Early Highstand Systems Tract Research Articles

Links between anhydrite precipitation and dolomitization during cycles of sea level change: Insights from the Late Jurassic Arab Formation, Abu Dhabi, United Arab Emirates

Despite extensive research in the field of evaporative dolomitization, the close link between anhydrite precipitation and dolomitization is still poorly explored within the context of sequence stratigraphy. This comprehensive diagenetic and sequence stratigraphic study of the Late Jurassic Arab Formation, Abu Dhabi, United Arab Emirates, provides important insights into the relationship between anhydrite precipitation and dolomitization within the 2nd, 3rd, and 4th orders of relative sea-level cycles. Initial good connectivity between the inner ramp and open sea during 2nd order late transgressive-early regression cycles accounts for the limestone-dominated lithology with limited anhydrite formation. Conversely, the following progressive 2nd order fall in the relative periodic restriction of the inner platform during 2nd order early regressive cycles (early highstand systems tracts, HST) caused seepage reflux dolomitization and the precipitation of scattered to pervasive anhydrite cement in the dolostones. Variations in the extent of dolomitization were controlled by the permeability and reactivity of the precursor limestones, resulting in the formation of what is known as dolomite fingers. The most laterally extensive dolomitization during late 2nd order regression resulted in the formation of microcrystalline dolostones with nodular and chickenwire anhydrite by a combination of evaporative sabkha pumping and the seepage reflux of lagoon brines. The δ1³C and δ1⁸O of calcite and dolomite reveal the influence of degree of restriction of the inner ramp, and related extent of seawater evaporation. The lower limestone-dominated interval (3rd order HST) is characterized by lower δ1³C values (+2.0‰ to +2.5‰) owing to periodic restriction of seawater circulation, which resulted in oxidation of organic matter during aging of the seawater. This study approach provides important insights into the genetic links between interbedded platform limestones, dolostones, and anhydrites, and hence a better understanding and prediction of reservoir quality distribution and compartmentalization.

Relative Sea-Level Fluctuations during Rhuddanian–Aeronian Transition and Its Implication for Shale Gas Sweet Spot Forming: A Case Study of Luzhou Area in the Southern Sichuan Basin, SW China

Most scholars believe that transgression events can contribute positively to organic matter enrichment and shale gas sweet spot development, while whether or not regression events are conducive to shale gas sweet spot development remains to be further discussed. Variations in organic carbon content (TOC), and major and trace elements at the Rhuddanian–Aeronian stage in the Luzhou area, the southern margin of Sichuan Basin, were analyzed in this paper. We discuss differences in paleoenvironment organic matter enrichment and shale sweet spot development during transgression and regression. A transgressive system tract (TST) occurred during the early Rhuddanian stage, while early highstand system tracts (Ehst-1 and Ehst-2) occurred during the late Rhuddanian stage and Aeronian stage, and a late highstand system tract (LHST) developed during the late Aeronian stage. A rise in sea level during the TST in the upper Yangtze resulted in an anoxic environment, where a continuous upwelling current brought about a large number of nutrients in the seawater, significantly increasing paleoproductivity. Strong tectonic subsidence, weak chemical weathering, and a rising sea level together led to a low terrigenous debris supply in the catchment area. Therefore, paleoproductivity and redox conditions were the primary controlling factors of organic matter enrichment at the TST stage, with a clastic supply of secondary importance. With the advance of the Guangxi orogeny, the organic matter enrichment at the EHST-1, EHST-2, and LHST stages was mainly controlled by redox conditions and debris supply. A comparison of the key physical parameters and geochemical indicators of shale reservoirs from these four system tracts suggests that two shale sweet spot types (type I and II) were developed during the Rhuddanian–Aeronian stage, occurring in the TST and EHST-2 stages. High TOC and high microcrystalline quartz content are key to developing type I sweet spots, while enhanced anoxic conditions in the bottom water caused by ephemeral, small-scale sea level rises are the main determinant of class II sweet spots in the later EHST stage.

High-resolution sequence stratigraphy of clastic shelves IX: Methods for recognizing maximum flooding conditions in shallow-marine settings

Maximum flooding conditions in high-frequency sequences are associated with a condensed section, a generally cryptic maximum flooding surface, and with facies contacts marked by grain size changes, shell beds and bioturbation; i.e., the downlap surface and the local flooding surface. Conditions of maximum water depth usually occur above the condensed section, in the early highstand systems tract. Due to its importance in sequence stratigraphic analysis, the identification of the precise position of the maximum flooding surface, which separates the transgressive and highstand systems tracts, is critical. This is particularly true in high-resolution studies based on field and core data, where only facies contacts that are older and younger than the maximum flooding surface are recognizable. An integration of several criteria, including sedimentological (facies analysis), diagenetic, micropaleontological, geophysical and geochemical, represents the best approach to define a relatively thin uncertainty interval within high-frequency sequences, in which the cryptic MFS should lie. Among the available criteria, those sedimentological and diagenetic are the ones best suited to recognize facies contacts, whereas those micropaleontological are the most reliable ones to identify the position of the maximum flooding surface. Future studies on the integration of these methods with those geophysical and geochemical have the potential to improve the ability to identify the maximum flooding surface, and improve the available tools in the high-resolution sequence stratigraphic analysis.

Depositional-diagenetic process and their implications for pore development of Wufeng-Longmaxi shales in the Jiangdong block, Fuling shale gas field, SW China

Depositional environment and diagenesis are important factors affecting pore characteristics, whereas how differential depositional and diagenetic processes affect pore development and preservation in shale remains controversial. In this study, based on geochemical analysis, low-temperature gas (including N2 and CO2) physisorption, core observation, thin section and field emission-scanning electron microscopy, the influences of various paleoenvironmental conditions and diagenetic pathways on pore development of the Wufeng and Longmaxi Formations are investigated. The results indicate that paleoenvironmental conditions determine primary mineral composition and content of organic matter, resulting in the various shale lithofacies. Subsequently, minerals in different shale lithofacies experienced various diagenetic events and evolution pathways, finally affecting pore generation and evaluation. Three system tracts, including transgressive systems tract (TST), early highstand systems tract (EHST) and late highstand systems tract (LHST) are identified in the Wufeng and Longmaxi Formations. Siliceous shale, mainly deposited in the TST, contains large amounts of biogenic authigenic quartz and abundant organic matter. Authigenic quartz aggregates effectively inhibit compaction and protect abundant pores. The contents of extrabasinal components increase during the EHST, while biogenic quartz and organic matter decrease to mainly develop mixed shale. Insufficient microcrystalline authentic quartz and high content of clay minerals lead to higher degree of compaction in mixed shale than siliceous shale, resulting in fewer residual pores. In contrast, clay minerals in clay-rich shale, deposited in the LHST, undergo strong compaction and illitization of smectite during diagenesis, resulting in strong deformation and loss of primary pores. As a result, higher amount of OM pores is generated and preserved in the organic-rich siliceous shales from the bottom of Wufeng and Longmaxi Formation, and gradually decreases in the mixed shale, while clay-rich shale in the upper section of the Longmaxi has the lowest amount of pore volume.

Terminal Ediacaran microbialite lithofacies associations with paleo-environmental constraints in a high-frequency sequence stratigraphic framework of Sichuan Basin, SW China

The terminal Ediacaran shallow marine microbialites are significant for paleo-environment reconstruction and petroleum exploration. Their depositional and paleo-environmental characteristics during high-frequency sea-level fluctuations were paid less attention, which limited the understanding of their evolution and related reservoir characterization. Microbialites from the fourth member of the upper Ediacaran Dengying Formation, Gaoshiti-Moxi area, Sichuan Basin, SW China are important records for studying their lithofacies associations and paleo-environmental constraints in high-frequency depositional sequences. Petrographic and geochemical analyses (in-situ major and trace elements and carbon and oxygen isotopes) indicate microbialites depositional and paleo-environmental evolution on a rimmed platform within a fourth-order depositional sequence. From late TST (transgressive systems tract) to early HST (highstand systems tract), thick-bedded dark-grey dolomudstone-thin-bedded spotted thrombolite dominates the platform margin; while thick-bedded dark-grey dolomudstone-thin bedded dark-grey wavy stromatolite dominates the platform interior. From late HST to early TST of the next fourth-order sequence, the platform margin is dominated by thick-bedded spotted thrombolite—dolograinstone, while the carbonate interior is dominated by thin-medium-bedded layered thrombolite—light-grey wavy stromatolite. Salinity correlates well with microbialite lithofacies associations: the platform interior layered thrombolite and wavy stromatolite enriched lithofacies associations are characterized by higher salinity than that of the platform margin spotted thrombolite enriched lithofacies associations. The microbialite lithofacies associations are less constrained by redox: platform interior layered thrombolite and wavy stromatolite enriched lithofacies associations are slightly more reduced than the platform interior spotted thrombolite enriched lithofacies associations. The redox analysis of the microbialites also indicates low oxygen levels of shallow marine in the study area during terminal Ediacaran, with possible oxygenation at the Ediacaran-Cambrian boundary. The study suggests that the terminal Ediacaran microbialite development was controlled by both high-frequency sea-level fluctuations and paleo-environmental factors such as salinity and redox. Platform margin spotted thrombolitic reefs from late fourth-order HST to next early fourth-order TST serve as favorable microbialite reservoirs. Platform interior layered thrombolites and wavy stromatolites are regarded as potential reservoirs, which deserve more detailed depositional-paleo-environmental research.

Multiphase dolomitization mechanisms of the Cambrian upper Changping Formation, North China Platform, China

The Cambrian dolomite reservoirs of the North China Platform offer good exploration prospects, but their occurrence and the impacts of dolomitization on reservoir quality are not yet clearly understood. The study herein assesses outcrop samples in the Cambrian upper Changping Formation, Dingjiatan area, and highlights the important role of multiphase dolomitization in the formation of paleo-reservoirs of acceptable porosity, where oil pools and fields may be discovered. A combination of petrology, fluid inclusion microthermometry, isotopes, and trace element compositions has been used to explain multiphase dolomitization mechanisms and their impacts on dolomite reservoirs. Five types of dolomites are identified through outcrop observation, thin section identification via transmitted light, and cathodoluminescence. The following geochemical analyses reveal various dolomitization mechanisms. In the (pene) contemporaneous stage, microbial dolomite is commonly related to microbial metabolic activities with significant carbon isotopic depletion compared to the Cambrian seawater values. With the influence of microbial dolomitization, dolomicrite corresponds to the sabkha dolomitization mode in a low-salinity seawater environment and early-stage dolomitization. The structureless dolomite (ssD) in the early highstand systems tract (EHST), characterized by elevated 87Sr/88Sr ratios and low oxygen isotopic values, forms from stratum brine water in the shallow-middle burial stage rather than in the (pene) contemporaneous stage. In contrast, ssD in the late highstand systems tract (LHST) undergoes (pene) contemporaneous dolomitization at salinities between 6% and 28% before later pore water transformation, with the participation of atmospheric freshwater through faults and unconformities exhibiting the lowest inclusion temperature and salinity values. The medium to coarse crystalline dolomite (MCD) in the LHST and the saddle dolomite (SD) in the EHST with low REE values are atypical hydrothermal dolomites caused by a combined superposition of middle-deep burial hydrothermal fluids at temperatures >150°C and stratigraphic brines. The MCD is also influenced by terrigenous water characterized by relatively low Eu anomaly values. Finally, the mechanisms of porosity increase are investigated, and it is concluded that the pore increase caused by the (pene) contemporaneous reflux interaction and the later pore retention both lead to better ssD reservoirs in the LHST than in the EHST.

Lithofacies Characteristics, Depositional Environment and Sequence Stratigraphic Framework in the Saline Lacustrine Basin-A Case Study of the Eocene Low Member of Xingouzui Formation, Jianghan Basin, China

The Jianghan Basin is a lacustrine basin in central China developing multiple salt bearing deposits in the Eocene with the success of shale oil exploration in the Qianjiang Formation. The lower member of the Xingouzui Formation in the Chentuokou Depression has become another exploration target. However, rapid changes in lithofacies and strong sedimentary heterogeneity limit the exploration progress. This study aimed to explore the sequence division, lithofacies characteristics, and sedimentary environment using sedimentological, X-ray fluorescence (XRF) and X-ray diffraction (XRD) analyses. The sequence stratigraphic analysis indicates that the low member of the Xingouzui Formation is divided into two third-order sequences, namely SQ1 and SQ2, and four system tracts, including highstand systems tract (HST), lake expanding system tract (EST), early highstand system tract (EHST), and late highstand system tract (LHST). Moreover, a total of nine major lithofacies and five lithofacies associations (LA1-5) were identified. The organic geochemical data show that the laminated argillaceous dolomite in EST and EHST developed the best oil content with an average TOC of 1.18% and S1 of 3.18 mg/g, The laminated argillaceous dolomite deposited in anoxic conditions with a humid climate, moderate salinity, and stratified deep waterbody is a favorable exploration facies for shale oil.

The heterogeneity of lithofacies types, combination modes, and sedimentary model of lacustrine shale restricted by high‐frequency sequence

The second Member of the Funing Formation in the Haian Sag of the Subei Basin is a set of organic‐rich fine‐grained sedimentary rocks, which is a key interval for shale oil exploration. Under the premise of establishing a high‐frequency sequence framework, the fine description of the lithofacies change characteristics of the continental fine‐grained sedimentary rocks is the basis for the prediction of shale oil favourable areas and the optimization of the production horizons. This study is established through wavelet transformation of the natural gamma curve of the high‐frequency sequence framework of the second Member of the Funing Formation, using thin‐section observation, total organic carbon analysis, and X‐ray diffraction experiments to study the lithofacies types and characteristics of fine‐grained sedimentary rocks, and discuss the combination of lithofacies under the high‐frequency sequence framework Mode and influencing factors. The results show that the second Member of Funing Formation is composed of three‐level sequence of low‐level system tract (LST), transgressive system tract (TST), and early high‐level system tract (EHST), corresponding to 6 fourth‐level sequences and 10 fifth‐level sequences. According to the division scheme of “abundance of organic matter + mineral composition + sedimentary structure”, it is found that 12 types of lithofacies were developed in the fine‐grained sedimentary rocks of the second Member of Funing Formation. The lithofacies type is affected by the palaeoenvironment. The organic matter facies are mainly controlled by palaeo‐water depth and palaeo‐redox conditions, the components are mainly controlled by palaeo‐water depth and palaeo‐salinity, and the tectonic facies are mainly controlled by palaeo‐redox and palaeo‐salinity. The lithofacies are regularly superimposed in the high‐frequency sequence framework, and three combination modes of A, B, and C are developed. Among them, the type A lithofacies are developed in the low‐stand system tract, and the type B lithofacies are developed in the transgressive system. The C‐type lithofacies assemblage developed near the largest flooding surface and the early highstand system tract, and combined with palaeoenvironment analysis to establish a lithofacies sedimentary model.

Analysis of Hybrid Facies, Hybrid Sequence, and Its Controlling Factors of the Devonian Yangmaba Formation in the Northwestern Sichuan Basin, Southern China.

Abundant hybrid sediments composed of clastic rocks and carbonate rocks were deposited in the Devonian Yangmaba Formation in the northwestern of Sichuan Basin. Based on the measurement of the Ganxi section in detail, combined with the observation of the 30 slices, the hybrid facies, and hybrid sequence, the hybrid deposit mechanism and its controlling factors of the Yangmaba Formation were analyzed. It shows that the hybrid facies consists of the hybrid shore at the lower and clastic hybrid shelf and carbonate hybrid shelf deposits at the middle-upper of the Yangmaba Formation. The hybrid sequence, which can be divided into four sedimentary system tracts: the shelf-margin systems tract (SMST), the transgressive system tract (TST), the early highstand systems tract (EHST), and the late highstand systems tract (LHST), was developed in the Yangmaba Formation. There are three hybrid mechanisms including punctuated mixing, facies mixing, and in situ mixing, and the first two are the main types in the Yangmaba Formation. The punctuated mixing and in situ mixing are the main hybrid mechanisms of hybrid shore and clastic hybrid shelf deposits, and the facies mixing is the main hybrid mechanism of clastic hybrid shelf and carbonate hybrid shelf deposits. The hybrid mechanisms are different among the system tracts: the punctuated mixing is the main mixing manner in the SMST and LHST and the facies mixing developed in the TST and EHST. Storm action is the most important controlling factor of punctuated mixing of the Yangmaba Formation. Relative sea level change, carbonate productivity or the rate of terrestrial clastic supply, and climate change are factors that control and affect hybrid deposits. SMST and HST are mainly related to changes in the relative sea level, while TST is controlled by sedimentary source recharge, and climate change affects the hybrid action of system tracts to varying degrees.

Sediment gravity-flow deposits in Late Cretaceous Songliao postrift downwarped lacustrine basin, northeastern China

The Songliao Basin is a large Cretaceous nonmarine petroliferous postrift downwarped basin in northeastern China. Deep lake channel-levee-lobe and slope fan deposits are identified within six depositional sequences that span from the Turonian to early Coniacian upper part of the fourth member of the Quantou Formation and Qingshankou Formation to the early Campanian second member of the Nenjiang Formation. Research on the sediment gravity-flow systems in the Songliao Basin has broad application for understanding the sedimentary processes and pattern of sediment gravity-flow systems, and predicting the presence of good sandstone reservoirs within postrift downwarped lacustrine basins. Research results show that the channel-levee-lobe deposits are present within sequences in the first member of the Qinshankou Formation and the first member of the Nenjiang Formation. The straight channel levees and slope fans were on high-gradient slopes, whereas the sinuous channel levees were on low-gradient slopes. The straight-to-sinuous channel levees, fed by northeastern deltaic subaqueous distributary channels, extend for 15–70 km, predominantly within transgressive-to early highstand system tracts, which terminated in bifurcated or small lobes. The channel-levee-lobe and slope fan deposits sourced from western deltas were deposited basinward of the syndepositional flexural slope-break zones controlled by deep-seated faults. The falling trajectories of the paleoshorelines during the highstand and lowstand systems tracts and large sediment fluxes during the transgressive systems tracts resulted in the development of the slope fans and channel-levee-lobes. Lakebed paleorelief influenced the channel forms. Sand bodies within the channel-levee-lobes and slope fans, encased within organic-rich source rocks, represent new significant targets for hydrocarbon exploration.

Continental input on shelfal deposits unveiled by integration of ichnology, sedimentology, and taphonomy: A case study in Givetian beds of the Paraná basin, Brazil

The Devonian succession in Paraná Basin encompasses marine deposits bearing fossils representative of the Malvinokaffric Realm, corroborated by sedimentological and ichnological data. Although the trace fossil assemblage preserved in these beds is archetypal of fully marine conditions, signs of environmental stressing conditions can be observed locally in the Givetian beds. This study aims to provide an integrated ichnological, sedimentological, and taphonomic analysis to test the hypothesis of a continental influence in the shallow marine environmental settings that sheltered the Malvinokaffric Realm in the Paraná Basin during the Lower Givetian. The integration among seven lithofacies, nine ichnofabrics, and three taphofacies allowed identifying a continental influence associated with salinity variations in a prograding section of an early highstand systems tract, above the maximum flooding surface (Kačák Evet). The stress apparently caused by salinity fluctuations, as indicated by the ichnological signatures, and the high organic input in the Itáytyba outcrop succession may be related to freshwater flows that were reworked by longshore currents. The results presented herein support the hypothesis that continental influxes along shoreline fed by rivers locally generated shoreline progradation and affected the Paraná Basin shelfal settings during the Lower Givetian. It contributed to reducing the benthic diversity in the Malvinokaffric Realm, as normally expressed in the basal intervals of Devonian beds from Paraná Basin.

Sequence stratigraphy study of the Dariyan Formation in the Gadvan and Zana sections in the northeast and northwest of Shiraz, Zagros Basin, SW Iran

In the present paper, sequence stratigraphy and microfacies of the Dariyan Formation in the two sections of the Gadvan and Zana Mountains located were studied in NE and NW of Shiraz (Fars province, SW Iran). An area of 429.5 m of the Dariyan Formation sediments was studied through 187 thin sections. Also, the index microfacies were closely identified. Generally, index microfacies of the studied thin section were mudstone, wackestone, packstone, and grainstone microfacies. Received paleontological data confirm Aptian–lower Albian age of the Dariyan Formation. According to the lithostratigraphic aspect, the lower contact of Dariyan Formation with the Gadvan Formation in the Gadvan Mountain was described as a gradual type and in Zana Mountain; disconformity was found due to the sign of water withdrawal. Given the upper contact of the Dariyan Formation concerning the formation percentage, contact disconformity was observed due to the changes in the Allochem element and the situation of the entity from water and also according to the high percentage of iron nodules and Gluconate. According to the sequence, stratigraphic studies in the section of Gadvan Mountain, the lower border of the Dariyan Formation with Gadvan Formation was in the gradual form [type 2 Sequence Boundary (SB2)] and there was a disconformity in the upper border of this formation with Kazhdumi Formation [type 1 Sequence Boundary (SB1)]. Dariyan Formation included the system tracts (transgressive system tract, early highstand system tract, and late highstand system tract). There was a disconformity in its lower of the Gadvan Formation and disconformity in its upper boundary with the lower of the part Kazhdumi Formation (SB1) and it included the system tracts (highstand system tract, transgressive system tract, and lowstand system tract). Overall, lap sequence grade3 was seen in the two sections.

Sequence stratigraphy in post‐rift river‐dominated lacustrine delta deposits: A case study from the Upper Cretaceous Qingshankou Formation, northern Songliao Basin, northeastern China

The Upper Cretaceous Qingshankou Formation in the northern Songliao Basin was deposited in river‐dominated lacustrine delta systems. Three third‐order sequences and two types of sequence architecture were recognized using core observations, logging curve interpretations, and three‐dimensional (3‐D) seismic profiles in Qingshankou Formation. One type of sequence architecture was developed in a syndepositional flexural slope break, and the other was deposited on a gentle slope. The studied sequences were deposited in a high accommodation setting, which was due to the large rate of subsidence of the basin basement, and a tropical to semi‐tropical and a humid to semi‐humid climate. The episodic tectonic/thermal subsidence of the basin controlled the development of the sequence architecture. Turbidite bodies within transgressive systems tracts and early highstand systems tracts were deposited predominantly in the basinward part of the syndepositional flexural slope breaks, and they were favourable exploration targets, particularly for stratigraphic traps.

Variations of the trace fossil Zoophycos with respect to paleoenvironment and sequence stratigraphy in the Mississippian Mobarak Formation, northern Iran

Zoophycos in the Mississippian Mobarak Formation (Central Alborz, Iran) shows its potential in interpreting depositional settings, ecological features and sequence stratigraphy. Zoophycos A (ZoA), Zoophycos B (ZoB), Zoophycos/Phycosiphon (Zo/Phy) and Phycosiphon (Phy) ichnocoenoses are identified in the middle and outer ramp facies of the Mobarak Formation on the basis of shape, ichnodensity, ichnocomplexity degree and size of Zoophycos and other trace fossils. Zoophycos portrays a proximal-to-distal trend in the storm-dominated ramp sequence. In deeper parts of the ramp with long-term stable conditions and organic enrichment, Zoophycos producers selected equilibrium strategies (K-selected), whereas into the shallower parts they opted for opportunistic behavior (r-selected) due to short-term stable conditions with low food flux. The feeding behavior proposed in this study is based on general morphology as well as mode of sediment exploiting, and includes deposit feeding and the caching model. Moreover, this study confirms that the distribution pattern of the Paleozoic Zoophycos producers within a sequence stratigraphic framework that was totally different from that of the Mesozoic and Cenozoic. During the late transgressive systems tract (TST) and early highstand systems tract (HST), the excess of benthic food and the environmental stability, together with the prevalence of dysaerobic conditions, caused intense burrowing by the Zoophycos producer in proximal outer ramp to distal middle ramp. Its ethology, paleoecologic strategies and taphonomic signatures suggest that erosionally-exhumed, dewatered muddy beds (firmground) were not able to provide appropriate conditions for deposit-feeding Zoophycos producers as a consequence of their lower environmental stability. Therefore, Zoophycos as a standard component of the resident fair-weather community cannot be a good representative of the distal expression of the Glossifungites ichnofacies.

Lithofacies and depositional mechanisms of the Ordovician–Silurian Wufeng–Longmaxi organic-rich shales in the Upper Yangtze area, southern China

The Ordovician–Silurian organic-rich shales in the Wufeng and Longmaxi Formations are widespread in southern China and are the most important shale gas producers in China. The shales display varying total organic carbon content ranging from 0.2% to 6% and are composed of black siliceous mudstone, shelly limestone, siliceous clay-rich mudstone, gray silty–shaly interlaminated mudstone, gray argillaceous mudstone, and calcareous or dolomitic mudstone. Detailed stratigraphic correlation of these formations was achieved, and a high-resolution sequence stratigraphy was constructed based on the integration of well-log data, mineralogical, geochemical, and petrophysical analyses. The Wufeng Formation and the Guanyinqiao Bed were interpreted to be a condensed section with four cycles of sea-level fluctuation. The Longmaxi Formation consists of three members. The Long-1 Member was interpreted to be a third-order depositional sequence that can be subdivided into a transgressive systems tract, an early highstand systems tract, and a late highstand systems tract. Within the constructed isochronal stratigraphic framework, the characteristics and genetic mechanism of different shale lithofacies were studied. The deposition of black siliceous mudstone of high total organic carbon in the Wufeng Formation and the Long-1 transgressive systems tract was likely influenced by volcanic activity. The Guanyinqiao shelly limestone or calcareous mudstone was associated with the Ordovician–Silurian transition when glaciation and mass extinction took place. The gray silty–shaly interlaminated mudstone of moderate total organic carbon in the Long-1 early highstand systems tract was the product of bottom flow intrusion. The deposition of gray argillaceous mudstone and calcareous or dolomitic mudstone with low total organic carbon in the Long-1 late highstand systems tract resulted from enhanced terrigenous input from peripheral uplifts. The distribution pattern of different lithofacies varies greatly in the Upper Yangtze area, and this difference is a result of dissimilar sediment provenances and variable hydraulic restriction levels that were controlled by basin geometry. A better understanding of the distribution pattern and depositional history of the Wufeng and Longmaxi Formations might provide guidance for future profitable shale gas exploration.

Multiscale petrographic heterogeneity and their implications for the nanoporous system of the Wufeng-Longmaxi shales in Jiaoshiba area, Southeast China: Response to depositional-diagenetic process

Abstract The organic matter-rich shales in Wufeng-Longmaxi Formation, Jiaoshiba area, Southeast China, are showing a notable petrographic heterogeneity characteristic within the isochronous stratigraphic framework, which lead to vast differences in the mineral composition and organic matter abundance in the adjacent sections of the shale reservoir. The studied shale has been divided into three systems tracts: a transgressive systems tract (TST), an early highstand systems tract (EHST), and a late highstand systems tract (LHST). Multiple-scale petrographic observation and detailed mineralogical and geochemical analyses were combined to investigate the manifestation, origin, and the ways by which the shale heterogeneity is affected. The results indicate that polytropic depositional environments lead to different components in sediment. Subsequently, these differences among shale sections become more apparent through different diagenetic pathways. During the deposition of the section TST, the Hirnantian glaciation and regional volcanism played a crucial role, contributing to the abundant accumulation of fine-grained intrabasinal silica and organic matter. In diagenesis stage, authigenic quartz aggregates derived from siliceous organisms are formed. They filled in primary interparticle pores, forming a rigid particle-bracing structure that provide effective resistivity against the compaction and spaces for organic matter migration and occlusion. Finally, the migrated organic matter left plenty of newly created pore spaces that constituted a great portion of the total porosity of shale reservoir. The depositional process of section EHST is strongly influenced by contour current, which brings about more extrabasinal influx and impoverishes organic matter. In diagenesis stage, the rigid particle-bracing structure could only be preserved in limited areas, since insufficient siliceous supply could not produce enough authigenic quartz. Primary interparticle pores are significantly reduced owing to compaction, leaving less space for later organic matter migration and occlusion. As a result, the total porosity of shale reservoir declines in this section. In a rapid tectonic-uplifting background, the deposition of section LHST is associated with a rapid increase in terrigenous clay minerals, which further dilutes organic matter. Ductile clay experienced strong compaction and then occupies most of the primary interparticle space. Rigid particles are wrapped by a large number of clays, which has destroyed the particle-bracing structure. As a result, the nanoporous system in the shale could not be well preserved.

Distribution and controls of petroliferous plays in subtle traps within a Paleogene lacustrine sequence stratigraphic framework, Dongying Depression, Bohai Bay Basin, Eastern China

The characteristics of petroliferous plays in subtle traps within a sequence stratigraphic framework in the Dongying Depression are investigated in this study. Sand bodies within lowstand systems tracts (LSTs) of sequences, comprising incised-channel fills, sublacustrine fans, deltas in LSTs, controlled by syndepositional normal faults, and sand bodies within transgressive systems tracts (TSTs) to early highstand systems tracts (HSTs), consisting of beach bars, and turbidites, controlled by the prodelta slope, paleorelief, and syndepositional normal faults, are good subtle reservoirs. Mudstones and shale of deep lake subfacies in TSTs to early HSTs of sequences are source and cap rocks. Abnormal overpressure is the dominant dynamic factor for hydrocarbon migration from source rock to the subtle traps. Normal faults, sand bodies, and unconformities function as conduit systems. Sand bodies distributed in the abnormal overpressure source rocks within LSTs to early HSTs are petroliferous plays in lithologic traps. The petroliferous plays in stratigraphic traps are controlled by unconformities at margins of the Depression.

Rare earth elements in the terminal Ediacaran Bambuí Group carbonate rocks (Brazil): evidence for high seawater alkalinity during rise of early animals

Rare earth elements plus yttrium (REY) mass fractions of ancient carbonate rocks are used to track changes in chemistry of past seawater. Here we investigate REY patterns in two carbonate sections from the Ediacaran Bambuí Group, São Francisco Basin (Brazil), which comprise its two lowermost transgressive-regressive second-order sedimentary sequences. Shale normalised distributions vary with the sequence stratigraphy framework. In the basal 2nd-order sequence, carbonate samples from the basal sequence transgressive systems tract display light REY (LREY) distributions slightly depleted to enriched that reflect input of freshwater, possibly in a post glacial episode. Upwards, carbonate rocks from the early highstand systems tract (EHST) yielded LREY enriched distributions, which progressively turns into LREY shale normalized depleted distributions on samples from the late highstand systems tract (LHST). This portion of the sequence also displays Y positive anomaly in some cases. Carbonate samples from the upper second-order sequence do not display coherent patterns. Ce/Ce* values > 1 in most samples throughout the two sections suggest permanent anoxia of seawater. The REY change from the EHST to LHST in the basal sequence marks an important paleoenvironmental overturn in the basin, with increasing alkalinity in seawater driving REY fractionation and LREY depletion. Confinement of the basin in the inner areas of West Gondwana due the uplift of marginal neoproterozoic orogens probably changed the weathering style of source areas to more congruent, thus delivering a higher ionic influx to a restricted setting, increasing alkalinity during LHST. Cloudina sp. fragments were reported in this stage of the Bambuí Group and in carbonate rocks with high Sr mass fractions in other West Gondwana basins, supporting the hypothesis that the high alkalinity of seawater during late Ediacaran may have driven the appearance of the first biomineralizing organisms.

Microfacies characteristics and depositional evolution of typical wave-dominated littoral deposits – A case of Carboniferous ‘Donghe sandstones’ in Hudson Oilfield, Tarim basin, NW China

Hudson Oilfield, which located in the northern Uplift of Tarim Basin, is considered to be the largest whole-shaped oilfield in China, and the extensive Carboniferous ‘Donghe sandstones’, one of the most important marine sandstones units for oil and gas exploration in Tabei Uplift, constitutes the main reservoir. However, not all this sandstones are with good physical properties, and hence, the sedimentary characteristics and depositional evolution is believed to be critical to prediction of reservoir. It is determined that wave-dominated clastic littoral deposits with no barrier are the main depositional environment based on the modern sedimentary theory and related data analysis including ancient landform characteristics. And transitional zone, shoreface, foreshore, backshore and dune are considered as the five subenvironments. Through data of high resolution dipmeter logging and imaging logging, different sedimentary structures and other facies marks observed from cores of research regions are demarcated, and thirteen microfacies are further identified. Based on the detailed microfacies types, the evolution process of Carboniferous ‘Donghe sandstones’ can be divided to four main periods, in which, early highstand systems tract (HST) deposits in Period 1-2, and late HST deposits in Period 3-4.

Recognition and significance of Upper Devonian fluvial, estuarine, and mixed siliciclastic-carbonate nearshore marine facies in the San Juan Mountains (southwestern Colorado, USA): Multiple incised valleys backfilled by lowstand and transgressive systems tracts

Abstract The Upper Devonian Ignacio Formation (as stratigraphically revised) comprises a transgressive, tide-dominated estuarine depositional system in the San Juan Mountains (Colorado, USA). The unit backfills at least three bedrock paleovalleys (10–30 km wide and ≥42 m deep) with a consistent stratigraphy of tidally influenced fluvial, bayhead-delta, central estuarine-basin, mixed tidal-flat, and estuarine-mouth tidal sandbar deposits. Paleovalleys were oriented northwest while longshore transport was to the north. The deposits represent Upper Devonian lowstand and transgressive systems tracts. The overlying Upper Devonian Elbert Formation (upper member) consists of geographically extensive tidal-flat deposits and is interpreted as mixed siliciclastic-carbonate bay-fill facies that represents an early highstand systems tract. Stratigraphic revision of the Ignacio Formation includes reassigning the basal conglomerate to the East Lime Creek Conglomerate, recognizing an unconformity separating these two units, and incorporating strata previously mapped as the McCracken Sandstone Member (Elbert Formation) into the Ignacio Formation. The Ignacio Formation was previously interpreted as Cambrian, but evidence that it is Devonian includes reexamined fossil data and detrital zircon U-Pb geochronology. The Ignacio Formation has a stratigraphic trend of detrital zircon ages shifting from a single ca. 1.7 Ga age peak to bimodal ca. 1.4 Ga and ca. 1.7 Ga age peaks, which represents local source-area unroofing history. Specifically, the upper plate of a Proterozoic thrust system (ca. 1.7 Ga Twilight Gneiss) was eroded prior to exposure of the lower plate (ca. 1.4 Ga Uncompahgre Formation). These results are a significant alternative interpretation of the geologic history of the southern Rocky Mountains.