Geochemical Characteristics and Controlling Factors of Organic Matter Enrichment in the Bagong Formation Source Rocks, Eastern Qiangtang Basin

To study the correlation between organic matter enrichment and sedimentary environment in the Ordos Basin, hydrocarbon source rocks in the region of the eastern edge of the central plain of the basin are selected for analysis. With reference to the related theories of sedimentary rocks and organic geochemistry, the means including pyrolyzed rocks, organic carbon determination, and detection of main trace elements are adopted. The relationship between the regional sedimentary environment and organic matter enrichment is examined after the features of the paleo-sedimentary environment in the study area are examined. At the same time, from the perspective of biomechanics, the concepts of fluid mechanics and material transport are introduced to analyze the influence of hydrodynamic conditions in sedimentary environments on the migration and enrichment of organic matter. The following results were obtained through the study of organic matter enrichment and sedimentary environment in the Ordos Basin: (1) In the analysis of microscopic features, the study area is mainly composed of matrix vitrinite (accounting for more than 68%) and sericite, and also found woody fibers, microbial debris, and pyrite. (2) In organic matter analysis, it was found that the hydrocarbon generation potential (S1 + S2) of the B-S group was divided into multiple levels. According to the relationship, coal can be classified as a good source rock, while the carbonaceous mudstone in the S group belongs to a poor source rock. Group B-T is mainly composed of type III kerogen (humic type). (3) In the analysis of geochemical indicators, the concentrations of Si, Al, and Fe are very high. Further analysis shows that Si and Ti are the main enriched elements, while Re and Mo are abnormally enriched elements. (4) In the analysis of sample physical properties, sample characterization indicates that the source rock is mainly microporous, with few micropores in sandstone, mainly consisting of fractures and medium to large pores. Research has found that there is a correlation between the pore conditions of the sample and organic matter, and the pore volume increases relatively with the increase of organic matter. (5) Organic matter, abdominal muscles, and sediment environment analysis, according to the examination of geochemical indicators, the research area’s environment changed significantly from humid to arid and semi-arid conditions at various points during the early, middle, and end of the study period. The salinity of the water body in different regions differed significantly, which was related to the regional climate. The study of the carbon content of organic matter and the characteristics of the sedimentary environment displayed that the redox reaction in the early stages of the region and the local climate had a major influence on the enrichment of organic matter. These two factors became the primary controlling factors affecting the region’s organic matter enrichment. The investigation of geological structures and the extraction of local rock gas and minerals can both benefit from the technological assistance this study can offer. At the same time, by introducing the principles of fluid mechanics and material transport in biomechanics, a new perspective is provided for understanding the migration and enrichment mechanism of organic matter in sedimentary environments.

The well-developed mudstone and gypsum mudstone in the Oligocene Linhe Formation (E3l) in the Hetao Basin are the main source rocks for gypsum. However, the sedimentary environment and organic matter (OM) enrichment factors of E3l are not clear, and this inhibits the prediction of hydrocarbon source rock distribution and resource calculation. Major and trace elements, total organic carbon (TOC), pyrolysis using the Rock-Eval II, and saturated hydrocarbon gas chromatography–mass spectrometry (GC–MS) analyses were performed in this study. The results show that E3l was deposited in brackish water and saline–ultrasaline water, with weak oxidation reduction in an arid and hot environment. Terrestrial input inhibits OM enrichment, while the redox, paleosalinity, paleoclimate, and paleoproductivity play a catalytic role. The main controlling factors of the same lithologic source rocks are different: terrestrial input and paleoclimate have a greater impact on mudstone, and the redox and paleosalinity were more favorable to gypsum mudstone. Although the main controlling factors are different for different lithologies, their OM enrichment characteristics are still consistent. The E3l water body was deep, and the contribution of nutrients from terrigenous debris to OM enrichment was less. In addition, the water retention environment changed significantly during the E3l sedimentary period, resulting in fewer nutrients, which limited the improvement of surface water paleoproductivity. The arid climate increased water evaporation and salinity, which to some extent prevented consumption and decomposition. Weak oxidation-reduction fluctuations and the stratification of the water body were obvious, and this was not only conducive to the enrichment of OM but also to its preservation.

Geochemical characteristics and classification of Oligocene source rocks with different facies in Bozhong Sag, Bohai Bay Basin, East China

The Xujiahe Formation (FM) is a significant source rock layer in the Sichuan Basin. In recent years, a growing number of scholars believe that the shale gas potential of the Xujiahe Formation is equally substantial, with the first member of the formation being the richest resource. The deposition of Member (Mbr) 1 of Xujiahe FM represents the first and most extensive transgression event within the entire Xujiahe Formation. This study investigates the sedimentary environment and organic matter (OM) enrichment mechanisms of the dark mud shales in the Mbr1 of Xujiahe FM on the southeastern margin of the Sichuan Basin, utilizing methods such as elemental geochemistry and organic geochemistry analyses. The results indicate that these dark mud shales possess a relatively high OM abundance, averaging 2.20% and reaching a maximum of 6.22%. The OM is primarily Type II2 to Type III. Furthermore, the paleoclimate during the Mbr1 period in the study area was warm and humid with lush aquatic vegetation. Intense weathering and ample precipitation transported large amounts of nutrients into the lacustrine/marine basin, promoting the growth and reproduction of algae and terrestrial plants. Correlation analysis between the Total Organic Carbon (TOC) content and various geochemical proxies in the Mbr1 mud shales suggests that OM enrichment in the study area was primarily controlled by the climate and sedimentation rate; substantial OM accumulation occurred only with abundant terrigenous OM input and a relatively high sedimentation rate. Redox conditions, primarily productivity, and terrigenous detrital input acted as secondary factors, collectively modulating OM enrichment. Event-driven transgressions also played an important role in creating conditions favorable for OM preservation. Synthesizing the influence of these multiple factors on OM enrichment, this study proposes two distinct composite models for OM enrichment, dominated by climate and sedimentation rate.

This study evaluated the hydrocarbon-bearing potential of Upper Jurassic marine source rocks in the Qiangtang (羌塘) Basin through a comprehensive organic geochemical analysis of the samples from a large number of outcrops in different structural units to predict the location of favorable hydrocarbon kitchens, based on the evaluation standards of Mesozoic marine source rocks in the Qiangtang Basin. Rocks’ depositional environment, thickness and organic geochemistry feature were analyzed in this study. The principal controlling factors of the occurrences of favorable source rocks were analyzed. Upper Jurassic Suowa (索瓦) Formation source rocks are mainly platform limestone in the Dongcuo (洞错)-Hulu (葫芦) Lake deep sag and Tupocuo (吐坡错)-Baitan (白滩) Lake deep sag. Lithologically, the Suowa Fromation is made up of a suite of marls in intra-platform sags, micrites and black shales, which were all deposited in the closed, deep and static water depositional environment. Marl could form hydrocarbon-rich source rocks and its organic matter type is mainly II type in mature to highly-mature stage, the limestone forms a medium-level source rock. In addition, the favorable source kitchen of limestone is larger than that of mudstone. This study provides an important reference for the evaluation of Jurassic marine source rocks and for prediction of petroleum resources in the Qiangtang Basin.

Organic matter enrichment of the Late Triassic Bagong Formation (Qiangtang Basin, Tibet) driven by paleoenvironment: Insights from elemental geochemistry and mineralogy

Geochemical characteristics and sedimentary setting of chang 9 shale in the Upper Triassic Yanchang Formation of southeastern Ordos Basin (NW China)

The Upper Triassic of the Qiangtang Basin is a key strata for oil and gas exploration in the Qinghai-Tibet Plateau. However, it has been controversial argued about the paleoenvironmental control mechanisms for the development of its source rocks. Based on the core samples from Well QK-9, this study reveals the climate and salinity on organic matter accumulation through comprehensive analysis of both organic geochemistry (TOC, Rock-Eval pyrolysis) and inorganic geochemistry (major and trace elements, pyrite content and isotopes). The results are: (1) The middle section consists of relatively good source rocks with relative high TOC (TOC = 0.70%–2.27%, with an average of 1.22%), while the upper and lower sections are poor source rocks for the low TOC (TOC = 0.07%–0.58%, with an average of 0.30%). (2) Paleoclimate (transitioning from humid to arid and back to humid) and paleosalinity (shifting from low to high and then to low) are the primary factors contributing to the vertical differentiation of source rocks. In contrast, the influence of oxidation conditions and biological productivity during that period was relatively minor. (3) In the upper and lower parts of the Bagong Formation, the accumulation of pyrite (with an average of 3.2%) is negatively correlated with low TOC (<0.3%), indicating that the sulfate-reducing bacteria (BSR) exacerbate the degradation of organic matter in a reducing environment. In the middle section, pyrite is less and the TOC increases significantly (with an average of 1.22%). This study reveals the negative correlation between pyrite accumulation and low TOC, challenging the traditional understanding that “reducing environments favor source rock formation” and proposing a “climate-salinity coupling” model for organic matter accumulation as a new theoretical basis for the evaluation of marine source rocks and oil and gas exploration in the complex tectonic background of the plateau.

Characteristics of the Middle Jurassic marine source rocks and prediction of favorable source rock kitchens in the Qiangtang Basin of Tibet

How marine incursions affect the sedimentary environment and the quality of source rocks in the Upper Cretaceous Songliao Basin, NE China

The Upper Triassic Chang 9 organic-rich sediments have been considered as effective hydrocarbon source rocks for the Mesozoic petroleum system in the Ordos Basin. Previous studies on the Chang 9 member mostly focused on the influence of their paleoproductivity and paleoredox conditions on the organic matter (OM) enrichment, whereas there are few studies on the influence of the paleoclimate condition and sediment provenance on the OM enrichment. In this study, a series of geochemical analyses was performed on the Chang 9 core samples, and their hydrocarbon generation potential, paleoclimate condition, and sediment provenance were assessed to analyze the effect of paleoclimate-provenance on OM enrichment. The Chang 9 source rocks are characterized by high OM abundance, type I−II OM type, and suitable thermal maturity, implying good hydrocarbon generation potential. Based on the C-values and Sr/Cu ratios, the paleoclimate condition of the Chang 9 member was mainly semihumid. In addition, the Th/Co vs. La/Sc diagram and negative δEuN indicate that the Chang 9 sediments were mainly derived from felsic source rocks. Meanwhile, the paleoweathering intensity of the Chang 9 member is moderate based on moderate values of CIA, PIA, and CIW, which corresponds to the semihumid paleoclimate. The relatively humid paleoclimate not only enhances photosynthesis of the primary producer, but also promotes chemical weathering intensity, leading to suitable terrestrial clastic influx to the lacustrine basin, which is beneficial for OM enrichment.

Differential enrichment mechanism of organic matters in the marine-continental transitional shale in northeastern Ordos Basin, China: Control of sedimentary environments

The organic matter (OM) enrichment mechanisms and depositional environment characteristics of lacustrine source rocks in the western Bozhong Sag, Bohai Bay Basin in Northeast China remain controversial. To address these issues, based on Rock‐Eval pyrolysis, kerogen macerals, H/C and O/C ratios, GC‐MS, major and trace elements, the Dongying Formation Member (Mbr) 3 (E3d3), the Shahejie Formation mbrs 1 and 2 (E2s1+2), and the Shahejie Mbr 3 (E2s3) source rocks in the western Bozhong Sag were studied. The above methods were used to reveal their geochemical properties, OM origins and depositional environments, all of which indicate that E2s1+2 and E2s3 are excellent source rocks, and that E3d3 is of the second good quality. E3d3 source rocks were formed under a warm and humid climate, mainly belong to fluvial/delta facies, the E3d3 sediments formed under weakly oxidizing and freshwater conditions. Comparatively, the depositional environments of E2s1+2 source rocks were arid and cold climate, representing saline or freshwater lacustrine facies, and the sediments of E2s1+2 belong to anoxic or suboxic settings with large evaporation and salinity. During the period of E2s3, the climate became warm and humid, indicating the freshwater lacustrine facies, and E2s3 was characterized by freshwater and abundant algae. Moreover, compared with other intervals, the OM origin of E3d3 source rocks has noticeable terrestrial input. The OM origin of the E2s1+2 and E2s3 are mainly plankton and bacteria. Tectonic subsidence and climate change have affected the changes of the depositional environment in the western Bozhong Sag, thus controlling the distribution of the source rocks, the geochemical characteristics in the three intervals of lacustrine source rocks have distinct differences. Overall, these factors are effective to evaluate the paleoenvironmental characteristics of source rocks by biomarkers, major and trace elements. The established models may have positive implications for research of lacustrine source rocks in offshore areas with few drillings.

The Lower Cretaceous shales of Chagan Sag, Yingen‐Ejinaqi Basin, are considered as the source rocks of hydrocarbons discovered in the sag. Borehole shales from the Lower Cretaceous were subjected to comprehensive geochemical study to evaluate the quality of source rock, to reconstruct the paleodepositional environment and to determine the controlling factors of organic matter enrichment. Content of present‐day total organic carbon, and hydrogen index, averaging 1.4% and 259.2 mg hydrocarbon/g total organic carbon, respectively reveal poor to fair quality of organic matter. Average value of vitrinite reflectance (Ro) of 1.0% indicates mature organic matter. Maceral composition and biomarker analysis provide evidence for a mixed aquatic and terrigeous organic matter input. Based on Sr/Cu, Sr/Ba, and gammacerane index, we propose that organic matter was deposited in closed hypersaline under hot and arid climate conditions. The ratios of V/(V + Ni), and pristane/phytane (Pr/Ph), as well as the presence of β‐carotene suggest a stratified water column with anoxic bottom water condition. Low to moderate productivity as a result of deficient nutrient supply, the excellent organic matter preservation condition under stratified anoxic water column and high sedimentary rate controlled the quality of source rock. In addition, influence of factors such as content of clay minerals on organic matter enrichment cannot be ignored.

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