Third-order Sequences Research Articles

Seismic sedimentology analysis of meter-scale thin sand bodies in the Jurassic Qigu Formation, Yongjin area, Junggar Basin

Seismic sedimentology is an interdisciplinary field that integrates sedimentary geology and geophysics to accurately characterize the spatial and temporal distributions of sedimentary systems. This paper focuses on the Qigu Formation in the Yongjin area of the Junggar Basin, China. The objectives are to establish a high-resolution sequence stratigraphic framework and to clarify the types of sedimentary systems present. Various technical methods, including 90° phase adjustment, attribute clustering, red-green-blue attribute fusion, stratal slicing, and seismic phase-controlled waveform indication inversion, are employed to restore the vertical evolution history of the sedimentary system and quantitatively characterize the spatial-temporal distribution of the thin sand bodies in the Qigu Formation. The results revealed that the Qigu Formation is divided into a third-order sequence, consisting of a lowstand systems tract and a lacustrine transgressive systems tract. During the lowstand systems tract, a thick channel sand body developed in the delta front, exhibiting a non-muddy, intermittent normal grading structure. The lacustrine transgressive systems tract features thin, discontinuous channel sand bodies with mud layers, also forming an intermittent normal grading structure. Two distinct types of delta fronts are identified: the first, formed during the lowstand systems tract, is characterized by branching subaqueous distributary channels in a shallow-water environment. The second, associated with the lacustrine transgressive systems tract, displays a network-like distribution of subaqueous distributary channels in a relatively deeper-water environment.

Improved Transient Modeling and Stability Analysis for Grid-Following Wind Turbine: Third-Order Sequence Mapping EAC

Improved Transient Modeling and Stability Analysis for Grid-Following Wind Turbine: Third-Order Sequence Mapping EAC

Investigating the potential of unconventional resources in the Midland Basin: A comprehensive chemostratigraphic analysis of a Cline Shale (Wolfcamp-D) core

Basinal mudrocks assigned to the Wolfcamp Group in the Midland Basin are prolific producers of oil and gas. Technological advancements in horizontal drilling and hydraulic fracturing have been successful in the exploitation of oil and gas from these tight mudrocks. However, many challenges remain unsolved in capturing their macroscale compositional variations to enhance recovery factors and delineate “sweet spots”. To gain improved insights into this fine-grained system, high resolution chemostratigraphic (XRF) data was integrated with mineralogical measurements (XRD), logs, thin sections, and TOC readings, as well as hierarchical cluster analysis to: 1) develop a sequence stratigraphic framework, and 2) reconstruct paleo-redox, as well as ancient water-mass, conditions within the Cline Shale (Wolfcamp-D) from a core in the Midland Basin.This study revealed the presence of three chemo-stratigraphically distinct depositional sequences in the Cline Shale, herein termed Lower, Middle, and Upper Cline from the base upwards. All three units exhibit distinct mineralogical and petrophysical characteristics, were correlated across the basin, and interpreted as third-order sequences. Superimposed onto the three interpreted third-order sequences within the Cline, are potential higher frequency (fourth-order) sequences. These fourth-order sequences typically consist of a lower interval, interpreted as lowstand deposits and an upper interval, which is interpreted as transgressive and highstand deposits. The lower interval is interpreted to be deposited during highly reducing conditions while the upper interval is interpreted to be deposited under suboxic conditions. The highest organic matter preservation is associated with the interpreted lowstand deposits, especially in the Upper Cline. Optimal landing zones (20 ft) are recognized in the upper zone of the Cline Shale based on their high resistivity, total organic richness, elevated siliceous content, and reduced percentage of clay minerals. These zones have been correlated with several nearby wells.The current study demonstrates the wealth of information that can be gained from a single core/well when properly investigated at high resolution with measurements on a cm-scale. The observed shifts in the chemistry of the sediments character and composition are distinct, with implications for sequence stratigraphic analysis, the reconstruction of ancient water-mass conditions, and the identification of “sweet spots” for hydrocarbon resources. The provided analysis lays a foundational basis for further studies in the region, serving as critical data input for expansive regional research efforts.

Retrogradation of carbonate platforms on a rifted margin: the late Ediacaran record of the northwestern Yangtze Craton (SW China)

The Late Ediacaran Dengying Formation, located in the Sichuan Basin of the northwestern Yangtze Craton, is of significant interest in oil and gas exploration due to its abundant pores and vugs within microbial mound-shoal complexes. However, there is still uncertainty regarding the spatiotemporal distribution and controlling factors of the platform margin. This study comprehensively analyzes the retrogradation pattern of the Dengying Formation platform margin using seismic data, well logs, field outcrops, and petrological characteristics. Our findings reveal that the Dengying Formation strata surrounding the rift basin at the northwestern of the Yangtze Craton can be divided into three main depositional facies: basin facies, slope facies, and platform margin facies. Additionally, based on the integration of lithological, log, and seismic characteristics, the Dengying Formation is subdivided into four third-order sequences, with five sequence boundaries and three seismic facies identified. Supported by sequence stratigraphy and geophysical data, we have reconstructed the tectono-sedimentary evolution of the multiple platform margins on the eastern side of the Deyang-Anyue rift in the Sichuan Basin during the late Ediacaran. Our findings indicate that the platform underwent two phases of retrogradation. The second-stage platform margin underwent retrogradation towards the interior, spanning a distance between 10 and 80 km, based on the initial configuration established by the first-stage platform margin. The main controls for progradation and retrogradation of carbonate platforms are eustatic sea-level changes and tectonic activity. Eustatic sea-level changes can be divided into constructive and destructive phases. Constructive phases are commonly observed in highstand systems tracts, while destructive phases are often associated with transgressive systems tracts and are related to platform retrogradation processes. However, sea-level changes alone cannot fully control the process of platform retrogradation. The thermal subsidence following mantle plume events likely played a significant role in the retrogradation of the platform in the study area. During this period, tectonic processes controlled the geometry of the platform and the deposition of carbonates in the platform margin-slope-basin environment. Additionally, karst-related mound-shoal complexes developed extensively along the platform margin of the Dengying Formation in the northwestern Yangtze Craton. The Lower Cambrian dark shales represent high-quality hydrocarbon source rocks, while the Dengying Formation exhibits an optimal source-reservoir configuration.

Ypresian–Lutetian platform evolution in the Indus Basin, Southwestern Pakistan: An interplay between local and regional tectonic changes

The present investigation aims to analyze the depositional environment, sequence stratigraphy, and geochemistry of the Early–Middle Eocene succession in the Laki Range of the Southern Indus Basin, Pakistan. The sequence is subsequently correlated with other sections within the Southern Indus Basin, Central Indus Basin, Upper Indus Basin, and Hazara Basin in Pakistan. For this study, the Lakhra and Laki formations were sampled from four sections in the Laki Range. Based on detailed outcrop observations, eight lithofacies were identified including four clastic lithofacies (CLF-1 to 4) and four limestone lithofacies (LLF-1 to 4). Similarly, based on detailed petrographic examination, seven microfacies were identified in limestone units. Considering the biotic paleoecology, facies texture, and chemical composition, depositional environments were assigned to the microfacies and lithofacies. All the microfacies, and the shale and sandstone lithofacies (CLF-1 and CLF-4) represent shallow inner to deeper outer-shelf settings whereas the other two lithofacies (CLF-2 and CLF-3) represent deltaic and tidal flat settings, respectively. All the microfacies and lithofacies information was used to establish sequence stratigraphy for the studied strata. The top of the Lakhra Formation and the whole of the Laki Formation in the Bara Nala Section (BNS) represent two complete, and two partial, third-order sequences, further divided into fourth order and small-scale cycles. The sequence's development is primarily influenced by regional and local tectonics. The regional correlation of the Indus Basin and Hazara Basin indicates that the regional basin's bathymetry and resultant depositional sequence were significantly influenced by the Himalayan Orogeny. The regional depositional pattern indicates that subsequent to India–Asia collision, the closure of eastern Tethys took place in a temporal succession from northwestern to the southwestern parts of the Indus Basin. In the Central Indus Basin, marine environments persisted until the Priabonian Stage (SBZ20) compared to those of the Upper (SBZ11–12) and Southern (SBZ13–14) Indus basins. This study offers valuable insights into both the local and regional depositional frameworks and the influence of local and regional tectonics on a carbonate platform evolution.

Evolution of a low-energy carbonate ramp, Lower Mississippian Pekisko Formation, northwestern Alberta, Canada

The lithofacies architecture and depositional evolution of the Lower Mississippian (Tournaisian) Pekisko Formation in the subsurface of the Hawk Hills area in northwestern Alberta have been established by integrating detailed core work and well log data. The formation is composed of skeletal-peloidal limestones and argillaceous limestones that were deposited along the northern flank of the Peace River Embayment, a semi-restricted and tectonically active oceanic re-entrant located along the western margin of Laurasia at low paleolatitude. Lithofacies associations recognized in the study area include the outer ramp to slope (LA 1), outer ramp (LA 2) and mid ramp (LA 3), which are stacked into three decameter-scale, deepening-upward and aggradational cycles that are of regional extent and have meter-scale deepening and shallowing-upward trends. A previously unrecognized paleosol horizon at the top of decameter-scale cycle 2, indicating widespread subaerial exposure of the ramp, is interpreted as a sequence boundary that divides the Pekisko and Shunda formations in the study area (and possibly elsewhere in the Peace River Embayment) into two third-order sequences, each consisting of transgressive and highstand systems tracts. The Pekisko Formation in the study area is interpreted to represent a low-energy, temperature-stratified ramp that was mainly homoclinal, but with transient, distal steepening occurring in the southern part of the study area. Ramp deposition was strongly affected by basement-fault reactivation causing differential subsidence and uplift in the Peace River Embayment. The paleogeography and paleoceanographic conditions of the embayment favored upwelling currents and development of a temperature-stratified ramp, as well as the formation of heterozoan carbonate deposits and mid ramp facies of predominantly packstones and wackestones. This depositional scenario is atypical, as most other documented examples of the Pekisko Formation and other Lower Mississippian ramp successions in western North America and western Europe are characterized by moderate to high-energy, mid to inner ramp facies deposited in open-ocean conditions. The results of this study contribute to an improved understanding of the range of depositional settings along the western margin of Laurasia during the Early Mississippian and demonstrate the applicability of the thermocline-stratified ramp model, with some modification, to ramps in semi-restricted embayments and other low energy settings.

The role of pre-existing faults in the early–middle eocene palaeogeographic evolution of the PY4 sag, Pearl River Mouth basin, South China Sea

The early–middle Eocene represents a pivotal geological phase in the Pearl River Mouth Basin, as it witnessed the emergence and transformation of numerous Cenozoic rifted basins. Nonetheless, the research on the palaeogeography of this period and its interplay with pre-existing faults within the pre-Cenozoic basement remains scant. This article delves into this topic through the lens of the PY4 Sag, a prominent ENE-striking rifted basin situated in the central region of the basin. Initially, we reconstruct the palaeogeography of five third-order sequences within the study interval, using 3D seismic data and drilling data. Then, by amalgamating characteristics of pre-existing faults in the pre-Cenozoic basement, garnered from the published literature, and analyzing their Cenozoic evolution and influence on lake basin structure, sedimentary environment, and sediment distribution, we delve into the correlation between these pre-existing faults and the stratigraphic palaeogeographic evolution in the study area. Our findings reveal that the PY4 Sag experienced five distinct stages of palaeogeographic evolution from the early to middle Eocene, including the initial formation of an ultradeep lake (SQ1), filling of an ultradeep lake (SQ2), early expansion of a shallow lake (SQ3), late expansion of a shallow lake (SQ4), and expansion of a semideep lake (SQ5). This process was accompanied by a gradual migration of the subsidence and sedimentation center of the lake basin from northeast to southwest. Since the Cenozoic, influenced by northwest extension and its clockwise rotation, two pre-existing thrust fault systems striking ENE and EW–WNW within the basement of the Pearl River Mouth Basin have been successively activated. This led to the emergence of numerous ENE-striking normal faults (exhibiting relatively stronger activity) during the early Eocene (SQ1) and EW–WNW-striking normal faults (mainly distributed in the west of the study area) during the middle–late Eocene. Consequently, the PY4 Sag developed a deep and expansive ENE-striking lake basin in the early Eocene, followed by the formation of several smaller EW–WNW-striking lake basins in the middle–late Eocene. The early ENE-striking deep lake progressively evolved into a shallow one due to continuous filling, while the later E–W-striking lake basins broadened and deepened, culminating in the amalgamation of these two distinctively oriented lake basins. In essence, the different spatio-temporal evolution of these two basement pre-existing fault systems during the Eocene, together with episodic magma intrusions in the middle Eocene, are the primary factors controlling the palaeogeographic evolution in the PY4 Sag. Analogously, other ENE-striking sags within the Zhu I Subbasin of the Pearl River Mouth Basin underwent a comparable palaeogeographic transformation during the early–middle Eocene.

Biostratigraphy and facies around the D/C boundary interval of the Tuye-Darvar section, Eastern Alborz Range, NE Iran

The 1st order mass extinction at the Devonian/Carboniferous transition, known as the Hangenberg Crisis, is characterized by major transgressive/regressive cycles which led to widespread ocean anoxia during the Hangenberg Black Shale Event, as well as to a global major sea-level fall and the worldwide deposition of regressive Hangenberg Sandstone equivalents. The Devonian/Carboniferous transition at the Tuye-Darvar section in the eastern Alborz Range is studied in terms of conodont biostratigraphy, litho-, microfacies and sequence stratigraphy. In order to examine the biostratigraphical framework, forty conodont samples were systematically taken from the studied interval. Thirty-two conodont species belonging to ten genera led to the discrimination of twelve conodont zones, ranging from the Pseudopolygnathus granulosus Zone to the Scaliognathus anchoralis-Doliognathus latus Zone. Due to facies, the conodont record also exhibits some hiatuses. Field observations and sedimentological and microfacies studies led to the identification of thirteen facies types from sub-tidal environments to the fully marine environments, including seven microfacies types. The sediments deposited in a mixed carbonate-siliciclastic platform, revealed four third-order sequences. The Hangenberg Black Shale is not recorded in the Tuye-Darvar section as a result of depositional facies. In comparison with other studied Devonian/Carboniferous Boundary (DCB) sections of the central and northern Iran, the Tuye-Davar section suggests a tectonic position, which is most likely placed on a separate tectonic block.

Sequence stratigraphy of the paleozoic successions in Ghadames Basin, southern Tunisia: Insights from integrated interpretation of well logs and seismic data.

The Ghadames Basin is considered as huge intracratonic structure, which preserved over 4000 m of Paleozoic sedimentary succession deposited over the Precambrian Basement. The present work consists of an integrated interpretation of the Paleozoic interval in the Ghadames basin of southern Tunisia using well logs and calibrated seismic data. Thus, the adopted sequence stratigraphic approach allowed the identification of a total of 3 s-order sequences, bounded by Taconic, Caledonian, and Hercynian unconformities recognized as major tectonic discontinuities in the study area. The determined second-order sequences have been further subdivided into fourteen third-order sequences with their associated systems tracts, which are limited by sequence boundaries (SB), and presenting transgressive surfaces (TS), and maximum flooding surfaces (mfs). Each sequence is characterized by a unique stacking pattern and distinctive facies, thus reflecting the lithology and the corresponding depositional environment. These sequences are composed of claystone and sandstone facies deposited in varying depositional environments, ranging from a shallow marine setting during the Ordovician period to a deep marine environment during the Early Silurian. A complex deltaic environment was evidenced during the Late Silurian involving to a fluvial environment during the Devonian period. The integration of well logs and seismic data proved to be an efficient approach in the delineation of sequence stratigraphy, revealing in detail the spatial distribution and sequence organizations within the study area. These later are controlled by the relative sea level variations, with eustasy is being the dominant factor. The obtained results are mainly correlatable with the sequence stratigraphy of North African basins at a regional scale and Snedden and Lee chart (2010) at a global scale.

Tectono-stratigraphic evolution in the Paleocene Lishui Sag, East China Sea Shelf Basin

The tectonic evolution of the Lishui Sag in the East China Sea Shelf Basin significantly influences its sequence stratigraphy. By integrating seismic reflections, well logs and core data, this study examines the quantitative activity of the Lingfeng fault, sequence framework and sequence architecture in the Lishui Sag. The Paleocene succession is divided into one second-order sequence (SSQ1), three third-order sequences (SQ1–SQ3) and nine system tracts, representing a complete syn-rift evolution consisting of initial syn-rift, rift climax and late syn-rift stages. The results indicate that the differential activity of the Lingfeng fault shapes the basin structure and controls the subsidence centre, leading to prominent variations in the spatial distribution and filling of the sequence stratigraphy during different tectonic stages. The initial syn-rift sequence (SQ1), characterised by a low activity of the segmented Lingfeng fault, is primarily found in isolated small sub-sags. The rift climax sequence (SQ2), with increasing displacement and longer segmented fault zones, exhibits a wedge-shaped filling pattern with substantial topographic elevation differences. The late syn-rift sequence (SQ3) shows a gradual decrease in tectonic activity and uniform filling, with all system tracts within the sequences fully developed. The sequence architecture of the hanging wall in the Lishui Sag can be classified into fault-scarp and fault-slope type, whereas the sequence architecture of the hanging wall dip-slope can be classified into sedimentary slope-break and faulted slope-break.

Sedimentary differentiation caused by glaciation and orogeny during the Ordovician–Silurian transition in the southern Sichuan Basin, China

The Wufeng–Longmaxi shales, which were deposited during the Ordovician–Silurian transition in the southern Sichuan Basin, exhibit distinct lithofacies transitions. The shift from organic-rich siliceous shale to organic-poor mixed shale and argillaceous shale is attributed to the influences of the Hirnantian glaciation and the Kwangsian Orogeny. Nevertheless, debates persist regarding the mechanisms underlying these significant lithofacies transformations. This study employs sequence stratigraphy, petrology, and geochemical analysis of key wells to unveil crucial insights. The findings indicate: 1) the Wufeng Formation to the Long-1 Member could be divided into two third-order sequences; 2) the waterbody underwent a transition from anoxic and strongly restricted conditions to oxic and moderately restricted conditions; and 3) the trend in formation thickness changed from an E–W orientation to a NE–SW orientation. The results suggest that considerable sediment condensation and anoxia resulting from glacier melting in the early Rhuddanian played a pivotal role in the formation of organic-rich siliceous shale. During the Middle Rhuddanian–Aeronian, the accelerated uplift caused by the Kwangsian Orogeny offset the transgression induced by glacier melting, leading to shallow water conditions and increased terrigenous influx. This diluted the organic matter content in the sediment, and subsequent turbidity currents altered sediment composition, driving the lithofacies transformation into organic-poor mixed shale and argillaceous shale. The study posits that the sedimentary differentiation in the southern Sichuan Basin during the Ordovician–Silurian transition is linked to the initiation and progression of the far-field effects of the Kwangsian Orogeny.

Sequence lithofacies paleogeography evolution of the Middle Permian Maokou Formation in the northwest margin of the Sichuan Basin

Due to the present tectonic and stratigraphic distribution characteristics in the northwest margin of the Sichuan Basin are complex, which restricts the understanding of sequence lithofacies paleogeography of the Middle Permian Maokou Formation. To investigate the process of tectonic–sedimentary evolution, basin-margin outcrop and intra-basin well and seismic data were used. The results show two structural sequences, namely, SSQ1 and SSQ2, and five third-order sequences, namely, SQ1–SQ5, in the Middle Permian Maokou Formation. SSQ1, with stable formation thickness, shows a slow transgression–fast regression sequence structure. SSQ2 shows a fast transgression–slow regression sequence structure in the area with large thickness and a sustained transgression–fast regression sequence structure in the area with small thickness owing to thickness differentiation caused by tectonic subsidence. Sedimentological analysis shows that sequences SQ1–SQ2 mainly consist of carbonate-ramp sediments, the sedimentary facies of which change from intermediate ramp to outer ramp as the area depressed topographically from south to north. Sequences SQ3–SQ5 mainly consist of rimmed carbonate platform sediments; the platform margin turned up along Shuangyushi–Jian’ge–Yuanba, and the region to the north subsided rapidly to form slope-basin facies. Lithofacies paleogeography in the Middle Permian epoch shows that the northwest margin of the Sichuan Basin was dominated by extensional stresses in northeast and northwest directions. Tectonic activities have a dominant control on the distribution of two structural sequences and the evolution of carbonate platform types in the Maokou Formation, and the sea-level change has a dominant control on third-order sequence boundaries and sedimentary facies migration. Thus, the Middle Permian tectono-sedimentary evolution model of the northwest margin of the Sichuan Basin is established. Research findings may offer new ideas and theoretical support to promising facies exploration in the Maokou Formation in the northwest margin of the Sichuan Basin.

Sequence Stratigraphic Analysis of the Volcanic Rock–Evaporite–Clastic Rock Symbiotic Zone in the Fourth Member of the Shahejie Formation in the South of Laizhou Bay

Laizhou Bay Sag is a typical continental rift lake basin, which is affected by strike–slip activity, salt rock activity, volcanic activity, and paleoclimate change in the Tanlu fault zone, where three major lithologies—volcanic rock, clastic rock, and evaporite—mainly develop. In order to determine the sequence correspondence between the different lithologies in the study area based on drilling, logging data, and seismic data, it is determined that the fourth member of the Shahejie Formation in the southern Laizhou Bay mainly develops three types of sequences: clastic rock type sequences, salt lake type sequences, and volcanic rock/clastic rock type sequences. Two second-order sequence boundaries (SBs3L, SBs4L) and one third-order sequence boundary (SBs4U) were identified, and the principle of three divisions of system tracts (LST, TST, HST) and sedimentary processes in different types of sequences were determined by analyzing the stacking patterns of different sequences. Compared with the previous research results, this study provides a more detailed analysis of the sequence boundary definition of different blocks and different lithologic strata, and it improves the previous general sequence stratigraphic division method. Based on the study of distinct types of sequences, a unified stratigraphic framework is established in the whole zone, and the stratum thickness distribution of the fourth member of Shahejie Formation is smaller in the southwest and larger in the northeast. The study area is generally filled with pyroclastic rocks–evaporite–clastic rocks, changing from fan delta–saline lake filling to braided river delta–freshwater lake–fan delta filling from bottom to top. A sequence stratigraphic model for the comparison of steep slope–gentle slope differences is established to provide guidance for future oil and gas field exploration and development.

Pore-Type Characterization and Reservoir Zonation of the Sarvak Formation in the Abadan Plain, Zagros Basin, Iran

Carbonate reservoirs are characterized by their complex depositional–diagenetic history and, consequently, intricate pore networks. In this study, pore types and reservoir zones of the Sarvak Formation, which is the second most important oil reservoir in Iran, are investigated by integrating core, thin-section, porosity–permeability, scanning electron microscopy (SEM), and mercury injection capillary pressure (MICP) data. Depositional, diagenetic, and fracture pores are identified in macroscopic to microscopic scales. Frequency analysis of pore types revealed that diagenetic pores, including vuggy, moldic, intercrystalline, and fracture pores, formed the majority of pore spaces of the Sarvak Formation. They are dominantly recorded in the regressive systems tracts of both the Cenomanian and Turonian sequences, just below the paleoexposure surfaces. Away from the disconformities, there are two general trends regarding the pore-types distribution: (a) grain-supported facies of shoal and talus settings with dominant primary (depositional) pores including interparticle, intra-skeletal, and growth-framework pores, which are concentrated in the middle part of the formation, and (b) microporous mud-dominated facies with various types of primary and secondary pore types in the TSTs of third-order sequences, especially in the lower part of the Sarvak Formation. Stratigraphic modified Lorenz plot differentiated reservoir, speed, barrier, and baffle zones in this formation. Depositional–diagenetic characters, petrophysical properties, and dominant pore types are defined for each zone in a sequence stratigraphic framework.

Sequence Stratigraphy of the Jurassic Strata and Occurrences of Potential Sandstone Reservoirs in the ST Gas Field, Northern West Siberia Basin

Jurassic strata in the ST gas field of the northern West Siberia Basin have been regarded as a potential exploration target with undiscovered hydrocarbon resources. However limited research has been performed on the sequence stratigraphy of the Jurassic strata, as well as its sandstone distribution controlled by variable sea level change and sediment input. In this paper, four third-order sequences (SQ1, SQ2, SQ3, and SQ4) and nine fourth-order sequences for the Jurassic strata are interpreted based on seismic facies analysis and the lithology stacking patterns of seven wells. SQ1 is characterized by the special Bazhenov Formation which is featured by regionally distributed deep marine shales. SQ2 (J1) is composed of a coarsening upward sequence, the base of which is an unconformable surface that can be recognizable in both seismic profiles and well logging data. SQ3 (J2-J8) is composed of a complete fining-upward and coarsening-upward sequence, showing a series of transgressive and regressive successions. A complete SQ4 has not been drilled through by all the seven wells, only showing a coarsening upward succession on its top (J9) which evolves into a fining upward succession at the base of SQ3. Combined with the seismic inversion result, which predicts sandstone distribution, a sequence evolution model was built for SQ3 showing a full unit of transgressive system tract and highstand system tract (TST-HST) which often occurs in shallow marine shelves. During sequence development, most reservoir sandstones are deposited in the shelf and tidal delta environment at the bottom of the TST and the top of HST, and mudstones are deposited as shelf mudstones, especially at maximum flooding surface. That is controlled by both accommodation and sediment input. Generally, under this sequence framework, the depositional architecture can be further analyzed with implications for source rock, reservoir sandstones, and sealing rock, which may guide future gas exploration and exploitation in this area.

Fine Division of Internal Cycles of Glutenite based on INPEFA Technology and Wavelet Transform

In order to study the internal cycle characteristics of glutenite reservoirs, this study conducted high-resolution sequence division of glutenite based on INPEFA technology and wavelet transform technology. In this paper, the basic principles of INPEFA technology and wavelet transform technology are introduced in detail, and the differences and advantages and disadvantages of the two methods in high-resolution sequence division are compared. Taking Jilantai oilfield as an example, combined with INPEFA technology and wavelet transform technology, INPEFA processing and wavelet processing are innovatively carried out based on acoustic time difference curve, and the accuracy is verified by each other. On this basis, combined with logging, seismic, lithology and other data, the top and bottom interfaces of the third-order sequence of the K1g1 in the study area were identified, and on this basis, K1g1 was further divided into two oil groups, a total of eight layers.

Tectonic-geomorphological evolution and provenance-sedimentary response: Insights from the Middle Jurassic–Lower Cretaceous, Junggar Basin, China

Understanding the complex depositional systems within lake basins requires a comprehensive analysis of the interplay between tectonic-geomorphological evolution and climate change. Based on three-dimensional seismic data, outcrops, well logging, cores, thin sections, and heavy mineral data, this study reconstructed the tectonic-geomorphological evolution of the Middle Jurassic–Lower Cretaceous in the Junggar Basin and analysed the responses of sequence stratigraphy, sediment provenance, and depositional systems. The Jurassic Xishanyao and Cretaceous Qingshuihe Formations developed third-order sequences (SQ1 and SQ2, respectively), which both consisted of lake transgressive (TST) and regressive system tracts (RST). During the SQ1 period, the Chepaizi-Mosuowan palaeo-uplift underwent substantial erosion and supplied sediment to the surrounding sub-basins, where deposited braided-river deltas were formed. Middle Yanshanian tectonism resulted in rapid subsidence, and the Chepaizi-Mosuowan palaeo-uplift submerged. During the early TST stage of SQ2, sediment from the northeast and northwest formed shallow braided-river deltas in topographic lows surrounding the residual Chepaizi-Mosuowan palaeo-uplift and reduced topographic roughness, resulting in a gentle slope to the south. The rapid lacustrine transgression from southeast to north in the late TST stage of SQ2 formed local beach-bars at shallow depths. During the RST stage of SQ2, a climate transition occurred from arid to humid, resulting in depositional systems that evolved from braided to meandering river deltas. The sedimentary fillings of the SQ1 and SQ2 stages were mainly controlled by tectonic-geomorphology and climate change, respectively. Collectively, our results suggest that the tectonic-geomorphological evolution and climate change jointly controlled the sediment sources and determined the depositional features in the basin.

Gamma-ray, stable carbon and oxygen isotope chemostratigraphy and sequence stratigraphy of the Lower Mahil Formation (KS-1 Khuff-Equivalent), Northern Oman

This research presents findings from a study focused on the Lower Triassic (Induan) Lower Mahil KS-1 Formation, situated on a homoclinal carbonate platform in Northern Oman. The sequence stratigraphy of this formation is characterized by a considerable thickness variation, slumps, and breccia deposits related to active normal faults coupled with intra-basin growth faults. The main objective was to establish a reliable stratigraphic framework for the Lower Mahil KS-1 Formation by integrating high-resolution carbon isotope data along with high-resolution spectral and total gamma-ray data. To achieve this, whole-rock samples were analyzed for δ13C and δ18O isotopes. Spectral and total gamma-ray records were obtained for the formation. Isotope sampling is conducted every 0.5 m in the Saiq Plateau and Wadi Sahtan sections. Furthermore, spectral gamma-ray measurements were taken at intervals of 10 cm from the logged sections. Within the third-order sequence, the spectral gamma-ray data revealed a distinct sea-level trend, leading to the division of KS1 into two different parts. Five fourth-order depositional sequences were identified by analyzing stable carbon isotopes, uranium, and total gamma-ray profiles. Four of these sequences displayed complete patterns, reflecting transgression and regression phases, while the fifth sequence was incomplete and solely comprised a transgressive phase. An essential outcome of the study is the correlation of the δ13C curve of the Lower Mahil KS-1 Formation with other similar formations around the Tethys region. This correlation indicates that the Lower Mahil KS-1 Formation captures the near-primary signal of carbon isotope variations in coeval seawater. As a result, it holds promise as a reference section for future investigations and studies in this field. Compared to the prior investigation, this study utilizes data with higher precision, capturing spectral gamma-ray measurements at 10 cm intervals and isotope measurements at 50 cm intervals. Furthermore, the study’s focus is confined explicitly to KS1.

Multi-scale sedimentary facies controls on fluvial sandstone-type uranium accumulation process in the Songliao Basin, NE China

The relationship between uranium mineralisation and sedimentary facies in a fluvial depositional setting remains a challenging topic. However, the specific effect of muti-scale sedimentary facies control on meandering sandstone-type uranium enrichment is poorly understood. Based on sedimentological, geochemical, mineralogical, and petrophysical analyses, three third-order sequences (SQ1, SQ2, and SQ3) composed of four systems tracts in meandering deposits were identified in the Sifangtai Formation of the northern Songliao Basin. Four subfacies (channel, levee, flood, and abandoned channels) and seven microfacies were identified in meandering rivers, exhibiting spatio-temporal migration of sedimentary fillings. Here, uranium deposits were mainly enriched in semi-abandoned channel deposits during the lowstand systems tract (LST) period of SQ1. Stable-distributed and thick-layered industrial-grade uranium deposits occurred in a positive rhythmic rock assemblages, exhibiting high-frequency variation from thicker medium–fine sandstone to thin charcoal-rich siltstone–fine sandstone. Rapid tectonic uplift and warm-semi humid climate during early Sifangtai period controlled sequence growth and sedimentary filling. Furthermore, sequence and sedimentation during the LST period mediated the formation of uranium-rich rock assemblage and fluid migration pathway, reacting on the abundance, location, and distribution of uranium enrichment. Ultimately, a conceptual model of the sequence and sedimentary control on uranium accumulation was established in the context of meandering rivers. This model revealed that semi humid-warm climate and tectonic inversion (74.9–72.2 Ma) control meandering river, sedimentary hiatus, and charcoal-rich uranium-hosted rock series formation in the LST. Enhanced Cenozoic tectonic uplift (38.0–10.0 Ma) and continental weathering prompted uranium-rich fluids along steep paleotopography, migrated to geochemical barriers of charcoal-rich rock series, and reductive mineralisation around semi-abandoned channel deposits. Our results illustrate the importance of examining multi-scale sedimentary facies to interpret the uranium accumulation process of meandering deposits, ultimately contributing to uranium exploration in similar settings.

Geodynamic reconstruction of offshore eastern Tunisia during the Cretaceous and Paleogene periods and its impact on hydrocarbon exploration

The analyses of structural, seismostratigraphic patterns and seismic anomalies were conducted using borehole and two-dimensional (2-D) seismic data to reconstruct the geodynamic evolution of offshore eastern Tunisia from the Cretaceous to the Paleogene times. Thirteen third-order sequences (S1 to S13) were identified, and several hiatuses were recognized, which were most likely related to the tectonic event associated with the convergence between Eurasian and African plates. Aptian buildups have also been recognized in the outer platform margin. The growth and distribution of these reefs are controlled by the uplift of fault blocks responsible for the relative sea-level fall which led to sub-aerial exposure and the development of a karstified carbonate platform. The structural features and the major tectonic events as well as the stratigraphic pattern, recognized in offshore eastern Tunisia, are related to the geodynamic evolution of the North African and western Mediterranean basins.The identification of several undrilled reef buildups with significant hydrocarbon potential could replenish the operators' portfolio.