Lacustrine Organic-rich Shale Research Articles

Boron Enrichment in Salinized Lacustrine Organic-Rich Shale of the Paleogene Biyang Depression, East China: Occurrence and Geological Controlling Factors

Although boron (B) is widely applied as a paleosalinity indicator for ancient lakes, the occurrence and geological controls of B enrichment in salinized lacustrine organic-rich shale (SLORS) are poorly understood. This study addresses this issue by comparing the mineral and element compositions of high-boron shale (HBS) and low-boron shale (LBS) from the Paleogene Biyang Depression, using integrated XRD, XRF, and ICP-MS analyses. The mineral composition of HBS is dominated by illite, whereas LBS primarily consists of albite; both are of detrital origin. Compared to the element composition of UCC, HBS is extremely enriched in Mo and W, whereas LBS is extremely enriched in W and U. Boron is positively correlated with Al2O3 and negatively correlated with Na2O, suggesting that B primarily occurs in illite. An enhanced extent of chemical weathering prevailed during the deposition of HBS, providing a greater supply of illite to the basin. Higher pH levels and greater reduction during HBS deposition encouraged illite absorption of B, ultimately leading to B enrichment in shale. Our findings suggest that pH and redox conditions, as well as the mineral compositions of shale, should be fully considered during the application of B and related ratios as paleosalinity indicators.

Biogenic “phosphorus” effect of terrestrial lakes and its significance to oil shale during the Carnian period in the late Triassic

The Triassic lacustrine organic-rich shale (LORS) in Member 7 of the Yanchang Formation (Carnian stage) in the southern Ordos Basin is relatively well developed. Additionally, its organic matter abundance, lake biomass and biodiversity, exceed the maximum values of the other strata deposited during the same period. Apatite is one of the most important minerals and is closely related to biological activities. However, the types and genesis of the continental sedimentary apatite and its significance to shale oil are not clear. Here, the stratigraphy, petrology, and sedimentology of the phosphorus-bearing rock series in the Ma Quan section are studied and its sedimentary environment and phosphorus formation are discussed. The current research shows that apatite in the study area can be divided into three categories: collophanite, bone fossils, and spherical microfossils. These three types of biogenic “phosphorus” products provide effective records of the transformation of the biological substances into sedimentary organic matter. There are two main formation mechanisms: the direct action of organisms and the interaction between the organisms. Bone fossils and spherical microfossils are formed by the direct action of organisms, while collophanite is formed by the indirect action of organisms. The phosphatization methods of the organisms mainly include “coating”, “replacement”, and “filling”, and the presence of a ubiquitous phosphate filler potentially reflects a sufficient phosphate supply. Apatite also is highly important for the development of organic matter in oil shale. First, it changes the productivity conditions. Second, the apatite in the shell of spherical microfossils has a certain protective effect on the organic matter in the inner cavity. In the current study, the geological processes of collophanite deposition and mineralization are explained, an important basis for prospecting phosphate resources is provided, and a new field for the study of the organic matter enrichment mechanism of source rocks is established. At the same time, the process of lake biogenic phosphorus formation provides evidence from the continental deposits in the eastern margin of the Tethys region for exploring deep global changes.

Quantitative Analysis of Pore Complexity in Lacustrine Organic-Rich Shale and Comparison to Marine Shale: Insights from Experimental Tests and Fractal Theory

Quantitative Analysis of Pore Complexity in Lacustrine Organic-Rich Shale and Comparison to Marine Shale: Insights from Experimental Tests and Fractal Theory

The occurrences and mobility of shale oil in the pore space of terrestrial shale

Movable oil, the critical factor for shale oil enrichment and effective production, is influenced by the pore structure and shale oil occurrence state. However, the pore spaces storing movable oil, the occurrence state of shale oil, and the lower flowing pore size for shale oil remained unclear. To address this knowledge gap, we chose the lacustrine organic-rich Chang7 shale of the Ordos Basin as the research target. By combining gas adsorption techniques (nitrogen and carbon dioxide) with Soxhlet Extraction, we investigated the pore structure alteration of Chang7 shale after Soxhlet Extraction. Based on a comprehensive analysis of mineralogical composition, TOC, and shale oil components, we discussed the pore space in which shale oil exists. Then, we established a slit-shaped clay model and clarified the occurrence state of multi-component shale oil in the pore space with the help of molecular simulation. Our investigation revealed that (1) Chang7 shale can be categorized into three distinct types. Type I, characterized by a high clay mineral content, possesses the greatest SSA and PV and has the highest abundance of movable oil, partially filling the “ink-bottle”-type pores. Shale oil in Type II and III shales mainly exists within slit-type pores. (2) The pore space of Chang7 shale is primarily comprised of mesopores rather than micropores. Moreover, mesopores’ PV and SSA are predominantly governed by clay minerals rather than organic matter. (3) A threshold pore size of 10 nm significantly impacts the occurrence state of shale oil in the Chang7 shale. Beyond this threshold, free oil accounts for over 80% of the total. Chang7 shale oil within micropores (<2 nm) is predominantly asphaltene, existing in an adsorbed state. Within pores sized 2 to 10 nm, shale oil is present in both adsorbed and free states, and the proportion of free-state shale oil increases with the pore size. Free-state shale oil prevails in pores exceeding 10 nm, with a smaller fraction adsorbed onto organic matter and mineral surfaces. Thus, we recommend adopting 10 nm as the lower flowing pore size limit for Chang7 shale.

Lamina-scale diagenetic mass transfer in lacustrine organic-rich shales and impacts on shale oil reservoir formation

Lamina-scale diagenetic mass transfer in lacustrine organic-rich shales and impacts on shale oil reservoir formation

The formation of the Paleocene lacustrine organic-rich shale in the Subei Basin, East China associated with the early late Paleocene event and marine incursions

The second member of the Funing Formation (E1f2) is a Paleocene lacustrine organic-rich shale and has been identified as a primary source rock for both conventional and unconventional oil and gas in East China. It is correlated with the Early Late Paleocene Event (ELPE), a significant temperature inflection point in the Paleocene. However, the impact of the ELPE on the lacustrine organic-rich sedimentation is still not well understood. This study pinpoints the ELPE in the lacustrine E1f2 shale of the Subei Basin, identifies the resultant marine incursions by biomarkers, correlates the ELPE, marine incursions, and the depositional process, and clarifies organic matter accumulation mechanisms in the E1f2 shale. The interval preceding the ELPE has relatively low organic matter content, having been deposited in an arid paleoclimate with saline water conditions and low productivity. Eukaryotes predominantly contributed to the organic matter accumulation. The ELPE is identified in the E1f2 shale by negative carbonate carbon and oxygen isotope excursions. Subsequently, increasing temperatures and rising global sea levels induced marine incursions into the Subei Basin, supported by the appearance of 24-n-propylcholestane and the C30/C27-30 steranes ratio. The interval following the ELPE shows relatively high organic matter content and was deposited in an arid to semihumid-semiarid climate and a saline to brackish waterbody. Marine incursions likely introduced nutrients and enhanced lacustrine productivity, as indicated by the consistency between signals of marine invasion and bioavailable nutrient elements. Different origins contribute to organic matter accumulation at this stage, with prokaryotes being more dominant than eukaryotes. The E1f2 shale provides valuable insights into lacustrine organic-rich sedimentary processes and the responses of lacustrine systems to global events.

Lake-Level-Fluctuation Control on Shale Oil Enrichment of the Salinized Lacustrine Organic-Rich Shale in the Paleogene Biyang Depression, East China

The paleolake level, which is controlled by the moisture balance (precipitation minus evaporation) within the lake basin, is a significant factor in determining the deposition of lacustrine organic-rich shale (LORS) across geological time, and hence influences shale oil enrichment. However, the impact of lake-level-fluctuations on shale oil enrichment of LORS is not well understood. Based on an integration of bulk geochemistry, organic petrography, pyrolysis gas chromatography, and element compositions, we address this issue using the Paleogene Biyang Depression in East China as an example. High lake levels, combined with anoxic–suboxic conditions, brackish–saline water, high productivity, and low detrital influx, are favorable for LORS deposition, which is characterized by a large distribution area and thickness, a high potential for oil generation and emplacement, and a high free shale oil content. In contrast, LORS deposited during low lake levels, with suboxic–dysoxic conditions, fresh–brackish water, low productivity, and high detrital influx, has a small distribution area and thickness, a low potential for oil generation and emplacement, and a low free shale oil content (a comparable maturity was present in all the studied LORS). Our data suggests that the elevated lake level led to higher salinity, stronger reduction conditions, higher productivity, and lower clastic inflow in the paleolake, forming LORS with higher shale oil potential. It has a positive effect on shale oil enrichment of LORS. The findings are also applicable to regional shale oil exploration.

Mineral diagenesis in lacustrine organic-rich shales: Evolution pathways and implications for reservoir characteristics

Porosity in shale reservoirs is profoundly influenced by diagenetic processes that shape the evolution of pore space. In the Songliao Basin, the first member of the Qingshankou Formation (K2qn1) shale serves as a typical lacustrine clay-rich shale oil reservoir. It exhibits variable mineralogy and thermal maturity, with its reservoir quality strongly influenced by these variations. This study integrates microscopic observations, including field emission scanning electron microscopy (FE-SEM), cathodoluminescence (CL), and organic petrology, with bulk rock investigations employing X-ray diffraction (XRD), Rock-Eval pyrolysis, and elemental analysis, to reconstruct the diagenetic pathways leading to changes in porosity. The findings reveal that K2qn1 shale is primarily composed of extrabasinal grain assemblages, such as detrital quartz and clay minerals, with sedimentary organic matter mainly as lamalginite. As maturity exceeds 0.8 %Ro, the presence of authigenic clay minerals, primarily characterized by chlorite, becomes widespread, often associated with bitumen impregnation. Additionally, authigenic dolomite and pyrite form during the synsedimentary to early diagenetic stages. In immature to marginally mature stages (<0.6 % Ro), mechanical compaction is the primary factor leading to porosity reduction. Pores associated with organic matter become visible when maturity reaches the oil window (>0.7 % Ro). At the peak of hydrocarbon expulsion (approximately 1.2 % Ro), a high frequency of organic matter shrinkage pores at interfaces to clay minerals is present, likely playing a significant role in enhancing permeability. Although localized occurrences of dissolution and cementation can be observed, the development of these diagenetic processes is limited and exerts minimal influence on the physical properties of the bulk rock. This study significantly contributes to our understanding of the mechanisms governing diagenetic processes and reservoir evolution in markedly heterogeneous lacustrine shale, considering the diverse origins of grain assemblages.

Organic matter enrichment and palaeo-sedimentary environment reconstruction of lacustrine shale in the second member of the Middle Jurassic Qiketai Formation in the Shengbei Sub-sag, Turpan–Hami Basin, NW China

Lacustrine organic-rich shales in the second member of the Jurassic Qiketai Formation (J 2 q 2 ) in the Shengbei Sub-sag, Turpan–Hami Basin, NW China exhibit strong heterogeneity due to frequent alternations of sedimentary environments. The distinct shale environments present in both upper and lower units of J 2 q 2 provide an ideal example for studying the enrichment mechanism of organic matter (OM) under a complex sedimentary background. In this study, petrological, mineralogical, major/trace element and isotopic analyses were used to reconstruct the palaeo-environment and reveal the mechanisms of OM enrichment. The results indicate that the J 2 q 2 shale was deposited in a lacustrine mixed carbonate–siliciclastic environment and the palaeo-environment indicators suggesting an oxic–dysoxic, mildly brackish water condition. Based on a comprehensive comparison of both members of J 2 q 2 , an OM enrichment model is established and the main controlling factor of the formation of the organic-rich shale is elucidated. Under the background of a warm-humid climate, the lower unit of J 2 q 2 was deposited in a deeper and more restricted water body with stronger chemical weathering, resulting in limited terrestrial input compared to that of the upper unit. Inorganic geochemical analysis indicates a higher primary productivity in the lower unit of J 2 q 2 with local fluctuations. High primary productivity and favourable preservation conditions domain the OM enrichment in the study area. Supplementary material: Raw data of carbon and oxygen isotopic analysis, TOC, Rock-Eval pyrolysis, and major and trace element analysis are available at https://doi.org/10.6084/m9.figshare.c.6901480

Palaeoclimatic influence on lake palaeoenvironment and organic matter accumulation in the Middle Jurassic Shimengou formation (Qaidam basin, NW China)

The formation and accumulation of lacustrine organic-rich shales are strongly related to palaeoclimate and palaeoenvironmental conditions. Middle Jurassic organic-rich lacustrine shales in the Qaidam Basin, NW China, are important source rocks for unconventional oil and gas exploration, but factors controlling their organic matter (OM) abundance and preservation are unclear. To understand how palaeoclimate and palaeoenvironmental conditions influenced depositional processes and elucidate the factors controlling OM accumulation of lacustrine organic-rich shales, we investigated total organic carbon (TOC), mineral compositions, and element geochemistry the Shale Member of the Middle Jurassic (J2s2) Shimengou Formation from the Yuqia Coalfield in the Qaidam Basin. From the bottom to the top, the Shale Member includes three significant stages based on their average TOC values of 5.45% (Stage I), 9.90% (Stage II), and 1.10% (Stage III). Paleoclimate proxies including C-values and Rb/Sr ratios indicate that in Stage I the lake developed under warm and humid conditions, while in Stage II climate changed to hot and arid/semiarid conditions before returning to warm and humid climates in Stage III. Correlation analysis among proxies for palaeoclimate, terrestrial influx (Al2O3 and TiO2 contents), sedimentation rate ((La/Yb) N ratios), palaeosalinity (Sr/Ba ratios and B/Ga ratios), palaeoproductivity (P/Ti ratios and Sibio contents), redox conditions (Corg/P ratios), and TOC values revealed that palaeoclimate had little direct impact on OM accumulation, with palaeoenvironmental conditions the main controlling factor for OM accumulation. OM accumulation in Stage I was primarily controlled by redox conditions, followed by palaeoproductivity, terrigenous influx, and sedimentation rate. In Stage II, high palaeoproductivity and anoxic bottom water conditions controlled OM accumulation, resulting in the deposition of oil shales with abundant hydrocarbon potential. In Stage III, poor OM preservation related to oxic redox condition was the primary controlling factor for OM accumulation. We suggest that variations in lake palaeoenvironments induced by palaeoclimate change controlled OM accumulation and petroleum potential.

Pore Structure and Geochemical Characteristics of Alkaline Lacustrine Shale: The Fengcheng Formation of Mahu Sag, Junggar Basin

Shale oil and gas are currently the major fields of unconventional hydrocarbon exploration and development. The Fengcheng Formation (FF) shale in the Mahu Sag of the Junggar Basin is an alkaline lacustrine organic-rich shale with an extremely prospective shale oil potential. However, its strong heterogeneity and complex pore structure greatly influence the development of shale oil. It is significant to investigate the pore and geochemical characteristics of shale reservoirs for shale oil extraction. In this study, the pore structure and geochemical characteristics of FF have been investigated using core analysis, Rock-Eval pyrolysis, X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), mercury injection capillary pressure (MICP), low-temperature gas adsorption (LTGA), and X-ray computed tomography (X-ray CT). The results show that the shale of FF has moderate organic matter abundance, and the kerogen is mainly of Type II, which is now at the peak of oil generation. Shale minerals are mainly composed of carbonate (dolomite and calcite) and siliceous (quartz and feldspar) minerals, with extremely low clay mineral content. The pore types are mainly intergranular pores (inter-P), intragranular pores (intra-P), and microfractures associated with mineral particles. The pore space is contributed predominantly by micropores of 0.5–1.2 nm and mesopores of 10–50 nm, whereas macropores are underdeveloped. The pores are mostly ink bottle- and slit-shaped, and the pore connectivity is relatively poor. The pore development of shale in the FF is influenced by organic matter abundance, thermal maturity, mineral composition, etc. Organic matter content (TOC), thermal maturity (Ro), and carbonate minerals have a positive effect on pore development, and the pore volume (PV) increases with TOC, Ro, and carbonate minerals. While clay minerals show a negative effect, the PV decreases with clay minerals. Additionally, the influence of the clay mineral content on the pore morphology of shale should not be ignored. This study investigates the pore structure and geochemical characteristics of the alkaline lacustrine shale of FF in Mahu Sag, which is significant to deepen the understanding of alkaline lacustrine shale and to improve the production of shale oil.

Characteristics and exploration targets of Chang 7 shale oil in Triassic Yanchang Formation, Ordos Basin, NW China

The geological characteristics and enrichment laws of the shale oil in the third submember of the seventh member of Triassic Yanchang Formation (Chang 73) in the Ordos Basin were analyzed by using the information of core observations, experiments and logging, and then the exploration potential and orientation of the Chang 73 shale oil were discussed. The research findings are obtained in three aspects. First, two types of shale oil, i.e. migratory-retained and retained, are recognized in Chang 73. The former is slightly better than the latter in quality. The migratory-retained shale oil reservoir is featured with the frequent interbedding and overlapping of silty–sandy laminae caused by sandy debris flow and low-density turbidity current and semi-deep–deep lacustrine organic-rich shale laminae. The retained shale oil reservoir is composed of black shale with frequent occurrence of bedding and micro-laminae. Second, high-quality source rocks provide a large quantity of hydrocarbon-rich high-quality fluids with high potential energy. The source-reservoir pressure difference provides power for oil accumulation in thin interbeds of organic-poor sandstones with good seepage conditions and in felsic lamina, tuffaceous lamina and bedding fractures in shales. Hydrocarbon generation-induced fractures, bedding fractures and microfractures provide high-speed pathways for oil micro-migration. Frequent sandstone interlayers and felsic laminae provide a good space for large-scale hydrocarbon accumulation, and also effectively improve the hydrocarbon movability. Third, sand-rich areas around the depression are the main targets for exploring migratory-retained shale oil. Mature deep depression areas are the main targets for exploring retained oil with medium to high maturity. Theoretical research and field application of in-situ conversion in low-mature deep depression areas are the main technical orientations for exploring retained shale oil with low to medium maturity.

Controlling factors of organic matter accumulation and lacustrine shale distribution in Lianggaoshan Formation, Sichuan Basin, SW China

The lacustrine shale, represented by the Lianggaoshan Formation, is widely distributed in oil and gas basins of China and will be a key target for unconventional hydrocarbon exploration in the future. Due to the complexity of geological conditions, the distribution of lacustrine shale and the mechanism of organic matter (OM) enrichment show significant differences between different basins. In this study, seismic interpretation, core observation, high-frequency geochemical analysis and other methods are integrated to reveal factors controlling lacustrine shale distribution and OM accumulation in lacustrine shale. The results suggest that six bottom-to-top organic-rich shale intervals are identified within the Lianggaoshan Formation due to lake-basin migration. The migration process of depocenters controls the planar distribution of lacustrine organic-rich shale. The organic-rich lacustrine shale within 1st Member and 2nd Member is characterized by relatively high paleoproductivity and dysoxic condition. The lacustrine organic-rich shale of the upper to the top of 3rd Member is characterized by relatively low paleoproductivity, relatively high terrestrial input, and dysoxic condition. Paleoproductivity and preservation condition caused by lake-level rise are generally the major influencing factor of organic matter accumulation in 1st Member and 2nd Member organic-rich shale. The input of terrestrial OM, and the condition of preservation caused by rapid deposition are the major factors controlling OM accumulation in 3rd Member of Lianggaoshan Formation.

Fractal characteristics of artificially matured lacustrine shales from Ordos Basin, West China

Nanometer scaled pores are critical to studying gas shale reservoirs. In order to obtain the information of the evolution mechanism of nanoscale pore within lacustrine organic-rich shales, artificially matured shale samples from the Ordos Basin were treated using hydrous pyrolysis experiment. Low-temperature nitrogen adsorption, inductively coupled plasma atomic emission spectrometry (ICP-AES), and field emission scanning electron microscopy (FE-SEM) experiments were used to investigate the nanopore evolution with migration and precipitation of materials. The results show that the pore sizes were distributed from 1.1 to 500 nm, and the overall porosity tends to increase first and then decrease. The micropores and fine mesopores (< 10 nm) increased gradually from the 250 to the 350 °C, calcite appeared dissolution following a small peak of feldspar dissolution at this stage, and the CO2 reaches a partial pressure peak at 350 °C. The micropores, mesopores and macropores increased steeply from the 370 °C to the 450 °C. Organic pores were not developed until 350 °C, and well developed at 370 and 400 °C. Organic pores, intergranular pores of clay and intragranular pores of pyrite were well developed at 370 °C. The cumulative specific surface areas increased at 400 °C caused by the dehydration and transformation reaction of clay minerals. This study could provide a reference for the exploration of shale gas in lacustrine shales with different thermal maturities.

The Depositional Environment of the Lacustrine Source Rocks in the Eocene Middle Number of the Liushagang Formation of the Weixinan Sag, Beibuwan Basin, China: Implications from Organic Geochemical Analyses

The Eocene middle number of the Liushagang Formation (LS2) of the Weixinan Sag, Beibuwan Basin, characterized by a thick succession of excellent quality source rocks, is composed of lacustrine organic-rich shales, mudstones, and shales (mudstones/shales). However, the complex and specific depositional environment in the source rocks of LS2 raise questions about the mainly controlling factors of lacustrine organic matter (OM) accumulation. In this study, total organic carbon (TOC) contents, Rock-Eval pyrolysis, as well as biomarker data are used to investigate the nature of the depositional environment and the enrichment mechanism of OM in the source rocks of LS2. The values of Tmax, CPI, C29 steranes αββ/(ααα+αββ), and the 22S/(22S+22R) ratios of the 17α, 21β(H)-C31 hopane together confirm that the OM in the source rocks of LS2 is immature to of low maturity, which suggests that the nature of biomarkers may not be affected by thermal maturity. The hydrocarbon potential was higher in the organic-rich shales (with a mean of 20.99 mg/g) than in the mudstones/shales (with a mean of 7.10 mg/g). The OM in organic-rich shales is type I and II kerogen and that in mudstones/shales is type II kerogen. The C27/C29 regular steranes ratios and 4-methylsterane indices (4MSI) further confirmed the difference in the source of OM between organic-rich shales and mudstones/shales; that is, that the OM of organic-rich shales is mainly derived from the lake algae and aquatic macrophytes and the OM of mudstones/shales is mainly from the higher plants. The values of the gammacerane index and ratios of C21/C23 TT and C24 Tet/C26 TT all indicate that the source rocks from LS2 are deposited in freshwater to a low salinity water column. Moreover, a cross-plot of C21–22/C27–29 sterane versus dia/reg C27 sterane ratios and Pr/Ph ratios suggests that the source rocks from LS2 are recorded to have sub-oxic to oxic conditions. Based on those analyses, two dynamical formation models were proposed: a high-productivity and oxic-suboxic dynamical formation model (Model A) and a low-productivity and oxic-suboxic dynamical formation model (Model B).

Effect of petroleum chemical fraction and residual oil content in saline lacustrine organic-rich shale: A case study from the Paleogene Dongpu Depression of North China

Halite and gypsum minerals in saline shale makes the retention mechanism and chemical fractionation of residual oil unique. The Dongpu Depression in North China is a typically saline lacustrine basin with developing halite and gypsum. The effect of gypsum minerals on residual oil content and chemical fractionation remains unclear. In this study, shale samples with different gypsum contents were used in organic geochemical experiments, showing that the high total organic matter (TOC) content and type Ⅱ kerogen leads to a high residual oil content, as shown by high values of volatile hydrocarbon (S1) and extractable organic matter (EOM). XRD and FE-SEM result indicate that the existence of gypsum in saline shale contributes to an enhanced pore space and a higher residual oil content in comparison to non-gypsum shale. Additionally, the increase in the gypsum mineral content leads to an increase in the saturated hydrocarbon percentage and a decrease in polar components percentage (resins and asphaltene). Furthermore, thermal simulation experiments on low-mature saline shale show that the percentage of saturated hydrocarbons in the residual oil is high and remains stable and that the storage space is mainly mesoporous (>20 nm) in the oil expulsion stage. However, the saturated hydrocarbons percentage decreases rapidly, and oil exists in mesopores (>20 nm and <5 nm) in the gas expulsion stage. In general, gypsum is conducive to the development of pore space, the adsorption of hydrocarbons and the occurrence of saturated hydrocarbon, leading to large quantities of residual oil. The data in this paper should prove to be reliable for shale oil exploration in saline lacustrine basins.

Cyclicity related to solar activity in lacustrine organic-rich shales and their significance to shale-oil reservoir formation

The formation mechanism of micron- to centimeter-scale sedimentary cycles in lacustrine shales is a hot topic of research, because these small-scale sedimentary cycles significantly influence shale-oil distribution heterogeneity. High-frequency paleoenvironmental evolution is an important controlling factor for the formation of small-scale sedimentary cycles. However, the driving factors of high-frequency paleoenvironmental evolution and the formation process of sedimentary cycles under its constraint remain speculative. In this study, which focuses on lacustrine shales, we find that the alternating deposition of variable thickness of organic-rich lamina (ORL) and silty-grained felsic lamina (SSFL) form sedimentary cycles on the micron to centimeter scale in the Chang 73 sub-member of the Yanchang Formation in the Ordos Basin. Based on detailed petrographic characterization, in-situ geochemical parameter testing, and high-resolution cycle analysis, the formation process of cyclical sedimentary records and related paleoenvironmental evolution are investigated. Three solar activity cycles were identified from the shales, namely the 360–500 yr, 81–110 yr, and 30–57 yr cycles (cycles I, II, and III, respectively). High-frequency paleoenvironmental evolution caused by solar activity induced lake-level fluctuation, which further controlled silty-grained sediment deposition and organic matter preservation in deep lake areas. Cycle I controlled relatively long-term lake-level fluctuation, driving several pairs of SSFL and ORL deposition at the centimeter scale. Cycles II and III were short-term cycles and acted on the millimeter to micrometer scale, further complicating the sedimentary strata forming during the period of lake-level fall induced by cycle I. The cyclic deposition of SSFL and ORL correspond to cycle III. Lake-level fluctuation influenced by cycle II mainly caused SSFL thickness variation in each lamina couplet. During the period of lake-level rise induced by cycle I, periods of lake level rise during cycles II, and III show cyclic variation in reducibility, and are not thought to control the supply of coarse-grained sediments in to the deep lake areas. Frequent lake-level fluctuation promotes lamina couplet formation in thickly-bedded shales, which creates favorable conditions for shale-oil accumulation. Oil produced from ORL can migrate-locally into dissolved feldspar porosity in SSFL and therefore is able to accumulate in shales, which creates high potential for future oil exploration in thickly-bedded lacustrine shales.

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.

Characteristics, Genesis, and Hydrocarbon Aspects of Lacustrine Siderite-Rich Marl (Early Jurassic): A Case Study in the Dachanggou Basin, Xinjiang, Northwest China

Thinly bedded marls are widely developed in the oil shale-bearing series of the Lower Jurassic Badaowan Formation in the Dachanggou Basin (Northwest China). In this study, the characteristics, genesis, and hydrocarbon prospects of marls in a lacustrine organic-rich shale series were analyzed via X-ray diffractometry, element geochemistry, carbon and oxygen isotope analysis, total organic carbon content analysis, rock pyrolysis, and sedimentary analysis. The results show that the main carbonate component of marl is siderite, which is mainly present in areas with lithologic changes at the top and bottom of organic shale, and that the development of a thick siderite-rich marl is closely related to the presence of the thick oil shale. The elemental indicators of the sedimentary environment show that the low initial organic matter abundance and low clastic input were conducive to the formation of siderite and that the water body in the marl development period provided an oxic–suboxic environment, although the formation of siderite was still disturbed by oxygen in the relatively reduced bottom sediments. The carbonate carbon isotopes in the marl are clearly positive, indicating that δ13C-rich carbonate provided by methanogens decomposing sedimentary organic matter contributed to the siderite formation. These siderite-rich marls formed in a relatively restricted environment; i.e., they were deposited close to a rich organic layer, with a sufficient methanogenic metabolite CO2 content and a small amount of terrigenous clastic input. These factors reached an appropriate balance, thus forming a narrow zone (partial shallow lake and delta front facies). Although the hydrocarbon generation potential of the marl is low relative to that of other source rocks, the thin marl that is widespread in continental shallow-water organic-rich sedimentary systems can improve the fracturing ability of shale reservoirs, which is of far-reaching significance for the development of shale oil and gas in continental shallow-water sedimentary systems.

Formation conditions and enrichment mechanisms of the Jurassic lacustrine organic-rich shale in the East Fukang Sag, Junggar Basin, NW China: A reassessment based on organic geochemistry

Chemical composition of sediments is often used to evaluate paleoclimate condition, provenance, tectonic setting, depositional condition, and paleoproductivity. However, the validity of these proxies has long been questioned. The comprehensive use of organic and inorganic multi-indicators in combination when interpreting issues related to terrestrial shales should be advocated. The paleodepositional environment, origin of organic matter (OM) and factor controlling OM accumulation in the Early Jurassic Badaowan (J1b) and Sangonghe (J1s) as well as Middle Jurassic Xishanyao (J2x) lacustrine shales in the East Fukang Sag are reassessed by using organic geochemical characteristics of the OM. Some previous knowledge is updated, and some knowledge is further supported by more evidence. The typical clay-rich shale developed under a lacustrine sedimental environment, and the thermal maturity of these organic-rich shales has entered the oil window and formed economic hydrocarbon potential for the tight-oil and shale-oil reservoirs. The paleoclimate conditions of the study area were warm and humid from the Early to Middle Jurassic periods but were colder and drier after the Middle Jurassic period. The salinity of the water column ranged from freshwater to brackish conditions. The J2x Formation was deposited under oxic conditions, while J1b and J1s formations developed under suboxic and reducing environmental conditions. The J2x Formation OM mainly derived from higher plants was deposited in a terrestrial environment,while the OM of J1b and J1s formations was a mixed OM derived from higher plants and bacteria with little algae deposited under bay/estuary environments alternated with terrestrial environments. It is effective to reflect the paleoclimate by element index and judge the salinity by the updated element thresholds, but it is not effective to evaluate the paleoredox conditions by common elemental ratios and to evaluate the paleoproductivity by Ba in the study area.