Lithofacies Variations Research Articles

Whole rock lithogeochemical analysis of the Mount Read Volcanics: a new tool for geochemical exploration

The Mount Read Volcanics (MRV) of western Tasmania is a belt of Middle to Late Cambrian submarine volcanics and volcaniclastics and one of the major base and precious metal provinces within Australia. Exploration success in the MRV has been in decline in recent years due to a variety of challenges including intensely altered rocks with complex stratigraphy and lithofacies variation, and a lack of accessible outcrop. As a result, local and regional mapping of the MRV is often restricted to Group-level stratigraphy, rendering geological mapping ineffective for identifying decametre-scale mine stratigraphy. Surface rock chip and drill hole samples were collected and analysed using commercially available geochemical assays to create a regional geochemical dataset of the MRV, covering the entire province and all major stratigraphic units. From these data, a lithogeochemical methodology was developed, utilizing Ti/Nb and V/Sc systematics that effectively characterizes the samples according to composition and magmatic processes. This method is shown to be robust to biases due to alteration and lithofacies, eliminating the need for highly selective sampling or extensive data filtering. To demonstrate the utility of this method in an exploration context, data from the K-lens of the Rosebery polymetallic volcanic-hosted massive sulfide (VHMS) deposit was used as a case study. Using Ti/Nb–V/Sc systematics, a rock composition classification was developed that allowed for discrimination of the footwall, hanging wall and host stratigraphy of the Rosebery mine sequence. A geochemical fingerprint for the footwall stratigraphy to mineralization was developed, and by correlating this signature to the regional unclassified samples, geochemical analogues for the Rosebery footwall were readily identified. This approach rapidly reduces the exploration search space and provides valuable information for future mineral exploration. The methodology is applicable in other terrains; however, appropriate orientation studies are required to investigate potential biases due to lithofacies, alteration and mineralogical variability.

Tracking environmental changes in an Early Cretaceous epicontinental sea: Sedimentology and geochemistry of the Romualdo Formation (Araripe Basin, NE Brazil)

The Early Cretaceous geological record contains evidence of major and abrupt global environmental changes. Understanding the past water-column redox fluctuations and paleoenvironmental evolution of Early Cretaceous environments is, therefore, pivotal for a better comprehension of this period as a whole. In this sense, to investigate the processes that modulated the deposition and preservation of the Romualdo Epicontinental Sea sediments (Aptian–Albian record of the Araripe Basin, Brazil), we present a multi-proxy study using samples from a new borehole drilled in the central area of the Araripe Basin. To unravel the origin, evolution, and demise of this shallow sea, a sedimentological and geochemical characterization was applied. We combine facies association, trace-fossil and petrographic analyses, bulk chemical data (pXRF), TOC and IR quantification (total organic carbon and insoluble residue, respectively), and SEM-EDS images. We identified twelve lithofacies that were grouped into four facies associations. The onset of the deposition of the Romualdo Formation is characterized by the transition from a fluvio-deltaic environment (FA-1) to an epicontinental sea (FA-2) that prevailed and further shifted into a deltaic environment (FA-3). The uppermost facies association (deltaic-fluvial; FA-4) reveals a continentalization process and the demise of the shallow sea. The variations of geochemical proxies were examined to assess terrigenous supply, salinity, redox conditions of bottom water, and primary bioproduction. Based on these proxies, we determined five chemostratigraphic units (U-A to U-E) that revealed a dynamic interplay between organic matter accumulation, paleoenvironmental shifts, and redox conditions. Our results demonstrate that the influx of nutrients from continental sources fostered pulses of biological productivity that, coupled with the low-oxygen environment, resulted in the preservation of organic-rich rocks (high TOC horizons). Notably, the enrichment of redox-sensitive trace elements (RSTEs) suggests that these organic-rich rocks were deposited under euxinic/oxygen-depleted environmental conditions, demonstrating that substantial variations in oxygen levels occurred. Overall, geochemical fluctuations indicate that climatic conditions and siliciclastic input primarily drove the lithofacies variation and organic matter accumulation. Lastly, the results provide constraints on the driving mechanisms that allowed the preservation of organic-rich mudstones of the Romualdo Formation, which is particularly relevant for other studies investigating similar processes in past epicontinental seas.

Carbon isotope sratigraphy and biostratigraphy, and associated organic carbon deposition during the Late Cenomanian to Early Turonian in the Tarfaya Basin, Morocco

The Cenomanian-Turonian (C/T) boundary event, also known as Oceanic Anoxic Event 2 (OAE2), is characterised by significant organic carbon preservation and a positive carbon isotope excursion that is recorded globally in sedimentary strata. In this study of the Tarfaya Basin, Morocco, three outcrop sections were logged - at Lagguing East, Tazra, and En-Naila, dated from Late Cenomanian to Early Turonian. New dating, based on ammonites and planktic foraminifera, combined with high-resolution δ13C isotope stratigraphy, yields a more precise delineation of the C/T boundary in the studied section. A total of 99 samples were collected for combined sedimentological, mineralogical, and geochemical analyses to allow a better understand of the palaeoenvironments of deposition and interpret controlling factors on organic enrichment. Five lithofacies have been identified, all indicative of a relative deep marine setting. Variations in lithofacies can be correlated with relative sea level changes, which align with the global eustatic curve.The OAE2 interval comprises black mudstones that signal anoxic conditions, leading to significant organic matter (OM) preservation, even though they appear weathered in the studied section. Notably, organic matter-rich mudstones were evident during the post-OAE2 Early Turonian maximum marine transgression. Trace element enrichment in these organic-rick mudstones indicates that organic matter accumulation was accompanied with increased sea surface productivity and associated with oxygen-depleted bottom water conditions, which facilitated organic matter preservation and enrichment. Upwelling has also been previously described as a process to deliver nutrients to the shelf, enhancing productivity. Some units with depleted organic content are the result of dilution from continental derived clastics. Others may originally have had high TOC, but were influenced by weathering or diagenetic process, changing from the typical black mudstones to yellow in colour. The identified carbon isotopic values and organic carbon enrichment in the C/T strata of the Tarfaya Basin reflect signals of global perturbations in the carbon cycles.

A revised stratigraphic model for the ∼ 1910–1835 Ma Tanami Group, the Northern Territory, Australia: Implications for exploration targeting

The Paleoproterozoic Tanami Group of the Granites—Tanami Orogen hosts a known endowment of > 20 Moz of gold mineralization across numerous deposits. Throughout the region, host lithologies impart a first-order control on the style, grade and endowment of mineralization. However, the genetic relationship and regional distribution of three regionally correlative formations (the Dead Bullock, Mount Charles and Stubbins formations) remains enigmatic. Here, we combine lithostratigraphic analysis, lithogeochemistry, detrital zircon geochronology and geophysical observations to propose a new evolutionary model for the Tanami Group. We find that the Tanami Group was deposited in a continental back-arc basin setting and developed in four stages. Package A marked the onset of the rifting of the continental Archean basement. Sedimentary rocks within Package A are dominantly felsic in composition and display a unimodal detrital zircon age population at ∼ 2530 Ma. Package B marks the onset of mafic volcanism; consequently, volcaniclastic and sedimentary rocks within Package B display a distinct mafic geochemical signature. Detritus shed from an Archean basement persisted throughout this period, and a unimodal detrital age component at ∼ 2530 Ma persists. Package C marked the waning stages of voluminous mafic volcanism across the Tanami Basin. Sedimentary volcaniclastic rocks within Package C display a mixed mafic and felsic composition. Detrital zircon age spectra record multiple peaks at ∼ 2500, 3000 and 3350 Ma, which may reflect erosion of older Archean basement domains or changes in paleo-drainage. Package D marked a significant change in the source of detritus entering the basin from dominantly Archean to Paleoproterozoic, with dominant detrital age peaks at ∼ 1860 Ma. Despite significant lithofacies variations, the presented model highlights multiple correlations between the Dead Bullock and Mount Charles formations. The Mount Charles Formation likely represents a volcanic source-proximal, high-energy depocenter. In contrast, the Dead Bullock Formation was deposited in a lower energy setting, distal to volcanic centers. These observations are important to mineral exploration. Notably, the Mount Charles Formation is distributed over a significantly greater area than previously thought, and the under-represented Hangingwall and Footwall sequences of this formation are interpreted to host significant gold mineralization at the Oberon and Groundrush deposits.

Lithofacies assemblage and effects on diagenesis in lacustrine tight sandstone reservoirs: Samples from Upper Triassic Yanchang Formation, Ordos Basin, China

Lacustrine basins are rich in unconventional oil and gas resources and the heterogeneity of matrix reservoir quality seriously affects its development. Previous studies have recognized that diagenesis is the key factor affecting the heterogeneity of matrix reservoir quality, but diagenesis controlled by lithofacies assemblages is poorly studied. In this study, tight sandstones in the Triassic (Ladinian-Carnian) Chang 7 Member of Ordos Basin were taken as examples. Lithofacies, lithofacies assemblages, diagenesis evolution and distribution were investigated by casting thin section analysis, scanning electron microscopy (SEM), X-ray diffraction (XRD), fluid inclusion and carbon-oxygen isotope. The results indicate that eight lithofacies assemblages in Chang 7 exhibit variations in lithofacies, sand-mud ratio and grain size rhythm, controlling the distribution and intensity of diagenesis. Variations in average proportion of lithofacies assemblages are linked to lake level cycles. Compared to cementation, the mechanical compaction has a higher adverse effect on primary porosity of tight sandstones in Chang 7. Siltstone lithofacies show stronger compaction and cementation (mainly clay cementation), whereas fine sandstone lithofacies exhibit stronger dissolution, especially FSpc (fine sandstone with parallel or cross-bedding). Modest proportions of illite (<4%) and chlorite (<1.5%) in fine sandstones tend to form clay coatings, resisting compaction and preserving more porosity. Conversely, abundant pore-filling illite (>4%) and chlorite (>1.5%) result in a noticeable decline in porosity. Particle size and sorting affect reservoir quality by controlling compaction strength. The origin of carbonate cement in sandstones is closely related to mud-rich lithofacies within lithofacies assemblages. Carbonate cementation preferentially occurs in fine sandstones within 0.4 m from the sand-mud interface and the thickness of adjacent mud-rich lithofacies is greater than 0.1 m. Interfaces between fine sandstones serve as fluid migration pathways and bidirectional erosion, facilitated by high porosity and feldspar content. In contrast, the clay-rich matrix in siltstones acts as a barrier hindering fluid migration, creating unidirectional erosion in fine sandstones near the interfaces between siltstones and fine sandstones. Ultimately, five evolution processes linking diagenesis with lithofacies assemblages are summarized, highlighting the differential reservoir quality. This study contributes to understanding the mechanism behind diagenetic variations in tight sandstones and the reservoir evaluation. Consequently, it offers geological reference for the development of unconventional oil and gas resources in comparable lacustrine basins.

Geochemical and palynological characteristics of the Maykop sections in the north-west of the Shamakha-Gobustan region (Azerbaijan)

The article provides a comparative analysis of the results of complex palynological and geochemical studies of rocks from natural outcrops of Lagich, Angekharan and Agsu in the north-west of the Shamakhi-Gobustan region. A comparison of the distribution features of palynomorph species with the lithofacies variability of sediments made it possible to reveal the conditions of sedimentation during the formation of the described sections. In addition, the age of the units identified in the Maikop deposits in Lagich and Angekharan was clarified by analogy with the formations and horizons of similar ages in the Western and Central Ciscaucasia. The boundary of Eocene and Oligocene deposits in the Lagich section is assumed to be in the interval of 80–115 m. Late Eocene deposits occurring at the base of the section are apparently an analogue of the boundary deposits of the Beloglinskaya and Pshekhskaya formations of the Ciscaucasia. The presence of cold-water dinocysts of the species I.recurvus and calcareous nannofossils Coccolithus pelagicus in the boundary interval corresponds to the existing ideas about climate cooling at the end of the Eocene - beginning of the Oligocene. The formation of sediments in the upper intervals of the section probably occurred in more open marine conditions, relatively warm water, but with low levels of nutrients (nitrates). This interval of sediments with a characteristic phytoplankton complex can be correlated with the middle part of the Khadum horizon, an analogue of the Nizhny Morozkinsky subformation and, possibly, with the upper clayey part of the Polbinsky formation of Western and Central Ciscaucasia. The accumulation of the lower units of the studied part in the Angekharan, dated to the Early Oligocene, most likely occurred in a marine, shallower environment with possible short-term transgressive impulses. Overall, it is possible to suggest a proximal environment of high nutrient content in well-mixed waters within the inner neritic zone. An analogue of the deposits of this interval can be the Lower Morozkinsky Formation and the lower parts of the Upper Morozkinsky Formation of the Ciscaucasia. The significant increase in deflandroids in the upper part of the Angekharan section is similar to the changes noted during the transition to the zone of Deflandrea spinulosa f. Majkopica in the upper part of the Batalpashinskaya Formation of the late Oligocene in Ciscaucasia. The accumulation of sediments probably occurred in a marine basin with signs of stagnation in brackish, eutrophic environment, with a short-term supply of material from the photic zone. The conclusions about the peculiarities of changes in the depositional conditions in the study area are also confirmed by the results of geochemical studies. The deposits of the studied sections are represented by both terrestrial and aquatic types of organic matter (OM). Terrigenous OM was more typical in rocks from the Lagich and Agsu outcrops. The OM of rocks from the Lagich section varies from marginally mature to mature. In the Angekharan section the OM is thermally immature, as evidenced by the low Tmax values. Despite the small thickness of the Agsu outcrop, rock samples from this section contain kerogen of various stages of transformation from immature to mature. The carbon isotope composition (CIC) of Corg in sediments within the studied sections is in the δ13C range, characteristic of Oligocene sediments in the Paratethys. There is a higher enrichment of the 13C isotope in OM in sediments from the Agsu section.

Volcanic lithofacies control the space in unconventional, rhyolitic hydrocarbon reservoirs: The Hailar Basin, NE China

Volcanic rocks are generally characterized by significant spatial variations of lithofacies. The characterization of volcanic lithofacies is an important tool to achieve accurate reservoir prediction during hydrocarbon exploration because of the type of lithofacies strictly controls the porosity and other physical and chemical properties of reservoir rocks. Studying the lithofacies of rhyolitic volcanic rocks is essential for uncovering the genesis and characteristics of volcanic reservoirs. The Lower Cretaceous Shangkuli Formation is located on the western boundary of the Hailar Basin in NE China. This basin hosts oil-bearing rhyolitic volcanic rocks and represents a natural laboratory for investigating the spatial distribution of different lithologies and volcanic lithofacies. Here, we present detailed field observations, TESCAN Integrated Mineral Analysis (TIMA), and fluorescence imaging to characterize seven types of rhyolitic volcanic rocks belonging to four main lithofacies associations. We examine the relationship between lithofacies associations and reservoir rocks. Field observations reveal the existence of volcanic conduit, pyroclastic, effusive, and extrusive lithofacies associations within the Hailar Basin. These lithofacies associations can be further subdivided into nine distinct subfacies. Among these, the extrusive lithofacies association is dominant and it is followed by pyroclastic, effusive, and volcanic conduit lithofacies associations. Intra-spherulite microfractures and pores, lithophysa cavities in the extrusive lithofacies associations, and inter-clast pores of the volcanic conduit lithofacies associations constitute the primary reservoir spaces of rhyolitic reservoirs. The lithofacies associations exert a control on the reservoir spaces of rhyolitic volcanic rocks by limiting the degree of supercooling (ΔT) and volatile content, which impact the formation of different textures in rhyolitic magma during syn-eruptive processes. We conclude that spherulitic and lithophysa rhyolites in the extrusive lithofacies associations, and the crypto-explosive breccias in the volcanic conduit lithofacies associations are promising targets for oil and gas exploration. The facies model we obtain from the Hailar Basin may be extended to similar rhyolitic rocks in other basins hosting unconventional reservoirs.

Lacustrine deposition in response to the middle eocene climate evolution and tectonic activities, Bohai Bay Basin, China

The lower unit of the third member of the Eocene Shahejie Formation (Es3L, 42.5 to ∼40Ma) is one of the most important petroleum exploration targets in the Bohai Bay Basin. Lacustrine deposits are widely distributed in the Es3Lunit of the Shulu Sag and display strong vertical and horizontal variations in lithofacies and geochemical characteristics. Based on core data, thin-section petrography, stable δ13C isotope analyses, and organic geochemistry data, eight lithofacies and six evolutionary lake stages are identified. Combining the results of the astronomical time scale (ATS) and the radiometric age, the correlation between the evolutionary lake stages of Es3Land the Eocene global climate curve was illustrated. Tectonic movement caused the basin to subside and resulted in shallow lake formation (Stage 1) during the cooling phase prior to the Middle Eocene Climatic Optimum (MECO); then, the lake level began to rise (Stage 2) with an increasingly warm and humid climate. Stage 3 corresponds to the peak of the MECO and was influenced by the warm and humid climate during the thermal maximum period, forming a deep, freshwater lake. This stage was followed by highly turbulent lake conditions (Stage 4), which were influenced by both the cooling climate and intensive tectonic activity. Subsequently, a stable lake (Stage 5) was formed, which was mainly controlled by the slightly warming climate since the tectonic movement was relatively stable, followed by another stage of turbulent lake conditions (Stage 6). The results show that the global mid-Eocene climate played an important role on the relative lake level changes and TOC enrichment. The MECO led to a rise in the relative sea-level and the obviously rise in relative lake-level in the Bohai Bay Basin. The evolution of the depositional settings was controlled by both global climate and tectonic activities and accounted for the deposition and distribution of lithofacies with distinctly different characteristics and origins.

Reservoir Quality, Lithotype Assessment, and Geochemical Source Rock Analysis: Insights from Well Logs and Pyrolysis Data, Karama Field, North-Western Desert, Egypt

Reservoir characteristics and source rock geochemistry are essential for petroleum system investigation as they reveal reservoir quality and hydrocarbon generation capability, respectively. The primary Karama oil field reservoir of Abu El Gharadig Basin is the limestone-sand-shale Abu Roash G (AR/G) Member. This study examines AR/G, analyzes source rocks for maturity and organic elements, and defines the main reservoir lithotypes and evaluates reservoir properties. Five well log datasets and an AR/F pyrolysis analysis on another well were used in this study to characterize the AR/G’s 168-foot effective thickness and assess the AR/F source-rock maturation. The effective porosity is up to 30%. The highest shale concentration was 24% in central and western parts of the field. Therefore, drilling development wells in this area, especially east and north, demands caution. The composition and vertical and lateral lithofacies variations of the defined reservoir in the Karama field region are a significant control of its petrophysical properties. The pyrolysis of AR/F revealed 1.32–5.84% content of organic matter. That content qualifies AR/F as a hydrocarbon source if thermal maturity is reached. Type I and type II kerogen in the Abu Roash F Member suggests oil production. The Abu Roash G Member and Upper Bahariya (UB) formation produce oil and gas due to their own type II and III kerogen. GC biomarker data suggests that the research area is predominantly maritime, with most samples showing environmental degradation. The area under consideration has one reservoir, AR/G, and three members of source rocks in AR/F&amp;G and UB. AR/G electrofacies revealed various lithotypes and flow units.

Diagenetic alterations and deep high-quality reservoirs within deltaic distributary channel facies: a case study from the Permian Shihezi formation in the Hongde area, southwestern Ordos basin, China

Deep tight gas reservoirs are considered important hydrocarbon exploration targets. High-quality reservoir prediction is critical for successfully exploring and developing deeply buried tight sandstone gas. Previous research has found that the reservoir quality of deeply buried tight sandstones is controlled by diagenesis and sedimentary facies. However, the variation of diagenetic alterations in different facies is still poorly studied on deltaic tight gas sandstone. In this study, core analysis, wireline log data, and 3D seismic were studied for the characterization of diagenetic alterations and sedimentary facies. The tight sandstones were formed in braided river delta deposits. Gravel-bearing coarse-grained sandstone facies and cross-bedded sandstone facies developed in tight sandstones. The Gravel-bearing coarse-grained sandstone is formed in the mid-channel bar of deltaic distributary channels. The major diagenetic processes developed in the tight sandstone include compaction, cementation, and dissolution. Constructive diagenesis can generate secondary pores, mainly including dissolution and kaolinite metasomatism, which can effectively improve reservoir physical properties. Through the diagenesis alterations linked to different lithofacies in cored wells, there are obvious diagenesis variations in different lithofacies. Despite strong compaction, the reservoirs in coarse sandstone facies have developed internal dissolution. After compaction, cementation, and dissolution to increase porosity, the reservoir retains intergranular and secondary pores and forms relatively high-quality reservoirs. The fine sandstone facies with cross-bedding are strongly compacted, with internal ductile particles being compacted and deformed, and the particle orientation is clearly arranged. The reservoir is tight, and the development of dissolution in the reservoir is weak, resulting in poor reservoir quality. In addition, reservoirs located at the interface between sandstone and mudstone are often affected by diagenesis, resulting in the development of calcareous cementation, leading to poor reservoir quality. Therefore, high-quality reservoirs are mostly distributed in coarse sand lithofacies, mainly distributed in the mid-channel bar of distributary channel deposits. Sedimentary facies control the original physical properties of the reservoir with different content and texture; the quality of the original reservoir is subject to diagenetic alteration in different ways. The points in this study could offer insights better to predict deep tight reservoir quality in continental basins.

Seismic interpretation and sequence stratigraphic analysis of the Bahariya Formation in the South Umbaraka oilfields (Western Desert, Egypt): insights into reservoir distribution, architecture, and evaluation

The integration of multiscale datasets, including seismic, well-logs and stratigraphy, is providing a precise picture about reservoir evaluation and spatial distribution. This study focuses on the seismic interpretation and sequence stratigraphic analysis of the Bahariya Formation in the South Umbaraka oilfields (Selkit, Khepri, and Sethos), located in the Western Desert of Egypt. Seismic interpretations reveal various structural closures, including anticlinal domes and faulted anticlines in different fields. The study area is divided into northern and southern blocks separated by a major fault, and structural cross sections provide valuable information on reservoir zones regarding lateral extension, thickness variation, and fault distribution. The Bahariya Formation is subdivided into two depositional sequences: Lower Bahariya sequence (BAH.SQ-1) and Upper Bahariya sequence (BAH.SQ-2) with distinct wireline-log characteristics, seismic reflections, and lithofacies variations. BAH.SQ-1 comprises mixed siliciclastic-dominated units with some carbonate units, while BAH.SQ-2 consists of mixed siliciclastic and carbonate units. Multiple sequence boundaries, depositional surfaces, and systems tracts are identified within BAH.SQ-1 and BAH.SQ-2, indicating changes in sedimentary environments and sea-level fluctuations. The petrophysical evaluation reveals promising reservoir zones in both the Lower and Upper Bahariya sequences. However, the Lower Bahariya sequence exhibits superior reservoir characteristics in terms of net-pay thickness, shale volume, effective porosity, and water saturation. Particularly, the sandstone-dominated LST-1 within the Lower Bahariya Formation is identified as a favorable reservoir zone. Conversely, the Upper Bahariya Formation has limited reservoir potential. This study underscores the significance of sequence stratigraphy and reservoir architecture, emphasizing the role of faulting, folding, and depositional environments in the distribution and quality of reservoir zones. The findings contribute valuable insights into the structural and stratigraphic characteristics of the Bahariya Formation, enhancing understanding of hydrocarbon plays and reservoir distribution in the studied area.

3D model of water saturation, effective porosity and volume of shale in upper Assam Shelf, India using multi-attribute regression and cascade-probabilistic neural network

Quantitative interpretation of geophysical data for reservoir identification to study the vertical and horizontal variation of lithofacies, involves integration of multiple disparate datasets. We use a 3D seismic data and two wells (Km and Te) to predict a petrophysical model within the Paleocene to Oligocene formations, and the Precambrian basement in the upper Assam Shelf. The conventional well data were examined in each well to identify reservoirs within the formations. This is followed by a model based seismic inversion of 3D post-stack seismic data resulting in a 3D impedance model. Next, the cascade-probabilistic neural network (CPNN) was trained using the well log derived attributes (target label data) at the collocated seismic observations (observed features). We were able to successfully train the network to predict water saturation, effective porosity, and volume of shale from the inverted seismic traces with the cross-correlation of range 0.96, 0.97, 0.97, respectively. The optimum set of multi-attributes at well-seismic tie locations are extracted from the seismic data and the selected optimum set of attributes are used at all locations in the application of CPNN. CPNN is found to train the target data better than probabilistic neural network and multi-layer feedforward neural network. Hydrocarbon reservoirs are found in shaly sandstones within the Kopili and Sylhet formations of Eocene and Paleocene ages, and Precambrian basement. The hydrocarbon reservoir shows the variation in water saturation 20% to 50%, with the volume of shale of range 40% to 60%, and the effective porosity of range 8% to 30% respectively.

Lithofacies variability and facies analysis of a Givetian reef in the southwestern Lahn Syncline (Rhenish Massif, Germany)

A 200 m thick drill core penetrating the Givetian Hahnstätten Reef in the southwestern Lahn Syncline (Rhenish Massif) was investigated. A range of different depositional environments is described based on lithofacies and microfacies analysis. All in all, nine lithofacies types (FTs) are distinguished, which can include subfacies types. The majority of lithofacies of these ultrapure carbonates is represented by lime mudstone and fenestral microbialites, all pointing to shallow subtidal, intertidal to even supratidal low-energy palaeoenvironments. In contrast, more high-energy parts of the reef were dominated by bioclastic rubble deposits (e.g. rudstone). Autochthonous, reef-building carbonates are represented by bafflestone and framestone. Diversity of reef building organisms (stromatoporoids and corals) is low and is dominated by Stachyodes, Actinostroma, Stromatopora, and Thamnopora and alveolitids, respectively. Other bioclasts are brachiopods, gastropods, ostracods, foraminifera, echinoderms, trilobites, and conodonts in descending order. Development of the Hahnstätten Reef is interpreted as having been controlled mainly by synsedimentary tectonics and volcanism with contributions from eustasy. The occurrence of Stringocephalus burtini in the entire section and conodont findings, which provide more precise biostratigraphic data confirm an early to middle Givetian age (Polygnathus rhenanus/varcus Zone to Polygnathus ansatus Zone) of the succession. The average quality of the ultrapure carbonates lies at 97.68% CaO (excl. loss of ignition), with 70% of the core ranging between 98% and 99% CaO. This extremely high purity makes it difficult to identify correlations between lithofacies and geochemical data.

Controls on preservation of organic matter during the Cenomanian Ocean Anoxic Event II (OAE2) and Turonian global sea-level rise: Agadir Basin, Morocco

The Ocean Anoxic Event II (OAE2) was a significant global event associated with a positive carbon isotope excursion, occurring from the Late Cenomanian to the Early Turonian (C/T). It has been commonly associated with enhanced organic carbon preservation. Carbon isotopic analysis carried out over the C/T interval in the Agadir Basin, Morocco, integrated with previous biostratigraphic data, allows a refined correlation of the OAE2 interval. Detailed sedimentary facies analysis identifies ten lithofacies that record the transition from shallow marine environments in the Late Cenomanian to relatively deeper conditions in the Early Turonian. The variation in lithofacies can be correlated to relative sea level changes that show a correlation to the global eustatic curve.The OAE2 interval comprises dark grey mudstones beds that display low total organic carbon (TOC) values. Trace element and facies analysis suggest dilution from high detrital influx, along with oxic water conditions and low productivity. OM-rich black mudstones are identified in post-OAE2 Early Turonian strata. Trace element analysis suggests this increase in organic matter accumulation was related to increased sea surface productivity and oxygen-depleted bottom water conditions, which facilitated organic matter preservation. Deposition of OM-rich black mudstones is widely reported during the global Early Turonian marine transgression, suggesting the very high sea level was a major control on organic matter generation and preservation.

Holocene evolution of the Banni Plain at the north‐east margin of the Arabian Sea: Constraints from a ca 50 m long sediment core

AbstractHolocene evolutionary history of the Banni Plain in the Great Rann of the Kachchh Basin is reconstructed from a subsurface sediment core of ca 50 m. Detailed data on textural and lithofacies variations, grain‐size analysis, environmental magnetism and accelerator mass spectrometry 14C dates on seven samples were generated on the sediment core retrieved from the Banni Plain near Berada. A high‐resolution record extending back to 10 ka has been reconstructed from the top ca 40 m of the core section comprising shallow marine sediments. The core is divisible into five depositional units. The basal part is a fluvial depositional unit followed upward by estuarine, sub‐tidal, intertidal and supra‐tidal environments. The sediment accumulation rate is highest in the sub‐tidal to intertidal facies (1.9 cm year−1) and decreases towards the supra‐tidal facies to 0.09 cm year−1. Environmental magnetic analysis, χlf coupled with the S‐ratio, indicates high magnetic mineral concentrations during the Early Holocene, suggesting a wet period accompanied by high monsoon precipitation. This is followed by the onset of semi‐arid conditions in the Great Rann of the Kachchh Basin as indicated by the low values of the χlf and S‐ratios. A westward and northward shift in the shoreline towards the deeper part of the basin is suggested during the Late Holocene, which is coupled with aridity and reduced monsoonal conditions. The change in depositional pattern from the retrogradational deposit of fluvial (Unit 1) to estuarine sediment (Unit 2), progressing to sub‐tidal (Unit 3), is attributed to sea‐level transgression followed by regressive intertidal (Unit 4) to supra‐tidal deposition (Unit 5), culminating in complete withdrawal of the sea, aided by tectonic uplift, during the Late Holocene. The results reveal that the sediment accumulation rates and depositional environments changed over time in response to changes in sea level from minima to maxima and then eventually to the present level.

Cyclostratigraphy of Lower Permian alkaline lacustrine deposits in the Mahu Sag, Junggar basin and its stratigraphic implication

The Lower Permian Fengcheng Formation of the Mahu Sag is one of the most potentially petroliferous sequences in China, and its unique alkaline lacustrine deposits provide important information on the paleoclimate and paleoenvironment of the early Permian. However, because of the complexity of the heterogeneous lithology and sedimentary facies in lacustrine deposits, the lateral correlation of lithofacies becomes challenging. Using cyclostratigraphy, we conducted a detailed astronomical cycle analysis of the Lower Permian Fengcheng Formation in the northern Mahu Sag, established an astronomical time scale, and constructed an isochronous sedimentary framework by collating the cycles of the different wells. Nine 405-kyr long-eccentricity cycles in the Fengcheng Formation were identified, and absolute astronomical time scales were established with the anchored point at ∼300 Ma in the Lower member of the Fengcheng Formation. Based on the identification of lithofacies, the spatio-temporal variation in the lithofacies within the Fengcheng Formation was reconstructed. The astronomical time scale has proven to be reliable, and the lithofacies distribution within the isochronal framework is effective for investigating the spatial variation of lithofacies in alkaline lacustrine deposits. Favorable dolomitic mudstones developed in the central and transitional zones, corresponding to the three long-eccentricity cycles in the middle member of the Fengcheng Formation. Tuffaceous mudstones with high potential mainly developed in the lower member of the Fengcheng Formation in the marginal zone of the Mahu Sag. This study demonstrates an approach that can be used to study lithofacies in lacustrine deposits.

Towards an improved discrimination and correlation of Permian-Lower Triassic sediments in Central Europe: A chemostratigraphic approach

The stratigraphic classification and correlation of Permian-Lower Triassic clastic beds in the Central European Basin System traditionally rely on geophysical, bio-, and lithostratigraphic methods. However, in basin margin areas, the accuracy of these tools is diminished due to the heterogeneous distribution of stratigraphic markers, lack of index fossils, and greater climatic and tectonic disturbance. To overcome these limitations, this study introduces a more versatile, multifaceted and chemostratigraphy-based approach. By integrating results from bulk X-ray fluorescence analysis, sandstone petrography, Raman heavy mineral analysis, lithofacies characterisation and borehole gamma-ray logs, we produce robust multi-stratigraphic schemes for selected sequences in the Franconian Basin (SE Central European Basin System). Hierarchical chemostratigraphic zones, comprising five chemozones and sixteen subzones, were designed using key elemental ratios (Ga/Rb, Rb/K, Na/K, Ca/Mg, Cr/Ti, Th/Ti, Nb/Th, Ti/Nb, P/Ti, Zr/P, Zr/Y), whose changes through time reflect modifications in the mineralogical composition of sedimentary rocks. To ensure precise interpretations of geochemical data, we established links between mineralogical and chemical components through statistical processing and direct comparison with mineralogical assemblages. Our results demonstrate that chemostratigraphic boundaries can identify major changes in palaeo-depositional environments and sediment sources induced by eustatic, tectonic and climatic disturbances. For example, changes in the P/Ti and Zr/P ratios across the CP3-CT1 document the increased abundance in apatite-rich granitic detritus, resulting from the exposure of new source lithologies in the catchment area due to Early Triassic synsedimentary tectonism. Since these perturbations had a regional impact on sedimentation, their geochemical fingerprints are correlated across the basin, regardless of lithofacies variability or distance between sections. Moreover, integrating characteristic geochemical signals into multi-stratigraphic schemes enables more precise placement of stratigraphic boundaries, identification of locally deposited/preserved stratigraphic markers, and overall refinement of outdated classification schemes. The chemostratigraphic approach introduced in this study offers a solution to the limitations of conventional stratigraphic tools, allowing robust characterisations and correlations of Permian-Triassic sequences in the marginal regions of the Central European Basin System. In addition, the multi-analytical workflow for processing geochemical data, refining stratigraphic schemes and attributing geochemical anomalies to environmental perturbations represents a further step towards more precise applications of chemostratigraphy to the depositional record.

Development and controlling factors of shale lithofacies cycles in a continental rift basin: A case study of Es4u in the Boxing Subsag of Dongying Sag, Bohai Bay Basin, China

The shale of the upper Es4 formation (Es4u), deposited during the Eocene in the Boxing Subsag of the Dongying Sag, is a typical set of lime-rich lacustrine shale in a continental rift basin. Through logging data interpretation, core and thin section observations, and geochemical elements [obtained by X radial fluorescence (XRF) mud logging] analysis, the development and controlling factors of lithofacies cycles of Es4u shale were analyzed. The results show that 22 types of lithofacies with typical characteristics are developed in Es4u, indicating the sedimentary environments, such as lower lakeshore (LL) slope zone; upper, middle, and lower shallow-lake slope zones; semi-deep-lake zone; and deep-lake zone. Lithofacies cycles in three scales are recognized in Es4u: small-scale lithofacies cycles indicated by alternate development of two lithofacies, mesoscale lithofacies cycles indicated by the repetition of lithofacies associations or regular variation of multiple lithofacies, and large-scale lithofacies cycles indicated by regular variation of well logging curves, which have the average thickness of approximately 0.4, 5, and 35 m, respectively. According to the vertical variation of astronomical parameters and lithofacies cycles, the periodic change in long eccentricity has a close relationship with the large-scale lithofacies cycle, the periodic change in slope is closely relative to the mesoscale lithofacies cycle, and the periodic change in precession is closely related to small-scale lithofacies cycle. However, the periodic change in short eccentricity correlates poorly with lithofacies cycles. The climate change reflected by long eccentricity and slope is quite distinct from the actual basin climate reflected by the paleoclimatic parameters (e.g., Rb/Sr and Sr/Cu), indicating that the basin paleoclimate was greatly influenced by the paleogeological conditions. For shale in Es4u of the Boxing Subsag, approximately 55% of the shale stratigraphic record is dominantly controlled by the astronomical climate and 45% by the paleogeological conditions. Affected by the paleogeological conditions, large-scale and mesoscale lithofacies cycles are different in lithofacies types and superimposition patterns, and just small-scale lithofacies cycles are found repetitive vertically. These findings suggest that the lithofacies cycles of shale in a continental rift basin are a product of astronomical driving mechanism (Milankovitch forcing) and variation of local paleogeological conditions, which is apparently different from the sedimentary record of marine shale that is evidently controlled by the Milankovitch cycle. Therefore, the lithofacies cycles should be comprehensively analyzed under the background of actual basin paleoclimate.

Rock mechanical properties of immature, organic-rich source rocks and their relationships to rock composition and lithofacies

Mechanical properties of layered rocks are critical in ensuring wellbore integrity and predicting natural fracture occurrence for successful reservoir development, particularly in unconventional reservoirs for which fractures provide the main pathway for hydrocarbon flow. We examine rock mechanical properties of exceptionally organic-rich, immature source rocks from Jordan, and understand their relationships with rock mineral composition and lithofacies variations. Four depositional microfacies were identified: organic-rich mudstone, organic-rich wackestone, silica-rich packstone and fine-grained organic-rich wackestone. The four types exhibit various mineralogical compositions, dominated by carbonates, biogenic quartz and apatite. Leeb hardness ranges between 288 and 654, with the highest average values occurring in silica-rich packstone and organic-rich mudstone. The highest uniaxial compressive strength (derived from the intrinsic specific energy measured using an Epslog Wombat scratch device), and compressional- and shear-wave velocities were measured in organic-rich mudstones (140 MPa, 3368 m s −1 and 1702 m s −1 , respectively). Porosity shows higher average values in organic-rich wackestones and fine-grained organic-rich wackestones (33–35%). Silica-rich packstone and organic-rich mudstone have brittle properties, while organic-rich wackestone and fine-grained organic-rich wackestone are ductile. High silica contents are correlated positively with brittleness. A strong hardness–brittleness correlation suggests that Leeb hardness is a useful proxy for brittleness. Our study allows a better understanding of the relationships between lithofacies, organic content and rock mechanical properties, with implications for fracking design to well completion and hydrocarbon production. Further work involving systematic sampling and a more rigorous study is still required to better understand the spatial distribution of target lithologies and their mechanical properties.

Tectonostratigraphic controls on pore fluid pressure distribution across the Taranaki Basin, New Zealand

AbstractSignificant variations in pore pressure across the Taranaki Basin, New Zealand, are attributed to changes in lithofacies and structure, usefully illustrated in terms of ten areas that we term geopressure provinces, each displaying individual pore pressure trends. Cretaceous to Early Miocene formations in different parts of the basin can be either normally pressured (near or at hydrostatic) or significantly overpressured (up to 28 MPa) at the same depth. Variations in Eocene–Oligocene facies types and thicknesses both within and between geopressure provinces provide first‐order control on the magnitude, distribution and maintenance of overpressure across the basin. Examples of hydraulic compartmentalisation due to sealing faults and stratigraphic architecture are identified within the basin. Deep pore pressure transitions are sealed by diagenetic, structural or stratigraphic mechanisms in different places and are associated with an increase in mudrock volume (reduced permeability) or gas generation. Thus, pore pressure distribution in the Taranaki Basin is controlled by a combination of sediment loading, lithofacies variations, fault zone permeability and structural architecture. This work represents an appraisal of the pore pressure distribution across the whole of a multiphase structurally complex basin, and the approach taken provides a framework for better understanding the distribution of pore fluid pressures and pore fluid migration in other sedimentary basins.