Terrigenous Organic Matter Input Research Articles

Sedimentology and geochemistry of the Upper Permian transitional shale, Western Guizhou, China: Implications for the environmental effect of the Emeishan large igneous province and organic matter accumulation

The marine-continental transitional shale of the Upper Permian Longtan Formation serves as a crucial target for both shale gas exploration and development, as well as a significant research object of the dramatic geological events during the Middle-Late Permian period. In this study, X-ray diffraction (XRD), polished section, scanning electron microscopy (SEM), energy spectrum analysis (EDS), total organic carbon content (TOC) and elemental geochemistry were conducted to elucidate the sedimentological and geochemical characteristics of the transitional shale in the Longtan Formation and the underlying Emeishan basalt in Western Guizhou. Additionally, the paleoenvironment conditions, the environmental effect of the Emeishan large igneous province and the organic matter enrichment mechanism of the transitional shale are investigated.The sedimentological and geochemical proxies reveal that influenced by seawater and river, the transitional shale of the Longtan Formation in western Guizhou was deposited in a marginal tidal flat-lagoon environment under a warm and humid paleoclimate, anoxic condition, brackish to saline bottom water, high terrigenous detrital input and high sedimentation rate condition. The mineral and elemental composition characteristics show that the transitional shale was predominantly derived from Emeishan basalt, supplemented by contributions from volcanic ash and hydrothermal activity. This suggests that following the eruption of the Emeishan large igneous province in Middle-Late Permian transition, the paleogeographic pattern of high in the west and low in the east was formed in the Upper Yangtze region. The weathering products of the Emeishan basalt were the main source of the Longtan Formation. Intense volcanic activities in the earliest Wuchiapingian potentially induced cooling of the climate, and the elements carried by hydrothermal input likely enhanced paleoproductivity and increased seawater salinity, and was conducive to the suboxic-anoxic water condition.The organic matter contents in transitional shale are extremely high with the highest organic matter content (>20%) of the shale deposited in mire, followed by the shale deposited in tidal flat (10%), whereas the lagoonal shale, predominant in the Longtan Formation, displays the lowest organic matter content (5%). The organic matter enrichment mechanism of transitional shale is primarily attributed to a “preservation model”. The suboxic-anoxic condition and brackish-saline water were conducive to the organic matter preservation. The primary productivity was low and organic matter mainly came from terrestrial higher plants. The warm and humid climate strengthened terrestrial weathering and erosion, consequently increasing terrigenous organic matter input and sedimentation rate. Elevated sedimentation rate reduced the decomposition time of organic matter, thereby also facilitating organic matter preservation. However, the correlations between TOC content and geochemical proxies in lagoonal shale suggest that the warmer and more humid paleoclimate may have increased terrigenous detrital influx and diluted the organic matter content.

Early Oligocene marine transgression and organic matter accumulation recorded in the upper Eocene to lower Oligocene Enping Formation of the Baiyun Sag, Pearl River Mouth Basin, South China Sea

The upper Eocene to lower Oligocene Enping Formation provides the primary source rocks for hydrocarbon accumulation in the Baiyun Sag, Pearl River Mouth Basin, South China Sea. Nevertheless, the depositional environment of this formation has been a subject of debate. This study confirms that the lower to middle members of the Enping Formation predominantly consist of lacustrine deposits, and a marine transgression occurred during the deposition of the Enping Formation’s upper member in the Early Oligocene period. This transgression is supported by an increase in paleosalinity proxies (B/Ga, Sr/Ba), greater negative S isotopic values of pyrite, and an increase in triaromatic dinosteroid concentrations in the upper member samples. A series of indicators (chemical alteration index, V/Cr and Ni/Co ratios) suggest that mudstones from the lower–middle member and the upper member were all deposited under warm-humid climate with the dysoxic-oxic water conditions. The mudstones from the lower–middle member and upper member were formed with low-to-moderate primary productivity under relatively strong dilution condition. The paleoredox and paleoproductivity were minimally influenced by marine transgression. Additionally, there is a weak correlation between the total organic carbon content and indicators of these depositional conditions. The input of terrigenous organic matter is a pivotal factor in regulating the accumulation of organic material within the Enping Formation. The multi-stage deltas likely played a significant role in supplying terrigenous organic matter, thereby influencing the distribution of organic-rich mudstone in the Baiyun Sag.

Controlling Factors of Organic-Rich Lacustrine Shale in the Jurassic Dongyuemiao Member of Sichuan Basin, SW China

Organic-rich continental shale, widespread in the Sichuan Basin during the deposition of the Jurassic Dongyuemiao Member (J1d), is considered the next shale hydrocarbon exploration target in southern China. To identify a shale gas sweetspot and reduce exploration risk, it is of great significance to determine the organic matter (OM) enrichment mechanism of J1d shale. In this study, based on sedimentological characteristics and organic matter content, high-resolution major and trace elements were systematically analyzed to demonstrate terrigenous influx, paleoredox, paleosalinity, paleoproductivity, and paleoclimate. The 1st section interval of the J1d 1st submember is dominated by shallow lake subfacies, while the other intervals have the characteristic of semideep to deep lake subfacies. The 1st submember interval of J1d lacustrine shale is characterized by the warmest-humid paleoclimate, strongest weathering degree, highest terrigenous input, moderate paleoproductivity, and paleoredox condition. Within the Dongyuemiao 1st submember, the 4th section interval has the highest paleoproductivity and the most oxygen-deficient condition in bottom water. During the deposition period of the 2nd submember, the sedimentary environment turned to a cold-dry paleoclimate, weak weathering degree, low terrigenous input, low paleosalinity, and high paleoproductivity. Under the background of semideep and deep lake, the terrigenous OM input plays the most critical role in controlling OM enrichment. Moreover, the high primary productivity of lake surface water and the suboxic condition of lake bottom water contribute to the formation of relatively higher TOC lacustrine shale interval in the 4th section of 1st submember.

Petroleum geochemistry and origin of shallow-buried saline lacustrine oils in the slope zone of the Mahu sag, Junggar Basin, NW China

Recently, significant oil discoveries have been made in the shallower pay zones of the Jurassic Badaowan Formation (J1b) in the Mahu Sag, Junggar Basin, Northwest China. However, little work has been done on the geochemical characteristics and origins of the oil in the J1b reservoir. This study analyzes 44 oil and 14 source rock samples from the area in order to reveal their organic geochemical characteristics and the origins of the oils. The J1b oils are characterized by a low Pr/Ph ratio and high β-carotene and gammacerane indices, which indicate that they were mainly generated from source rocks deposited in a hypersaline environment. The oils are also extremely enhanced in C29 regular steranes, possibly derived from halophilic algae. Oil-source correlation shows that the oils were derived from the Lower Permian Fengcheng Formation (P1f) source rocks, which were deposited in a strongly stratified and highly saline water column with a predominance of algal/bacterial input in the organic matter. The source rocks of the Middle Permian lower-Wuerhe Formation (P2w), which were deposited in fresh to slightly saline water conditions with a greater input of terrigenous organic matter, make only a minor contribution to the J1b oils. The reconstruction of the oil accumulation process shows that the J1b oil reservoir may have been twice charged during Late Jurassic–Early Cretaceous and the Paleogene–Neogene, respectively. A large amount volume of hydrocarbons generated in the P1f source rock and leaked from T1b oil reservoirs migrated along faults connecting source beds and shallow-buried secondary faults into Jurassic traps, resulting in large-scale accumulations in J1b. These results are crucial for understanding the petroleum system of the Mahu Sag and will provide valuable guidance for petroleum exploration in the shallower formations in the slope area of the sag.

The organic geochemistry of Permian high latitude sediments in two boreholes in the southern Sydney Basin, New South Wales, Australia

Twenty six samples of Permian high latitude sedimentary rocks from the southern Sydney Basin were analysed using organic geochemical methods for their thermal maturity, organic matter inputs, and depositional environment. Fourteen samples are from the Elecom Clyde River (ECR) DDH01 borehole (inland; Southern Highlands), and 12 are from the ECR-DDH07 borehole (coastal; near Nowra). The analysed formations are the Berry Siltstone, the Nowra Sandstone, the Wandrawandian Siltstone, the Snapper Point Formation and the Pebbley Beach Formation. This study is the first organic geochemical study of samples from the Berry Siltstone and Nowra Sandstone. The formations mostly have total organic carbon varying from 1 to 2 wt%, but the Snapper Point Formation is leaner in TOC (0.2–0.7 wt%). The samples vary from low to very high thermal maturity. The samples from the ECR-DDH01 borehole are in the early to middle of the oil window (vitrinite reflectance equivalent = 0.60–0.80%), based on hopane, sterane, alkylnaphthalene, alkylphenanthrene, alkylbiphenyl, and alkyldibenzothiophene thermal maturity parameters. The samples from the ECR-DDH07 borehole are much more thermally mature, in the late part of the oil window to the early gas window (vitrinite reflectance equivalent = 1.2–1.7%). This high thermal maturity has led to the loss of nearly all polycyclic biomarkers, and the thermal rearrangement of many groups of compounds, including monomethylalkanes. The ECR-DDH01 borehole location and time-equivalent outcrop samples south-west of Ulladulla have similar thermal maturities and are close to the margins of the Sydney Basin, and it is estimated to have previously been covered by about 3 km of overburden. In contrast, the samples in the ECR-DDH07 borehole were buried much deeper, with about 4–5 km of overburden, which means that only limited information about their source inputs and depositional environment could be recovered. Aliphatic biomarker, n-alkane and aromatic data are consistent with an overall strong terrigenous organic matter input for the samples from the ECR-DDH01 borehole. Sterane/hopane ratios are mostly low, but only a few compounds diagnostic of specific source inputs were identified. There is a microbially-dominated sample near the base of the Snapper Point Formation (high C29 αβ hopane/C30 αβ hopane ratio and 2α-methylhopanes), and high amounts of parent polycyclic aromatic hydrocarbons in some samples from the Berry Siltstone, likely reflecting an occasional pyrogenic signature from palaeo-forest fires. Several parameters, including sterane carbon number distributions, terpane ratios and the relative abundance of dibenzofuran compared to phenanthrene show that the samples from the ECR-DDH01 borehole have a more strongly terrigenous signal than the previously analysed outcrop samples. This is inferred to be because the ECR-DDH01 location was closer to the palaeo-shoreline than the likely more distal outcrop samples. Based on the pristane/phytane ratio and hopane distributions, deposition of the sediments was under varying oxicity conditions, from highly oxic to suboxic. The inferred environments are marine shale/lacustrine and fluvial/deltaic, so the organic geochemical data agrees with the Permian depositional history of the southmost part of the Sydney Basin previously inferred from sedimentological and palaeogeographic studies.

Global sea level changes or local tectonics? Pliocene, Miocene and Oligocene biomarkers in cored sedimentary rocks from IODP Expedition 317, Canterbury Basin, New Zealand

The various influences of global sea level (eustasy) and local tectonic changes on sedimentation processes in continental margin deposits are important topics in sedimentary research. International Ocean Discovery Program Expedition 317 to the Canterbury Basin, on the eastern margin of the South Island of New Zealand, provided an opportunity to study sediment geochemistry in contrasting depositional settings, from mid-shelf to upper continental slope sedimentary rocks from the Eocene to the Holocene. This paper provides the first examination of the organic geochemical record from Pliocene, Miocene, and Oligocene sediment samples recovered during the expedition from the U1351, U1352, and U1353 sites, using bulk geochemistry and hydrocarbon distributions, including biomarkers. The main aim of this research was to correlate changes in organic accumulation with local tectonic activity and global climate changes that occurred in the Miocene. There is good preservation of C11 to C35n-alkanes, with varying predominance of odd-over-even chain length n-alkanes, and much variation in the terrigenous/aquatic ratio over the cores. Based on biomarker and Rock Eval parameters, thermal maturity varies from low in the Pliocene, to early oil window in the deeper Miocene and Oligocene sedimentary rocks. The pristane/phytane ratios for the three cores indicate anoxic to oxic depositional environments. Marine organic matter input is indicated by the high C30 sterane index, whilst high relative amounts of oleanane, C24 tetracyclic terpane, and C29 steranes indicates major terrigenous organic matter input. Local tectonic activity is suggested to have had a significant influence on the accumulation of the organic matter in the Canterbury Basin. The influence of eustasy and global sea temperature variations are suggested for the periods < 14 Ma, 12–7 Ma, and ∼ 6 Ma. An increase in terrigenous organic matter input during the late Miocene correlates with the uplift of the Southern Alps and an increase in the continental slope angle for the South Island of New Zealand.

Impact of Terrigenous Organic Matter Input on Organic Matter Enrichment of Paleocene Source Rocks, Lishui Sag, East China Sea

To clarify the organic matter (OM) enrichment of the Lishui Sag, the factors influencing the variable abundance of OM in the Lingfeng Formation are studied using organic geochemical data. The source rocks of the Lingfeng Formation have medium–high total organic carbon (TOC) values (0.53–3.56%). The main type of kerogen is II2-III. Compared to the shallow marine subfacies source rocks, the TOC of the delta front subfacies source rocks is higher. The distribution of biomarkers shows that the redox environment of the delta front subfacies source rock is the sub-oxidizing and oxic environment, and the source rock is mainly supplied by terrigenous higher plants; the redox environment of shallow marine subfacies source rocks is a sub-reducing and suboxic environment, and the OM mainly comes from algae. The link between OM input and OM abundance demonstrates that terrigenous OM (TOM) input has a considerable influence on OM abundance. However, there is no obvious relationship between preservation and OM abundance, which suggests that preservation is not the determining element in OM enrichment. The strong sediment flux decreases the amount of time that OM is exposed to oxygen. As a result, delta front subfacies with large TOM input have a huge number of excellent source rocks. This paper proposes a “delta front-OM input model” for excellent source rocks.

Development of a novel sea surface temperature proxy based on bacterial 3-hydroxy fatty acids

Gram-negative bacterial 3-hydroxy fatty acids (3-OH-FAs) have received recent attention for their potential as palaeoclimate proxies. A novel temperature proxy, the ratio of anteiso to normal C13 3-OH-FA (RAN13), has been proposed for sea surface temperature (SST) reconstruction in the North Pacific Ocean. However, whether this newly proposed temperature proxy is applicable to marginal seas with significant terrigenous input or tropical oligotrophic seas requires further investigation. Here, we analyzed the composition and distribution of 3-OH-FAs and evaluated the possible impact of various environmental parameters (SST, water depth, dissolved oxygen, salinity and nutrient concentration) on their distribution in marine surface sediments from the Bohai Sea (BS) and the South China Sea (SCS). In the BS, the potential source proxy, fractional abundance of anteiso 3-OH-FAs (average 17%), indicates 3-OH-FA geochemical signature are not greatly overprinted by terrigenous inputs. The relative abundance of long-chain 3-OH-FAs (C15-C18) are higher in the SCS (average 41%) compared to those in other seas (average 33% for all marine samples). Massive inputs of terrigenous organic matter to the BS likely result in overestimation of SSTs based on the RAN13 proxy, and limited abundance of anteiso and normal C13 3-OH-FAs in the oligotrophic SCS may increase the uncertainty of the RAN13 estimated SSTs. More importantly, we find that most short-chain 3-OH-FAs are temperature dependent, especially the fractional abundance of i-C12, a-C13, i-C14 and n-C14 with a high determination coefficient (R2 &amp;gt; 0.60). Based on these newly found correlations, we propose a novel proxy: RANs. The RANs index shows a strong linear relationship with SST (R2 = 0.92, p&amp;lt; 0.001, n = 85) and more accurate prediction than the RAN13, especially in tropical samples. Furthermore, the RANs proxy is significantly correlated with TEX86, and RANs-based SSTs are approximate to LDI derived temperature in the SCS, which support the reliability of RANs as a temperature proxy. These findings further suggest 3-OH-FA based proxies have potential for paleo-SST reconstruction, especially at higher and lower ends of the ocean temperature spectrum and even in cases where marginal inputs of terrestrial organic matter and nutrients are high.

Origin, Depositional Environment and Thermal Maturity of some Source Rocks from Niger Delta Basin, Nigeria

Eleven (11) core samples from Agbada and Akata formations in the Niger Delta, Nigeria, were analyzed using gas chromatography-mass spectrometry to evaluate their paleodepositional conditions and source input as well as to determine their thermal maturity. The distribution of n-alkanes, isoprenoids, and biomarkers indicate a mixture (marine/terrigenous) organic matter source input for Agbada and Akata formation sediments deposited in marine environment. Low values of Phytane/nC18 (0.45 to 1.32) and Pristane/nC17 (0.37 to 1.8) indicates Marine organic matter contribution and anoxic-suboxic (Type II/III kerogens) depositional conditions of extract for Agbada and Akata Formations. The Carbon Preference Index (CPI) values for all analysed samples are slightly greater than 1.0 indicate a mixed input of marine and terrigenous organic matter deposited under relatively reducing (dysoxic) conditions. The high concentration of Oleanane in studied samples reveals that source rocks are tertiary age and belong to marine deltaic depositional environment. Moderate to higher Gammacerane index values ranging from 0.06 to 0.11, suggests water-column stratification, potentially due to hypersalinity for Isan oilfield well samples from Agbada formation. Biomarker maturity parameters suggest that the sediments have reached the early stages of hydrocarbon generation.

Extremely High Primary Productivity and Organic Matter Enrichment in Lacustrine Sediments: Late Triassic Chang 73 Sub-Member, Western Ordos Basin

The 7th member of the Late Triassic Yanchang Formation is the most important Mesozoic source rocks and shale oil reservoir in the Ordos Basin. The hydrocarbon potential and organic matter enrichment mechanism of this organic-rich member are questioned because of the high heterogeneity of continental sediments. In particular, the lack of drilling in the Chang 73 sub-member of the western Ordos Basin (central Tianhuan Depression) hinders this understanding and the progress of shale oil exploration in this region. In this study, 87 samples were collected from the well DP1 in the Tianhuan Depression, western Ordos Basin, and analyzed for sedimentology and geochemistry to unravel the paleoenvironment, paleo-productivity, and hydrocarbon potential of the Chang 73 sub-member. The results show that the Chang 73 shale is characterized by ultra-high organic matter abundance (up to 10.60%), type II kerogen, and good hydrocarbon generation potential. An intermediate chemical weathering intensity suggests that this set of organic-rich shale was deposited in a warm and humid paleoclimate setting. Extreme organic matter enrichment in study area may be attributed to ultra-high productivity (hypereutrophic lake), superimposed terrigenous organic matter input, and transgression, rather than volcanism and anoxic bottom water. This model is different from those of Chang 73 interval with higher organic matter content in the central and southern parts of the Ordos Basin. Therefore, this study proposes that the Chang 73 shale in the central Tianhuan Depression is not a good target layer for shale oil exploration and that the favorable area may be located in an undisturbed deep lake with higher organic matter abundance (e.g., southwestern Ordos Basin).

SOURCE ROCK POTENTIAL OF THE UPPER TRIASSIC CHANG 7 MEMBER IN THE WESTERN ORDOS BASIN, CHINA

The Chang 7 member of the Upper Triassic Yanchang Formation is an important source rock in the Ordos Basin, NW China. Previous studies of the unit have in general focused on the relatively deep‐water (∼25 m) anoxic, OM‐rich lacustrine mudstones which form the main Mesozoic source rock at fields in the south of the basin. However, this paper presents an integrated geochemical investigation of the relatively shallow‐water Chang 7 facies in a study area around Jiyuan field in the western part of the basin in order to evaluate its source rock potential. The results show that the Chang 7 source rock in the study area has a high content of Type II OM, and is interpreted to have been deposited in a suboxic‐anoxic lacustrine setting with a mixed input of aquatic and terrigenous organic matter. The results of 1D modelling of a well in the study area showed that the Chang 7 member entered the oil generation window from the Middle Jurassic. Oil‐oil correlations based on hierarchical cluster analysis and correspondence analysis showed that crude oils generated by deep‐water Chang 7 source rocks from wells in the south of the basin can be distinguished from Soxhlet‐extracted petroleum from reservoir sandstones in the study area. The compositional differences are inferred to be due to variations in source rock facies compositions. An oil – source rock correlation study showed that the shallower‐water Chang 7 source rock in the western part of the basin generated the hydrocarbons in core extracts of reservoir sandstones from wells in this area. The shallow‐water Chang 7 facies may therefore constitute an effective oil‐prone source rock in the western Ordos Basin.

Unscrambling the source kitchen and hydrocarbon prospect information of the ultra-deepwater area in southern Pearl River Mouth basin, South China Sea: Insights from the first ultra-deepwater exploration well W21

Recent breakthroughs in Pearl River Mouth basin (PRMB) regarding offshore hydrocarbon exploration have shifted the focus from deepwater area to ultra-deepwater area (>1500 m water depth), and the southern sub-sag of Baiyun sag, Liwan sag, and the Shunhe Uplift between them constitute the front area for PRMB ultra-deepwater hydrocarbon exploration. This study investigated the source-rock strata (i.e., Eocene Wenchang Formation and Enping Formation) and reservoir fluids (mainly CO2 with small amounts of oil and alkane gas) in the first exploratory well (namely W21, drilled at Shunhe Uplift), aiming to unscramble the source kitchen and hydrocarbon prospect information in this ultra-deepwater area. Biomarker and transgressive history analysis of source-rock strata jointly determined the Wenchang Formation and Enping Formation to be marine sediments in the Liwan sag, while marine and lacustrine sediments in the Baiyun southern sub-sag, respectively. The oil biomarkers and isotopes of alkane gas evidenced prospect for mature oil and oil-associated gas in this ultra-deepwater area, and the correlated source analysis determined the source rocks in the two sags have an advantage of aquatic organic input, indicating a good hydrocarbon-generating potential. Additionally, the regional distribution of terrestrial source-specific biomarkers (i.e., oleanane and bicadinanes) in PRMB deep to ultra-deep water areas traced the terrestrial source area and revealed a significant less input of terrigenous organic matter at the two ultra-deepwater sags. The isotopic analysis of reservoir non-hydrocarbon gases determined the CO2 to be mantle-derived magmatic origin. The present reservoirs are a result of displacement of early charged oils by posterior migrated CO2. In conclusion, this PRMB ultra-deepwater area has a considerable prospect for mature oil; however, the frequent volcanism since the Pliocene poses great risks to hydrocarbon exploration.

Coupled Oceanic and Atmospheric Controls of Deglacial Southeastern South America Precipitation and Western South Atlantic Productivity

Various mechanisms were proposed as substantial drivers of (sub)tropical South American hydroclimate changes during the last deglaciation. However, the interpretation of past precipitation records from the regions affected by the South American Summer Monsoon, the dominant hydroclimatic system in (sub)tropical South America, still insufficiently consider feedbacks between oceanic and atmospheric processes evident in modern observational data. Here, we evaluate ocean-atmosphere feedbacks active in the region from 19 to 4 ka based on a multi-proxy record comprising lipid biomarker, bulk sediment elemental composition and foraminiferal geochemistry from a sediment core retrieved from the tropical western South Atlantic offshore eastern Brazil at ~22°S. Our proxy data together with existing paleoclimate records show that the consideration of large scale synoptic climatic features across South America is crucial for understanding the past spatio-temporal rainfall variability, especially during the last deglaciation. While the paleohydrological data from our study site show relatively stable precipitation across the deglaciation in the core region of the South Atlantic Convergence Zone, distinct hydroclimatic gradients developed across the continent during Heinrich Stadial 1, which climaxed at ~16 ka. By then, the prevalent atmospheric and oceanic configuration caused more frequent extreme climatic events associated with positive rainfall in the northern portion of eastern South America and in the southeastern portion of the continent. These climatic extremes resulted from substantial warming of the sub(tropical) western South Atlantic sea surface that fostered oceanic moisture transport towards the continent and the reconfiguration of quasi-stationary atmospheric patterns. We further find that enhanced continental precipitation in combination with low glacial sea level strongly impacted marine ecosystems via enhanced terrigenous organic matter input in line with augmented nutrient release to the ocean. Extreme rainfall events similar to those that occurred during Heinrich Stadial 1 are likely to recur in South America as a consequence of global warming, because the projected reduction of the intra-hemispheric temperature gradient may lead to the development of atmospheric patterns similar to those in force during Heinrich Stadial 1.

Spatial and Seasonal Variations in the Stable Isotope Values and Trophic Positions of Dominant Zooplankton Groups in Jiaozhou Bay, China

The spatial and seasonal distributions of stable carbon and nitrogen isotopes (δ13C and δ15N) in dominant zooplankton groups were investigated in Jiaozhou Bay. Zooplankton δ13C values ranged from −22.89‰ to −15.86‰, and δ15N values ranged from 3.18‰ to 13.57‰, respectively. The δ13C and δ15N values generally followed the order of small zooplankton &amp;lt; large calanoids &amp;lt; small Sagitta &amp;lt; large Sagitta. Spatial distribution patterns of zooplankton δ13C and δ15N values varied in different seasons. Our results suggested that the spatial variation of δ13C was mainly controlled by terrigenous organic matter (OM) input and phytoplankton biomass, but water temperature may have played a key role in the seasonal variation of δ13C. In spring, the high phytoplankton biomass might increase the δ13C value of small zooplankton in the inner bay. During other seasons, the δ13C values of zooplankton generally increased from the inner bay to the outer bay, which might be associated with the influence of 13C-depleted terrigenous OM carried by the river discharge. Small zooplankton stable isotope values were significantly correlated with that of particulate organic matter (POM). The influence of anthropogenic nutrient input on isotopic baseline can be cascaded to the zooplankton, and the effect might be weak at higher trophic levels. The Bayesian standard ellipse areas of dominant zooplankton groups were generally smallest in the winter, suggesting a narrow niche width during that time. The niche partition between small zooplankton, large calanoids, and Sagitta was most distinct in winter, and followed by summer. The relative trophic level of Sagitta ranged from 2.23 to 4.01, which generally declined from the inner bay to the outer bay during the spring, autumn, and winter seasons. High anthropogenic nutrient loading might reduce the difference in trophic niches among zooplankton groups. This study provided detailed information on the distribution of zooplankton δ13C and δ15N in a coastal bay, which will be useful for understanding the anthropogenic influence on the ecosystem structure and function.

Organic petrologic and geochemical characterization of petroleum source rocks in the Middle Jurassic Dameigou Formation, Qaidam Basin, northwestern China: Insights into paleo-depositional environment and organic matter accumulation

The organic matter-rich mudstones of the Middle Jurassic Dameigou Formation in the northern Qaidam Basin are regarded as the principal petroleum source rocks for unconventional resource exploration in China. In this study, a detailed investigation on the organic petrography, inorganic geochemistry as well as bulk and molecular organic geochemistry is presented. Three different source rock units were identified with TOC contents and VRr ranging from 1.5–39.2% and 0.35–0.57%, respectively. Carbonaceous mudstone and organic matter-rich mudstone in unit 1 with high Pr/Ph (4.79–7.25) and low Gam index (0.02–0.03) values mainly developed in an oxic freshwater column under warm and humid climatic conditions and strong weathering intensity. Organic matter in unit 1 with high proportions of C29 ααα 20R steranes (61–74%) is interpreted to be derived from terrestrial higher plants. By contrast, units 2 (Pr/Ph: 2.56–4.23; Gam index: 0.03–0.14) and 3 (Pr/Ph: 0.73; Gam index: 0.48) were deposited in sub-oxic to oxic freshwater and anoxic saline environments, respectively. Progressive aridification during the deposition of units 2 and 3 are reflected by decreasing CIA values (59.97–86.68) resulting in weakened weathering intensity and lower terrestrial influx. A mixture of terrestrial higher plants and algal biomass was the main source of organic matter in unit 2 with moderate C19/C23 Tri (0.96–5.44), C20/C23 Tri (1.28–3.87) and C24 Tet/C26 Tri (1.10–2.54) ratios, while unit 3 with the lowest C19/C23 Tri (0.25), C20/C23 Tri (0.95), and C24 Tet/C26 Tri (0.65) ratios and highest proportions of C27 ααα 20R steranes (69%) is characterized by a decrease in terrigenous organic matter content. The dominant source of organic matter is lake plankton, including halophilic algae and cyanobacteria. Organic matter accumulation in unit 1 is associated with a warm and humid climate, weathering intensity, and terrigenous organic matter input. Primary productivity is the most important factor controlling the enrichment of organic matter in units 2 and 3.

Biomarker and compound-specific isotope records across the Toarcian CIE at the Dormettingen section in SW Germany

The Toarcian oceanic anoxic event (T-OAE) is associated with a prominent negative carbon isotope excursion (CIE; ~ 183 million years (Myr)). About 10-m-thick organic matter-rich sediments accumulated during the T-OAE in the Southwest German Basin (SWGB). Rock–Eval, maceral and biomarker analysis were used to determine variations of environmental conditions across the CIE interval. Carbon isotope records were determined for various n-alkanes, pristane and phytane to contribute to the reconstruction of the paleo-environment and to study the factors controlling molecular δ13C values. Geochemical redox indicators provide evidence for photic zone anoxia during the Toarcian CIE, which reached its maximum after deposition of the “Unterer Stein” marker horizon. The 2α-methylhopane index suggests enhanced activity of diazotrophic cyanobacteria, which is also supported by nitrogen isotope data. This distinguishes the SWGB from other basins with Toarcian black shale. Oxygen-depleted conditions, albeit with lower intensity continued after the CIE. All investigated compounds replicate the negative CIE, but the magnitudes vary considerably. The largest shift is observed for n-C27 (9‰) and reflects the combined effect of the global CIE and a major change in organic matter input (termination of terrigenous organic matter input). The shift for short-chain n-alkanes, pristane, and phytane, interpreted to reflect marine biomass, varies between 4.5 and 5.0‰. This is the highest value observed so far for any Toarcian section. δ13C values of pristane and phytane reach a minimum near the base of the CIE interval and increase upsection. Thus, the maximum negative isotope shift predates the strongest basin restriction by about 450 thousand years (kyr).

A detailed chronology of the sedimentation in the Danube abyssal fan records the major episodes of the late-Holocene Black Sea evolution

We have constructed a high-resolution Bayesian sedimentation model spanning the last 5500 years based on 25 AMS radiocarbon dated sediments of bulk organic matter (OM) sampled from the NW Black Sea anoxic waters of the continental slope. The corrections for the 14 C ages due to marine reservoir effect (MRE) and detritus organic carbon are correlated with exogenous information such as 210 Pb dating, metallurgy pollution and human-induced soil erosion, highlighting the Danube influence on the geochemistry and chronology of the NW Black Sea sediments through the input of terrigenous organic matter. The results show excellent agreement with some of the previous studies, supporting a total age offset for the bulk OM of 60 years as MRE and 580 years as detritus organic carbon influence. The revisited chronology pinpoints the first and second invasion of the coccolithophores Emiliania huxleyi at 2524 ± 87 and 625 ± 65 years cal. BP. Sedimentation rate shows an increase of about three times with the starting of the late Medieval, which correspond to the highest observed sediment discharge of the Danube as are considered the last 500-300 years. This type of high-resolution sedimentation model is an important step for constructing the carbon budget in bottom waters of variable oxygen concentration.

A comparative study of organic-rich shale from turbidite and lake facies in the Paleogene Qikou Sag (Bohai Bay Basin, East China): Organic matter accumulation, hydrocarbon potential and reservoir characterization

Organic-rich shale (ORS) was developed in various sedimentary facies in lacustrine basins, displaying different organic matter (OM) accumulation processes, hydrocarbon potential and reservoir characterizations. Taking the Paleogene Qikou Sag (Bohai Bay Basin, East China) as an example, the ORS from two typical sedimentary facies in lacustrine basins, i.e., turbidite and deep to semi-deep lake were comparatively studied, based on the integration of mineralogical, bulk geochemical, organic petrographic, isotopic and elemental geochemical, and shale reservoir analyses. Under the influence of turbidity currents, the Es 3 2 ORS was formed in the paleo-lake characterized by low paleoproductivity, suboxic-oxic and freshwater conditions, and high detrital influx. However, high terrigenous OM input led to moderate TOC contents (avg. 1.58 wt%) and low HI values (avg. 193 mg/g TOC) in the Es 3 2 ORS of turbidite facies. In contrast, the Es 1 z ORS of deep to semi-deep lake facies was deposited in the paleo-lake with high paleoproductivity, suboxic and brackish water conditions, and low detrital influx, which resulted in high TOC contents (avg. 2.77 wt%) and HI values (avg. 335 mg/g TOC). Combined with the analysis of hydrocarbon potential and reservoir characterizations, the Es 1 z ORS of deep to semi-deep lake facies could serves as main source rocks for crude oil, whereas a significant shale oil potential is likely to observe in the Es 3 2 ORS of turbidite facies in areas with favorable maturity and high brittleness. Our results are conductive to the formation mechanisms of ORS from various sedimentary facies in lacustrine basins, and provide scientific guidance for hydrocarbon exploration in similar areas. • Accumulation models of organic matter in different sedimentary facies were established. • Hydrocarbon potential of ORS from different sedimentary facies was comparatively studied. • Reservoir characterization of ORS was revealed by multiple methods. • The ORS from different sedimentary facies plays different roles during hydrocarbon exploration.

Geochemistry of oils and condensates from the lower Eagle Ford Formation, south Texas. Part 2: Molecular characterization

Fifty-five petroleum samples from unconventional wells in the self-sourced lower Eagle Ford Shale were analyzed using routine geochemical methods. The samples are broadly distributed, geographically and vertically, and range in maturity from mid-oil to wet gas. Their physical and chemical properties span most of the range observed in the entire play. Two source facies are identified: west of San Marcos Arch is a marine marl facies deposited under anoxic to euxinic conditions containing abundant type II/IIS kerogen and east of the Arch is a marine shale to marl facies deposited under mostly oxic to anoxic conditions that received an additional input of terrigenous organic matter. The unconventional oils are too mature to detect any subtle differences that could distinguish a marl from a pure carbonate source facies. Saturated and aromatic biomarkers rapidly decrease with depth and are at or near the limits of detection below ∼9,000 feet. Condensates >13,500 ft show evidence of advanced hydrocarbon cracking. All properties and molecular parameters that are dependent on thermal maturity for oils/condensates west of the San Marcos Arch exhibit two trends with true vertical depth (TVD). One is defined by samples between the Karnes Trough area and the Maverick Basin; the other trend defined by samples near the Karnes Trough is offset with equivalent maturity occurring at deeper depths. This ∼2,000 ft offset is likely due to differences in the amount of uplift and erosion experienced in the two regions after the source rocks achieved their maximum maturity.

Geochemical and petrological characteristics of the Middle Jurassic organic-rich siliciclastic sediments from the central part of the Polish Basin

Organic-rich claystones and mudstones can be very good source rocks for hydrocarbons. Middle Jurassic organic-rich siliciclastic horizons are known from the Polish Basin area, however, there are no hydrocarbon accumulations associated with these horizons. The main scientific goal of this paper is to propose why these horizons did not produce significant hydrocarbons quantities. To deal with it, we characterised organic matter origin, maturity level, and hydrocarbon potential of Middle Jurassic strata. Palaeoenvironmental conditions of the Middle Jurassic Polish Basin were also described. The samples were examined by geochemical (Rock-Eval and GC–MS) and petrological (macerals content, vitrinite, and fusinite reflectance values) analyses. The analyses revealed that the Middle Jurassic strata at the study area, in general, contain mixed terrigenous/aquatic organic matter with a significant content of pyrogenic origin polycyclic aromatic hydrocarbons (PAHs). Inertinite and liptinite group macerals dominate in maceral composition. This suggests that analysed samples contain a mixture of type IV and II kerogens. Most of the analysed samples contain immature or early-mature organic matter. In general, the maturity level is higher in the southern part of the study area, where higher subsidence of Polish Through allowed to reach greater burial depths. Sediments were deposited in anoxic basement conditions of the epicontinental sea with the serious influence of deltaic/estuarine systems. The water and air transport are responsible for significant input of terrigenous organic matter, mostly reworked in frequent palaeo-wildfire events. The high content of charred organic matter is the main reason why despite high total organic carbon (TOC) values, the hydrocarbon potential of analysed horizons is poor/fair. Fire maceral index (FMI) was proposed and juxtaposed with pyrolytic input ratio (PIR) to propose that PIR is sensitive mostly to higher-temperature palaeo-wildfires products.