High Total Organic Carbon Content Research Articles

The spatial distribution of sedimentary compounds and their environmental implications in surface sediments of Lake Khar Nuur (Mongolian Altai)

AbstractLake sediments are valuable natural archives to reconstruct paleoclimate and paleoenvironmental changes which consist of inorganic and organic sediment compounds of allochthonous origin from the catchment and of autochthonous production in the lake. However, for robust paleo‐reconstructions it is important to develop a better understanding about sedimentation processes, the origin of inorganic and organic sediment compounds and their distribution within the lake. In this context, modern process studies provide important insights, although environmental and anthropological changes can affect the spatial distribution of sediment compounds through time. Therefore, in this study the spatial distribution of grain size and geochemical proxies in 52 surface sediment samples from Lake Khar Nuur, a small high‐altitude lake in the Mongolian Altai with a small and anthropogenically used hydrological catchment, is investigated. The results show a distinct sediment focussing in the two deep basins of the lake, which therefore act as accumulation zones. In those accumulation zones, total organic carbon (TOC), total nitrogen (N) and their isotopic composition (δ13CTOC, δ15N) as well as n‐alkanes indicate that organic sediment compounds are a mixture of both allochthonous and autochthonous origin. While the recent catchment vegetation consists of grasses/herbs and the shrub Betula nana (L.) with distinct differences in their n‐alkane homologue patterns, those differences are not reflected in the sediment surface samples which rather indicates that grass‐derived n‐alkanes become preferentially incorporated in the lake. Extensive anthropogenic activity such as grazing and housing in the southern part of the catchment causes soil erosion which is well reflected by high TOC, N and sulphur (S) contents and 15N depleted δ15N values at the central southern shore, i.e. increased allochthonous sediment input by anthropogenically‐induced soil erosion. Overall, the surface sediments of Lake Khar Nuur origin from allochthonous and autochthonous sources and are focussed in the accumulation zones of the lake, while their distribution is both environmentally and anthropogenically driven.

Characteristics of major and trace elements in surface sediments of the Makran Accretionary Prism, Pakistan and their implications for natural gas hydrates

To accurately identify the natural gas hydrates (NGH) in the sea area of the Makran Accretionary Prism, Pakistan, this paper presents the testing and analysis of major and trace elements in sediment samples taken from two stations (S2 and S3) in the area by the China Geological Survey. As shown by testing results, all major elements are slightly different in content between the two stations except SiO2 and CaO. This also applies to the trace elements that include Sr and Ba primarily and Cr, Ni and Zn secondarily. It can be concluded in this study that the tectonic setting of the Makran Accretionary Prism is dominated by oceanic island arc and that provenance of the Makran Accretionary Prism is dominated by felsic igneous provenance, which is at the initial weathering stage and mainly consists of granodiorite. Besides terrigenous detritus, there are sediments possibly originating from Makran-Bela Ophiolite from the northwestern part and Murray Ridge igneous rocks from the southeastern part. The V/Cr, Ni/Co, and V/(V+Ni) ratios indicate that sediments of the two stations are in an oxidation-suboxidation environment. However, the authors infer that the sedimentary environment of the sediments 3.0 m below the seafloor tends to be gradually transformed into a reduction environment by comparison with the Qiongdongnan Basin in the South China Sea where NGH has been discovered. The sediments in the Makran Accretionary Prism are rich in organic matter, with total organic carbon (TOC) content greater than 1%. According to comprehensive research, the organic matter in the sediments mainly originates from marine algae and has high TOC content, which is favorable for the formation of NGH.

Analysis of Developmental Characteristics and Dominant Factors of Pore-Fracture Systems in Lower Cambrian Marine Shale Reservoirs: A Case Study of the Niutitang Formation, Fenggang Block, Southern China.

Due to breakthroughs in the Lower Silurian Longmaxi Formation in the Sichuan Basin and multiple strata around the basin, the northern part of Guizhou adjacent to the Sichuan Basin has become a key area for shale gas exploration. Compared with the Longmaxi Formation, the Niutitang Formation displays greater TOC (total organic carbon) content, depositional thickness and distribution area, but the details remain undetermined. In the study area, the Lower Cambrian Niutitang Formation typically has high TOC content, maturity and brittle mineral content. The study area has experienced multiple periods of tectonic movement, which have great influence on the fracture and pore characteristics. The fractures are mainly structural fractures and have obvious zoning. The primary types of pores are intraparticle pores, organic matter pores, and interparticle pores. Further, macropores and mesopores less than 50 nm contribute most of the pore volume, while pores less than 2 nm contribute most of the specific surface area. Many factors affect the pore-fracture system, such as tectonism, TOC content, mineral composition, and sedimentary environment. Tectonic movements produce fractures based on the changing stress field, but the degree of fracture development does not agree well with the degree of pore development. The TOC content has good positive correlations with the development of fractures and micropores, especially for nanoporosity, while clay minerals show a negative correlation with the development of fractures but a strong positive correlation with the growth of micropores. Quartz displays a positive correlation with the development of fractures but no good correlation with pore development. Finally, the lithofacies, lithologies and mineral compositions under the control of sedimentary environments are internal factors that can impact the development of pore-fracture systems.

Stock and indices of carbon management under different soil use systems

The aim of this study was to evaluate the stock of total organic carbon (TOC) and to perform the physical-granulometric fractionation of soil organic matter (SOM) in different management systems (MS). Three MS and one reference area of Native Forest (NF) were studied, and the three systems were sugarcane (SC), permanent pasture (PP) and no-tillage system (NTS). Soil samples were collected in the 0–0.05, 0.05–0.10, 0.10–0.20-m layers. Soil density (Sd), TOC, stratification index (SI), carbon stock (StockC), variation in StockC (∆StockC), carbon content of particulate organic matter (C-POM) and mineral organic matter (C-MOM), carbon stock index (CSI), lability (L), lability index (LI), and carbon management index (CMI) were determined. The MS presented higher Sd than the NF area. The NF area had higher TOC contents in the first layers, reaching 25.40 g kg-1 in the 0–0.05-m layer, with the PP area having higher values than the NF in the 0.10–0.20-m layer. The NF area showed the highest levels of C-POM (15.25 g kg-1) and C-MOM (10.15 g kg-1) in the first layer. In the 0.10–0.20-m layer, the PP and NTS systems were superior to the others. Regarding the C-MOM content, SC and PP showed higher levels in the 0.10–0.20-m layer. The highest CMI values were observed in the NTS and PP areas in the 0.10–0.20-m layer. The MS increased the Sd and reduced the TOC levels. The different MS modified the POM fraction, and the MOM fraction was most impacted by the SC area. The lability of the SOM was altered by the MS in the most superficial layers.

Total organic carbon and the preservation of organic-walled microfossils in Precambrian shale

Abstract Much of our understanding of early eukaryote diversity and paleoecology comes from the record of organic-walled microfossils in shale, yet the conditions controlling their preservation are not well understood. It has been suggested that high concentrations of total organic carbon (TOC) inhibit the preservation of organic fossils in shale, and although this idea is supported anecdotally, it has never been tested. Here we compared the presence, preservational quality, and assemblage diversity of organic-walled microfossils to TOC concentrations of 346 shale samples that span the late Paleoproterozoic to middle Neoproterozoic in age. We found that fossil-bearing samples have significantly lower median TOC values (0.32 wt%, n = 189) than those containing no fossils (0.72 wt%, n = 157). Preservational quality, measured by the loss of surface pattern, density of pitting, and deterioration of wall margin, decreases as TOC increases. Species richness negatively correlates with TOC within the ca. 750 Ma Chuar Group (Arizona, USA), but no relationship is observed in other units. These results support the hypothesis that high TOC content either decreases the preservational quality or inhibits the preservation of organic-walled microfossils altogether. However, it is also possible that other causal factors, including sedimentation rate and microbial degradation, account for the correlation between fossil preservation and TOC. We expect that as TOC varies in space and time, so too does the probability of finding well-preserved fossils. A compilation of 13,940 TOC values spanning Earth history suggests significantly higher median TOC levels in Mesoproterozoic versus Neoproterozoic shale, potentially biasing the interpreted pattern of increased eukaryotic diversity in the Tonian.

Different accumulation mechanisms of organic matter in Cambrian sedimentary successions in the western and northeastern margins of the Tarim Basin, NW China

Lower Cambrian black shales are widely distributed in the Tarim Basin, but those in the Keping area in the west have a high total organic carbon (TOC) content different from those in the Kuluketage area in the northeast of the basin. The Cambrian Yuertusi Formation in the Shiairike section (Keping) and the Xishanbulake Formation in the Yaerdangshan section (Kuluketage) are time-equivalent and can be correlated. Black shales of the Yuertusi Formation have lower δ13CKer values (<−34‰) than the Xishanbulake Formations (δ13CKer values > − 4‰). These values suggest that black shales in the Yuertusi Formation were dominated by benthic algae, whereas rocks in the Xishanbulake Formation contain substantial amounts of planktonic algae. Analyses of trace elements demonstrate an anoxic depositional environment, with high productivity in both the Yuertusi and Xishanbulake Formations. The enhanced productivity was promoted by upwelling fluids and hydrothermal events, leading to the enrichment of nutrient elements such as barium, cadmium, copper, phosphorus, nickel, vanadium, and zinc. More terrigenous inorganic detritus input to the Xishanbulake Formation, which led to less amounts of organic matter and degradation of planktonic algae during sedimentation, resulted in a lower TOC content than the Yuertusi Formation.

Geochemistry and shale gas potential of the lower Permian marine-continental transitional shales in the Eastern Ordos Basin

Marine–continental transitional strata were widely developed in the Ordos Basin in Upper Carboniferous - Lower Permian. The Taiyuan - Shanxi Formation possesses promising shale gas exploration layers. Shale samples from two drilling wells of Shanxi-Taiyuan Formation in Shilou and Xixian, Ordos Basin, were investigated to study their carbon–sulfur contents and distribution characteristics of organic components using carbon/sulfur analyzer and gas chromatography–mass spectroscopy. Using results of total organic carbon analyses, Rock-Eval pyrolysis, X-ray diffraction analysis, shale gas desorption experiments, and other relevant experimental data, the shale samples were comprehensively analyzed. The exploitability of the shale in the study area was evaluated. The Shanxi-Taiyuan Shale in the Shilou and Xixian areas was characterized by high total organic carbon contents of 7.1% and 2.1% and high Tmax values of 499 and 505 °C, respectively. The organic matter of the shale is types II2 and III. Moreover, biomarker parameters including n-alkanes, Paq, Pwax, average carbon chain length, and the ternary diagram of C27-C28-C29 steranes show the organic matter constituted terrestrial higher plants and aquatic low biological algae. Multiple n-alkane parameters show the organic matter input in the Shilou area is mainly derived from terrestrial higher plants. The Pr/Ph value and trace element indicators show the deposition environment is dominated by weak oxidation–reduction conditions. A shale gas desorption experiment shows the average desorbed gas contents of the shale samples in the Shilou and Xixian areas were 1.79 and 0.37 m3/t, respectively. The organic matter content determined the differences in shale gas properties between the two areas in Ordos Basin. The composition and content of inorganic minerals affect the reservoir physical properties. According to the analyses, the shale in the Shilou area has good shale gas reservoir characteristics in terms of desorbed gas content and the above-mentioned geochemical parameters. Furthermore, the Shanxi shale has good potential for shale gas industrial exploitation.

Geochemical and organic petrographic characteristics of high bituminous shales from Gurha mine in Rajasthan, NW India

A high bituminous shale horizon from the Gurha mine in the Bikaner sub-basin of the Rajasthan District, NW India, was studied using a collection of geochemical and petrological techniques. This study investigated the nature and environmental conditions of the organic matter and its relation to the unconventional oil-shale resources of the bituminous shale. The analyzed shales have high total organic carbon and total sulfur contents, suggesting that these shale sediments were deposited in a paralic environment under reducing conditions. The dominant presence of organic matter derived from phytoplankton algae suggests warm climatic marine environment, with little connection to freshwater enhancing the growth of algae and other microorganisms. The analyzed bituminous shales have high aquatic-derived alginite organic matters, with low Pr/Ph, Pr/n-C17, and Ph/n-C18 ratios. It is classified as Type II oil-prone kerogen, consistent with high hydrogen index value. Considering the maturity indicators of geochemical Tmax (< 430 °C) and vitrinite reflectance values less than 0.40%VRo, the analyzed bituminous shale sediments are in an immature stage of the oil window. Therefore, the oil-prone kerogen Type II in the analyzed bituminous shales has not been cracked by thermal alteration to release oil; thus, unconventional heating is recommended for commercial oil generation.

Redox fluctuations and organic accumulation on the outer shelf of the Early Cambrian (Ages 2–3) Yangtze sea: Geochemical records on a seaward submarine sill

The Early Cambrian was a critical interval marked by dramatic oceanic, geochemical, and biological changes in Earth's history. Although numerous studies have documented oceanic redox changes and revealed a heterogeneous pattern from the inner shelf to outer shelf margin in the Yangtze Block, less attention has been paid to their palaeogeographic‐specific occurrences, particularly in the deeper basinal regime during that time. To better understand marine redox variations and organic accumulation across the entire outer shelf‐basin transect, we selected a coeval organic‐rich section (i.e., Guantang section, western Hunan) deposited on a farther seaward submarine sill for a comparison. Integrated geochemical data of iron speciation, redox‐sensitive trace element [i.e., molybdenum (Mo) and uranium (U)] concentrations and total organic carbon (TOC) contents as well as P/Al and Ba/Al ratios, in combination with previously published data from the outer shelf slope‐basin, reveal an overall high TOC content on the sill and further attest to the occurrence of a fluctuating euxinic water wedge overlying the deep ferruginous water on the outer shelf slope to basin transect in the presence of oceanic upwelling system. The fluctuating euxinic watermass could be dynamically advanced upslope and retreated downslope in the pace of sea‐level rise and fall. All these signify the critical roles of palaeogeographic setting and eustatic changes in controlling the spatiotemporal productivity and redox fluctuations in the Yangtze sea during the Early Cambrian.

Soil organic carbon and biological indicators of uncultivated vis-à-vis intensively cultivated soils under rice–wheat and cotton–wheat cropping systems in South-Western Punjab

Soil carbon (C) pools and biological indicator plays an important role in maintaining soil quality. Land use change from naturally undisturbed to intense cultivation had significant impact on C storage due to change in soil biological properties. The present study was conducted to understand the impact of land use change from the uncultivated to intensively cultivated soils under rice–wheat and cotton–wheat cropping system on labile and stable C pools, soil microbial and enzymatic activity, and a change in nutrient availability. The Olsen’s P concentration was significantly (p < 0.05) higher in soils under rice–wheat, while ammonium acetate (NH4OAc)-K was higher in the uncultivated soils, compared with others. Soils under cotton–wheat cropping system had significantly lower total organic carbon (TOC) in contrast to rice–wheat soils. The uncultivated soils had the highest TOC concentration, which was ∼20.2% higher than rice–wheat and ∼46.4% than the cotton–wheat soils. The water extractable organic carbon (WEOC) was the smallest organic C fraction (∼0.31%–0.40% of TOC), and was significantly lower in cotton–wheat soils. The basal soil respiration (BSR) and mineralization quotient (qM) were significantly higher for rice–wheat, compared with the cotton–wheat soils. These microbial indices were improved due to increased alkaline phosphates (Alk-P) and dehydrogenase (DHA) enzymatic activity in soil. Soil microbial biomass C (MBC) was significantly higher (by ∼31.7%–57.3%) in the uncultivated than the cultivated soils which has increased the C mineralization in soil. The stable C pool comprised ∼68.8% of TOC in the uncultivated soils, almost similar to rice–wheat soils (∼68.5%), but higher than cotton–wheat soils (∼61.9%). The stable C pool leads to C rehabilitation indicated by higher C management index for rice–wheat soils. The principal component analysis (PCA) distinct the uncultivated soils from the cultivated soils under two cropping systems based on BSR and labile C (Fract. 2) in PC1 (explaining ∼82.7% of total variability), and the microbial (qmic) and mineralization (qM) quotient in PC2 (∼17.3% variability). These indicators were most influential for studying C dynamics of the natural ecosystem and restoring the cropland ecosystem by suitable soil management practices.

Primary productivity and basin redox conditions within the Mesoproterozoic Hongshuizhuang Formation from Chaoyang area, Liaoxi sag

The Mesoproterozoic Hongshuizhuang Formation in Liaoxi sag, northern China, is an important hydrocarbon source unit having good potentials to generate oil and gas. The total organic carbon (TOC) content ranges from 0.54% to 4.07% in the Chaoyang area. Trace element geochemistry was utilized to study the primary productivity and basin redox conditions during the deposition of Hongshuizhuang Formation black shales. The results suggest that primary productivity during the deposition of the Hongshuizhuang Formation was relatively high. The redox-sensitive elements results indicate that the Hongshuizhuang Formation was deposited under euxinic or anoxic bottom water conditions. The degree of environment restriction, diagenesis, reoxygenation, and the presence of free H2S can make some of the redox-sensitive elements proxies and primary productivity proxies not applicable. The high TOC content of the Hongshuizhuang Formation is contributed by high primary productivity in the surface water combined with euxinic/anoxic in the bottom water.

Characterization of methane adsorption on shale of a complex tectonic area in Northeast Guizhou, China: Experimental results and geological significance

Complex tectonic settings are generally considered to be unfavorable for shale gas accumulation, but a favorable desorption-gas content has been detected in Lower Cambrian Lujiaping Shales in Dabashan fold-thrust belt in northeastern Chongqing, Southern China, which suggests a good exploration potential. High-pressure (~30 MPa) and high-temperature (~120 °C) (HP-HT) methane adsorption isotherms were conducted on seven samples collected at 2665–2707 m below the ground surface from the Lower Cambrian Niutitang Shale (equivalent to the Lujiaping Shale) from northeast Guizhou Province, Southern China to assess methane adsorption capacity and the influence of total organic carbon (TOC) content, thermal maturity, mineralogy, and temperature-pressure conditions on methane adsorption in over-mature shales in complex tectonic areas. The samples contain high TOC contents, ranging from 5.1% to 8.4%, and equivalent vitrinite reflectance Ro values ranging from 3.62% to 3.85%. Over the experimental pressure range up to 30 MPa, Langmuir volumes correlate positively with TOC contents, which may be related to the increase in specific surface area and micropores (<2 nm) volume with an increasing TOC content. Organic matter (OM)-hosted micropores and nanometer-sized intergranular pores in clay-OM nanocomposites mostly contribute to the adsorption capacity of this over-mature Niutitang Shale. In addition, the different behaviors of other minerals (quartz, carbonate and illite) under intensive tectonic compression also affect the adsorption capacity of shales. Due to the low porosity of the Niutitang Shale, the free gas content in these samples is relatively low, and the adsorbed gas always makes up the majority of gas-in-place (GIP), hence the “equivalent point” of adsorbed gas and free gas does not exist at depths of <6 km. This suggests that the shallow (<3 km) Niutitang Shale should be primarily targeted for adsorbed gas under low-temperature and high-pressure geological conditions.

Paleoclimatic indication of X-ray fluorescence core-scanned Rb/Sr ratios: A case study in the Zoige Basin in the eastern Tibetan Plateau

The X-ray fluorescence (XRF) core-scanned Rb/Sr ratios of lake sediments have been widely used as a proxy for chemical weathering intensity and past climate change. However, some factors could affect Rb/Sr ratios, causing misinterpretation and limiting its application. In this study, we present a high-resolution XRF core-scanned Rb/Sr record of core ZB13-C1 from the Zoige Basin in the eastern Tibetan Plateau. To validate its application, we correlated this record with the chemical index of alteration (CIA) and other paleoclimatic proxies. Our results showed that (1) the core-scanned Rb/Sr ratios and CIA were reliable proxies of chemical weathering intensity in fine-grained sedimentary sequences; (2) the low values of core-scanned Rb/Sr ratios and CIA were significantly correlated with high total organic carbon content, arboreal pollen content, carbonate content, and C/N ratios, confirming its reliability as a proxy for the Asian summer monsoon intensity; (3) the core-scanned Rb/Sr ratios at core depths of 25–0.3 and 56–17 m were unable to reliably reflect chemical weathering intensity due to both the grain-size effect and the low test accuracy. Our study highlights the need for mutual verification of multiple indicators before accurately applying Rb/Sr as a paleoclimatic proxy in other similar study areas.

Late Toarcian continental palaeoenvironmental conditions: An example from the Cañadón Asfalto Formation in southern Argentina

The Cañadón Asfalto continental Basin preserves terrestrial deposits interbedded with volcanic rocks, providing a unique opportunity for palaeoenvironmental reconstructions of the Jurassic continental realm. This study presents a sedimentological, mineralogical, and geochemical analysis of three lacustrine successions in the Cerro Cóndor area of the Cañadón Asfalto Basin, Argentina. New CA ID-TIMS U–Pb data from tuffaceous deposits indicate a late Toarcian age (179.481 ± 0.059, 179.41 ± 0.13, and 177.27 ± 0.40 Ma), suggesting that the palaeolakes developed contemporaneously to the Chon Aike volcanic activity. The sedimentary successions are composed of carbonate, organic matter-rich mudstone — with up to 8 wt% total organic carbon (TOC) content — sandstone and conglomerate, all with an important contribution of volcanic and volcanogenic material. The clay mineral assemblage dominated by corrensite (chlorite-vermiculite mixed layers), smectite and vermiculite is likely related to hydrothermal activity and alteration of volcanic material, rather than to change in weathering processes and climate. Organic matter preserved in the sedimentary successions has a lacustrine and terrestrial/reworked origin. Changes in the source of carbon dioxide (CO2) and in the type and/or degree of preservation of the organic matter had a major impact on the δ13Corg values. This further exemplifies the necessity to evaluate the influence of the depositional environment before interpretation of the organic carbon isotope records. The combined high phosphorus and TOC contents suggest that episodes of increased nutrient availability into the basin enhanced lacustrine primary productivity, which favoured the development of oxygen-depleted conditions bottom waters and ultimately organic matter preservation and burial. High Corg/Ptot ratios indicate that phosphorus was likely released back into the water column in a positive feedback loop, further sustaining primary productivity. This study provides an important clue to understand the palaeoenvironmental conditions prevailing on land during the late Toarcian and on how volcanism exerted a control on the depositional conditions.

Effects of the type of forest alteration on gross nitrogen mineralization in soils of southern India

Change in land-use practices can result in major shifts in the cycling of various elements, particularly nitrogen (N), which is prone to anthropogenic perturbations. For quantifying these shifts, accurate measurements of rates of biogeochemical transformations of N are needed. We used the (15N) isotope dilution technique to understand the effects of the types of forest alteration on (N) transformation rates by comparing gross N mineralization and ammonium (NH4+) consumption rates in soils of a managed forest, an unmanaged forest, and a rubber plantation in Kerala, India. Overall, nitrate (NO3−) dominated soils of the managed and unmanaged forests, whereas soils in the rubber plantation showed relatively higher NH4+ concentration. Total N (TN) and total organic carbon (TOC) concentrations were the highest under the rubber canopy (TN: 1.49 ± 0.02 mg N g−1; TOC: 7.96 ± 0.86 mg C g−1). In soils of all three forest types, gross N mineralization rates were higher compared to NH4+ consumption rates. Despite high TN and TOC concentrations, the rates of gross N mineralization and NH4+ consumption were considerably lower in the rubber plantation (mineralization: 1.08 ± 0.08 mg N kg−1 d−1; consumption: 0.85 ± 0.09 mg N kg−1 d−) compared to the managed (mineralization: 3.71 ± 0.35 mg N kg−1 d−1; consumption: 2.20 ± 1.41 mg N kg−1 d−1) and unmanaged (mineralization: 2.20 ± 1.07 mg N kg−1 d−1; consumption: 1.39 ± 0.27 mg N kg−1 d−1) forests. The lower NH4+ consumption rates in the rubber plantation led to significantly higher (p < 0.05) residence time of NH4+ (~ 4 days) compared to the managed and unmanaged forests (< 2 days), possibly contributing to acidification of rubber soils (pH ~ 4.8). These results together suggest that replacement of naturally grown forests with a mono-cropped plantation such as rubber negatively impact rates of N transformation processes in tropical soils and imply that change in tree species composition of naturally grown forests can adversely affect soil microbial activity. We recommend intercropping these plantations with commercial crops to maintain soil microbial diversity and biogeochemical cycling for sustainable forest management.

Foraminiferal communities of a mid-Holocene reef: Isla Colón, Caribbean Panama

The distribution of mid-Holocene, tropical benthic foraminiferal assemblages, including species' proportions, diversity, dominance and wall type, were analyzed for their correspondence to marine habitats and invertebrate facies. Benthic foraminifera are useful for paleoenvironmental reconstructions because the modern ecology of many species found as Quaternary fossils is known. Samples were collected from trenches in ~30,000 m2 of an excavated Acropora cervicornis-dominated, mid-Holocene reef with an age of ~6 kyr on Isla Colón (Colon island), bordering Almirante Bay in Bocas del Toro, Caribbean Panama. Bulk sediment samples were collected from a maximum depth of ~7 m below modern mean sea level and classified in the field into five invertebrate biofacies: 1) A. cervicornis-dominated reef, 2) molluscan mud, 3) Porites-Agaricia reef, 4) mixed coral, and 5) Lobatus-dominated seagrass. Sediment carbon and grain size analyses, a cluster analysis performed on the relative abundance of species per sample, and the Fisher's alpha diversity index were used to compare sample similarity and environmental variables to determine habitat relationships. Most samples contained high total inorganic carbon and poorly sorted, medium-coarse sediments. Principle component analysis of sediment grain size and carbon values did not show a clear correspondence among samples, habitat type or location of trenches. Foraminiferal assemblages in A. cervicornis and other reefal samples (categories 1, 3 and 4, above) had the greatest diversity and did not distinguish between the three reefal types, suggesting similar, normal marine conditions and/or mixing of coral fragments. Molluscan mud samples with high total organic carbon content were least diverse with dominant Ammonia and Elphidium taxa, though foraminifera in other molluscan mud samples showed a transition to proximal reef corals. Seagrass samples were differentiated from molluscan mud samples and had similar diversities and species assemblages to the Porites-Agaricia samples, and taxa known to be temporary grazers and living temporarily attached to seagrass blades are present in relatively greater amounts in these samples.Based on the distribution of foraminiferal species across this mid-Holocene coral reef, we conclude that it was a patch reef that included high-organic and seagrass facies similar to those of modern Almirante Bay. Results from this study can be compared to modern foraminiferal studies to investigate whether the modern habitats of BDT are significantly different from the pristine reefs of the mid-Holocene.

An integrated sequence stratigraphic–geochemical investigation of the Jurassic source rocks in the North Yellow Sea Basin, eastern China

The North Yellow Sea Basin (NYSB) is one of the petroliferous basins in the offshore region of eastern China. An integrated sequence stratigraphic–geochemical investigation was carried out to identify and assess the effective source rocks of recent oil discoveries in the NYSB. A total of four third-order sequences (the first [J2SQ1] and second [J2SQ2] third-order sequence within the Middle Jurassic, and the first [J3SQ1] and second [J3SQ2] third-order sequence within the Upper Jurassic) were identified in the Jurassic succession, in which four types of probable source rocks were distinguished, namely, lacustrine deep-water mudstones, lacustrine shallow-water mudstones, prodeltaic mudstones, and deltaic-plain mudstones. The lacustrine deep-water mudstones are widely distributed within the J2SQ1, J2SQ2, and J3SQ2 sequences in the gentle tectonic slope and depression area. They are characterized by moderate total organic carbon (TOC) contents (0.79−3.72 wt. %, with an average of 1.71 wt. %), lower hydrocarbon potential indexes (the amount of free hydrocarbons before analysis [S1] and the amount of hydrocarbons that results from cracking of kerogen [S2], i.e., S1+ S2, in a sample during Rock-Eval pyrolysis) (0.97 mg HC/g rock < S1 + S2 < 7.89 mg HC/g rock, with an average of 3.10 mg HC/g rock), and predominantly type II to type III kerogen. In contrast, the prodeltaic mudstones in J3SQ2 have higher TOC contents (1.5−4.0 wt. %, with an average of 2.71 wt. %), higher S1 + S2 (5.83−25.51 mg HC/g rock, with an average of 14.89 mg HC/g rock), and predominantly type I to type II kerogen. The oil–source correlation results indicate that the oils discovered in the NYSB mainly originated from the prodeltaic J3SQ2 mudstones. The effective source kitchen is mainly located in the northwestern part of the study area. The factors controlling the occurrence of high-quality prodeltaic source rocks are a sufficient supply of nutrients and terrigenous organic matter, high depositional rates, and a suboxic environment.

Molecular differences in sequential extracts obtained by core flooding of the early mature ultra-tight Posidonia Shale

The here analyzed Posidonia Shale (PS) samples have relatively high total organic carbon (TOC) contents greater than 2 wt% and are thermally early mature. They, thus, already experienced some generation of petroleum and contain abundant bitumen in the pore space. Solvent core flooding extraction was performed on five clay- and carbonate-rich early-mature Posidonia Shale plugs using dichloromethane (DCM) as permeating fluid under controlled confining pressure in a tri-axial flow cell. Molecular characteristics and differences were observed in original, sequential and residual extracts obtained by core flooding. Furthermore, porosity and permeability changes due to removal of soluble organic matter were recorded. Our results show that core flooding extraction of the clay-rich shales failed due to their low permeability (3.0–5.0 nD), while sequential extraction was successfully performed on the carbonate-rich shales. Acidic compounds were flushed out first, which indicate that these compounds might be preferentially expelled during the early stage of primary migration. Pristane and phytane are preferentially expelled in the whole process of expulsion as compared to n-alkanes. Composition of the other aliphatic (including n-alkanes, hopanes and steranes) and aromatic (including methylnaphthalenes, phenanthrenes, dibenzothiophenes and methyldibenzothiophenes) compounds of the sequential, original and residual extracts do not exhibit much difference. Statistical data indicate that the original and residual extracts have higher similarity and show lower apparent maturity than the sequential extracts indicating that expelled oil might show higher maturity than the related source rock. This observation might be important for interpreting oil/source rock correlations based on geochemical parameters. Porosities of the carbonate-rich shales were slightly smaller than those of the clay-rich rocks, while the permeability is much higher in the carbonate-rich lithologies. It is worth noting that porosity and permeability did not increase during the extraction of soluble organic matter. On the contrary, due to counteracting compaction effects that reduce pore volume extraction lead to an overall porosity and permeability decrease.

Organic geochemistry of Late Cretaceous Mukalla shales from the Mukalla-Sayhut Basin, Southern Yemen—terrestrial organic matter inputs and gas-resource potential

The Late Cretaceous Mukalla Formation is preserved in the Mukalla-Sayhut Basin of Southern Yemen. Using a combination of organic geochemical and petrological methods, we investigated shale intervals in the Mukalla Formation using the Masilah-1 well. We focused on identifying the organic facies and their hydrocarbon generating potential, which has relevance in terms of attracting future hydrocarbon exploration in this basin. The geochemical analysis was conducted on bulk cutting shale samples and indicated fair–good source rock potential, with moderate–high total organic carbon (TOC) contents of 0.51–3.49 wt%. Rock-Eval hydrogen index (HI) pyrolysis results suggested that the organic matter in the analyzed Mukalla shales was mainly type III kerogen; thus, it is likely to have shale-gas generation potential. The high proportions of type III kerogen are consistent with the distributions of lipid biomarkers and visual kerogen typing. Visual kerogen results showed that the analyzed Mukalla shales mainly contained vitrinite organic matter derived from land plants. The biomarker fingerprints of normal alkane, isoprenoid, sterane, and terpane in the saturated hydrocarbon fraction further suggested high contributions of terrestrial organic matter that deposited in fluvial to fluvial-deltaic environments under oxic conditions. The thermal maturation level was inferred from both chemical and optical maturity indicators and indicated that the shale samples, which were buried at relatively shallow depths in the Masilah-1 well, were of low maturity, i.e., within early-mature oil window.

Environmental response to volcanic activity and its effect on organic matter enrichment in the Permian Lucaogou Formation of the Malang Sag, Santanghu Basin, Northwest China

Lacustrine organic-rich fine-grained rocks are widespread in the second member of the Permian Lucaogou Formation (P2l2) in the Malang Sag, Santanghu Basin, with complex lithology and significant changes of lithofacies due to frequent volcanic activity. In this study, the mineralogical and petrological characteristics, types of organic matter (OM), and effect of volcanism on the sedimentary environment and OM accumulation were analyzed using organic, carbon, and oxygen isotopes, as well as major and trace element data. The results revealed that the P2l2 was characterized by mixed deposition of volcanic ash and carbonate minerals, with four main rock types: tuff, dolomitic tuff, tuffaceous dolomite, and dolomite. The fine-grained rocks dominantly contain type I and II1 kerogens, which were in a low-mature stage and exhibited excellent original hydrocarbon generative potential. When volcanic activity was strong, large amounts of nutrients were released by the volcanic ash, which then entered the lake, in turn promoting the blooming of phytoplankton, and causing the δ13C values in the tuff to decrease. In addition, the paleotemperature decreased for a short time, and the suboxic-euxinic environment was conducive to the preservation of OM, resulting in high total organic carbon (TOC) content in the tuff. In contrast, during intermittent or weak periods of volcanic activity, the climate was hotter and drier, and evaporation resulted in a sustained increase of salinity. Consequently, the growth of algae was restricted, which brought about an increase in the δ13C values and relatively low TOC content in the dolomite. The relationships among primary productivity, redox conditions, clastic influx proxies, and TOC content illustrate that primary productivity was the most vital control on the accumulation of OM. High primary productivity was mainly driven by volcanic ash falls. Meanwhile, high salinity and suboxic-euxinic conditions were beneficial to the preservation of OM, giving rise to a high degree of OM enrichment.