Depositional Paleoenvironment Research Articles

Organic-Inorganic Geochemical Characteristics of the Upper Permian Pusige Formation in a High-Saline Lake Basin, Tarim Basin: Implications for Provenance, Paleoenvironments, and Organic Matter Enrichment

The third member (M3) of the Upper Permian Pusige Formation is a prominent organic-rich lacustrine mudstone sequence within the Yecheng-Hetian Sag, Tarim Basin, hosting major petroleum resources. However, its depositional history and organic matter (OM) enrichment mechanism have received little attention. Therefore, various organic and inorganic geochemical analyses were performed on thirty-four core samples from the Well DW1, to elucidate their depositional paleoenvironments, provenance, and tectonic setting, as well as the controlling factors of OM enrichment. Results showed that the M3 mudstones are classified as poor- to fair-quality hydrocarbon source rocks with mature type II-III kerogen, considering their low organic geochemical parameters. Paleosalinity indexes (e.g., Beq, Sr/Ba, and B/Ga) indicated the typical high-saline lacustrine water body, in which redox state was the oxic-dysoxic as suggested by multiple indicators. Many paleoclimate and weathering proxies suggest a dominant semiarid condition and low weathering degree in the Yecheng-Hetian Sag, which led to that weathered felsic rocks from the West Kunlun Orogen to the southwest of basin were quickly transported into the lake basin. Detrital materials carrying nutrient elements finally promoted the development of relatively high paleoproductivity indicated by fairly high P/Ti and Ba/Al ratios. The negative relationship between P/Ti and total organic carbon (TOC) indicates that paleoproductivity was not the main controlling factor. The correlations among TOC and P/Ti and other multiple proxies suggest that the OM enrichment can be interpreted as both the “preservation model” and “dilution model.” Although the water body was relatively oxygen-riched, high sedimentation rate could largely shorten the exposure time of OM with oxygen, thus decreased the decomposition of OM. In particular, the high-saline, stratified lake water may also restrain the degradation of OM. Furthermore, detrital dilution exerted a potential effect on TOC abundances. On the basis of the above results, a developing model was established to decipher the formation mechanism of OM in these M3 mudstones.

Multi-disciplinary approach to sedimentary facies analysis of Messinian Salinity Crisis tectono-sequences (South-Mansoura Area, Nile Delta): Incised-valley fill geological model reconstruction and petroleum geology–reservoir element delineation

The quality of a hydrocarbon reservoir is strongly controlled by the depositional and diagenetic facies nature of the given rock. Therefore, building a precise geological/depositional model of the reservoir rock is critical to reducing risks while exploring for petroleum. Ultimate reservoir characterization for constructing an adequate geological model is still challenging due to the in general insufficiency of data; particularly integrating them through combined approaches. In this paper, we integrated seismic geomorphology, sequence stratigraphy, and sedimentology, to efficiently characterize the Upper Miocene, incised-valley fill, Abu Madi Formation at South Mansoura Area (Onshore Nile Delta, Egypt). Abu Madi Formation, in the study area, is a SW-NE trending reservoir fairway consisting of alternative sequences of shales and channel-fill sandstones, of the Messinian age, that were built as a result of the River Nile sediment supply upon the Messinian Salinity Crisis. Hence, it comprises a range of continental to coastal depositional facies. We utilized dataset including seismic data, complete set of well logs, and core samples. We performed seismic attribute analysis, particularly spectral decomposition, over stratal slices to outline the geometry of the incised-valley fill. Moreover, well log analysis was done to distinguish different facies and lithofacies associations, and define their paleo-depositional environments; a preceding further look was given to the well log-based sequence stratigraphic setting as well. Furthermore, mineralogical composition and post-depositional diagenesis were identified performing petrographical analysis of some thin sections adopted from the core samples. A linkage between such approaches, performed in this study, and their impact on reservoir quality determination was aimed to shed light on a successful integrated reservoir characterization, capable of giving a robust insight into the depositional facies, and the associated petroleum potential. The results show that MSC Abu Madi Formation constitutes a third-order depositional sequence of fluvial to estuarine units, infilling the Eonile-canyon, with five sedimentary facies associations; overbank mud, fluvial channel complex, estuarine mud, tidal channels, and tidal bars; trending SW-NE with a Y-shape channel geometry. The fluvial facies association (zone 1 and 3) enriches coarse-grained sandstones, deposited in subaerial setting, with significantly higher reservoir quality, acting as the best reservoir facies of the area. Although the dissolution of detrital components, mainly feldspars, enhanced a secondary porosity, improving reservoir quality of MSC Abu Madi sediments, continental fluvial channel facies represent the main fluid flow conduits, where marine influence is limited.

Cambrian (Series 2 to Miaolingian) platform facies from central Sonora, Mexico and the regional correlation

This study provides new insights about depositional paleoenvironments through siliciclastic microfacies, carbonate microfacies, and biofacies analysis from sedimentary formations of the lower and middle Cambrian (Stage 4–Wuliuan), exposed in central Sonora, northern Mexico. Results of the petrographic analysis of 48 samples revealed the following lithologies: quartzarenite, oncolytic rudstone, grainstone-packstone, wackestone, mudstone, and to a lesser extent sandy limestone. Two siliciclastic microfacies were identified: (A) quartzarenite with cross-bedded and horizontal stratification deposited in an intertidal and supratidal environment; and (B) massive quartzarenite with Skolithos ichnofacies deposited in subtidal and intertidal environments. Four carbonate microfacies were identified: microfacies 1 is a sandy limestone with trilobite fragments; microfacies 2 is a grainstone with intraclasts, salterellids, hyolithids, trilobites, and echinoderms plates; microfacies 3 is an oncolytic rudstone consisting of microbes and abundant echinoderms plates; and microfacies 4 is a packstone-grainstone with abundant ooids, trilobite fragments, and echinoderm plates. Two biofacies were identified: Agnostid-polymeroid biofacies with predominance of the trilobites Pentagnostus, Bathyuriscus, Oryctocephalites, and Elrathina; and Pagetia biofacies with abundant trilobites of the genera Pentagnostus, Pagetia and Elrathina. It is concluded by the sedimentation model that changes in sea level is the most important parameter in determining the siliciclastic microfacies, carbonate microfacies and biofacies; as well as the depositional environments that vary from the coastline (subtidal to supratidal) to shallow-water open circulation marine platform with low and high energy waters. The Cambrian deposits of northern Mexico are correlated with the deposits of California and Nevada (USA), as well as to the Precordillera (Argentina), where the species in common show a strong affinity.

Lowermost Cretaceous biostratigraphy and paleoenvironmental features of the central-western Getic Carbonate Platform (Pui-Bănița zone, Southern Carpathians, Romania): A holistic approach

We carried out combined micropaleontological, microfacies and multivariate statistical analyses of benthic foraminiferal assemblages on a previously undocumented lowermost Cretaceous carbonate succession from the western part of the Getic Carbonate Platform (Pui-Bănița zone, Southern Carpathians). The microfacies analysis and interpretations suggest that these limestones were developed under peritidal settings within different zones of a shallow-water inner platform paleoenvironment (bioclastic grainy bars, subtidal–intertidal flats and intertidal–supratidal ponds), where benthic foraminifera, microbial microstructures and calcareous algae were the main carbonate producers. The well-preserved micropaleontological content (mostly benthic foraminiferal assemblages) is important for lowermost Cretaceous Neotethyan biostratigraphy and allows the recognition of the Valanginian stage within this part of the Getic Carbonate Platform. The multivariate data processing (diversity indices, cluster analyses and principal components analyses) performed on the benthic foraminifera assemblages provide valuable information about their compositional patterns, diversity trends and spatial distribution with respect to the main facies zones. The results obtained through these analyses are extremely useful for the recognition of common/different paleoecological features and for a more precise interpretation of depositional paleoenvironments.

A combined geochemical, Nd, and stable Ca isotopic investigation of provenance, paleo-depositional setting and sub-basin connectivity of the Proterozoic Vindhyan Basin, India

Geochemical and isotopic investigation of Proterozoic clastic and chemical sedimentary rocks provide insights into surface conditions of the early Earth. The mid-Proterozoic Vindhyan Basin of central India, comprising ~5 km thick, mostly undeformed and unmetamorphosed clastic and chemical sediments, is one such archive which is exposed in two sub-basins, the Son Valley Vindhyan sub-basin in the east (SVV) and Chambal Valley Vindhyan sub-basin in the west (CVV). Geochemical and Nd isotopic compositions of siliciclastic sedimentary rocks from the CVV sub-basin are reported in order to understand the provenance of the Vindhyan sediments in this sub-basin and the results are compared with the provenance of the sediments exposed in the SVV sub-basin to evaluate whether the two sub-basins had shared a common provenance. Elemental ratios, such as Th/Co, La/Sc suggest that the clastic sedimentary rocks from the CVV have dominantly upper crustal silicic/felsic provenances and are broadly similar to those of equivalent stratigraphic horizons from the SVV. Initial Nd isotopic composition (εNd(t)) of the siliciclastic rocks from the Lower Vindhyans of the CVV, calculated at their respective times of deposition, suggest sediment derivation from the Banded Gneissic Complex, Berach Granitoids, 1.82 Ga continental arc granitoids of the Aravalli Range and the Hindoli Volcanics. In contrast, the εNd(t) values of the Upper Vindhyan siliciclastics suggest that the Delhi Belt granitoids were the primary source of detritus during the deposition of these sediments. Paleo-depositional environment and connectivity between the SVV and CVV sub-basins are evaluated using geochemical, radiogenic Nd, and stable Ca isotopic compositions of carbonates from both sub-basins. Non-redox sensitive REY anomalies and Y/Ho ratios of carbonates from the two sub-basins suggest deposition in an epeiric sea with limited connection to the open ocean. Neodymium isotopic compositions of the carbonate units suggest that the two sub-basins were connected during the deposition of both Lower and Upper Vindhyan sediments. Carbonates from both sub-basins record a significant shift towards more radiogenic Nd isotopic compositions around 1.6 Ga, possibly as a result of an Andean-type arc magmatism in near vicinity of the Son Valley sub-basin. The δ44/40Ca compositions of the SVV carbonates are typically lower than the CVV carbonates. This difference is likely due to greater freshwater input in the SVV sub-basin which is consistent with the existence of paleo-gradients in the Vindhyan Basin.

Multiple Controls on the Accumulation of Organic-Rich Sediments in the Besa River Formation of Liard Basin, British Columbia, Canada

The Late Devonian Besa River Formation is an organic-rich shale sequence in Liard Basin, northeastern British Columbia, Canada, with significant natural gas reserves. High-resolution elemental geochemistry of three long continuous cored intervals of the Besa River Formation was used to better understand the paleodepositional environment of organic-rich intervals in this thick marine shale. The studied core intervals were divided into five chemostratigraphic units based on organic and inorganic geochemical proxies. The highest total organic carbon (TOC) content (up to 13 wt.%) was identified in the upper part of the Patry member (Unit III) within the Liard Basin. During the deposition of Unit III, low clastic influx and euxinic bottom conditions mostly contributed to the high accumulation of organic carbon. Moreover, a high productivity and organic influx may have increased organic-rich basinal sediments, which further depleted the seawater column oxygen content in the presence of a large amount of organic matter. This took place within the oxygen-deficient bottom water from the Patry–Exshaw stratigraphic units. This high TOC interval was most likely deposited through abundant biogenic silica production by radiolarians, thereby utilizing the supply of nutrients from the upwelling. Sea level change was also an important factor that controlled organic matter accumulation in the Besa River Formation. The transgression in sea level changed the residence time of the organic matter in oxic zones within the water column, which limited its supply in deeper water; this decreased the TOC content in Unit IV. Before the deposition, silica production collapsed and was replaced by terrestrial sedimentation of clay minerals in the upper part of the Exshaw member, which caused organic matter dilutions in Unit V (under 5 wt.%). These results provide new insights into the effects of relative sea level changes on redox conditions, productivity, and detrital flux, which are related to organic matter enrichment patterns and their geographic variations. Unit III is characterized by an organic-rich interval as well as an abundance of biogenic silica that is closely related to fracturing. Thus, Unit III is expected to have the highest shale gas potential in the Devonian Besa River Formation. The high-resolution geochemical data integrated with well log and/or seismic data can be used to determine the distribution of the perspective interval for shale gas production in the Liard Basin.

Biostratigraphic and paleoecological significance of benthic foraminiferal morphogroups in the uppermost Turonian–upper Campanian of Gebel Duwi Range, Red Sea, Egypt

Identifications of the benthic foraminiferal assemblages (Gavelinella, Stensioeina, and Brotzenella) from one stratigraphic section measured from the platform of the uppermost Turonian-upper Campanian successions of Gebel Duwi Range, Quseir Region, allowed the recognition of seven biozones and two subzones. The Duwi Basin through the uppermost Turonian to upper Campanian increased in depth, the benthic components indicating a time of the Egyptian stable continental shelf. The gradual increase in depth had occurred with no dramatic structure or tectonic changes. The benthic foraminiferal content reveals significant correlates of biostratigraphical and paleoecological changes of the same interval through Eastern, Western, and Central European provinces, and Tethyan epicontinental facies. The benthic foraminiferal contents are of diversified epibenthic and endobenthic taxa suggesting epicontinental inner to outer shelf depositional paleoenvironments and mesotrophic habitat.

Palynostratigraphic and palynofacies analysis of x and y wells, offshore Niger Delta, Nigeria

A detailed palynostratigraphic and palynofacies study was carried out on two shallow offshore wells, codenamed Well X and Well Y, from the Niger Delta Basin, Nigeria. The study is aimed at establishing the age and palaeoenvironment of the sedimentary sequences penetrated by the wells. Standard palynological/palynofacies laboratory procedures were used to free the palynomorphs and palynomacerals from the embedding matrices.Prepared palynological slides were studied using the Leitz Ortholux II transmitted light microscope. The palynofloral assemblages recorded from the two well sequences include well-preserved palynomorphs such as Zonocostites ramonae, Monoporites annulatus, Retitricolporites irregularis, Cyperaceaepollis sp., Echitricolporites spinosus and Nymphaeapollis clarus among others. The recognition of diagnostic pollen of Cyperaceaepollis sp. and Nymphaeapollis clarus as well as the associated palynomorph assemblage aided the delineation of the well sequences into the P830, P840-P850 and P860 subzones of the P800 palynological zone in the late Miocene. The relative abundance of particulate organic matter (Amorphous Organic Matter (AOM), Phytoclasts and Palynomorphs) revealed the delineation of three palynofacies assemblages, PF-A, PF-B and PF-C, suggesting three distinct paleoenvironments of deposition. These paleoenvironments range from distal shelf through marginal to proximal with varying conditions from oxic, suboxic to anoxic, thus suggesting marginal marine to coastal deltaic environments of deposition for the sedimentary sequences penetrated by the two wells.
 Keywords: Late Miocene, Niger Delta, Paleoenvironment, Palynostratigraphy, Palynofacies.

Changes in depositional paleoenvironment of black shales in the Permian Irati Formation (Paraná Basin, Brazil): Geochemical evidence and aromatic biomarkers

Physicochemical changes that occurred during deposition of Irati black shales were investigated via outcrop samples collected in the northeastern and central-eastern Paraná Basin. Various traditional geochemical indicators were used, in addition to newly detected biomarkers: C32 lanostane, simonellite, sempervirane, and C32–C35 benzohopanes. Bulk geochemistry parameters, mineralogy, and conventional saturated biomarkers, such as tetracyclic polyprenoids (TPPs), reflect restricted and hypersaline marine deposition with freshwater input and increased terrigenous contribution toward the top of the studied outcrops. This led to stratification of the water column, establishing euxinic conditions in the photic zone and reducing conditions at the bottom of the water column, which enhanced preservation of organic matter. Principal component and hierarchical cluster analyses were performed to group the samples according to their similar geochemical characteristics: prevailing marine environment (basal outcrop samples) and higher freshwater inflow with greater terrigenous contribution (top outcrop samples). These geochemical observations were corroborated by aromatic biomarkers such as triaromatic steroids, trimethylnaphthalenes, simonellite, and sempervirane. The aryl isoprenoid ratio (AIR) indicates persistence of photic zone euxinia (PZE) during Irati black shale deposition. The presence of C32 lanostane and C32–C35 benzohopanes is directly related to the hypersaline depositional environment of the analyzed samples. Finally, the C32-33/C32-35 benzohopane ratio is proposed as a thermal maturity proxy because variations in the ratio can be linked to the presence of a diabase sill and temperature catalyst minerals near one of the outcrops.

Petrographic and diagenetic study of siliciclastic Jurassic sediments from the northeastern margin of Africa: Implication for reservoir quality

Northeastern Egypt is one of the most important exposures in the Middle East and North Africa to study Jurassic facies. The present study analyzes 130 thin sections from a subsurface well (Well X) in the northern Gulf of Suez, and from two surface sections at Gebel Maghara (north Sinai) and Khashm Elgalala (North Eastern Desert), Egypt for petrographic and lithofacies analyses. In the present study, detailed petrographic analysis is used as a tool to better understand the diagenetic history of the Early to Late Jurassic siliciclastic sediments; the identified diagenetic elements are inferred concerning the reservoir quality. For this, sidewall cores, scanning electron microscopy and thin sections are used to detail detrital and authigenic mineralogy; these are then used to infer the depositional framework, factors controlling reservoir characteristics, and the operating diagenetic processes. The inferred depositional paleoenvironment is a prograding and retrograding linear siliciclastic shoreline within a shallow marine carbonate platform with coal swamps and occasional cross-cutting rivers. Diagenesis, petrographical characteristics and depositional conditions are the main factors controlling continental and marine reservoir architecture. The diagenetic processes affecting siliciclastic sedimentation are near the surface, shallow to intermediate burial, and deep burial cycles with different pore fluid filling at each stage. The siliciclastic sediments have been categorized into seven depositional lithofacies - calcareous claystone (S1a), carbonaceous claystone (S1b), siltstone (S2), planar and trough cross-bedded sandstone (S3), coarse, well rounded, large scale trough cross-bedded sandstone (S4), ooid sandstone (S5) and coal (C). Grain compaction, various phases of syntaxial quartz overgrowth, carbonate cementation and replacement, creation of dissolution porosity, and clay authigenesis are the most important diagenetic processes that have affected the siliciclastic continental and marine sediments. Many different diagenetic events, both destructive and constructive, have modified porosity. The destructive events include compaction and cementation (of silica, kaolinite, ferroan dolomite, Illite, and anhydride). The constructive events include silica cementation, feldspar dissolution, and ferroan dolomite dissolution. Additionally, present data also suggest that the continental sandstones have excellent reservoir quality whereas the marine sandstones have good to very good reservoir potential. Despite the excellent reservoir quality of the continental sandstone lithofacies, the effective stratigraphic seal is leaking. The interbedded calcareous and carbonaceous claystone lithofacies may provide an excellent stratigraphic seal for these siliciclastic reservoirs. Based on these analyses, a model is proposed that can be used as a template for subsurface Jurassic reservoir characterization and reservoir discrimination.

Changing the paleo-depositional environment in the last 2300 years: a study through sedimentology and geochemistry of a sediment core, western Bay of Bengal

The sediment core collected at a water depth of 260.2 m off Mahanadi, western Bay of Bengal was analyzed for grain size fractions, clay minerals, and major and trace elements to understand changing depositional environments, source and processes with time, from ~ 2300 cal yr before present (BP) to the year of the collection (2014). Age was estimated using accelerator mass spectrometer (AMS) 14C dates of two planktonic foraminifera samples. The core was divided into six distinct zones based on the significant changes in the sediment components and clay minerals. The presence of higher than average clay and smectite and lower silt and illite, in Zone II (2000 years BP–1070 cal yr BP) and Zone V (300–130 cal yr BP) revealed their deposition in low energy conditions, while, Zone III (1070 to 530 cal yr BP) and Zone VI (132 cal yr BP to present) exhibited higher silt and illite and lower clay and smectite than average suggesting their deposition in a high energy environment. Similar depositional environments observed between Zones II and V and also between Zones III and VI indicated the reoccurrence of similar hydrodynamic conditions with time. Zone II represents the Roman Warm Period and part of the early Medieval Warm Period, while, zone V corresponds to the retreating phase of the Little Ice Age. High Al in zone III and fluctuating trend in zone VI indicates higher Al release along with silt and illite from physical weathering in a cold and arid climate, while, smectite showed a positive correlation with Ti indicating the source of Ti and smectite as basic igneous rocks released during the enhanced southwest monsoon and warm climate. Mn and Zn distribution revealed fluctuations in the Oxygen Minimum Zone (OMZ) intensity to changing hydrodynamic conditions and depositional environments. The present study revealed changing depositional environments in the western Bay of Bengal with time with respect to changing material influx, reflecting fluctuating monsoons and climate.

SEDIMENTARY ARCHITECTURAL ELEMENTS AND SANDY BRAIDED FLUVIAL SUCCESSIONS IN AJALI SANDSTONE RIDGES, WESTERN AFIKPO BASIN, UTURU, NIGERIA

Lithofacies, bounding surfaces and sedimentary architectural elements exposed in two ridges at Uturu being quarried for construction sands were analysed to determine the paleoenvironment of deposition and the factors that control the deposition of sand units. Mainly outcropped is Ajali Formation overlying locally exposed Mamu Formation in Western Afikpo basin. Lithofacies identified include: Trough cross bedded medium- to coarse-grained (St), Planar cross bedded fine- to coarse-grained sandstone (Sp), Small scale planar cross bedded sandstone (SSp), Lenticular mudstone (Fm), Heterolithic sandstone/mudstone (Fsm), Horizontal stratified sandstone (Sh), Cross ripple laminated sandstone (Sr), Reddish muddy sand (Fl), Siltstone (SSm) and Shale (Fsh). The associations of lithofacies and bounding surfaces gave four fluvial and one marine architectural element. The fluvial elements which mainly characterized the Ajali Formation include: Channel-fill (CH), Macroforms Accretion (MA). Flood-Plain Fines (FF) and Channel Abandonment Fines (CAF). Offshore-shoreface fines (OSF) element defined marine Mamu Formation. The profiles of the ridges show dominance of MA followed by CH while FF is limited in occurrence and in some zones pinch-out to lenticular inter-bar mudstone. The MA is characterized by planar cross beddings, reactivation surfaces, internal grading, steep dipping ferruginized accretion surfaces and abrupt flat top which indicate mid-channel bars deposition in typical sandy braided fluvial depositional system. Generally, there is vertical aggradation/ amalgamation of channel deposits and dominance of sheet alluvial architecture. Low rate of channel avulsion, moderate rate of lateral migration and aggradation, variable discharge rate and high rate of sediment supply and subsidence were considered as factors that controlled the deposition and preservations of sand units. This study provided an understanding of mesoscopic heterogeneities and compartmentalization style inherent in hydrocarbon bearing sandy braided reservoirs which can be used as analog model for its development.

Grain Size and Heavy Minerals Analysis of Maastrichtian Sandstone, Anambra Basin, Nigeria: Implication for Aquifer Properties

The Maastrichtian sediments of Anambra Basin, south western Nigeria was studied for grain characteristics and maturity of the sediments, so that by combining sedimentological characteristics, maturity and its paleoenvironment of deposition of the Ajali sandstone, in order to ascertain the aquifer properties of the groundwater within the study area and by extension the availability of drinking water for both the urban and rural settlers. Grain size analysis (51 samples) and heavy mineral analysis (15 samples each) were analyzed. The textural parameters show that the Sandstone are medium sand, poorly to moderately sorted, coarse to strongly coarse skewed with mesokurtic to leptokurtic grains. The heavy minerals present in lithologic sand and sandstone units of Ajali Formation are mainly zircon, rutile, tourmaline, apatite, staurolite, and opaque minerals (Goethite, hematite, ilmenite), these heavy minerals found in the sandstone are associated with igneous and metamorphic source which indicates that these maastritchian sediments are from basement complex rock The ZTR index of 67.96% and the values of the kurtosis indicates a sub-mature sandstone to mature sandstone, which gives an excellent aquifer properties. Also, the paleoenvironment of deposition of the Maastrichtian sediments is Fluvial. The sand and sandstone bodies deposited in a fluvial system have sheet-like geometries and due to the differences in the extent of aquifer compartmentalization, Sandstone deposited in fluvial environment normally has better hydraulic conductivity which again reveals a good aquifer property. The sedimentological characteristics for the Sandstone exposed at Fugar and environs, is a coarsening upward sequence (increasing grain sizes) with a progressive increase in sorting. since hydraulic conductivity increases with increasing grain sizes and sorting, hence, excellent aquifer characteristics for the groundwater within the study area. The foregoing revealed that the aquifer geometry and properties of the study areas is favourable for the availability of drinking water for both the Urban and rural dwellers within these communities.
 Keywords: Aquifer geometry, groundwater, hydraulic conductivity, Anambra Basin

Use of rare earth elements and internal transition elements for characterization of the depositional paleo environment in samples of the experience of the Pimenteiras Formation, Parnaíba Basin, Brazil

The quality of the organic matter together with the conditions of the depositional paleoenvironment at the top of the Pimenteiras Formation (Devonian), were geochemically characterized using 11 (eleven) samples of shales collected from meter to meter in outcrop located on the western edge of the Parnaíba Basin, the northeast region of Brazil. The inorganic geochemical characterization was carried out through the analysis of rare earth elements and uranium, and transition elements (Ni, V, Co, and Mo) by ICP OES for Ni and V. For the rest of the elements, the determinations were made by ICP MS. The transition elements allowed to evaluate the origin of the organic matter using the diagram of the concentration ratio between the Ni/Co elements versus the concentration ratio for the V/Ni elements and the depositional paleoenvironment using the V concentration versus Ni concentration diagram. The uranium and molybdenum relationships established the presence or not of sulfides (euxinic or non-euxinic environment). The first group consists of samples from the lower third (base) of the outcrop, which has mixed organic matter, predominantly terrestrial, with the contribution of fresh/brackish water organisms, deposited under paleoenvironmental conditions from oxic to suboxide. The second group consists of samples collected in the upper third (top) of the outcrop containing mixed organic matter with a predominance of marine organic matter and little terrestrial contribution, deposited under anoxic paleoenvironmental conditions. The sum of the concentrations of rare earth elements suggested the existence of two sets of samples containing organic matter from different sources. The organic geochemical characterization was performed using analytical techniques to evaluate the percentage of total organic carbon, total sulfur, Rock-Eval pyrolysis data, and for diagnostic reasons of saturated biomarkers. The shales of the first group poor in total organic carbon (TOC), have the sum of the concentrations of rare earth elements lower than the second group, which are rich in organic matter. According to analyzes of the saturated biomarkers, about the depositional paleoenvironment of the Pimenteiras Formation in the area of that outcrop, the organically poorest shales of the first group were deposited in a transitional depositional paleoenvironment, with a large supply of continental organic matter, while the most enriched in organic matter. organic matter represents a marine flood surface, where the organic material is predominantly marine.

Subseafloor Archaea reflect 139 kyrs of paleodepositional changes in the northern Red Sea.

The vertical distribution of subseafloor archaeal communities is thought to be primarily controlled by in situ conditions in sediments such as the availability of electron acceptors and donors, although sharp community shifts have also been observed at lithological boundaries suggesting that at least a subset of vertically stratified Archaea form a long-term genetic record of coinciding environmental conditions that occurred at the time of sediment deposition. To substantiate this possibility, we performed a highly resolved 16S rRNA gene survey of vertically stratified archaeal communities paired with paleo-oceanographic proxies in a sedimentary record from the northern Red Sea spanning the last glacial-interglacial cycle (i.e., marine isotope stages 1-6; MIS1-6). Our results show a strong significant correlation between subseafloor archaeal communities and drastic paleodepositional changes associated with glacial low vs. interglacial high stands (ANOSIM; R=.73; p= .001) and only a moderately strong correlation with lithological changes. Bathyarchaeota, Lokiarchaeota, MBGA, and DHVEG-1 were the most abundant identified archaeal groups. Whether they represented ancient cell lines from the time of deposition or migrated to the specific sedimentary horizons after deposition remains speculative. However, we show that the majority of sedimentary archaeal tetraether membrane lipids were of allochthonous origin and not produced in situ. Slow post-burial growth under energy-limited conditions would explain why the downcore distribution of these dominant archaeal groups still indirectly reflect changes in the paleodepositional environment that prevailed during the analyzed marine isotope stages. In addition, archaea seeded from the overlying water column such as Thaumarchaeota and group II and III Euryarchaeota, which were likely not have been able to subsist after burial, were identified from a lower abundance of preserved sedimentary DNA signatures, and represented direct markers of paleoenvironmental changes in the Red Sea spanning the last six marine isotope stages.

LITHOLOGY AND PALEODEPOSITIONAL ENVIRONMENT ANALYSIS OF PERMIAN SEDIMENTS IN CONNECTION WITH OIL AND GAS POTENTIAL IN THE SOUTH-KHRESTYSHCHE AREA IN THE DNIEPER-DONETS BASIN

The aim of this work to study the integration of modern geological and geophysical methods for identifying stratigraphic traps. Permian deposits of the Dnieper-Donets depression were selected for the analysis. These deposits are characterized by inconsistency of productive horizons and the sandstone bodies formation. Direct indicators of the hydrocarbons, various characteristics of the seismic wave field, lithological classification and analysis of the reservoir properties of rocks were used. The available literature data on the oil and gas development and production from the Permian deposits and geophysical data were used for a comprehensive analysis. Geophysical data was presented by well logging data and seismic data. consolidated geological section across the Dnieper-Donets depression was constructed by geophysical data. The geological section gave information about the modern occurrence of the Permian sediments. Seismic surveys was used for attribute analysis. Attribute analysis give us zones with possible hydrocarbon contents and improved reservoir properties. This method shows the role of seismic studies in determining the lithology and oil and gas content of the studied strata. The combination of lithology, seismic data and information about paleodepositional environment allowed us to identificate unconventional hydrocarbon traps.

PALEOAMBIENTE DEPOSICIONAL E ORIGEM DA MATÉRIA ORGÂNICA DOS FOLHELHOS BETUMINOSOS DA FORMAÇÃO IPUBI, PORÇÕES LESTE E SUDESTE DA BACIA DO ARARIPE, NE DO BRASIL: EVIDÊNCIA DE N-ALCANOS E ISOPRENOIDES

Condições paleoambientais dos folhelhos betuminosos cretáceos da Formação Ipubi (Bacia do Araripe-NE do Brasil) são continuamente debatidos. O principal ponto de controvérsia é se essas rochas foram depositadas em condições lacustres ou marinhas restritas. O presente estudo se desenvolveu com amostras representativas desses folhelhos betuminosos da unidade sedimentar supracitada, a partir da análise de biomarcadores saturados (n-alcanos e isoprenoides) para se reconstruir o paleoambiente deposicional e a origem da matéria orgânica. O predomínio de n-alcanos entre n-C13 e n-C19 são indicativos de uma contribuição algálica. Entretanto, picos das concentrações dos n-alcanos n-C27, n-C28 e n-C29 sugerem uma influência de plantas continentais. As relações Pr/Fi e Pr/Fi vs. (Pr + n-C17)/(Fi + n-C18) indicam condições redutoras (baixa oxigenação) e hipersalinas durante a deposição destes folhelhos. O diagrama Pr/n-C17 vs. Fi/n-C18, além de sugerir também condições redutoras durante a deposição destes folhelhos, classifica a contribuição algálica como de origem marinha-transicional. Sendo assim, baseado nos compostos orgânicos moleculares, este estudo sugere que os folhelhos estudados foram depositados em condições anóxicas e hipersalinas em um paleoambiente marinho restrito à transicional.

Paleo-depositional environment, origin and characteristics of organic matter of the Triassic Chang 7 Member of the Yanchang Formation throughout the mid-western part of the Ordos Basin, China

A comprehensive geochemical and petrographic study was performed on 23 clay-rich shale samples collected from 18 wells drilled in the mid-western part of the Ordos Basin to investigate the characteristics and thermal maturity of organic matter as well as the paleo-depositional environment of the Chang 7 Member within the Triassic Yanchang Formation. The samples are characterized by moderate to very high organic carbon (TOC) contents ranging from 0.24% to more than 24.83% (2.72% on average) and total sulfur (TS) contents of 0.06% to 6.51%. Rock-Eval S1 and S2 values range from 0.13 to 3.70 mg HC/g rock and 0.62 to 92.50 mg HC/g rock, respectively, implying good to very good hydrocarbon generation potential for most of the samples. Rock-Eval data and petrographical observations reveal that the kerogen is mainly composed of type II to type III, consisting of a mixture of aquatic and terrigenous organic matter. The aliphatic fractions show variable monomodal to bimodal distributions of normal alkanes. Different parameters such as TOC/TS and molecular biomarker data imply that the Chang 7 member is a typical clay-rich source rock deposited in a sulphate-poor lacustrine to fluvio-deltaic environment under oxic to sub-oxic conditions with substantial input of organic matter derived from higher land plants, mainly conifers. Low values of gammacerane further indicate a low salinity of the paleo-lake. Average Tmax values of 445 °C, vitrinite reflectance values between 0.68% and 0.88%, biomarker parameters, such as sterane ratios of C29 ααα 20S/(20S + 20R) and C29 ββ/(ββ + αα), and maturity indices based on aromatic compounds, indicate a thermal maturity within the oil window for the studied samples, showing some regional variability within the mid-western Ordos Basin.

Lithofacies, particle size analysis and paleodepositional environment of the Eze-Aku Group (Cenomanian–Turonian) in the Itigidi-Ediba area, Afikpo Synclinorium, southeastern Nigeria

Field, lithofacies and particle size analysis of sediments of the Eze-Aku Group in the Itigidi-Ediba localities, Afikpo Synclinorium, were carried out to describe the textural attributes of the sediments and reconstruct the paleodepositional environment of the study area. Field studies show that the geology is characterized by a succession of marine shales and shaly-limestone overlain by elongate sandstone ridges that trend in a northeast–southwest direction. In certain cases, the sandstone ridges alternate with shales. These deposits are characterized by wave and tide generated sedimentary structures with trace fossil assemblages consisting mainly of Ophiomorpha, Skolithos, and Cruziana ichnofacies that are suggestive of a moderately disturbed environment. Lithofacies studies of the sedimentary succession indicates five facies associations: conglomeritic consolidated sandstone, laminated sandstone, bioturbated sandstone, laminated shaly-limestone and cross-bedded sandstone facies associations suggesting deposition in proximal sub-marine, shelf edge/sub-marine fan, distal sub-marine fan/distal shelf, shallow/deep marine, and shoreface/foreshore sub-environments, respectively indicating a tidal shallow shelf to deep marine environment. Analysis of the grain size spectrum in some selected outcrops shows sediments that are moderate to poorly sorted, generally positively and very positively skewed and mesokurtic, platykurtic to leptokurtic, deposited in a shallow marine environment under moderate to high energy conditions, with fluvial influence.

Geochemical characteristics of gypsum lithofacies in northeastern of Muş (Eastern Anatolia-Turkey): an indication of the Neotethys closure

The study investigates the origin, age, paleo-depositional environment and formation conditions of gypsum lithofacies deposited in northeastern Mus in Eastern Anatolia using element analysis and sulfur, oxygen and strontium isotope combinations. The correlation diagrams of major ions and trace elements plots in the gypsum lithofacies indicate that carbonates and clastics carried by terrestrial waters (streams, rivers, etc.) significantly contributed to the evaporitic phases. Furthermore, the effects of hydrothermal solutions, increased salinity of the basin and presence of biological activity were associated with high values for major ions and trace elements. While the δ18O and δ34S isotope contents of some gypsum lithofacies samples show that they are of terrestrial or marine origin, others do not indicate either origin because they are affected by other factors such as water salinity variations, bacterial sulfate reduction, new terrestrial water and detrital inputs carried into the basin. However, 87Sr/86Sr isotope concentrations analyzed in gypsum lithofacies in the study area indicate Rupelian (Lower Oligocene) time, which is compatible with the Cenozoic age curve. Based on all these data, gypsum lithofacies in this basin were deposited in a transition zone (shallow sea-lagoon-inland sabkha-mudflats) under the influence of both marine and terrestrial conditions. Therefore, we claim that this basin formed during the Rupelian (Early Oligocene), at a time when the sea connection between the Indian Ocean and the eastern Mediterranean was restricted; that is, the southern branch of the Neotethys was extremely shallow during this period and almost terminated in the study area.