Cretaceous Qingshankou Formation Research Articles

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.

Enrichment and movability of lacustrine tight shale oil for the first member of the Upper Cretaceous Qingshankou Formation in the Sanzhao Sag, Songliao Basin, NE China: Insights from saturated hydrocarbon molecules

The Sanzhao Sag is one of the main hydrocarbon generation sags with good shale oil exploration and exploitation potential in the northern Songliao Basin, and the first member of the Upper Cretaceous Qingshankou Formation (K2qn1) is regarded as the most important target layer. In this paper, the complete K2qn1 rock cores from the first shale oil parameter well named the SYY3 well in the study area were taken to investigate tight shale oil enrichment and movability from saturated hydrocarbon molecules combined with bulk organic geochemical characteristics. The results show that the K2qn1 mudstones/shales in the SYY3 well are of high organic matter abundance (1.28–5.62 wt% TOC), good organic matter types (I and II1 kerogens) and medium maturity (0.82%–0.98% Ro), creating excellent material basis of hydrocarbon generation for shale oil enrichment. The molecular compositions of saturated hydrocarbons suggest that the K2qn1 were deposited in reducing brackish lacustrine water with mainly prokaryotes and secondly eukaryotic algae as the biotic input, and are characterized by obvious two-stage distribution with buried depth. The lower unit (>2015.00 m) is generally more enriched in shale oil than the upper unit (<2015.00 m). Relative to the upper unit, the lower unit has lower ratios of pristane/phytane, gammacerane/C30 hopane, C19-20/C21-26 tricyclic terpanes, C24 tetracyclic terpane/C26 tricyclic terpanes, 4-methylstane/C27-29 αααR steranes, higher ratios of regular steranes/17α-hopanes, C27/C29 and C28/C29 αααR sterane, and higher contents of diadrimanes, diahopanes and diasteranes. This indicates that the lower unit can be deposited in a more reducing, lower salinity and more clay-rich sedimentary environment, and contains more eukaryotic algae and less prokaryote biotic input than the upper unit, which can be conducive to the formation and retention of shale oil in the lower unit. The S1 values showing certain positive correlations with regular steranes/17α-hopanes and C27/C29 αααR sterane indicates that the biotic input contribution of eukaryotic algae can be an important positive factor influencing free oil content. The negative correlation between (nC21 + nC22)/(nC28 + nC29) and OSI (S1 × 100/TOC) can be due to diffusion fractionation effect of hydrocarbon molecules in micro-nano reservoir spaces. Low (nC21 + nC22)/(nC28 + nC29) values usually indicate good shale oil movability and reservoir space connectivity under the similar thermal maturity.

Enrichment model and major controlling factors of below-source tight oil in Lower Cretaceous Fuyu reservoirs in northern Songliao Basin, NE China

Based on the geochemical, seismic, logging and drilling data, the Fuyu reservoirs of the Lower Cretaceous Quantou Formation in northern Songliao Basin are systematically studied in terms of the geological characteristics, the tight oil enrichment model and its major controlling factors. First, the Quantou Formation is overlaid by high-quality source rocks of the Upper Cretaceous Qingshankou Formation, with the development of nose structure around sag and the broad and continuous distribution of sand bodies. The reservoirs are tight on the whole. Second, the configuration of multiple elements, such as high-quality source rocks, reservoir rocks, fault, overpressure and structure, controls the tight oil enrichment in the Fuyu reservoirs. The source-reservoir combination controls the tight oil distribution pattern. The pressure difference between source and reservoir drives the charging of tight oil. The fault-sandbody transport system determines the migration and accumulation of oil and gas. The positive structure is the favorable place for tight oil enrichment, and the fault-horst zone is the key part of syncline area for tight oil exploration. Third, based on the source-reservoir relationship, transport mode, accumulation dynamics and other elements, three tight oil enrichment models are recognized in the Fuyu reservoirs: (1) vertical or lateral migration of hydrocarbon from source rocks to adjacent reservoir rocks, that is, driven by overpressure, hydrocarbon generated is migrated vertically or laterally to and accumulates in the adjacent reservoir rocks; (2) transport of hydrocarbon through faults between separated source and reservoirs, that is, driven by overpressure, hydrocarbon migrates downward through faults to the sandbodies that are separated from the source rocks; and (3) migration of hydrocarbon through faults and sandbodies between separated source and reservoirs, that is, driven by overpressure, hydrocarbon migrates downwards through faults to the reservoir rocks that are separated from the source rocks, and then migrates laterally through sandbodies. Fourth, the differences in oil source conditions, charging drive, fault distribution, sandbody and reservoir physical properties cause the differential enrichment of tight oil in the Fuyu reservoirs. Comprehensive analysis suggests that the Fuyu reservoir in the Qijia–Gulong Sag has good conditions for tight oil enrichment and has been less explored, and it is an important new zone for tight oil exploration in the future.

Hydrocarbon micro‐migration and differential enrichment mechanism of shale oil in upper Cretaceous Qingshankou Formation in Songliao Basin

Shale reservoir has strong heterogeneity in mineral composition and oil content even at a short distance in the same interval. To better understand the accumulation mechanism of shale oil and hydrocarbon migration tendency in the interval, and explore the main influencing factors of distribution. This paper employed various methods, including thin‐section observation, TOC (total organic matter (OM)) analysis, rock pyrolysis, Soxhlet extraction, group component separation, and GC–MS (gas chromatography mass spectrometry) analysis of saturated hydrocarbons. The shale oil samples were collected from an exploratory well in the Songliao Basin, and the distribution and enrichment characteristics of shale oil interlayers were analysed. The results show that the first member of Qingshankou Formation (K2qn1) could be divided into six small layers. From Q1 to Q6, the lithofacies could be divided into three types: high‐frequency laminar shale, massive shale, and bioclastic shale. The lacustrine matrix shale of K2qn1 in Songliao Basin has medium TOC, and high oil contents, OM conversion rate. The TOC ranges from 1.5 to 4.2 wt%, S1 varies between 1.4 and 4 mg/g, S2 falls within the range of 4 to 10.6 mg/g, Tmax ranges from 351 to 497°C, and the OSI varies between 57 and 115.34 mg HC/g rock. The lower part of the K2qn1, Q1–Q4, characterized by high‐frequency laminar shale, contained residual shale oil, while the upper part of the K2qn1, Q5 and Q6, characterized by bioclastic shale and massive mudstone, and are dominated by in‐situ type shale oil. The crude oil in K2qn1 is homologous, and primarily undergoes micro‐migration. The Q5–Q6 section has the highest light oil content, while other regions with notable light components are located in the middle of Q1, the upper sections of Q2, Q3, and Q4.Q1–Q4 are primarily composed of residual shale oil, whereas Q5–Q6 is primarily composed of in‐situ shale oil, and the primary micro‐migration direction of shale oil occurs laterally within Q1–Q4. The lateral migration of each section mainly occurs at the upper part of Q3 and Q4, the lower part of Q1 and Q2, the bottom of Q5, and the interface between Q6 and the second member of Qingshankou formation (K2qn2). The block mudstone retains in‐situ shale oil, especially light hydrocarbons. Within the high‐frequency laminar shale, there is strong horizontal connectivity, and lateral hydrocarbon expulsion serves as the primary mechanism for micro‐migration in Q1–Q4. This paper can provide reference values for K2qn1 Formation shale oil and the migration and differential reservoir formation of medium and high‐maturity matrix shale oil.

Vertical heterogeneity of Gulong shale oil governed by the OM precursors and oil accumulation: Implications for shale oil exploration in the Songliao Basin, Northeastern China

Shale oil has been discovered and produced in the Cretaceous Qingshankou Formation (K2qn) of the Gulong sag in the Songliao Basin (the Gulong shale oil). However, the selection of favorable layers for shale oil development remains uncertain due to significant variations in oil production of different intervals in the Formation (Q1–Q9). In this study, the geochemical features of shale, the content and chemical features of extracted oils in different shale intervals are analyzed in detail, and the favorable intervals and mechanism for shale oil accumulation are also addressed. It is shown that the Gulong shale has total oil contents of 200–400 mg/g TOC with considerable contents of light hydrocarbons in extracted oils. In the shale, there is a “W” trend for the evolution of chemical and carbon isotopic compositions (δ13C) of shale oil with depth. It is deduced that this evolution trend of shale oil is governed by the variation of organic matter precursor associated with the deposition environment. The distributions of δ13C of mud gas and individual n-alkanes with depth reveal that the Gulong shale oil can be separated as two independent petroleum systems at the Middle–Lower Q7. Mass balance calculations imply Q2–Q3 and Q8–Q9 are two intervals with lower oil expulsion efficiency. Combined with the morphology of calcite cements and veins as well as the homogenization temperature of inclusions, an intralayer accumulation model of the Gulong shale oil was established. In addition, the statistical analysis of the oil content, lithological properties, and oil production capacity of different layers from 95 wells, demonstrates that Q2–Q3 and Q8–Q9 can be selected as the preferential target layers for future development of the Gulong shale oil. More importantly, this study may provide new insights into the enrichment of shale oil with tight shale type in China and around the world.

Geological characteristics of unconventional tight oil reservoir (109 t): A case study of Upper Cretaceous Qingshankou Formation, northern Songliao Basin, NE China

The Daqing exploration area in the northern Songliao Basin has great potential for unconventional oil and gas resources, among which the total resources of tight oil alone exceed 109 t and is regarded as an important resource base of Daqing oilfield. After years of exploration in the Qijia area, Songliao Basin, NE China, tight oil has been found in the Upper Cretaceous Qingshankou Formation. To work out tight oil’s geological characteristics, taking tight oil in Gaotaizi oil layers of the Upper Cretaceous Qingshankou Formation in northern Songliao Basin as an example, this paper systematically analyzed the geological characteristics of unconventional tight oil in Gao3 and Gao4 layers of the Qijia area, based on the data of the geological survey, well drilling journey, well logging, and test. It is that three sets of hydrocarbon source rocks (K2qn1, K2qn2+3, and K2n1) develop in the examined area, and exhibit excellent type I and II kerogens, high organic matter abundance, and moderate maturity. The reservoir is generally composed of thin-bedded mudstone, siltstone, and sandstone, and presents poor porosity (average 8.5 vol.%) and air permeability (average 4 mD). The main reservoir space primarily includes intergranular pores, secondary soluble pores, and intergranular soluble pores. Three types of orifice throats were identified, namely fine throat, extra-fine throat, and micro-fine throat. The siltstone is generally oil-bearing, the reservoirs with slime and calcium become worse oil-bearing, and the mudstone has no obvious oil-bearing characteristics. The brittleness indices of the sandstone in the tight oil reservoir range from 40% to 60%, and those of the mudstone range from 40% to 45%, indicating a better brittleness of the tight oil reservoir. Based on the study of typical core hole data, this paper gives a comprehensive evaluation of the properties of the tight oil and establishes a tight oil single well composite bar chart as well as the initial evaluation system with the core of properties in the tight oil reservoir. This study has theoretical guiding significance and practical application value for tight oil exploration and evaluation in the Qijia area.©2024 China Geology Editorial Office.

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

Application of nano-scratch technology to identify continental shale mineral composition and distribution length of bedding interfacial transition zone - A case study of Cretaceous Qingshankou formation in Gulong Depression, Songliao Basin, NE China

Bedding is the smallest macroscopic layer in a sedimentary sequence, usually containing different mineral compositions and ranging in thickness from a few tens of micrometers to a few millimeters. The microscale mineral composition and mechanical properties of the bedding interfacial transition zone (BITZ) have a significant influence on the crack propagation mode, which further leads to changes in hydraulic fracturing efficiency. At present, most of the identification of the shale mineral composition and the distribution length of the bedding interface transition zone relies on scanning electron microscope, but this method can only obtain the surface morphology and cannot further evaluate the mechanical properties. In this paper, the mechanical parameters such as friction coefficient and fracture toughness of shale samples from Songliao Basin were measured by nano-scratching technique. The damaged shapes and mineral composition in the scratched area were analyzed by scanning electron microscope. Indentation depth and fracture toughness data obtained from scratch tests were fitted with a Gaussian function to determine mineral composition. The slope of the d2-L curve is closely related to the mechanical behavior of minerals and is considered an effective parameter for identifying mineral composition. Based on the fluctuation characteristics of friction coefficient and fracture toughness curve, a quantitative method of shale BITZ distribution length based on linear regression is proposed, and the upgrade from single-parameter to multi-parameter combination is realized by using the method of dimensional analysis. The results of the study show that plastic minerals can effectively absorb energy and convert it into volume deformation and the rapid release of strain energy from brittle minerals results in the collapse of rock debris. The identification of mineral composition based on fracture toughness is a more accurate and effective method compared to the scratch depth and the slope of the d2-L curve. As the influence of mechanical performance was not considered, the length of BITZ distribution determined by SEM was significantly smaller than the analysis results of mechanical parameters. Compared with the friction coefficient method and the fracture toughness method, the dimensionless parameter method integrates several mechanical parameters and has high accuracy and applicability in identifying the distribution length of the interfacial transition zone of shale bedding. The results will help study the mechanical properties of different mineral interfaces of shale at the microscopic scale using the nano-scratch technique.

An Improved Method for Evaluating Hydrocarbon Generation of Shale: A Case Study of the Lower Cretaceous Qingshankou Formation Shale in the Songliao Basin

AbstractBecause of the influence of hydrocarbons, especially adsorbed hydrocarbons, on the detection of cracked hydrocarbon (S2) and total organic carbon (TOC), the hydrogen index (HI)‐based hydrocarbon generation model deviates from actual practice. In this study, the shale in the first member of the Qingshankou Formation in the central depression of the Songliao Basin, where in northeastern China, was taken as the research object and a correction method for S2 and TOC was established. By correcting the experimental results of different maturity samples, the actual hydrocarbon generation model has been revealed, the differences before and after correction compared, and the evolutionary characteristics of the adsorbed hydrocarbon content were clarified. The results show that the organic matter enters the hydrocarbon generation threshold at Ro–0.5% and reaches the hydrocarbon generation peak at Ro–1.0% and that the hydrocarbon generation process ends at Ro–1.3%. The hydrocarbon generation model established based on the measured values has a ‘lag effect’ compared to actual values, and this extends the hydrocarbon generation window of organic matter and delays the hydrocarbon generation peak. With the increase of maturity, adsorbed hydrocarbon content shows the characteristics of ‘first increasing, then stabilizing, and then decreasing’, and reaches the most abundant stage at Ro of 0.9%–1.3%.

Identification, evolution and geological indications of solid bitumen in shales: A case study of the first member of Cretaceous Qingshankou Formation in Songliao Basin, NE China

On the basis of sorting out current understanding of solid bitumen (SB) in shales and taking organic-rich shales in the first member of the Cretaceous Qingshankou Formation in the Songliao Basin as an example, the definition, classification, occurrence and evolution path of SB are systemtically studied, and the indicative significance of SB reflectance (Rob) on maturity and its influence on the development of reservoir space are discussed and summarized. The results show that the difference of primary maceral types is primarily responsible for the different evolution paths of SB. Most of the pre-oil bitumen is in-situ SB with only a small amount being of migrated SB, while most of the post-oil bitumen and pyrobitumen are migrated SB. From the immature to early oil maturity stage, bituminite, vitrinite, and inertinite can be distinguished from SB based on their optical characteristics under reflected light, and alginite can be differentiated from SB by their fluorescence characteristics. Under scanning electron microscope, in-situ SB and migrated SB can be effectively identified. Rob increases linearly with increasing vitrinite reflectance (Ro), as a result of a decrease of aliphatic structure and the enhancement of aromatization of SB. Within the oil window three types of secondary pores may develop in SB, including modified mineral pores, devolatilization cracks and bubble holes. At a high maturity stage spongy pores may develop in pyrobitumen. Scanning electron microscopy combined with in-situ SEM-Raman spectroscopy can further reveal the structral information of different types of SB, thus providing crucial data for understanding for understanding OM migration paths, dynamics, and distances at micro-scale.

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

Astronomical forcing has an important effect on organic matter enrichment in marine mudstones, but this effect remains poorly constrained in lacustrine mudstones. Here, we investigate the astronomical control of organic matter enrichment in lacustrine black mudstone from the first member of the Qingshankou Formation in the Songliao Basin. Detailed mineralogical, lithological, geochemical, and cyclostratigraphic studies were carried out. The results show that there are high-frequency variations in the organic matter content in the mudstones of the Qingshankou Formation. The upper and lower sections are further divided according to the organic matter content level and the variation frequency. The organic matter content in the lower section is high, up to 6.08 wt%, with long-period variation and individual cycles around 10 m. The organic matter content in the upper section is relatively low, the highest organic matter content is only 2.91 wt%, with short-period variation and individual cycles mostly less than 5 m. Cyclostratigraphic studies indicate that significant astronomical signals are recorded in the mudstone stratigraphy. 100 kyr short eccentricity and 38 kyr obliquity strongly control the variation of organic matter content in the lower and upper sections, respectively. Organic matter enrichment has a closely synchronized phase relationship with paleoproductivity, preservation conditions, and dilution indicators. Astronomical cycles influence the depositional conditions of organic matter enrichment by controlling climate change, which eventually forces organic matter enrichment. At high values of eccentricity and obliquity, enhanced insolation, monsoon, precipitation, and river runoff caused lake expansion, increased input of terrestrial nutrients, and increased bottom water reduction conditions, which are conducive to organic matter enrichment. On the contrary, the depositional conditions for organic matter enrichment become poor.

Widespread development of bedding-parallel calcite veins in medium–high maturity organic-rich lacustrine shales (Upper Cretaceous Qingshankou Formation, Northern Songliao Basin, NE China): Implications for hydrocarbon generation and horizontal compression

Bedding-parallel calcite veins (BPCVs) occur widely in Upper Cretaceous lacustrine shales of the Qingshankou Formation in the Northern Songliao Basin, China. Research on the formation processes and geological significance of BPCVs has focused on the sources of fluid, timing of formation, initiation, dilation and growth mechanisms, and on relationships with hydrocarbon generation. In this study, we addressed these issues through a comprehensive study of drill cores containing BPCVs using petrographic observations, isotope geochemistry, rare earth element (REE) analysis, fluid inclusion analysis, in situ U–Pb dating, and basin modeling. BPCVs were observed in medium–high maturity shales with high organic matter contents with heterogeneous compositions. Similarities in δ13C, δ18O, 87Sr/86Sr, and REEs contents between BPCVs and the host shale indicate that locally derived formation fluids driven by chemical gradients were involved in BPCV formation. The in situ U–Pb age dating (70 ± 8.9 Ma and 70.6 ± 9.7 Ma) constrained BPCVs formation to the late Cretaceous, corresponding to a burial depth of ∼1600 m, indicating that the BPCVs formed in the oil window. The latter is also supported by fluid inclusions-derived temperatures (∼76–109 °C). BPCV formation included vein initiation and dilation, and the microstructures, e.g., domal and pincer structures, sigmoidal calcite veins, conjugate en-échelon calcite vein arrays, as well as mechanical crystal twins of the BPCVs, all indicate that tectonic compression influenced fracture development as also indicated in other basins where beef occur. Fluid overpressure caused by hydrocarbon generation, tectonic compression, and clay mineral transformation (mainly smectite to illite conversion) was responsible for vein dilatation. Antitaxial fibrous crystals with smooth crystal boundaries and high aspect ratios, containing sinusoidal solid inclusions as well as median planes, indicate that crystallization force promoted vein growth. Thus, BPCVs can be used as a good indicator of hydrocarbon generation, and horizontal tectonic compression.

Links between marine incursions, lacustrine anoxia and organic matter enrichment in the Upper Cretaceous Qingshankou Formation, Songliao Basin, China

There is a general relationship between the formation of terrestrial hydrocarbon source rocks and marine incursions during the Mesozoic–Cenozoic in China. To better understand this scientific issue, this study focused on the little-studied Upper Cretaceous Qingshankou Formation (early Late Cretaceous) in the Songliao Basin, China, which was investigated using organic petrological and organic and inorganic geochemical methods. The mudstone and shale in the Qingshankou Formation were deposited in an anoxic, reducing, slightly brackish water environment and contain benthic macroalgae, which is a typical marine fossil in Qingshankou Member I. The δ34S values are generally similar to those of Late Cretaceous marine sulfate, and the total N contents and isotopic compositions (δ15N) are both high, suggesting that the Qingshankou Formation was affected by marine incursions that triggered lake anoxia. The total organic carbon (TOC) contents of the mudstones and shales in the Qingshankou Formation are 0.21–3.86 wt%, with an average value of 1.91 wt%, which meet the criterion of a very good–excellent source rock. The organic matter varies from type I to III kerogen but is mainly types I and II, which generate oil. The hydrocarbon-generating bio-precursors are mainly planktonic and benthic algae. The hydrocarbon generation potential of the Qingshankou Member I is high, mainly due to the high productivity during its deposition. Increased productivity caused by marine intrusions, water stratification caused by high salinity, and reducing bottom-water environment provided favorable conditions for organic matter enrichment. The marine incursions during deposition of the Qingshankou Formation (especially Member I) improved the quality of the source rocks by enhancing the primary productivity, which might be a common feature of terrestrial source rocks affected by such incursions in general.

Gulong shale oil enrichment mechanism and orderly distribution of conventional– unconventional oils in the Cretaceous Qingshankou Formation, Songliao Basin, NE China

Through the study of organic matter enrichment, hydrocarbon generation and accumulation process of black shale of the Cretaceous Qingshankou Formation in the Songliao Basin, the enrichment mechanism of Gulong shale oil and the distribution of conventional–unconventional oil are revealed. The Songliao Basin is a huge interior lake basin formed in the Early Cretaceous under the control of the subduction and retreat of the western Pacific plate and the massive horizontal displacement of the Tanlu Fault Zone in Northeast China. During the deposition of the Qingshankou Formation, strong terrestrial hydrological cycle led to the lake level rise of the ancient Songliao Basin and the input of a large amount of nutrients, resulting in planktonic bacteria and algae flourish. Intermittent seawater intrusion events promoted the formation of salinization stratification and anoxic environment in the lake, which were beneficial to the enrichment of organic matters. Biomarkers analysis confirms that the biogenic organic matter of planktonic bacteria and algae modified by microorganisms plays an important role in the formation of high-quality source rocks with high oil generation capability. There are four favorable conditions for the enrichment of light shale oil in the Qingshankou Formation of the Gulong Sag, Songliao Basin: the moderate organic matter abundance and high oil potential provide sufficient material basis for oil enrichment; high degree of thermal evolution makes shale oil have high GOR and good mobility; low hydrocarbon expulsion efficiency leads to a high content of retained hydrocarbons in the source rock; and the confinement effect of intra-layer cement in the high maturity stage induces the efficient accumulation of light shale oil. The restoration of hydrocarbon accumulation process suggests that liquid hydrocarbons generated in the early (low–medium maturity) stage of the Qingshankou Formation source rocks accumulated in placanticline and slope after long-distance secondary migration, forming high-quality conventional and tight oil reservoirs. Light oil generated in the late (medium–high maturity) stage accumulated in situ, forming about 15 billion tons of Gulong shale oil resources, which finally enabled the orderly distribution of conventional–unconventional oils that are contiguous horizontally and superposed vertically within the basin, showing a complete pattern of “whole petroleum system” with conventional oil, tight oil and shale oil in sequence.

Quantitative evaluation model of shale oil adsorption: A case study of the first member of Cretaceous Qingshankou Formation in northern Songliao Basin, NE China

A quantitative evaluation model that integrates kerogen adsorption and clay pore adsorption of shale oil was proposed, and the evaluation charts of adsorption-swelling capacity of kerogen (Mk) and adsorbed oil capacity of clay minerals (Mc) were established, taking the 1st member of Cretaceous Qingshankou Formation in the northern Songliao Basin as an example. The model and charts were derived from swelling oil experiments performed on naturally evolved kerogens and adsorbed oil experiments on clays (separated from shale core samples). They were constructed on the basis of clarifying the control law of kerogen maturity evolution on its adsorption-swelling capacity, and considering the effect of both the clay pore surface area that occupied by adsorbed oil and formation temperature. The results are obtained in four aspects: (1) For the Qing 1 Member shale, with the increase of maturity, Mk decreases. Given Ro of 0.83%–1.65%, Mk is about 50–250 mg/g. (2) The clay in shale adsorbs asphaltene. Mc is 0.63 mg/m2, and about 15% of the clay pore surface is occupied by adsorbed oil. (3) In the low to medium maturity stages, the shale oil adsorption is controlled by organic matter. When Ro>1.3%, the shale oil adsorption capacity is contributed by clay pores. (4) The oil adsorption capacity evaluated on the surface at room temperature is 8%–22% (avg. 15%) higher than that is held in the formations. The proposed evaluation model reveals the occurrence mechanisms of shale oils with different maturities, and provides a new insight for estimating the reserves of shale oil under formation temperature conditions.

Petroleum Retention, Intraformational Migration and Segmented Accumulation within the Organic‐rich Shale in the Cretaceous Qingshankou Formation of the Gulong Sag, Songliao Basin, Northeast China

AbstractIn this study, organic geochemical and petrological analyses were conducted on 111 shale samples from a well to understand the retention, intraformational migration and segmented accumulation (shale oil enrichment in different intervals is unconnected) features of shale oil within the organic‐rich shale in the Qingshankou Formation of the Gulong Sag. Our study shows that retained petroleum characteristics in the investigated succession are mainly influenced by three factors: organic richness, intraformational migration and segmented accumulation. Organic matter richness primarily controls the amount of retained petroleum, especially the ‘live’ component indicated by the S2 value rather than the total organic carbon (TOC) figure alone. The negative expulsion efficiencies determined by mass‐balance calculations of hydrocarbons reveal that petroleum from adjacent organic‐rich intervals migrates into the interval of about 2386–2408 m, which is characterized by high free hydrocarbon (S1), OSI and saturated hydrocarbons content, along with a greater difference in δ13C values between polar compounds (including resins and asphaltenes) and saturated hydrocarbons. The depth‐dependent heterogeneity of carbon isotope ratios (δ13C) of mud methane gas, δ13C of extracts gross composition (SARA), δ13C of kerogen and SARA content of extracts suggest that the studied succession can be subdivided into four intervals. The shale oil sealing enrichment character in each interval is further corroborated by the distinct δ13C values of mud methane gas in different intervals. Due to the migration of petroleum into the 2386–2408 m interval, the S1, OSI and saturated hydrocarbons content of the interval show higher relative values. The maturity of organic matter in the 2471–2500 m interval is at the highest with the smaller size molecular components of the retained petroleum. Thus, favorable ‘sweet spots’ may be found in the 2386–2408 m interval and the 2471–2500 m interval, according to the experiment results in this study.

Phase behavior and GOR evolution using a natural maturity series of lacustrine oil-prone shale: Implications from compositional modelling

For the exploration of shale oil, there is still an urgent need to relate hydrocarbon generation reactions and changing phase behavior to exact levels of thermal maturity. Six organic-rich shale samples from the Cretaceous Qingshankou Formation, Songliao Basin spanning the maturity range with vitrinite reflectance (Ro) 0.58%–1.16% were studied for petroleum generation characteristics using the PhaseKinetics, PhaseSnapshot, and GORFit approaches. All samples show very good petroleum potential and contain Type I kerogen with total organic carbon contents ranging between 1.5% and 5%. A full spectrum of light to heavy hydrocarbons, consistent with live oils, occurs in thermal extracts of all samples. Gas chromatography fingerprints of samples with Ro > 0.7% closely resemble waxy oils derived from lacustrine organic matter (OM); fingerprints of lower maturity samples are not yet as wax rich. The inferred petroleum type organofacies of all samples is paraffinic high wax with low organic sulfur levels. Hydrocarbon generation from the least mature sample is characterized by one dominant activation energy at 55 kcal/mol, accounting for 87% of the total bulk reaction. Assuming a simplified geological scenario, i.e., a 3 K/My linear heating rate, primary petroleum is generated (transformation ratio, i.e., TR: 10%–90%) over a narrow temperature interval of only 23 °C. Onset (10% TR), maximum (Tpeak), and completion (90% TR) temperatures and Ro values are 141 °C at Ro 0.85%, 155 °C at Ro 1.08%, and 164 °C at Ro 1.2%, respectively. Primary gas generation (148–172 °C) occurs 10 °C later (300 m deeper) than the generation of C6+ compounds (138–161 °C). Secondary gas formation from oil cracking begins at 182 °C, maximizes at 197 °C, and is completed well before 225 °C (beginning of late gas generation from the secondary cracking of thermally matured, macromolecular organic matter) at 206 °C. Thus, primary and secondary oil cracking processes do not overlap under geological conditions. In line with a paraffinic high-wax oil petroleum type organofacies, cumulative fluids generated from the least mature sample fall within the black oil class over most of the primary kerogen conversion range up to 90% TR as indicated by low saturation pressures (Psat) and gas-oil-ratios generally below 160 Sm3/Sm3. Maturation results in relatively heavy fluids with Psat values in the range 40–70 bar when the free hydrocarbons, i.e., already present prior to pyrolysis, are included, and in lighter fluids with Psat values in the range 70–90 bar, when the free, mostly waxy hydrocarbons, are excluded. Systematics of physical properties and fluid compositions evolution reproduce the general behavior of naturally occurring petroleum sourced from lacustrine OM very well and fit physical properties and compositions of produced shale oils in the Songliao Basin at comparable maturity levels.

Whole petroleum system and hydrocarbon accumulation model in shallow and medium strata in northern Songliao Basin, NE China

Based on the oil and gas exploration practice in the Songliao Basin, combined with the latest exploration and development data such as seismic, well logging and geochemistry, the basic geological conditions, oil and gas types and distribution characteristics, reservoir-forming dynamics, source-reservoir relationship and hydrocarbon accumulation model of the whole petroleum system in shallow and medium strata in the northern part of Songliao Basin are systematically studied. The shallow–medium strata in northern Songliao Basin have the conditions for the formation of whole petroleum system, with sufficient oil and gas sources, diverse reservoir types and well-developed transport system, forming a whole petroleum system centered on the source rocks of the Cretaceous Qingshankou Formation. Different types of oil and gas resources in the whole petroleum system are correlated with each other in terms of depositional system, lithologic association and physical property changes, and they, to a certain extent, have created the spatial framework with orderly symbiosis of shallow–medium conventional oil reservoirs, tight oil reservoirs and shale oil reservoirs in northern Songliao Basin. Vertically, the resources are endowed as conventional oil above source, shale oil/tight oil within source, and tight oil below source. Horizontally, conventional oil, tight oil, interlayer-type shale oil, and pure shale-type shale oil are developed in an orderly way, from the margin of the basin to the center of the depression. Three hydrocarbon accumulation models are recognized for the whole petroleum system in northern Songliao Basin, namely, buoyancy-driven charging of conventional oil above source, retention of shale oil within source, and pressure differential-driven charging of tight oil below source.

Introducing Novel Correction Methods to Calculate Sedimentary Basin Overpressure and Its Application in Predicting Pressure Value and Origin

Overpressure is an important phenomenon observed in sedimentary basins. To enhance the precision of identifying the origin of overpressure and predicting it accurately, this paper presents four practical methods for analyzing formation overpressure, taking into account various factors, such as burial history, rock properties, mechanical models and data quality. By illustrating the application examples in typical basins, this paper demonstrates the practicality and potential of each model. Based on the analysis of organic matter content and denudation, our understanding of the origin of overpressure in the first member of the Cretaceous Qingshankou Formation in Songliao Basin has been revised. A novel model based on the static equilibrium equation was established, which not only demonstrates theoretical rationality but also exhibits practical advantages in the Junggar Basin and Southeast Hainan Basin. The incorporation of lateral loading in the correction model and the utilization of colored inversion in seismic velocity processing model have significantly enhanced pressure prediction accuracy in Junggar Basin and Southeast Hainan Basin, respectively. In conclusion, these correction models demonstrate a high level of reliability and possess significant potential for widespread adoption.

The role of water in the laboratory thermal advancement of immature type I kerogen from the Cretaceous Qingshankou Formation in China

To understand variations in geochemistry, organic petrology, and chemical composition of crude oil and byproducts, an immature sample from the Cretaceous Qingshankou Formation in the Songliao Basin, China, was analyzed by anhydrous and hydrous pyrolysis (AHP/HP) at a wide range of temperatures ranging from 300 °C to 450 °C. The geochemical parameters: TOC, S2, HI, and Tmax obtained from Rock–Eval pyrolysis showed both a decrease and an increase as thermal maturity progressed under HP and AHP conditions. Gas chromatography (GC) analysis showed the presence of n-alkanes in the C14 to C36 range in both expelled and residual byproducts, a Delta-shaped configuration although many samples had a gradually reducing (tapering) trend toward the high range. Gas chromatography-mass spectrometry (GC–MS) analysis revealed both an increase and a decrease in biomarker and very small changes in aromatic compound variations with increasing temperature during pyrolysis. To be more specific, C29Ts biomarker increased with temperature for the expelled byproduct, while the opposite trend was observed for the residual one. Next, The Ts/Tm ratio initially increased and then decreased with temperature while the C29H/C30H ratio fluctuated for the expelled byproduct but increased for the residual. Moreover, the GI and C30 rearranged hopane to C30 hopane ratio remained unchanged whereas the C23 tricyclic terpane/C24 tetracyclic terpane ratio and the C23/C24 tricyclic terpane ratio showed varying trends with maturity alike the C19/C23 and C20/C23 tricyclic terpane. Ultimately, based on organic petrography observations, increasing the temperature resulted in higher bitumen reflectance (%Bro, r) and optical and structural alterations in the macerals. The findings of this study provide valuable insights for future exploration endeavors in the studied region. Moreover, they contribute to our understanding of the significant role of water in the generation and expulsion of petroleum and associated byproducts, thereby facilitating the development of updated models in this field.