fractal dimensions D1 (ranging from 2.558 0 to 2.710 2) and D2 (ranging from 2.541 5 to 2.765 2). The pore structure of the Niutitang Formation shale is primarily controlled by quartz and clay content. Fractal dimensions are able to characterize the pore structure complexity of Niutitang Formation shale because D1 and D2 correlate well with average pore diameter and quartz content.

During the late Eocene-early Oligocene, major changes in the global climate and environment occurred, including decreases in global temperature and the appearance of Antarctic glaciation. These changes marked the transition from early Cenozoic greenhouse conditions to icehouse conditions. Research on this cooling trend in the Atlantic and high-latitude regions has increased, but few reports have studied low-latitude regions (e.g., the northern South China Sea), and it remains unclear whether sediment records in these areas preserve signals related to this global cooling event. In this study, we studied records from a lacustrine well in the Wenchang Sag, Pearl River Mouth Basin (PRMB), northern South China Sea. We conducted palynological and cyclostratigraphic analyses to reconstruct the depositional environment and assess the influence of orbital forcing. The cyclostratigraphic analysis showed that depositional cycles (including fining-upwards and coarsening-upwards sequences) were controlled mainly by long eccentricity, forcing the expansion and contraction of the lake system at similar to 1.2 Myr time intervals. In addition, changes in the pollen-spore assemblage before and after the Eocene-Oligocene (E-O) transition show a dramatic shift from a moderately deep lake to a shallow lake with delta facies, as recorded by the Wenchang to Enping Formations in the Wenchang Sag, PRMB. These findings suggest that the climate deteriorated in response to global cooling. The amplification of obliquity forcing and the coevolution between obliquity power and global pCO(2) during the E-O transition suggest that the low-latitude sedimentary records from China contain a high-latitude signal. Furthermore, the low amplitudes of the precession, obliquity, and eccentricity cycles are coupled with the initial decreases in the global sea-level and pCO(2) and a positive bias in the carbon-oxygen isotopic compositions at similar to 34.3 Ma. These results suggest that orbital forcing played a role in elevating pCO(2) across a critical threshold during the E-O transition (similar to 33.9 Ma).

Based on numerous microfossils in Mesozoic of North Carnarvon Basin in Australia, the Mesozoic paleoclimate and paleoenvironment were analyzed, the paleogeographic background and development model of Mesozoic high-quality source rocks were investigated and discussed. Variation of microfossil species and their relative contents in Mesozoic different intervals indicates changes of paleoclimate and paleoenvironment. The paleoclimate shows a cycle variation of arid–humid–arid–humid, the regression occurred in the Early Triassic–Late Triassic and the transgression occurred in the Late Triassic–Late Cretaceous. Four sedimentary facieses including delta facies, littoral facies, restricted sea facies and open sea facies were developed in the Mesozoic. The open sea was mainly developed in the Early Triassic, the delta was distributed in the Middle–Late Triassic, the restricted sea was especially well developed in the Jurassic, and the restricted sea and open sea were mainly distributed in the Cretaceous. Characteristics of microfossil assemblage in these four sedimentary facies are dramatically different. From the delta facies, littoral facies, restricted sea facies to open sea facies, content of pollen gradually decreases, but content of the dinoflagellate + acritarch gradually increases. The delta facies and littoral facies are dominated by the pollen. In the restricted sea facies, content of the pollen is equivalent to that of the dinoflagellate + acritarch. The open sea facies is dominated by the dinoflagellate and acritarch. Supply of sediment and formation of organic matter are influenced by the paleoclimate and paleoenvironment, and type of organic matter is controlled by the microfossil assemblage. Based on the palaeogeographic background, paleoclimatic condition and microfossil assemblage, two developmental models of the Mesozoic source rocks such as the development model of terrestrial organic matter under the background of large delta and the development model of mixed organic matter under the background of the restricted sea, were proposed.

利用地层中包含的大量微体化石,结合沉积学分析,恢复了澳大利亚North Carnarvon盆地中生代的古气候与古环境,综合中生界优质烃源岩发育的古地理背景,探讨了其发育模式.研究结果表明,中生代不同层段微体化石属种及其相对含量的变化,对应于古气候与古环境的变迁;古气候呈干旱—湿润—千旱—湿润的旋回性变化;早三叠世—晚三叠世为水退过程,晚三叠世—晚白垩世整体为水进过程.中生界可识别出4个沉积相类型:三角洲相、滨海相、局限海相和开阔海相.早三叠世主要发育开阔海相,中—晚三叠世则主要发育三角洲相,侏罗纪局限海相尤为发育,白垩纪主要发育局限海相和开阔海相.4个沉积相的微体化石组合特征差异明显,三角洲—滨浅海—局限海—开阔海孢粉含量逐渐下降,沟鞭藻+疑源类含量逐渐升高;三角洲相与滨浅海相孢粉占主导地位,局限海相孢粉含量与沟鞭藻+疑源类含量相近,开阔海相则以沟鞭藻+疑源类占主导地位.古气候与古环境影响沉积物的供给及有机物的形成,微体化石组合决定了有机质的母质类型,两者最终决定了盆地有机质的发育模式,综合中生界烃源岩发育的古地理背景、古气候条件及微体化石组合特征,将其有机质发育模式总结为大型三角洲背景下陆源有机质发育模式和局限海背景下混合有机质发育模式.

Brittle minerals, especially quartz, play an important role in the quality of shale reservoir. The precipitation of authigenic quartz caused a significant change in the rock stiffness and porosity. However, the formation mechanism of authigenic quartz in shale is still unclear. Lower Cambrian Niutitang shale from Guchengcun outcrop has been examined with respect to quartz cement. Four different types of quartz have been identified using SEM/CL/EDS. Type 1 occurs as angular to sub-rounded silt-size quartz grains with uniform luminescence, indicative of detrital origin. Type 2 appears as relatively large quartz aggregates approximately 30–150 μm in size, which is composed of small quartz grains (1–3 μm) and abundant in siliceous shale. The petrological evidence and consideration of potential diagenetic process suggest that Type 2 quartz was formed by local re-crystallization of biogenic silica in syndiagenesis and preferentially precipitated in the axial cavities of sponge spicules. Type 3 occurs as quartz overgrowths that are essentially non-luminescent at the periphery of high luminescence detrital quartz grains. This type quartz is present in most investigated samples. Type 4 represents microcrystalline quartz embedded in the illitized clay matrix as discrete, short chains or small clusters/nests and they are most commonly observed in argillaceous shale. The petrological evidence and CL responses indicate that both Type 3 and Type 4 quartz are derived from precipitation of silica released during the smectite to illite dissolution–precipitation reaction. S-I transition can be driven by microbial metabolic processes in shallow buried diagenesis or by heat and pressure in late diagenesis. Some of Type 4 quartz may be originated from low-temperature microbial pathway in shallow buried diagenesis, while Type 3 quartz and the remaining Type 4 quartz were formed in late diagenesis. Siliceous shale contains less abundant detrital quartz than argillaceous shale, indicating that siliceous shale was deposited in deeper water and far from terrigenous input than argillaceous shale. Although identifying the exact cause for diagenetic silica deposition in shale is not a trivial matter, its recognition is important for the reconstruction of the depositional history of shale sequences and understanding the role of diagenesis in shale reservoirs development, moreover, it also has great potential for enhancing prediction of mechanical rock properties in shales.

A large-scale braided delta is developed in the late stage of the Enping Formation in the Baiyun sag,but the sedimen tary process,the internal development stages and the plane distribution of the delta remain unclear.Based on the nine wells which penetrated the third member of the Enping Formation,the detrital zircon U-Pb dating and the 2D and 3D seismic data analyses were carried out in this paper.The sediment supply patterns of the Enping Formation in different depositional periods were determined.During the first-second members of Enping Formation (SQ1-SQ2),sediments derived from the paleo-uplift were mainly composed of Mesozoic igneous rocks and are interpreted to have sourced via relatively short-distance transportation.During the third member of Enping Formation (SQ3),the sediments oriented from South China folded belt (SCFB) which mainly consists of Precambrian-Paleozoic metamorphic rocks.The sediments which are derived from SCFB and transported long-distance pathways through northwest-southeast orientation have deposited in Baiyun sag and the large-scale delta developed subsequently.Meanwhile,the sedimentary characteristics of large-scale delta of the SQ3 within Baiyun sag are described through typical seismic facies and the energy half-time seismic attribute analysis.The delta developed three stages of progradational wedges continuously to the southeast direction and with increasing scale initially,followed by decreasing scale until the rift basin disappeared.The results reveal that the deltaic coal-bearing and terrigenous-marine source rocks are mainly developed in the SQ3 and thus the large-scale delta has an excellent accumulation condition of source-reservoir-cap rock assemblages.And the delta front sand bodies tend to develop the updip pinch-out reservoir which will become an important target for oil and gas exploration in Paleogene of Baiyun deep-water region.

North Carnarvon盆地是澳大利亚最主要的产油气盆地,也是世界上主要的富气盆地之一.三叠纪North Carnarvon盆地属于克拉通边缘坳陷,地层厚度巨大且平面分布广.中三叠世晚期澳大利亚西北陆架发生强制性海退,造成North Carnarvon盆地沉积格局发生转变,沉积环境由滨浅海突变为海陆过渡环境,从而沉积了两套差异性明显的地层,即Locker shale和Mungaroo组.本文从宏观和微观、有机与无机入手,阐述了Locker shale和Mungaroo组的古地理背景、沉积充填演化及其相关的烃源岩与有机质显微组分发育特征.宏观上,Mungaroo组发育大型三角洲,中粗粒分支流水道砂岩与暗色泥岩频繁互层,其中近端三角洲平原泥岩厚度较薄,薄煤层只局部发育,陆源有机质含量较高,平均TOC含量为1.59％;远端三角洲平原泥岩厚度较大,薄煤层广泛发育,陆源有机质含量高,TOC含量最高,达4.11％;三角洲前缘泥岩厚度较大,陆源有机质含量低,TOC含量低,为1.05％.LockerShale以滨浅海相沉积为主,局部发育小型三角洲,其中开阔浅海亚相烃源岩厚度较大,前三角洲、滨浅海、开阔浅海三个亚相TOC含量相当,平均值为1.16％.微观上,Mungaroo组由近端平原亚相—远端平原亚相—前缘亚相壳质组含量逐渐增多,镜质组先增加后下降,惰质组先减少后增加;远端三角洲平原镜质组含量最高.Locker shale由前三角洲相—浅海相壳质组增加,镜质组和惰质组均减少.研究区三叠系烃源岩及有机质显微组分的分布规律与特提斯南缘特殊的古地理、季风洪水的古气候背景密切相关,Mungaroo组的远端三角洲平原为最有利的烃源岩发育环境.North Carnarvon盆地三叠系岩相古地理格局与沉积充填演化规律体现了温室气候效应下阵发性水流主导的沉积特色,为类似地质背景下油气资源评价提供了新的思路.

North Carnarvon basin is the main production in Australia petroleum basin, and it is also the one of main rich-gas basins. The Triassic strata are of big thickness and widely distributed, potential of nature gas resources are huge and the reservoir quality is good. The early Triassic, the entire northwest continental shelf for shallow sea covers. The basin deposits Locker shale. In the middle-late Triassic, the northwest continental shelf generally occurred regression retreat, the basin is the river delta facies in Mungaroo group, the lithologic features for thick layer of sandstone with thin layer of mudstone, and contain coal seam. The upper Triassic Mungaroo group is the main purpose of the exploration layer, it is also the main hydrocarbon source rocks of the development of the layer section, the reservoir and hydrocarbon source rock in a "sandwich" type, and sand development zone and the high quality hydrocarbon source rocks distribution are folded. Throughing the sedimentary characteristics and spatial distribution analysis of the organic matter in Mungaroo group, the rule of the Triassic organic matter space distribution has been made clear, and the Triassic terrestrial organic matter hydrocarbon source rocks of the development of the pattern has been put forward, for Australia North Carnarvon basin oil and gas geologic research and offshore oil and gas field development similarly in China provide geologic foundation.