Lower Triassic Inorganic Carbon Isotope Excursion in Chaohu, Anhui Province, China

This paper reports a Lower Triassic inorganic carbon isotope profile from the North Pingdingshan Section in Chaohu, Anhui Province, China, which was situated in a deep part of the Lower Yangtze carbonate ramp. The δ ( 13 C) excursion shows two periods from the Permian Triassic boundary to the lower Spathian substage, corresponding to the ecosystem undergoing evolution and recovery after the end Permian mass extinction and related events. The first period starts at the δ ( 13 C) depletion caused by the mass extinction and evolves with a gradual δ ( 13 C) increase resulting from the development of some disaster taxa during the Induan. The strong Smithian δ ( 13 C) depletion in the second period might be formed by the collapse of the disaster ecosystem and the biotic recovery occurred with the explosive increase of bioproductivity in the Spathian. Thus the δ ( 13 C) excursion in the Lower Triassic expresses patterns of biotic evolution and recovery during the erratic ecosystem that followed the great end Permian mass extinction.

Lower Triassic and Induan-Olenekian Boundary in Chaohu,Anhui Province,South China

Since the West Pingdingshan Section in Chaohu was proposed as the candidate of the Global Stratotype Section and Point of the Induan-Olenekian boundary in 2003, the Lower Triassic of Chaohu has been extensively studied. Based on the studies on the Lower Triassic of Chaohu, （1） a continuous conodont zonation is established, which has become an important reference for Lower Triassic stratigraphic correlation over the world; （2） the First Appearance Datum of conodont Neospathodus waageni was suggested and has been basically accepted as the primary marker to define the InduanOienekian boundary; （3） a characteristic Lower Triassic excursion of carbon isotopes was brought to light and has been proven to be not only an excellent index for the stratigraphic correlation but also a unique indication for the perturbation of ecological environments in the aftermath of the end-Permian mass extinction; （4） a magnetostratigraphic sequence is constituted with a certain biostratigraphic control in the low-latitude region and it presents an important correlation to the Boreal sequence; （5） a cyclostratigraphic study provides an alternative method to constrain the age of the chronostratigraphic units; and （6） a scheme of the Olenekian subdivision is recently suggested to define the boundary between the Smithian and Spathian Substages. In addition, Chaohu is also the type locality of the Chaohuan Stage, the upper stage of the Lower Triassic in the China Chronostratigraphic System.Thus, the Lower Triassic of Chaohu is not only a classic sequence in South China, but also a key reference sequence to the investigation of the corresponding stratigraphy and geological events over the world. The recent achievements are viewed here for an overall understanding of the sequence. Then the current situation of the Induan-Olenekian and Smithian-Spathian boundaries is discussed to provide a reference for later works.

Comparison of basic features and origins of oolitic shoal reservoirs between carbonate platform interior and platform margin locations in the Lower Triassic Feixianguan Formation of the Sichuan Basin,southwest China

The oolitic shoal reservoirs of the Lower Triassic Feixianguan Formation carbonates in the Sichuan Basin of southwest China are an important target for gas exploration in the basin.Their occurrence,like other cases worldwide,can be divided into two locations in general,i.e.,platform interior and platform margin locations.Their differences of reservoir features and origins,however,have not been investigated comprehensively due to different exploration degrees.This issue is addressed in this paper,to provide basic data and information for the basin＇s hydrocarbon exploration and for the study of carbonate platform sedimentology and reservoir geology worldwide.We compared the features of these two types of reservoirs in detail,including the depositional and diagenetic features,pore types and petrophysical features.Based on the comparison,the origin of the reservoirs was further discussed.It is shown that the reservoirs in platform interior and platform margin locations differ significantly.The interior carbonates were deposited in moderate to high energy settings and the dominant lithologic type was limestone,which was weakly compacted and intensely cemented and has undergone meteoric dissolution.Pore types include intragranular dissolution and moldic pores,with low porosities（6%） and low permeabilities（0.1 mD）.By contrast,the platform margin carbonates were deposited in relatively high energy settings and mainly consisted of dolostones with some limestones.The rocks were strongly compacted but incompletely cemented.As a result,some primary intergranular pores were preserved.Both meteoric solution and burial solution have taken place.There are various types of pore spaces including intergranular and intercrystalline solution pores and residual intergranular pores.This type of reservoir generally has better petrophysical properties（9% porosity and 0.1 mD permeability） and pore-throat structures than the interior reservoirs.These differences were influenced by both primary depositional features and secondary diagenesis.For the interior carbonate reservoirs,early meteoric dissolution,weak compaction and strong cementation are important controlling factors.By contrast,the factors controlling the formation of the margin carbonate reservoirs mainly include dolomitization,preservation of primary pores and burial dissolution.

Formation mechanism of reservoir oolitic dolomite in Lower Triassic Feixianguan formation, northeastern Sichuan Basin, southwest China

A series of marine natural gas fields were recently discovered in oolitic dolomites of the Lower Triassic Feixianguan formation, northeastern Sichuan Basin, southwest China. The mechanism forming these reservoir dolomites is debatable, limiting the ability to characterize these reservoir successfully. Based on the investigation of the representative Dukouhe, Luojiazhai, and Puguang areas, this issue was addressed by examining the distribution, petrology, and geochemistry of the dolomites, the most comprehensive study to date was provided. Dolomitization occurred at a very early stage of diagenesis, as shown by the petrological features of the rock fabric. Vadose silt, which is composed primarily of dolomitic clasts, is found in the primary and secondary pores of the oolitic dolomite. This indicates that the overlying strata were subjected to dolomitization when the Feixianguan formation was located in the vadose zone. Therefore, it may be inferred that the dolomitization which occurred before the formation was exposed to meteoric conditions. The spatial distribution and geochemical characteristics of the dolomite indicate that dolomitization occurred as a result of seepage reflux. The degree of dolomitization decreases with increasing distance from the evaporative lagoon. Furthermore, the type and porosity of the dolomite vary in different zones of the upward-shoaling sequence, with the porosity gradually decreasing from the highest layer to the lowest layer. This reflects a close relationship between dolomitization and seawater evaporation during the formation of the dolomite. Geochemical analysis provided further evidence for the relationship between the dolomitization fluid and the coeval seawater. The 87Sr/86Sr and 813C isotopes, as well as the abundances of trace elements, Fe and Mn, indicate that seawater concentrated by evaporation acted as the dolomitization fluid. These results also show that dolomitization most likely occurred in a semi-closed diagenetic environment. Therefore, the main mechanism of oolitic dolomite formation is seepage reflux, which occurred at an early stage of diagenesis.

Sedimentary system characteristics and depositional filling model of Upper Permian——Lower Triassic in South Yellow Sea Basin

In the South Yellow Sea Basin,Mesozoic–Paleozoic marine strata are generally well developed with large thickness,and no substantial breakthroughs have been made in hydrocarbon exploration.Through research,it is believed that the Upper Permian–Lower Triassic can be regarded as a long-term base-level cycle.Based on drilling data,characteristics of the lithology–electric property combination cyclicity,and the special lithology,the long-term base-level cycle was divided into five medium-term base-level cycles(MC1–MC5).On this basis,the Permian–Triassic sedimentary systems and their filling model were analyzed in accordance with the change of base-level cycle and transition of sedimentary environment,as well as characteristics of the drilling sedimentary facies and seismic facies.The results show that there were six sedimentary systems(fluvial,delta,tidal flat,open platform,restricted platform,and continental shelf)developed in the Upper Permian–Lower Triassic,the sedimentary systems were distributed such that the water was deep in the northwest and shallow in the southeast,and there were two base-level cycle filling models(a relatively stable tidal flat facies and a rapidly transgressive continental shelf facies to stable platform facies)developed in the Upper Permian–Lower Triassic.These models can provide a basis for evaluation of the Mesozoic–Paleozoic hydrocarbon geology in the South Yellow Sea Basin.

Paleogeographical Distribution and Stratigraphical Range of the Lower Triassic Claraia,Pseudoclaraia and Eumorphotis(Bivalvia)

On the basis of a worldwide review of Claraia , Pseudoclaraia and Eumorphotis , especially their important species , two range zones are recognized:the Pseudoclaraia wangi range zone —— Upper Griesbachian——and the Claraia stachei- C. aurita - Eumorphotis multiformis acme zone——Upper Griesbachian to Lower Smithian This result revises both the tradition-al Chinese concept regarding these fossils as limited in Lower Scythian and the viewpoint of some Eurasian researchers who hold Eumorphotis multiformis to be Upper Scythian .The paleogeographical distribution of the three genera is controlled primarily by temperature (latitude) and secondarily by physiographic separations .Distribution of these fossils is subdivided into four realms and five provinces . The Eastern Asia province of the Tethys realm is suggested as the source area of these fossils .The striking contrast between the composition of the species along the southern and northern margins of the Tethys , together with other distributional characters ,supports the plate tectonic theory .

Porosity Formation and Evolution of the Deeply Buried Lower Triassic Feixianguan Formation, Puguang Gas Field, NE Sichuan Basin, China

The deeply buried shoal dolomite reservoirs of the Lower Triassic Feixianguan Formation, giant Puguang Gas Field, NE Sichuan Basin, are exceptionally porous. The influences of diageneses on pore evolution are studied. Through petrologic investigation, diagenetic phases are divided into four stages, i.e., near-surface, pre-oil window, oil window, and gas window. The Adobe Photoshop system is used to quantify the rock texture components, porosity constitutions and the influences of diageneses on reservoir porosity. Porosity evolution curves are quantitatively recovered. The Feixianguan reservoir porosities are mainly created by early meteoric dissolution, dolomitization, and organic acids dissolution. Dissolution during deep burial is insignificant. Pores are formed in near-surface and pre-oil window stages and effectively preserved till present. This result may be of great significance to the further exploration of deeply buried carbonate reservoirs not only in NE Sichuan Basin, but also around the world.

A Taxonomic Re-Assessment of the Novispathodus waageni Group and Its Role in Defining the Base of the Olenekian (Lower Triassic)

Lower Triassic conodont biostratigraphy has been well studied around the world in the past decades, but the Induan-Olenekian boundary （IOB） remains undecided. The Novispathodus waageni group has been taxonomically re-assessed based on abundant new materials from the Jianshi and Chaohu sections, South China. New study shows that Nv. waageni typically possesses： （1） an approximately equi-dimentional P1 blade element, （2） an accurate upper profile with denticle height descending in both directions, （3） a denticulated posterior edge （lower denticles posterior of the highest denticle）, and （4） a round basal cavity outline. Of the three proposed subspecies of the waageni species, both Nv. waageni waageni （Sweet, 1970） and Nv. w. eowaageni （Zhao and Orchard, 2005） are valid, and the former differs clearly from Nv. w. eowaageni in having （1） a slightly higher length/height ratio （holotype=1.30 ： 1.23）, （2） a thicker blade, sometimes with medial thickening, （3） fewer （broader） denticles per unit length, （4） generally recurved denticles, not straight and upright, （5） highest denticles closer to posterior, （6） common differentiation of a posterior cusp, and （7） more sinuous basal profile, with increased posterior upturning. A third subspecies illustrated as Nv. waageni n. subsp. A sensu Goudemand, 2014 is not conspecific with older individuals of Nv. w. eowaageni, and also cannot be as- signed to the Nv. waageni group. Abundant new materials demonstrate a clear ontogenic process for Nv. w. eowaageni, indicating that Nv. w. eowaageni occurring in the Induan-Olenekian boundary （IOB） succession is rather stable. Small, earlier individuals （i.e., those from Bed 225 in Jianshi） are referred to as Nv. w. eowaageni Morphotype A, and are thought to have likely evolved from Ns. dieneri Morphotype 3, and to be the precursor of mature elements ofNv. w. eowaageni. The first appearance datum of Nv. w. eowaageni therefore is an ideal mark defining the IOB.

Proposed new Lower Triassic stages in South China

As the global Lower Triassic stages are not properly defined, a Chinese scheme of the Lower Triassic subdivision was recently proposed to meet the insistent need of the general investigation in China. The Lower Triassic sequences of South China are briefly summarized and the twofold Lower Triassic subdivision is discussed. The type sections of the two Chinese Lower Triassic stages, Yinkengian and Chaohuan, and their definitions are stated.

Characteristics and origin of a new type of polyhalite potassium ore in the Lower Triassic Jialingjiang Formation, Puguang area, northeastern Sichuan Basin, SW China

A new type of polyhalite potassium ore(NTPPO) was found in the Lower Triassic Jialingjiang Formation, NE Sichuan Basin, SW China. It is water soluble, therefore can be exploited using the water-solution method, and is of great potential of economic value and research significance. Based on cores, thin sections, energy spectrum and SEM analyses, its microfeatures, macrofeatures and origin are discussed, and a genetic model is established to provide a scientific basis for future evaluation, prediction and exploration of potassium ore in the Sichuan Basin. It is proposed that the NTPPO was caused by storm activities:(1) the storm broke the original sedimentary polyhalite–gypsum beds, whose fragments were transported into the salt basin with high content of K+ and Mg2+;(2) in the basin, the polyhalite continued to be formed from gypsum by metasomatism with K-and Mg-rich brine;(3) during diagenesis, under high temperature and high pressure, K–Mg-rich brine from halite continued to replace anhydrite(or gypsum) to form polyhalite.

Recovery of Brachiopod and Ammonoid Faunas Following the End-Permian Crisis:Additional Evidence from the Lower Triassic of the Russian Far East and Kazakhstan

After the End-Permian mass extinction, ammonoids reached levels of taxonomic diver- sity higher than in the Changhsingian by the Dienerian Substage of the Induan. However, brachiopods exhibit a prolonged delay in recovery, and their taxonomic diversity had not recovered to Late Permian levels even by the Olenekian. The differential patterns of recovery between these two clades may reflect fundamental differences in physiology and behavior. Brachiopods were benthic organisms that were dependent on specific trophic sources, and their general reduction in size during the Early Triassic may have been a response to a relative paucity of food resources. In contrast, ammonoids were sluggish- nektic organisms that utilized a wider range of trophic sources and that suffered no comparable size decrease during the Early Triassic. Brachiopods may have been at a disadvantage also due to vulner- abilities associated with their larval stage, during which they had to locate a suitable substrate for set- tlement. In contrast, ammonoids had no larval stage and juveniles may have been dispersed widely into favorable habitats. These factors may account for differences in the relative success of ammonoids and brachiopods at high-latitude regions following the End-Permian mass extinction： ammonoids success- fully reeolonized the Boreal region during the Early Triassic whereas brachiopods did not.

Carbon isotope composition of the Lower Triassic marine carbonates, Lower Yangtze Region, South China

Studies on three Lower Triassic sections located on the shallow water platform, the deep water slope and in the deep water basin in the Lower Yangtze Region, South China, show the similar trend of carbon isotope evolution. Biostratigraphic correlations among the Lower Triassic sections on the basis of standard conodont zones indicate that three negative shifts occurred in the Griesbachian, the Smithian and the late Spathian stages respectively, and one distinctly positive shift occurred in the early Spathian stage. Trend of carbon isotope evolution of the Lower Triassic reflects some significant changes in the global carbon cycle. Moreover, δ 13C background values are intensively controlled by palaeogeographic environment. In general, δ 13C values from deep-water slope carbonates are lighter than those from carbonate platform and heavier than those from deep-water basin carbonates. The positive carbon isotope excursion may be induced by a significant amount of organic carbon burial in marine sediments and increase in primary productivity. The large negative carbon isotope excursions during the Early Triassic in Lower Yangtze Region are interpreted to relate to volcano eruptions based on tuffaceous claystone interlayers observed near the Permian-Triassic boundary, the Induan- Olenekian boundary and the Lower Triassic-Middle Triassic boundary.

First record of Lower Triassic Undichna spp.fish swimming traces from Emei,Sichuan Province,China

我们从记录从的 ichnospecies Undichna unisulca 讨论踪迹三叠纪那在这份报纸第一次被报导。这些石块也是一些迄今为止在中国发现的最旧的脊椎动物 ichnites。游泳踪迹(Undichna unisulca ) 描述了的鱼发源从更低三叠纪。这些踪迹在 Jialingjiang 形成被发现并且在峡被定位从 Emei 的 7.5 km，四川省，中国。ichnospecies Undichna unisulca 由一个常规正弦曲线波浪组成。波长和振幅在分别地在 2832 公里和 912 公里之间变化的每个标本之间是相当不变的。当 hypichnial 在下面隆起，游泳跟踪的鱼和在一样的床以内的分发被保存。

Study of the Early and Middle Triassic Lower Yangtze Sea Basin

The Early and Middle Triassic primary lower Yangtze sea basin was formed before the Yangtze and Sino. Korean blocks collided and were assembled. showing the characteristics of an open continental shelf.continental margin sea. In order to provide evidence useful for oil and gas exploration in the studied region, this paper centres on the features of the sediments and their facies framework in the basin and the sedimentation parameters such as the deposition rate, palaeotemperature, palaeosatinity, palaeodepth of water and palaeocurrents of the basin.

Sedimentary evolution models of Lower Triassic deep-water carbonate rocks of west Qinling Mts.

Based on the study of Lower Triassic deep-water carbonate rock sequence of west Qinling Mts . deep-water carbonate rock sequence is divided into lower fine-grain euxinic deposits of a carbonate gentle slope type and upper bathyal and abyssal sediments of carbonate steep slope type. The upper member is emphatically analysed and synthesized into five fades associations. They comprise four fining- and thinning-upward megacycles, each of them representing a sedimentary column which accumulated after a tensional fault subsidence event, which recorded a whole rifting process of west Qinling ocean trough during Lower Triassic.

Advance in the study of the Lower Triassic conodonts at Meishan Section, Changxing, Zhejiang Province

Meishan Section in Changxing County of Zhejiang Province is one of the four candidates of the global stratotype section and point (GSSP) of the Permian and Triassic boundary (PTB), and currently the conodonts have been widely accepted as the index fossils to define the Permian and Triassic boundary. The previous studies on the Lower Triassic conodonts at Meishan Section are summarized and the recent discovery of the Lower Triassic conodonts there is reported. The recognized data from the section indicate that Meishan is the best selection of the GSSP of PTB.

The relationship between dolomite textures and their formation temperature: a case study from the Permian-Triassic of the Sichuan Basin and the Lower Paleozoic of the Tarim Basin

Study of dolomite texture can contribute to understanding the process of dolomitization.This research reports textures and homogenization temperatures of dolomites from the Permian-Triassic strata in the Sichuan Basin and the Lower Paleozoic strata in the Tarim Basin,which provided insights into relationships between dolomite textures and their formation temperatures.Our results are summarized as follows：1） dolomites with well-preserved texture indicate low dolomitization temperature.However,in certain diagenetic environments,the hydrothermal dolomitization may completely or partially preserve the original texture of dolomites.2） The formation temperatures of non-planar dolomites are always higher than those of planar dolomites.3） The formation temperatures of dolomite cements are generally higher than those of replacive dolomites.4） Although the formation temperatures of saddle dolomite cements have a wide range,they show higher values than those of the planar subhedral to euhedral dolomite cements.Thus,saddle dolomites could generally be an indicator of high precipitation temperature.5） The fluid Mg/Ca ratio is another element controlling dolomite morphology.Micritic dolomites,which precipitate from hypersaline fluids with a high Mg/Ca ratio in a subaerial environment could also have features of non-planar anhedral crystal shape because of rapid nucleation and crystallization during dolomitization.

A New Triassic Primitive Ichthyosaur from Yuanan, South China

Based on the systematic study of two fossil skeleton specimens collected from the top of the third member of the marine Lower Triassic Jialingjiang Formation of Yuanan, Hubei Province, South China, a new Early Triassic primitive ichthyosaur Chaohusaurus zhang]iawanensis sp. nov. is reported and described. The beds yielding the type material are correlated with the Neospathodus homeri-N, triangularis Conodont zone. The new taxon is most similar to Chaohusaurus geishanensis Young and Dong, 1972 in the shape and configuration of the scapula, forefin, pelvic girdle and hindfin, presacral vertebral count and well-developed caudal peak, but distinguished from the latter by its larger size, the position of the pineal foramen in the centre of the parietal, the occurrence of a larger calcaneum in hindfin and the first sacral rib with distal expansion. The new species exhibits common features of primitive ichthyosaurs such as： （1） the prefrontal has an antero-dorsal shelf projecting towards the orbit; （2） the upper temporal fenestra is small; （3） the postorbital and the squamosal meet laterally to the upper temporal fenestra; and （4） cylindrical centra. However, more derived ichthyosaur characters are seen with the frontal separated from the orbital dorsal margin by the contact of the prefrontal and postfrontal, which offer new clues to the early radiation of ichthyosaurs.

川东北宣汉地区海相“新型杂卤石钾盐矿”的发现、突破与前景

全球探明钾盐资源主体为古代海相可溶性固体钾盐矿床(氯化钾、光卤石),仅少量来自现代盐湖卤水钾矿。与国外不同的是,目前我国探明钾盐资源主体为现代陆相盐湖钾矿,古代海相可溶性固体钾盐找矿40余年,未获突破。四川盆地是我国主要大型海相成钾盆地之一,前人在中—下三叠统发现大量与硬石膏和白云石共伴生或互层的杂卤石,但绝大部分埋深超过2000 m,目前尚难以开发利用。2017年以来,笔者通过对四川盆地东北部宣汉盐盆普光地区卤水探井(恒成2、3井)岩芯复查,发现杂卤石主要与石盐共伴生,大量破碎的杂卤石碎屑颗粒分布于石盐基质中,KCl含量达边界工业品位以上,不同于钾石盐和光卤石,亦不同于前人发现的与硬石膏和白云石共伴生或互层的杂卤石(石膏型杂卤石),将之命名为“新型杂卤石钾盐矿”。小试和中试结果表明,该“新型杂卤石钾盐矿”可以采用对接井注水溶采,是能够得到规模化开发利用的“活矿”,是一种全新的硫酸盐+氯化物复合型海相可溶性固体钾盐矿床新类型。通过成钾条件、成矿规律综合研究,笔者提出了新型杂卤石钾盐矿“双控复合成矿”理论新认识、建立了“三高、两低、一大”综合测井识别新模型,创新理论技术指导设计部署“钾锂兼探”基准井——川宣地1井,探获厚29.46 m、氯化钾(KCl)平均含量12.03%(最高20.5%)的“新型杂卤石钾盐矿”厚层高品位工业矿层。与此同时,采用岩芯样品实测数据和测井解释数据拟合的方法,建立矿石K含量和密度参数的测井定量预测模型,结合“气钾兼探”,利用33口钻井数据,依据古代固体盐类勘查规范(2020),运用“几何法”,初步估算“新型杂卤石钾盐矿”氯化钾(KCl)资源规模:推断资源量2.45亿t(超大型)、潜在资源4.65亿t,合计7.1亿t,奠定了川东北达州市宣汉地区形成中国首个亿吨级海相钾盐基地的资源基础,取得了我国海相可溶性固体钾盐找矿的重大突破。

基于机器学习的低含油饱和度砂岩储层参数预测——以准噶尔盆地夏子街油田夏77井区下克拉玛依组为例

准噶尔盆地夏子街油田夏77井区块下克拉玛依组(简称克下组)特低孔特低渗油藏油水关系复杂、产量低、储层含水高,且具有低含油饱和度、孔渗相关性差、储层参数与测井响应关系不清晰、油水层识别困难等特征,常规储层参数评价及预测方法适用性差。通过对岩性、物性、含油性分析,明确了克下组储层岩性为砂砾岩、砂质砾岩,黏土矿物以伊蒙混层为主;储层为以原生粒间孔和残余粒间孔为主要储集空间的低孔隙度、特低渗透率储集层。通过建立含油饱和度解释模型,确定了本区油藏属于低饱和度油藏,含油饱和度一般为36%~55%。砂砾岩储层物性和含油性优于中细砂岩,储层物性控制含油性,呈现低饱和度特征,电性受含油性和岩性双重影响。通过低含油饱和度油藏形成机理研究,认为储层微观孔隙结构是形成低含油饱和度的主要原因。通过对敏感参数优选,基于自然伽马、电阻率和声波时差测井等资料,引入基于机器学习的BP神经网络技术,对夏子街油田夏77井区块克下组油藏进行了孔隙度、渗透率和含水饱和度的计算及预测,储层参数预测精度均高于80%,相关结论及方法可为低含油饱和度致密砂岩储层的物性参数预测提供依据和参考。