Facies Variations Research Articles

Stratigraphy and sequence stratigraphy of the Neoproterozoic (Cryogenian–Ediacaran) Stuart Shelf, South Australia

The Stuart Shelf is part of the Adelaide Superbasin overlying the Gawler Craton and Cariewerloo Basin in South Australia. The basin is of interest for sediment-hosted copper mineralisation known to be hosted in numerous stratigraphic intervals across the region. Therefore, facies analysis and understanding of spatial distribution of host units are essential for exploration targeting and mineral systems research. Our study presents improved and new definitions of Cryogenian and Ediacaran Stuart Shelf stratigraphy, and a detailed, regional-scale sequence stratigraphic analysis. The Cryogenian non-glacial interlude was of particular interest, as it includes the Tapley Hill Formation, a known host for copper mineralisation. The succession of Tapley Hill Formation (including Sturtian cap carbonates), Brighton Limestone and Angepena Formation represents a third-order depositional cycle, an equivalent to the lowermost cycle of the Cryogenian non-glacial interlude in the adjacent Adelaide Rift Complex. The Ediacaran post-glacial succession includes the Nuccaleena Formation and Tent Hill Formation and is interpreted as a second-order depositional cycle. The Sturtian and Marinoan cap carbonates form regional stratigraphic marker horizons and were deposited in a transgressive systems tract. Variations in facies and thickness can be linked to development of localised depocentres and topographic highs, such as the Pernatty High. Modelling of 3D surfaces reveals a shift in the basin orientation between the Cryogenian and Ediacaran sedimentation from an NNW–SSE axis towards a N–S axis. The Stuart Shelf sequence stratigraphic framework provides new insights in the basin sedimentology and evolution, which aids sediment-hosted mineral systems analysis and improves the understanding of Cryogenian and Ediacaran basin evolution in Australia.

Downslope variability in deep-water slope channel fill facies and stacking patterns

Lithologic variations in deep-water slope channel systems provide critical insight into sedimentary processes on deep-water slopes, and are a key control on natural resource distribution and connectivity in subsurface reservoirs associated with ancient deep-water systems. While many studies have described these variations at outcrop-scale along depositional strike (across-channel), few have focused on documenting along-depositional-dip changes due to: (1) the dearth of along-slope perspectives afforded by outcrops; or (2) limited resolution and coverage of subsurface data sets. In this study, slope channel elements (≤30 m thick; ≤400 m wide) and composite channelform bodies composed of ≥2 stacked channel elements (channel complexes and channel systems; >30 m thick; >400 m wide) were characterized along a 50-km-long depositional-dip-oriented outcrop belt of the Campanian-Maastrichtian Tres Pasos Formation (Chile) to constrain longitudinal changes in channel fill character and stacking patterns. Results show that channel elements become more sandstone-rich downdip. Outcrop observations are supported by channel element net-to-gross ratios (measures of sandstone proportion), which increase downdip. Sandstone-prone channel elements within channel complexes and channel systems are more prevalent downdip; however, variability in lateral and vertical offset between successive channel elements downdip results in a poor correlation between net-to-gross values and paleoslope position when composite channelform bodies are considered. These findings suggest that the along-slope distribution of coarse-grained sediment in channel systems is tied to the degree of coarse-grained sediment bypass and erosion that occurs along a slope. The results of this study provide a detailed perspective into downdip changes in slope channel fill and stacking patterns, and help clarify aspects of analogous subsurface reservoirs, including sandstone proportion trends for several scales of deep-water slope channel architecture.

New fossiliferous sites from the mid-Cretaceous Tendrara dome (High Plateaus, Morocco): biostratigraphical, paleoenvironmental and paleogeographical implications

Sedimentological and stratigraphical studies in the Cretaceous series of the Tendrara dome led to the discovery of fossiliferous levels in the pre-Cenomanian and Cenomanian deposits, providing significant biostratigraphic, paleoenvironmental, and paleogeographic results. In the SW part of the Tendrara dome, the predominantly terrigenous deposits at the base of the pre-Cenomanian (Dekkar 1 Formation) yielded dinosaur eggshell fragments and charophytes, in particular Clavatoraceans, pointing to the Barremian-Aptian. Above this unit, two beds with fish remains were discovered in the Dekkar 2 Formation and at the base of the Dekkar 3 Formation, respectively. To the NE of the Tendrara dome, a fossiliferous site with diversified benthic fauna and abundant fish remains was discovered in a thin marly unit unconformably overlying the Middle Jurassic basement. Elasmobranch micro-remains indicate a Cenomanian age for this unit. The SW-NE correlations indicate a marked thickness reduction and lateral facies variations, implying a strong asymmetry in the Cretaceous paleogeography of the dome. The first continental and lagoonal basins of the Barremian-Aptian and Albian?-Cenomanian are located in the southern part of the Tendrara dome. The Cenomanian transgression, initiated from the south, gradually covered the dome, depositing reduced coastal plain elasmobranch-rich facies in its northern part. The Tendrara dome constituted a paleogeographic barrier, limiting the first transgressions of the Cenomanian sea. This paleostructure is part of an emerged area (Idrissides High) located between the Tethyan Ocean and the Saharan epicontinental sea.

Impact of reservoir quality on the carbon storage potential of the Bunter Sandstone Formation, Southern North Sea

The Lower Triassic Bunter Sandstone Formation is a major prospective reservoir for carbon capture, utilization and storage in the UK Southern North Sea, and is likely to play a pivotal role in the UK reaching mid-century Net Zero targets. A knowledge gap in reservoir quality exists between previous detailed, but highly focused front-end engineering and development projects, and large-scale regional analysis. This study integrates a regional approach with locally derived reservoir characterization, offering a holistic analysis of the prospectivity of the Bunter Sandstone Formation for subsurface CO 2 storage. Petrophysical analysis of ninety-six wells across the UK Southern North Sea is coupled with seismic interpretation to understand spatial variations of reservoir thickness, facies and quality that underpin theoretical CO 2 storage capacity models. Electrofacies classification is employed to identify and correlate baffles and barriers to permeability over areas currently licensed for geological carbon storage. Our findings point to variable, but broadly favourable reservoir conditions, though identification and correlation of laterally extensive intraformational mudstones and halite-cemented horizons will likely present challenges to CO 2 injection. Within carbon storage license blocks CS001, CS006 and CS007, the Bunter Sandstone Formation has the potential to store 5700 MCO 2 t, the equivalent of seventy-nine years of the UK's 2022 business and industrial CO 2 emissions. A further 434 MCO 2 t is offered by Triassic closures within license CS005, with many neighbouring moderate (100–1000 MCO 2 t) and small (<100 MCO 2 t) closures forming part of newly awarded carbon storage licenses that will likely form part of the UK SNS CCUS portfolio in the future. Supplementary material: well-correlation panels and tabulated velocity and storage capacity modelling parameters are available at https://doi.org/10.6084/m9.figshare.c.7027450

Scree conglomerate and its derivatives in the Upper Cretaceous Kallankurichchi Limestone, Ariyalur Group, Cauvery Basin, South India

The repetitive influx of coarse clastics of mixed composition, siliciclastics and carbonates, locally common in the lower part of the Upper Cretaceous marine Kallankurichchi Limestone specifies a resurgence of tectonic unrest in the Cauvery rift basin, India. The basin-margin scree and its derivatives elicit diverse modes of emplacement and differ in many ways from denoting it only as basal conglomerate. The study meticulously reveals the depositional history of these basin-margin coarse clastics. The scree conglomerate bodies are wedge-shaped in appearance and often have flat, eroded tops. At places, their surfaces, tops and flanks, are encrusted with Inoceramus although internally, they are mostly unfossiliferous. They are clast-supported and extremely poorly sorted, having interstitial spaces filled by sand-sized grains at the basal part. The clasts can be traced into the underlying Sillakuddi Sandstone and the granitic basement. The clasts derived from the sandstone are angular and measure up to 60 cm in length, while the basement-derived clasts dominate the smaller (maximum diameter measured 5 cm) and more rounded population. The elongated clasts are chaotically arranged, even oriented sub-vertically, reclining on other clasts. The lowermost scree conglomerate has a sharp base, and the pebbles sunk into the underlying sandstone, although no discernible impact laminae wrapping their bottom were ever observed. Evidently, the scree fans were initiated under the sea, which permitted the slow sinking of assorted rock fragments dropped from above. The sporadic occurrence of marine fossils further corroborates this contention. The scree conglomerates at different stratigraphic levels transitioned laterally into conglomerates of mass flow origin and then to massive calcarenite, together forming wedge-shaped bodies. The initial alignment of clasts parallel to bedding transforms to a chaotic alignment representing the transition from internally sheared flow to debris flow and associated shapes. Matrix-supported fabric grades into massive calcarenite, suggesting gravity-driven transformation. Further down the wedge, the massive calcarenite turns into cross-stratified facies, making it evident that laminar flow turned turbulent in the course of body transformation of the sediment-driving flows. It can be presumed that these flows were triggered by subsidence, which resulted from renewed gaps in formation and led to the collapse of scree cones. Facies variability suggests scree deposits giving way downslope to debris flow and related deposits. Eventually, all the scree products pass laterally into the carbonate formation basinwards.

Geological structure and metallogeny of Precambrian sedimentary structures of western Aldan-Stanovy Shield

The article presents the results of the study by various geologists of the geological structure and prospects of ore-bearing trog complexes performing structures such as throgs, which were identified at different stages of geological research in the western part of the Aldan-Stanovy shield. The main types of traction complexes and their sections were determined. The nature of the occurrence of sedimentary (terrigenous), volcanogenicsedimentary and metavolcanic rocks performing throgs and forming throg complexes was established. The nature of the ratio of these rocks in the context of the trog complex in the studied trogs was described. The results of the study of trog complexes showed that they are generally characterized by facies variability, heterogeneity of metamorphism and dynamothermal processing of rocks (especially in the sides of trogs) with the formation of mylonites and cataclasites. It was established that the throgs are confined to three regional systems of discontinuous disturbances. Both consonantal and secant discontinuities were marked inside the throgs, defining their block tectonics. The results of determining the geological age of the rocks of the trogovoi complex were presented. The trog structures are characterized by ore mineralization associated with the processes of sedimentation (ferruginous quartzites, copper sandstones) and volcanogenic ore formation (copper-crusted and polymetallic mineralization), as well as with the processes of metamorphogenic-hydrothermal redistribution of ore matter (stratiform scheelite mineralization, ore-bearing quartz veins). It was also found that the metallogeny of the trog structures has not been studied sufficiently. Nevertheless, the majority of geologists who studied the trogovye complexes noted in their reports that Precambrian trogovye complexes can be considered as important and metallogenically promising geological objects in the western part of the Aldan-Stanovy Shield and deserve more in-depth study.

Platform Evolution in an Oligo–Miocene Back‐arc Basin: An Example from the Central Iran Basin

AbstractThe Qom Formation is the most important hydrocarbon reservoir target in Central Iran. The Qom platform developed in a back‐arc basin during the Oligo–Miocene due to the closing of the Tethyan Seaway. This formation consists of a variety of carbonate and non‐carbonate facies deposited on a platform ranging from supratidal to basin. A combination of tectonic and eustatic events led to some lateral and vertical facies variations in the study area. Six third‐order depositional sequences and related surfaces were identified regarding vertical facies changes in the studied sections of this Oligo–Miocene succession. According to all results and data, this succession was initially deposited during the Chattian upon a distally steepened ramp of siliciclastic‐carbonate composition, including the Bouma sequence. Then, from the late Chattian to the Aquitanian, the platform changed into a homoclinal carbonate ramp with a gentle profile. With respect to tectonic activity, this phase was a calm period during the deposition of the Qom Formation. Finally, a drowned carbonate platform and a rimmed shelf emerged during the Burdigalian, terminated by the continental deposits of the Upper Red Formation. Regarding all geological characteristics, three main tectono‐eustatic evolutionary phases have been recognized in the Qom back‐arc basin.

Facies Variability in Paleogeographic Trends from Core Data in the Lower and Middle Frio Formation in South Texas

This core-based study of the Oligoene lower and middle Frio formation in South Texas documents facies variability, depositional systems, and sandstone-body architecture in the lower and middle Frio formation in South Texas. A previous study by the Bureau of Economic Geology (BEG) in 1982 characterized the Frio Formation in terms of depositional systems, structural framework, hydrocarbon origin, migration, and exploration potential. Paleogeographic trends in the 1982 study were delineated for the lower, middle, and upper Frio Formation, based mostly on wireline-log data. Core data in this study augment and refine facies interpretations and clarify depositional styles in four paleogeographic trends from the 1982 study in the lower and middle Frio Formation. These trends include (1) lower delta plain/margin, (2) aggradational fluvial, (3) aggradational (multistoried) barrier, and (4) floodplain. With minor exceptions, this study validates and refines interpretations of the regional distribution of lower and middle Frio paleogeography in South Texas in the 1982 study. Four (4) main results of this study include (1) documenting lithology and facies types in these paleogeographic trends, (2) demonstrating that these paleogeographic trends are a composite of multiple transgressive-regressive (T–R) cycles, (3) providing evidence that these fluvial systems are mixed-load, meander belt in origin rather than coarse-grained bedload in origin, based on vertical grain-size profiles and the presence of paleosols that indicate stable, well-developed floodplains, and (4) identifying anomalous occurrences of sandstone-rich fluvial-channel systems in areas previously mapped as sandstone-poor and interpreted to be primarily floodplain in origin. Finally, this study serves as an example of the value of core data in refining interpretations of paleogeography and depositional systems in stratigraphic units that are a composite of multiple T–R episodes.

Hydrogeological and hydrochemical condition in planned tunnel B route of the Yogyakarta-Bawen toll road

The Yogyakarta-Bawen toll road tunnel, located in Magelang Regency, Central Java Province, is a national strategic project that connects Central Java and the Special Region of Yogyakarta. It has geology resulting from volcanic processes, resulting in rocks with moderate aquifer properties and low to moderate permeability. The study covers 24 observation points, with 11 locations sampling groundwater for chemical analysis. The analysis results of hydrochemical facies are Na-K-HCO3. Water table depth varies from 623 m to 704 m above sea level, with the water table along the tunnel route at depths of 13-40 m above mean sea level, generally above the tunnel’s elevation. The groundwater flow pattern tends to move from southeast to northwest, perpendicular to the tunnel’s direction. Groundwater quality testing indicates that water in the research area falls under the category of fresh groundwater, based on electrical conductivity and total dissolved solids values. The overall acidity level is generally acidic, except at specific points. In general, the interaction of groundwater with concrete results in weak erosion. It can be concluded that the groundwater at the research location is shallow, situated above the planned tunnel elevation, exhibits hydrochemical facies variations, and has the potential to weakly erode concrete.

On depositional processes governing along‐strike facies variations of fine‐grained deposits: Unlocking the Little Ice Age subaqueous clinothems on the Adriatic shelf

ABSTRACTDepositional processes recorded by shelf deposits may vary widely along‐strike, depending largely on the mode of delivery and deposition of sediments to the basin. In fine‐grained systems in particular, depositional processes are difficult to reconstruct with standard facies analysis of sediment cores due to the ostensibly featureless and homogenous appearance of muds. In this study, sedimentological, palaeontological, geochemical, and oceanographic data were combined in a detailed characterization of depositional conditions via sedimentary structures, type of organic matter, trace‐metal geochemistry, and benthic fauna assemblages (foraminifera and ostracods) along the 600 km long shelf delta clinothems of the West Adriatic shelf (Italy). Processes inferred from sedimentary facies and micro‐structures were then considered in the context of the modern Adriatic oceanographic regime. Specific attention was given to the Little Ice Age stratigraphic unit (1500–1850 ce), which contains a continuum of genetically related fine‐grained strata. The Little Ice Age deposit offers the opportunity to examine a source‐to‐sink system with the high resolution typical of modern analogues, at a time interval when Apennine rivers were not yet hydraulically engineered with man‐made sediment traps along their trunks. Individual beds within the Little Ice Age muddy prodelta form hectometre to kilometre‐wide bedsets that reflect the interplay between energetic meteo‐ocean conditions (storm‐dominated beds), flood supply (river‐dominated beds or hyperpycnites) and along‐shelf bottom‐current dispersion (drift‐dominated beds). The multidisciplinary approach applied at different scales of observations helped in understanding sediment provenance and the relative timing of sediment transport before final burial that strongly promoted organic matter oxygen exposure and the loss of carbon by microbial degradation. Overall, the distinctive depositional processes that acted in concert along the prodelta slope produced a subtle lateral heterogeneity of preserved sedimentary structures, faunal associations, and organic matter composition in a laterally‐continuous lithostratigraphic unit deposited at centennial scale. These findings have implications on the forcing conditions that ultimately control the location and nature of fine‐grained beds in both modern and ancient, mud‐dominated depositional systems.

Facies distribution and depositional cycles in lacustrine and palustrine carbonates: The Lutetian–Aquitanian record in the Paris Basin

AbstractThe difficulty of correlating continental deposits hinders predicting lacustrine and palustrine carbonate facies variations in time and space. This study aims to understand better the factors governing these facies heterogeneities by measuring carbonate isotopes and conducting facies, petrographic and sequence stratigraphic analyses of the Lutetian–Aquitanian deposits of the Paris Basin, that record the transition from marine to lacustrine environments. Large‐scale correlations enabled the definition of two lacustrine–palustrine carbonate facies models. (1) The coastal lacustrine system (Bartonian to Rupelian), consists of fine‐grained brackish carbonate exhibiting episodic marine inputs during short‐term relative sea‐level maxima and evaporite sedimentation during relative sea‐level minima. Lacustrine sediments differ notably from marine ones with more negative δ13C and δ18O compositions that co‐vary and a biota adapted to low salinity conditions. In the associated palustrine environment, depositional sequences evolve upwards from micritic lacustrine deposits to nodular and then laminar calcretes. Microbial‐coated grains and rhizoliths indicate biological processes during repeated subaerial exposure phases in sub‐tropical to arid climates. (2) The inland lacustrine system (Rupelian and Aquitanian) was disconnected from the marine domain and showed evidence of microbial activity with microbial crusts and oncoidal rudstones. Facies rich in micritic intraclasts composed of palustrine and lacustrine facies indicate the reworking of already lithified sediments along the margins. In the palustrine domain, the calcrete facies are less abundant than breccias formed in‐situ by desiccation, limestones with root traces, or organic‐rich wackestones and marls. This system reflects a more temperate climate with more developed microbial structures and less exposed carbonates than the coastal lacustrine system. The southward migration of the depocentre and the transition from marine environments to (1) coastal and then (2) inland systems are controlled by uplift phases induced by Pyrenean and Alpine orogenesis. Third‐order relative sea‐level variations appear to control only short‐term cycles in coastal systems.

An Integrated Approach for Saturation Modeling Using Hydraulic Flow Units: Examples from the Upper Messinian Reservoir

The Upper Messinian reservoirs located in the Salma Field of the Nile Delta area contain variable facies. The key reservoir interval of the Abu Madi Formation was deposited in fluvial to deltaic environments. These fine-grained facies form significant reservoir heterogeneity within the reservoir intervals. The main challenges in this study are reservoir characterizing and predicting the change in reservoir water saturation (SW) with time, while reservoir production life based on the change in reservoir capillary pressure (Pc). This work applies petrophysical analysis to enable the definition and calculation of the hydrocarbon reserves within the key reservoir units. Mapping of SW away from the wellbores within geo-models represents a significant challenge. The rock types and flow unit analysis indicate that the reservoir is dominated by four hydraulic flow units. HFU#1 represents the highest flow zone indicator (FZI) value. Core analysis has been completed to better understand the relationship between SW and the reservoir capillary pressure above the fluid contact and free water level (FWL), which is used to perform saturation height function (SHF) analysis. The calculated SW values that are obtained from logs are affected by formation water resistivity (Rw) and log true resistivity (RT), which are influenced by the volume of clay content and mud salinity. This study introduces an integrated approach, including evaluation of core measurements, well log analysis covering cored and non-cored intervals, neural analysis techniques (K-mode algorithm), and permeability prediction in non-cored intervals. The empirical formula was predicted for direct calculation of dynamic SW profiles and predicted within the reservoir above the FWL based on the change in reservoir pressure.

3D structural modeling using seismic data and well logs for Khatatba reservoir in Matruh-Shushan Basin, North Western Desert, Egypt

Middle Jurassic reservoirs present challenges in the northern segment of the Western Desert due to geometric uncertainties arising from structural configurations, lateral facies variations, diverse lithologies, and heterogeneous reservoir quality. Consequently, this study employed an intricate approach, constructing detailed 3D geostatic models by amalgamating diverse datasets, including 2D seismic sections and digital well-logs. The focus of these 3D models was on the Khatatba Formation (Upper-Safa Member, Kabrit Member, and Lower-Safa Member) in Matruh—Shushan Basin in the North Western Desert. The objectives encompassed assessing hydrocarbon potential, precisely estimating reserves, formulating development and exploration strategies, and identifying prospective drilling locations. The resultant structural model revealed a compartmentalized region marked by major and minor NE–SW trending normal faults, establishing structurally advantageous locations for hydrocarbon trapping within the study area. Petrophysical analyses highlighted the promising potential of the Upper-Safa Member as a reservoir, featuring porosity values ranging from 10 to 18%, peaking in the northeast sector, volume of shale (Vsh) between 15 and 24%, water saturation (Sw) spanning from 18 to 53%, and increased sand thickness towards the eastern section. Similarly, the Lower-Safa Member demonstrated favorable reservoir attributes, including porosity values ranging from 10 to 16%, with higher values in the southeastern part, Vsh between 17 and 28%, and Sw varying from 15 to 47%. The study findings underscored the hydrocarbon potential in the northeast block of the study area for the Middle Jurassic Khatatba Formation. These insights contribute valuable information for decision-making in exploration and production endeavors within the basin.

Carbonate sediment dynamics in the Abu Dhabi lagoon - implications for low-angle inner-to-middle ramp models

Carbonate ramps are prominent features of continental margins and intrashelf basins throughout much of Earth's history. Among the limited number of recent analogues of ancient carbonate ramps, the Persian/Arabian Gulf stands out due to its morphologically complex array of open coastal to sabkha environments. This study presents detailed field and laboratory data on spatial facies variability linked to different hydrodynamic regimes spanning from open coastal to inner lagoon settings in Abu Dhabi. Sediment transport and deposition are determined by waves and tidal currents, with spatially and temporally complex variations in the hydrodynamic regime, resulting in a complex facies mosaic. The open coastal domain is subject to persistent north-westerly trade (Shamal) winds and episodic winter storms, generating waves that affect the deposition of ooids, peloids, lithic clasts and bioclasts. In the outer lagoon, these waves interact with tidal currents and thus become less dominant towards the middle lagoon. Tidal currents are more important in the restricted inner lagoon. The outer lagoonal sediments primarily consist of ooids, bioclasts and lithic clasts and are peloid- and micrite-lean. Bimodal tidal currents, locally with velocities of two or more metres per second, are present in the relatively open regime of the middle lagoon and accumulate bar complexes dominated by skeletal carbonates. Seagrass and biofilms locally stabilise muddy carbonates and limit sediment entrainment even where significant tidal currents exist. In a landward direction, and with decreasing hydrodynamic levels, carbonate mud (micrite) is increasingly abundant, reflecting the lower hydrodynamic conditions in the restricted regime of the mid-and inner lagoon. Data shown here provide a facies distribution model in the shallow coastal portions of what might be one of the few modern analogues of a ‘marginal epeiric’ sea, shedding light on the complex interplay between coastal morphology, wave and tide energy and biological processes. This study discusses how hydrodynamics and seafloor bio-stabilisation influence the spatial carbonate facies of this highly dynamic ramp system by comparing data shown here with data from the Kuwait and Doha coasts, which have different orientations to the prevailing winds. The quantitative and refined composite inner ramp model shown here has wider significance for coastal dynamics and carbonate stratigraphy.

Synthetic seismic-stratigraphic interpretation from a sedimentological forward model in a pre-salt field in the Santos Basin

Forward modelling of sedimentary systems is a method that simulates sedimentation processes over geological time to generate a set of facies distributed in a depositional space. The objective of using forward modelling in this work was to build a 3D facies model from which a synthetic seismic simulation was generated, and then to analyse the relation of seismic-stratigraphic interpretations with the knowledge of the a priori generated sedimentological model. This modelling methodology was applied in a pre-salt field in the Santos Basin, Brazilian offshore, focused on the Barra Velha Formation. The modelling parameters used were: (i) the initial surface of bathymetric depth; (ii) the lake-level variation; (iii) the subsidence map; and (iv) the deposition rates of the facies. Average-constant of acoustic impedance values were assigned to each facies and a synthetic seismic was obtained. With the facies and synthetic models available, it was possible to analyse: (i) the distribution of thicknesses and proportion of facies by region; (ii) the vertical stacking pattern and lateral facies variation; (iii) the Wheeler distance × time diagram; and (iv) the seismic reflector patterns through the seismic facies classification. Through these analyses it was possible to better understand the possibilities and limitations of seismic stratigraphy as an interpretation auxiliary tool in pre-salt carbonate environments.

High-resolution lithostratigraphy and reconnaissance sedimentology of Changotaung structure, Chittagong Tripura fold belt, Bengal Basin, Bangladesh

Unlike other structures in the vicinity of the Chittagong Tripura Fold Belt, the Changotaung anticline is one of Bangladesh's least explored structures. An attempt has been made for the first time to understand and document sedimentary deposits, environments, structure, and tectonic activity based on the high-resolution outcrop and reconnaissance study with the knowledge of broad-brush geology. We found that Changotaung is a symmetrical box-folded structure with an extensive western flank where the amount of dip varies between 11° and 45°. The exposed Cenozoic succession was categorized into three separate sedimentary sequences and correlated with the conventional stratigraphic unit. A first-order simple Markovian approach was presented for the exposed litho-section in an effort to illustrate vertical facies variations in the Upper Surma group. We quantified that heterolithic bed mostly overlies both trough cross-bedding ({P}_{ij}U = 0.706) and parallel laminated bed ({P}_{ij}U=0.955) according to the facies transition probability matrix. According to the results of the stationary distribution, there is a 40% chance of coming across heterolithic beds within the Upper Surma group during any given event that is completely random whereas trough cross-bedding, parallel laminated bed, cumulative sandstone facies, and cumulative shale facies shows around 10.8%, 15.2%, 20.6%, 13.4% probability. We hypothesized, based on the interpretive facies analysis, that the Chittagong Tripura fold belt region's Upper Surma Group underwent three interrelated depositional settings (wave-dominated shallow marine, tide-dominated shallow marine, and fluvio-deltaic distributary).

Geochemical characteristics and origin of the ultra-deep hydrocarbons from the Shunbei Oilfield in the Tarim Basin, China: Insight from molecular biomarkers and carbon isotope geochemistry

To better evaluate the exploration prospects of ultra-deep hydrocarbon resources in the Tarim Basin, origin of the newly discovered ultra-deep hydrocarbons from the Shunbei Oilfield are investigated thoroughly concerning the maturity, secondary variations, and source kitchens based on molecular biomarkers and carbon isotope geochemistry. Given the physical and chemical properties of the studied oils and gases, they are classified into five families: A, B, C, D, and E. Newly proposed maturity parameters are employed for evaluating the maturity of the studied oils and gases as Family D > E > B > A, except for Family C hydrocarbons, which may be a mixture of relatively high-maturity natural gas with relatively low-maturity oil. Secondary alterations are concentrated in the southern part of the Shunbei region, primarily including oil cracking and TSR effects, both of which may occur in deeper strata than in situ. Furthermore, the simulated maturity evolution and variation of sedimentary facies of the Yuertusi Formation (Є1y) source rocks in the Shuntuoguole Low Uplift (STLU) area match well with the genesis of the studied hydrocarbons, which suggests that the studied hydrocarbons may be derived from the Є1y source rocks in the STLU area. Collectively, Family A hydrocarbons originate from the western Є1y source kitchen in the STLU region where the water column is shallow, rich in planktonic algae, and the maturity is relatively low; in contrast, Family E hydrocarbons are derived from the eastern Є1y source kitchen in the STLU region where the water column is deep, rich in benthic algae, and the maturity is relatively high; Families B and C hydrocarbons originate between them. This research also contributes to the establishment of a geochemical evaluation system for ultra-deep hydrocarbons.

The Mondot-1 Core, Aínsa Basin, Spanish Pyrenees: a deltaic reservoir teaching set with augmented reality

Abstract Well Mondot-1 was drilled, cored and logged behind an outcrop of the Eocene Sobrarbe deltaic complex in the Aínsa Basin, Spanish Pyrenees. The data acquired represent a fluvial-dominated delta system prograding over a carbonate slope in the presence of active tectonism, and form part of a basin-to-grain teaching set for subsurface professionals. This paper highlights the workflow step(s) to move from a 1D core description to 2D and 3D correlations, including some of the pitfalls caused by rapid lateral facies variability, which is a common challenge in subsurface reservoir description. The core allows direct log-to-lithology comparison across a full deltaic succession. In the proximal and distal delta fronts, there are clear differences from the closely adjacent outcrop that are ascribed to lateral facies changes and delta front slumping. Two approaches to core display are described, as means towards making such data available to a wider audience: an augmented reality display run over a slabbed core, and a virtual whiteboard display carrying easily scalable viewing possibilities. These approaches can be deployed for other cores, allowing visualization and analysis where people are otherwise unable to travel to the physical location, or in circumstances where the core is now inaccessible.

The distributions and variations of Quaternary Thames River Terrace deposits of Greater London

A database containing c. 27 000 records of Quaternary lithostratigraphy extracted from c. 5800 boreholes drilled to a variety of depths has been compiled from information contained in logs recorded and held by the British Geological Survey (BGS) and from the Crossrail project. After ‘cleaning’ and quality assessment the raw borehole log data have been investigated for patterns in distribution, and variations in the Quaternary facies across Greater London. In particular, the analysis of Quaternary River Terrace Deposits has been focused on here and a summary of relevant published terminology and nomenclature is also included in this Technical Note, as an aid to understanding. The spatial distributions of each major Quaternary lithostratigraphic interval, as identified in the log descriptions, are also presented as point locations in map form, and in comparison with the corresponding BGS mapped outcrop patterns of the respective lithostratigraphic units. The presence of various specific clasts in the log descriptions (e.g. cobbles and chalk clasts) is also presented at stratigraphic member levels against their apparent source outcrops. For instance, cobbles recorded from the Boyne Hill Terrace intervals are concentrated in the eastern outcrops and reflect the Wealden source and former river system of the Darrent–Wid towards the Essex coast at Maldon. Similarly, the presence and distribution of shell and chalk clasts within river terrace sands and gravels reflect the geology of the underlying subcrop strata of Lower Shelly Clay and Chalk respectively. The Crossrail data also include some grain-size information from sieve analysis and some engineering in situ test values, which require careful analysis beyond the scope of this work but are potentially useful. The grain-size distributions for some of the samples from the river terrace sands and gravels show fining-up and coarsening-up sequences, and geographical grain-size trends in various stratigraphic intervals. The recorded presence of faults, fractures and fissures, as identified by the drilling contractors or logging geologists, is also extracted and mapped. Supplementary evidence, from repeated stratigraphy or improbable spatial proximity changes in subcrop depths, suggests a cause other than deposition for such changes and, on the presumption that their cause is natural, is interpreted as likely to be representative of faulting of some kind, including low-angle thrust faults. Faulting is also considered to have controlled the distribution of several interglacial silt members and is the subject of considerable research in the deeper intervals of London stratigraphy, so is only briefly dealt with here. To define the period over which tectonic structures, now known to exist within the strata of the London Basin, were active requires an analysis of Quaternary deposits that overlie them. This Technical Note provides an initial review of the data available from boreholes for such work and for any other studies requiring knowledge of these deposits. Supplementary material : Spreadsheets (and other files), and notes on the name and significance of each, are available at https://doi.org/10.6084/m9.figshare.c.6700313 Thematic collection: This article is part of the Geology of London and its implications for ground engineering collection available at: https://www.lyellcollection.org/topic/collections/geology-of-london-and-its-implications-for-ground-engineering

Study and analysis of seasonal soil degradation in Lower Guinea and Forest Guinea

Seasonal degradation of roads has become a recurring concern over the last two decades in Guinea. The most visible causes are those of their supports: the soil. Out of concern for the consequences of this scourge on the daily life of the population, the aim of this study is to understand the causes and propose solutions. On the Coyah_Kindia road, a terrible traffic jam was recorded on September 14, 2020 (∼3H/Km). The humidity of the road surface layers gives them very slippery characteristics, reflecting a plastic fringe of their fine fractions. The Péla-Yomou section, during rainshowers on the 22 km section along the Yomou-Nzérékoré national road, shows enormous degradation of the roads, with very acute soil slippage (even for pedestrians). Our GIS and remote sensing work removes the ambiguity about the main reasons for the degradations, which are more attributable to erosion than to the destruction of the vegetation cover (seen as a whole and not in the analysis of the classes that make it up), which, for the intervals of years [1982, 2021], fluctuates from 70%, 63%, 67% and 71% in Coyah, against 70%, 61%, 57% and 68% in Kindia, then 68%, 88%, 69% and 73% in Yomou. Our geoscientific analyses show a diversity of facies in terms of age, nature and reaction to the permanent constraints (meteorological and anthropogenic) to which they are subjected, while the soils, which are often less indurated and weathered in places, show great sensitivity to landslides that cover the low-lying areas often crossed by roads.