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Lacustrine mixed carbonate/siliciclastic sediment is an important type of oil and gas reservoir with significant potential. Although previous studies have investigated the sedimentary characteristics of the mixed depositional system in numerous oil and gas-bearing basins worldwide, a detailed sedimentary architecture model is still lacking for guiding reservoir characterization at the hydrocarbon reservoir scale. In this paper, a typical lacustrine mixed carbonate/siliciclastic sedimentary system, preserved in the lower member of the ShangGanchaigou Formation, Neogene, in the western Qaidam Basin, Western China, was deeply investigated based on well logging data from 640 wells, 438 m of cores from 3 core wells, and outcrop studies. The results demonstrate that 1) seven types of architecture elements, namely, distributary channel, channel mouth bar, distal bar, sheet-like sand, shallow water mud, algal mound, and marl flat characterized by different lithofacies associations, were recognized based on core and well logging data. 2) The lacustrine mixed carbonate/siliciclastic depositional system can be divided into three facies belts. Along the lakeward direction, the proximal facies belt is dominated by delta front deposits and characterized by gradually downstream bifurcating distributary channels and associated lateral amalgamated delta lobes. The middle facies belt is characterized by isolated and small-scale delta lobes and inter-lobe deposits, including sheet-like sand, small-scale algal mounds, and marl flats, and the distal facies belt is a combination of large-scale algal mounds and marl flats. 3) Within a depression, short-term base-level cycles controlled the facies belt transition, and the proximal, middle, and distal facies belts formed under relatively low, middle, and high base-level conditions, respectively. 4) The scale and connectivity of reservoirs gradually decreased from the proximal to the distal belt. [ABSTRACT FROM AUTHOR]

The storm deposition types within the Dabeiwang area of Xuzhou City, Jiangsu Province, China, showcase a wide range of diversity and distinctly apparent characteristics. Based on measured sections and thin-section observations, we have studied the typical characteristics of storm deposition in this area, divided the sedimentary sequence, and summarized a comprehensive model of storm deposition. Various rock types such as thick-bedded dolomitic limestone, oolitic limestone, and calcirudite are developed in the study area. The calcirudite exhibits bamboo leaf-like morphology and various distribution forms. Typical storm sedimentary structures such as scour surfaces and mound cross-bedding were identified in the study area. Based on the distance of the gravel layer from the storm center and the depositional environment, the storm sedimentary sequence in the study area can be divided into allopatric-proximal-lagoon facies storm deposit, allopatric-distal-oolitic shoal facies storm deposit, autochthonous-supratidal facies storm deposit, allopatric-distal-interbeach marine storm deposit, sub-allopatric-tide zone facies storm deposit, and autochthonous-tide zone facies storm deposit. We summarize the comprehensive model of Cambrian storm deposition in the Dabeiwang area of Xuzhou, which has enriched the research results of Cambrian storm deposition in Xuzhou and provides support for the construction of a lithofacies paleogeographic pattern. [ABSTRACT FROM AUTHOR]

Lacustrine mixed carbonate/siliciclastic sediment is an important type of oil and gas reservoir with significant potential. Although previous studies have investigated the sedimentary characteristics of the mixed depositional system in numerous oil and gas-bearing basins worldwide, a detailed sedimentary architecture model is still lacking for guiding reservoir characterization at the hydrocarbon reservoir scale. In this paper, a typical lacustrine mixed carbonate/siliciclastic sedimentary system, preserved in the lower member of the ShangGanchaigou Formation, Neogene, in the western Qaidam Basin, Western China, was deeply investigated based on well logging data from 640 wells, 438 m of cores from 3 core wells, and outcrop studies. The results demonstrate that 1) seven types of architecture elements, namely, distributary channel, channel mouth bar, distal bar, sheet-like sand, shallow water mud, algal mound, and marl flat characterized by different lithofacies associations, were recognized based on core and well logging data. 2) The lacustrine mixed carbonate/siliciclastic depositional system can be divided into three facies belts. Along the lakeward direction, the proximal facies belt is dominated by delta front deposits and characterized by gradually downstream bifurcating distributary channels and associated lateral amalgamated delta lobes. The middle facies belt is characterized by isolated and small-scale delta lobes and inter-lobe deposits, including sheet-like sand, small-scale algal mounds, and marl flats, and the distal facies belt is a combination of large-scale algal mounds and marl flats. 3) Within a depression, short-term base-level cycles controlled the facies belt transition, and the proximal, middle, and distal facies belts formed under relatively low, middle, and high base-level conditions, respectively. 4) The scale and connectivity of reservoirs gradually decreased from the proximal to the distal belt.

The Shan1 member in the Qingyang Gas Field, Ordos Basin, constitutes one of the primary production layers. However, debates persist regarding its sedimentary environment and facies types. Divergent interpretations have resulted in discrepancies in sand body correlation patterns and distribution within each sedimentary facies, posing challenges in well-location exploration. Leveraging existing research, the author integrates seismic, logging, core, outcrop, and laboratory data. Employing diverse methods, including seismic reflection structure analysis, core-based examination of lithological grain size and bedding structure, logging sedimentary cycle characteristics, and comparisons with modern sedimentology, the sedimentary facies of the Shan ³1 strata were discussed, analyzed, and described comprehensively. Results indicate that the lithology in the study area is predominantly medium to coarse sandstone with planar and small trough cross-bedding. Logging facies encompass sections with a characteristic upward thinning sequence, indicative of meandering fluvial depositions, and sections with coarse-grained sediments, associated with the sedimentation of braided rivers. Particle size analysis reveals a prevalence of coarse particles, indicating robust paleohydrodynamic forces. Field outcrop data demonstrate the concurrent development of both meandering and braided rivers during the deposition period of the Shan1 member. Modern satellite images showcase the widespread development of meandering and braided rivers. In conclusion, the study identifies the sedimentary environment of Shan ³1 as fluvial facies, characterized by the coexistence of meandering and braided rivers. Key factors contributing to the complexity of sand body distribution and rapid lateral facies changes include frequent lateral accretion of meandering rivers, planform changes in river patterns, and the multiple-phase stacking of vertical channels. [ABSTRACT FROM AUTHOR]

By using equipment such as time-lapse cameras and 3D laser scanners, this study conducted flume simulation experiments to obtain topographic elevation, the amount of sediment increment, sedimentary cross sections, and other related data and diagrams. Quantitative software was then utilized for sedimentological analysis to clarify the depositional evolution, architecture, and depositional model of lobate shallow-water deltas. The research results show that under the influence of hydraulic conditions and main distributary channels, lobate shallow-water deltas primarily undergo three evolution stages: initial channel formation, distributary channel system formation, and continuous stable evolution stage. The architecture elements of lobate shallow-water deltas can be divided into distributary channels dominated by channel deposition, proximal river mouth bars, transitional bars, and distal bars. The lateral composition and downstream deposition of the river-bar combination are the main structural units of the shallow water delta. The proximal profile distributary channels exist in the form of thick channel sand bodies with multiple repetitive intercalation cycles, and thick proximal bar deposits form on both sides of the distributary channels. The bifurcation of distributary channels in the middle profile leads to a reduction in channel scale, while large-scale proximal bars still develop on both sides of the channel. In the distal profile, a substantial river mouth bar complex is formed at the break of the delta front slope, and with the gradual progradation of the delta, the river mouth bar deposition zone becomes flattened. Based on the insights of the flume simulation experiments, a sedimentary model for lobate shallow-water delta was established. [ABSTRACT FROM AUTHOR]

The Qixia Formation in the foothills of Longmen Mountain in the western Sichuan Basin is gaining attention for oil and gas exploration. However, exploration of the Qixia Formation in the middle Longmen Mountain is insufficient. Consequently, it is essential to understand its stratigraphic sedimentary characteristics for guiding natural gas exploration. This study focuses on the Beichuan Tongkou section, combines field measurements, thin-section observation and trace element analyses to investigate the sequence stratigraphy and sedimentary features of the Qixia Formation, thereby defining its sedimentary environment. Within the Beichuan Tongkou section, which spans from the Liangshan Formation to the Qixia Formation, researchers have identified two third-order and one fourth-order sequence boundaries, delineating one third-order sequence and two fourth-order sequences. The sedimentary facies found in this interval include shore-swamp, open platform, and platform edge environments, which are further classified into four subfacies including open sea, intertidal sea, intra-platform beach, and platform edge beach. Additionally, three microfacies are identified including intra-platform bioclastic beach, platform edge bioclastic beach, and platform edge sand debris and bioclastic beach. The Qi1 member and lower part of the Qi2 member is located on an open platform, while the upper-middle part of the Qi2 member is found at the platform edge. This setup defines a sedimentary model for a carbonate-rimmed platform in the Qixia Formation of the middle Longmen Mountain. During the sedimentary period of the Qixia Formation, a warm, humid, and oxygen-rich marine environment supported abundant ancient organisms. In the highstand system tract of the upper-middle part of the Qi2 member, the Sr/Ba ratio is lowest, while the MgO/CaO ratio is highest, indicating elevated ancient water temperatures and specific chemical compositions. The m value, reflective of paleosalinity, is notably high, suggesting a shallow, high-energy platform edge environment away from the mainland. Thick, high-energy platform edge sand debris and bioclastic beaches in this area promote the development of high-quality porous dolomite reservoirs of beach facies. The sedimentary facies and reservoir conditions of the Qi2 member in the middle Longmen Mountain are comparable to those in other sections, making it a promising target for oil and gas exploration within the Qixia Formation and a potential strategic replacement area for reservoir expansion.

Seasonal delta is a sandy fan-shaped sedimentary system formed by seasonal river flooding in continental basins. Modern sedimentary finds that seasonal deltas mainly develop in an arid-semi-arid climate environment with gentle terrain. However, the study of seasonal deltas in ancient rock records is still insufficient, leading to misinterpretations as other sedimentary systems, which poses challenges to the correct understanding of the origin and spatial distribution of sand body deposits. In this study, the red bed deposits of the Cretaceous Baxigai Formation in the west of the Tabei Uplift were selected as the research object. Based on core observation and description, a comprehensive analysis of the sedimentary environment, sedimentary characteristics and sedimentary model of the Baxigai Formation was carried out using data such as well-logging and analytical testing, combined with modern depositional characteristics. Research shows that the sedimentary sequence of the Baxigai Formation red bed developed a seasonal delta sedimentary system in the arid climate. The seasonal delta deposits provide an opportunity to understand the influencing of river flow and total sedimentary loading vary greatly on the sedimentary model, sedimentary mechanism, and distribution of sand bodies in arid climates. The sedimentary process of the seasonal delta in the Baxigai Formation was controlled by the dual-stage sedimentation process in an arid paleoclimate. During the flood period, the supercritical flow sedimentary structures are widely developed in the major distributary channels, and the distributary channels migrate and bifurcate frequently. On the delta plain, crevasse channels are formed through the avulsion of major distributary channels. During the dry period, due to the consumption of water seepage and evaporation transfer, terminal distributary channels are abandoned, crevasse channels are not developed, and the sediment is exposed. The density of the distributary channels is reduced, and the total sedimentary loading is low. The distributary channels sand body is the skeleton sand body of the seasonal delta. This study is of great significance for accurately explaining the formation process of seasonal deltas and provides a useful reference for further exploration and deployment of oil and gas reservoirs and paleogeographic reconstruction in the study area. [ABSTRACT FROM AUTHOR]

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The Lower Cambrian Qiongzhusi Formation in Sichuan Basin and on its periphery is rich in shale gas resources, and it has become one of the important shale gas development intervals in Sichuan Basin. Controlled by complex sedimentary setting, the silty shale reservoir in Qiongzhusi Formation has various lithofacies and strong heterogeneity longitudinally, which restricts the exploration and development of shale gas. Therefore, it is urgent to study the sedimentary characteristics, sedimentary facies and sedimentary models. Based on the previous understanding of tectonic setting and sequence stratigraphy, and through detailed analysis of rock data, well logging data, geochemistry data and paleontology data, this paper clarifies the Early Cambrian sedimentary background of combined ocean-paleoland influence, moderate water depth and widespread reduction of Sichuan Basin and its periphery. The paper also believes that the Qiongzhusi Formation is mainly deposited in shore facies and continental shelf facies. The continental shelf facies are dominated, including shallow water continental shelf subfacies and deep water continental shelf subfacies. Based on the analysis of the combined action of wave and tide, seawater evaporation, suspension, rising ocean current, sedimentary paleogeomorphology and other fine-grained sedimentary influencing factors, and combined with the spatial distribution characteristics of sediments, the fine-grained sedimentary model of the Lower Cambrian Qiongzhusi Formation in Sichuan Basin and on its periphery is established, i.e., in the early stage of sedimentation, the effect of differential sedimentation is significant, the sequence thickness varies greatly, and the sedimentary characteristics and influencing factors differ greatly in different regions. In the middle stage of sedimentation, the effect of differential sedimentation is diminished and the difference of sequence thickness decreases continuously. In the late stage of sedimentation, with steady sedimentation, it enters the "open basin" stage, and the sequence thickness tends to be consistent.

The lower play of the Junggar Basin has huge oil and gas potential, which is the most important strategic succession field for oil and gas exploration. Large-scale delta sand bodies are developed in Permian and Triassic in the four sags in the hinterland, in which new discoveries of oil and gas have been made in succession. In order to reveal the sedimentary characteristics, sedimentary model, and distribution patterns of sand bodies in the hinterland, the studies on basin prototype, sequence framework, paleogeomorphic restoration and sedimentary system were systematically carried out based on a large number of newly drilled cores, logging and geophysical data. The results show that: the development background of large depression lake basin can be identified in the development period of the Upper Permian-Lower Triassic shallow water braided river delta in the hinterland of the basin, and the study area has the characteristics of flat terrain, small slope, sufficient material supply, extremely shallow and frequently turbulent water bodies, alternating oxidation and reduction environments, and overall oxidation environment during the sedimentation period. The shallow braided river delta is characterized by coarse grain size, low impurity content, medium texture maturity, long distance transportation, strong hydrodynamic scouring, cross bedding and parallel bedding. The advantageous water system in the sedimentation period of the Upper Wuerhe and Baikouquan formations in the hinterland mainly comes from the northwest and northeast, and extends from north to south as a whole, with three provenance systems developed, namely Wuerhe provenance, Karamay provenance and Kelameily provenance. The terrain within the basin has obvious zoning characteristics, forming four facies belts namely fan delta area, braided river delta plain area, front area and lake area, thus forming a shallow water braided river delta sedimentary pattern of "large plain, small front" in the hinterland, where plain area and front area are favorable sand body development zones.

Seasonal delta is a sandy fan-shaped sedimentary system formed by seasonal river flooding in continental basins. Modern sedimentary finds that seasonal deltas mainly develop in an arid-semi-arid climate environment with gentle terrain. However, the study of seasonal deltas in ancient rock records is still insufficient, leading to misinterpretations as other sedimentary systems, which poses challenges to the correct understanding of the origin and spatial distribution of sand body deposits. In this study, the red bed deposits of the Cretaceous Baxigai Formation in the west of the Tabei Uplift were selected as the research object. Based on core observation and description, a comprehensive analysis of the sedimentary environment, sedimentary characteristics and sedimentary model of the Baxigai Formation was carried out using data such as well-logging and analytical testing, combined with modern depositional characteristics. Research shows that the sedimentary sequence of the Baxigai Formation red bed developed a seasonal delta sedimentary system in the arid climate. The seasonal delta deposits provide an opportunity to understand the influencing of river flow and total sedimentary loading vary greatly on the sedimentary model, sedimentary mechanism, and distribution of sand bodies in arid climates. The sedimentary process of the seasonal delta in the Baxigai Formation was controlled by the dual-stage sedimentation process in an arid paleoclimate. During the flood period, the supercritical flow sedimentary structures are widely developed in the major distributary channels, and the distributary channels migrate and bifurcate frequently. On the delta plain, crevasse channels are formed through the avulsion of major distributary channels. During the dry period, due to the consumption of water seepage and evaporation transfer, terminal distributary channels are abandoned, crevasse channels are not developed, and the sediment is exposed. The density of the distributary channels is reduced, and the total sedimentary loading is low. The distributary channels sand body is the skeleton sand body of the seasonal delta. This study is of great significance for accurately explaining the formation process of seasonal deltas and provides a useful reference for further exploration and deployment of oil and gas reservoirs and paleogeographic reconstruction in the study area.

Understanding controls on river planform changes can help to build predictive models for distributive fluvial systems, and then guide the oil and gas exploration. To do this we have undertaken a detailed investigation of the modern Great Halten River distributive fluvial system from the Sugan Lake Basin, Qinghai, China. Unmanned aerial vehicle (UAV) photography, satellite remote sensing data and elemental analysis were used to determine differences in the sedimentary characteristics of the distributive fluvial system. From the apex to the toe, the changes in the slope, river morphology, sedimentary characteristics and element content in different regions were determined and three facies belts: "proximal", "medial" and "distal" were identified. We found that the sedimentary structure and elemental content characteristics of each facies differ greatly. We compare the large-scale evolution of rivers from braided to meandering rivers, and the fine description of sedimentary characteristics in combination with each observation, we strengthen our overall understanding of the modern DFS from macro to micro scale. At the same time, we summarize the sedimentation model of the Great Halten River DFS, and our study provides a reference for establishing the sedimentary model in continental petroliferous basins. [ABSTRACT FROM AUTHOR]

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Based on core observation, rock thin sections, logging data, and testing data, taking the shale of the upper submember of the 4th Member to the lower submember of the 3rd Member of Paleogene Shahejie Formation in Jiyang Depression of Bohai Bay Basin as an example, we determine the lithofacies division scheme, divide the main lithofacies types, analyze the sedimentary origin and development location of different shale lithofacies, establish the continental lake basin sedimentary model, determine the types and enrichment areas of favorable lithofacies, and provide guidance for the exploration and development of Shale oil. The results show that: (1) According to the mineral composition, sedimentary structure, and organic matter abundance, the division scheme of shale lithofacies in the study area is proposed, and the shale lithofacies of the study area was mainly divided into 17 types. (2) Based on the lithologic changes, the lacustrine sedimentary shale area was divided into muddy water area, transition area, and clear water area. (3) Under the background of locally uplifted slope paleogeomorphology, considering the combined effects of climate, topography, hydrodynamic, mechanical, and chemical differentiation of sediments and biological habits, the sedimentary model of shale was established. (4) Organic-rich shale was mainly deposited between the clear water area and the end of the muddy water area, with the characteristics of water, brackish water, strong reduction, and water stratification, and was mainly enriched in the low-lying parts of paleotopography. [ABSTRACT FROM AUTHOR]

This article studies the sedimentary characteristics and models of the delta and gravity flow system of the third member of the Shahejie Formation in the Niuzhuang Sag area. Through seismic, logging, and core observation methods, a thorough investigation is conducted to examine the lithologic characteristics, grain size characteristics, sedimentary structure characteristics, and sedimentary facies distribution characteristics of this region. The results show that the third middle member of the Shahejie Formation in the Niuzhuang Sag can be classified into four sedimentary types: sliding, collapse, clastic flow, and turbidity flow. This article aimed to establish the distribution characteristics and depositional models of the deltaic and gravity flow depositional systems within the study area. The findings reveal that slip deposition primarily occurs near the delta front, while collapse and clastic flow depositions are concentrated near the far slope. Moreover, turbidity flow deposition is found near the far slope. This study significantly contributes to our understanding of the sedimentary characteristics and models associated with deltas and gravity flow systems in faulted lacustrine basins. Furthermore, it enriches existing theories related to gravity flow and provides a valuable reference for the investigation of deep-water sedimentation in continental faulted lacustrine basins. [ABSTRACT FROM AUTHOR]

Based on core, well-logging and seismic data, the gravity flow characteristics and sedimentary model of the 1st member of the Tengger formation in the eastern steep slope belt of the Sai-4 tectonic belt in the Saihantala depression (Erlian Basin, China) have been systematically studied; further, the sublacustrine fan sedimentary model in the study area has been established. The main types of gravity flow identified in the study area are sandy debris flow deposits and turbidity flow deposition. Affected by paleo-geomorphology, provenance and other factors, sublacustrine fans in the eastern steep slope zone are formed by gravity flows from different periods. The sublacustrine fan is divided into three subfacies: inner-fan, mid-fan and outer-fan. The subfacies can be subdivided into six types of microfacies: main channel, levee of main channel, distributary channel, inter-channel, levee of distributary channel and terminals. The formation of the sublacustrine fan is caused by steep slopes and triggered by earthquakes. The sedimentary model of the sublacustrine fan and its sedimentary characteristics provide a foundation for exploring and developing oil and gas in the study area. [ABSTRACT FROM AUTHOR]

For black shales, laminae and bedding are hard to identify, grain size is difficult to measure, and trace fossils do not exist. Taking the Ordovician Wufeng – Silurian Longmaxi shale in southern Sichuan Basin, China, as an example, the types, characteristics and models of microfacies in epicontinental shale are analyzed by means of full-scale observation of large thin sections, argon-ion polishing field emission-scanning electron microscopy (FE-SEM), and kerogen microscopy. The epicontinental sea develops delta, tidal flat and shelf facies, with black shale found in microfacies such as the underwater distributary channel and interdistributary bay under delta front facies, the calcareous and clayey flats under intertidal flat facies, the calcareous and clayey shelfs under shallow shelf facies, the deep slope, deep plain and deep depression under deep shelf facies, and the overflow under gravity flow facies. Basinward, silty lamina decreases and clayey lamina increases, the grain size changes from coarse silt to fine mud, the silica content increases from about 20% to above 55%, the carbonate and clay minerals content decreases from above 40% to around 10%, and the kerogen type changes from type II2 to type II1 and type I. Provenance and topography dominate the types and distribution of shale microfacies. The underwater distributary channel, interdistributary bay, clayey flat, clayey shelf, and overflow microfacies are developed in areas with sufficient sediment supply. The calcareous flat and calcareous shelf are developed in areas with insufficient sediment supply. The deep shelf shale area is divided into deep slope, deep plain, and deep depression microfacies as a result of three breaks. The formation of epicontinental shale with different microfacies is closely related to the tectonic setting, paleoclimate, and sea level rise. The relatively active tectonic setting increases the supply of terrigenous clasts, forming muddy water fine-grained sediment. The warm and humid paleoclimate is conducive to the enrichment of organic matter. The rapid sea level rise is helpful to the widespread black shale.

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Mahu sag is an important hydrocarbon-rich sag and a large stratigraphic lithologic reservoir exploration field in the northwestern Junggar Basin, forming a large stratigraphic lithologic reservoir with multiple layers. In order to find out the distribution law of oil and gas in this area and provide geological basis for the next exploration deployment, based on core observation and comprehensive analysis of seismic, logging and other data, the structural sequence analysis method was applied to study the main oil and gas reservoir series from Permian to Triassic. The results show that there are many large stratigraphic unconformity surfaces from Permian to Triassic in Mahu sag, which can be divided into Lower Permian, Middle Permian, Upper Permian-Triassic and other structural sequences. From Permian to Triassic, it experiences three major evolution stages of fault depression, depression and large depression slope, forming the superposition of three major structural sequences. The Lower Permian is a dustpan structural sequence formed under the extensional tectonic background; the northern stratum is thick, and the southern stratum is thin; it is deposited under the dustpan structure background of deep north and shallow south; the depression area is widely distributed fine-grained mixed rock, which is constructed of high-quality hydrocarbon source rock, and gradually becomes sandstone and conglomerate towards the slope area. The Middle Permian is a structural sequence with thick central and thin peripheral margins under the tectonic background of depression; the basin margin fan delta is developed, which is characterized by large basin, shallow water, limited basin margin fan body, and interbedded sediment of sand and mud in the basin. In Upper Permian-Triassic, Mahu area is located in the northwest slope area of Junggar large depression basin; under the background of shallow water, gentle slope and lacustrine transgression, the large retrogressive fan delta deposits are formed, and the sand bodies are vertically superimposed and horizontally distributed. The vertical superposition of the three major structural sequences forms the multi-layer distribution of stratum-lithologic or structure-lithological reservoirs in the area. [ABSTRACT FROM AUTHOR]

In order to clarify the characteristics of fine-grained sedimentary lithofacies and the depositional models in lacustrine environments of the Chang 7 Member of the Upper Triassic Yanchang Formation in the Ordos Basin, we focus on the lacustrine lithofacies classification and controlling factors. Based on the typical field sections of the southern Ordos Basin, combined with the methods of organic geochemical analysis, polarizing microscopic observation, XRD and elemental geochemistry, we summarize the main controlling factors of the lithofacies under different sedimentary environments and establish the sedimentary model under different sedimentary backgrounds. Results show that the Chang 7 Member of the Ordos Basin includes six major lithofacies types, i.e., quasi-laminated clayey shale facies, blocky silty mudstone facies, quasi-laminated silty shale facies, laminated silty shale facies, blocky mixed mudstone facies, and laminated argillaceous siltstone facies. Al2O3, MgO, CaO, Na2O, K2O, and SiO2 are relatively depleted, while TiO2, TFe2O3, and P2O5 are relatively enriched in the Chang 72+3. Meanwhile, the trace elements are relatively enriched in Cd, Cu, Mo, U, and V. The fine-grained sedimentary facies were influenced by various sedimentary environments, such as paleoclimate, redox conditions, productivity and terrigenous input. According to lithofacies types and genetic mechanisms of the Chang 7 Member in the study area, two types of lithofacies assemblages are identified. The rapid rise in lake level during the deposition of the Chang 72+3 resulted in an anoxic water column, high productivity, and low terrigenous input under a humid climate and weak-to-moderate weathering conditions. Therefore, clayey shale lithofacies assemblage is developed in the lower Chang 7 Member. [ABSTRACT FROM AUTHOR]

Cap carbonate of the early Ediacaran period represents an abrupt climate shift from severe Marinoan glaciation to a post‐glacial warm environment. Some consensuses on the distinctive negative excursion of δ13Ccarb and the origin of its unique sedimentary structures have been reached over the past decades, but certain aspects such as the distribution and redox conditions of cap carbonate still remain controversial. To resolve these issues, we collected detailed samples of the Doushantuo cap carbonate from the Ganlong‐Fanyang section and the Poutoushang reservoir section in the Danzhai County, southeast Guizhou Province, respectively. We conducted carbon and oxygen isotope analysis and trace element analysis in this study. The Ganlong‐Fanyang section was possibly situated in a submarine highland because of its higher carbon isotope than that of the Potoushang section according to the carbon isotopic gradient. Negative carbon isotope excursions at the basal Doushantuo cap carbonate from the two sections correlate well with those from other equivalent cap carbonates and were corresponding to the first one (SN1 or EN1) of the carbon isotopic curve during the Ediacaran period, implying a remarkable global event after the post‐Marinoan glaciation. Geochemical features of trace elements and rare earth elements support an overall oxygen‐rich condition with syn‐depositional or pene‐contemporaneous hydrothermal activities and methane seeps during the deposition of cap carbonate. We reconstruct a new sedimentary model that the Doushantuo cap carbonates were mainly precipitated in relatively shallow‐ to deep‐water shelf environments without disturbance by abundant terrestrial materials, which may be on isolated platforms over the early developed grabens in a deep‐water basinal condition, rather than within the entire oceanic basin. [ABSTRACT FROM AUTHOR]

On the basis of systematic study of sedimentary facies, microfacies and geochemistry of measured strata sections and wells, the ooid shoals of the Cambrian Miaolingian Zhangxia Formation in the southern Ordos Basin are composed of both high- and low-energy ooid shoals. The characteristics of the high-energy shoals are sparry cementation, with moderate to well sorting, large grain size, high ooid content and low micrite, weak micritization, and well-preserved internal textures of the ooids. Macroscopically, the high-energy ooid shoals display a thick-bedded aggradational stacking pattern in upward coarsening and thickening sequences. The low-energy ooid shoals are mainly made up of micritic cementation, with moderate to poor sorting, relatively small grain size, lower ooid content and higher micrite, strong micritization, and poorly-preserved internal textures of the ooids. Macroscopically, the low-energy ooid shoals show a thin-bedded, interbedded stacking pattern in upward fining and thinning sequences. The sedimentological evidence and carbon isotope data show that the differences of internal fabric and sedimentary evolution of both high- and low-energy ooid shoals are closely related to the sea-level change. [ABSTRACT FROM AUTHOR]

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The rise of the mantle asthenosphere and tectonic activity are linked to travertine and mid-ocean ridges, although their relationship has not been clarified. To investigate the connection between travertine and mid-ocean ridges, we gathered information on the geographic distribution of travertine from around the world, plotted it, and linearized it. Making a map of the locations of linearized travertine, mid-ocean ridges, and 2016-2020 (M>5) earthquakes (which may or may not constitute seismic belts) and assessing its global distribution. The sedimentary models of travertine and mid-ocean ridges, on the other hand, were drawn based on previous studies, comparing and analyzing the similarities and differences between the sedimentary models of travertine and mid-ocean ridges. The results indicate that: 1) travertine and mid-ocean ridges are both primarily distributed in the seismic belts, 2) their deposition is closely related to the mantle asthenosphere, and 3) travertine occurs on land but mid-ocean ridges are in the ocean, which is a significant difference. This study examines the relationship between travertine and mid-ocean ridges are related analogs regarding geographical location and sedimentary model and suggests that travertine, like mid-ocean ridges, may be a driving force for plate drift. Researchers may gain a better understanding of the otherwise difficult-to-study mid-ocean ridge system by analyzing similarities between travertine and mid-ocean ridges. [ABSTRACT FROM AUTHOR]

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The Miocene is a crucial epoch in the evolution of the Gulf of Suez basin. Its deposits experienced rapid and distinct spatial and temporal variations in facies and thickness due to deposition in a complex system of fault-bounded sub-basins that were continuously active throughout most of the Miocene. This review addresses the evolution of Miocene facies in the Gulf of Suez, which remains a major challenge, by integrating a detailed stratigraphic and sedimentological study. Two markedly different facies characterize Miocene deposits: marginal and deeper marine facies. Regionally traced unconformities intersect these facies, reflecting the nature of the tectonically active rift basin in which they accumulated. They define seven depositional sequences in the lower and middle Miocene facies exposed along the rift margins. Unconformities and related sea-level falls formed in response to major rift events, although some are associated with eustatic sea-level falls. The latter played an important role during the rift abandonment, when tectonic subsidence was slow. The evolution of Miocene facies reveals three distinct phases of rifting, each showing unique subsidence rate, sedimentation rate, structural relief, depositional systems, and facies architectures. These phases define four depositional stages, including the shallow marine stage, the open marine stage, the transitional stage, and the restriction stage. The first stage displays slow tectonic subsidence and initial marine invasion during the Aquitanian–early Burdigalian (Nukhul Formation). The second stage shows a rapid increase in rates of crustal extension, tectonic subsidence, and sedimentation during the early–late Burdigalian (Rudeis Formation), and a marked uplift of the rift shoulders in the late Burdigalian. The third stage indicates reduced rates of extension, subsidence, and sedimentation during the Langhian (Kareem Formation). The final stage shows a very slow subsidence with periodic water inflows from the Mediterranean during the Serravallian (Belayim Formation), and ends with an uplift in the northernmost Gulf of Suez basin that permanently isolates it from the north during the Tortonian–Messinian (South Gharib and Zeit formations). Facies attributed to the Gharamul and Gemsa formations, and the Sarbut El-Gamal Formation developed simultaneously along the western and eastern rift margins, respectively, during the second, third, and the lower part of the fourth evolutionary stages. [Display omitted] • This review accurately correlates marginal and deeper marine facies of the basin. • It provides additional chronostratigraphic data for marginal facies. • Eight unconformities attributable to rift events define seven exposed sequences. • Locally induced sea-level changes control sequences; some show eustatic signals. • Four distinct evolutionary stages characterize the Miocene facies. [ABSTRACT FROM AUTHOR]

The Shanxi Formation Shan-2 Lower Sub-member transitional shale in the eastern Ordos Basin is characterized by high total organic carbon value, wide distribution, and large single-layer/cumulative thickness; it is the key target interval for transitional shale gas exploration. Based on drilling, logging, 3D seismic, and natural energy spectrum data, this study discusses the spatial distribution and controlling factors of complex lithology in the transitional shale stratum. Using the multi-lithological eigenvalue method, the lithologic differential distribution characteristics of the Shan-2 Lower Sub-member were identified through seismic attribute analysis and post-stack seismic parameter inversion. On this basis, the controlling factors of lithology spatial difference distribution are revealed by paleogeomorphology restoration, the sedimentary environmental indicator characteristics of shale, and seismic facies division. The results indicate that the lithology distribution of transitional shale strata in the Shan-2 Lower Sub-member is significantly different, and the geomorphological pattern of interjacent depressions and highlands lays a foundation for the formation of the sedimentary system. The environmental and energy differences of paleo-sedimentary water form a sedimentary environment with frequent facies transitions, which controls the distribution of sediments. Th/U and Th/K radios, which indicate redox conditions and sedimentary water energy during the early sedimentary stage of the Shan-2 Lower Sub-member, show that the geomorphic uplift area is characterized by a high-energy transitional environment of mainly developed sandstone and silty mudstone, while the depression is relatively low-energy under a brackish water reduction environment, mainly composed of both dark and carbonaceous shale. During the late sedimentary stage of the Shan-2 Lower Sub-member, when geomorphic control was weakened, it presented a shallow-water, high-energy, and oxidized transitional environment, with a lithology dominated by the complex interbedding of fine-grained sandstone, silty shale, shale, carbonaceous shale, and coal seam. Therefore, it is inferred that the Shan-2 Lower Sub-member may have developed as a barrier island-lagoon sedimentary model. The early depositional period was dominated by barrier island-lagoon-tidal flat depositional combination, thence the sedimentary environment gradually evolved into a tidal flat-swamp environment.

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. [ABSTRACT FROM AUTHOR]

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This article studies the sedimentary characteristics and models of the delta and gravity flow system of the third member of the Shahejie Formation in the Niuzhuang Sag area. Through seismic, logging, and core observation methods, a thorough investigation is conducted to examine the lithologic characteristics, grain size characteristics, sedimentary structure characteristics, and sedimentary facies distribution characteristics of this region. The results show that the third middle member of the Shahejie Formation in the Niuzhuang Sag can be classified into four sedimentary types: sliding, collapse, clastic flow, and turbidity flow. This article aimed to establish the distribution characteristics and depositional models of the deltaic and gravity flow depositional systems within the study area. The findings reveal that slip deposition primarily occurs near the delta front, while collapse and clastic flow depositions are concentrated near the far slope. Moreover, turbidity flow deposition is found near the far slope. This study significantly contributes to our understanding of the sedimentary characteristics and models associated with deltas and gravity flow systems in faulted lacustrine basins. Furthermore, it enriches existing theories related to gravity flow and provides a valuable reference for the investigation of deep-water sedimentation in continental faulted lacustrine basins.

The Dongfang1-1 gas field (DF1-1) in the Yinggehai Basin is currently the largest offshore self-developed gas field in China and is rich in oil and gas resources. The second member of the Pliocene Yinggehai Formation (YGHF) is the main gas-producing formation and is composed of various sedimentary types; however, a clear understanding of the sedimentary types and development patterns is lacking. Here, typical lithofacies, logging facies and seismic facies types and characteristics of the YGHF are identified based on high-precision 3D seismic data combined with drilling, logging, analysis and testing data. Based on 3D seismic interpretation and attribute analysis, the origin of high-amplitude reflections is clarified, and the main types and evolution characteristics of sedimentary facies are identified. Taking gas formation upper II (IIU) as an example, the plane distribution of the delta front and bottom current channel is determined; finally, a comprehensive sedimentary model of the YGHF second member is established. This second member is a shallowly buried "bright spot" gas reservoir with weak compaction. The velocity of sandstone is slightly lower than that of mudstone, and the reflection has medium amplitude when there is no gas. The velocity of sandstone decreases considerably after gas accumulation, resulting in an increase in the wave impedance difference and high-amplitude (bright spot) reflection between sandstone and mudstone; the range of high amplitudes is consistent with that of gas-bearing traps. The distribution of gas reservoirs is obviously controlled by dome-shaped diapir structural traps, and diapir faults are channels through which natural gas from underlying Miocene source rocks can enter traps. The study area is a delta front deposit developed on a shallow sea shelf. The lithologies of the reservoir are mainly composed of very fine sand and coarse silt, and a variety of sedimentary structural types reflect a shallow sea delta environment; upward thickening funnel type, strong toothed bell type and toothed funnel type logging facies are developed. In total, 4 stages of delta front sand bodies (corresponding to progradational reflection seismic facies) derived from the Red River and Blue River in Vietnam have developed in the second member of the YGHF; these sand bodies are dated to 1.5 Ma and correspond to four gas formations. During sedimentation, many bottom current channels (corresponding to channel fill seismic facies) formed, which interacted with the superposed progradational reflections. When the provenance supply was strong in the northwest, the area was dominated by a large set of delta front deposits. In the period of relative sea level rise, surface bottom currents parallel to the coastline were dominant, and undercutting erosion was obvious, forming multistage superimposed erosion troughs. Three large bottom current channels that developed in the late sedimentary period of gas formation IIU are the most typical. [ABSTRACT FROM AUTHOR]

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Unconventional natural gas resources in the Lower Yangtze region have caused plenty of interests of the Chinese energy companies in recent years. The developing characteristics and controlling factors of natural gas source rocks in the Middle Permian Gufeng Formation were studied by analyzing the typical field profiles and boreholes. The thickness center of Gufeng Formation shale locates in the Wuhu area, and the maximum of thickness is greater than 150 m. The total organic carbon (TOC) content of shale is relatively high, mainly ranging from 1.0% to 4.0%, with an average value of about 3.4%, and there are significant differences for the TOC content of different systematic tracts. Kerogens are mainly of types Ⅱ2 and Ⅲ. The Gufeng Formation shale is in the late stage of thermal maturity, and within the range of the dry gas window. The development of organic-rich beds in the Gufeng Formation was related to volcanism and its closely associated hydrothermal fluid activities. Volcanism and hydrothermal fluid activities brought materials from deep earth to surface, which enriched water body with nutrient-rich elements, caused biological bloom and formed high productivity, thus provided a good material for good source rocks.

During the Middle Permian, Earth experienced a period of profound environmental changes, including the final stages of the Late Paleozoic Ice Age (LPIA), global seawater regression, the Guadalupian mass extinction event, and the eruption of the Emeishan Large Igneous Province (ELIP) in South China. These events collectively induced significant shifts in both the environment and biosphere, yielding several geochemical anomalies, including deposition of manganese (Mn) deposits. The link between Middle Permian Mn deposits and the prevailing environmental conditions, however, remains unclear. Here, four sections spanning the Middle Permian Maokou Formation to the Late Permian Longtan Formation in the Zunyi area of northern Guizhou, South China are investigated. Through sedimentological and geochemical analyses, this work aims to elucidate the evolution of sedimentary environments over the Middle to Late Permian. During this period, a transition from a shallow-water carbonate platform in the 1st Member of the Maokou Formation to a deeper, siliceous deposit in its 2nd Member was recorded. This shift results in a pronounced paleogeographic differentiation in the 3rd Member, juxtaposing the shallow-water carbonate platform against a deeper Mn-enriched basin. A number of geochemical proxies indicate a strong hydrothermal signal in the Mn deposits. Combined with positive Eu anomalies and low Sr isotopes, the data underscore the profound influence of hydrothermal input linked with early subaqueous eruptions from the ELIP on the nearby marine environment. A detailed examination of carbon isotope profiles from the Maokou Formation indicates that volcanic influences, shifts in marine ecosystems, and changes in primary production caused the fluctuations of δ13C carb and δ13C org. Moreover, the formation of Mn carbonate in deeper waters effectively sequestered organic C. Collectively, our new data demonstrate that the essential conditions for the development of the carbonate-hosted Mn deposits include drowning of the carbonate platform through an increase in sea level, the availability of a sufficient Mn source, and the presence of a redox-stratified water column. [ABSTRACT FROM AUTHOR]

Dolomite is widely present in geological history, but its origin has always been a prominent problem that troubles sedimentologists. For lacustrine dolomite, current research has not yet provided a reliable explanation for its complex genesis mechanism. The Early Cretaceous lakes in Northwest China host various morphological dolomites, providing valuable materials for exploring the origin of dolomites. According to their petrological and mineralogical characteristics, it can be divided into thick laminated dolomite, thin laminated dolomite, dolomitic mudstone, and vein dolomite. The ratios of trace elements and rare earth elements show that these dolomites precipitated in a brackish–suboxic environment. The high δ13C values (>8 ‰VPDB) of thick laminated dolomite and some thin laminated dolomite suggest the involvement of methane-producing microorganisms in the precipitation of dolomite, and the appearance of microscale/nanoscale spherical dolomite aggregates and the dispersed organic matter around dolomite particles jointly confirm that microbial-mediated biological activity promotes dolomite precipitation. The dolomite stoichiometry (mole % MgCO 3) confirms that thick laminated dolomite was deposited in a restricted shallow water environment, while dolomitic mudstone is mainly deposited in relatively open water areas. The thin laminated dolomite in the shale laminae represents short-term or seasonal climatic and environmental fluctuations. In addition, some carbonate minerals of dolomitic mudstone in shallow water environment recrystallized by post-depositional hydrothermal effect, resulting in δ18O value decreased (<−10 ‰VPDB). The vein dolomite is characterized by high rare earth content and low δ13C and δ18O values, and its Sr isotope (0.712894 ± 0.000374) values reflect that the hydrothermal fluid may have been formed by the mixing of infiltrating lake water and crustal magmatic water. According to the characteristics of fluid inclusions, it is inferred that the hydrothermal fluid has the characteristics of low temperature (108.3 °C–159.8 °C), medium salinity (3.5 wt%–14.3 wt% NaCl) and high density (0.95–1.00 g/cm3). The microbial mediation and tectonic hydrothermal fluids play an important role in the formation of the Early Cretaceous lacustrine dolomite. • Microscale and nanoscale spherical dolomite aggregates appear in laminated dolomite. • The high δ13C values of dolomite suggest the involvement of methanogenic bacteria. • The strontium isotope ratios reveal that hydrothermal fluids originate from the crust. [ABSTRACT FROM AUTHOR]

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Take the Jurassic Sangonghe Formation in Mobei Area, Junggar Basin as the example, the sedimentary characteristics and models of the Jurassic Sangonghe Formation were analyzed based on core data, logging, thin sections, well logs and 3D seismic data. The results show that the coarse-grained shallow water delta deposits, which are mainly coarse and medium-coarse sandstone, are widely developed in the Mobei Area. Two grade siope breaks and three zones controliing sedimentation are deveioped in the western depression of well Pen 1. Coarse-grained shallow braided silver delta-lake system develops in the uplift-lope area, forming two subfasies of the inner and outer front, which distribute vertically or obiiquely aiong the lakeshore as a whole in the shape of fiowers or btrds feet. The underwater distsibutary channel sandbody which are mainly coarse and medium-coarse grained feldspar ilthic sandstone is the frontal skeleton. Block and traction flow origin structures are well developed, and thetr physical properties are relatively good. Muli-stage sand bodies are well developed in the vertical and cut and o-vellap each other in the lateral, and continuous distribution. Four front underwater distributary channels which are widely distirbuted and extends far away are developed in the plane near east-west dtrection. The type is the transgressive-regressive, coarse-grained shallow water delta which has a large area and abundant source supply in wet climate. The results may serve as a reference for deepening expiora-ion and development in the Mobsi Area and simllar fields in the Junggar Basin. [ABSTRACT FROM AUTHOR]

Although the potential of lake facies shale oil and gas resources is enormous, the lithology is complex, heterogeneous, and obviously controlled by a sedimentary environment. By integrating cores, thin sections, X-ray diffraction analysis, trace elements, organic geochemical indexes, and paleontological data, this work studies sedimentary environment elements and sedimentary model of shale in the E1f2 member, Gaoyou Depression, Subei Basin. The shale in the E1f2 member is completely shore lacustrine deposits forming into the hot dry climate of tropical central and south Asia. The lake basin extended continuously, with brackish water, strong reduction, and medium water stratification. The transgression resulted in the rapid rise of water, salinity, reducibility, and water stratification. As the climate becomes wetter, the E1f2 water level, salinity, reduction, and stratification decrease. The shale in E1f2 members is characterized by sedimentary zones, which are divided into muddy water desalination deposit in the flooded zone, clear water salinization deposit in the transition zone, and still water salinization deposited in the clear water zone. Under palaeotopography settings of E1f2 members, combining characteristics of vertical stratification and the plane zone of the lake, an integrated sedimentary model of lacustrine shale in a flat platform has been established under climate and transgression control. The plane area of the lake could be divided into the slow slope zone, sub-depression zone, processes zone, deep slope zone, and deep depression zone, corresponding roughly to the sedimentary zones of the flooded zone, transition zone, and clear water zone. Moreover, the oil shale enriched in low-lying parts of the paleotopography is mainly deposited at the end of muddy water areas and clear water areas.

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Abstract: Taking the Cambrian Yuertus Formation outcrop profiles in the Aksu-Keping-Wushi areas of northwestern Tarim Basin as examples, the depositional environments of organic rich fine sediment were analyzed by examining the outcrop profiles macroscopically and microscopically. The study reveals that: (1) The lower part of the Yuertus Formation consists of organic-rich fine sediment or thin rhythmic interbeds of organic-rich fine sediment and siliceous sediment, the formation transforms to terrigenous diamictic grain shoal and inverse grading carbonate rocks upward. (2) The thin limestone interbedded with dark shale rhythmically has inverse grading. (3) The thin-bedded siliceous rock has metasomatic residual granular texture, stromatolithic structure and cementation fabric in vugs. (4) There are iron crust layers at the top of the shallowing diamictic grain shoal, beneath which exposed karst signs, such as karrens, dissolved fissures, sack-like vugs, near surface karst (plastic) breccia, breccia inside the karst system and terrigenous clastic fillings, can be seen. (5) Both the outcrops and seismic profiles show that organic-rich fine sediments above the unconformities or exposed surfaces are characterized by overlapping. The organic-rich fine sediment of the Cambrian Yuertus Formation was deposited in the anoxic-suboxidized restricted gulf lagoon environment, and its formation was controlled by high paleoproductivity and poor oxygen exchange jointly, then a shallow-water overlapping sedimentary model has been established. The results will help enrich and improve the sedimentary theory of organic-rich fine sediments.

Abstract In order to reconstruct the sedimentary environment of marly strata related to the upper parts of Qom Formation, one stratigraphic outcrop have been studied. Studied section is located in the Zarrin-Abad area (South of Zanjan) and is composed of 135 meters of marls and argillaceous limestones. Theses strata are limited by thick bedded limestone (related to the F Member of the Qom Formation) at the base. There is a thick bed of evaporate layer at the top of these strata with sharp boundary that can be stratigraphically related to the Upper Red Formation. Based on petrographic and texture studies, eight microfacies have been identified. These microfacies are related to the distal parts of inner shelf, patch reef, slope, toe of slop and deep shelf sub environments. Based on microfacies analysis we can propose an open shelf as sedimentary environment for these strata. Relative abundance of allochems show one cycle (rise and fall) in relative sea level along the stratigraphic column that can be correlated with eustasy curve. Keywords: Microfacies, Sedimentary model, Qom Formation, Zarrin-Abad, Zanjan. Introduction The Qom Formation (Oligocene–Miocene) in central Iran is composed of carbonate and silisiclastic strata that is related to the lagoon, reef and basin sedimentary environments (Reuter et al. 2009). Due to the hydrocarbon potential, this formation has been considered by many geologists. Different sedimentary environment (ramp and/orshelf) have been proposed for this formation. Thus, this study try to reconstruct the sedimentary environment for these strata in the south of Zanjan to complete the comprehensive plan for the sedimentary environment of the Qom Formation. This formation in the studied section (Zarrin-Abad) is unconformably underlain by silisiclastic strata of the Lower Red Formation (Aalipour et al. 2017) and overlain (covered boundary) by the evaporitic package of Upper Red Formation. This study focused on upper parts of Qom Formation (marls and argillaceous limestones) in order to reconstruct sedimentary environment. Material & Method One stratigraphic section has been sampled in the South of Zanjan (Sothwest of Zarrin-Abad). Forty two samples from 135 meters of marls and argillaceous limestones (Upper parts of Qom Formation) have been collected and thin sections were prepared. Sampling intervals were about three meters in average and included hard and semi-hard sediments. The microfacies analysis using polarized microscope have allowed the reconstruction on carbonate environment of upper parts of the Qom Formation in this section. Carbonate rock classification, facies study and environmental interpretation on thin sections was based on Dunham (1962) and Flugel (2010). All thin sections have been housed in Paleontological Laboratory, Department of Geology, University of Zanjan. Discussion of Results & Conclusions Biostratigraphic studies confirm a Burdigalian age for these strata based on two index fossils (Borelis meo curdica, Meandropsina Iranica). One biozone has been identified based on first occurrence of Borelis melo curdica in the base of section (S1) and last occurrence of ­Meandropsina Iranica in the top of section (S39). This biozone can be correlated with (Adams and Bourgeois 1967) and (Daneshian and Ramezani Dana 2019) zonations. Microfacies analysis lead to identification of eight microfacies (1- Bioclast benthic foraminifera algal grainstone, 2- Coral boundstone, 3- Bioclast wackestone, 4- Sandy bioclast grainstone, 5- Sandy bioclast packstone, 6- Bioclast mudstone, 7- Pelagic mudstone/wackestone and 8- Pelagic packstone) related to the middle to outer carbonate shelf environment. Coral reef facies in lower parts of studied section can approve a carbonate shelf as sedimentary environment. Moreover, bioclast particles that consist of corals, benthic foraminifera, algae and some other fossils beside coral boundstone facies show fore reef facies in front of the platform margin. Based on presence of clastic particles in pelagic facies, we can conclude that there were some channels in coral reef areas that connect back reef environments to the open marine areas. Based on field observation and microfacies analysis, we proposed non-rimmed carbonate shelf as sedimentary environment for the studied strata. Based on quantitative analysis on environmental data, most of samples (strata) are related to the deep shelf environment. Quantitative analysis diagrams (allochems percentage) shows in each samples that there were rising trend in relative sea level from the base of section to the middle part. Following this trend the falling trend towards the end of the section have been recorded. This trends can be correlated with the global sea level changes during this time (Burdigalian). Thus it can be concluded that the Burdigalian basin in this area had been connected to the open oceans.

In order to clarify the characteristics of fine-grained sedimentary lithofacies and the depositional models in lacustrine environments of the Chang 7 Member of the Upper Triassic Yanchang Formation in the Ordos Basin, we focus on the lacustrine lithofacies classification and controlling factors. Based on the typical field sections of the southern Ordos Basin, combined with the methods of organic geochemical analysis, polarizing microscopic observation, XRD and elemental geochemistry, we summarize the main controlling factors of the lithofacies under different sedimentary environments and establish the sedimentary model under different sedimentary backgrounds. Results show that the Chang 7 Member of the Ordos Basin includes six major lithofacies types, i.e., quasi-laminated clayey shale facies, blocky silty mudstone facies, quasi-laminated silty shale facies, laminated silty shale facies, blocky mixed mudstone facies, and laminated argillaceous siltstone facies. Al2O3, MgO, CaO, Na2O, K2O, and SiO2 are relatively depleted, while TiO2, TFe2O3, and P2O5 are relatively enriched in the Chang 72+3. Meanwhile, the trace elements are relatively enriched in Cd, Cu, Mo, U, and V. The fine-grained sedimentary facies were influenced by various sedimentary environments, such as paleoclimate, redox conditions, productivity and terrigenous input. According to lithofacies types and genetic mechanisms of the Chang 7 Member in the study area, two types of lithofacies assemblages are identified. The rapid rise in lake level during the deposition of the Chang 72+3 resulted in an anoxic water column, high productivity, and low terrigenous input under a humid climate and weak-to-moderate weathering conditions. Therefore, clayey shale lithofacies assemblage is developed in the lower Chang 7 Member.