Depositional Paleoenvironment Research Articles

Palynostratigraphy and palaeoenvironment of the Danian sediments from the Nsukka Formation of the Anambra Basin at the vicinity of Ikpankwu, southeastern Nigeria.

Detailed investigations of biostratigraphy of the Nsukka Formation in the Ikpankwu area are paramount due to the fluctuation of depositional environment, fossil flora abundance and diversity interchange, and lateral facies change dynamics from the southern to northern sections of this formation. The microflora assemblages, chronostratigraphic age and depositional paleoenvironment of index palynomorphs suggest a Danian age for the studied strata, based on global dinoflagellate cyst events. The persistent abundance of monocolpate assemblages and the biostratigraphic resolution attained, suggest that the studied sediment interval is continuous. The marker species of terrestrial microflora (Constructipollenites ineffectus, Echitriporites trianguliformis, Foveotriletes margaritae, Longapertites group, Proxapertites group, Retidiporites magdalenensis, Scabratriporites annellus, Spinizonocolpites group, Syncolporites marginatus) and dinocysts (Carpatella cornuta, Danea californica, Fibrocysta licia, Kenleyia pachycerata, Spiniferella cornuta subsp. kasira), compared with several reference sections from the tropical to subtropical province, engineered a refined geologic age and paleoenvironment of deposition of the sediments. These dinocysts taxa are global Danian index fossils which are valuable in the identification of earliest Palaeocene sequence of the formation. The depositional paleoenvironment alternately fluctuates from continental setting to marginal marine inner neritic and outer neritic conditions, based on the presence of land-derived miospores and dinocysts. These macroenvironments are the products of effective vacillating depositional environment systems between the lower and upper deltaic plains, oscillating from tidal flat, lagoon, tidal bar to nearshore open marine influenced settings. Lycopodiaceae-Zlivisporis blanensis fern spore with fragile perispore substantiate the autochtonous vegetation and least transport and palynomorph hydrodynamic system inherent in the predominant pollen and spore species and close proximity to fresh water settings visible in medium to coarse grained sandstone units of the middle section. These microenvironment syntheses of the Nsukka Formation revealed a confirmed and pronounced transgressive systems tract (TST), articulated in the shale sample from the Ikpankwu section of the Nsukka Formation, Anambra Basin. The palaeoenvironments and palynostratigraphic dynamics illustrate that overall progradation was trailed by sporadic retrogradation of the delta with a high biostratigraphic resolution system indicative of continuous biological imprints of the terrestrial and marine microflora of the Nsukka Formation during the Danian.

Fossil woods from Corcovado (Eocene?), Argentinean Patagonia: angiosperm diversity and biodeterioration

Summary Angiosperm fossil woods of putative Eocene age from Corcovado, Chubut Province are described. They are placed in four taxonomic units: Weinmannioxylon trichospermoides and cf. Caldcluvioxylon of the Cunoniaceae, Myrceugenellites maytenoides of the Myrtaceae, and Nothofagoxylon ruei of the Nothofagaceae. The taxa in the assemblage are in accordance with similar fossil wood assemblages previously described from the Eocene and Oligocene of Patagonia. All the taxa are anatomically similar to trees that live today in Patagonia. Some of the woods show decay patterns by different saproxylic organisms. These include arthropod borings with coprolites and fungal remains in fungi-decayed tissues that are consistent with an active saproxylic community involved in the recycling of wood in the depositional paleoenvironment.

Geochemical evaluation of the carbonaceous shale associated with the Permian Mikambeni Formation of the Tuli Basin for potential gas generation, South Africa

Abstract The increasing demand for energy and global commitment to reduce CO2 emissions to mitigate climate change has spurred countries to pursue unconventional hydrocarbon generation for power production. The investigation of the carbonaceous shale from the Permian Mikambeni Formation of the Tuli Basin in South Africa aims to assess the potential for gas generation. The Rock-Eval 6 pyrolysis of the studied samples shows total organic carbon (TOC) content ranging from 10.10 to 37.61 wt%, indicating an excellent source rock quality since it exceeds the threshold limit of 5 wt%. The produced gas that emanated from the thermal breakup of remaining hydrocarbon (S2) ranged from 14.30 to 65.55 mg HC/g rock while the hydrogen index (HI) ranged from 56.55 to 198.22 mg/g TOC. The plot of the HI against T max indicates the predominance of type-III kerogen, prone to gas generation. The thermal maturity parameters, T max (430–466°C), and vitrinite reflectance, VRo (0.6–1.2), showed a matured organic material. The production index value (0.06) of the samples indicates a moderately mature source generating a biogenic gas. The carbon isotopic composition, δ 13C (−20.0 to −23.4‰), indicates a predominant influx of organic matter derived from a vascular higher plant. The studied samples showed relatively depleted values of oxygen isotopic composition δ 16O (14.1–18.8‰), suggesting a brackish water depositional environment. Additionally, the Ba concentration (303.15–331.27 ppm) exceeds the 200 ppm threshold for the marine environment, indicating a non-marine environment characterised by low marine paleo-productivity and high detrital influx. Based on the evaluation, the studied coaly-shale is found with a matured and type-III kerogen that is prone to gas generation in a fluvial, paleo-depositional environment.

Maastrichtian – Paleocene environmental and climatic conditions in Koum Basin (northern benue trough, Cameroon) inferred using a multiproxy approach including sedimentology, mineralogy, and geochemistry

The Koum Basin is one of major intracontinental basins in northern Cameroon. It is part of the Upper Benue Trough and has genetic ties to the West and Central African Rift System. The main purpose of studying the Koum Basin sedimentary deposits was to re-examine the time of the deposition and constrain the palaeoenvironmental and paleoclimatic conditions during deposition. To accomplish this, we used a combination of sedimentologic, palynologic, mineralogic, and geochemical data. Claystone units interbedded with calcareous siltstones dominate sedimentary sequence, including few clay beds, fine to medium-grained sandstone, and rare marl layers. The palynomorph assemblage suggests a previously unknown Maastrichtian – Paleocene age in this basin. The sediments originated from intermediate igneous parent rocks with a relative contribution of felsic rocks. These rocks underwent low detritism (Si/Al: 2.51–4.43; K/Al: 0.11–0.36; Ti/Al: 0.05–0.10) and chemical weathering (CIA: 55–64; PIA: 56–68) in the source region. Consequently, deposits are immature (ICV values > 1) and weakly recycled (Si2O3/Al2O3 ratios ≤5). The organic matter analysis and the palynoflora assemblage, reveal Type III kerogen with significant contributions of Type II-III and type II kerogens, as well as a continental paleo-depositional environment with open rainforest to savannah vegetation. The clay minerals distribution suggests moderate illitization and chloritization processes and thus moderate burial diagenesis at temperatures ranging from 70 to 90 °C, also attested by immature to mature Tmax values (≤436 °C) marginally. The low CIA values and very low kaolinite contents, indicate dry climatic conditions. In addition, two major-element-based discrimination diagrams show that the parent rocks of clastic sediments formed primarily in a continental arc system, with a moderate contribution from the rift system and a low contribution from the oceanic island arc, implying compressional conditions, which is consistent with previous studies on the tectonic background of the region during the late Maastrichtian to Paleocene period.

Paleodepositional environment and hydrocarbon generation potential of the Paleogene lignite in the Barmer Basin, Rajasthan, India

The Barmer Basin in Rajasthan is considered one of the most important petroliferous sedimentary basins of India. Apart from a hydrocarbon-producing source rock, the basin also hosts some significant lignite deposits of Giral fen and Kapurdi bog developed during the late Paleocene to early Eocene, being currently under exploitation. Applying organic petrology, geochemical, mineralogical techniques, and Rock-Eval pyrolysis on samples obtained from seams in both mines, the paleodepositional settings in both areas were reconstructed, and the hydrocarbon generation potential was assessed. The Tmax (377–427 °C) and PI (0.01–0.09) values in the studied samples from the Barmer Basin indicate an immature nature of the organic matter. HI values range from 66 to 476 mg HC/g TOC, revealing that the Giral and Kapurdi lignite contains mostly Type III and mixed Type II-III kerogen and can generate mainly gaseous hydrocarbons, although few samples have liquid hydrocarbon generation potential. Significant values of TOC (20–60 wt%) reveal that they may act as a good source of hydrocarbons and make them favourable for the generation of hydrocarbons. The concentrations of major oxides and selected trace elements indicate that the Giral and Kapurdi lignite was developed in warm humid to some extent arid warm climate and oxic conditions prevailed in the basin. Petrographic indices suggest the peat accumulation occurred in a dry forest mire under limnic to limnotelmatic conditions. The indicators further suggest the lignite originated from the bog and fen under mesotrophic to ombrotrophic hydrological condition. The Giral and Kapurdi lignite were determined to be ortho-lignite or low-rank lignite C rank, immature for hydrocarbon generation. The lignite derived mainly from herbaceous vegetation with a higher contribution of woods in the Giral fen compared to the Kapurdi bog. Geochemical indicators suggest the presence of freshwater conditions and a shift from freshwater to brackish water conditions within the basin.

Hydrostratigraphic decomposition of fluvio-deltaic sediments inferred from seismic geomorphology and geophysical well logs in the Pannonian Basin, Hungary

Heterogeneity and anisotropy of lithological units influences hydraulic conductivity on several scales. The purpose of this study is to develop a novel method supporting hydrostratigraphic classification by considering horizontal and vertical variations of sand content in relation with the paleo-depositional environment. The workflow was applied for the uppermost ca. 1800 m thick fluvio-deltaic deposits (Great Plain Aquifer) of late Neogene to Quaternary age basin-fill succession in the eastern part of the Pannonian basin in Hungary. Five combined 3D seismic volumes, seven master horizons, and 30 well logs were analyzed. First, RMS amplitude maps were extracted to interpret the seismic geomorphological features and depositional architectures. Their associated lithology was inferred from wireline logs (GR and SP) by calculating the shale volume and the net-to-gross sand ratio for 30 m thick intervals. These were used to calibrate the seismic facies as a proxy for the horizontal distribution of sand versus shale. This method allowed the identification of sand bodies, i.e.: deltaic lobes, complex channel belts, simple fluvial channels behaving as aquifers, and the dominantly muddy delta plain to flood plain suits as aquitard. The vertical pattern of sand distribution was also evaluated. Three major stratiform and some corridor-like minor hydrostratigraphic units were defined instead of the former regional (basin) scale aquifer unit. 1) Laterally extended interval of stacked deltaic lobes of high sand ratios and high rate of connectedness, at the bottom of the studied interval. 2) General presence of extended muddy floodplains with anastomosing river systems, characterized by 100–200 m wide channels, low sand ratio, and limited connectedness: 3) widespread meandering and/or braided river systems with high sand ratios and high connectedness in the Quaternary. Within unit 2) spatially and temporally variable appearance of major 500–3600 m wide meandering channel belts produce locally high sand-ratio corridors in the NE-SW direction. A workflow adapted from the oil-and-gas industry was successfully applied to distinguish units of varying sand content and hydraulic conductivity. This approach can be used on basin to local scale to build a spatially complex facies-based model of hydrostratigraphy. Thus, a robust heterogeneous geological model serves as a base for investigating fluid flow on the required scale.

Paleoenvironment of deposition and hydrocarbon potential of Maastritchian Shale Sediments, Southern Anambra Basin, Edo State, Nigeria

Paleoenvironment of deposition and hydrocarbon potential of Maastritchian Shale Sediments, Southern Anambra Basin, Edo State, Nigeria

Analysis of redox-sensitive trace elements to the reconstruction of the depositional paleoenvironment of the Ponta Grossa Formation in southern Brazil

In the Devonian period, several intense climatic changes occurred on Earth, such as global cooling and intense volcanic eruption events, increasing the amount of organic matter in water bodies and forming a bituminous sedimentary material containing redox-sensitive trace elements, called shale. Among the places with this type of rock formation from the Devonian period, one of them is the Ponta Grossa region, located in the Paraná Basin, southern Brazil, which lacks studies that use metals to understand the depositional environment of its formation. Therefore, this study aims to analyze the redox-sensitive trace elements together with the total organic carbon to understand the depositional environment at the time of the formation of these shales. For this, outcrop samples were collected and analyzed regarding the total organic carbon and the redox-sensitive trace elements, generating ratios between them. From the results obtained, it was observed that the environment was warmer and humid, with very low salinity, a greater tendency to anoxia, periods of greater primary productivity, and marine transgressions. Thus, it was possible to know the depositional environment of organic matter that later originated the shale deposits in this region of the planet.

Depositional environment variations and organic matter accumulation of the first member of the Qingshankou formation in the southern Songliao Basin, China

The organic-rich shale in the first member of the Qingshankou Formation, which is located in the southern Songliao Basin, is regarded as a high-quality source rock in East China. Geochemistry parameters were utilized to illustrate the formation and preservation conditions of the Qing1 Member in the Changling Sag, southern Songliao Basin. In the present study, from longitudinal continuity, the samples of organic geochemistry and elemental geochemistry were collected and systematically analyzed. This aims at determining the paleosalinity, paleoclimate, paleoredox conditions, and paleoproductivity and reconstructing its depositional paleoenvironment. According to total organic carbon (TOC) content, the Qing1 Member in the Changling Sag can be classified into two intervals, which are the lower part and the upper part, with high TOC content and low TOC content, respectively. It can be proved from the results of geochemical indicators that under warm and humid climatic conditions the relatively lower part was generated in the anoxic environment. Terrigenous input brought nutrients to the water body of the lake, made algae flourish, and had a relatively high paleoproductivity of the lake, which imposed a vigorous impact on the accumulation of organic matter. The upper part is mainly deposited under weakly oxidizing conditions, with gradually enhanced oxidation and reduced productivity. In contrast to the lower shale, the terrigenous inflow is relatively low. TOC content in the Qing1 Member has a positive relevance with the paleoredox conditions, as well as the paleosalinity conditions, indicating that good preservation is favorable for the accumulation of organic matter. A depositional model is proposed for the organic matter accumulation of this shale. The upper part was in a relatively hot and dry paleoclimate, with a low degree of organic matter enrichment, whereas the lower part had a warm and humid paleoclimate, with the input of terrestrial organic matter into the primary productivity of the lake basin and a high degree of organic matter enrichment.

Stratigraphic architecture of the Tethyan Cenomanian-Turonian succession and OAE2 in the Dokan Area, Kurdistan Region, northeast Iraq

This study provides a detailed examination of a condensed Cenomanian-Turonian (C-T) succession of two sections (Dokan Dam and Khalakan) in the Kurdistan Region, Northeastern Iraq, based on biostratigraphy (calcareous nannofossils and planktic foraminifera), carbon and oxygen isotope geochemistry, and facies analysis. The C/T boundary in this region is characterized by a hiatus noticeable due to the absence of the Globigerinelloides benthonensis and Dicarinella hagni subzones and the lack of positive carbon isotope excursion (CIE) peak b during the Oceanic Anoxic Event 2 (OAE2). Positive CIEs in our studied succession include three peaks (a, c, and d) from the four established positive global CIE peaks (a, b, c, and d). Peak a is recognized at the upper part of the Cenomanian Dokan Formation, while peak b is missing within the hiatus across the C/T boundary. Peak c is found within the lowest Turonian above the Gulneri/Dokan formational boundary, and peak d appears at the base of the middle Turonian Kometan Formation. Twelve microfacies and four submicrofacies types are identified and grouped by depositional paleoenvironments into five facies associations, ranging from peritidal to basinal settings. Three incomplete third-order depositional sequences (DSs) and their transgressive-regressive cyclic stacking pattern have been identified in the Cenomanian-Turonian succession studied herein. These DSs are associated with hiatuses, suggesting tectonic control combined with a global sea-level fall. Hiatuses and extreme condensation recorded in the latest Cenomanian-early Turonian successions on many Neo-Tethys platforms resulted from platform exposure, subsequent drowning, and a warming climate.

Facies association and Paleo depositional environment of the exposed Neogene succession in the Sitapahar Anticline of Rangamati Area, Chittagong Tripura Folded Belt (CTFB) region, Bengal Basin, Bangladesh

Neogene sedimentary rocks are exposed in the Chittagong Tripura Folded Belt (CTFB) Region in Bengal Basin. Sitapahar anticline is situated in the middle part of the CTFB Region. In this structure, eleven litho-facies have been identified in fluvial, overbank, shallow marine, deltaic, and deep marine depositional environments. Surma Group is composed of Bhuban and Boka Bil formation during Miocene time and was deposited in shallow marine, deltaic, and deep marine environments and characterized by the identification of interbedded shale facies (ShI), fine sandstone facies (SFs), lenticular laminated silty shale to shale facies (Zw), wavy bedded sandstone siltstone facies (Slsh), flaserbedded sandstone siltstone facies (Sxhs), ripple laminated sandstone facies (Sr), black Shale facies (Shb) and nodular shale facies (Zn). While, Pliocene Tipam group and Plio-Pleistocene Dupi Tila Formations were deposited in fluvial intervention and characterized by crossbedded sandstone facies (Sxbt), mudstone facies (Mf), interbedded mudstone and sandstone facies (MI).

Paleoenvironmental characterization of the Ponta Grossa formation (Devonian) at the northwest border of the Paraná Basin, Brazil

Previously uncharacterized Devonian shale samples from the Ponta Grossa Formation (Paraná Basin) were collected from three outcrops in Mato Grosso (DA01 and DA02) and Mato Grosso do Sul (PG02) along the northwest of the basin. The depositional paleoenvironment and hydrocarbon source potential were assessed using Rock-Eval pyrolysis, saturate and aromatic biomarkers, and carbon isotope data. For the DA01 shales, poor total organic carbon (TOC) contents and low hydrogen index (HI) indicate a high predominance of type III kerogen. The biomarker analysis results suggest a transitional depositional paleoenvironment with a greater contribution of continental than marine organic matter, pointing to a regressive marine event. In DA02, the TOC contents ranged from 0.24 to 4.46%, with kerogen types II and III, S2 ranging from poor to good, HI ranging from 78 to 410 mg HC/g TOC, and biomarkers indicating a marine transitional environment and transgressive marine event. Shales from outcrop PG02 had the highest TOC contents, particularly at the top, with predominantly kerogen types II and III, S2 varying from low to excellent, and HI ranging from 100 to 350 mg HC/g TOC. Two types of paleoenvironments were identified based on the biomarkers of PG02: regressive at the base and transgressive in the middle and top of the outcrop. The DA01 outcrop was located at a more proximal position to the coast, the DA02 were at more distal position, while PG02 was located in a transitional position between DA01 and DA02. Ratios of C29αββ/(αββ+ααα) and 20S/(20S + 20R) steranes, the presence of unresolved complex mixture, and a higher proportion of polar compounds to the detriment of saturated and aromatic compounds indicated that the shales were immature for the generation of hydrocarbons. This study represents the first broad organic geochemical evaluation of the northwest Ponta Grossa Formation.

Sedimentological study of Bima sandstone in Dumne and Environs, Yola Sub-basin, Upper Benue Trough, Northeastern Nigeria.

Field mapping was carried out on Bima sandstone from Dumne and its environs with the aim to understand the geology and determine the paleoenvironment of deposition. The integrated (outcrop study with granulometric and pebble morphometric) studies were used to achieve this aim. Based on outcrop studies, nine (9) lithofacies were identified from the proximal alluvial fan facies to the mid-fan braided river: massive, matrix-supported, gravel-dominated facies (Gmm); matrix-supported gravel (Gmg); clast-supported gravel stratified (Gh); gravel stratified trough cross-bedded sandstone (Gt); trough cross-bedded sandstone (St); planar cross-bedded sandstone (Sp); ripple cross-laminated sandstone (Sr); massive sandstone (Sm) and silt and mud (Fsm). The outcrop sampled materials were subjected to granulometric and morphometric analyses. The result of the granulometric shows that the Bima sandstones of the study area have an average graphic mean (Mz) of 0.82 (coarse grains), an inclusive graphic standard deviation (δi) of 1.33 (poorly sorted), a graphic kurtosis (K) of 1.11 (leptokurtic), and an inclusive graphic skewness (Ski) of 0.17 (positive skewed). The result of the pebble morphometrics shows that the grains have an average elongation ratio (ER) of 0.80, coefficient of flatness (FI) of 60.6%, oblate–prolate index (OPI) of 0.35, maximum projection sphericity index (MPSI) of 0.77 and roundness (ρ) of 0.43. The roundness versus elongation ratio plot indicates a fluvial to transitional environment. Several bivariate and ternary plots that include the granulometric and pebble morphometric data suggest a river environment for the Bima sandstone in the studied area.

Sabkha and salina dolomite preserves the biogeochemical conditions of its depositional paleoenvironment

Precipitation of Mg-carbonate minerals, ranging from Mg-calcite to Ca-rich dolomite, is typically associated with microbial activity, and commonly takes place in saline environments along with other evaporitic minerals. The geochemistry of such evaporitic minerals can contribute to the interpretation of the environmental conditions and those (bio)geochemical processes occurring at the time of deposition. We use a multiproxy approach that combines stratigraphic, petrographic, mineralogical and geochemical data, such as Fe and Sr isotope values, to describe the environmental and biogeochemical conditions under which dolomite precipitated in a sabkha/salina environment using the Miocene Vilobí Gypsum Unit (Penedès Basin, NE Spain) as a case study.The lithofacies of Vilobí Gypsum Unit are interpreted to indicate sedimentary environments alternating between sabkha and coastal salina. The δ13C and δ18O values of Vilobí dolomites (from −6.3 to −0.4‰ and from 0.2 to 2.3‰, respectively) compare well with microbially influenced dolomite from modern sabkha and coastal environments. The dolomite 87Sr/86Sr ratios (0.70816 to 0.70874) show significant variations and indicate that the coastal sabkha/salina was strongly influenced by seawater. Microbial activity is evidenced by petrography (microbially induced sedimentary structures interpreted as cyanobacterial mats, and spheroidal dolomite with empty bacterial voids), and geochemical data (elemental concentrations enrichment in Fe2+ and Mn2+, up to 1388 and 605 ppm, respectively; supported by δ56Fe values of between −0.16 and 0.44‰). Collectively, these data suggest that the studied dolomite is a primary precipitate that records the geochemistry of the saline depositional environment influenced by microbial activity. The Vilobí Gypsum Unit is here dated to 16.05 Ma and 16.30 Ma (Burdigalian) by correlating the 87Sr/86Sr values of dolomite and shell of the oysters (shells between 0.70870 and 0.70872) with the global Sr curve. Moreover, dolomite records marine (porewater) values (C-O-Fe-Sr) that can be used to infer the depositional environment and geochemical conditions at the time of deposition. Results of non-traditional isotopes and trace metal concentrations support using dolomite formed in evaporitic environments as an archive of paleoenvironmental conditions.

Paleoenvironment and diagenetic aspects of the limestone (Gboko Formation) around Igumale, Central Benue Trough, Nigeria: Evidence from petrography

Limestone core samples from Igumale-1 and Igumale-2 wells, Central Benue Trough, Nigeria were petrographically studied in detail to determine their paleo-depositional environment and diagenetic history. Petrographic examination of the limestone shows that the limestone comprised monocrystalline quartz, ferroan calcite (3-15 %), peloids (3-20 %), ooids (7-10 %), micrite (33-48 %) and sparites (10-50 %). Fossil fragments consist of foraminifera and gastropods (17-57%), and suggest subtidal to shallow marine environment. Four sedimentary facies (oobiosparite, biosparite, biopelmicrite, and biomicrite) were recorded and characterised by moldic and vuggy porosity. This study has provided a petrographic synopsis of the limestone for further consideration and may impact both the exploration and development of the limestone deposit.

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.

Palynozonation of Akukwa-2 Well, Anambra Basin: Implications for Age and Paleoenvironment

Palynological examination of twenty (20) ditch shale samples from sedimentary succession penetrated by Akukwa-2 Well at depth interval of 3,296 - 3,850 m was undertaken to investigate the occurrence of Palynomorph in the sediments of Anambra Basin with a view to determining the age of the sediments and reconstruct the depositional paleoenvironment. Standard Palynological procedure was followed and the samples were processed using the conventional acid maceration, alkali treatment and staining methods. The oxidized residues were sieved with 5micron nylon sieves using digital Sonifier 450 machine and mounted on glass slides with Norland adhesive gel. Laborlux 12 (Ernst Leitz) transmitting light microscope was used for identification and counting or the palynomorphs. The palynological analysis was done using the population and species of pollen grains, spores and dinoflagelate. The analyzed palynological slides yielded forty nine (49) palynomorphs species identify in the ditch samples, forty (40) were Miospores (pollen and spores) and nine were Dynocysts. However, some diagnostic forms that are stratigraphically useful are present and are used for the palynozonation. Delineation of the well section produced three main palynological zones based on stratigraphic distribution. Zone 1 characterized by the quantitative base appearance of Triorites Africaensis Assemblage indicating Marginal Marine paleoenvironment and of Turonian age; Zone 2, marked by Droseridites Senonicus assemblage also of Marginal Marine but of Coniacian age, Zone 3, marked by Milfordia spp. Acme Zone, purely Marine and of Campano- Maastrichtian age. The study further reveled that Palynomorph recovery in Akukwa-2 well was not that impressive both in terms of abundance and preservation. In most cases, the macerals present seems to indicate high geothermal effect suggesting that some of the kerogen might have been overcooked to imply inability  to generate hydrocarbon or such generated hydrocarbon have turned into carbon.

Depositional controls in an ancient, closed lake system: A high-resolution and multi-scalar case study from the Yacoraite Formation (Salta Basin, Argentina)

The Yacoraite Formation (Cretaceous–Paleogene) is a mixed carbonate–siliciclastic microbialite-bearing lacustrine succession deposited in the intra-continental Salta rift basin (Argentina). In the northern sub-basin of Tres Cruces, the spectacular exposures and high lateral continuity of the outcrops provide a suitable setting to investigate and reconstruct the paleoenvironments of deposition and the resulting facies architecture of this closed, saline lake system. The Yacoraite Formation deposited in a predominantly shallow-water ramp-like lake system, characterized by eighteen main facies, classified within four main facies associations, each defining a particular depositional environment along the lake profile. From proximal (shallow) to distal: i) Palustrine Facies Association; ii) Littoral Facies Association; iii) Sub-littoral Facies Association; and lastly iv) Profundal Facies Association. In the Yacoraite paleo-lake extensive fringing mud-flat environments surrounded the lake margins, while the littoral marginal areas hosted extensive wave-dominated oolitic–bioclastic grainstones forming shoals and bars parallel to the lake margins and provided a sheltered environment in the back-shoals that allowed prolific microbialite development. In proximity to river inputs, shallow-water deltas and shoreline sandstones accumulated in the littoral margins. In the sub-littoral zone, mud-supported carbonates such as ostracod wackestones and mudstones deposited below the fair-weather wave base, and in the profundal settings deep-water organic-rich shales deposited. The Yacoraite paleo-lake evolved from a relatively stable and perennial lake system (lake stage 1) into a rapidly fluctuating ephemeral lake system (stage 2), as a result of progressively changing environmental and climatic conditions toward more arid settings. Climate control is also critical in the facies architecture, with the development of short-term cyclicity, represented by meter-scale transgressive–regressive (T–R) cycles being the result of lacustrine expansion–contraction cycles. The regressive hemicycles represent shallowing-upward succession of sub-littoral and littoral carbonate-dominated facies, whereas the transgressive hemicycles are mostly represented by sub-littoral and profundal mudstones and shales. Stacking of short-term cycles resulted in the development of medium- and long-term cyclicity at the scale of tens of meters, which represent the long-term evolution of the lake system. Lastly, a comparison with two well-documented closed lake systems highlights the role of allogenic factors acting at a regional scale (e.g., climate and tectonics) in controlling the lake-basin type, facies association and the resulting architecture. Factors at the lake-basin scale (e.g., lake margin profile, bathymetry) modulate the expression of facies and cyclicity. Ultimately, the characterization of the kilometer-scale outcrops of the Yacoraite paleo-lake provides a reference analog study to support the characterization of similar depositional systems and reservoirs of other ancient, closed lake systems, including the Pre-Salt Carbonates of the South Atlantic.

Genesis and Geological Significance of Siderite in the First Member of the Nantun Formation of Dongming Sag, Hailar Basin

Multiple siderite beds developed in the first member of the Lower Cretaceous Nantun Formation (K1n1) in the basin. The results show that the siderites in K1n1 of the study area are mostly stratiform or massive, with three micromorphological features (dense micronized crystals, bands, and paragenesis with quartz and calcite). The siderite beds are mainly composed of siderite, clay, quartz, calcite, and feldspar. Under the microscope, charcoal, algal fossils, granular pyrite crystals, vein-like siliceous bands, etc., were observed. The oxides in the siderite beds include Fe2O3, SiO2, Al2O3, etc. The trace elements are typically characterized by high Mn and Be contents; low Sr/Ba, Th/U, and Al/Ti ratios; and high V/Cr ratios. These indicate weakly reducing, freshwater depositional paleoenvironments. The δ13Cv-PDB and δ18Ov-PDB values of siderite are −0.20–1.11‰ (mean: 0.62‰) and −18.22‰ to −10.14‰ (mean: −14.23‰), respectively, which shows that the carbon in siderite came mainly from carbonate dissolution. The Fe-bearing rocks in the source area migrated to the basin after undergoing physical and chemical weathering, and when the resultant Fe2+ concentration reached saturation, Fe2+ combined with CO32− in the water bodies to form authigenic siderite.

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.