Qiongzhusi Shale Research Articles

Sedimentological and geochemical characteristics of Lower Cambrian Qiongzhusi shale in the Sichuan Basin and its periphery, SW China: Implications for differences in organic matter enrichment

Few studies have systematically investigated the factors controlling organic matter enrichment in shales from the Qiongzhusi Formation, within and surrounding the Sichuan Basin, under different depositional environments. This has resulted in different academic understandings and limited clarity on the mechanisms of organic matter enrichment. On this premise, in this study, the basic geological characteristics and depositional paleoenvironments of shales along the passive continental margin, the western Hubei Trough, and the western Sichuan Trough were compared and analyzed using core, outcrop, and mineral testing. Furthermore, data from organic geochemical and elemental analyses were utilized to investigate the different enrichment mechanisms and formation modes of the organic matter in different periods. The results reveal that the organic matter enrichment in this region should be mainly influenced by the preservation conditions, paleo-productivity, and terrigenous input. However, there were clear differences in the main controlling factors in the different periods. In the Q1 phase, the region had a high sea level, had the strongest rifting, had the largest accommodation space, and exhibited characteristics of low terrestrial input and bottom water hypoxia. The changes in the paleo-productivity caused by upwelling currents were the main factors controlling the variations in the organic matter enrichment. In the Q2 phase, the weakened decreasing sea level co-occurred with a reduction in the accommodation space across the region. The organic matter enrichment was significantly controlled by the paleo-productivity, preservation conditions, and terrigenous inputs, and the organic matter enrichment conditions deteriorated from the passive continental margin to the western Hubei Trough and western Sichuan Trough. The total organic carbon (TOC) content of the shale decreased significantly. In the Q3 phase, the entire region entered an infilling stage, which was dominated by an oxygen-rich environment, and the preservation conditions were the decisive factor controlling the organic matter enrichment. The TOC content was low overall, and there were no evident differences across the different zones.

Pore development characteristics and controlling factors of Lower Cambrian Qiongzhusi shale gas reservoir in Sichuan Basin, China

Recently, the shale gas exploration of Lower Cambrian Qiongzhusi Formation in Sichuan Basin has made a great breakthrough. However, the pore characteristics and controlling factors of the shale-gas reservoir in the Qiongzhusi Formation are not clear, which hinders further exploration and development of shale gas. This paper focuses on the Qiongzhusi Formation in the Lower Cambrian of the Sichuan Basin. On the basis of mineral composition and the lithofacies types of shale-gas reservoirs in Qiongzhusi Formation were classified by microscopic observation. Additionally, focused ion beam-scanning electron microscope (FIB-SEM), advanced mineral identification and characterization system (AMICS) analysis, large-field stitching scanning electron microscope technology (MAPS) and nitrogen adsorption experiment were employed to comprehensively determine the differential characteristics and controlling factors of pore development. The results indicate that the pore types in Qiongzhusi Formation are mainly composed of interparticle pores and intercrystalline pores. The pore structure of Qiongzhusi Formation is complex. Significant differences in pore structure parameters are observed among different lithofacies. Quartz and feldspar contribute to the preservation of primary pores due to their strong compressive resistance. Clay minerals and carbonate minerals impede pore development by occluding pores. The influence of organic matter on the pore development is relatively small. According to the mineral composition and pore parameters, laminated shale is a relatively favorable reservoir facies among different lithofacies types.

Pore characteristics and preservation mechanism of over-6000-m ultra-deep shale reservoir in the Sichuan Basin

Investigation of pore characteristics and their preservation mechanism of over 6000 m ultra-deep shale reservoirs is of significance for shale gas exploration and development in the Sichuan Basin. In this study, the pores structure and multifractal characteristics of pores for the Longmaxi shale and Qiongzhusi shale in the Sichuan Basin are well studied by using field emission scanning electron microscopy (FE-SEM), gas adsorption and multifractal analysis. The results show that: (1) Extremely strong mechanical compaction resulting from ultra-deep burial depth can lead to the homogenization of pore structure, which is characterized by the change of bubble-like OM pores to silt or fracture shape pores and the higher average pore diameter (APD) value. (2) The Longmaxi shale and Qiongzhusi shale reservoirs have the obvious multifractal nature for different pore sizes. Samples from the Longmaxi shale and Qiongzhusi shale in well CS#1 demonstrate the higher average H values and smaller average ΔD values compared with those samples in well MS#1, indicating that shale reservoir in over-pressure condition has higher connectivity and less heterogeneity. The pore preservation in over 6000 m ultra-deep shale reservoirs are influenced by several geological factors, including 1) quartz is beneficial for the preservation of pores especially for OM pores due to its supporting effect; 2) the shale reservoirs in over-pressure show many more OM pores and higher surface porosity than those in normal pressure; 3) the direct floor of Qiongzhusi shale is likely the critical geological factor affecting the pores preservation. Therefore, the Longmaxi shale with both over-pressure condition and high quartz content is likely the best target zone for deep shale gas exploration in the Sichuan Basin.

Effects of different types of fractures on shale gas preservation in Lower Cambrian shale of northern Sichuan Basin: Evidence from macro-fracture characteristics and microchemical analysis

The Lower Cambrian Qiongzhusi shale is the main producing layer of shale gas in the northern Sichuan Basin. Natural fractures are considered an important factor that influences the exploration and development of shale gas. In this study, outcrops, drill cores, thin sections, fluid inclusions and rare earth element (REE) were analyzed to determine the characteristics, changes in fluid chemistry, and fluid source of different types of fractures and then reveal their effects on the accumulation and preservation of shale gas. Natural fractures of Qiongzhusi shale are mainly composed of non-strata-bounded fractures (NSBFs), strata-bounded fractures (SBFs), and bed-parallel fractures (BPFs). NSBFs span multiple layers, which can significantly enhance the vertical connectivity of shale reservoirs. There are three dominant orientations of NSBFs, as N-S, NNE-SSW, and E-W trends. The E-W trending fractures, which intersect with the maximum principal stress (NWW-SEE) at a small angle, have larger permeability and control the vertical leakage of shale gas. SBFs are confined to individual layers. Homogenization temperatures of aqueous inclusions in SBF veins vary from 149.7 to 172.3 °C. Calcite veins exhibit positive Eu anomalies (δEu = 11.71, 27.9). Analysis of fluid inclusions and rare earth elements indicated that SBF vein-precipitating fluids were derived from the surrounding wall rock. The development of SBFs contributes to shale gas enrichment. BPFs are parallel to bedding planes or intersect at a low angle (<20°). Microstructural analysis suggests that cemented BPFs form from the multiple crack-seal processes. The temperature of fluid during mineral precipitation exceeds 200 °C. REE concentrations vary significantly across the BPF veins (δEu = 2.31–119.37), donating that fluid characteristics episodically changed during vein growth. The methane inclusions trapped in cements and mixing externally sourced fluids indicate that BPFs are the preferential fluid-flow pathway of shale gas and controls the lateral migration of shale gas.

Triaxial testing on water permeability evolution of fractured shale.

A sound understanding of the water permeability evolution in fractured shale is essential to the optimal hydraulic fracturing (reservoir stimulation) strategies. We have measured the water permeability of six fractured shale samples from Qiongzhusi Formation in southwest China at various pressure and stress conditions. Results showed that the average uniaxial compressive strength (UCS) and average tensile strength of the Qiongzhusi shale samples were 106.3 and 10.131 MPa, respectively. The nanometre-sized (tiny) pore structure is the dominant characteristic of the Qiongzhusi shale. Following this, we proposed a pre-stressing strategy for creating fractures in shale for permeability measurement and its validity was evaluated by CT scanning. Shale water permeability increased with pressure differential. While shale water permeability declined with increasing effective stress, such effect dropped significantly as the effective stress continues to increase. Interestingly, shale permeability increased with pressure when the pressure is relatively low (less than 4 MPa), which is inconsistent with the classic Darcy's theory. This is caused by the Bingham flow that often occurs in tiny pores. Most importantly, the proposed permeability model would fully capture the experimental data with reasonable accuracy in a wide range of stresses.

The Fluid Evolution of Ancient Carbonate Reservoirs in Sichuan Basin and Its Implication for Shale Gas Exploration

Sichuan Basin is the only successful basin for shale gas exploration in China. In addition to the main shale in the Lower Silurian Longmaxi formation, the lower Cambrian Qiongzhusi shale is an important potential formation. However, it was once considered that shale gas is difficult to enrich because of its poor sealing conditions and hydrocarbon migration to adjacent reservoirs. With the increasing research on hydrocarbon generation and reservoir in shale of Qiongzhusi Formation in recent years, it has become an important exploration target in Sichuan Basin. The enrichment of oil and gas is closely related to fluid activities. Limited by the degree of exploration, there is little analysis of fluid activities in Qiongzhusi Formation, and there is little analysis of shale gas enrichment potential from the perspective of fluid. The hydrocarbon generated from Qiongzhusi shale in the rift could migrate laterally to the uplift area and form a reservoir in Dengying Formation. The fluid activities from source rock to reservoir are basically the same. Therefore, this paper reconstructed the history of hydrocarbon activities in Dengying reservoirs based on fluid inclusion analysis. Then the fluid activity process in Qiongzhusi shale was studied, and its enrichment conditions of shale gas was discussed. The results show that the hydrocarbon activities of Dengying Formation can be divided into three stages: 1) oil charging stage, 2) oil cracking gas generation stage and 3) gas reservoir adjustment stage. The first stage is under normal pressure, and the second and third stages developed overpressure with pressure coefficients of 1.3 and 1.2, respectively. High pressure coefficient is an important indicator of shale gas enrichment. Because the source rock of Qiongzhusi Formation has always been the main source rock of Dengying Formation, it can supply hydrocarbon to Dengying Formation only with overpressure in gas generation stage. Therefore, overpressure in the last two stages of gas generation indeed existed. As long as the sealing condition of shale itself is not particularly poor, shale gas “sweet points” would be formed. Therefore, the thick shale in Deyang-Anyue rift is the focus of shale gas exploration in Qiongzhusi Formation.

A method for automatic shale porosity quantification using an Edge-Threshold Automatic Processing (ETAP) technique

Scanning electron microscopy (SEM) is one of the most prevalent methods used to image and quantify the pore size distribution of shale rock, critical in understanding unconventional petroleum systems and production. Generally, digital greyscale SEM images of shale are currently processed for pore quantification either by a manual drawing method, manual threshold method, automatic threshold method, edge detection or watershed methods, all of which have some limitations that impact the quality of pore extraction results. A new, Edge-Threshold Automatic Processing (ETAP) method is reported here to enable robust extraction and quantification of pore data in shale images. Image pre-treatment makes the greyscale of regions brighter than that of kerogen set to the peak value of kerogen greyscale. The pore image is subsequently obtained using an edge detection method. A discriminant function has been designed to determine the best threshold of the greyscale image to obtain the pore image. Finally, combination of both processed pore images gives the final pore image. Our new method overcomes the impact of kerogen, mineral, roughness and artificial debris caused by pre-treatment of samples, which potentially introduce errors using alternative methods. We compare our new method to a systematic manual drawing method. The processing results through ETAP provide reliable results, and gets the highest value of 0.7466 using a discriminant function Qt, compared with the automatic threshold methods, the edge detection method and watershed method. The application of the ETAP method on shale samples of the Longmaxi Formation and Qiongzhusi Formatiosn in Sichuan basin shows that samples from the Longmaxi Formation have more organic pores than that of the Qiongzhusi Formation, however a larger size of inorganic pores develop in the Qiongzhusi shale. This indicates that shale of the Longmaxi Formation has better reservoir properties and reliable preservation conditions.

Maturity and thermal evolution differences between two sets of Lower Palaeozoic shales and its significance for shale gas formation in south‐western Sichuan Basin, China

AbtractThe Lower Cambrian Qiongzhusi shale and the Upper Ordovician–Lower Silurian Wufeng‐Longmaxi shale are major source rocks and important shale gas exploration targets in the Sichuan Basin. Cutting and core samples from two shale gas wells in south‐western Sichuan Basin have been selected for organic matter (OM) reflectance measurement and Raman spectra analysis with the aim to determine maturity. The results were incorporated for basin modelling to construct the thermal evolution of this area. Graptolite fragments and pyrobitumens are the dominant form of OM in Wufeng‐Longmaxi shale. A limited number of the OM as pyrobitumens and vitrinite‐like particles is presented in Qiongzhusi shale. According to the measured reflectance of OM in conjunction with the maturity calculated using Raman spectra, the equivalent vitrinite reflectance (EqVRo) values based on solid bitumen and graptolite is 2.53–2.81% for the Wufeng‐Longmaxi shale. The EqVRo value is 2.85–3.53% within 300 m depth intervals for the Qiongzhusi shale. A significant difference in oil generation occurred before the Late Permian between Qiongzhusi shale and Wufeng‐Longmaxi shale based on basin modelling and reassessment of previous published data. Compared with the Qiongzhusi shale, the Wufeng‐Longmaxi shale would have lower maturities before Late Permian and less hydrocarbon could be generated/expelled at that time, therefore, this shale remained with a large amount of oil, which could crack into gas. This difference had shown a favourable time relationship for generation and accumulation of shale gas in Wufeng‐Longmaxi shale.

Hydrocarbon generation and expulsion characteristics of the Lower Cambrian Qiongzhusi shale in the Sichuan Basin, Central China: Implications for conventional and unconventional natural gas resource potential

In recent years, significant progress and breakthroughs have been made in the exploration of conventional and unconventional natural gas in the Lower Cambrian petroleum system in the Sichuan Basin in Central China, showing excellent exploration prospects. However, detailed evaluation of the Qiongzhusi shale, the main source rocks in the Lower Cambrian petroleum system, including their hydrocarbon generation and expulsion characteristics, is lacking, even though such evaluation is critical for understanding conventional and unconventional natural gas resource potential. To investigate the geological and geochemical characteristics of the Qiongzhusi shale, experiments including pyrolysis, total organic carbon (TOC), vitrinite reflectance (Ro), gas chromatography (GC) and gas chromatography–mass spectrometry (GC–MS) were conducted. Based on modeling work and fluid inclusion analysis, the hydrocarbon generation and expulsion characteristics of the Qiongzhusi shale and their control on hydrocarbon accumulation was analyzed, and the conventional and unconventional natural gas resource potential of the Lower Cambrian petroleum system was systematically evaluated. The results indicate that the organic matter of the Qiongzhusi shale is mainly derived from bacteria and algae deposited in a saline, reducing marine environment. The shale is predominantly composed of type I kerogen with an average TOC value of 1.85 wt%, and is widely distributed in the basin with an average thickness of 212.7 m. The Qiongzhusi shale is primarily post-mature with an average vitrinite reflectance (Ro) of 2.53%. Modeling work suggests the Qiongzhusi shale reached the hydrocarbon expulsion threshold (HET) at Ro = 0.9%, and the main hydrocarbon expulsion stages are Triassic, Jurassic, and Cretaceous. The maximum hydrocarbon generation and expulsion intensities at present day are 32 × 10 6 t/km 2 and 18 × 10 6 t/km 2 , respectively. The conventional natural gas resource potential of the Longwangmiao Formation derived from the Qiongzhusi shale is estimated to be between 9.51 × 10 12 m 3 and 19.03 × 10 12 m 3 . The unconventional shale gas resource potential of the Qiongzhusi shale is predicted to be 1.4 × 10 12 m 3 , showing significant prospects for natural gas exploration. • Geochemistry characteristics of the Qiongzhusi Formation were systemically evaluated. • Hydrocarbon generation and expulsion characteristics of the Qiongzhusi Formation were first analyzed. • Conventional and unconventional natural gas resource potentials of ancient Lower-Cambrian petroleum system were calculated.

Prediction of the gas-generating characteristics of the Qiongzhusi and Longmaxi Formations, Yangtze Platform, southern China, using analogues

China has been said to have the largest putative shale gas resources in the world. The highest potential occurs in the Sichuan Basin, with the overmature Qiongzhusi (Cambrian) and Longmaxi (Silurian) Formations as prime exploration targets. Here, the likelihood of late gas formation is examined using less mature equivalents from the Georgina Basin (Australia) and the Baltic Basin (Lithuania). We consider the respective functions of kerogen and polar bitumen in gas generation with reference to the Eagle Ford, Yanchang, Niobrara, and Vaca Muerta Formations. Both of the lower Paleozoic shales are bitumen poor in a geochemical sense, this being in stark contrast to the Mesozoic shales, which are bitumen rich. Kerogen is, therefore, the major gas precursor in the Cambrian and Silurian of the Sichuan Basin. Graptolites and solid bitumen are petrographically dominant. The solid bitumen exhibits flow structures and is deduced to be highly polar because it is insoluble in dichloromethane. Secondary cracking kinetics determined for the Arthur Creek are closely similar to source rocks containing predominantly paraffinic oil. Late gas generation from very stable refractory kerogen structures via alpha-cleavage reactions at maturities above 2% equivalent vitrinite reflectance (Ro) was verified, and importantly, the upper ceiling for late gas generation in Paleozoic shales of the Sichuan Basin is set at 3% Ro. As far as the Qiongzhusi shale is concerned, raising the prospective acreage to a 3% Ro limit brings an additional contribution of 40 mg HC/g total organic carbon of late gas charge. The same is true for the extensive fairway of the Longmaxi shale along the western flank of the basin, close to the subcropping erosional edge.

Geological controls on the accumulation of shale gas: A case study of the early Cambrian shale in the Upper Yangtze area

Abstract The results of sedimentological, mineralogical, geochemical and petrophysical studies on the early Cambrian shale in the Upper Yangtze area are summarized to provide insights on the paleoenvironmental conditions, diagenetic evolution and preservation conditions that give rise to shale gas accumulation. The lower Cambrian organic-rich Qiongzhusi shale is mainly developed in the outer shelf facies of an intracratonic sag [average total organic carbon (TOC): 1.3–2.5%] and in the southeast margin (average TOC: 3.5–5.5%) of the Upper Yangtze area. The thermal maturity values (Reqv) of the Qiongzhusi shale range from 2.19 to 4.23%, while first increasing and then decreasing from the center of the Sichuan Basin outward. The average porosities of the Qiongzhusi shale from the outer shelf facies association (FA5) in the intracratonic rift range from 1.5 to 3.41%, and those from the outer shelf facies association (FA4) in the southeast Sichuan and Guizhou areas range from 1.32 to 2.56%. The organic matter (OM) content and the development degree of OM pores are the main factors that control the porosity and pore structure of the Qiongzhusi shale. For the outer shelf deposits in the intracratonic rift, a quartz (detrital origin) content ranging between 25 and 35% is most favorable for providing an effective support framework for OM pore development due to the mixture of brittle and clay minerals. For the outer shelf deposits in the southeastern margin of the Upper Yangtze area, the high OM content and poor compaction resistance cause a poor development of OM pores, with a smaller average pore diameter than that in the outer shelf shale of the intracratonic rift. In addition, the carbonization of OM in the overmature Qiongzhusi shale change the microstructure and decrease the OM pore volume. Due to poor preservation conditions, the gas content is generally low in the organic-rich shale in the eastern and southeastern parts of the Upper Yangtze region. The Qiongzhusi shale in the interior of the Sichuan Basin, with good preservation conditions, high pressure coefficients, and high gas contents, is a favorable area for shale gas accumulation and potential exploitation.

Biological sources of sedimentary organic matter in Neoproterozoic–Lower Cambrian shales in the Sichuan Basin (SW China): Evidence from biomarkers and microfossils

Study of the occurrence and formation of sedimentary organic matter in the Sichuan Basin, southern China, provides valuable insights into the biological sources and compositions of Neoproterozoic to Lower Cambrian shales. Rock samples from two sets of shales in the Sichuan Basin and adjacent areas, including the Ediacaran Doushantuo shale and the Lower Cambrian Qiongzhusi shale in the Gaoshiti-Moxi and Ziyang-Weiyuan areas were collected and geochemically analyzed in this study.The distribution of biomarkers, stable carbon isotope compositions and microfossil characteristics reveal that eukaryotic algae including chlorophytes, pelagophyceae, dinoflagellates, diatoms (or their ancestors) and sponges proliferated widely during the Late Neoproterozoic to Early Cambrian in the Sichuan Basin. Cyanobacteria, some α-Proteobacteria, rhizobiales and aerobic methanotrophs also made a contribution to organic matter in the studied shales. Aerobic methanotrophs that utilize 13C-depleted substrates such as CH4 as carbon sources may have been the cause of anomalous 13C-depleted kerogen in the study area.

Effect of lithofacies on pore structure and new insights into pore-preserving mechanisms of the over-mature Qiongzhusi marine shales in Lower Cambrian of the southern Sichuan Basin, China

Research on shale pore structures has furthered the understanding of shale oil and gas accumulation. However, understanding of mechanisms of shale lithofacies that affect pore structures remains poor, restricting evaluations of marine shale and accurate estimations of gas content. This study focuses on the over-mature Qiongzhusi marine shales in Lower Cambrian of the Southern Sichuan Basin. A series of laboratory experiments were conducted on core samples, including X-ray diffraction (XRD), scanning electron microscopy (SEM), low-pressure gas adsorption (N2 and CO2) and high-pressure mercury intrusion porosimetry (MIP). XRD results show that four main types of shale lithofacies are found in the Qiongzhusi Formation according to the content of siliceous, carbonate and clay minerals, including silica-rich argillaceous shale lithofacies (SAL), argillaceous-rich siliceous shale lithofacies (ASL), carbonate/argillaceous containing siliceous shale lithofacies (C/ASL) and argillaceous/siliceous mixed shale lithofacies (A/SML). SEM images reveal that three kinds of pores within the shale samples (organic matter, intergranular and dissolved pores) with the majority being slit-like and narrow slit-like pores. Silica-rich argillaceous shale lithofacies exhibit the highest pore surface areas and volumes, while other shale lithofacies and carbonate/clay containing siliceous shale lithofacies exhibit relatively lower pore surface areas and volumes. It can be inferred that silica-rich argillaceous shale lithofacies facilitate shale gas storage and flow. Both TOC and clay mineral content are more positively correlated with the microscopic pore structure parameters of micropores than that of mesopores, and thus further interpreted as two dominant factors controlling the development and distribution of micropores. A relatively high quartz content in Qiongzhusi shale of Lower Cambrian are conductive to contribute a large macropore volume, but most importantly, the development of rigid frameworks established by authigenic quartz/pyrite and clay minerals are favorable for the preservation of primary organic matter micropores by providing effective grain supporters. Clay-rich siliceous shale could be deemed as one of the most typical lithofacies types displaying this unique pore-preserving mechanism. Shale lithofacies with different TOC and mineral compositions exhibit distinct differences in pore structures.

Porosity-preserving mechanisms of marine shale in Lower Cambrian of Sichuan Basin, South China

Porosity preservation is of great significance in the capacity of gas shale reservoir. Whether pore space can be preserved at the later diagenetic stage controls the capacity of shale reservoir. In this study, porosity-preserving mechanisms of marine shale from Lower Cambrian, both inside and at the margin of Sichuan Basin, South China, were investigated via a combination of the X-ray diffraction (XRD), organic geochemistry analysis, gas adsorption analysis and focused ion beam milling-scanning electron microscopy (FIB-SEM) in both two-dimension (2-D) and three-dimension (3-D). The results showed that there was little difference in mineralogical compositions between samples of Qiongzhusi shales and Niutitang shales that brittle minerals and clay were predominate components. The mean value of the total organic carbon (TOC) of Qiongzhusi shales was 1.44 wt% lower than that of Niutitang shales (4.37 wt%), whereas Qiongzhusi shales were at relatively lower thermal maturity stage with equal-vitrinite reflectance (Ro) ranging from 2.66% to 2.8% compared with Niutitang shales (3.06%<Ro<3.59%). Nitrogen (N2) and Carbon dioxide (CO2) adsorption revealed that micropore-mesopore (0.4–60 nm) volumes (PV) of Qiongzhusi shale samples (averaging 0.0305 ml/g) were higher than those of Niutitang shale samples (averaging 0.0251 ml/g), whereas the mean micropore-mesopore surface area (PSA) of Qiongzhusi samples (20.77 m2/g) was relatively lower than that of Niutitang samples (26.71 m2/g). FIB-SEM (2-D) observation manifested three distinctive pore types in terms of pores of rigid framework, pores associated with clay minerals and organic matter (OM)-hosted pores. FIB-SEM (3-D) reconstruction illustrated that pore networks possessed appropriate connectivity in the upside of Qiongzhusi shale samples.These observations suggest that brittle mineral grains can function as rigid frameworks or grain supporters due to high brittleness which preserves ductile matrix relating pores (OM-hosted pores and clay associated pores) from collapse by sharing the effective stress; in addition, either excessive OM abundance (TOC>5.0 wt%) or over thermal maturity (Ro>3.3%) is detrimental to pore preservation; moreover, poor sealing ability of shale systems might deteriorate pore preserving ability through losing overpressure environment or stress supporting mechanism.

Hydrocarbon generation capability of Sinian–Lower Cambrian shale, mudstone, and carbonate rocks in the Sichuan Basin, southwestern China: Implications for contributions to the giant Sinian Dengying natural gas accumulation

The giant natural gas accumulation recently discovered within the (Neoproterozoic) Dengying Formation in the Sichuan Basin of southwestern China represents a new and important supplement to the ancient Neoproterozoic–Cambrian petroleum systems worldwide. However, the source of this gas is controversial because there are five possibilities based on geology, from bottom to top including the Lower Sinian Doushantuo mudstone, the Upper Sinian Dengying algal dolomite, the mudstone of the third member of the Dengying Formation, the Lower Cambrian Maidiping argillaceous dolomite, and the Lower Cambrian Qiongzhusi shale. To improve the understanding of this issue, 33 samples from the five possible source rock units in the study area were investigated for their character and maturity via petrology, total organic carbon (TOC), pyrobitumen reflectance, kerogen carbon isotope, and biomarker analyses. Based on the results, we address their hydrocarbon generation capabilities and contributions to the giant gas accumulation. Results show that present-day TOC values of the samples range from 0.03% to 2.36% and vary both regionally and stratigraphically, with the Qiongzhusi shales in the central basin and the Weiyuan–Ziyang area ranking the highest. In contrast, the organic matter source and maturity within these rocks are relatively homogenous with slight variation throughout the study area. The organic matter is predominantly derived from algae and bacteria with different compositions deposited in marine reducing environments. The rocks range in thermal maturity from postmature to overmature. These results imply that the generation capability of source rock had been high in the past, and the gas in the study area is most likely originated from oil cracking and the late generation directly from the kerogen of source rocks, with the Qiongzhusi shales being the dominant source rock. Thus, the hydrocarbon migration pattern in the study area is a lateral migration from deep-seated and younger Cambrian source rocks in sags to shallow-seated and older Sinian dolomite reservoirs in uplifts. The migration process occurred in the oil phase with the oil being later cracked. The differences in natural gas geochemistry may be controlled by complex factors, e.g., thermal maturity, the migration–accumulation process, and differing contributions from source rocks besides the dominant Qiongzhusi. Regionally, the central basin and the Weiyuan–Ziyang area have had high hydrocarbon generation capability and thus should be considered as favorable targets for natural gas exploration in the future. This study of China provides an additional example of Neoproterozoic–Cambrian petroleum systems worldwide and thus can help to synthesize the systematics of such systems and understand the sedimentary transition across the Precambrian–Cambrian boundary.

Pore structure characteristics of marine Silurian shales in the Sichuan Basin, China: Insights to reserve assessment and production design

Due to shortage of conventional resources, tight/shale oil/gas reservoirs have become increasingly important in balancing the energy consumption in China. As the most important shale reservoirs in China, marine Silurian shales in the Sichuan Basin have received intensive investigation during recent years in the area of pore structure characterization and reserve assessment. However, no work has been conducted to compare the China shale plays with other mature shale plays in the North America using multiple experimental methods based on the mineralogy analysis and organic geochemistry. In this paper, pore structure, formation porosity, total organic carbon (TOC), thermal maturity, and mineralogy of 36 samples in Longmaxi shale and 7 samples in Qiongzhusi shale have been investigated using multiple experimental methods. FIB-SEM imaging results show that Longmaxi shale gas reservoir include primary pores, residual intergranular pores, dissolution pores, and intergranular pores. The majority of the pores are distributed in residual intergranular pores, secondary dissolution pores, and intercrystalline pores, but less in primary intergranular pores. X-ray diffraction analysis shows that the clay minerals of Longmaxi shale reservoir mainly consist of illite and kaolinite, then followed by chlorite. The content of kaolinite is from 7.2% to 16.87%, and the average is 12.21%. The content of illite is from 8.03% to 26.8%, with the average of 16.1%. TOC tests show that most of the samples have their TOC more than 0.5%, which means Longmaxi formation has a potential to produce shale gas. Clay minerals mainly include quartz, K-feldspar, calcite, dolomite, anorthose, and pyrite. The quartz has the highest content value, followed by calcite, which are 29% and 22% average respectively. The content of pyrite and anorthose are less than 10%. The average content of brittle minerals of Longmaxi formation samples is 66% (>50%), which is suitable for hydraulic fracturing. Thermal maturity tests show that Vitrinite reflectance of each sample is more than 2.0%, which indicates that it is high enough to generate shale gas in the formation. Comparing with other mature shale plays, the Longmaxi and Qiongzhusi shale plays have lower porosity and lower TOC than that of the U.S. shale plays, and the quartz content and thermal maturity are similar. The Qiongzhusi shale and Longmaxi shale are over-mature (Ro>2.5%) compared with U.S. shale plays. The results of this study will contribute to the formation characteristics understanding and reserve assessment of China shale plays. Also, it will facilitate the development of suitable and efficient production plan for China shale plays.

Characterization and Evolution of Nanoporosity in Superdeeply Buried Shales: A Case Study of the Longmaxi and Qiongzhusi Shales from MS Well #1, North Sichuan Basin, China

The nanopore characteristics of shale samples from the superdeeply buried Longmaxi Shale (drillcore recovered from 6604–6920 m below ground level), Wufeng Shale (6920–6926 m), and Qiongzhusi Shale ...

Investigation of the Methane Adsorption Characteristics of Marine Shale: A Case Study of Lower Cambrian Qiongzhusi Shale in Eastern Yunnan Province, South China

Marine organic-rich shale in South China has considerable exploration potential, and the shale adsorption capacity has a great impact on the accumulation of shale gas. To study the methane adsorption capacity of marine shale, ten shale samples from the Lower Cambrian Qiongzhusi Formation in eastern Yunnan province were investigated by organic geochemical analysis (total organic carbon content, thermal maturity, and kerogen type), X-ray diffraction (XRD) analysis, field emission scanning electron microscopy (FE-SEM), and low-pressure nitrogen adsorption and methane adsorption experiments. Based on the different adsorption mechanisms of various pores, the Dubinin–Radushkevich and Langmuir–Freundlich models were used to construct a supercritical adsorption model of shale. Combined with this model, the mechanisms and characteristics of shale adsorption under supercritical conditions were analyzed. The maximum absolute methane adsorption capacities of micropores (V1) and mesopores-macropores (V2) were also cal...

Further discussion about the origin of natural gas in the Sinian of central Sichuan paleo-uplift, Sichuan Basin, China

The origin of Sinian natural gas in the paleo-uplift in the central Sichuan is controversial. The major geo-chemical evidence to prove the gas origin was studied again, and it indicates that: the relationship of the Cambrian and Sinian ethane carbon isotope and ethane content complies with the Rayleigh distillation model, hence, the differences in the δ13C2 was not due to the different parent materials. As for the Sinian methane, the δD1 abnormally decreases with the increase of the δ13C1. This can be caused by the isotopic exchange with the formation water at an extremely high maturation stage rather than the different water salinities of depositional environment of their source rocks. Meanwhile, for the Sinian bitumen, the alkyldibenzothiophene molecular ratios 4-/1-MDBT is higher in the southwest and lower in the northeast, showing the hydrocarbon laterally migrated from the aulacogen Cambrian Qiongzhusi shale. The C7 light hydrocarbon was dominated by methylcyclohexane and a lack of toluene, demonstrating that the gas is from oil cracking instead of carbonate source rock. Therefore, the point that there is significant contribution from the Sinian source rock lacks evidence at present, and the major contribution of the Sinian gas is from the aulacogen of the Cambrian shale.

Shale Gas in China: Reality and Dream

China extracted shale gas of 23, 500∼24, 600 m3/d from the Lower Cambrian Qiongzhusi Shale Formation in Weiyuan gas field of Sichuan Basin in 1966. This shale interval, however, was considered less prospective because the Sinian Doushantuo Formation dolomite reservoir was defined as the target layer for drilling at that time. Forty years later, this black shale interval became attractive as triggered by the “Shale Gas Evolution” in North America. In 2005, by learning from shale gas mode of the United States, PetroChina Research Institute of Petroleum Exploration and Development (RIPED) reviewed the existing well data of the region where the Weiyuan and Longchang gas fields and the Changning structure were located within the Southern Sichuan Basin. In order to re-examine the gas shows and geochemical property and to evaluate the gas potential of shale strata within the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation and the Lower Cambrian Qiongzhusi Formation. On that basis, for the entire Sichuan Basin and adjoining regions, PetroChina has carried out a geologic survey on organic-rich shales, drilled shallow shale gas appraisal wells, evaluated the shale gas resource potential, and defined some favorable targets for shale gas exploration and development by 2008. W201, the first shale gas appraisal well of China, drilled by PetroChina in 2009, initially produced 17, 000 m3/day from the Wufeng-Longmaxi Formation shale and 10, 800 m3/day from the Qiongzhusi Formation shale, marking the commencement of shale gas exploration and development activities in China. Thus far, China has received shale gas flow from marine, transitional-limnetic coaly and lacustrine shales in many regions, such as Sichuan, Chongqing, Guizhou, etc. The commercialization of marine shale gas has been achieved in Sichuan and Chongqing totaling 0.2 billion cubic meters of shale gas production in 2013. Through an overall review on the progress of shale gas exploration and development in China, we concluded that the geological conditions for shale gas in China are extremely complex. First, the most sedimentary basins in China belong to superimposed basins, all of them have experienced the depositional evolution from marine to terrestrial facies, and three types of shales were well developed including marine, transitional, and terrestrial organic-rich shales. Second, the conditions of shale gas formation and accumulation in China are favorable and shale gas resource has a great potential for further development in China. Third, like North America, the marine organic-rich shales of China have the best and most realistic shale gas exploration and development prospects. But it faces technical, environmental, regulatory, infrastructural and other challenges that are likely to delay rapid shale gas development. Fourth, until now, although the exploration and exploitation of shale gas in China are relatively low, very good signs have been emerged to prove that China has good prospects in the future.