Reconstruction Of Depositional Environments Research Articles

Petrography and geochemistry of the Late Devonian Pirispiki Formation in northern Iraq: Implications for Reservoir Characteristics and reconstruction of depositional environments

Petrographic, mineralogical and whole-rock geochemical investigation of siliciclastic and interbedded carbonate rocks from the Devonian Pirispiki Formation in the Nazdur and Ora sections, extreme northern Iraq, were conducted to assess reservoir potential and to study the effects of Devonian deformation (Middle Paleozoic, Caledonian event) on paleogeography and sediment transport in northern Gondwana. Traditional petrographic and scanning electron microscopy, X-ray diffraction and X-ray fluorescence were used. The main results are: sandstones from the Ora section contain significantly more quartz and less lithic fragments than those from the Nazdur section. This variation in sandstones composition may reflect uplifting effect due to Middle Paleozoic event and/or different provenance. The most notable feature of the major and trace elements distribution in the studied mudstones and sandstones is that they are significantly enriched in U but significantly depleted in Zr and Hf. The relative absence of zircon in the samples could account for the depletion of Zr and Hf, two elements linked to zircon. The extremely high U concentrations seen in mudstones and sandstones may relates to precipitation by circulating groundwater where it transported into the sediment prior to cementation. No fauna or flora have previously been reported from the Pirispiki Formation at outcrop in Iraq. The widespread occurrence of shell fragments in sandstones from the Nazdur section combined with the presence of eight dolomite and dolomitic marl beds suggest that the lower and middle parts of this section were deposited in a marine environment. This has not previously been recognized. Previous correlations between the Chalki basalts in the Chia Zinnar anticline and conglomerates in the Ora anticline are not supported by our data. We propose a new correlation between the sections in northern Iraq and those from southern Turkey and western Iraq that suggests that the Pirispiki Formation was deposited during a relative sea-level fall. The paleo-shoreline advanced northward through the study area during the Late Devonian. The Pirispiki Formation has been proposed as a potential reservoir for hydrocarbons sourced in underlying Paleozoic units. However, the sandstones and carbonates investigated during this study have no potential as a reservoir due to pervasive carbonate cements.

Petrographic and geochemical analysis of Barmer Basin Paleogene lignite deposits: Insights into depositional environment and paleo-climate

The current study attempts to characterize the samples from Kapurdi, Giral and Sonari lignite mines of Barmer Basin within Rajasthan Tertiary Lignite deposits using petrographic and geochemical techniques. The data generated has been studied in order to better understand the geochemical properties, hydrocarbon potential, depositional environment and paleo-climatic condition of the paleo-mires of these lignites. The proximate analysis of the samples reveals that the average ash content of the Barmer Basin is 11.93 %, the average moisture content is 14.08 %, average volatile matter yield is 42.03 % and the average fixed carbon is 31.95 %, which suggesting that the physical properties of lignite are dominated by volatile matter on dry basis. The ultimate investigation reveals the chemical properties of the lignite with the average carbon content found to be 58.79 wt%, hydrogen value 3.83 wt%, nitrogen percentage 1.60 wt% and sulphur content 2.75 wt%. This clearly indicates high sulphur concentration in Barmer lignite deposits, based on dry basis. The lignite samples subjected to FTIR studies identified aliphatic CH group, aromatic CO group, hydroxyl OH and Aldehyde HCO group. The Rock-Eval pyrolysis studies have been carried out to know the coalification profile of the deposits and hydrocarbon source rock potential. The varying behaviour between brown coal lithotype are primarily reflected by changes in oxygen index values, whereas hydrogen index values demonstrate a more pronounced correlation with coal type rather than the level of maturation. The results show the average total organic carbon (TOC) 47.29 %, S1 peak 2.18 mg HC/g, S2 peak 88.70 mg HC/g, S3 peak 19.22 % mg CO2/g, Tmax 410.53 °C, hydrogen index 219.2 mg HC/g TOC and oxygen index 46.33 mg CO2/g TOC. According to petrographic investigations, the huminite macerals dominate followed by liptinite and inertinite maceral groups in mineral matter-free basis. In all the three different lignite mines, the huminite group dominates with preserved sub-maceral ulminite indicating less transformation of structured plant matter. Reconstruction of depositional environment and paleo-climatic condition of the paleo-mires has been attempted by using organic petrographic indices. The high gelification index (GI) and low tissue preservation index (TPI) values suggest a prolonged wet environment during the decomposition of organic material, accompanied by a gradual subsidence process. The ground water influence index (GWI) and vegetation index (VI) denote ombrotrophic to mesotrophic hydrological conditions of basin at the time of deposition.

From shallow-water carbonate ramp to hemipelagic deep-marine carbonate deposition: Part 2. Sirogojno (Klisura quarry) - the reference section of the Middle to Late Anisian Bulog sedimentary succession in the Inner Dinarides (SW Serbia)

The Middle to Late Anisian sedimentary succession preserved in the Klisura quarry in Sirogojno (Zlatibor Mt., SW Serbia) preserves the most complete deepening depositional history in the Inner Dinarides in Serbia. In this section, extensional tectonic movements that took place in connection with the Neo-Tethys break-up are reflected in the exceptional well-preserved record of the depositional history. The Pelsonian Ravni Formation at the base of the section is dissected by two generations of neptunian dikes formed in A) the late Pelsonian, and B) the late Illyrian. Above the late Pelsonian drowning unconformity a deepening trend of the depositional environment is mirrored by the litho- and microfacies characteristics of the more than 21 meter thick red nodular limestones of the Bulog Formation. Periods of starvation with deposition of Fossillagerstatten beds and hardgrounds mark characteristic changes in deposition. Two phases of distinct tectonic motions result in unconformities and a stepwise deepening of the depositional realm. The first and major unconformity is the drowning surface on the top of the Ravni Carbonate Ramp which led to the nearly total demise of shallow-water carbonate production, i.e. the change in deposition from shallow- to deepwater limestones. The second phase of tectonic motions is expressed by the formation of an angular unconformity after a phase of starvation in the basin. No Bulog Limestone section in the Dinarides, even not in the type region near Sarajevo, preserves all these characteristic sedimentological features as preserved in the Klisura quarry section in Sirogojno. Therefore this section is proposed to be the reference section for the sedimentological evolution and depositional environment reconstruction of the Bulog Formation in the Inner Dinarides, located in SW Serbia.

Sedimentology and New Paleogeographic Data of the Lower Eocene Chouabine Formation in the Gafsa Basin (Tunisia), Southern Tethyan Margin

Abstract Facies and paleo-environment reconstruction applied to the lower Eocene Chouabine deposits occurring in the Gafsa area in Tunisia provided important information on sediment cyclicity and paleogeographic conditions. From west to east, four sections were surveyed: Tamerza (25 m), Oued Thelja (48 m), Jebel Ong (25 m), and Jebel Chamsi (45 m). Furthermore, the facies stacking pattern allowed for the reconstruction of depositional environments in relation to sea level changes. The Chouabine Formation is formed by the succession of three sequences composed of marls, phosphorites, and carbonates. There are five main facies organized into two transgressive cycles. The gradual transition of facies from intertidal to circatidal environments unraveled the sedimentation on a homoclinal ramp, episodically swept by upwelling currents. The Chouabine Formation is made of deepening–upwards cycles. Each cycle ranges from 2 to 5 m in thickness. The proximal-distal transition of depositional environments was gradually increasing from intertidal to offshore environments. Lateral variation of thickness and facies is generated by the reactivation of inherited NW-SE extensive and/or transtensive faults, acting since the late Cretaceous during the intermediate closing phase of the Neotethys. Synsedimentary tectonics has created trap structures favouring confinement conditions in the center of the basin, permitting the accumulation and preservation of large thicknesses of phosphate deposits. Based on new litho- and biostratigraphic data, this study presents for the first time a clear and detailed regional stratigraphic correlation and paleogeographic maps.

INSIGHTS FROM FOSSIL FISH TAPHONOMY INTO THE DEPOSITIONAL ENVIRONMENT OF THE LOWER MILWAUKEE FORMATION (BERTHELET MEMBER, LATEST MIDDLE DEVONIAN)

ABSTRACT The fossiliferous, argillaceous dolostone of the latest Middle Devonian (Givetian) Milwaukee Formation contains abundant disarticulated fish skeletal elements. This study refines depositional environment reconstruction of the Milwaukee Formation through taphonomic analysis of this fish assemblage. Robust skeletal elements of placoderm grinding teeth, fin spines, and armored plating dominate the assemblage. Specimens display variation in taphonomic attributes including color, luster, corrasion (chemical corrosion and/or physical abrasion), presence of pyrite and phosphate permineralization, and epibionts. Rare occurrences of teeth belonging to osteichthyan fish are also taphonomically variable. Associated invertebrates are diverse and include abundant cephalopod internal molds and pyritized brachiopods often randomly oriented in shell hashes. In sum, these observations suggest deposition on a marine shelf between normal and storm wave base. During low-energy background periods, fish plates exposed on the seafloor would undergo initial disarticulation, epibiont attachment, and color- and luster-altering corrasion and phosphatization. Buried skeletal elements were ‘protected' from epibiont attachment and corrasion, but occasional low oxygen conditions in the substrate led to early diagenetic pyrite permineralization. Periodic high-energy storm events were responsible for complete disarticulation as well as reworking and reexposure of buried skeletal elements (and burial of others), resulting in a time-averaged assemblage with a wide range of taphonomic attributes. Robust placoderm skeletal elements were best suited to survive fossilization in this setting and therefore the placoderm-dominated fossil assemblage appears to be an artifact of taphonomic processes and energy of the depositional environment.

Carbonate microfacies and depositional model of Triassic Pha Kan and Doi Long Formations, Lampang Group, Sukhothai Zone, Northern Thailand

Triassic carbonate rocks of Pha Kan and Doi Long Formations of Lampang Group consist mostly of carbonate successions deposited in the Sukhothai Zone, northern Thailand. These formations are mixed carbonate and siliciclastic sediments and widely exposed in the Lampang area. These deposits are important for the reconstruction of depositional environments and regional correlations of the Triassic carbonate rocks with respect to the volcanic arc of the Sukhothai Zone during the Triassic. The goal of this study is to analyze the lithology and microfacies of the carbonate rocks of the Pha Kan and Doi Long Formations and reevaluate the existing depositional models and carbonate settings. Both formations have diverse skeletal fossils, including foraminifers, sponges, calcimicrobes, calcareous algae, bivalves, echinoderm spines and fragments, ostracods, gastropods, and other fossils. Based on field observations and microfacies analysis, twelve major facies, corresponding to specific depositional environments within a carbonate setting, have been established. The inferred depositional environment is a carbonate platform with a well-developed lagoon, small-scale reefs, and bioclastic and oolitic shoals, as parts of a carbonate ramp. The microfacies and sedimentological study show that this carbonate system was influenced by the nearby volcanic arc. Consequently, our study shows that the Lampang Group in northern Thailand can be correlated with the Lincang Massif of the western Yunnan Province, PRC and the northwestern Lao PDR.

Reconstruction of Depositional Environment of Sarchahan Formation (Silurian) in the Persian Gulf

Sarchahan Formation (Silurian) consists of shale and sandstone interbeds in the studied well in the Persian Gulf. Sedimentological characteristic of this formation is presented here for the first time. Based on petrographic studies and wire line logs studies, five petrofacies and three facies associations were identified in the Sarchahan Formation. Variations in facies association in combination with palynological studies represent that deposition of the Sarchahan Formation took place in different subenvironments of a delta environment from prodelta to delta plain. The lower part of the formation is represented by a high amount of black shale and higher total organic carbon values which are the result of deposition in the deeper parts of the basin. Meanwhile, the upper part is composed of sandstone and red mudstone which are the result of deposition in the shallower part of delta environment. Cementation, compaction and dissolution are the main diagenetic features which affected the Sarchahan Formation. Silica and sericite cements are the most abundant cement types in quartzarenite and subarkose petrofacies the development of which resulted in porosity reduction. The comparison with the Arabian plate, the deposition took place in a southward deepening basin into which sediments were introduced from the northern parts. .

A multi-proxy reconstruction of depositional environment of a Late Pleistocene submerged site from the Central Coast of Chile (32°): Implications for drowned sites

Site GNL Quintero 1 (GNLQ1), located on the central Chilean coast (32°S), remains the only submerged fini-Pleistocene landscape reported for the southeastern Pacific coast to date. This unique site is characterized by a high frequency and diversity of extinct terrestrial faunal remains supported within a fine-grained sedimentary matrix. Several studies have been performed on the faunal assemblage; however, the sedimentary environment in which the site was formed still remains unclear. In this work, stratigraphic, sedimentological and geochemical analyses conducted on two marine cores are combined with seismic data to determine the depositional conditions affecting the formation of this exceptional shallowly buried submerged landscape. Previous results indicated that site GNLQ1 formed, at least between 28 and 21 ka, in an aquifer-fed freshwater lagoon that developed on a fluvial plain. Here, we show a three-stage sedimentological evolution. In the first stage, a shallow restricted freshwater lagoon formed under relatively arid conditions. Subsequently, the lagoon expanded and deepened under the humid climatic conditions that affected central Chile during the Last Glacial Maximum (LGM). Finally, around 8.5-8 ka, the most recent post-glacial marine transgression (PMT) submerged the site, probably eroding the Late Pleistocene – Early Holocene part of the sequence. Currently, the site lies at ca. 13 m b.s.l. within the shoreface of present-day Quintero Bay, 650 m from the current shoreline. This enhanced understanding of GNLQ1's evolution offers a new perspective on resource availability on the coastal shelf of western South America during the Late Pleistocene, as well as highlighting the unexpected paleontological potential of this area and encouraging future archaeological research of potential prehistoric evidence within these drowned landscapes.

Facies analysis and sequence-stratigraphic control on reservoir architecture: Example from mixed carbonate/siliciclastic sediments of Raha Formation, Gulf of Suez, Egypt

Mixed carbonate-siliciclastic systems are complex, and their development can sometimes be difficult to understand, given the various responses to each sub-environment to specific forcing, such as variation in sediment supply, climatic conditions, or available accommodation. This work integrates a detailed facies, associated with a depositional environment reconstruction, all of which is placed in a sequence stratigraphy framework in order identify the different controls on the reservoir architecture of the Upper Cretaceous (Cenomanian) Raha Formation, Gulf of Suez. The study utilized outcrop data from Gabal Ekma and Gabal Nezzazat, as well as subsurface data from five wells drilled through the October oil field.Our findings revealed that the Raha Formation succession is subdivided into eleven facies associations (FA) deposited as one depositional sequence. The Abu Had Member of the Raha formation consists of mixed siliciclastic/carbonate packages, arranged in a retrogradational stacking pattern, which were deposited during a period of pulsative shoreline regression. As a result of the transgression, the siliciclastic input in the system increased, potentially due to increased precipitation and weathering in the hinterland of the study area. The upper part of the Raha Formation belongs to the Mellaha Sand Member, which is mostly composed of bioturbated quartz arenite with bioturbation, and is the main reservoir in the October Field. It was deposited during a period of shoreline progradation as testified by its progradational stacking pattern.The reservoirs of the Raha Formation can be divided into five distinctive types (G, C, Q, P, and M), based on the factors controlling the permeability (Diagenetic criteria and compositional framework). The outcrops-subsurface correlations indicated that these reservoirs in the Mellaha Sand Member are laterally extensive, and thicken from 25 m in the SE to 91 m northwestwards. It is predicted that these reservoir intervals thicken toward the unexplored areas northern of October Field. Pore-filling cement and detrital matrix decreased the permeability and the reservoir quality to minimum while, the grain coating prevented the collapse of the reservoir quality. Pore-filling cement and detrital matrix decreased the permeability values, consequently lowering the reservoir quality to minimum, whereas the grain coating prevented the collapse of the reservoir quality, retaining the highest permeability values in the overall pore system. The aforementioned study provides significant insight into the understanding of depositional facies and sequence-stratigraphic control on reservoir architecture. Additionally, it provides insights for further exploration activities in these mixed carbonate/siliciclastic intervals along the Gulf of Suez and adjacent basin, and the improved understanding of these reservoirs units could also be applied to future Carbon Capture and Storage projects.

Interpretation and Reconstruction of Depositional Environment and Petroleum Source Rock Using Outcrop Gamma-ray Log Spectrometry From the Huai Hin Lat Formation, Thailand

Gamma ray logs are most useful in identifying subsurface lithology and interpreting depositional environments. This study highlights the use of outcrop gamma-ray logs along with outcrop observations and total organic carbon (TOC) analysis to provide the stratigraphic framework of the organic-rich rocks of Huai Hin Lat Formation in central Thailand. The study reveals five sedimentary facies including (1) structureless sandstone, (2) structured sandstone, (3) interbedded sandstone and siltstone, (4) interbedded mudstone and siltstone and (5) calcareous mudstone. These facies can be grouped into two facies associations; mudstone-dominated and sandstone-dominated facies associations. The depositional environment was interpreted as lacustrine basin-fill subdivided into deep lacustrine environment and sublacustrine fan associated with the turbidity currents. The total gamma-log characteristics are closely related to the lithologies controlled primarily by clay mineral compositions. Whist, the use of spectral gamma-ray can reveal more details on depositional environments and conditions. In this study, U concentrations is proven to be useful in highlighting organic-rich rocks in low K and Th concentration successions due to its ability to be fixed in clay minerals and organic materials under an anoxic condition. Thus, the U spectral gamma ray is suggested to combine with conventional gamma ray log for depositional environment and recognition of organic-rich rocks.

Dysodiles from the lower Barremian of Lebanon: Insights on the fossil assemblages and the depositional environment reconstruction

The Recent re-/discovery of unusual Lower Cretaceous continental deposits (dysodiles) brings important and exceptional paleontological assets. Dysodile beds are found in many localities across Lebanon and are characterized by their high organic matter content and their richness in exceptionally well-preserved fossils. We focus our study on five sampling localities in the lower Barremian sandstone beds, deposited when Lebanon was located in the north-eastern part of the Gondwana supercontinent. The localities are found associated with volcanic deposits suggesting a close relation between volcanism and their deposition and/or preservation. The present study provides, for the first time, insights on the fossil assemblages found in the dysodiles, including fishes, turtles, gastropods, insects, plant debris (macroflora and palynomorphs), ostracods and coprolites. We provide a preliminary reconstruction of the depositional environment that is common to the five sampling localities. It was a freshwater lake or possibly a series of interconnected lakes, in the vicinity of volcanic edifices, surrounded by a typical Lower Cretaceous flora (open forest environment), and inhabited by basal actinopterygian fishes and teleosts that will constitute the core of subsequent freshwater assemblages. The Lebanon localities have important implications for local palaeoenvironments as well as worldwide biogeography. In addition, the fossil assemblages show some taxonomic and taphonomic differences between the sampling sites, suggesting local differences in environmental conditions during deposition.

Model showing influence of tectonics and energy condition of the fluvial environment on facies architecture: A case history of Quaternary alluvial deposits of Purna basin, central India

Present work includes identification of lithofacies and reconstruction of depositional environments including controls of tectonics, and energy condition of the medium on sedimentation pattern of alluvial deposits of Purna basin, central India. The basin exhibits good development of dominantly areno-argillaceous sediments along with restricted occurrences of boulder-pebbly lithounits, covering together an area of 6,522 km2. These sediments are also characterized by preservation of various calcrete morphotypes, vertebrate remains and Youngest Toba Tuff (YTT) ash in certain stratigraphic units. The work is based on detailed field and lithological studies of 20 river-cut sections exposed along the course of Purna river, representing entire alluvial deposits in both vertical and lateral profiles. Three lithofacies associations have been identified, i.e., (1) gravelly facies association (FA-1), (2) sandy facies association (FA-2) and, (3) silty-clayey facies association (FA-3). These associations consist of total 10 lithofacies distributed as: (i) matrix supported massive gravelly (Gmm) facies, (ii) matrix supported gravelly (Gmg) facies, (iii) horizontally stratified gravelly (Gh) facies, (iv) planar stratified gravelly (Gp) facies (FA-1), (v) horizontal sandy (Sh) facies, (vi) planar cross stratified sandy (Sp) facies, (vii) low angle cross bedded sandy (Sl) facies, (viii) massive sandy (Sm) facies (FA-2), (ix) laminated sandy (Fl) facies, and (x) palaeosol (P) facies (FA-3). Various architectural elements have also been identified. Based on lateral and vertical profiling of the lithofacies architecture, it is interpreted that gravel dominated facies in the mountainous region of the basin area are deposited by medium to high energy debris flow,whereas sandy and silty-clayey facies in the plain area are because of saltation and suspension modes of deposition under medium to low energy condition of depositing medium. The tectonic and topographical controls are well reflected on the sedimentation pattern and the same has been illustrated by proposing a model. Discussions along with the model have also been made on the progressive development of bazada or, pediment zone along the northern margin of the basin marked by the foothills of Satpura mountain ranges.

Depositional environments of limestones from the Taiyuan Formation in the North China Block interpreted from REE proxies

The predominantly calcareous Taiyuan Formation was formed during the Late Paleozoic period in the southern part of the North China block in the eastern Palaeotethys. We present the rare earth elements (REE) and petrography data from nine limestone beds in the Zhangji mine of the Huainan coalfield as proxies to interpret the depositional environment of Taiyuan Formation. Thin-section petrography indicates that the limestones are mainly bioclastic, in-situ and are characterized by matrix-supported cementation. The REE concentrations and elemental patterns in the limestones reflect deposition in a normal marine environment. The REE analyses show the depletion of light rare earth elements, positive La anomalies, and negative Gd anomalies. Moderately negative δCe anomalies indicate that the limestones are most likely formed in an oxidized setting, typically a shallow marine environment. The positive Eu anomaly suggests deposition in a coastal environment with influx of clastics and terrestrial organic matter, thus indicating a more southerly provenance. Results from this study highlight the utility of REE and trace-element analyses of carbonate rocks for the reconstruction of depositional environments.

A compositional approach to the reconstruction of geochemical processes involved in the evolution of Holocene marine flooded coastal karst basins (Mljet Island, Croatia)

Geochemical patterns in the environment are always the result of certain processes. Therefore, it is essential to decipher a process to properly evaluate the environmental role and potential of chemical elements/compounds. This allows the distinction between natural and anthropogenic influence on elemental concentrations. However, if the compositional nature of geochemical data is neglected, erroneous or misleading conclusions regarding the processes involved are probable. In this study the reconstruction of depositional environments and processes through the Holocene in two sediment cores obtained from submerged sinkholes located on the island of Mljet, Croatia, was performed by taking into account the compositional nature of geochemical, mineralogical and grainsize data. Problems involving compositional data are always multivariate; for example, the concentration of a single element does not carry any interpretative information, as only the ratios between elements do. This has led to the discovery of a large number of geochemical proxies based on elemental ratios, which describe certain environmental conditions and processes involved. Nevertheless, some proxies have been found to be restricted to only some specific environments, thus preventing them from being used in general; therefore, some kind of relation between different proxies is necessary to obtain final conclusions. However, when using simple elemental ratios, those correlations cannot be obtained due to the nature of compositional data. With a sequential binary partition of a compositional vector, orthonormal log ratio (olr) coordinates (proxies) can be constructed. When based on expert knowledge, those proxies fully acknowledge the geochemical properties of the chosen elements with one major difference - that the correlation between newly obtained variables is mathematically well grounded. As a result, the final conclusion is more accurate. In this research, geochemical proxies obtained as a representation in olr coordinates of the elements that are enriched compared to the local soil were used to perform principal component analyses. It helped to unravel the evolution of sedimentary environments. Mineralogical (XRD and heavy mineral data) and grain size analyses supported the conclusions obtained based solely on geochemical data. Furthermore, data analysis suggests that the proxies for redox conditions described in the literature should be used with caution, as their use is somewhat limited.

Depositional environment and reservoir characterization of the deep offshore Upper Miocene to Early Pliocene Agbada Formation, Niger delta, Nigeria

Recently, continued global demand for energy has necessitated more research for a better understanding of depositional environments and petrophysical parameters of reservoir sands in order to discover more petroleum accumulation. Therefore, the integration of lithological and well log data analyses of ditch-cutting samples from Wells A, B and C from the Niger Delta region are utilized to delineate the Upper Miocene and Early Pliocene Agbada Formation's depositional environments and to determine the quality of the delineated reservoir sands. The detailed well log analysis revealed two major electrofacies namely, shale and sand, while lithofacies analysis identified four facies, which are sandstone, siltstone, claystone and mudstone that were deposited in various depositional environments and sequences. Shale (claystone and mudstone) lithology is characterized by irregular and bell gamma-ray log motifs, which may be deposited in basin plain marine and transgressive marine shelf environments that are associated with transgressive depositional sequences. In contrast, sand (sandstone and siltstone) lithology is typified by funnel and cylindrical gamma-ray log motifs that are deposited in crevasse/prograding delta (sub-unit of fluvial/deltaic environment), and slope/inner fan channel and turbidite environments that are correlated to regressive and aggradation depositional sequences, respectively. Furthermore, sedimentology analysis evidenced twelve, four and seven reservoir sands in wells A, B and C, respectively, while qualitative analysis of well logs reveals eighteen (notated as A to R), four (A to D) and fifteen (A to P) reservoir sands in wells A, B and C, respectively. Moreover, palynological analysis shows alternation of transgressive and regressive sequences in wells A, B and C in this study. The prevalence of mangroves, spores, freshwater swamp and lowland rainforest pollen with rare occurrence of savanna pollen characterized the transgressive sequences, while low percentages of these assemblages with high occurrence of savanna pollen signified the regressive sequences in this study. Alternatively, quantitative analysis showed that volumes of shale (Vsh) are generally low and allow distinction of delineated reservoir sands into clean sand zones (<10% Vsh) and shaley sand zones (10–28% Vsh). Furthermore, water saturation (Sw) ranges from 13.0% to 63.0%, 5% to 27% and 37%–71% (wells A, B and C), and hydrocarbon saturation (Sh) ranges from 37.0% to 87.0%, 64%–95% and 29–63% in wells A, B and C, respectively. Estimated porosity values are relatively high and a porosity cutoff value of <30% permits to distinguish between reservoir and non-reservoir rocks. Furthermore, hydrocarbon saturation (37–87%) and permeability (10–50%) are relatively moderate to high and fair to moderate values, respectively. However, a strong linear relationship exists between permeability and porosity with positive coefficients of correlation that are ranging from near perfect to very significant in this study, and is an indication of permeable and porous sand facies. In conclusion, the economic importance of some of the delineated depositional environments and the outcome of reservoir characterization in this study has been previously highlighted in the Niger Delta Basin and other basins of the world. Therefore, the reconstruction of depositional environments and resulting petrophysical parameters in this study will help in understanding the geology of Agbada Formation, which will further enhance hydrocarbon exploration and exploitation in the Niger Delta region.

Bolsovian (Pennsylvanian) tropical peat depositional environments: The example of the Ruhr Basin, Germany

A set of 11 Bolsovian coal seams from the German Ruhr Coal Basin were analyzed in profiles for their organic geochemical and petrographic composition, aiming at the reconstruction of depositional environments, mainly in terms of water supply and redox conditions. For this purpose, established maceral indices and A/I ratio were employed. The seams show the same basic depositional pathways of terrestrialisation and paludification as reported for Duckmantian coal seams from the same basin. The presence of coals formed as ombrotrophic peat indicates that climate was still wet enough for the formation of domed bogs until the Upper Bolsovian, although the typical densosporite facies does not occur within the uppermost third of the analyzed succession. Very high TS values for seam Parsifal II indicate the influence of marine waters at the peat stage. A marine ingression event was not reported before for this stratigraphical level within the Ruhr Basin. Data also reveal some variability in vitrinite reflectance (0.65–0.74%) within a given coal seam which cannot be due to thermal maturation. For seams with decreased VRr values, a correlation with high liptinite contents was found. Rock-Eval HI values do not show any strong variation with depths in the recorded maturity interval. Tmax values vary between 418 °C and 444 °C showing dependence on both, thermal maturity and liptinite content. Decreasing contents of inertinite macerals throughout the Pennsylvanian in the Ruhr Basin suggest climatic changes during that time span.

Depositional Environment Reconstruction of OM-4 and OM-A Wells, Niger Delta, Nigeria, Using Palynomorphs and Wireline Logs

Microfacies analysis combined with palynological studies were carried out on two wells (OM-4 and OM-A wells) in the Niger Delta to reconstruct the depositional environment of the penetrated wells from 8200 – 11350 ft each in both wells. The succession studied yielded rich terrestrial components of the palynomorph assemblage, which gave zero to low PMI values in both wells, suggesting a paralic (freshwater to marginal marine) environment. Also, the abundance of mangrove swamp species such as, Zonocostites ramonae, Acrostichum aureum, Botryococcus braunii and Psilatricolporites crassus in both wells (which are brackish water indicators) denotes an environment of deposition with higher terrestrial influences and pockets of marine intrusions, confirming a paralic environment of deposition. The facies established from the wireline log signatures gave an alternating cylindrical to funnel shaped log motif with few units of bell shapes. This was interpreted as continental to marginal marine environment. The results of the facies analyses were correlated with the PMI values of the wells and generally cylindrical to funnel shaped facies were established with corresponding nil to low PMI values, confirming a paralic environment of deposition in both wells.

HYDROCARBON POTENTIAL AND PALAEO‐DEPOSITIONAL ENVIRONMENT OF LACUSTRINE SOURCE ROCKS: MIDDLE JURASSIC SHIMENGOU FORMATION, NORTHERN QAIDAM BASIN, NW CHINA

The Middle Jurassic Shimengou Formation in the Qaidam Basin, NW China, includes coals and lacustrine source rocks which locally reach oil shale quality (i.e. yielding &gt;3.5 % oil on low‐temperature distillation). In the present study, the palaeo‐depositional environment and hydrocarbon potential of the 84.5 m thick Shale Member of the Shimengou Formation are investigated based on bulk geochemical parameters, organic petrographic data, biomarker analysis, and stable isotope geochemistry of 88 core samples.The Shale Member was deposited in an anoxic freshwater lake which formed following the drowning of a precursor low‐lying mire. Variations in bulk geochemical parameters allow four informal units to be identified, referred to (from the base up) as Units 1 to 4. These contain intervals of oil shale of varying thicknesses. In Unit 1, mudstones in the interval referred to as oil shale Layer 1 (true thickness [TD]: 2.06 m) are OM‐rich as a result of algal blooms and photic zone anoxia, and correspond to an initial flooding event. Subsequently, productivity of aquatic organisms decreased, resulting in the deposition of organic‐lean mudstones in Unit 2. Oil shale Layers 2 (TD: 2.03 m) and 3 (TD: 8.03 m) near the base of Unit 3 were deposited during maximum water depths. As with Layer 1, high productivity by algal blooms resulted in photic zone anoxia in a stratified water column. The shales in the upper part of Unit 3 are characterized by high TOC contents and a gradual increased input of terrigenous OM, and were deposited in a stable semi‐deep lake. Finally, organic‐lean mudstones in Unit 4 were deposited in shallow lacustrine conditions. The reconstruction of depositional environments in thick, non‐marine shale‐rich successions by mineralogical, petrographic and inorganic geochemical methods may be challenging as a result of the homogenous composition of component mudstones. The results of this study indicate, however, that sub‐division and basin‐wide correlation of such intervals can be achieved by organic geochemical analyses.Oil shales and organic‐rich mudstones in Units 1 and 3 of the Shimengou Formation Shale Member are excellent oil‐prone source‐rocks with a Source Potential Index of 3.2 t HC/m2. Considering the large area covered by the Shimengou Formation in the northern Qaidam Basin (∼34,000 km2), the results of this study highlight the regional significance for future petroleum exploration. They indicate that variations in organic productivity and dilution by minerals are key factors controlling the abundance and type of organic matter in the formation. An understanding of these factors will assist with the identification of exploration targets.

Integration of Seismic Facies and Seismic Sequence analysis for Depositional Environment Reconstruction of nandy Field, Niger Delta

An integrated seismic sequence stratigraphy and facies analysis has been carried out with a view to understanding the depositional environment and stratal stacking pattern of Nandy Field of Niger Delta. Well logs and biostratigraphic information were used to identify the lithologies, stratal stacking patterns, stratigraphic surfaces, system tracts and reservoir potentials of the field. Seismic sequence stratigraphy and seismic reflection patterns were used to identify the seismic facies. Facies-related attributes were employed to identify the continuity and amplitude of seismic events. Three seismic facies packages and three reservoir sands were identified. The environments of deposition of the area consist of marginal marine to continental depositional settings predominantly made of shoreface deposits with few channel sands. The depositional model of the area shows that the deposition went down the axis of fluvial depositional system, which transports sediments through channels down to the shelf into the base of the slope down the basin. The seismic reflection patterns in this field are parallel to divergent reflections, high-continuity and high-amplitude facies, low-amplitude facies, low-continuity and variable-amplitude reflections, chaotic reflections and wavy reflection patterns. The seismic stratigraphic analysis of this field revealed the presence of low stand system tract (LST), high stand system tract (HST) and transgressive system tract (TST). Sands 1 and 2 were delineated within high stand systems tract (HST) while sand 3 was within low stand systems tract (LST). The results of this study show that the Nandy Field was of high hydrocarbon potentials and highlight the importance of using seismic reflection parameters (reflection configuration, amplitude, frequency and continuity) in the division and classification of seismic facies.

Комплексный подход к восстановлению обстановок осадконакопления терригенных пород парфеновского горизонта Ангаро-Ленской и горизонта B10 Непско-Ботуобинской нефтегазоносных областей

Комплексный подход к восстановлению обстановок осадконакопления терригенных пород парфеновского горизонта Ангаро-Ленской и горизонта B10 Непско-Ботуобинской нефтегазоносных областей