Tuff-rich mixed shales, representing an important class of unconventional hydrocarbon resources, exhibit diverse components, rapid lithofacies variations, complex diagenetic evolution, and strong heterogeneity. For these shales, there remains a lack of a systematic understanding of the diagenetic evolutionary processes and reservoir formation mechanisms of different lithofacies, restricting target area selection and evaluation for shale oil exploration. In this study, we investigate the diagenesis, reservoir formation, and favorable exploration targets of mixed shales with unique components in the 2nd Member of the Lucaogou Formation (also referred to as the Lu 2 Member) in the Santanghu Basin. To this end, a range of test methods are employed, including core characterization, thin section observation, scanning electron microscopy (SEM), whole-rock X-ray diffraction (XRD) analysis, electron probe microanalysis, high-pressure mercury injection (HPMI), and nuclear magnetic resonance (NMR), along with measurements of porosity, permeability, and oil saturation. The results indicate that the mixed shales of the Lu 2 Member are composed primarily of tuffaceous materials, carbonates, and organic matter and can be classified into 10 lithofacies, which are frequently interbedded. The mixed shales mainly contain nano-to micro-scale intercrystalline pores in dolomites, devitrification-induced pores in volcanic ash, and dissolution pores, suggesting complex pore structures. Distinct lithofacies exhibit significant differences in physical and oil-bearing properties. Among these, massive lithofacies featuring low organic matter abundance and composed primarily of a single component display the most favorable properties, followed by lamellar transitional lithofacies dominated by dolomites, while lamellar transitional lithofacies composed primarily of tuffaceous materials show the poorest physical and oil-bearing properties. The shale component types and their differential diagenetic evolution govern reservoir quality. Rapid deep burial and compaction in the early stage represent primary factors responsible for the deterioration of reservoir physical properties. However, they occurred earlier than other diagenetic processes. Furthermore, dolomitization and devitrification occurred before organic acid-induced dissolution. This diagenetic evolutionary sequence provides effective spaces for organic acid migration while also offering a material basis for dissolution, serving as the key mechanism behind the formation of high-quality reservoirs. A comprehensive analysis reveals that favorable exploration targets in the Lu 2 Member include the basin margin zone and the slope zone near the basin margin.

The geometry and topology of shale pore-fracture systems govern hydrocarbon migration and control the feasibility of geological carbon dioxide storage in shale reservoirs. This study examines lacustrine shale across a range of maturities by integrating (ultra) small-angle neutron scattering, repeated mercury intrusion capillary pressure, field-emission scanning electron microscopy, and computed tomography following Wood’s metal impregnation. The pore system is divided into four pore-size classes, and their volumes and connectivity are tracked with increasing thermal maturity. At low maturity, mechanical compaction and early cementation reduce the total pore volume and concentrate connected porosity in fractures. As maturity increases, newly formed organic-matter pores lead to a modest increase in total pore volume, while liquid hydrocarbons generated within the oil window occupy part of the pore space and weaken pore-fracture connectivity. At high maturity, the secondary cracking of liquid hydrocarbons to gas raises pore pressure, partially reopens previously sealed pores and fractures, and enhances both total pore volume and pore-fracture connectivity. These results indicate that mature to high-mature lacustrine shales provide more pore surface area, storage space, and connected pathways for the long-term storage of carbon dioxide than low-maturity shales.

In this work, the potential influences of grain-coating clays on water-CO₂-rock interactions in sandstones and subsequent ramifications for CO₂ storage were investigated using reactive transport simulations. The results indicated that, compared to pore-filling smectite, grain-coating smectite leads to significant pH decrease, increases in the CO₂-species concentrations, and decreases in smectite dissolution and the precipitation of secondary minerals. Moreover, it was revealed that smectite and chlorite coats dissolve preferentially over detrital K-feldspar being covered, while K-feldspar is dissolved preferentially over illite and kaolinite coats. While the mineral trapping mechanism is only important for smectite and chlorite coats, sandstone porosity is significantly reduced for chlorite coat but increased for the other three clay coats. The main causes of the differences between pore-filling and grain-coating scenarios for smectite and chlorite coats are ascribed to the inhibitory effect of clay coats on the growth of secondary quartz and the dissolution of clay. In addition to the above two factors, the decelerating effect of clay coats on the dissolution of K-feldspar is also important for illite coat; meanwhile, for the kaolinite coat, the dissolution of clay is less important and the other two factors are more critical. Furthermore, the coverage and thickness of clay coats, fluid flow rate, detrital grain size, detrital lithology, partial pressure of CO₂, and temperature may all impact the role of clay coats.

Volcanic ash input holds significant impacts on the reconstruction of sedimentary environments in saline lacustrine basins. Specifically, when abundant trace elements from volcanic ash enter lakes and are preserved in tandem with sediment, the element contents in lacustrine strata cannot accurately reflect the original sedimentary setting. Focusing on the 2nd member of the Permian Lucaogou Formation (P₂l²) in the Tiaohu-Malang Sag within the Santanghu Basin, we investigate the mechanisms by which volcanic ash alteration affects element anomalies in fine-grained mixed deposits using major, trace, and rare earth element analyses, gas chromatography-mass spectrometry (GC-MS) of saturated hydrocarbons, whole-rock X-ray diffraction (XRD), total organic carbon (TOC) content determination, and thin section observation. Accordingly, the saline lacustrine sedimentary environment under the action of volcanic ash is reconstructed. The results indicate that the P₂l² consists of fine-grained tuffaceous materials and carbonates, with the total content of felsic and carbonate minerals reaching up to 95 % on average, suggesting gradational mixed sedimentation at facies margins in a broad sense. Under the influence of alteration such as early dissolution in water and mid-late devitrification and organic acid corrosion, substantial volcanic ash thermodynamically unstable released various high-abundance nutrient ions (including metal ions) into pore fluids, which significantly interfered with the elements including Ni, Co, S, P, and Ga in the fine-grained mixed rocks of the P₂l². As a result, these elements cannot accurately reflect the original sedimentary environment. Biomarker compounds, together with the analysis of major and trace element data after screening and correction, reveal a hot and arid climate during the deposition of the P₂l², featuring limited water supply to the lacustrine basin and extremely high salinity and low oxygen concentration of the lake. Volcanic activity played a significant role in regulating the sedimentary environment of the P₂l². The transition from intense to intermittent volcanic eruptions corresponded to an increasingly hot and arid climate, along with elevated water evaporation, water salinity, and oxygen concentration, leading to the deposition of various types of rock assemblages.

Shales in the first member of the Qingshankou Formation (the Qing 1 Member) in the Songliao Basin exhibit a large variation of organic matter maturity and strong heterogeneity of hydrocarbon mobility. How to analyze the shale oil phase and physical properties has posed a major challenge for efficient shale oil exploitation and production. We investigate the characteristics of shale oil composition evolution by carrying out closed and semi-closed organic matter pyrolysis experiments on low-maturity shale samples, in which compensation correction for light hydrocarbon loss of retained hydrocarbons is performed based on the composition of generated and expelled hydrocarbons. By integrating the burial and thermal evolution histories of typical wells in main source kitchens in the Central Depression, we explore the phase evolution pattern of shale oil. Furthermore, we identify play fairways for light shale oil exploration and outline protection conditions for maintaining production pressure. The results reveal that under geological conditions, light components’ proportions and gaseous hydrocarbon content in shale oil increase progressively with the maturity of organic matter. Concurrently, the phase envelopes evolve from high dew point temperature (DPT) and low bubble pointpressure(BPP) toward low DPT and then to high BPP with an increase in the organic matter maturity. In the Qijia-Gulong Sag, the shale oil reservoirs of the Qing 1 Member evolved into light oil reservoirs in the middle stage of the Nenjiang Formation deposition. In the Changling Sag, these reservoirs began to evolve into light oil reservoirs at the end of the Nenjiang Formation deposition. In contrast, these reservoirs in the Sanzhao Sag have been consistently present as black oil reservoirs. Shale oil within both black and volatile oil reservoirs remains in a single liquid phase. In the Central Depression of the Songliao Basin, the light shale oil reservoirs of the Qing 1 Member are primarily distributed at the center of the Qijia-Gulong Sag and in the northern Changling Sag, with maturity of organic matter(R_o) varying from 1.3 % to 1.6 % and formation pressure from 12.2 to 22.4 MPa.

Describing the organic-inorganic pore evolution influenced by mineral composition is crucial for characterizing shale oil storage capacity and flow in shale, and it also helps predict storage capacity for sequestered CO₂. Using laboratory pyrolysis experiments to artificially mature shale samples at relatively high temperatures and short times, this study compares a series of natural samples with different thermal maturations, which can better reflect the real underground pore evolution. A total of 30 natural shales spanning from low to high maturity were collected from the Cretaceous Qingshankou Formation of the Songliao Basin, and the analysis results revealed four main typical shales, namely argillaceous shale, felsic shale, calcareous shale, and mixed shale. The existence of clay minerals, quartz and feldspar promote the development of > 50 nm pores, while 0-20 nm pores are mainly developed in clay minerals and organic matter. When the content of total organic carbon is less than 2.5 wt.%, it displays a positive correlation with the specific surface area, but the correlation becomes negative for samples with a content of total organic carbon greater than 2.5 wt.%. The organic pores are most developed at the peak oil maturity, while inorganic pores are most developed during the oil window, and tend to be stable at high maturity. Argillaceous shale in the high maturity stage may be favorable for petroleum exploration in the Qingshankou Formation of the Songliao Basin. Mixed shale and calcareous shale may not be conducive to the short-term storage of CO₂ due to strong reactions with CO₂ at the beginning. On the other hand, argillaceous shale and felsic shale may be conducive to the long-term storage of CO₂.

Organic matter (OM) hosted pores are crucial for the storage and migration of petroleum in shale reservoirs. Thermal maturity and macerals type are important factors controlling the development of pores therein. In this study, six lacustrine shale samples with different thermal maturities from the first member of the Qingshankou Formation in the Songliao Basin, of which vitrinite reflectance (R_o) ranging from 0.58% to 1.43%, were selected for a comparative analysis. Scanning electron microscopy (SEM) and reflected light microscopy were combined to investigate the development of organic pores in different macerals during thermal maturation. The results show that alginite and liptodetrinite are the dominant primary macerals, followed by bituminite. Only a few primary organic pores developed in the alginite at the lowest maturity (R_o = 0.58%). As a result of petroleum generation, oil-prone macerals began to transform to initial-oil solid bitumen at the early oil window (R_o = 0.73%) and shrinkage cracks were observed. Initial-oil solid bitumen cracked to oil, gas and post-oil bitumen by primary cracking (R_o = 0.98%). Moreover, solid bitumen (SB) was found to be the dominant OM whenR_o > 0.98%, which indicates that SB is the product of oil-prone macerals transformation. Many secondary bubble pores were observed on SB, which formed by gas release, while devolatilization cracks developed on migrated SB. Additionally, at the late oil window (R_o = 1.16%), migrated SB filled the interparticle pore spaces. With further increase in temperature, the liquid oil underwent secondary cracking into pyrobitumen and gas, and spongy pores developed on the pyrobitumen at higher levels of maturity (R_o = 1.43%), which formed when pyrobitumen cracked into gas. Vitrinite and inertinite are stable without any visible pores over the range of maturities, verifying their low petroleum generation potential. In addition, it was concluded that clay minerals could have a catalytic effect on the petroleum generation, which may explain why organic-clay mixtures had more abundant pores than single OM particles. However, afterR_o> 0.98%, authigenic minerals occupied the organic pore spaces on the organic-clay mixtures, resulting in fewer pores compared to those observed in samples at the early to peak oil window.

The low mature shale oil resources of Lucaogou Formation in Jimusar Sag have a great potential, but the heavy oil quality limits large-scale economic development significantly. Ultrasonic is a typical representative of heavy oil viscosity reduction and anhydrous fracturing technology, and how to understand the action characteristics and mechanism of ultrasonic effect on reservoir is a critical issue to enhance shale oil production in the industrialized application of power ultrasonic. Therefore, the comparative experiments with different time of power ultrasonic loading were conducted to analyze the response mechanism of reservoir characteristics and the change of fluid mobility. The results indicate that the ultrasonic treatment is ameliorative to the pore-fracture structure, and the improvement degree is controlled by the mechanical vibration and cavitation of ultrasound. Generally, the location with weak cementation strength or relatively developed microcrack is preferred to pore expansion. After the ultrasonic treatment, the shale oil quality becomes lighter, and the transformation of shale oil from adsorbed to free, is accelerated due to enhanced fluidity. Pore-expanding effect and fluid mobility enhancement are essential aspects of the power ultrasonic loading to improve the recovery of low mature shale oil. The results of this study support the feasibility analysis of ultrasonic enhanced shale oil exploitation theoretically.

The low mature shale oil resources of Lucaogou Formation in Jimusar Sag have a great potential, but the heavy oil quality limits large-scale economic development significantly. Ultrasonic is a typical representative of heavy oil viscosity reduction and anhydrous fracturing technology, and how to understand the action characteristics and mechanism of ultrasonic effect on reservoir is a critical issue to enhance shale oil production in the industrialized application of power ultrasonic. Therefore, the comparative experiments with different time of power ultrasonic loading were conducted to analyze the response mechanism of reservoir characteristics and the change of fluid mobility. The results indicate that the ultrasonic treatment is ameliorative to the pore-fracture structure, and the improvement degree is controlled by the mechanical vibration and cavitation of ultrasound. Generally, the location with weak cementation strength or relatively developed microcrack is preferred to pore expansion. After the ultrasonic treatment, the shale oil quality becomes lighter, and the transformation of shale oil from adsorbed to free, is accelerated due to enhanced fluidity. Pore-expanding effect and fluid mobility enhancement are essential aspects of the power ultrasonic loading to improve the recovery of low mature shale oil. The results of this study support the feasibility analysis of ultrasonic enhanced shale oil exploitation theoretically.

Underbalanced perforation can substantially reduce formation damage and improve the efficiency of production operation. The field in question is a giant oil field in Southwest Iran, with over 350,000 bbl/day production rates. Reservoir X is the main reservoir of the field and includes 139 horizontal wells out of the total of 185 production wells drilled in the field. Despite its technical difficulties, under-balance perforation has been proven to result in high productivity ratios and has been shown to reduce workover costs if appropriately conducted. Therefore, this study investigated a customized underbalanced tubing conveyed perforation to enhance oil production. First, post-drilling formation damage was estimated using Perforating Completion Solution Kits. Next, high-density guns (types 73 and 127) with high melting explosives were selected based on the reservoir and well specifications. By conducting a sensitivity analysis using schlumberger perforating analyzer program, shot angles of 60 ${}^{\circ }$ and 90 ${}^{\circ }$, shot densities of 16 and 20 shots per meter, perforation diameters of 8 and 10 mm, and helix hole distribution were selected as optimized perforation parameters and resulted in productivity ratios up to 1.18. The current study provides a case study of applying a combination of two previously proven technologies, tubing convoyed and underbalanced perforation, in Iran’s giant oilfield. The method used and the outcome could be used to analyze the efficiency of applying the technology in other green or mature fields.