Depositional Setting Research Articles

Highlighting relationships between sand thicknesses, reservoir-seal pairs and paleobathymetry from a sequence stratigraphic perspective: An example from Tortonian Serravallian deposits, onshore Niger Delta Basin

The utilization of sequence stratigraphic concepts in identifying sands and their spatial continuity in distinct gross depositional settings is key, especially in frontier settings where data paucity is a common challenge. In the Baka field, onshore Niger Delta, detailed reservoir correlation guided by sequence stratigraphic framework analysis showed the distribution of sand and shale units constituting reservoir-seal pairs (RSP) correlatable across the field. Within the 3rd-order packages, it is observed that the lowstand systems tract (LST) and highstand systems tract (HST) contain more RSPs and thicker 4th- and 5th-order sands than the transgressive systems tract (TST). In terms of bathymetry, it is noted that irrespective of systems tracts, the RSP Index (RI) decreases from the proximal shallow/inner shelf settings to the more distal outer shelf areas. Amongst all three systems tracts, intervals interpreted as lowstand prograding complexes contain the best developed sands and highest RSP. Sand development within the LSTs has been controlled by a pronounced growth fault regime accompanied by high subsidence and sedimentation rates. This is linked to the basinward migration of the sands during prolonged sea-level fall, creating significant accommodation space for sand deposition. On the other hand, the TSTs known to mark periods of progressive sea-level rise and landward migration of sandy facies, show thinner sands enclosed in much thicker, laterally extensive, and better-preserved deeper marine shales. Interpreted seismic sections indicate intense growth faulting and channelization that influenced the syn- and post-depositional development of the sand packages across the field. The initial timing of deformation of sub-regional faults in this area coincides with periods of abrupt falls in sea level. This approach could be useful for predicting sand-prone areas in frontier fields as well as possible reservoir-seal parameters required for some aspects of petroleum system analysis and quick-look volume estimation.

Development and Application of the Branched and Isoprenoid GDGT Machine Learning Classification Algorithm (BIGMaC) for Paleoenvironmental Reconstruction

AbstractGlycerol dialkyl glycerol tetraethers (GDGTs), both archaeal isoprenoid GDGTs (isoGDGTs) and bacterial branched GDGTs (brGDGTs), have been used in paleoclimate studies to reconstruct environmental conditions. Since GDGTs are produced in many types of environments, their relative abundances also depend on the depositional setting. This suggests that the distribution of GDGTs also preserves useful information that can be used more broadly to infer these depositional environments in the geological past. Here, we combined existing iso‐ and brGDGT relative abundance data with newly analyzed samples to generate a database of 1,153 samples from several modern sedimentary settings. We observed a robust relationship between the depositional environment and the relative abundances of GDGTs in our samples. This data set was used to train and test the Branched and isoGDGT Machine learning Classification (BIGMaC) algorithm, which identifies the environment a sample comes from based on the distribution of GDGTs with high precision and recall (F1 = 0.95). We tested the model on the sedimentary record from the Giraffe kimberlite pipe, an Eocene maar in subantarctic Canada, and found that the BIGMaC reconstruction agrees with independent stratigraphic and palynological information, provides new information about the paleoenvironment of this site, and helps improve its paleotemperature reconstruction. In contrast, we also include an example from the PETM‐aged Cobham lignite as a cautionary example that illustrates the limitations of the algorithm. We propose that in cases where paleoenvironments are unknown or are changing, BIGMaC can be applied in concert with other proxies to generate more refined paleoclimate records.

SEDIMENTOLOGY, PALAEOGEOGRAPHY AND DIAGENESIS OF THE UPPER PERMIAN (Z2) HAUPTDOLOMIT FORMATION ON THE SOUTHERN MARGIN OF THE MID NORTH SEA HIGH AND IMPLICATIONS FOR RESERVOIR PROSPECTIVITY

This paper provides an updated understanding of the reservoir stratigraphy, sedimentology, palaeogeography and diagenesis of the Upper Permian Hauptdolomit Formation of the Zechstein Supergroup (“Hauptdolomit”) in a study area on the southern margin of the Mid North Sea High. The paper is based on the examination and description of core and cuttings data from 25 wells which were integrated with observations based on existing and new 3D seismic.Based on thin‐section petrography of cuttings and core from the wells studied, it is evident that Hauptdolomit microfacies are distributed in a relatively predictable way, and well‐defined platform interior, platform margin, slope and basin settings can be distinguished. Platform margins are typically characterised by the development of ooid shoals and, to a lesser‐extent, by microbial build‐ups. High‐energy back‐shoal settings are characterised by a more complex combination of peloid grainstones, thrombolitic and microbial build‐ups, and fine crystalline dolomites. Lower energy lagoons which developed further behind the platform margin are characterised by a variety of microfacies types; fine crystalline dolomites are common in this setting as well as peloidal facies and local microbial build‐ups. Intertidal and supratidal settings are typified by increased proportions of anhydrite and the development of laminated microbial bindstones (stromatolites). Platform margins are in general relatively steep and pass into slope and basinal settings. Only a few wells have penetrated Hauptdolomit successions deposited in a slope setting, and these successions are characterised by a range of resedimented shallow‐water facies together with low‐energy laminated dolomicrites and fine crystalline dolomites. Slope zones in the study area are interpreted from seismic data to be typically 1‐1.5 km in width. Basinal Hauptdolomit deposits have been strongly affected by post‐depositional diagenesis and are dedolomitised to variable degrees. The original depositional facies are rarely preserved.Diagenetic studies show that dolomitisation has affected almost the entire Hauptdolomit Formation throughout the study area in both basinal and platform settings. The dolomite is considered to result from seepage‐reflux processes and is an early diagenetic phase. Mouldic porosity is present in many facies types as a result of dissolution, especially in ooid grainstones, thrombolitic build‐ups and peloidal facies. The dissolution cannot be associated with any one diagenetic phase but was most likely a result of the dolomitisation process itself. Stable isotope analyses indicate that all dolomites were precipitated from Permian marine‐derived pore fluids. Fluid inclusion analyses of dolomite cements indicate that cementation continued into the burial realm. Anhydrite cementation occurs in two phases: early anhydrite precipitation was associated with dolomitisation, and can be distinguished from a later, pore‐filling cement which is highly detrimental to reservoir quality.The Hauptdolomit succession in basinal wells (and in some slope wells) in the study area has undergone significant dedolomitisation. Dedolomitisation was a shallow burial process which affected precursor dolomites, whereby excess calcium from the transition of gypsum to anhydrite during burial combined with CO2 and organic acids derived from basinal sediments. The process was triggered by excess calcium reacting with excess carbonate ions from dissolution.3D seismic volumes supplemented by numerous 2D lines were available in the study area and allowed an interpretation to be made of Hauptdolomit gross depositional settings; platform margins and base‐of‐slope polygons were mapped, with the greatest confidence in areas of 3D seismic. The basin, slope and platform settings were distinguished using seismic data integrated with the results of micro‐facies analysis and incorporating seismic‐to‐well ties. The data shows that large parts of the study area are characterised by the presence of polyhalites within the overlying (Z2) Stassfurt Halite Formation, which may create particular seismic geometries at the Hauptdolomit slope. These are interpreted to be intra‐Stassfurt Halite features, providing an alternative model to the thickened, prograded Hauptdolomit which has been suggested in previous publications.Because few wells drilled in the study area had the Hauptdolomit as the primary target, cores were limited but significant data was obtained from cuttings analyses. More than 400 thin sections were evaluated, allowing depositional models based on microfacies observations to be developed, verifying the seismic‐scale observations.

A Bayesian hierarchical age model for optical dating of single grains of quartz

In optical dating, the last time that a sample of sediment was exposed to sunlight is determined by dividing its equivalent dose (De) by the dose rate. For single-grain dating, the sample De is based on the statistical analysis of the distribution of De values estimated for individual grains, whereas the dose rate is usually determined from measurements of the environmental radioactivity of the bulk sample, together with allowances for radiation sources internal to the grains and cosmic rays. Conventionally, the De and dose rate are measured and analysed separately to produce an estimate of the depositional age of a sample, but this approach may result in loss of information because distributions of single-grain De values are influenced by several factors. Existing statistical models do not incorporate all the key information contributing to age estimation, such as the pattern and scale of dispersion of single-grain De values and dose rates, the associated measurement uncertainties, the effect of natural variability among grains, and the outlier probabilities of De and dose rate estimates. Here we propose an empirical Bayesian hierarchical age model (BHAM) for optical dating of quartz samples that incorporates the above information to estimate their depositional ages. The BHAM is based on the implementation of standardised growth curve and LnTn methods to integrate information on the full distribution of single-grain De values, sources of measurement uncertainty, beta-dose heterogeneity (observed or modelled), and detection of outliers. We present the results of validation tests using data sets of optically stimulated luminescence measurements and dose rates obtained for quartz samples dated previously from Denisova Cave (Russia) and for simulated samples. We conclude that the BHAM represents a robust and flexible approach to dealing with data for single grains of quartz within a Bayesian hierarchical framework and is suitable for application to sediments deposited in a variety of depositional settings.

Depositional Setting of the Shiranish Formation (Campanian-Maastrichtian) in Selected Sections from Northern Iraq

The Shiranish Formation (Campanian-Maastrichtian) was studied in two surface sections (Diana and Garuta) in Erbil Governorate. Both sections lie within the High Folded Zone of Zagros Fold-Thrust Belt in Iraq. Stratigraphically, the formation is divided into three lithostratigraphic units depending on the field characteristics with varied thicknesses of the formation in both sections. The lower unit (Unit A) consists of blue to grey well-bedded marly massive limestone, the middle unit (Unit B) is composed of blue to grey marl, marly limestone and stratified shale, and finally, the upper unit (Unit C) is composed of grey limestone and blue marly limestone. The lower contact of the formation is unconformable with the Bekhme Formation in the Diana section and conformable with the Kometan Formation in the Garuta section, while the upper contact is conformable with both Tanjero and Kolosh formations in Diana and Garuta sections respectively. X-ray diffraction analysis has revealed that calcite is the main mineralogical component with less of quartz and clay minerals as represented by chlorite and illite. Four lithofacies were distinguished (Marl facies (Sml), Shale facies (Ssl), Conglomerate limestone facies (Scl)) and red clay layers (Srl), whereas, three main microfacies were recognized; Fossiliferous Lime Mudstone Microfacies (Sml), Lime Wackestone Microfacies (Sw) and the Packstone Lime Microfacies (Sp), which in turn were subdivided into seven secondary facies, according to their components, which are; Planktonic Foraminiferal Lime Wackestone Submicrofacies (Sw1), Keeled Planktonic Foraminiferal Lime Wackestone Submicrofacies (Sw2), Calcispheres Lime Wackestone Submicrofacies (Sw3), Bioclastic Lime Wackestone Submicrofacies (Sw4), Benthonic Foraminiferal Lime Wackestone Submicrofacies (Sw5), Keeled Planktonic Foraminiferal lime Packstone Submicrofacies (Sp1) and Planktonic Foraminiferal Globular Chamber Packstone Submicrofacies (Sp2 ). Based on these facies, the formation was deposited in a relatively deep marine environment and within two secondary environments; the outer shelf and upper bathyal environments as a result of the widespread transgression in the beginning of the Campanian.

Implications of a geochemical approach to reconstruct the Maastrichtian–Danian seaways of central India: The planktic foraminifera-bearing Jhilmili intertrappean revisited

Maastrichtian–Danian sedimentary sequences associated with Deccan volcanism are commonly interpreted to be deposited in fluviatile-lacustrine environments, except for the nearshore marine outcrops in Razol province (Rajahmundry). Discovery of Danian zone-P1a planktic-foraminifera, from a thin-horizon of an otherwise entirely lacustrine-palustrine Jhilmili intertrappean section in central India, makes this section unique, with reliable age-constraints. The presence of this foraminifera-bearing unit was used to postulate the existence of an ∼800 km marine-seaway from the west up to central India. Occurrences of non-marine biota in the foraminifera-bearing horizon and the absence of associated lithotopes and biotopes characteristic of marine transgressive events in this section is noteworthy. This study focuses on trace-element geochemistry and clay-mineralogy of the Jhilmili section to explore changes in depositional conditions along this section and obtain insights on the ambient waters from which Unit 3 was deposited. High-resolution sampling has been conducted and geochemical proxies such as Sr/Ba, Y/Ho (whole-rock, clay-fractions) and REE+Y (carbonates) have been utilized. Geochemical parameters suggest a non-marine environment for the entire section except the middle-unit showing brackish conditions because of storm/tsunami induced seawater mixing into freshwater depositional settings. Lithofacies, fossil-assemblage and variations in geochemical ratios indicate short-term episodic mixing of marine elements, possibly through storm/tsunami related activity in the lacustrine-palustrine freshwater ecosystem. Previous palaeobiological records from outcrops in the Godavari Rift indicate that seawater possibly entered through the Godavari rift. This study provides additional insights for the Maastrichtian–Danian paleogeography of central India.

Geochemical evaluation and hydrocarbon generation potential of the Upper Cretaceous–Pliocene succession, offshore Nile Delta, Egypt

The offshore Nile Delta is a prominent petroleum province; nevertheless, previous research focused mainly on evaluation of Miocene–Pliocene possible source rocks because of lack of data. The current study is the first to evaluate organic geochemical features of the Upper Cretaceous–Pliocene sequence in the western offshore Nile Delta, applying TOC, Rock-Eval pyrolysis and GC-MS procedures for 137 ditch cuttings collected from three wells. The main objectives are to infer organic richness, type of kerogen, maturation, and depositional paleoenvironment of organic matter for these possible source intervals.The analysed rock samples generally display fair to good organic enrichment. Most of these samples are enriched in Type-III and Type-II/III kerogen, suggesting a high influx of terrigenous organic matter. However, Abu Madi, Appollonia, and Khoman formations contain Type-II oil-prone kerogen, reflecting more marine organic matter, whereas Kafr El Sheikh and El Wastani cuttings display the lowest TOC contents and are more enriched in non-generating inert organic matter.Biomarker ratios indicate that Kafr El Sheikh, Abu Madi, Qawasim, and Qantara formations are rich in Type-II/III kerogen (mixed organic matter) preserved under suboxic nearshore or lacustrine depositional environments. In contrast, Sidi Salem, Tineh, Appollonia, and Khoman formations contain abundant Type-II marine-algal organic matter deposited under anoxic marine depositional settings. Vitrinite reflectance data and biomarker ratios indicate immature to peak mature rock samples. The stable carbon isotopic composition of condensate oil samples suggests nonmarine waxy oils. Ratios of n-alkanes and isoprenoids for a condensate sample from the study area reveal no similarity with the Upper Cretaceous–Pliocene rock extracts.

Palynofacies analysis of the JOG16N-8 borehole, K5 formation from the Maniamba Basin (middle Permian), Mozambique: A contribution for hydrocarbon exploration

The Maniamba Basin in Mozambique is surveyed mainly in regard to coal exploration. However, this basin is also known for containing well preserved fossil plants and vertebrate fossils that shed light to the understanding of Gondwana ecosystems. Nevertheless, organic matter analysis (palynofacies) and its implication for depositional environment and hydrocarbon source rock have not yet been performed. Here, we report for the first time in the Maniamba Basin, results from palynofacies analysis combined with thermal maturation and kerogen typification. Palynofacies analysis was applied on 69 samples from the sediments of the JOGN16N-8 borehole, K5 Formation (middle Permian).The results indicate that sediments are dominated by phytoclasts with less abundant sporomorphs and sparse amorphous organic matter (AOM). Three palynofacies were identified suggesting depositional settings in a general proximal and oxic lacustrine-fluvial deltaic environment. The dominance of opaque phytoclasts may be due to wildfires, oxidation conditions before or post deposition in proximity to terrestrial settings and an environment under obliquity cycle controlled monsoon climate. The organic matter composition combined with Spore Colour Index (SCI), pointed out the existence of an abundant type IV kerogen followed by type III in immature phase suggesting very little or no potential for gas exploration. Very few horizons suggest the existence of kerogen type II. This study, gives significant information for better understanding of the variation of organic matter, palaeoenvironment and source rock potential of the studied area.

Sedimentary provenance in continental rifts: U–Pb detrital zircon, Nd and Sr isotopes and lithogeochemistry of the Eocene alluvial sandstones of the Resende Basin, SE–Brazil

Multiproxy analysis of detrital minerals is a powerful tool for unravelling the source-to-sink evolution of syn-rift sedimentary basins that represent important depositional settings during major rift events. The Cenozoic Resende half-graben in SE Brazil is a classic example of a syn-rift basin developed under oblique extension, which preserves evidence of tectonic episodes and palaeoclimatic conditions during the opening of the Atlantic. Sediment in the basin was sourced from the surrounding basement rocks, which to the faulted margin consists of older Proterozoic orthogranulites and orthogneisses, whilst to the flexural margin is characterised by Neoproterozoic passive margin paragneisses. Cretaceous intra-plate alkaline stocks crop out at the western edge and in the central-south basement of the basin and represent a distinct provenance fingerprint. This study uses new detrital zircons U–Pb ages combined with previously published elemental and Nd and Sr isotopic data to characterise the sedimentary provenance of Eocene alluvial deposits and to understand how sediment sourcing has changed throughout the evolution of the basin. U–Pb detrital zircon analyses from samples collected across the Resende Basin are presented herein. Samples yield a spread of detrital age spectra from the Archaean to the Cretaceous. Precambrian zircons range from ca. 3228 Ma to ca. 542 Ma. Cretaceous zircons yield ages between ca. 138 Ma and ca. 67 Ma. The sediment source location is interpreted by comparing the yielded detrital zircon spectra with the age spectra of zircons from the basement rocks. The dominance of detrital zircons yielding ages of ca. 1500 Ma and ca. 1000 Ma is interpreted to be sourced from the Neoproterozoic basement rocks to the flexural border of the half-graben. Cretaceous zircons from samples with whole rock εNd40Ma > −10 are found in both the western and eastern portions of the half-graben but not in the central portions. It is interpreted that an along-axis fluvial system was responsible for a small increase in the compositional maturity of the Resende sandstones from west to east, resulting in the progressive dilution of Cretaceous alkaline igneous detritus along the basin axis. Preserved metamorphic rims in zircon grains indicate low transport abrasion, suggesting that Cretaceous zircons in the eastern depocentre could be associated with other small proximal sources. U–Pb zircon analyses and Nd and Sr isotopes provide reliable insights regarding sediment provenance and transport processes, whilst element geochemistry supplies evidence for hydraulic sorting effects. Consequently, the data represents a more robust estimation of the provenance patterns of the Resende Basin, showing chemical and geochronological differences that support the division of the basin into three depocentres (western, central, and eastern) connected by a longitudinal fluvial system sourced mostly by sediments coming from the south-western flexural border. This study highlights the usefulness of using multiple provenance proxies to unravel the complex development of rift basins, assessing multiple source terrains and the variable degrees of mixing in sedimentary basins.

Lithofacies and depositional settings of Mesoproterozoic shales: Insights from the ∼1.65 Ga Chuanlinggou Formation, Yanliao Basin, North China Craton

Precambrian black shale has become a significant trend in petroleum exploration, and the Chuanlinggou Formation is the oldest potential hydrocarbon source rock deposit in the North China Craton. The Mesoproterozoic was a unique transitional stage in the evolution of the environment and life in Earth's history. Research on lithofacies and depositional settings in this stage has important scientific and oil and gas exploration significance. Outcrop observations, thin sections, and elemental and organic geochemical analyses were used to study the mineralogy, sedimentary structures, lithofacies assemblages and their relationships with the depositional environment. Three endmembers (silicate, carbonate and clay minerals) based on X-ray fluorescence were employed to establish the lithofacies classification suitable for the Chuanlinggou Formation in the Yanliao Basin. The lithofacies of the Chuanlinggou Formation were classified into six types, including calcareous mudstone, siliceous/calcareous mixed mudstone, argillaceous/siliceous mixed mudstone, silicate-rich argillaceous mudstone, clay-rich siliceous mudstone and siliceous mudstone. Four lithofacies assemblages (LAs) were identified according to the integrated lithofacies analysis from the bottom to the top of the section. Lithofacies assemblages 1 and 2 (LA1 and LA2), which were deposited in deep subtidal to shelf environments, have high petroleum potentials and are advantageous lithofacies assemblages for organic matter accumulation. The black shale developed early rift may not be as favorable as in the late rift, while the siliceous shale developed in the continental shelf-slope is a favorable lithofacies. A comprehensive cognition of the LA and depositional settings of the Chuanlinggou Formation might provide guidance for further petroleum exploration and sedimentary environments in the Mesoproterozoic.

The characteristics and origin of chlorite coats in the Oligocene gas sandstone reservoirs of the Upper Huagang Formation, Xihu Depression, East China Sea Shelf Basin

Clay coats are widespread in the Oligocene braided river channel sandstones in the Xihu Depression on the East China Sea Shelf Basin. High clay coat coverage in braided channel sandstones is typically associated with good reservoir quality. The morphology, origin and distribution of clay coats and their effect on reservoir quality in different braided channel facies are investigated by core description, core analysis and a series of petrographic techniques. The present clay coats consist of two parts, the inner and outer coats. The inner coats are mainly composed of illite and chlorite components. The inner illite coats are transformed from smectitic clay precursors, while the inner chlorite coats may originate from thermally driven recrystallization of Fe-rich clay precursors. The outer clay coats, composed of well-crystallized chlorite, are possibly the result of neoformation directly from Fe- and Mg-rich pore water. The distribution of inner clay coats is controlled by sediment transport and deposition processes. The inner illite coats that are better developed in terms of content and thickness typically form in low energy braided channels due to relatively less transport abrasion of the smectitic clay precursors. The inner chlorite coats are observed exclusively in high energy mud clast-rich braided channels. The abundance of mud clasts and Fe-rich clay precursors are likely derived from the intense erosion of adjacent overbank mudstones, which are enriched in flocculated fluvial Fe oxides and hydroxides. The high coverage outer chlorite coats tend to occur in high energy braided channels with well-developed primary pores, since they provide sufficient growth space for chlorite. High energy braided channels with relatively low ductile component content generally maintain better porosity and permeability, which facilitates the development of the outer chlorite coats, thereby preserving more pore space by inhibiting quartz cementation and promoting subsequent dissolution of unstable silicate minerals, further improving reservoir quality. The results of this study provide new insights into the prediction of chlorite-coated reservoir quality of gas fields in Xihu Depression while also being relevant to braided river sandstones having comparable depositional settings worldwide.

Reconstruction of Tournaisian-Viséan tectonic and climatically induced event histories of the Mobarak carbonate platform along depositional strike in the northeastern margin of Gondwana: Constraints from high-resolution cycle and sequence stratigraphy

Cyclic successions are one of the most important types of deposits in the carbonate platform of the Mississippian (Tournaisian-Viséan) in Gondwanan and Laurussian margins. The importance of climatically induced events impact on the amplitudes and frequencies of sea-level fluctuations in Mississippian of Iran is significant because the beginning, extent, duration, and significance of periods of climatically induced events in northern margin of Gondwana are not well recognized. A high-resolution sequence architecture in integration with paleoenvironmental reconstruction and foraminiferal biozonation from the Tournaisian-early Visean carbonate successions of the Mobarak Formation in Alborz basin along depositional strike (E-W- a distance of c. 340 km) suggests a mosaic-like pattern of deposition and compartmentalization of the depositional settings (homocline ramp to distally steepened ramp). According to the recognized facies, the stacking pattern of high-frequency cycles across the platform, and major accommodation events recognized on the correlated Fischer plots, six 3rd-order sequences (DS1–DS6) were identified. Detailed analysis on high-frequency cycles representing a highly variable number of shallowing-upward cycles, types, thicknesses, and facies heterogeneity along depositional strike. A combination of the episodic tectonic events along faults interacting with autocyclic processes, and inherited topographic irregularities are suggested as the driving mechanism for the cycle development. Six stages of Mobarak carbonate platform evolution based on understand the interplay of autocyclic and allocyclic controls are recognized in a hierarchical 3rd-order sequence stratigraphic framework and a chronological order: (1) early Hastarian corresponds to post-glacial transgressive episode with progressive backstepping of facies belts of early TST of DS1 throughout the region; (2) middle to late Hastarian corresponds to a major transgressional phase of DS1 following global temperature rise superimposed on different subsidence/uplift rates with the deformation of the platform along depositional strike; (3) Hastarian–Ivorian boundary corresponds to TST of DS2 with facies heterogeneity along depositional strike related to continuation of different subsidence/uplift rates. The absence of any systematic changes in the amplitude and period of cyclicity or lowstand deposits and subaerial surfaces during this phase shows the lack of influence of mid-Tournaisian glaciation and concurrent eustatic regression in northern margin of Gondwana. (4) early to middle Ivorian coincides with the RST of DS2 and TST of DS3 showing by remarkable topographic irregularities in the basin physiography and change in the position of the depocenter axis from a E–W trend to the center; (5) late Ivorian-early Moliniacian coincides with the RST of DS3 and DS4 showing by the Late Tournaisian thermal period and eustatic rise superimposed on uplift and subaerial exposure of the western part and subsidence in the central and eastern parts of the basin; (6) Moliniacian coincides with DS5-DS6 sequences showing by change from fault-related topography to major tilted fault-block topography accompanied by eustatic sea-level fall caused indirectly by the Viséan and Serpukhovian glaciations phases.

Geochemical assessment and hydrocarbon potential of Oligocene–Pliocene source rocks from northeast onshore Nile Delta, Egypt

Recent exploration in the Nile Delta Basin has led to major oil and gas discoveries; however, source–reservoir relationships in the onshore part of the basin are still ambiguous. This work involves a comprehensive geochemical assessment of possible Oligocene–Pliocene source rocks, using TOC/Rock-Eval pyrolysis and gas chromatography–mass spectrometry. The aim is to investigate quantity, quality, thermal maturity, sources, and depositional paleoenvironment of the disseminated organic matter, and to correlate rock samples with hydrocarbons retrieved from the study area. Moreover, the chemical and isotopic compositions of gases were employed to examine origin, maturity, mixing and secondary alteration processes.Results reveal fair to good organic richness (TOC ∼1 wt%) for the Oligocene–Pliocene rocks, with the highest TOC content from the Oligocene Tineh Formation. The kerogen is generally gas-prone Type-III and to a lesser extent Type-IV and Type-II/III. Molecular and biomarker results indicate mixed source facies with variable contributions from higher plants, algae, bacteria, and plankton, deposited under suboxic to anoxic nearshore marine or lacustrine depositional settings. Significant biomarkers include elevated C26/C25 tricyclic terpane ratios (0.82–3.62), low C31 homohopane (22R)/C30 hopane ratios (0.17–0.63), and low oleanane and gammacerane contents. Maturity-related biomarkers, Rock-Eval Tmax and vitrinite reflectance values are consistent and suggest immature to early mature rock samples.Molecular and isotopic compositions of mud gases indicate complex origins and mixing histories ranging from primary microbial to pure thermogenic, where thermogenic processes dominate the pre-Miocene intervals.Chemometric analysis of 18 source-related biomarker ratios for rock extracts revealed four genetic families. Hierarchical cluster analysis (HCA) of biomarker data for rock extracts and condensate oils from the onshore Nile Delta indicates no correlation between Miocene–Pliocene rocks and condensates or oils in the area. Therefore, pre-Miocene source rocks are suggested to be the most probable candidates for hydrocarbons in the onshore Nile Delta.

Facies and architectural element analysis of the Upper Bokabil Sandstone in the Bengal Basin

The Upper Bokabil Sandstone is a geologically significant unit in the Bengal basin, hosting the most prolific hydrocarbon reservoirs and serving as a source of groundwater. Despite its economic importance, the depositional setting of this unit has not yet been thoroughly studied, largely due to the limited accessibility to the outcrops. This research aims to provide the first modern sedimentological interpretation of the Upper Bokabil Sandstone in the Bengal basin utilizing data from outcrops and well logs. A generic model of the depositional environment is proposed based on sedimentary facies and architectural elements. The sedimentary facies of the Upper Bokabil Sandstone were examined at outcrops in the Sari River section of Jaintiapur (northern Bengal basin) and Paglir Bill in Ukhiya (southern Bengal basin). A total of 12 fluvial lithofacies and multiple high and low order erosional surfaces were identified. Facies associations and their relationship with their bounding erosional surfaces have been utilized to identify the architectural elements. The unit is characterized by laterally and vertically amalgamated channels and bar complexes, with downstream and lateral accreted barforms being the dominant architectural elements. Lateral Accretion (LA) is more prevalent in the southern part of the basin. The presence of the fluvial facies, distinct architectural elements associated with typical braided river and lack of aeolian facies lead to the interpretation of a low sinuosity braided river with seasonal fluctuation in discharge as the depositional environment of the Upper Bokabil Sandstone.

Geochemical approach for decoding the paleoenvironmental and depositional evolution of a coastal lacustrine Konservat-Lagerstätte (Early Cretaceous, south-Central Pyrenees)

The lithographic limestone of La Pedrera de Meià (LPM) in south-Central Pyrenees (NE Spain) is considered one of the best preserved lacustrine-coastal successions of the Early Cretaceous in Europe, hosting a taxonomically diverse record of Barremian biota. While this Konservat-Lagerstätte has been extensively surveyed for paleontological purposes, little is known about the paleoenvironmental and depositional conditions prevailing during the formation of this fossil lagerstätte deposit. LPM limestone is made up of a homogeneous, dark gray to pale brown, faintly laminated 50 m-thick succession of micrite-rich intervals. To shed light on the environmental conditions of deposition, 303 rock samples were studied using a multiproxy approach based on different techniques including petrography, SEM-EDX, XRD, ICP-MS, XRF, Rare Earth Element distributions and C and O stable isotope geochemistry. The combination of sedimentological and geochemical data reveals a vertical shift of facies from littoral to profundal environments and then returning to littoral conditions. Profundal facies are made up of filament-rich mudstones originated from in situ precipitation of micrite either through biotically mediated or physico-chemical processes in undisturbed low energy settings, under anoxic conditions, characterized by the presence of transported terrestrial and lacustrine organisms that are exquisitely well-preserved. Littoral areas are characterized by the presence of in situ charophytes, smooth ostracods, and miliolid foraminifera that accumulated in mud-rich facies indicating low/moderate-energy, shallow-water, and relatively oxygenated conditions. Stable isotopic data and elemental geochemistry suggest a hydrologically closed to semi-closed lacustrine system with a restricted water circulation encouraging dysoxic to anoxic conditions in profundal sediments. Paleoproductivity proxies suggest a good correlation between catchment-derived siliciclastic discharge to the lake and primary productivity peaks. Furthermore, a decline in terrigenous input coupled with a reduction of meteoric water entrance to the lake is interpreted based on elemental proxies derived from paleosalinity and paleoredox analysis. Collectively, the strontium/barium (Sr/Ba) ratios, the sparse presence of miliolid foraminifera, and the isotopic signatures indicate an increase in salinity toward the younger intervals of the lake infilling, although their ultimate causes are unclear (i.e., increased evaporation, or a combination of reduced meteoric input and sporadic marine inputs might explain our data). The present work shows the importance of performing detailed multiproxy geochemical-sedimentological studies to better constrain the unique paleoenvironmental and paleohydrological conditions associated with the exceptional preservation of fossil biotas in Konservat-Lagerstätte deposits in analogous depositional settings.

Geochemistry of granulite facies BIFs from the Luis Alves Craton (Southern Brazil): Implications for Archean-Paleoproterozoic depositional and tectonic settings

Banded Iron Formations (BIF) developed strongly during the Neoarchean – Paleoproterozoic Eras linked with global environmental processes such as the Great Oxidation Event (GOE) and changes in geodynamics, sea level, geobiology, hydrothermal systems, and sediment deposition conditions. BIF development and distribution worldwide are also related to supercontinent cycles, being a powerful tool to understand the evolution of plate tectonics. In this paper, we used field-based geological-structural data, petrography, X-ray diffraction, and lithogeochemical analyses to perform the first characterization of Banded Iron Formations at the Luis Alves Craton (LAC), Southern Brazil. The LAC BIF are classified as oxide and silicate facies, both with granoblastic texture. Oxide facies consists of quartz and magnetite. Besides these minerals, silicate facies contains olivine and ferrosilite, evidencing granulite metamorphism. Based on major elements oxide facies has a hydrothermal signature, with low content of Al2O3 and TiO2, whereas silicate facies register a continental sediment contribution been more enriched in these elements, MgO, MnO, and CaO. Trace-elements also indicate the main source for both silicate and oxide facies, which is seawater with a major influence of low-T hydrothermal sources, besides continental sedimentation sources. The Ce/Ce × anomaly is true negative, representing oxidizing waters, which also may represent a shallow water column depth. The depositional environment is inferred as a shallow, restricted basin. Oxide facies represents sedimentation closer to hydrothermal vents and with weak influence from continental clastic sedimentation, while silicate facies represent sedimentation closer to an adjoining continental landmass. Oxide facies has more Algoma-like characteristics, such as higher Fe and Eu contents, Sm/Yb and Eu/Sm typical of higher hydrothermal fluid inputs, and low siliciclastic contribution. On the other hand, silicate facies resembles Superior-type BIF, with higher siliciclastic contribution, negative Ce/Ce × anomalies, and Eu/Eu* <1.8. In summary, we propose the LAC BIF is representative of a transitional environment from Algoma-to Superior-type. The first depositions of the LAC BIFs may have developed within an Algoma-like environment during the Neoarchean. The waning tectonic and volcanic activity during the Siderian shifted to a platformal environment, with shallower waters that may induce the deposition of carbonates typical of Superior-type deposits. The depositional basin may have been closed by the Rhyacian and those sediments were then pushed down to the lower crust and underwent granulite facies metamorphism.

Integrated Litho-, Chemo- and Sequence Stratigraphy of the Ediacaran Gametrail Formation Across a Shelf-Slope Transect in the Wernecke Mountains, Yukon, Canada

The Ediacaran Gametrail Formation of northwestern Canada chronicles the evolution of a complex carbonate ramp system in response to fluctuations in relative sea level and regional tectonic subsidence alongside exceptional global change associated with the Shuram carbon isotope excursion (CIE). Here, we use extensive outcrop exposures of the Gametrail Formation in the Wernecke Mountains of Yukon, Canada, to construct a shelf-slope transect across the Shuram CIE. Twelve stratigraphic sections of the Gametrail Formation are combined with geological mapping and a suite of geochemical analyses to develop an integrated litho-, chemo-, and sequence stratigraphic model for these strata. In the more proximal Corn/Goz Creek region, the Gametrail Formation represents a storm-dominated inner to outer ramp depositional setting, while slope depositional environments in the Nadaleen River region are dominated by hemipelagic sedimentation, turbidites, and debris flows. The magnitude of the Shuram CIE is largest in slope limestones which underwent sediment-buffered diagenesis, while the CIE is notably smaller in the inner-outer ramp dolostones which experienced fluid-buffered diagenesis. Our regional mapping identified a distinct structural panel within the shelf-slope transect that was transported ~30 km via strike-slip motion during the Mesozoic–Cenozoic Cordilleran orogeny. One location in this transported structural block contains a stromatolite reef complex with extremely negative carbon isotope values down to ~ -30‰, while the other location contains an overthickened ooid shoal complex that does not preserve the characteristic negative CIE associated with the Shuram event. These deviations from the usual expression of the Shuram CIE along the shelf-slope transect in the Wernecke Mountains, and elsewhere globally, provide useful examples for how local tectonic, stratigraphic, and/or geochemical complexities can result in unusually large or completely absent expressions of a globally recognized CIE.

Geochemical evolution of kerogen type III during hydrous pyrolysis: A case study from the Damoguaihe Formation, Hailar Basin, China

In order to evaluate changes in geochemistry, organic petrology, and chemical composition of petroleum and byproducts, an immature sample from the Damoguaihe Formation in Hailar Basin in China was examined using hydrous pyrolysis (HP) at the following temperature steps: 300 °C, 325 °C, 350 °C, 365 °C, 400 °C, and 450 °C. The final product from each pyrolysis step was assessed with several analytical methods including programmed pyrolysis, Gas Chromatography (GC), Gas Chromatography-Mass Spectrometry (GC-MS) and organic petrology. Rock-Eval pyrolysis revealed that the geochemical parameters including TOC, S2, HI, and Tmax changed as thermal maturity advanced. Both un-extracted and extracted organic matter (OM) showed a steady decrease in TOC, HI, and OI as the temperature increased. On the other hand, a general increase in Tmax values with increasing temperature was observed. Regarding GC analysis, fluctuations were observed in n-alkanes during pyrolysis. In general, lower molecular weight n-alkanes of <C17 increased between 325 °C and 400 °C, and heavier n-alkanes fluctuated with increasing temperature. Similarly, GC-MS results showed variations in biomarkers with increasing pyrolysis temperature. These variations could stem from the GC-MS instrument's inability to detect light hydrocarbons, specifically at higher temperatures beyond the oil generation window into the gas window. Some biomarkers, including Ts/Tm, C29H/C30H, Gammacerane Index (GI), C23 tricyclic terpane/C24 tetracyclic terpane showed that the Damoguaihe Formation has a carbonate lithology and was deposited in a freshwater environment with no water stratification. Based on organic petrography, an increase in temperature caused optical and structural changes in the macerals. Organic petrography also showed that the Damoguaihe Formation contains different maceral groups such as liptinite, vitrinite, and inertinite, and hence a combination of kerogen types II, III, and IV. Based on these observations, a deltaic depositional setting is envisaged for the Damoguaihe Formation in the study area, which is also confirmed by previous studies.

New insights into the depositional facies of the Early Eocene Rus Formation, Bahrain

Detailed geological field measurements combined with petrographic and mineralogical analyses of the Horse Racing Club outcrop, Bahrain, revealed new insights into the composition, facies distribution, and depositional settings of the Early Eocene Rus Formation, which is poorly constrained in Bahrain. The mineralogical analysis revealed that the rock-forming units are dominated by dolomite, dolomitic marl, claystone, and minor evaporites. The integration of field data and lab measurements resulted in the identification of thirteen facies that are grouped into five facies associations comprising i) lagoonal complexes, ii) sabkha complexes, iii) transgressive lag, iv) peritidal channel complexes, and v) barrier island and washover complexes. Combined paleocurrent analysis of field-observed clinoformal structures, lateral accretion surfaces, and trough cross-bedding indicate a NNW–SSE paleoshoreline trend with tidal channels trending dominantly towards NE–SW and NW–SE directions. The depositional facies model developed for the Rus Formation in Bahrain and within the surrounding region suggests that during the Early Eocene, two landmasses existed to the north and south of Bahrain, restricting the Neo-Tethys Ocean flow through a relatively narrow strait that was influenced by periodic storms. The results of this paper challenge the role of the Rus Formation as an effective regional hydrocarbon seal and, as such, highlight the need for a critical evaluation of the associated petroleum system potential in Bahrain and neighboring countries.

Obliquity‐driven mountain permafrost‐related fluvial magnetic susceptibility cycles in the Quaternary mid‐latitude long‐term (2.5 Ma) fluvial Maros Fan in the Pannonian Basin

Magnetic susceptibility (MS) of the Quaternary long‐term mid‐latitude Maros fluvial fan (Pannonian Basin) was recorded to understand the stratigraphical features of source‐proximal fluvial depositional settings. Three fully cored 500‐m‐deep boreholes were sampled at 0.5‐m intervals; low‐field and frequency dependent MS were measured, and complementary hysteresis and SEM‐EDAX investigations were performed on selected samples. Logged susceptibility data were also used to log correlations established by a simultaneous comparison of wireline log and laboratory measurements. Time‐series analyses of the susceptibility records reveal a ~41‐ka and ~100‐ka cyclicity. Towards the source‐distal sections the intensity of the ~41‐ka cycles decreases, while that of the ~100‐ka cycles remains strong. Stratigraphical and spectral similarities were observed between the Maros fluvial fan and Chinese loess records; however, based on complementary magnetic data, the magnetic phase of the Maros Fan sections is related to the detrital magnetite that originates from the catchment during early postglacial permafrost degradations. The amplification of the ~41‐ka cycles can be attributed to the very high susceptibility values in source‐proximal settings and to the special stratigraphical feature of the distributive fluvial systems. This comprises the increased avulsion frequency on the fluvial fans in ‘glacial recession periods’, in concert with the ‘early postglacial’ occurrence of the permafrost‐related magnetite originating from the catchment. As a local phenomenon, this is significant since it records the obliquity‐driven variations in permafrost development in a catchment. However, fluvial and alluvial fans are widespread depositional landforms within the Eurasian mountain range and were possibly the same during the Quaternary deglaciations. Thus, obliquity‐driven magnetic susceptibility variations in source‐proximal fan deposits attached or adjacent to regions of loess deposition should also be considered when scanning for potential source material of aeolian deposits.