Fluvio-deltaic Environment Research Articles

Organic geochemistry and 1D-basin modeling in the Taranaki Basin, New Zealand: Implications for deltaic-source rocks of the cenozoic oil and condensate reservoirs

The Taranaki Basin in New Zealand is an area of active exploration for oil and condensate. This research focuses on integrating geochemical characteristics and 1-D basin modeling to Late Cretaceous to Miocene source rock systems, along with oil and condensate data from fifty-seven wells in the Taranaki Basin. The geochemical study reveals that the oil and condensate samples were generated from clay-rich source rocks, containing mixed organic matter, with large amounts of terrestrial organic matter input. These source rocks were deposited in fluvial to fluvio-deltaic environments under oxic conditions. The presence of oleanane in both oil and condensate samples suggests that the source rocks had a significant terrestrial component and deposited during the Late Cretaceous to Cenozoic. Using various biomarker proxies, oil-source rock correlation along with 1-D basin modeling revealed that the oil and condensate were mainly derived from the Late Cretaceous Rakopi and Paleocene Farewell formations at different maturity stages. The oils were generated within the peak-mature oil window, while the condensates primarily resulted from the secondary cracking of oil taking place in the source rock within the gas generation window. This finding is consistent with the 1-D basin modeling results. The model shows that the Paleocene Farewell source rock has achieved the primary stage of oil generation (0.55–0.95 Easy %Ro), contributing to most of the discovered oils in the Cenozoic reservoir rocks. Meanwhile, the Late Cretaceous Rakopi source rock reached the gas window with a higher vitrinite reflectance of more than 1.30 Easy %Ro, indicating greater gas generation potential.

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

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

Investigating the hydrocarbon potential of the Paleogene coal seams in Kapuni field, Taranaki basin, New Zealand: Implications for coal-oil correlation

Hydrocarbon generation in Taranaki Basin, New Zealand, has been proven in several source rocks within the syn- and post-rift basin-fill sedimentary successions. These source rocks are mainly hosted in the marginal and deep marine Upper Cretaceous shales, which generate marine and terrigenous oils. Some of the terrigenous oils could have been linked to the Paleogene coals; however, the capability of these coals to generate and expel liquid hydrocarbons is poorly-investigated. In this study, we present organic geochemistry, kerogen petrography, wireline logging, and basin modelling to evaluate the hydrocarbon generation potential of the Paleogene coals and coal-rich facies of the Kapuni Group sediments in Kapuni Field, Taranaki Basin. A compositional correlation between oil in the Kapuni Group reservoirs and coal extracts was performed to decipher the oil origin and define its parent source rock. Kapuni oils display high pristane/phytane (Pr/Ph) > 5, low dibenzothiophene (DBT)/phenanthrene <0.1 and high relative abundance of C29 regular steranes >60%, typifying their generation from a terrigenous source rock deposited in an oxic, fluvio-deltaic environment. Similar values were also observed in the muddy coal extracts of Farewell Formation, thereby reflecting a positive coal-oil correlation. The maturity-relevant biomarkers (Ts/Tm trisnorhopanes; C29 steranes isomers, and 22S-, 22R C32 homohopanes) are similar in both oils and Farewell coal extracts confirming oil expulsion from Farewell coals during the peak oil window maturity. These results demonstrate the generation and expulsion of coal-oil, thereby shedding light on the prospectively of the Paleogene plays in the Taranaki Basin.

Sedimentary Facies, Architectural Elements, and Depositional Environments of the Maastrichtian Pab Formation in the Rakhi Gorge, Eastern Sulaiman Ranges, Pakistan

An integrated study of sediments was conducted to examine the facies architecture and depositional environment of the Cretaceous Pab Formation, Rakhi Gorge, and Suleiman Ranges, Pakistan. This research focused on analyzing architectural elements and facies, which are not commonly studied in sedimentary basins in Pakistan. To identify lithofacies, outcrop analysis and section measurement were performed. The identified lithofacies were then categorized based on their depositional characteristics and facies associations, with a total of nine types identified within a stratigraphic thickness of approximately 480 m. These facies were mainly indicative of high-energy environments, although the specifics varied by location. Sedimentary structures such as planar and trough crossbedding, lamination, nodularity, load-casts, and fossil traces were found within these facies, indicating high-energy environments with a few exceptions in calm environments. The identified facies were grouped into seven architectural elements according to their depositional environments: delta-dominated elements, including laminated shale sheet elements (LS), fine sandstone elements (SF), planar cross-bedded sandstone elements (SCp), trace sandstone elements (ST), and paleosol elements (Pa); and river-dominated elements, including trough cross-bedded sandstone elements (SCt), channel deposit elements (CH), and paleosol elements (Pa). These architectural elements, along with their vertical and lateral relationships, indicate a transitional fluvio-deltaic environment within the Pab Formation. In conclusion, by interpreting facies and architectural elements, it is possible to gain a better understanding of the depositional history of the formation and the distribution of reservoir units.

Near equatorial paleoclimatic evolution and control on organic matter accumulation during the Cenomanian in the Abu Gharadig Basin, southern Tethys: Insights from palynology, organic petrography, and geochemistry

The beginning of the Late Cretaceous (Cenomanian) attested to a major marine transgression interrupted by sea level falls in north Africa, which led to significant environmental perturbation and widespread accumulation of organic matter-rich sediments. During this period, northern Egypt was located at the southern edge of the Tethys Ocean and deposition of thick siliciclastic alternations between shales, siltstones, and sandstones of the Bahariya Formation took place in the BED2 gas field of the Abu Gharadig Basin, north Western Desert. An integrated approach of palynofacies analysis and palynomorph composition, organic petrography, and geochemical analysis of total organic carbon (TOC)/Rock-Eval pyrolysis, carbonate content, XRD analysis, and clay mineralogy of thirty-nine cutting samples obtained from the lower-middle Cenomanian Bahariya Formation was conducted. This approach was used to characterize organic matter and possible occurrence of source rock intervals, paleoenvironmental and paleoclimatic conditions and their role in organic matter accumulation. Moderate to high proportions of pteridophyte and water fern spores and pollen grains of Araucariaceae (Araucariacites), Cheirolepidiaceae (mainly Classopollis), and Gnetalean (Elaterate pollen) and Afropollis versus the sparsely recorded xerophilous ephedroids (Ephedripites) indicate prevalent conditions of a warm and humid climate. This is consistent with clay mineral composition that is dominated mainly by kaolinite and smectite compared to low illite and the lack of chlorite, which indicated a predominantly warm-humid climate state. Palynofacies analysis showed two assemblages; the first one is dominated by phytoclasts and indicates gas-prone Type III kerogen and deposition in a fluvio-deltaic environment. The second palynofacies assemblage is characterized by moderate abundance of phytoclasts and AOM mostly of terrestrial origin, suggesting a mixed Type III/II kerogen and deposition in a marginal to shallow neritic shelf environment. The organic geochemical characterization showed relatively low average values of TOC, S2, and HI, indicating that the Bahariya Formation is characterized by Type III gas-prone kerogen. Thermal maturity of the Bahariya Formation ranges from immature to the early stage of the oil window. Additionally, organic matter characteristics of the Bahariya Formation are qualitatively and quantitatively similar to their equivalents from nearby basins. Low TOC contents are attributed to a high sedimentation rate and an increased terrestrial input of sand- and silt-sized fractions at shallow water column settings that controlled strong dilution and decomposition versus low burial efficiency of the organic matter.

Depositional and diagenetic controls on porosity evolution in sandstone reservoirs of the Stuttgart Formation (North German Basin)

The Mesozoic succession of the North German Basin comprises permeable sandstones operated by ‘conventional’ open doublet systems for geothermal heat production. The high permeability, exceeding 6 Darcy at individual localities, is the result of undercompacted grain fabrics, low abundance of authigenic minerals and large secondary porosity volumes. The discrepancy of undercompacted grain fabrics and porosity volumes of >20% at depth of up to 2500 m gave rise to a basin-scale study, in which classical petrographic methods were employed to reconstruct the diagenesis of sandstones on the example of the Stuttgart Formation (Schilfsandstein, Late Triassic). Associated to the deposition of sandstones in distinct fluvio-deltaic environments, four diagenetic pathways are indicated by systematic variations of detrital and authigenic assemblages. These four pathways are herein introduced as Delta Channel, Delta Plain, Fluvial Channel, and Floodplain Diagenesis Types. In response to the history of the North German Basin, high permeabilities of sandstones of the Delta Channel and Fluvial Channel Diagenesis Types are related to the subsequent control of depositional and diagenetic regimes on porosity evolution. Pervasive eogenetic calcite cementations contributed to preservation of pre-burial intergranular volumes and effectively prevented the mechanical compaction of grain fabrics during late Triassic–late Jurassic burial diagenesis. The uplift from deep to shallow burial depth, associated to latest Jurassic–Cenozoic structural differentiation and inversion of the North German Basin, triggered substantial dissolution processes that mainly affected carbonate cementations. This opened pores and contributed to the evolution of secondary porosity, in particular in sandstones of the Delta Channel and Fluvial Channel Diagenesis Types. Moderate post-inversion reburial rates resulted only in negligible mechanical compaction and reduction of secondary porosity. The improved knowledge of diagenetic pathways, herein exemplified by sandstones of the Stuttgart Formation, will significantly contribute to improved predictions of Mesozoic hydrothermal reservoir in the North German Basin.

Oil biomarker signature and hydrocarbon prospectivity of Paleozoic versus Mesozoic source rocks in the Faghur–Sallum basins, Egypt's Western Desert

Exploration campaigns focusing on Paleozoic plays in the Egyptian north Western Desert Sallum and Faghur basins have been limited to date, and little has been done to investigate their hydrocarbon generation potential. A total of 284 shale, calcareous shale, coaly shale and siltstone side-wall core and cuttings samples, covering the rock succession that ranges from the Silurian Kohla Formation to the Turonian Abu Roash Formation were analyzed for source rock potential. In addition, eight reservoired Paleozoic–Mesozoic oil samples were geochemically analyzed. This paper systematically investigates (1) organic enrichment, hydrocarbon generation potential, and thermal maturity of Mesozoic as well as possible Paleozoic source rocks, and (2) composition and biomarker characteristics of crude oil samples in the study area from the far north Western Desert basins of Egypt.The Silurian Kohla and Basur and the Carboniferous Desouqy and Dhiffah siltstone and shale rock samples show overall poor and fair to good source rock potential for the Lower and Upper Cretaceous rock units, respectively. In contrast, high TOC (up to 4.83 wt%) and Rock-Eval S2 (up to 6.57 mg HC/g rock) for the Devonian Zeitoun Formation samples indicates fair to very good source potential. The Middle Jurassic Khatatba Formation samples, compared with the Devonian Zeitoun Formation or other rock units in the stratigraphic column of the study area, have the highest TOC (30.1 wt%) and the highest S2 (26.9 mg HC/g rock), indicating fair to excellent source rock potential. These high values of the Khatatba Formation are locally related to organic-rich coaly shales and calcareous shale intervals. The Rock-Eval results indicate an overall dominance of Type-III kerogen with strongly varying OI values. This highly gas-prone character for most samples was confirmed petrographically by abundant vitrinite phytoclasts and recycled organic matter. Coals and coaly shale facies from the Alam El Bueib and Khatatba formations that contain liptinitic materials may represent Type-III kerogens that are capable of generating gas with minor oil. Based on Rock-Eval Tmax and HI results, the Zeitoun and Khatatba samples range from early to peak oil window. This is consistent with the Ro measurements of the Paleozoic Zeitoun samples (0.65–1.02%) and the Middle Jurassic Khatatba samples (0.59–1.01%).The Faghur oils are light (35°–46° API gravity), belong to a single genetic family and are non-biodegraded based on wide-ranging n-alkanes. Abundant tricyclic diterpanes, C24 tetracyclic terpane, C27 Ts trisnorneohopane, diahopane, cyclohexanes, C29 steranes and rearranged steranes and low to very low gammacerane and extended hopanes are common molecular signatures of the analyzed light oil and condensate samples. These findings indicate a relatively oxic fluvio-deltaic environment for the corresponding clay-rich coaly shale and calcareous shale source rocks that contain predominantly higher land-plants with negligible algal/bacterial organic materials. Based on saturated- and aromatic-maturity parameters, the analyzed oils were inherited from peak–late mature source rock.

From apex to shoreline: fluvio-deltaic architecture for the Holocene Rhine–Meuse delta, the Netherlands

Abstract. Despite extensive research on alluvial architecture, there is still a pressing need for data from modern fluvio-deltaic environments. Previous research in the fluvial-dominated proximal and central Rhine–Meuse delta (the Netherlands) has yielded clear spatial trends in alluvial architecture. In this paper, we include the backwater length to establish architectural trends from apex to shoreline. Channel-belt sand body width / thickness ratios and interconnectedness were determined, and the proportions of fluvial channel-belt deposits, fluvial overbank deposits, organics and intertidal deposits were calculated for the complete fluvio-deltaic wedge based on high-resolution geological cross sections. It was found that the average width / thickness ratio of channel-belt sand bodies in the proximal delta is 5 times higher than in the distal delta. Other down-valley trends include an 80 % decrease in the channel deposit proportion (CDP) and a near-constant proportion of overbank deposits. Additionally, interconnectedness in the proximal delta is 3 times higher than in the distal delta. Based on the Rhine–Meuse dataset, we propose a linear empirical function to model the spatial variability of CDP. It is argued that this relationship is driven by four key factors: channel lateral-migration rate, channel-belt longevity, creation of accommodation space and inherited floodplain width. Additionally, it is established that the sensitivity of CDP to changes in the ratio between channel-belt sand body width and floodplain width (normalized channel-belt sand body width) varies spatially and is greatest in the central and distal delta. Furthermore, the proportion of fluvial channel-belt sands is generally an appropriate proxy for the total sand content of fluvio-deltaic successions, although its suitability as a total sand indicator rapidly fades in the distal delta. Characteristics of the backwater zone of the Rhine–Meuse delta are (1) sand body width / thickness ratios that are lower as a consequence of channel narrowing (not deepening), (2) a rapid increase and then a drop in the organic proportion, (3) an increase in the total sand proportion towards the shoreline, and (4) a drop in the connectedness ratio. For this paper, unique high-resolution quantitative data and spatial trends of the alluvial architecture are presented for an entire delta, providing data that can be used to further improve existing fluvial stratigraphy models.

Provenance of organic matter in an intracratonic rift basin: Insights from biomarker distribution in Palaeogene crude oils of Cambay Basin, western India

The biomarker distributions of crude oils are a valuable and complementary tool for understanding the provenance of source organic matter (OM) in intracratonic rift basins such as the Cambay Basin, India, which has five tectonic blocks from north to south. Distinctive biomarkers recorded in Cambay oils included tetrapolyprenoids, C29 28-nor-spergulane, diaryl isoprenoids and methylated chromans. Other diagnostic markers including oleanane, A-ring degraded hexanortriterpanes, bicadinanes and aromatic oleanoid/ursanoid triterpenoids, suggest substantial inputs to the OM from tropical angiosperm families. The biomarker distributions and various plots of the data, including a principal component analysis, indicate two distinct OM facies present in the basin. The depositional environment of the source OM that prevailed in the north blocks (NB) and south blocks (SB) of the basin are different, despite similar ages and geography. The results suggest that the NB oils were generated from mixed algal/microbial and higher plant-derived organic biomasses, deposited in suboxic to anoxic conditions, probably in a restricted marine or lacustrine environment. Contrarily, the oils of SB were generated from OM substantially derived from tropical lowland angiosperm rainforest trees, which were deposited under a fluvio-deltaic environment with a suboxic to oxic setting. The various maturity parameters reveal that northern oils are marginally less mature than the southern oils.

Depositional paleoenvironment and source rock characterization across the Toarcian Oceanic Anoxic Event from the eastern Tethys, Tibet, SW China

An integrated approach based on organic petrographic and geochemical proxies along with palynological and palynofacies analysis were employed using 24 rock samples representing the upper part of the Quse and Sewa formations in the southern Qiangtang Basin, Tibetan Plateau, SW China. This interval was deposited during the Early Jurassic Toarcian Oceanic Anoxic Event (T-OAE) and is investigated to assess the biostratigraphic age control based on palynomorphs composition. The current approach allows for the interpretation of the local paleoenvironmental response to T-OAE climate variability that still unraveled based on palynofacies analysis of the Bilong co succession. Visual palynofacies analysis, vitrinite reflectance (VRo), and total organic carbon (TOC)/Rock-Eval pyrolysis parameters enables a detailed assessment of probable source rock characterization.The palynomorph content comprises mainly terrestrially-derived land-plants, mostly sphaeromorphs pollen grains, with minor content of marine dinoflagellate cysts (dinocysts). Marker dinocysts and pollen taxa allow the subdivision of the studied succession into two age-distinctive units of early Toarcian and late Bajocian-Bathonian. Additionally, carbon isotope profile of the studied succession observed a large negative carbon isotope excursion (CIE) within the lower Toarcian oil shale interval, which is compatible with regional δ13Corg records from adjacent basins at this time. The regional TOC contents of lower Toarcian reached up to 21 wt% in areas of the eastern Tethys compared to lower values in the western Tethys and Boreal (ca. 3 wt%). Two palynofacies assemblages revealed deposition of these deposits in distal inner neritic to fluvio-deltaic environment and to some extent in brackish marginal marine setting. Geochemical screening indicated that most samples were dominated by very good to excellent organic matter richness of kerogen Type II and mixed Type II/III with very good to excellent hydrocarbon generation. The calculated VRo-eq from Rock-Eval Tmax and measured VRo values are in good agreement and showed that the organic matter of the studied interval is in the early to middle stages of the oil window.

Palynomorph biostratigraphy, palynofacies, thermal maturity and paleoenvironmental interpretation of the Bajocian–Aptian succession in the OBA D-8 Well, Matruh Basin, Egypt

A palynological study of 35 cutting samples from the Middle Jurassic–Lower Cretaceous section encountered in the OBA D-8 Well in the north Western Desert of Egypt yielded palynomorphs and particulate organic matter used to interpret the biostratigraphy, palynofacies, paleoenvironment, thermal maturation, and hydrocarbon potential of the Matruh Basin. The stratigraphically significant dinoflagellate cyst and sporomorph taxa recovered, which include Murospora florida, Aequitriradites spinulosus, Pilosisporites trichopapillosus, Callialasporites trilobatus, Tenua anaphrissa, Muderongia simplex, Systematophora sp., Gonyaulacysta jurassica, Korystocysta gochtii, Pareodinia ceratophora, and Dissiliodinium sp., permit an age assignment of the studied section as Bajocian to Aptian. Seven formal dinoflagellate cyst zones and four formal sporomorph zones are proposed for this time interval, in addition to differentiating four palynofacies assemblages and using the Rock-Eval Pyrolysis method to visually and geochemically evaluate the source rock potential. The 0.45–2.82 wt% TOC values of the studied samples indicate fair to good source rocks. An inner neritic depositional environment due to relatively high sea level is deduced for the Khatatba and Masajid formations using palynological proxies, as reflected in the lower percentages of sporomorphs relative to dinoflagellate cysts. During the Lower Cretaceous, a predominance of terrestrial sporomorphs in the Alam-El Bueib Formation suggests the advance of a fluvio-deltaic environment, possibly related to a significant sea level fall and regressive phase.

Sedimentological and Geochemical Charectesization of Manaveli and Cuddalore Formations, Puducherry Basin, India

Sedimentological and Geochemical Charectesization of Manaveli and Cuddalore Formations, Puducherry Basin, India &#x0D; &#x0D; Shyam N. Mude, Shyam Yawale and Vishal Choudhari &#x0D; &#x0D; Department of Geology, Fergusson College (Autonomous), Pune- 411004, India&#x0D; Email: shyam.mude@fergusson.edu; shyammude25@yahoo.co.in&#x0D; &#x0D; Abstract: The lithostratigraphically rocks of Puducherry basin (Sub-basin of Cauvery Basin) have been classified as Valudavur, Mettuveli, Karasur, Manaveli, Cuddalore formations in ascending order of deposition. The sedimentological and geochemical studies of sediments from Manavali (Paleocene) and Cuddalore (Mio-Pliocene) formations were carried out to understand grain size variation and distribution of major oxides and trace elements. &#x0D; The sieve analysis was used to study grain size variation and on the basis of grain sizes various geo-statistical parameters were calculated to understand depositional environment. The discriminant function analysis of the sediments from the Cuddalore Formation infers that mostly the sedimentation was occurred in fluvio-deltaic environment with incursions of shallow marine environment whereas bivariate plots suggests that the deposition was occurred in riverine to deltaic environment.&#x0D; The major element geochemistry of the Manaveli and Cuddalore formations have been carried out to understand the provenance, paleoclimate and source area weathering conditions. The values of CIA (Chemical Index of Alteration), CIW (Chemical Index of Weathering), ICV (Index of Compositional Variability) and PIA (Plagioclase Index of Alteration) of the sediments infer high weathering was occurred in source area of sediments for Manaveli and Cuddalore formations. The ratio (Al2O3+K2O+Na2O)/SiO2 indicate semi-humid climatic conditions during deposition of Cuddalore Formation whereas Manaveli Formation was deposited in semi-arid climatic conditions. The discriminant function diagram (A-CN-K Plot), suggest Quartzose sedimentary provenance for Cuddalore and Mafic igneous provenance for Manaveli Formation. &#x0D; &#x0D; Keywords: Grain Size, Geochemistry, Manaveli, Cuddalore, Puducherry Basin

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.

Sedimentary and tectonic controls on Lower Carboniferous (Visean) mixed carbonate–siliciclastic deposition in NE England and the Southern North Sea: implications for reservoir architecture

Discovery of the Breagh gas field in the Southern North Sea (SNS) has demonstrated the potential that the Lower Carboniferous (Visean, 346.7–330.9 Ma) Farne Group reservoirs have to contribute to the UK's future energy mix. New biostratigraphic correlations provide a basis to compare Asbian and Brigantian sedimentary cores from the Breagh Field and age-equivalent sediments exposed on the Northumberland Coast, which has proved critical in gaining an understanding of exploration and development opportunities. Thirteen facies associations characterize the mixed carbonate–siliciclastic system, grouped into: marine, delta front, delta shoreface, lower delta plain and upper delta plain gross depositional environments. The facies associations are interpreted as depositing in a mixed carbonate and siliciclastic fluvio-deltaic environment, and are arranged into coarsening- and cleaning-upward cycles (parasequences) bounded by flooding surfaces. Most cycles are characterized by mouth bars, distributary channels, interdistributary bays and common braided rivers, interpreted as river-dominated deltaic deposits. Some cycles include rare shoreface and tidally-influenced deposits, interpreted as river-dominated and wave- or tide-influenced deltaic deposits. The depositional processes that formed each cycle have important implications for the reservoir net/gross ratio (where this ratio indicates the proportion of sandstone beds in a cycle), thickness and lateral extent. The deltaic deposits were controlled by a combination of tectonic and eustatic (allocyclic) events and delta avulsion (autocyclic) processes, and are likely to reflect a changing tectonic regime, from extension within elongate fault-bounded basins (synrift) to passive regional thermal subsidence (post-rift). Deep incision by the Base Permian Unconformity across the Breagh Field has removed the Westphalian, Namurian and upper Visean, to leave the more prospective thicker clastic reservoirs within closure. Thematic collection: This article is part of the Under-explored plays and frontier basins of the UK continental shelf collection available at: https://www.lyellcollection.org/cc/under-explored-plays-and-frontier-basins-of-the-uk-continental-shelf

Mechanisms of riverbank failure and channel instability on the Nkisi River, Southeast Nigeria

Sedimentation hazards pose several geoenvironmental problems in active alpine regions and fluvio-deltaic environments. Therefore, this research seeks quantitative knowledge of the mechanisms that trigger riverbank erosion which result in high volumes of sediment yield in the downstream reaches of the Nkisi River. Study of the mechanisms of mass wasting of the riverbanks was motivated by the high rate of siltation and accretion that are currently occurring at the mouth of the Nkisi River, occupying a total surface area of about 40,000 m2. Many riverbank stability and erosion assessment studies have been done using a wide range of geomorphometric and geotechnical analyses which consider the shear strength of the bank materials in relation to the geomorphometric characteristics of the riverbanks. With this insight, this research focuses on investigating the potential causes of riverbank retreat on the river, with particular interest on the effects of soil properties in triggering mass failures. Field surveys and laboratory investigations, coupled with Bank Erosion Hazard Index (BEHI) and Bank Stability and Toe Erosion Model (BSTEM) characterized the riverbanks as unstable and conditionally stable, revealing a potentially high rate of sediment loading from the composite banks, with a factor of safety that varies from 0.76 to 1.04.

Organic petrography, Rock\u2013Eval pyrolysis and biomarker geochemistry of Maastrichtian Gombe Formation, Gongola Basin, Nigeria

The Maastrichtian Gombe Formation is located within one of the inland basins of Nigeria (Gongola Basin). Surface and borehole samples of coal and interbedded shale of this formation were evaluated to determine the organic matter richness, quality, maturity, source and depositional environment through organic geochemical techniques such as total organic carbon (TOC) determination, Rock–Eval pyrolysis, chromatography and organic petrography. The genetic potential and average TOC value of the coals, shales and coaly shales are 38.03 wt%, 3.14 wt% and 12.1 wt%, respectively, indicating moderate-to-good source potential. HIs are less than 200 mgHC/gTOC for all the samples indicating Type III kerogen for most of the samples (gas prone) and plant contributions from terrestrial sources. The source rocks were characterized by high molecular weight n-alkanes which range from C10–C41 in the coals and C9–C41 in the shales. The n-alkanes distributions in the coals maximize at C29 indicating terrestrial sourced organic matter, while the shales maximize at C16 and C29 indicating mixed sources. Pr/Ph of the coals and shales ranges from (2.15–3.51) to (0.95–2.37), respectively, suggesting fluvio-deltaic deltaic and coastal swamp depositional environments. The predominance of pristane over phytane suggests humic origin of the organic matter. The sterane/hopane values (< 0.6) indicate incorporation of bacterial inputs commonly associated with terrigenous organic matter in the coals. The CPI is greater than unity, Tmax is generally < 435 °C and vitrinite reflectance less than 0.55 Ro for most of the samples and suggest the samples are of low maturity status to expel liquid hydrocarbon.

Seeking enlightenment of fluvial sediment pathways by optically stimulated luminescence signal bleaching of river sediments and deltaic deposits

Abstract. Reconstructing sediment pathways in fluvial and deltaic systems beyond instrumental records is challenging due to a lack of suitable methods. Here we explore the potential of luminescence methods for such purposes, focusing on bleaching of the optically stimulated luminescence (OSL) signal of quartz sediments in a large fluviodeltaic system across time and space. We approach this by comparing residual doses of sand and silt from the modern Mississippi River channel with estimated residual doses of sand isolated from Late Holocene Mississippi Delta mouth bar and overbank deposits. Further insight is obtained from a comparison of burial ages of paired quartz sand and silt of Mississippi Delta overbank deposits. In contrast to some previous investigations, we find that the bleaching of the OSL signal is at least as likely for finer sediment as for coarser sediment of the meandering Mississippi River and its delta. We attribute this to the differences in light exposure related to transport mode (bedload vs. suspended load). In addition, we find an unexpected spatiotemporal pattern in OSL bleaching of mouth bar sand deposits. We suggest this may be caused by changes in upstream pathways of the meandering channel belt(s) within the alluvial valley or by distributary channel and coastal dynamics within the delta. Our study demonstrates that the degree of OSL signal bleaching of sand in a large delta can be highly time- and/or space-dependent. Silt is shown to be generally sufficiently bleached in both the modern Mississippi River and associated paleo-deposits regardless of age, and silt may therefore provide a viable option for obtaining OSL chronologies in megadeltas. Our work contributes to initiatives to use luminescence signals to fingerprint sediment pathways within river channel networks and their deltas and also helps inform luminescence dating approaches in fluviodeltaic environments.

Reconstruction of the Upper Gondwana palaeoclimates based on palynostratigraphy, palynofacies and sedimentology of the Jurassic sequences in the Tabbowa Basin, Sri Lanka

The palynological study of the Tabbowa sediments from Sri Lanka reveals predominant Jurassic gymnospermous pollen assemblage (Araucariacites australis, A. cooksonii, A. fissus and Callialasporites dampieri with spore Murospora florida) belongs to the Callovian-Kimmeridgian age. The collective palynoflora can be correlated to the Classopollis–Araucariacites–Shanbeipollenites assemblage zone of Africa and the Murospora florida zone of the Indian subcontinent and Australia, showing close phytogeographic relationships from the middle to late Jurassic periods. Palynofacies and sedimentological investigations inferred oxic-dysoxic conditions that prevailed when the sediments and organic matter deposited in shallow brackish water conditions in a fluvio-deltaic environment.

Geomorphology and environmental change around the Punic sites of the Terralba plain (Oristano Gulf, Sardinia, Italy)

We report a geomorphological study of the coastal plain of Terralba (SW coast of the island of Sardinia, Italy) in order to contextualize the high concentration of Punic rural settlements and analyze the environmental changes. The methodology is based on a detailed geomorphological study and sedimentological analysis of several records from different environments in artificial cuts and cores (chronology from archaeological remains and 14C dating). The study was also completed with information from various cartographic sources, and historical and geoarchaeological data. The Punic sites are located on a wide alluvial plain, modelled by aeolian sand mantles and wetlands, between the floodplain of the Sitzerri-Mannu and Mogoro rivers. Extensive aeolian units (Holocene and Pleistocene) are the most characteristic features of the geomorphology of the area. The results provide useful information on the environmental context of the Punic settlements on the coastal plain of Terralba, while evidence of significant geomorphological and environmental changes were found for over two millennia affecting the coastline, lagoon, marsh, aeolian and fluviodeltaic environments. The emergence of fresh water in the “paulis” or small depressions interspersed between dunes and well-drained soils suitable for cultivation have been a factor in the settlement of Punic farms.

Lithostratigraphy of Tabul Formation and Onshore Geology of Nunukan Island, North Kalimantan

Nunukan Island is located in Northeast Kalimantan Basin, one of prolific basin in western Indonesia. However, this island lies in northern verge of the basin and hydrocarbon exploration were limited before the 1940s. No detail geological fieldworks has been published. This paper attempts to share geological information obtained from few accessible outcrops, particularly from the western part of the island where previous oil drilling were carried out. Most of the island have been built by Tabul Formation, which was also the main objective of reservoir for hydrocarbon exploration. It is composed of claystones, siltstones, and sandstones interbeds, deposited in transitional environment during Late Miocene. They exhibit coarsening upward sequence, the lower part composed of heterolitic sandstones deposited in tidal flats, and the upper part composed of arkosic sandstones deposited in distributary channels and mouthbars. The eastern coastal strip was constructed by Tarakan conglomerates, which have been deposited in fluvio-deltaic environment in Pliocene, unconformably over the Tabul clastics. Those Neogene deposition of paleo-Simengaris Delta apparently deformed and inverted in Plio-Pleistocene, contemporaneously with basaltic volcanism which spread widely over northeastern Borneo, which also observed in the northern half of the Nunukan Island