Dilution Of Organic Matter Research Articles

Organic Matter Enrichment Mechanisms in the Lower Cambrian Shale: A Case Study from Xiangandi #1 Well

In order to investigate the effect of primary productivity, organic matter dilution, and preservation on the accumulation of organic matter, geochemical data, and proxies of primary productivity, clastic influx, and redox conditions were obtained for organic-rich shales in the Cambrian Niutitang Formation. The primary productivity (total organic carbon [TOC], Mo, P, Ba, and Babio) and redox (Ni/Co, V/Cr, U/Al, and Th/U) proxies suggest the organic-rich shales were deposited in anoxic-euxinic conditions during periods of high primary productivity. Pyrite in the Niutitang Formation comprises spherical framboids, which also indicate that anoxic bottom waters were present during organic matter deposition. High primary productivity enhanced the organic C flux into the thermocline layer and bottom waters, which lead to the development of anoxic bottom waters owing to O2 consumption by microorganisms and organic matter degradation. The anoxic bottom waters were beneficial for the preservation of organic matter. In addition, Ti/Al ratios correlate well with TOC contents throughout the Niutitang Formation, indicating that clastic input increased the burial rate and prevented organic matter degradation during deposition. Therefore, the accumulation of organic matter in the Niutitang Formation was controlled mainly by primary productivity rather than bottom-water redox conditions.

Response to palaeoclimate by Early Cretaceous terrestrial organic-rich shales in the Yin'e Basin: Evidence from sporopollen, n-alkanes and their compound carbon isotopes

The Cretaceous period is a typical representative of a ‘greenhouse climate’ in geological periods, and is also an important period for the formation of hydrocarbon source rocks, which has become a hot spot in the study of palaeoclimate and organic matter enrichment. In order to further explore the mechanism of action between paleoclimate and organic matter enrichment, the response mechanism of organic matter enrichment to palaeotemperature and palaeorainfall is finely characterised by synthesising the analysis of sporopollen, biomarkers and n-alkane carbon isotopes in this study. The sporopollen indicates that the palaeoclimate is generally temperate, semiarid to semihumid, and the n-alkane and n-alkane carbon isotopes indicate that the palaeoclimate and palaeorainfall have obvious fluctuations and synchronous characteristics of rainfall and heat. Based on the differences in palaeotemperature and palaeorainfall, the palaeoclimate can be further divided into three stages: cool-temperate and semiarid, warm and semihumid, and cool-temperate and semiarid. The trend of palaeoatmospheric pCO2 recovered from the carbon isotopes of normal alkanes C27, C29, and C31 is roughly the same as that of palaeotemperature, indicating that pCO2 is important in regulating palaeotemperature. According to the distribution pattern of organic carbon, different layers are divided into three sedimentary Units: I, II and III. Different sedimentary Units have good corresponding relationships with different palaeoclimatic stages. Under the cool-temperate and semiarid climate, lower palaeotemperature and higher palaeosalinity are not conducive to the prosperity of lake organisms. The formation of light grey mudstone, silty mudstone and carbonate has lower TOC, and the organic matter types are mostly II1 and II. Under a warm and semihumid climate, higher palaeotemperature and moderate palaeorainfall are conducive to the prosperity of lake organisms. The formation of oil shale and shale has higher TOC, and the organic matter types are mostly I and II1. However, excessive palaeorainfall brings much terrigenous debris, leading to the organic matter dilution. Therefore, palaeoclimate and palaeorainfall not only affect the abundance of organic matter, but also affect the types of organic matter. In the paper, the mechanism of organic matter enrichment is improved by delicately characterising the palaeotemperature and palaeorainfall.

Low organic matter abundance and highly efficient hydrocarbon generation of saline source rock in the Qaidam Basin, NW China

The geochemical analysis and experimental simulation are comprehensively used to systematically study the hydrocarbon generation material, organic matter enrichment and hydrocarbon generation model of Paleogene source rock in the Western Qaidam Depression, Qaidam Basin, NW China. Three main factors result in low TOC values of saline lacustrine source rock of the Qaidam Basin: relatively poor nutrient supply inhibits the algal bloom, too fast deposition rate causes the dilution of organic matter, and high organic matter conversion efficiency causes the low residual organic carbon. For this type of hydrogen-rich organic matter, due to the reduction of organic carbon during hydrocarbon generation, TOC needs to be restored based on maturity before evaluating organic matter abundance. The hydrocarbon generation of saline lacustrine source rocks of the Qaidam Basin is from two parts: soluble organic matter and insoluble organic matter. The soluble organic matter is inherited from organisms and preserved in saline lacustrine basins. It generates hydrocarbons during low-maturity stage, and the formed hydrocarbons are rich in complex compounds such as NOS, and undergo secondary cracking to form light components in the later stage; the hydrocarbon generation model of insoluble organic matter conforms to the traditional “Tissot” model, with an oil generation peak corresponding to Ro of 1.0%.

Organic petrographic, geochemical, and sequence stratigraphic analyses for evaluating the hydrocarbon potential of Middle Jurassic–Lower Cretaceous rocks in Shushan Basin, northwestern Egypt

Integrated organic petrographic and geochemical analyses were made on organic-rich marine carbonate and mixed clastic-carbonate rocks of Middle–Late Jurassic and Early Cretaceous age from the Shushan Basin, Egypt to evaluate their hydrocarbon potential. Analyses allowed the identification of depositional settings, paleoclimate, and three third order genetic stratigraphic sequences (SQ) with deposits assigned to highstand (HST), lowstand (LST) and transgressive systems tracts (TST). Deposition of the source rocks in the rifting Shushan Basin resulted from the interaction between Neotethyan sea level changes, tectonic, and climate. The good reducing conditions developed during the Neotethyan Middle–Late Jurassic (Bajocian–Kimmeridgian) second order sea level rises and the climatically induced carbonate sedimentation resulted in the deposition of the organic-rich carbonates of the Khatatba Formation (SQ 1, early–middle TST) in inner–middle shelf settings under anoxic–dysoxic conditions. The Late Jurassic (late Kimmeridgian) uplifting resulted in the deposition of the organic-lean mixed clastic–carbonate strata of the Masajid Formation (SQ 1, latest TST) in the same shelfal and reducing conditions, which experienced a notable dilution of organic matter. The late TST deposits of SQ 1 are good to very good oil-producing source rocks, where they show average good to very good generative potential of late mature (late oil-to early wet gas-window) highly oil-prone organic matter. The Early Cretaceous (Valanginian–Albian) uplifting associated with the rifting of the Shushan Basin overprinted the Neotethyan late Valanginian–Hauterivian second order sea level rises, Aptian second order highstand sea level, and Albian second order sea level rise. The coeval climatic shift toward more humid conditions resulted in the clastic-dominated deposition of the organic-lean regressive units of SQ 2 (HST and LST of Alam El Bueib, Alamein, and Dahab formations) and SQ 3 (HST and LST of the lower–upper Kharita Formation) in marginal marine settings under anoxic–dysoxic to oxic conditions. The HST and LST deposits of the SQ 2 and SQ 3 show poor to good organic richness of early–mid mature (early–peak oil-window) oil/gas-prone and gas/oil-prone organic matter, respectively and exhibit average fair oil source rock potential with no gas generation.

Palaeoenvironmental Evolution Based on Elemental Geochemistry of the Wufeng-Longmaxi Shales in Western Hubei, Middle Yangtze, China

The organic-rich shales found in the Wufeng–Longmaxi Formation are typically deposited in oxygen-deficient reducing environments. One of the primary sources of debate revolves around the question of whether the anoxic bottom water found in these shales is either euxinic or ferruginous, and this matter remains unresolved. Previous studies have mostly focused on the Wufeng–Longmaxi Formation as a whole in order to understand the key factors that control organic matter accumulation (OMA). However, research on OMA for each member, including the Wufeng Formation (WF), the lower Longmaxi Formation (LLM), and the upper Longmaxi Formation (ULM), has been insufficient. This paper aims to investigate the palaeoenvironmental conditions and OMA mechanisms of the Wufeng–Longmaxi shales in western Hubei by integrating data on total organic carbon (TOC) content, mineral compositions, major and trace elements, and iron speciation. The results indicate that the Wufeng–Longmaxi shales were deposited under highly restricted hydrographic conditions, except for relatively open and upwelling conditions in the upper WF. Silica in the upper WF was primarily biogenic origin and not hydrothermal. Ferruginous conditions were the primary redox conditions for organic-rich shales except for minor formations in the lower LLM that were deposited under euxinic conditions. Due to the tectonic uplift caused by the Kwangsian Orogeny in the upper LLM, the palaeoenvironment was characterized by a warmer and wetter climate, high terrigenous influx, oxic conditions, and low productivity as the result of the insufficient nutrients caused by the weak upwelling, leading to the turnover of graptolite biozones from LM5 to LM6. The factors influencing OMA changed vertically. TOC contents have a highly positive correlation with Al content, indicating that terrigenous influx was the main factor affecting OMA in the WF, which significantly differed from patterns found in other regions. This suggests that the sedimentation rate of organic matter was higher than the terrigenous dilution rate during the WF stage. The combination of redox conditions and productivity were the main factors affecting OMA in the LLM, while terrigenous influx was the key factor controlling OMA in the ULM, resulting in the dilution of organic matter. Regions in the eastern Yiling block, which are close to the Qinling Ocean, show better prospects for shale gas exploration. This research will further facilitate the development of shale gas in this area.

Multi-Proxies Analysis of Organic Matter Accumulation of the Late Ordovician–Early Silurian Black Shale in the Lower Yangtze Region, South China

The evolutional process of palaeoceanic environment and its effect on the accumulation of organic matter during the Ordovician–Silurian transition in Lower Yangtze region has been overlooked compared to that in Upper Yangtze region of South China, although their paleogeographic settings were expected to be discrepant. This paper documents the marine depositional environment, paleoclimate, and sediment supply changes, and discusses their roles in controlling the organic matter enrichment in sedimentary rocks within the Ordovician–Silurian transition of the Lower Yangtze region, using the latest geochemical data of the continuous drilling core. The stratigraphic framework of the Ordovician–Silurian transition in the Lower Yangtze region is composed of two third-order sequences, each of which can be subdivided into a lower TST (transgressive systems tract) and an upper RST (regressive systems tract). TST1 represented an evident depositional transition stage which was marked by the ending of the underlying carbonate sediments and the initiation of the terrigenous clastic-dominated sediments. Geochemical proxies indicate that the relatively low productivity, dysoxic water column condition, and high sediment supply flux collectively resulted in inadequate organic matter hosted in deposits of the TST1. During the depositional period of RST1, the global sea level declined due to the Hirnantian glaciation age. The icehouse also caused the decrease in overall river flux and, thus, the terrigenous clastic sediment supply. The icehouse also strengthened the upwelling that occurred in the Lower Yangtze sea. The upwelling boosted the marine algae explosion through the delivery of abundant nutrients, which not only enhanced paleoproductivity but also led to an anoxic environment by oxygen consumption. Such high paleoproductivity, anoxic water column environment, and low sediment supply flux caused the deposition of organic-rich shale. The sea level rose during the TST2 due to the ending of an ice age. The relatively large water depth and high paleoproductivity associated with volcanic eruptions are the main factors that caused the enrichment of organic matter during this stage. During the deposition of RTS2, the increase of sediment supply flux resulted in a decrease in accommodation space and water depth and the dilution of organic matter in deposits, which was the primary constraint of organic matter accumulation.

Climate fluctuations during the Ordovician-Silurian transition period in South China: Implications for paleoenvironmental evolution and organic matter enrichment

The Ordovician-Silurian (OS) transition witnessed dramatic tectonic, climatic, marine, and biological coevolution, during which organic-rich black shale was widely deposited and is considered an important source rock. However, the potential links between the differential enrichment pattern of organic matter and climate fluctuations and paleoenvironmental evolution during this period are still unclear. In this paper, the Late Ordovician-early Silurian black shale in a JieLong (JL) section in South China was identified by graptolitic fossils, total organic carbon (TOC) content, X-ray diffraction (XRD), scanning electron microscopy (SEM) and geochemical analysis. The results show that the provenances of sedimentary rocks are all granodiorite-like. According to the variations in the TOC content and geochemical indicators, four layers are delineated. Under warm and humid climatic conditions, sea level rise and unstable seasonal upwelling led to Unit 1 organic matter enrichment controlled by high primary productivity, anoxic conditions and effective smectite production. Unit 2 organic matter burial was controlled by cold, arid and oxic environmental conditions, but primary productivity was still relatively high. After the end of the glacial period, warm and humid climatic conditions and rapid transgression were the main factors affecting the burial of organic matter in Unit 3. Under hot and humid climate conditions, higher terrigenous detrital inputs and unfavourable preservation environments, the dilution of organic matter in Unit 4 was accelerated. The corrected chemical index of alteration (CIAcorr) values show that the study area experienced a climate change of alternating cold and warm conditions in response to the climate fluctuation during the glacial-interglacial period. The CIAcorr value is negatively correlated with the TOC content, while the generation of available clay minerals (smectite) in warm and humid climates is usually consistent with the enhanced burial flux of organic matter, supporting the important role of climate fluctuations in the accumulation of organic matter. Finally, a comparison of the data from the present study with geochemical data from different profiles in South China reveals the role of climatic fluctuations during the OS transition in controlling paleoenvironmental evolution and organic matter enrichment, while such a heterogeneous factor is usually associated with global or regional geological events. Our study highlights this differential enrichment mechanism as a combined response of multiple factors and provides important implications for glacial-interglacial organic matter enrichment mechanisms.

Controls on the accumulation of Early Cretaceous organic-rich fine-grained deposits in a mixed marine siliciclastic-carbonate distal settings of the Neuquén Basin, central-western Argentina

Deposition of black shales occurred under specific conditions that need to combine in a precise way the rate of production and the rates of destruction and dilution of organic matter. Whereas good progress has been made in the determination of the role of production and destruction of organic matter, the role of dilution of the organic components still remains comparatively understudied. In mixed siliciclastic-carbonate marine systems, the dilution of organic matter by non-organic components could be detrital and/or biogenic in origin, adding more complexity to the understanding of these particular types of black shales. In the present contribution, we studied the mixed siliciclastic-carbonate distal deposits of the late Hauterivian Agua de la Mula Member (Agrio Formation) in the Neuquén Basin. Through a combination of sedimentological, petrographic, SEM, and geochemical analyses we define the origin of the fine-grained components, reconstruct the environmental evolution, and determine the role of the production, preservation, and dilution of the organic matter. The results indicate that the unit is predominantly composed of calcite (>47%), whereas illite is the major component among the clay minerals. Thin sections and SEM studies suggest that the carbonate fraction is dominantly composed of nannofossils, i.e. biogenic in origin, and subordinately micarbs (diagenetic, but with a potential biogenic precursor). Thus, the composition of the distal deposits is mostly controlled by the input of detrital illite. The organic geochemistry and XRF data reveal that the most appropriate conditions for the accumulation of organic matter are the transgressive intervals, near the base, with a predominant effect of the rate of the production over the preservation. On the other hand, the dilution of organic matter is mainly driven by the rate of production of biogenic calcite. Interestingly, previous studies showed that in more proximal settings (but still offshore to basin realms) the dilution is controlled by siliciclastic components. Finally, organic geochemistry suggests that most of the organic matter produced is marine in origin (type II), but towards the top of the section it switches to mixed marine and terrestrial (type III to IV), likely related to a progradation of the shallower system and the consequent increase in the input of terrestrial-derived components.

Differential enrichment mechanisms of organic matter in the Chang 7 Member mudstone and shale in Ordos Basin, China: Constraints from organic geochemistry and element geochemistry

Source rocks in the seventh member of the Upper Triassic Yanchang Formation (Chang 7 Member) are characterised by lacustrine organic-rich mudstone and organic-rich shale in the Ordos Basin. There are obvious differences in organic and inorganic geochemical characteristics between mudstone and shale in the Chang 7 Member, indicating that they are significantly different in genetic mechanism. Presently, the difference of organic-matter enrichment mechanism between shale and mudstone in the Chang 7 Member is still obscure. In this study, we explored the organic-matter accumulation mechanisms in the Chang 7 Member mudstone and shale by investigating their parent-rock properties, palaeoproductivity, palaeoenvironment and dilution conditions using biomarkers and elemental geochemical data. According to our analysis results, the mudstone and shale parent rocks are composed of sedimentary rock and granite. The material source of the mudstone was mainly from the south of the Ordos Basin, whilst the source of the shale was mainly from the south and northeast of the basin. The organic-matter enrichment of the shale was affected by high palaeoproductivity and anoxic environment conducive to the preservation of organic matter. The palaeoproductivity during the deposition of the mudstone was moderate, and the dilution caused by the high sedimentation rate was the main reason for its low abundance of organic matter. Affected by frequent volcanic activities, a grater algae bloom occurred in the process of the shale deposition, which made the palaeoproductivity during shale deposition higher than that of the mudstone deposition. In addition, the degradation of a large quantity of algae resulted in a reducing environment during shale deposition. Therefore, the high palaeoproductivity and anoxic preservation conditions resulted in large amounts of aquatic organisms in the shale, resulting in high organic-matter abundance in the shale. The palaeoclimate during mudstone deposition was warmer and wetter than during shale deposition, which was more conducive to parent-rock weathering and the transportation and deposition of terrigenous detritus. Therefore, the organic matter of mudstone is a mixture of terrestrial higher plants and aquatic organisms. However, the high input rate of terrigenous detritus caused the dilution of organic matter in the mudstone. The mudstone was mainly deposited under weakly oxidising to weakly reducing conditions. The moderate palaeoproductivity, oxygen-containing preservation conditions and dilution caused by the high sedimentation rate made the aquatic and terrestrial organic matter in the mudstone easily oxidised, degraded and diluted. Therefore, the differences of primary productivity and preservation conditions make the organic-matter abundance of the mudstone lower than that of the shale.

Tracing Lower Cretaceous organic-rich units across the SW Barents Shelf

On the Barents Shelf, the northernmost and least explored hydrocarbon province of the Norwegian Continental Shelf, Upper Jurassic organic-rich shales have traditionally been given most attention as these represent the most prolific source rock unit of the region. However, in the western frontier areas of the Barents Shelf, the Upper Jurassic is too deeply buried. By combining high-resolution 2D seismic data, well logs, and digitalized Rock-Eval data, this study documents the lateral distribution and variability of alternative source rock units within the Lower Cretaceous succession on the SW Barents Shelf. Negative high-amplitude anomalies are traced from shallow basins on the platform westward into deeper basins along the western shelf margin. The anomalies are tied to intervals of increased total organic carbon contents in several exploration wells, and we thus establish the presence of four potential source rock units; these are the (1) upper Hauterivian, (2) Barremian, (3) lower Aptian, and (4) upper Cenomanian units. Based on the distribution of the associated seismic anomalies, we infer that the deposition and preservation of these organic-rich units are coupled to localized, fault bounded depocenters, mainly controlled by Late Jurassic – Early Cretaceous rifting and local reactivation events. The lower Aptian stands out as the most significant source rock unit, particularly in the Fingerdjupet Subbasin, where it displays a kerogen Type II composition. The distribution and development of this oil-prone source rock unit is linked to an early Aptian fault reactivation event. Due to increased sediment influx in combination with high subsidence rates during the Albian to Cenomanian, potential pre-Aptian source rock units appear to have undergone too deep burial in the Tromsø and Bjørnøya basins to be presently generative. Furthermore, organic matter dilution due to increased sedimentation rates seems to have reduced the overall potential of the upper Cenomanian unit.

Predicting marine organic-rich deposits using forward stratigraphic modelling: The Jurassic Najmah source rock – Case study

Predicting the distribution and heterogeneity of marine Mesozoic organic-rich rocks is a challenging task that requires multi-disciplinary data integration supported by innovative numerical modelling. This study aims at investigating the factors controlling marine organic matter production, accumulation, and preservation along the eastern Arabian intrashelf basins. Such types of basins exhibit an enhanced lateral and vertical facies variability caused by the interplay between shallow and deeper water sedimentation. The work focuses on characterizing the Mid-Upper Jurassic Najmah Formation that presents source rock, reservoir, as well as seal characteristics along Kuwait. It comprises high maturity levels (>0.6% Ro) and TOC values ranging between 2 and 30%. Forward stratigraphic modelling that includes organic matter production, transport, and preservation (i.e., degradation and dilution) was utilized. Models corresponding to two end-member basin types (restricted and fully open marine) have been simulated to mimic the organic matter deposition within a 3rd-order sequence stratigraphic framework. Simulation outputs provide a qualitative and quantitative assessment of organic matter characteristics (TOC, HI, and OI) at the time of deposition. Results show that (1) a combined variable eustatic sea level change and tectonic subsidence are the main driving mechanisms for sediment accommodation during the Mid-Upper Jurassic time, (2) strong anoxic conditions in the restricted basin simulations promote the preservation of organic-rich deposits in the distal northern sectors while (3) the influence of open marine bottom current oxygenation is not expected to impact the relatively shallower conditions affecting the intrashelf basin (<200 m depth), (4) organic matter dilution can be observed along the southernmost sectors of Kuwait due to high sedimentation rates related to the development of gravity driven deposits. Sensitivity analysis on input parameters (i.e., primary productivity vs water depth) supports the partially restricted basin conditions with occasional connection to the open ocean during eustatic sea level rise. This study contributes to the improvement of basin models in conventional as well as unconventional petroleum system exploration.

What happened to the organic matter from the Upper Cretaceous succession in Guatemala, Central America?

Organic palynological content in rocks, which commonly occurs in aquatic and terrestrial sediments, is essential for sediment dating, palaeoenvironmental reconstructions and determination of hydrocarbon potential. Unfortunately, the palynological signal may be blurred or even completely destroyed by intra- and post-depositional diagenetic processes. Fossil shells of foraminifera constitute another tool for dating and/or palaeoenvironmental reconstructions and, unlike palynological matter, they are not affected by post-depositional oxidation or thermal alteration. Herein, we present a preliminary palynological study of Upper Cretaceous formations in northern and central Guatemala and we compare it to foraminiferal data. Rocks that are often full of foraminifera, visible without magnification, yielded monotonous palynofacies dominated by black-opaque equidimensional phytoclasts. The palynological signal may potentially be explained by: (i). unfavourable aquatic conditions and habitat; (ii). a high-sedimentation rate combined with organic matter dilution; (iii). organic matter oxidation related to subaerial exposure; (iv). post-depositional weathering and oxidation; (v). thermal conversion of organic matter during burial and/or interaction with volcanic intrusions. Due to the complicated tectonic history of the Yucatán region, all of these factors need more extensive discussion.

The Middle Jurassic to lowermost Cretaceous in the SW Barents Sea: Interplay between tectonics, coarse‐grained sediment supply and organic matter preservation

AbstractSyn‐rift successions contain both prolific organic‐rich and coarse‐grained rocks, but these rocks are usually studied independently. Thus, the interplay among organic matter deposition, tectonics and sediment supply is not commonly assessed. In this study, we use well logs, Total Organic Carbon (TOC) content, biostratigraphy and seismic data from the SW Barents Sea to unravel the variability of organic‐rich successions and its interaction with potential reservoir rocks in areas affected by active normal faulting versus tectonically stable areas. Four Transgressive‐Regressive (T‐R) sequences were defined for the Middle to Upper Jurassic Fuglen Formation and for the organic‐rich Upper Jurassic to lowermost Cretaceous Hekkingen Formation. Three TOC trends, controlled by palaeobathymetric variations, and organic matter dilution, were identified within the two main organic‐rich sequences (sequences 2 and 3): (a) wells with the highest TOC values (&gt;10 wt %) at the base of the succession; (b) wells with the highest TOC values at the top of the succession and (c) wells with high TOC values at the base and top of the succession. Our interpretation indicates that fault activity controlled the TOC trends in two different ways: Firstly, by creating a sharp topographic contrast‐triggering hyperpycnal flows, and shallowing the footwalls, where higher oxygen content and less developed stratified waters lowered organic matter preservation. Secondly, by significantly increasing the input of clastic material, which inherently led to dilution of organic matter and resulted in lower TOC values (&lt;6 wt %). We interpret that sand deposition was controlled by the size and geomorphology of the sediment source areas. The western part of the study area (i.e. the Loppa High) was characterized by uplifted footwall islands and localized sands along their flanks, whereas the southern part (i.e. the Finnmark Platform) constituted a larger sediment source area for the Volgian age. This work has implications for a better understanding of the distribution of reservoir and source rocks in active rift basins.

Turbidite‐induced re‐oxygenation episodes of the sediment‐water interface in a diverticulum of the Tethys Ocean during the Oceanic Anoxic Event 1a: The French Vocontian Basin

AbstractWidespread anoxic events affected the Tethys Ocean during the Mesozoic. The Ocean Anoxic Event 1a (Early Aptian), expressed as the Selli Level or Goguel Level (GL) in European basins. The GL was deposited in the French Vocontian Basin, a semi‐enclosed basin connected to the Tethys Ocean. This study presents an integrated approach (Rock Eval, clay minerals, grain‐size, inorganic geochemistry and molecular biomarkers), applied to four sections in the basin distributed along a proximal‐distal transect. This study shows that the GL was perturbed by turbidites. In particular, the results demonstrate: (a) a homogeneous redox status of the basin that experienced oxic to suboxic conditions, according to trace element distributions and (b) low organic matter contents (total organic carbon ca 1 wt%) in the three sections where turbiditic deposits are observed. The distal, condensed section exhibits the highest organic matter contents (total organic carbon &gt;3 wt%). In addition, the presence of gammacerane and isorenieratene derivatives in the distal sections suggests that the water‐column was intermittently stratified, with hypoxia developing in the photic zone. This stratification did not result from strong surface productivity but more likely from: (a) limited renewal of deep water in the basin; (b) reputedly high surface‐water palaeotemperatures during the Early Aptian; and (c) the influx of waters, possibly depleted in free oxygen and in some dissolved trace elements, into the basin. The turbiditic inputs, in addition to organic matter dilution in the sediments and a brief rupture of the water‐column stratification in the proximal areas of the basin, ventilated the sea floor and more specifically re‐oxidized the sediment‐water interface as well as underlying sediments. Such episodes of benthic re‐oxygenation could have altered the long‐term palaeoredox record, even in the distal sections where reducing conditions prevailed during deposition. In the area deprived of turbiditic input, sedimentary condensation, coupled with low oxygen conditions, furthered organic matter preservation and concentration.

Depositional and organic carbon-controlled regimes during the Coniacian-Santonian event: First results from the southern Tethys (Egypt)

Abstract Geochemical and palynological proxies were determined for 31 samples representing the upper Coniacian-lowermost Campanian Abu Roash A Member, collected from the BED2-3 well. This was done to assess the prevailing paleoredox conditions, productivity levels, and water-column settings to: (1) understand controls and mechanisms related to accumulation of organic carbon within the sedimentary record; (2) assess principal environmental processes that triggered the preservation of organic matter-poor, calcareous shale and limestone facies of the Abo Roash A Member; and (3) to interpret the paleoceanographic settings and confirm the occurrence of either Oceanic Anoxic Event 3 (OAE3) or Cretaceous Oceanic Red Beds (CORBs) within the Abu Gharadig Basin in the southern part of the Tethyan Ocean, Egypt. Sedimentation rates were reconstructed based on the biostratigraphic-age constraints of marine dinoflagellate cysts (dinocysts). Trace elements-based ratios and relationships were used in order to study the role of clastic terrestrial input. Three groups of dinocysts and freshwater Pediastrum algae were used to study the water-column conditions. Results showed that the Abu Roash A Member was deposited under oxic to intermittently dysoxic conditions in a distal inner to middle neritic environment, with varying sedimentation rates from 32 to 119 mm kyr−1. Productivity appeared to be a significant factor for determining magnitudes of the organic carbon accumulation; however, low productivity was prevalent during deposition. Carbonate content was high, which resulted in dilution of organic matter during varied rates of sediment input. Such results indicate that the paleoceanographic conditions in the southern Tethys witnessed the absence of prevalent oceanic anoxia (i.e., the Coniacian-Santonian OAE 3) and deposition of the CORBs as in several parts of the Tethys, the Pacific, and the Indian oceans.

Sedimentary environment and the controlling factors of organic-rich rocks in the Lucaogou Formation of the Jimusar Sag, Junggar Basin, NW China

The Lucaogou Formation in the Jimusar Sag of the eastern Junggar Basin is an important sedimentary stratum accumulating huge amounts of lacustrine tight oil in China, where organic-rich rocks are commonly observed. Focusing on the Lucaogou Formation, a precise analysis of the inorganic and organic petrology and the inorganic geochemistry characteristics was conducted. The paleoclimate and paleoenvironment during sedimentation of the Lucaogou Formation were established, and the key factors that were controlling the accumulation of organic matter during this time were identified. The results of this study suggest that during the sedimentation of the Lucaogou Formation, the paleoclimate periodically changed from a humid environment to an arid environment. As a result, the salinity of the water and the redox environment fluctuated. During the sedimentation period, the lake showed sufficient nutrient supplies and a high primary productivity. The interval studies in the Lucaogou Formation were divided into five sedimentary cycles, where the first, second, and fifth sedimentary cycles consisted of cyclical paleoclimate fluctuations varied from a humid environment to an arid environment and shifted back to a humid environment with levels of salinity from low to high and decreased again. The third and fourth cycles have cyclical fluctuations from a humid to an arid environment and corresponding salinity variation between low and high levels. During the period when organic-rich rocks in the Lucaogou Formation deposited in the Jimusar Sag, the paleoclimate and the water body were suitable for lower aquatic organisms to flourish. As a result, its paleoproductivity was high, especially during the early period of each cycle. A quiet deep water body is likely to form an anoxic environment at the bottom and is also good for accumulation and preservation of organisms. Fine-grained sediments were accumulated at a low deposition rate, with a low dilution of organic matter. Therefore, high paleoproductivity provided a sufficient volume of organisms in the studied area in a quiet deep water body with an anoxic environment and these were the key factors controlling formation of organic-rich rocks.

Bottomset and foreset sedimentary processes in the mixed carbonate-siliciclastic Upper Jurassic-Lower Cretaceous Vaca Muerta Formation, Picún Leufú Area, Argentina

The Upper Jurassic-Lower Cretaceous Vaca Muerta Formation at the Picún Leufú area constitutes bottomset and foreset marine deposits comprising open bay, siliciclastic basin and mixed carbonate-siliciclastic shelf facies. Detailed sedimentological, ichnological and sequence stratigraphic analysis of six stratigraphic sections allows establishing two depositional sequences. Depositional Sequence 1 (DS1) begins with lowstand eolian deposits included in the Quebrada del Sapo Formation, and continues with thin, retrograding, transgressive open bay facies of the Vaca Muerta Formation encompassing coastal sand sheet and dune, marginal marine, bay margin and distal bay facies. Above the transgressive deposits, the regressive hemicycle of DS1 consists of bottomset deposits representing a siliciclastic basin facies association. Lobe and lobe fringe hyperpycnal flow sedimentation alternated with hemipelagic basinal deposition in an oxygen-deficient environment. A relative sea-level fall generated a sequence boundary, which coincides with an angular unconformity that marks the base of Depositional Sequence 2 (DS2). This sea-level fall triggered the formation of an extensive lowstand channel-fill and lobe complex at the base of the slope, followed by retrogradational lobe facies during the subsequent transgression. The regressive hemicycle of DS2 represents foreset mixed carbonate-siliciclastic shelf facies, forming a slope mud belt and slope sand bodies. Near the top, sedimentation in a foreset-topset transition resulted in the accumulation of bioturbated mixed slope and sandy shoal deposits. Sand bars and lagoonal facies occur on top of all sections and are truncated by a sequence boundary. The combined analysis provides insights into the sedimentary processes affecting bottomset and foreset deposition, underscoring the role of wave-influenced hyperpycnal flows, and the effect on organic matter dilution in one of the most important unconventional reservoirs from Argentina.

Control actions of sedimentary environments and sedimentation rates on lacustrine oil shale distribution, an example of the oil shale in the Upper Triassic Yanchang Formation, southeastern Ordos Basin (NW China)

Abstract Oil shale is an important form of unconventional resources. It is significant to determine the control factors on oil shale distribution. Abundant oil shale resources are hosted in the Member Chang 7–3 (Ch7-3), Triassic Yanchang Formation in southeastern Ordos Basin (NW China). This study was conducted to investigate the sedimentary environment, distribution and organic matter source of the Ch7-3 Oil shale by a combined study of outcrops, core data, organic and element geochemical indices. The controlling factors of the oil shale distribution were determined by an analysis of sedimentary environments and geochemical features of the oil shale. Sedimentary facies analysis based on outcrops and core data indicated that the Ch7-3 Oil shale was deposited in lacustrine-delta environments. Warm and moist climates and reducing conditions indicated by major and trace elements were favorable to organic matter production and preservation. Rock-Eval pyrolysis showed that the organic matter of the Ch7-3 Oil shale is from both lacustrine phytoplankton and terrigenous higher plants. The organic matter from terrigenous higher plants was probably transported into the deep lake by gravity flows. Sedimentary facies and (La/Yb)N values show that the spatial distribution of oil shale was controlled by sedimentary environments and sedimentation rates. In the deep lake area near to delta-turbidite fan deposition systems, terrestial organic matter input was bounteous, and sedimentation rates were moderate, which were beneficial to organic matter accumulation and preservation, providing favorable conditions for oil shale formation. In delta-turbidite fan deposition systems, high sedimentation rates led to organic matter dilution. In the deep lake area far from delta-turbidite fan deposition systems, sedimentation rates were relatively low, and organic matter was easily degraded, in addition only lacustrine organic matter input with no terrestial organic matter input inhibited organic matter enrichment.

НАКОПЛЕНИЕ УРАНА И ИТТРИЯ КОСТНЫМ ДЕТРИТОМ В УГЛЕРОДИСТЫХ ПОРОДАХ П-ОВА КАМЧАТСКИЙ МЫС

The carbonaceous rocks composing the beds in the Cretaceous carbonate-siliceous paleoceanic sediments of the Kamchatsky Mys Peninsula (Eastern Kamchatka) were studied using a scanning electron microscope. In the matrix, which consists of organic and siliceous matter, abundant bone debris and phosphate coprolites have been revealed. In the fragments of fish bones, the authors have revealed microinclusions enriched with uranium and, to a lesser extent, yttrium. The accumulation of these elements is associated with their sorption from seawater and from sediment in bone debris. The concentration of uranium was boosted by euxinic conditions in the near-bottom waters caused by high biological productivity in the surface waters of the ocean, as well as low sedimentation rate, which prevented the dilution of organic matter and biogenic phosphates by terrigenous material and promoted long-term exposure of bone debris at the bottom.

Geochemical and petrographic characteristics of Wufeng-Longmaxi shales, Jiaoshiba area, southwest China: Implications for organic matter differential accumulation

Abstract The organic matter-rich shales in Wufeng-Longmaxi Formation have been considered to be a high-quality source rock and a main shale-gas producer in southern China. As the material basis of shale gas, the organic matter content in shales exhibits a notable vertical difference in the Jiaoshiba area of Fulin shale gas field. This study integrated detailed geochemical analysis and sedimentary observations to investigate organic matter enrichment and depletion mechanism. Three intervals are identified by organic matter contents and their according variations. Six major lithofacies are identified based on mineral composition X-ray diffraction (XRD) analyses, microscopic observations and Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) data. Geochemical redox proxies (Mo-EF, U-EF, V/Cr and Corg/P) indicate that organic matter-rich shales of Wufeng and bottom Longmaxi formation are deposited in anoxic conditions, whereas organic matter-lean shales of the middle to top Longmaxi formation are deposited in oxic environment. This pattern is influenced by both worldwide marine transgression and regional tectonic movement. The productivity proxies (P/Al, Cu/Al, Ni/Al, SiO2-bio) indicate that the organic matter-rich Wufeng-Longmaxi shales were deposited under moderate to high surficial primary productivity. Increasing terrigenous influxes dominated organic matter dilution and may have been promoted by regional tectonic uplift. The organic matter enrichment or depletion mechanisms in each unit are controlled by different geological events. Enrichment was controlled by multiple reasons, including both global and regional factors, whereas depletion was mainly dominated by regional tectonic uplift. Additionally, the sequence stratigraphic boundaries coincide with organic matter shifting interfaces, indicating that high-resolution sequence stratigraphy of fine-grained sediments may have a certain effect on optimal reservoir predication. Conversely, boundaries of TOC and geochemical proxies may also help to subdivide sequence boundaries.