Organic Matter Enrichment Mechanism Research Articles

Sedimentary Paleoenvironment and Organic Matter Enrichment Characteristics of Lacustrine Shahezi Shale in Songliao Basin: Insights from the Continental Scientific Drilling.

The lacustrine shale of the Shahezi Formation in the Songliao Basin has obvious organic matter enrichment characteristics and great potential for oil and gas resources. At present, the understanding of the sedimentary paleoenvironment and organic matter enrichment characteristics of the lacustrine shale of the Shahezi Formation is relatively weak. Therefore, taking the international continental scientific drilling Program (ICDP) borehole Songke-2 (SK-2) with continuous and whole Shahezi cores as the research object, combined with organic geochemistry, elemental geochemistry, and logging data, the sedimentary paleoenvironment and organic matter enrichment mechanism were studied. The results show that the organic matter of Shahezi shale is generally in the high to over mature stage. The kerogen type of organic matter is mainly II2-III. The organic matter is mainly derived from the lake basin's own algae and terrestrial higher plants. The total organic carbon (TOC) content of Shahezi shale is relatively high, and the TOC is mainly distributed between 1% and 2%. The Shahezi Formation is dominated by clay shale and siliceous shale, and experienced moderate chemical weathering during deposition. According to the analysis of organic geochemistry and elemental geochemistry data, the paleoenvironment of organic matter deposition in Shahezi shale was dominated by warm and humid climate in the early stage, and then experienced multiple cooling and arid periods. The climate type turned to semihumid-semiarid, with stable terrigenous debris input, low deposition rate, brackish water salinity and oxygen-rich-oxygen-poor water environment. The sedimentary period of Shahezi Formation is in the Cretaceous oceanic anoxic Aptian-Albian stage. During the oceanic anoxic event, the anoxic sedimentary environment and the frequent volcanic activity have an important impact on the organic matter enrichment. The oceanic anoxic event and volcanic activity are the main causes of water body hypoxia in the lake basin. The nutrients brought by volcanic activity are also one of the reasons for promoting the growth of lake basin organisms, creating good conditions for organic matter enrichment. The enrichment of organic matter in Shahezi Formation is the result of the interaction and coupling of various factors such as paleoclimate, paleoenvironment, paleoproductivity, water environment, terrestrial input, and major geological events. And, the organic matter enrichment model of Shahezi Formation shale is established.

Mechanism of organic matter enrichment in a basin with shallow biogenic gas: a case study of Pleistocene shale in the Qaidam Basin

ABSTRACTOrganic matter is the material basis of shale gas. The Qaidam Basin is a key exploration and development area for shallow biogenic shale gas in China. In this study, we have focused on Quaternary Pleistocene shale in the Qaidam Basin, and the mechanism of organic matter enrichment was investigated in terms of water column stratification and paleoclimate. The results show that the K9–K7 section has greater biological productivity than the K5–K4 section. During the early–middle Pleistocene (K9–K7 deposition period), due to a warm and humid climate, the water column was strongly stratified and herbaceous plants developed, resulting in increased biological productivity. Stronger stratification also led to a reducing environment in the lower layer, which was conducive to the preservation of organic matter from the upper layer. During the late Pleistocene (K5–K4 deposition period), with a dry and hot climate, stratification became weaker and the vegetation evolved into woody plants, reducing biological productivity. Weaker stratification led to destruction of the reducing environment, which was not conducive to enrichment of the sedimentary organic matter. Moreover, the increased temperatures increased the activity of methanogenic bacteria, which consumed a lot of the organic matter.

Sedimentary paleoenvironment and its control on organic matter enrichment in the Mesoproterozoic hydrocarbon source rocks in the Ordos Basin, southern margin of the North China Craton

The black shale of the Mesoproterozoic Cuizhuang Formation in the Changcheng System in Yongji city, North China Craton, is a potential source rock. Understanding the organic matter enrichment mechanism is crucial for evaluating source rock resources and understanding oil and gas accumulation mechanisms. In this study, we evaluated the sedimentary paleoenvironment and organic matter enrichment mechanisms of shale using thin section observations, mineral composition analysis, organic geochemistry, and elemental geochemistry. We found significant differences in the sedimentary paleoenvironment and organic matter enrichment mechanisms between the lower Cuizhuang Formation and the Beidajian Formation shale. The Cuizhuang Formation was deposited in a late-stage, restricted basin environment during the rift phase, and elemental and geochemical indicators showed that the Cuizhuang Formation was in a suboxic–anoxic water environment, that was influenced by a warm and humid paleoclimate and submarine hydrothermal activities, which promoted the accumulation of organic matter. However, the enrichment of organic matter in the Cuizhuang Formation was mainly controlled by redox conditions. The formation of suboxic-anoxic water environments may be closely related to the warm and humid paleoclimate and submarine hydrothermal activities. Warm conditions promote continental weathering and increase marine productivity, thereby consuming oxygen in the bottom water. Moreover, acidic hydrothermal activity also helps to establish an anoxic environment. Our results reveal the effects controlling various coupled mechanisms dominated by redox conditions, which may explain the development of source rocks in the Cuizhuang Formation.

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.

Coastal upwelling and redox variations in the northwestern Tarim Basin (northwest China) during the Middle-Late Ordovician: implication for paleo-depositional conditions of the organic matter enrichment in the Saergan Formation

The black shales of the Saergan Formation, which represent one of the main hydrocarbon source rocks in the Tarim Basin, witnessed a time span of organic matter enrichment by profound changes in the Earth System. A multi-proxy geochemistry study was carried out on the samples of the Saergan Formation to reconstruct the depositional environment and to explore the mechanism of organic matter enrichment of the unit at the Yingshanbeipo section, Keping area, northwest Tarim. Elemental and TOC data are suggestive of an upwelling setting, with a less pronounced oxygen minimum zone (OMZ) compared to the highly productive Peru and Namibian margins. Ferruginous anoxic bottom water conditions prevailed during most time of the deposition, with suboxic conditions dominating the basal and the top parts of the studied unit. As a whole, primary productivity seems to be the dominating factor that controlled the micronutrients and OM accumulations in the Saergan Formation whereas the role of benthic redox conditions may have been subordinate. The variations in primary productivity and bottom water redox conditions were resulted by the multiple, interacting environmental factors including nutrient supply regulated by oceanic circulation and climate changes, and relative sea-level fluctuations.

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.

Biological sources, paleoenvironment, and organic matter enrichment in source rocks of the Ordovician Majiagou Formation, Ordos Basin, China: Evidence from biomarkers, microfossils, and inorganic geochemical analyses

Carbonate deposits in the Ordovivian Majiagou Formation are significant source rocks for natural gas generation in the Ordos Basin, northwestern China. Previous studies mainly focused on the organic matter enrichment mechanism of shales rather than carbonate rocks. The biological sources and paleoenvironment of carbonate source rocks, and the main controlling factors of organic matter enrichment in the carbonate source rocks were studied in this paper in combination with evidence from biomarkers, microfossils and inorganic geochemistry analysis. The results show that four types of microfacies were identified in the Majiagou Formation, respectively as: mud flat microfacies, mud–dolomite flat microfacies, dolomite flat microfacies, and open marine microfacies. The biological sources of organic matter are chiefly planktonic algae, followed by bacteria. The mud flat and mud–dolomite flat contain a high abundance of terrigenous detrital inputs, as indicated by the high content of Al2O3, TiO2, Th, and Zr. The low Sr/Cu and high Rb/Sr values reveal warm and humid paleoclimate conditions in the mud flat and mud–dolomite flat, whereas the dolomite flat and open marine were likely formed in hot and arid paleoclimate conditions. The mud flat and mud–dolomite flat deposits were characterized by high paleoproductivity of the Majiagou Formation. Low Sr/Ba values were found in the mud flat samples, indicating fresh to brackish water condition, whereas samples of other facies have a relatively high degree of salinity. Based on Uauth, Moauth, Crauth, Coauth, δCe, and δEu values, the mud flat microfacies was formed in a suboxic and anoxic environment, whereas the mud–dolomite flat, dolomite flat and open marine microfacies were within dysoxic to oxic conditions. A model of organic matter enrichment in the Majiagou Formation is thus established. The level of terrigenous detrital inputs is the principal factor of organic matter enrichment in the Majiagou Formation, secondly are redox condition and then paleoproductivity. The mud flat and mud–dolomite flat microfacies show abundant terrestrial detrital inputs and nutrient elements, indicative of warm and humid climate that facilitated biotic productivity, including an abundance of planktonic algae (microfossils). The suboxic and anoxic environments promoted the preservation of organic matter, as evidenced by the relatively high TOC content. The mud flat as well as the mud–dolomite flat of the Majiagou Formation is prospecting for forming source rocks.

Organic matter enrichment in shale deposited proximal to paleo-uplifts and its impact on shale gas exploration

Much of the global exploration and development of shale gas is focused on organic-rich shale deposited in basin or slope/basin margin settings. There is a clear need to assess the degree to which organic-rich shale deposited in other settings, notably proximal to paleo-uplifts, are promising shale gas prospects. Organic-rich sediments that accumulated proximal to paleo-uplifts are recognized globally and include the Cambrian Niutitang Formation, Ordovician-Silurian transition “hot shales”, and modern sediments of the Black Sea. Though the subject of recent research efforts, much remains to be learned of the mechanisms of organic matter enrichment of fine-grained sediment deposited adjacent to paleo-uplifts. Our study focuses on the Ordovician Wufeng Formation that accumulated proximal to uplifted areas of the Upper Yangtze platform. Though the TOC content of the analyzed Wufeng Formation samples recovered from near the paleo-uplifts is less than that of Wufeng samples collected from basin or slope/basin margin settings, average TOC value of the former can still reach more than 2%. Wufeng Formation shale deposited proximal to paleo-uplifts accumulated under conditions of medium to high paleoproductivity, elevated terrigenous clastic input and sedimentation rate, and oxic-suboxic water column conditions. Principal component analysis suggests that the factor most critical to organic matter enrichment of Wufeng deposits proximal paleo-uplifts was paleo-redox conditions. The TOC abundance in Wufeng Formation shale deposited during late Katian stage is higher than that of shale deposited during Hirnantian stage. It was at the Hirnantian stage that the paleo-uplifts became most active in association with the Kwangsian Orogeny resulting in diminished paleoproductivity and preservation conditions as reflected in the reduced organic matter abundance of associated shale. Uplift of the paleo-uplift of the Yangtze Platform, a manifestation of the Kwangsian Orogeny during the Ordovician-Silurian transition, appears to have been largely responsible for spatial and temporal distribution differentiation of organic matter abundance of the Wufeng Formation of the Upper Yangtze Platform. Results of the present study contribute supplementary information regarding the mechanism of organic matter enrichment in shale and can provide guidance for shale gas exploration and development in geologically similar regions.

Hydrocarbon generation potential and organic matter accumulation patterns in organic-rich shale during the mesoproterozoic oxygenation event: evidence from the Xiamaling formation shale

A significant deposition of black shales occurred during the Mesoproterozoic Oxygenation Event (MOE). In order to investigate the hydrocarbon generation potential and organic matter enrichment mechanism of these shale deposits, we studied the Xiamaling Formation shale in the North China region as a representative sample of the Mesoproterozoic shale. The research involved organic petrology, organic geochemistry, mineralogy, and elemental geochemistry. The following observations were made: (1) The depositional environment of the Xiamaling Formation shale can be categorized as either oxic or anoxic, with the former having shallow depositional waters and high deposition rates, while the latter has deeper depositional waters and slower deposition rates. (2) Anoxic shales exhibited significantly better hydrocarbon generation potential compared to shales deposited in oxic environments, although the latter still demonstrated high hydrocarbon generation potential. (3) Shales deposited in anoxic environments displayed higher paleoproductivity compared to those deposited in oxic environments. The high deposition rate in oxic environments slowed the decomposition and mineralization of organic matter, leading to the formation of high-quality shales. In contrast, the strong paleoproductivity, along with favorable preservation conditions, accounted for the high hydrocarbon potential of anoxic shales.

Paleoenvironmental characteristics and organic matter enrichment mechanisms of the upper Ordovician-lower Silurian organic-rich black shales in the Yangtze foreland basin, South China

The Wufeng Formation-Longmaxi Formation (O3w-S1l1) black shales within the Yangtze foreland basin (South China) hold significant potential as unconventional oil and gas resources. However, several challenges, including sluggish sedimentation rates, thin sedimentary layers, and notable homogeneity, hinder a comprehensive grasp of the organic matter enrichment (OME) mechanism within these shales. These challenges impede the exploration of unconventional oil and gas resources. This study aims to identify the O3w-S1l1 black shales in the Weiyuan area through a combined approach of organic and inorganic geochemical analyses. By doing so, it delves into the controlling mechanism behind OME in the black shales of the Yangtze foreland basin. The findings of this research reveal that the O3w-S1l1 black shales primarily consist of sapropelic kerogen and exhibit signs of over-maturation, with TOC content ranging from 0.43% to 8.21%. These shales are classified as organic-rich, mainly composed mixed and siliceous lithofacies. The presence of silica in the shales originates mainly from biogenic sources, and the highest TOC content coincides with a Sixs contribution of around 30% to the total silica content. During the late Katian, a global sea level drop led to an oxic paleoenvironment, transitioning into a euxinic paleoenvironment as sea levels rose in the early Rhuddanian. Notably, TOC content exhibited a significant correlation with paleoceanographic conditions (e.g., V/Cr, Ni/Co) and paleoproductivity levels (e.g., P/Al, Babio), while displaying a negative correlation with paleoclimate conditions (e.g., CIA, C-value), terrigenous detrital input intensity (e.g., Sixs, Ti/Al), and sedimentation rate ((La/Yb)N). Specifically, favourable conditions for OME encompass robust reducing seawater conditions, high paleoproductivity, a humid climate, reduced influx of terrigenous debris, and relatively elevated sedimentation rates. Regional tectonic movements (Guangxi movement) and fluctuations in relative sea levels exert influence over the deposition of O3w-S1l1 black shales. This study provides a valuable case study for exploring and developing shale gas resources within the Yangtze foreland basin.

Organic matter enrichment mechanism in saline lacustrine basins: A review

Organic‐rich shale in saline lacustrine basins holds significant importance as a source rock for conventional hydrocarbon exploration and has emerged as a prominent target for unconventional hydrocarbon exploration and development in recent years. Based on saline lacustrine basins, this paper provides a summary of the organic matter enrichment mechanisms in saline lacustrine basins, considering sedimentary characteristics, biological activities, factors for the organic matter enrichment and consumption, and hydrocarbon generation. The implications for these factors are discussed in relation to the distribution prediction of high‐quality lacustrine shale district settings and the exploration and development of shale oil. Saline lacustrine basins undergo distinct evolutionary stages, each corresponding to different sedimentary stages involving carbonate minerals, sulphate minerals, and alkaline minerals. Moreover, these basins exhibit diverse biological types and experience extensive biological activities. The prosperity of organisms and the accumulation of sedimentary organic matter are ensured by halophilic organisms. Organic matter enrichment in saline lacustrine basins is influenced by two main aspects: the primary productivity of organic matter, which is promoted by the proliferation of halophilic organisms, and the efficient preservation of organic matter facilitated by the strong reducing environment resulting from promoted water salinity stratification. The organic matter consumption in saline lacustrine basins involves bacterial sulphate reduction (BSR) in the early stages, thermochemical sulphate reduction (TSR) in the late stages, and dilution of salt minerals with higher depositional rates. of the presence of salt beds and saline minerals positively influences hydrocarbon generation and expulsion in organic‐rich shale within saline lacustrine deposition. Consequently, continental saline lacustrine basins in China offer favourable conditions for the formation of organic‐rich shale and present broad prospects for the exploration of shale oil and gas resources.

Organic matter enrichment and palaeoenvironmental comparison of Alinian fine-grained sedimentary rocks in lake facies

The Yan'an Formation in the Ordos Basin and the Xintiangou Formation in the Sichuan Basin are typical lake facies sedimentary formations developed in the Alinian period. They contain a large amount of organic-rich fine-grained sedimentary rocks and are the main sites for shale oil and gas enrichment. In this study, geochemical tests, including major, trace, and rare earth elements and TOC tests, were carried out on the fine-grained sedimentary rocks of the shallow lake Yan'an Formation and deep lake Xintiangou Formation to reconstruct their depositional palaeoenvironments, and to explore their organic matter enrichment mechanisms in different depositional environments under lithological differences. The conclusions of this study are as follows: 1) The total organic carbon (TOC) of fine-grained sedimentary rocks increases with increasing mud content and decreasing grain size; 2) The palaeoenvironments of the fine-grained sedimentary rocks in the shallow and deep lake facies are generally consistent, but the redox conditions and deposition rate of the water column in the deep lake facies are stronger than those in the shallow lake facies, while the input of terrigenous detrital, palaeosalinity, and stagnant environment are relatively weaker; 3) The organic matter enrichment mode of lake facies fine-grained sedimentary rocks is a typical palaeoclimatic and palaeoproductivity-dominated “Production mode”; 4) The difference in depositional environments under lithological differences is mainly related to productivity factors, such as palaeoclimate (CIA) and palaeoproductivity (P/Ti). The results of this study help to understand the formation mechanism of lake facies fine-grained sedimentary rocks, distinguish the hydrocarbon generation potential of different lithologies, and provide a basic geological theoretical basis for the exploration of shale oil and gas in the study area.

Destruction mechanism of organic matter enrichment controlled by tectonism in the lower Silurian organic-rich marine mudstone in the western South China Block

This destruction mechanism of organic matter enrichment plays an important role in improving the mudstone sedimentary structure system. For the lower Silurian marine mudstone in the western South China Block, the destruction mechanism of organic matter enrichment is unclear, with previous studies ignoring the control from paleogeography and spatial differences. Lamination is a typical sedimentary structure in mudstone and different types of lamination in mudstones can reflect a range of depositional environments. Lenticular lamination, composed of arranged lenses of variable composition, is a common type of lamination in marine mudstone and mainly occurs in organic-lean mudstone. The origin of lenticular lamination remains controversial, and a multidisciplinary approach is needed that integrates sedimentology, petrography, and paleogeography. The main objectives of this approach are to (1) interpret the formation mechanism of lenticular laminations in the Longmaxi marine mudstone, and (2) discuss the significance of lenticular lamination to organic matter destruction. The results not only allow for a complete understanding of the destruction mechanisms of the Silurian marine mudstones but also provide new insights into the sedimentary processes of marine mudstone.

Organic matter enrichment in Asia's palaeolake controlled by the early and middle Eocene global warming and astronomically driven precessional climate

The sediments deposited in continental lake systems record sensitive palaeoclimate signals and provide better perspectives for understanding rapid palaeoclimate changes and their implications. To better understand the terrestrial record and climate-driven organic matter enrichment mechanism in a low-latitude region, which have rarely been investigated, we studied the Eocene terrestrial records from the Beibuwan Basin, South China Sea (SCS). Cyclostratigraphic analysis was conducted to assess the orbital forcing. Milankovitch-scale astronomical signals were determined by spectral analyses, evolutionary Fast Fourier transform, and sedimentation rates estimation of the natural gamma ray logging data in the depth domain. A floating astronomical time scale (ATS) was constructed based on astronomical tuning that converts the depth domain to the time domain by sedimentation rates. The ATS was calibrated using published dating result of 35.2 Ma from the Eocene to the Oligocene. This ATS constrains the ages of two organic-rich lacustrine petroleum source intervals of the Eocene Liushagang Formation in the Beibuwan Basin at ∼49-47 Ma and ∼40.9–40.2 Ma, respectively. A comparison of the timing of these two source rocks with a published global benthic carbon and oxygen isotope series shows that they correspond to Eocene positive carbon isotope excursions. This finding suggests that organic carbon burial in Eocene lacustrine basins responded to early and middle Eocene Climatic Optimum (i.e., EECO and MECO). We used geochemical data based on X-ray fluorescence core scans for further investigation of the effect of orbital forcing on organic matter enrichment. The high Rb/Sr, K + Ti + Rb and Sr/Ca geochemical ratios and stable Mo and U/Th values all correspond to the precessional minimum, suggesting that the orbital precessional minimum is responsible for a warm and humid climate, and brackish and hypoxic water conditions. Thus, four schematic hypotheses were proposed to explain the effect of the precessional monsoonal climate on organic matter enrichment. This study provides a terrestrial record that documents the mid-Eocene global warming event well and provides evidence that the orbital precessional climate controlled organic matter enrichment in Asia's palaeolake.

Palaeoenvironment evolution and organic matter enrichment mechanisms of the Wufeng-Longmaxi shales of Yuanán block in western Hubei, middle Yangtze: Implications for shale gas accumulation potential

The deposition of black shale is influenced by several factors, including organic matter input, which is controlled by primary productivity; preservation, which is governed by redox conditions and deposition rates; and dilution, which is controlled by terrigenous input. However, determining the individual contribution of these factors to organic matter enrichment (OME) is complex due to significant variations in palaeodepositional conditions across different regions. In this study, we aimed to investigate the palaeoenvironmental conditions and OME mechanisms in the Wufeng-Longmaxi Formation in western Hubei by integrating data on total organic carbon (TOC) content, mineral compositions, and major and trace elements. We also examined the rapid variations in the palaeoenvironment, which led to the mass extinction in the Late Ordovician. Our findings indicate that dolomite siltstone and silty dolomite are present in the Guanyinqiao Formation (GYQ), which differs from other regions. The silica in the Wufeng-Longmaxi Formation has a mainly nonhydrothermal origin, while the upper Wufeng Formation (WF) has an entirely biogenic source. The organic-rich shales were primarily deposited in a warm and humid climate with low terrigenous input, low salinity, dysoxic conditions, and moderate to high palaeoproductivity due to moderate to strong upwelling. The GYQ serves as a transitional section from the WF to the Longmaxi Formation, which was characterized by a cold climate, oxic conditions, weak upwelling, and deposition in a strongly restricted setting, potentially leading to the mass extinction. Overall, OME was primarily dominated by preservation, with factors that influenced OME changing vertically. Palaeoproductivity, combined palaeoproductivity and palaeoredox conditions, and palaeoredox conditions were the main controlling factors in the WF, the Lower Longmaxi Formation (LLMX), and the Upper Longmaxi Formation (ULMX), respectively. Additionally, the Huangling basement, a submarine high, and the Xuefeng uplift's uplift and compression played important roles in OME. However, shale gas exploration prospects in the study area are limited due to the thinness of the organic-rich shales, oxic to dysoxic conditions, and relatively low palaeoproductivity.

Organic geochemistry and organic matter enrichment mechanisms of the lacustrine source rocks from a small-faulted Basin: A case study of the Bayindulan, A'er and Wuliyasitai sags in the Erlian Basin, Northern China

Lacustrine organic-rich shales in the lower Tengger member (K1bt1) are the most important source rocks in the Erlian Basin. The Erlian Basin contains many sags with independent subsidence-sedimentation histories, structure styles and hydrocarbon exploration potential. A comprehensive study was carried out to investigate the organic geochemical characteristics of the shales from three typical sags (the Bayindulan, A'er and Wuliyasitai sags) in the Erlian Basin to define the differences to their depositional environment, dominant source material, and hydrocarbon generation potential in small-faulted basins. Kerogen types in these shales in the study area can be classified into type Ⅱ1-Ⅰ kerogen in the Bayindulan Sag, type Ⅱ2 to type Ⅱ1 in the A'er Sag and type Ⅱ2 in the Wuli Sag. Biomarker characteristics indicate that the dominant algae-sourcing organic matter (OM) in the shales in the Bayindulan Sag was formed under the salty and reducing conditions. Source rocks from the A'er Sag were formed under the fresh-brackish and weak reducing-weak oxidizing conditions with a mixing input of algae and higher plants. Organic-rich shales from the Wuli Sag were deposited under the fresh water and oxidizing environment with a terrestrially dominated organic matter input. The correlation among parameters including average chain length (ACL) of C25–C33n-alkanes, the aquatic/macrophytes proxy (Paq), TAR, the proportion of saropelinite, the wax indexes, Pr/Ph, and TOC were used to investigate the main controlling factors of the enrichment of the organic matter in these three sags. The organic matter accumulation models on the formation of high-quality shales in the Erlian Basin are anisotropic due to various depositional environments and biological sources of source rocks in different regions. The characteristics of organic matter accumulation in the Erlian Basin can be classified into three patterns. Firstly, the good preservation condition that favored the accumulation of organic matter is the main controlling factor resulting in the organic matter enrichment in the Bayindulan Sag. Secondly, the bloom of algae in nutrient-rich surface water favored the accumulation of OM in the A'er Sag. Finally, large influx of vascular plants in the oxygen-rich condition is the main factor leading to the formation of the high-quality shales in the Wuli Sag. Further, these three organic matter accumulation models could be used for reference in the assessment of the petroleum potential of the faulted basins that are similar with the Erlian Basin.

Mechanisms for the Accumulation of Organic Matter in Sediments of the Middle Permian around Bogda Mountain, Southern Junggar Basin, NW China

The organic-rich shales and mudstones of the Middle Permian are the most important unconventional petroleum targets in the southern Junggar Basin of northwestern China. Although numerous studies have been vigorously conducted on paleoenvironment reconstructions, the organic matter enrichment mechanism is still controversial due to the utilization of problematic geochemical proxies established in early studies. In this study, major and trace elemental compositions, molecular markers, stable carbon isotopic compositions, and organic petrology were used to study multiple factors affecting the accumulation of organic matter in sediments. According to the results, a relatively hydrostatic, brackish, anoxic environment is proposed in the northern periphery of the Bogda Mountain, where the original structure of organic matter could be preserved with non-correlations between the productivity proxies (P and Ba) and TOC. In the western Bogda area, organic-rich sediments were deposited under suboxic conditions, with frequent fluctuation occurring between the top and bottom water columns. The accumulation of organic matter was the result of productivity blooms and rapid sedimentation. This improved study of the enrichment of organic matter in the Middle Permian around the Bogda Mountain can contribute to improved evaluations of the petroleum potential and distinguishing the characteristics of different organic matter enrichment models.

Geochemical characteristics of shales from the first member of Qingshankou Formation in Gulong Sag, Songliao Basin, China: Implication for mechanism of organic matter enrichment

Commercial oil production from the first member of the Qinshankou Formation in the Songliao Basin, China has proven it can be a promising target for shale oil recovery. However, geochemical characteristics and the mechanism of organic matter (OM) enrichment in this layer remain unknown. Herein, a large number of samples collected from the first member were analyzed through a combination of organic and inorganic geochemical methods. Results show, bedding oil shale and laminated shale are prominent facies in this member. Moreover, this member mainly consists of type-I and –II kerogen at highly thermal mature stage, belonging to good to excellent source rock. Overall, this member has been deposited in the semi-humid to humid and warm paleoclimate, fresh to brackish water mass, and dysoxic to anoxic conditions. This member is enriched in P2O5, indicating the high primary productivity. Moreover, lack of relation between P2O5, with Al2O3 and Fe2O3 concentrations, suggests that the contribution of terrigenous detritus influx and the effect of Fe cycle for P enrichment in sediments can be disregarded. The terrigenous detritus influx, rapid sedimentation rate and marine transgression were found to be detrimental to the OM enrichment. Moreover, more serious oxygen deficiency in water mass along with slightly lower sedimentation rate, made the lower part of this member to yield better preservation conditions and weaker dilution effect, which resulted in the formation of OM with better quality and higher abundance. Collectively preservation conditions were found to be significant for the OM enrichment.

Organic matter enrichment mechanism of Youganwo Formation oil shale in the Maoming Basin

The Youganwo Formation oil shale located in the Maoming Basin represents a large commercially valuable lacustrine oil shale resource and a potential bio-shale gas reservoir in South China. With the aim of deepening the understanding of factors that influence organic matter enrichment, this research conducted a geochemical investigation to reconstruct the depositional paleoenvironment of bioproductivity, preservation and dilution. Youganwo Formation oil shale is mainly deposited in semi-deep to deep-lake environments with relatively warm and humid paleoclimate in the subtropical-temperate zone. The total organic carbon (TOC) content (1.46–11.85%), S2 values (4.79–115.80 mg HC/mg rock) and HI (328–1040 mg HC/mg TOC) indicate that the oil shale has a good oil source rock potential. TOC content, (S1 + S2) values and vitrinite reflectance values show that its marginally mature organic matter (OM) belongs to kerogen type I-III with good oil-generating potential.A 3rd order sequence was identified in the Yougnwo formation. Subsequently, the multiple factors including bioproductivity, preservation and dilution that control the OM enrichment of oil shale within system tracts were discussed. Moderate-quality oil shales (Oy-1) were developed in the transgressive systems tract (TST) in an oxidizing condition with abundant detrital input. High-quality oil shales (Oy-2) were deposited during the high-stand systems tract (HST) with increased accommodation space, improved preservation conditions, warm and humid climate, higher water bioproductivity and minimum detrital matter input. During the regressive systems tract (RST, Oy-3), higher detrital matter input and fresher water led to lower TOC values. Among these multiple factors, dilution condition was the major one that influences OM abundance and variation on the basis of sufficient organic matter input. Thus, OM enrichment models of Oy-1, Oy-2 and Oy-3 sub-members were established. The OM enrichment and quality in oil shale were controlled by the combined effect of bioproductivity, preservation, and dilution.