Southern Songliao Basin Research Articles

Nanoscale pore structure and fractal characteristics of lacustrine shale: A case study of the Upper Cretaceous Qingshankou shales, Southern Songliao Basin, China.

The Upper Cretaceous Qingshankou Formation's lacustrine shales in the Songliao Basin are among China's most promising shale oil reservoirs. To elucidate their pore and fractal characteristics, a comprehensive set of analyses encompassing total organic carbon (TOC), X-ray diffraction (XRD), and low-temperature N2 adsorption (LTNA), Rock-Eval pyrolysis experiments and two-dimensional nuclear magnetic resonance (2D-NMR) were conducted. Using the Frenkel-Halsey-Hill (FHH) method, fractal dimensions (D) were calculated, and their relationship with pore metrics and shale compositions were explored. Two distinct fractal dimensions, D1 (0 < P/P0 < 0.5) and D2 (0.5 <P/P0 <1.0), were derived from LTNA isotherms via the FHH approach. D1 values fluctuated between 2.5715 and 2.7551 (mean 2.6564), while D2 spanned from 2.3247 to 2.4209 (mean 2.3653). Notably, D1 consistently surpassed D2, signifying that smaller pores exhibit greater homogeneity compared to their larger counterparts. D1 gradually increases with the increase of clay content. A direct correlation was observed between pore volume (PV), specific surface area (SSA), and D (both D1 and D2), whereas the association between average pore diameter (APD) and D was inverse. Both D1 and D2 escalated with diminishing TOC, 2D-NMR solid organic matter (OM), S1 content and 2D-NMR light oil. Intriguingly, D1 showed a stronger association with key pore and "sweet spot" parameters, highlighting its utility in assessing pore structural complexity and shale oil potential. This study illustrates how fractal theory enhances our understanding of pore structures and the shale oil enrichment process for the lacustrine shale.

Analysis of Controlling Factors of Pore Structure in Different Lithofacies Types of Continental Shale—Taking the Daqingzi Area in the Southern Songliao Basin as an Example

Due to the influence of terrigenous debris, the internal pore structure of continental shale is highly heterogeneous, and the controlling factors are complex. This paper studies the structure and controlling factors of shale reservoirs in the first member of the Qingshankou Formation in the Southern Songliao Basin using core data and various analytical test data. The results show that the original deposition and subsequent diagenesis comprehensively determine the shale reservoirs’ pore structure characteristics and evolution law. According to the severity of terrigenous debris, the shale reservoirs in the study area are divided into four categories and six subcategories of lithofacies. By comparing the characteristics of different shale lithofacies reservoirs, the results show that the lithofacies with a high brittle mineral content have more substantial anti-compaction effects, more primary pores to promote retention and a relatively high proportion of mesopores/macropores. Controlling the organic matter content when forming high-quality reservoirs leads to two possibilities. An excessive organic matter content will fill pores and reduce the pore pressure resistance. A moderate organic matter content will make the inorganic diagenesis and organic hydrocarbon generation processes interact, and the development of organic matter mainly affects the development of dissolution pores. The comprehensive results show that A3 (silty laminated felsic shale) reservoirs underwent the pore evolution process of “two drops and two rises” of compaction, cementation and pore reduction, dissolution and pore increase, and organic matter cracking and pore increase, and they are the most favourable lithofacies of the shale reservoirs in the study area.

Sedimentary Reservoir Characteristics of Fuyu Oil Formation in the Middle of Fuxin Uplift, Southern Songliao Basin

Currently, the Fuyu reservoir in the middle of the Fuxin uplift, located in the southern Songliao Basin, is in a high water cut development stage. The understanding of this reservoir is limited, and the distribution of favorable reservoirs remains unclear, necessitating detailed reservoir characterization. This study employs a combination of well and seismic technologies to accurately depict the sedimentary and reservoir characteristics of the Fuyu oil reservoir. By conducting detailed correlations of oil zones, a regional high-resolution isochronous stratigraphic framework is established. The division of sedimentary microfacies enables fine characterization of single sand bodies, identifying channel boundaries, clarifying lateral connectivity, and detailing the vertical contact relationships of sand bodies across different deposition periods. The results demonstrate that root-mean-square amplitude and total energy metrics effectively correspond to sand bodies in the study area and can be used to analyze sand body distribution. Provenance is primarily from the southwest to the northeast, with shallow deltas predominantly developed. Delta plain subfacies are found in zones 4-12, while delta front subfacies are in zones 1-3. Microfacies types mainly include the main distributary channel, distributary channel sides, crevasse splay, overbank sand, and underwater distributary channels. The main distributary channel reservoirs exhibit the best physical properties, with porosity ranging from 20-25%, while sand bodies at the sides of distributary channels reach 15-20%. Predicted favorable areas are mainly located at the intersections of distributary channels and structural highs, where drilling is likely to yield high oil flow.

Seismic Prediction Technology for Channel Sand Body of Fuyu Oil Layer in Xinmin Area, Southern Songliao Basin

The Fuyu reservoir in Xinmin area is a delta plane-front facies deposit, and the channel sand body in this area is the main reservoir of oil and gas. Due to the complex sedimentary environment, the sand body shows great variation in thickness and distribution. In this study, the method of combining well vibration, model forward modeling and stratum slicing are used to predict the sandstone bodies of different thickness successfully. Especially for the thin sand body with a thickness of less than 5 meters, this study found that the GR curve can effectively distinguish sandstone and mudstone through the intersection analysis of logging curves, and applied the GR curve co-simulation inversion technology to perform fine characterization. In addition, the study also reveals that the northeastward twisting fault zone in Xinmin area is a favorable accumulation area of the reservoir, which provides a new direction for oil and gas exploration. This study not only improves the accuracy of sandstone prediction, but also provides a valuable reference for oil and gas exploration under similar complex geological conditions.

Impact of CO2 influx on sandstone reservoir quality: A case study of the Quantou Formation, southern Songliao Basin, China

Impact of CO₂ influx on sandstone reservoir quality: A case study of the Quantou Formation, southern Songliao Basin, China

Characteristics of rock fissure fillings and their relationship with the accumulation of uranium and associated elements in the Kailu Sag of the southern Songliao Basin, Northeast China

The relationship between deep fluids and the enrichment of uranium and associated elements in sedimentary basins remains unclear. Sandstone-type uranium deposits in the southern Songliao Basin exhibit abundant fault systems, and evidence of hot fluid activity is present in the ore-bearing strata, including the presence of hydrothermal minerals such as galena and sphalerite. To elucidate this relationship, we conducted detailed thin-section observations and micro-X-ray fluorescence spectroscopic analysis on the fissure fillings of mafic rocks, granite, sandstone, and mudstone in or near the ore-bearing strata. Our findings indicate that rock fissures are filled with minerals such as pyrite, quartz, ankerite, and calcite, suggesting that deep fluids have multiple sources and stages. Specifically, ankerite, rich in Fe and Mn, is closely associated with uranium mineralization, and is observed in both the fissure fillings and the ore-bearing strata. Fluids enriched in elements such as Tb and Dy likely originate from or are related to the mantle, while fluids with few other components may come from deep strata. However, U and Re contents are low in the fissure fillings, being insufficient for their mineralization. Furthermore, given the high content of carbonate minerals in the ore-bearing strata and their close association with uranium mineralization, we analyzed fluid inclusions and carbon–oxygen isotope composition of the carbonate minerals present in the rock fissures. Fluid inclusions in carbonate minerals indicate moderate to low temperatures (ranging from 145.5 to 298.5 ℃, with an average of 215.3 ℃) and low salinities (ranging from 0.18 to 7.99 wt%, with an average of 2.22 wt%). δ13CV-PDB values of carbonate minerals ranges from − 15.1 ‰ to 0.4 ‰, and δ18OV-SMOW values range from 0.5 ‰ to 14.4 ‰, suggesting a composite genesis of carbonate fluids involving magma and atmospheric precipitation. These test results on fissure fillings match roughly with those of the ore-bearing strata, indicating that deep fluids have infiltrated into the ore-bearing strata and contributed to uranium mineralization. Therefore, we propose that the U and Re enrichment in the ore-bearing strata originates from supergene fluids, whereas deep fluids may provide ore-forming materials such as ankerite, reducing fluids, and heat sources for uranium mineralization.

The contradiction between lithospheric thermal and seismic structures reveals a nonsteady thermal state – A case study from Northeast China

The nature of the lithosphere, as the outermost layer of the solid Earth, has long been a focus of the Earth sciences. Many methods have been used to study the properties and structure of the lithosphere, including gravity, magnetism, electricity, seismic, and geothermics. Since the 1970 s, a coupled heat-flow heat-production steady-state model has been employed to study the thermal structure of the continental lithosphere and has helped researchers explain many geothermal observations. However, the steady-state model does not work well in some areas. Taking Northeast China as a case study, we systematically calculated the lithospheric thermal structure, compared the result with seismic research, and concluded that the lithospheric thermal state is nonsteady in the southern Songliao Basin, the southern Greater Xing’an Range and the Changbai Volcano. The lithosphere in southern Songliao Basin features a hot top and a cold bottom. The lithosphere in southern Greater Xing’an Range and the Changbai Volcano feature a cold top and a hot bottom. Our numerical simulation has demonstrated that the response of surface heat flow to thermal disturbance at the bottom of the lithosphere often lags by more than 20 Myr, resulting in a significant disparity between the thermal structure calculated based on surface heat flow and the velocity structure of seismic waves. In addition, this study of the thermal structure in Northeast China shows that the temperature of the Moho in Songliao Basin is very high (>600 °C), which may be an important cause of the lack of Cenozoic volcanism in the Songliao Basin.

The source and preservation of lacustrine shale organic matter: Insights from the Qingshankou Formation in the Changling Sag, Southern Songliao Basin, China

Organic-rich shale in the Cretaceous succession of the large non-marine Songliao Basin in northeastern China, especially the Qingshankou Formation of the Turonian and Coniacian stages, provides a unique record for studying the role of global climate control on organic matter accumulation. Organic enrichment in the Qingshankou Formation along the Southeastern Uplift of the basin has been widely reported and is implicated in the formation of algal blooming and a saline anoxic water environment. However, studies on the Qingshankou Formation in the Changling Sag are relatively few, which leaves a significant gap to understand the controlling factors of organic enrichment at a basin scale. In this study, the sedimentary environment of the first member of the Upper Cretaceous Qingshankou Formation (K2qn1) is reconstructed from petrological and geochemical data to discuss the sources and preservation mechanisms of lacustrine shale organic matter in the Changling Sag (southern Songliao Basin). The K2qn1 is subdivided into three stratigraphic units: Sq1 phase, Sq2 phase, and Sq3 phase. Biomarker (abundance of tricyclic terpane and regular steranes) and petrographical (maceral composition) data indicate that the organic matter in K2qn1 was mainly from terrigenous plants, with some algal input. Inorganic minerals (major and trace elements) and petrological (framboidal pyrite size) data suggest a humid climate and saline anoxic water environment at the Changling Sag during the most organic-rich Sq1 phase. During the Sq2 and Sq3 phases, a relative low lake level and semi-arid climatic conditions likely prevailed. Moreover, these results imply that apart from a saline anoxic water environment, the persistence of continentally derived organic matter in lacustrine shale is mainly controlled by clay minerals. Accordingly, this study proposed a new enrichment model for lacustrine shale organic matter. The new model complements existing lacustrine shale sedimentary models for sources of organic matter and emphasizes the role of clay minerals in preserving organic matter.

Coupling control of organic and inorganic rock components on porosity and pore structure of lacustrine shale with medium maturity: A case study of the Qingshankou Formation in the southern Songliao Basin

Shale components affect organic–inorganic diagenesis, control the formation and evolution of pore structure, and lead to heterogeneity of shale reservoirs. Therefore, revealing the coupling control of shale components on the porosity and pore structure is crucial for selecting the dominant lithofacies. In this study, the lacustrine shale with medium maturity in the first member of the Qingshankou Formation (Qing1 Member) in the Changling Sag, southern Songliao Basin, China, was analyzed. Using pressurized sealed original shale samples and 2-D nuclear magnetic resonance (2D-NMR) analysis, the effective porosity of shale was accurately determined. The pore structure was characterized via experiments such as low-temperature nitrogen adsorption, NMR, and scanning electron microscope (SEM) analyses. In addition, the joint influences of the shale rock components, sedimentary structure, and maturity on the porosity and pore structure were investigated. Finally, the favorable reservoirs in the lacustrine shale with medium maturity and their formation conditions were identified. Studies have shown that for lacustrine shale with medium maturity: (1) a moderate total organic carbon (TOC) content (between 1% and 2.5%) is a key factor for improving the effective porosity and the proportions of mesopores (100–1000 nm) and small-pores (30–100 nm). This is related to the generation of organic acids and the cracking of organic matter into pores. An excessive TOC content can lead to increased plasticity of the rock and filling of the pores by bitumen. (2) The porosity and pore structure are controlled by the organic matter and inorganic minerals as well as sedimentary structure. Organic matter has dual impacts on the porosity improvement related to clay minerals, promotion of clay conversion and filling of clay-related pores, and the ratio of the clay content to the TOC (Vsh/TOC) can reflect the degree of influence of organic matter, the greater the Vsh/TOC ratio is, the greater the effective porosity and proportion of micropores are. (3) Organic matter and felsic minerals jointly effect the improvement of the effective porosity and larger pores (>30 nm), which is related to their good compression resistance, organic pore retention, and dissolution-enhanced porosity effect. The product of the TOC and felsic mineral content (Vsi*TOC) can reflect the degree of their coupled effect. For moderate TOC contents, a higher felsic mineral content, and moderate shell laminae (Vca>10%) are conducive to the development of the effective porosity and larger pores. (4) The organic medium-rich argillaceous laminated felsic shale and organic-medium argillaceous laminated clayey shale have a higher effective porosity and higher proportion of larger pores, making them the dominant lithofacies for shale oil sweet spots.

A novel hybrid CNN–SVM method for lithology identification in shale reservoirs based on logging measurements

Shale oil and gas reservoirs are lithologically complex and heterogeneous and accurate identification of lithology is necessary for the correct identification of lithofacies. There is also a poor mapping relation between logging response and lithology, making reliable lithological identification difficult. We propose a hybrid CNN-SVM model for lithological identification of the shale reservoir in the southern Songliao Basin of Northeast China. As training data, seven conventional logging curves are used, including spontaneous potential (SP) and deep and shallow lateral resistivity (RD, RS) logs. The CNN automatically extracts feature information from well log data and lithology, while the SVM overcomes the problem of limited sample size. Using the receiver operator characteristic (ROC) curves and the area under the curve (AUC) values, we assess the effect of lithological classification. The accuracy of lithological identification for test well H2 is 91.95%, and the AUC of the hybrid model is 0.94, 0.98, and 0.99 in mudstone, shale, and sandstone, respectively. The hybrid model outperforms CNN and SVM in terms of the identification of three types of lithologies and is more stable in terms of AUC value and ROC curve shape. The lithological identification accuracy for test well H3 is 89.49%, which demonstrates that the method has much capacity for generalization and may be extensively utilized in the study area. Finally, from the perspective of model interpretability, SHapley Additive exPlanations (SHAP) is developed to increase transparency and further confirm the reliability of the hybrid model.

Pore Structure Characterization of Lacustrine Shale and its Coupling Relationship with Sedimentary Structure from the Upper Cretaceous Qingshankou Formation, Southern Songliao Basin, NE China

Pore Structure Characterization of Lacustrine Shale and its Coupling Relationship with Sedimentary Structure from the Upper Cretaceous Qingshankou Formation, Southern Songliao Basin, NE China

Origin of quartz and its implications for different phase fluids in lacustrine shales from the Qingshankou Formation, southern Songliao Basin, China

Quartz, an important mineral that occurs in lacustrine shales, can be of different types and origins. Yet, only few studies have investigated how quartz from different sources contribute to phase fluids, such as adsorbed and free fluids present within shale matrix pores. This study integrates various experimental techniques, including X-ray diffraction analysis, geochemical analysis, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and high-frequency two-dimensional nuclear magnetic resonance analysis to quantitatively characterize the different phase fluids associated with various types of quartz occurring in the Qingshankou Formation shale. The characterization results indicated four distinct quartz types: silt-sized quartz, microcrystalline quartz linked with clay minerals, organism-skeletal quartz, and quartz aggregates comprising multiple crystals. The Fe–Al–Mn ternary diagram and SiO2 versus Zr scatter plot indicate that terrigenous clastic material is the primary source of SiO2 in shale quartz. Smectite-to-illite transition plays a pivotal role in authigenic quartz formation. Based on quartz concentration data obtained using major element analysis, we estimated the ranges of terrigenous clastic quartz, diagenetic quartz formed by the smectite-to-illite transition, and biogenic quartz concentrations to be from 55.49% to 92.03% (average 84.70%), 6.67%–20.30% (average 13.12%), and 0%–24.21% (average 2.17%), respectively. The solid organic matter in shale is affected by both terrigenous detrital quartz and authigenic quartz transformed from the clay in the basin. Authigenic quartz, formed by the smectite-to-illite transition, considerably enhances the development of solid organic matter in the Qingshankou Formation. By contrast, an abundance of terrigenous detrital quartz can lead to a dilution of the solid organic matter. Thus, this study provides the first direct evidence that along with a high value of bio-organic matter yield factor, the source of quartz plays a significant role in controlling organic matter abundance.

Deformation and seismic anisotropy of the lithospheric mantle in southern Songliao Basin: Constraints from peridotite xenoliths

Deformation and seismic anisotropy of the lithospheric mantle in southern Songliao Basin: Constraints from peridotite xenoliths

Paleoenvironment and petroleum potential of the lacustrine shales in the Lower Cretaceous Shahezi Formation of the Southern Songliao Basin, NE China

Paleoenvironment and petroleum potential of the lacustrine shales in the Lower Cretaceous Shahezi Formation of the Southern Songliao Basin, NE China

Paleoenvironment and petroleum potential of the lacustrine shales in the Lower Cretaceous Shahezi Formation of the Southern Songliao Basin, NE China

Paleoenvironment and petroleum potential of the lacustrine shales in the Lower Cretaceous Shahezi Formation of the Southern Songliao Basin, NE China

Depositional environment variations and organic matter accumulation of the first member of the Qingshankou formation in the southern Songliao Basin, China

The organic-rich shale in the first member of the Qingshankou Formation, which is located in the southern Songliao Basin, is regarded as a high-quality source rock in East China. Geochemistry parameters were utilized to illustrate the formation and preservation conditions of the Qing1 Member in the Changling Sag, southern Songliao Basin. In the present study, from longitudinal continuity, the samples of organic geochemistry and elemental geochemistry were collected and systematically analyzed. This aims at determining the paleosalinity, paleoclimate, paleoredox conditions, and paleoproductivity and reconstructing its depositional paleoenvironment. According to total organic carbon (TOC) content, the Qing1 Member in the Changling Sag can be classified into two intervals, which are the lower part and the upper part, with high TOC content and low TOC content, respectively. It can be proved from the results of geochemical indicators that under warm and humid climatic conditions the relatively lower part was generated in the anoxic environment. Terrigenous input brought nutrients to the water body of the lake, made algae flourish, and had a relatively high paleoproductivity of the lake, which imposed a vigorous impact on the accumulation of organic matter. The upper part is mainly deposited under weakly oxidizing conditions, with gradually enhanced oxidation and reduced productivity. In contrast to the lower shale, the terrigenous inflow is relatively low. TOC content in the Qing1 Member has a positive relevance with the paleoredox conditions, as well as the paleosalinity conditions, indicating that good preservation is favorable for the accumulation of organic matter. A depositional model is proposed for the organic matter accumulation of this shale. The upper part was in a relatively hot and dry paleoclimate, with a low degree of organic matter enrichment, whereas the lower part had a warm and humid paleoclimate, with the input of terrestrial organic matter into the primary productivity of the lake basin and a high degree of organic matter enrichment.

Characteristics and Key Controlling Factors of the Interbedded-Type Shale-Oil Sweet Spots of Qingshankou Formation in Changling Depression

Different types of shale-oil sweet spots have developed and are vertically stacked in multiple layers of the Qingshankou Formation in the Changling Depression, southern Songliao Basin. Furthermore, this area lacks a classification standard in the optimization of its shale-oil sweet-spot area/layers. Through relevant tests of the region in question’s organic geochemistry, physical properties, oiliness, and pore structure, this paper investigates the formation elements of shale-oil sweet spots. In addition, summaries of its enrichment-controlling factors are given, and the classification standard and evaluation method for understanding the comprehensive sweet spots of the interbedded-type shale oil are then established. The interbedded-type shale oil is enriched in the Qingshankou I Member in the Changling Depression, and it has the features of medium-to-high maturity, the development of inorganic pores and micro-cracks, as well as higher oil saturation and better oil mobility. The sweet-spot enrichment is affected by lamina type, sedimentary facies, maturity, and sand–shale combinations. Both silty-laminated felsic shale and argillaceous-laminated felsic shale, which are developed in semi-deep lakes, are favorable shale lithofacies as they have excellent brittleness and oil mobility. The high maturity and the interbedded combination of sand and shale ensure the efficient production of shale oil, among which the pure-shale section issues a continuous contribution to the production process. Combined with oil testing, sweet-spot classification standards and a comprehensive evaluation of interbedded-type shale oil were established. An area of 639.2 km2 for the interbedded-type shale-oil sweet spots was preferred, among which type I (193 km2) belonged to the combination of “good shale and good siltstone interlayers adjacent”, and type II belonged to “good shale and medium siltstone interlayers adjacent” combination (which have long-term low and stable production prospects). The research provides theoretical guidance on the effective exploration and development of the shale oil of the Qingshankou Formation in the Changling Depression.

Subaqueous volcanic eruptive facies, facies model and its reservoir significance in a continental lacustrine basin: A case from the Cretaceous in Chaganhua area of southern Songliao Basin, NE China

The conventional lithofacies and facies model of subaerial and marine pyroclastic rocks cannot reflect the characteristics of subaqueous volcanic edifice in lacustrine basins. In order to solve this problem, the lithofacies of subaqueous eruptive pyroclastic rocks is discussed and the facies model is established by taking the tuff cone of Cretaceous Huoshiling Formation in the Chaganhua area of the Changling fault depression, Songliao Basin as the research object. The results indicate that the subaqueous eruptive pyroclastic rocks in the Songliao Basin can be divided into two facies and four subfacies. The two facies are the subaqueous explosive facies and the volcanic sedimentary facies that is formed during the eruption interval. The subaqueous explosive facies can be further divided into three subfacies: gas-supported hot pyroclastic flow subfacies, water-laid density current subfacies and subaqueous fallout subfacies. The volcanic sedimentary facies consists of pyroclastic sedimentary rocks containing terrigenous clast subfacies. A typical facies model of the tuff cone that is formed by subaqueous eruptions in the Songliao Basin was established. The tuff cone is generally composed of multiple subaqueous eruption depositional units and can be divided into two facies associations: near-source facies association and far-source facies association. The complete vertical succession of one depositional unit of the near-source facies association is composed of pyroclastic sedimentary rocks containing terrigenous clast subfacies, gas-supported hot pyroclastic flow subfacies, water-laid density current subfacies and subaqueous fallout subfacies from bottom to top. The depositional unit of the far-source facies association is dominated by the subaqueous fallout subfacies and contains several thin interlayered deposits of the water-laid density current subfacies. The gas-supported hot pyroclastic flow subfacies and the pyroclastic sedimentary rocks containing terrigenous clast subfacies are favorable subaqueous eruptive facies for reservoirs in continental lacustrine basins.

Channelised magma ascent and lithospheric zonation beneath the Songliao Basin, Northeast China, based on surface-wave tomography

The Songliao Basin in Northeast China is located within the eastern part of the Central Asian Orogenic Belt and to the north of the accretionary belt of the North China Craton. The lithosphere beneath the basin and the surrounding region has undergone a series of complex deformation processes due to sequential interactions among different tectonic plates since the Palaeozoic. Here we construct a three-dimensional lithospheric shear-wave (S-wave) velocity model for the region via surface-wave tomography to better understand the deep dynamic processes that shaped the Songliao Basin. The S-wave velocity model highlights strong linkages between the surface geology and mantle lithospheric structures. The Baicheng–Changchun uplift (or Song-Liao watershed), which divides the basin into two river systems, is underlain by a dome-like area of uplifted crust. The central Songliao basin is underlain by a weak, thin lithosphere that is ∼80 km thick, whereas the eastern Great Xinggan Range to the northwest is underlain by a normal crust and lithosphere that is ∼100 km thick. The northern North China Craton and southern Songliao Basin are also underlain by normal crust and a lithosphere that is up to ∼150 km thick, with a low-velocity zone penetrating the high-velocity mantle lithosphere at ∼80–100 km depth. Such structural features imply that the far-field effect of Pacific Plate subduction since the Cenozoic varies across Northeast China and is dependent on the lithospheric structures that existed prior to subduction.

The sedimentary filling differences of subsags and their impacts on oil shale and coal accumulation in a rift lacustrine sag: A case study from the Lower Cretaceous in the Zhangqiang sag, southern Songliao Basin, Northeast China

In order to characterize the sedimentary filling differences of two rift lacustrine subsags and their control factors and to construct the depositional model for coal and oil shale, the Lower Cretaceous Jiufotang and Shahai formations in the Qijiazi and Zhanggutai subsags, Zhangqiang sag, Songliao Basin, one of the most petroliferous basins in Northeast China have been analyzed. Seven facies were identified in the Lower Cretaceous strata of two subsags and were grouped into three facies associations, namely alluvial fan, fan delta and lacustrine. During the deposition of Jiufotang Formation, the Qijiazi subsag is mainly characterized by alluvial fan deposits, while the Zhanggutai subsag is dominated by fan delta and littoral-sublittoral lacustrine deposits. During the deposition of Shahai Formation, the Qijiazi subsag is characterized by fan delta, littoral-sublittoral lacustrine, and deep lacustrine deposits, whereas the Zhanggutai subsag is filled with fan delta and littoral-sublittoral lacustrine deposits. Coal and oil shale deposits are only preserved in the Shahai Formation of the Qijiazi subsag. Total organic carbon content, organic matter type, vitrinite reflectance, thermal alteration index and total reduced sulfur content also show that there are differences in organic matter preservation and redox conditions between two subsags. In the Jiufotang Formation, the total organic carbon contents are relatively low, the kerogen type is dominated by type III and the source rocks entered a mature phase in the Zhanggutai subsag, and the total reduced sulfur contents show a suboxic to oxidized environment in the Zhanggutai subsag and an oxidized environment in the Qijiazi subsag. In the Shahai Formation, the total organic carbon contents are relatively high, the kerogen type is dominated by type II and the source rocks entered an immature to mature phase in the Zhanggutai subsag, whereas the kerogen type is dominated by type III and the source rocks display an overall mature phase in the Qijiazi subsag. The total reduced sulfur contents of the Shahai Formation show the reduced environment in two subsags. Tectonics, sediment supply and redox conditions control the temporal and spatial differences in sedimentary filling and distribution of organic-rich rocks in two subsags. A comparison of rift lacustrine Qijiazi subsag and Fushun Basin shows that the rift basins are both characterized by deepening-upward to shallowing-upward successions controlled by tectonic subsidence and climate changes. The relatively high subsidence along with warmer climate resulted in the enhanced input of lake plankton and the decrease in higher plant input and triggered the transition from coal to oil shale formation.