Organic matter enrichment patterns and sedimentary environments of shale oil: a case study of second member of Funing Formation in Gaoyou Sag, Subei Basin

The second member of the Paleogene Funing Formation (E1f2) in the Gaoyou Sag, Subei Basin, is a promising shale oil target, yet its organic matter (OM) enrichment mechanisms remain poorly understood. This study integrates petrological and multi-proxy geochemical analyses to investigate lithofacies, paleoenvironmental evolution, and OM enrichment of the E1f2 shale. Seven lithofacies types transition upward from laminated (submembers III to V) to blocky structures (submembers I to II). TOC and hydrocarbon potential increase stepwise from bottom to top, with Type II OM dominant. The paleoenvironment evolved from arid, saline, and semideep lacustrine with strong terrigenous input (V); through semiarid to arid, brackish to saline, and semideep to deep lacustrine with peak productivity (III to IV); to humid to semiarid, fresh to brackish, and deep lacustrine with minimal terrigenous input (I to II). Anoxia persisted throughout. OM enrichment is jointly controlled by paleoclimate, water depth, paleosalinity, and terrigenous input, with paleoproductivity subordinate and redox conditions insignificant. Critically, terrigenous input exerts a non-linear dual control, defining an optimal window where nutrient supply, dilution, and oxidation are balanced. The highest OM enrichment in submembers I to II results precisely from terrigenous input falling within this window. This challenges productivity/preservation-dominant paradigms and provides a new framework for shale oil sweet-spot prediction in saline lacustrine basins.

Key factors controlling oil contents in different lithofacies shales from the Funing Formation, Subei Basin: Evidence from scanning electron microscopy

Reducing marine and warm climate conditions during the Late Cretaceous, and their influence on organic matter enrichment in the oil shale deposits of North Jordan

In the Gaoyou Sag located within the Subei Basin, the hybrid shales from the second member of the Funing Formation (E1f2) have been identified as a prolific source of shale oil production, despite their characteristically low organic matter content (TOC < 1.5%). This observation suggests that specific macerals within these hybrid shales demonstrate a pronounced hydrocarbon generation potential, thus unveiling a new frontier for shale oil exploration endeavors. In this study, 16 samples were rigorously extracted from both mudstone and hybrid shale strata within the E1f2. An exhaustive suite of organic and inorganic geochemical analyses was conducted on these specimens. The analyses elucidated several key findings: (1) The maceral composition within the hybrid shales is predominantly comprised of alginite, solid bitumen, and inertinite. Remarkably, the variability of alginite content within the hybrid shales is more pronounced than that observed in high-TOC mudstones. High-TOC mudstones are characterized by a preponderance of lamalginite and a paucity of telalginite, leading to a diminished aggregate hydrocarbon potential. (2) Biomarker ratios (e.g., sterane/hopane, C23 tricyclic terpane/αβ C30 hopane, C24 tetracyclic terpane/C26 tricyclic terpane, etc.) suggest a primary derivation from lower aquatic organisms, with a secondary contribution from terrigenous organic matter within the hybrid shales. (3) The accretion of macerals is governed by an intricate set of factors, including the input of terrigenous detritus, paleosalinity, and paleoproductivity. (4) Alginite is identified as the principal constituent responsible for hydrocarbon genesis in hybrid shales. The proliferation of alginite facilitates the concurrent enrichment of shale oil and organic matter within the hybrid shales of the Subei Basin, illustrating a cooperative mechanism underlying the accumulation of shale oil and organic matter. This indicates that even hybrid shales with scant organic content exhibit considerable potential for shale oil exploration.

Controls on organic matter enrichment of the early-middle Miocene lacustrine oil shale, central Tibet (SW China): New insights from clay minerals and geochemistry

Restoring the sedimentary environment of paleolakes is of great significance to the formation of laminated calcareous shale deposited in paleolakes and the prediction of shale oil reservoir distribution. This article focuses on the second section shale of the Paleogene in the Funing Formation in the Gaoyou Sag, Subei Basin, China, and uses X-ray fluorescence diffraction technology and core lithology analysis methods to obtain the content datum of major and trace elements such as Sr, Cu, Ba, Ga, V, and Ni in shale at different depths. Based on the empirical values of Sr/Cu, Sr/Ba, V/(V + Ni), and total organic carbon, paleoenvironmental evolution of the continental shale was determined and studied, including the changes in paleoclimate temperature, paleosalinity, paleowater depth, and strong or weak redox intensity. The research results indicate that the sedimentary environment of the paleolake in the Paleogene Funing Formation, second section, in the Gaoyou Sag is mainly characterized by a dry and hot climate; the salinity of paleolake water is that of stable brackish water, and the entire sedimentary period of the Funing Formation, second section, is dominated by a reduction environment, which is conducive to the preservation of sedimentary organic matter. The frequent changes in the depth of sedimentary water and the alternating dry and hot climate are the main reasons for the development of laminated calcareous shale in the second section of the Paleogene Funing Formation of the Gaoyou Sag and have also contributed to the abundant commercial resources of laminated calcareous shale oil in the second section of the Funing Formation.

Differential adsorption of clay minerals: Implications for organic matter enrichment

The interactions between clay minerals and organic matter, specifically adsorption, are widely recognized as a crucial mechanism for promoting the preservation of organic matter within sedimentary environments. This paper discusses the genesis and adsorption of clay minerals, especially their influence on the process of organic matter enrichment in sedimentary environments. The composition of clay minerals found in sediments serves as an indicator of both the climatic and lithological characteristics of the provenance area. Smectite formation is favored in mafic provenance, while the coexistence of illite and chlorite suggests cold and arid conditions. Additionally, an abundance of kaolinite indicates intense chemical weathering of the provenance area. The physical and chemical properties of clay minerals and organic matter significantly influence adsorption. Smectite is thought to adsorb more organic matter due to its greater surface area. Selective adsorption of high molecular weight, aromatic, and aliphatic compounds dissolved organic matter onto mineral surfaces may have played a role in kerogen formation. Moreover, the preservation of organic matter via clay mineral adsorption is intricately linked to the sedimentary environment. Disparities in nutrient elements during the clay minerals synsedimentary period can impact biological productivity. Furthermore, the pH and solution metal ions of the water column influence the quantity and type of organic matter adsorbed. The contribution of adsorption to organic matter preservation is influenced by redox conditions, and this contribution can be observed through density separations. Additionally, we present several recommendations for future research. Firstly, diagenesis alters the clay mineral assemblage in sedimentary rocks. However, it may be possible to reconstruct the clay mineral assemblage based on morphological, mineralogical, and chemical composition characteristics. Secondly, studies have indicated that clay minerals can facilitate the precipitation of carbonate minerals, providing insights into the formation of abiotic carbonate minerals. Finally, the practical applications of clay minerals in shale oil and gas exploration are underscored, such as their ability to predict sweet-spot areas and control reservoir properties.

Gradually replacing fossil fuels with renewable energy constitutes a long-term strategy for achieving sustainable development. In the short term, it is necessary to explore unconventional oil and gas resources to support current economic sustainability and to secure essential time for the energy transition. With the continuous growth in global energy demand, unconventional resources such as shale oil and shale gas have become important alternative energy sources. Lacustrine mudrock successions demonstrate significant potential for unconventional oil and gas resources. However, the unclear understanding of how paleoenvironmental evolution influences lithofacies and organic matter enrichment restricts the optimization of shale oil reservoirs and evaluation of shale oil resources, thereby hindering the progress of lacustrine shale oil exploration and development. The mudrocks in the Chang 7 Member of the Triassic Yanchang Formation, Ordos Basin, were deposited in a pro-delta to a deep lacustrine environment and are rich in shale oil resources. Through petrographic, sedimentological, sequence stratigraphic, and geochemical analyses, this study reveals how the evolution of the paleoenvironment controlled the development of mudrocks and the enrichment of organic matter, and establishes a sedimentary model for freshwater lacustrine systems. Six lithofacies have been identified within the mudrock interval of the Chang 7 Member. According to the T-R (transgressive–regressive) sequence model, the Chang 7 Member can be subdivided into three fourth-order sequences, termed Parasequence Set 1–3 (PPS1–3). Mudrock is predominantly developed in the fourth-order sequences PSS1 and PSS2. The PSS1 and the lower part of PSS2 consist of lithofacies 1–4, representing semi-deep to deep lacustrine deposits. The upper part of PSS2 develops lithofacies 5, representing shallow lacustrine to pro-delta deposits. Fluctuations of the lake level controlled the vertical stacking of lithofacies and the transition in depositional mechanisms. During lake-level rise, bottom currents shifted to suspension settling, whereas the opposite occurred during lake-level fall. The organic matter is derived from algae, and its enrichment is jointly controlled by productivity and the redox conditions. Volcanic–hydrothermal activity and a humid climate promoted high productivity in the water body. This high productivity promotes dyoxic conditions in the bottom water. Fourth-order relative lake-level fluctuations also influence organic matter enrichment. During lake-level rise, increased productivity coupled with reduced consumption and dilution favors organic matter enrichment. Conversely, organic matter accumulation is inhibited during lake-level fall. Ultimately, a depositional model for a freshwater lacustrine basin under a humid to semi-humid climatic background was established. This paper elucidates the influence of sedimentary environment on mudrock lithofacies and organic matter enrichment, providing a theoretical basis for optimizing shale oil reservoir selection and resource assessment, thereby promoting efficient exploration and low-carbon development of shale oil in lacustrine basins.

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.

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.

Astronomical control on organic matter enrichment of lacustrine mudstones in the first member of the Late Cretaceous Qingshankou Formation, the Songliao Basin, NE China

Lacustrine depositionalsystems are more sensitive to climaticfluctuations than marine systems, producing significant stratigraphicand sedimentological heterogeneity that affects shale oil distribution.However, the mechanisms linking paleoclimatic variability to shaleoil enrichment remain insufficiently constrained. This study investigatesthe coupling between hydrological variations, organic matter enrichment,and pore structure development under different climatic conditionsin the Second Member of the Funing Formation from Well A1 in the QintongDepression. Using a combination of total organic carbon (TOC) analysis,total sulfur (TS) analysis, X-ray diffraction (XRD), high-resolutiongeochemical profiling, field-emission scanning electron microscopy(FE-SEM), and helium porosimetry, we reveal how paleoclimate influencesshale oil accumulation. The Second Member of the Funing Formationis subdivided into four stratigraphic units based on TOC variations,with units 1 and 3 corresponding to extremely arid conditions, unit2 to arid conditions, and unit 4 to a humid climate. Both arid andhumid conditions promoted elevated lacustrine primary productivityand moderate sedimentation rates, thereby favoring efficient organicmatter accumulation. Increased water depth and optimal salinity alsofacilitated bottom-water anoxia, enhancing organic matter preservation.The study demonstrates that climatic variations in terrigenous clasticinput and lake salinity affect the mineral composition of the lacustrineshale, influencing pore development. Under arid conditions, favorablemineralogical proportions enhances the formation of intercrystallinepores within the clay matrix, improving hydrocarbon storage potential.This research establishes that paleoclimate plays a critical rolein shale oil enrichment by influencing both organic matter accumulationand pore structure. Among the analyzed units, unit 2, deposited underarid conditions, shows the most favorable source–reservoircharacteristics and is identified as the most promising interval forfuture shale oil exploration.

Oil shale formation in the Upper Cretaceous Nenjiang Formation of the Songliao Basin (NE China): Implications from organic and inorganic geochemical analyses

Paleoclimatic and paleoenvironmental constraints on organic matter enrichment in the Paleocene strata in the Lishui Sag, East China Sea Shelf Basin