2017 Vol. 42, No. 7
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2017, 42(7): 1045-1056.
doi: 10.3799/dqkx.2017.084
Abstract:
In view of the characteristics of strong temporal and spatial heterogeneity, high thermal maturity, complex tectonic evolution, differential enrichment and various preservation conditions of shale gas in southern China, multidisciplinary methods are applied to further study the Wufeng-Longmaxi marine shale gas, and a number of key technologies are developed to characterize the evaluation parameters of marine shale gas with over-high maturity in the complex geologic areas of southern China. The results show that argillaceous siliceous shales and argillaceous/siliceous mixed shales are the advantageous lithofacies in the Wufeng Formation-Longmaxi Formation. The meso-pore and macro-pore make a great contribution to the pore volume, and the micro-nano pore structure is controlled by mineral composition and porosity development. The enrichment and evolution of shale gas are characterized by high paleotemperature, high thermal evolution, complex evolution of temperature and pressure, complicated conditions for shale gas occurrence and preservation. The Wufeng-Longmaxi marine shale gas could be effectively evaluated in terms of technologies of lithofacies optimization, the characterization of pore space from macroscopic and microscopic perspectives, the complex evolution process of marine shale, and the evaluation parameters for sweet spots of shale gas. The research results can provide technical support for the basic geological survey of shale gas in southern China.
In view of the characteristics of strong temporal and spatial heterogeneity, high thermal maturity, complex tectonic evolution, differential enrichment and various preservation conditions of shale gas in southern China, multidisciplinary methods are applied to further study the Wufeng-Longmaxi marine shale gas, and a number of key technologies are developed to characterize the evaluation parameters of marine shale gas with over-high maturity in the complex geologic areas of southern China. The results show that argillaceous siliceous shales and argillaceous/siliceous mixed shales are the advantageous lithofacies in the Wufeng Formation-Longmaxi Formation. The meso-pore and macro-pore make a great contribution to the pore volume, and the micro-nano pore structure is controlled by mineral composition and porosity development. The enrichment and evolution of shale gas are characterized by high paleotemperature, high thermal evolution, complex evolution of temperature and pressure, complicated conditions for shale gas occurrence and preservation. The Wufeng-Longmaxi marine shale gas could be effectively evaluated in terms of technologies of lithofacies optimization, the characterization of pore space from macroscopic and microscopic perspectives, the complex evolution process of marine shale, and the evaluation parameters for sweet spots of shale gas. The research results can provide technical support for the basic geological survey of shale gas in southern China.
2017, 42(7): 1057-1068.
doi: 10.3799/dqkx.2017.085
Abstract:
Sichuan basin has been taking the lead in China for the shale gas business development in recent years. Ordovician Wufeng Formation-Silurian Longmaxi Formation has been the main layer of marine shale gas development in the south. Based on the analysis of shale gas deposition and distribution, tectonic evolution, tectonic patterns and preservation, shale reservoir characteristics, the main controlling factors on enrichment and high-production of marine shale gas are summarized. The rich organic shale formed in deep shelf sedimentary environment is taken of the main factor as the material basis. The time and style of tectonic uplift are the key factors of shale gas accumulation and preservation. Three types of shale gas tectonic styles are determined, including intact type, residual type and destructive type. Overpressure of formation is a necessary condition of high shale gas production. In this paper the prospect of shale gas in South China has been forecasted. Edges of paleo-highs and Yunnan-Guizhou-Guangxi region are predicted as the favorable areas for future exploration and development of shale gas.
Sichuan basin has been taking the lead in China for the shale gas business development in recent years. Ordovician Wufeng Formation-Silurian Longmaxi Formation has been the main layer of marine shale gas development in the south. Based on the analysis of shale gas deposition and distribution, tectonic evolution, tectonic patterns and preservation, shale reservoir characteristics, the main controlling factors on enrichment and high-production of marine shale gas are summarized. The rich organic shale formed in deep shelf sedimentary environment is taken of the main factor as the material basis. The time and style of tectonic uplift are the key factors of shale gas accumulation and preservation. Three types of shale gas tectonic styles are determined, including intact type, residual type and destructive type. Overpressure of formation is a necessary condition of high shale gas production. In this paper the prospect of shale gas in South China has been forecasted. Edges of paleo-highs and Yunnan-Guizhou-Guangxi region are predicted as the favorable areas for future exploration and development of shale gas.
2017, 42(7): 1069-1082.
doi: 10.3799/dqkx.2017.086
Abstract:
Shale sequence division and composition remain highly controversial, and the existing research on the characteristics and genesis of sequence framework is inadequate. The Wufeng-Longmaxi Formation Shale is one of the most important horizons of shale gas exploration in the Yangtze region. On the basis of research on well-core, outcrop, conventional log, a spectral gamma-ray log, geochemical parameters and other information are used for quantitative sequence stratigraphy analysis of black shale in the upper Ordovician Wufeng Formation to the lower Silurian Longmaxi Formation. Three third-order sequences are divided based on the recognition of four third-order boundaries. The Wufeng Formation is equivalent to a third-order sequence and is subdivided into a transgressive system tract (TST) (black shale of lower Wufeng Formation) and a highstand system tract (HST) (Guanyinqiao member of upper Wufeng Formation). the first member of Longmaxi Formation is equivalent to a third-order sequence and is subdivided into a TST, an early highstand system tract (EHST) and a late highstand system tract (LHST); the second member and third member of Longmaxi Formation are combined to be one third-order sequence and is subdivided into a lowstand system tract (LST), a TST and a HST. Sequence development and sedimentary environment characteristics are analyzed within each system tract unit.TOC is correlated to V/Cr and EF-Ni differently within different system tract units, suggesting paleoproductivity and water redox condition in different system tract units have different effect on the enrichment of organic matter in the studied shale. Based on the changes of depositional processes and water column conditions, we develop a depositional model to interpret the evolution of depositional environments of the Wufeng-Longmaxi Formation.
Shale sequence division and composition remain highly controversial, and the existing research on the characteristics and genesis of sequence framework is inadequate. The Wufeng-Longmaxi Formation Shale is one of the most important horizons of shale gas exploration in the Yangtze region. On the basis of research on well-core, outcrop, conventional log, a spectral gamma-ray log, geochemical parameters and other information are used for quantitative sequence stratigraphy analysis of black shale in the upper Ordovician Wufeng Formation to the lower Silurian Longmaxi Formation. Three third-order sequences are divided based on the recognition of four third-order boundaries. The Wufeng Formation is equivalent to a third-order sequence and is subdivided into a transgressive system tract (TST) (black shale of lower Wufeng Formation) and a highstand system tract (HST) (Guanyinqiao member of upper Wufeng Formation). the first member of Longmaxi Formation is equivalent to a third-order sequence and is subdivided into a TST, an early highstand system tract (EHST) and a late highstand system tract (LHST); the second member and third member of Longmaxi Formation are combined to be one third-order sequence and is subdivided into a lowstand system tract (LST), a TST and a HST. Sequence development and sedimentary environment characteristics are analyzed within each system tract unit.TOC is correlated to V/Cr and EF-Ni differently within different system tract units, suggesting paleoproductivity and water redox condition in different system tract units have different effect on the enrichment of organic matter in the studied shale. Based on the changes of depositional processes and water column conditions, we develop a depositional model to interpret the evolution of depositional environments of the Wufeng-Longmaxi Formation.
2017, 42(7): 1083-1091.
doi: 10.3799/dqkx.2017.087
Abstract:
Shale gas enrichment is uneven, and production is different whether for different shale reservoirs or in different shale oil and gas reservoirs. It is necessary to systematically analyze compare the relationship between geology, petroleum system and oil and gas enrichment of different shale oil and gas plays in the world, to find out the main enrichment factors of shale oil and gas for the purpose of best choice of exploration and development technology. Based on the observation and description of outcrop and core, property testing of shale samples, reservoir characterization, petroleum system analysis, and production test, the shale reservoirs in the United States, Argentina, and China were studied. Results show that the organic-rich shale with content of high brittle minerals is mainly distributed in the shallow water sedimentary and tectonic background far away from the orogenic provenance. The high quartz content and high gamma value are not the evaluation criterion of high quality shale reservoir. The high quality carbonate-rich shale reservoir should be rich in carbonate and of low gamma value instead of being rich in quartz and high gamma value. The "shale reservoirs" are actually fine grained organic-rich shale reservoirs or the organic-lean fine-grained reservoirs adjacent to the source rocks. The natural fractures have both positive and negative effects on shale oil and gas enrichment. Therefore, the shale oil and gas accumulation is controlled by the coupling of tectonic and sedimentary environment, lithofacies, mineral composition, and natural fracture. These controlling factors vary in different basins and different shales with specific properties.
Shale gas enrichment is uneven, and production is different whether for different shale reservoirs or in different shale oil and gas reservoirs. It is necessary to systematically analyze compare the relationship between geology, petroleum system and oil and gas enrichment of different shale oil and gas plays in the world, to find out the main enrichment factors of shale oil and gas for the purpose of best choice of exploration and development technology. Based on the observation and description of outcrop and core, property testing of shale samples, reservoir characterization, petroleum system analysis, and production test, the shale reservoirs in the United States, Argentina, and China were studied. Results show that the organic-rich shale with content of high brittle minerals is mainly distributed in the shallow water sedimentary and tectonic background far away from the orogenic provenance. The high quartz content and high gamma value are not the evaluation criterion of high quality shale reservoir. The high quality carbonate-rich shale reservoir should be rich in carbonate and of low gamma value instead of being rich in quartz and high gamma value. The "shale reservoirs" are actually fine grained organic-rich shale reservoirs or the organic-lean fine-grained reservoirs adjacent to the source rocks. The natural fractures have both positive and negative effects on shale oil and gas enrichment. Therefore, the shale oil and gas accumulation is controlled by the coupling of tectonic and sedimentary environment, lithofacies, mineral composition, and natural fracture. These controlling factors vary in different basins and different shales with specific properties.
2017, 42(7): 1092-1106.
doi: 10.3799/dqkx.2017.088
Abstract:
The evaluation system of shale hydrocarbon generation is mainly addressed in the static condition at present, ignoring the dynamic process of hydrocarbon generation. Consequently, the original hydrocarbon generation potential of the shale cannot be properly evaluated. In this study, a typically marine shale sample with relatively low maturity and its kerogen were artificially matured by a half closed pyrolysis system and a closed pyrolysis system. Samples with different maturity levels obtained from the two systems were then pyrolyzed for gas generation in sealed gold tubes (i. e., pyrolysis experiment in sealed gold tubes). The quantitative analysis based on the products of C1-C5 gases, C6-C12 light hydrocarbons and carbon isotopes of gases from the simulation experiments indicates that the generation process of methane in kerogen can be divided into four stages:oil-generation ( < 1.0% EasyRo), condensate-generation (1.0%-1.5% EasyRo), wet-gas-generation (1.5%-2.2% EasyRo) and dry-gas-stage (>2.2% EasyRo). Kerogen, expelled bitumen and residue bitumen contributes 22.7%, 57.6% and 19.6% of maximum yield of methane in shale, respectively. Abundant soluble bitumen still exists in the shale matrix after the hydrocarbon expulsion, which becomes the major source of shale gas by interacting with kerogen and insoluble bitumen at high maturity levels.
The evaluation system of shale hydrocarbon generation is mainly addressed in the static condition at present, ignoring the dynamic process of hydrocarbon generation. Consequently, the original hydrocarbon generation potential of the shale cannot be properly evaluated. In this study, a typically marine shale sample with relatively low maturity and its kerogen were artificially matured by a half closed pyrolysis system and a closed pyrolysis system. Samples with different maturity levels obtained from the two systems were then pyrolyzed for gas generation in sealed gold tubes (i. e., pyrolysis experiment in sealed gold tubes). The quantitative analysis based on the products of C1-C5 gases, C6-C12 light hydrocarbons and carbon isotopes of gases from the simulation experiments indicates that the generation process of methane in kerogen can be divided into four stages:oil-generation ( < 1.0% EasyRo), condensate-generation (1.0%-1.5% EasyRo), wet-gas-generation (1.5%-2.2% EasyRo) and dry-gas-stage (>2.2% EasyRo). Kerogen, expelled bitumen and residue bitumen contributes 22.7%, 57.6% and 19.6% of maximum yield of methane in shale, respectively. Abundant soluble bitumen still exists in the shale matrix after the hydrocarbon expulsion, which becomes the major source of shale gas by interacting with kerogen and insoluble bitumen at high maturity levels.
2017, 42(7): 1107-1115.
doi: 10.3799/dqkx.2017.089
Abstract:
Well TX1 is the first well aimed at Niutitang Formation that was fractured outside of Sichuan basin. The field gas content of well TX1 core in Guizhou Cengong area is high at 1.00 to 3.06m3/t, showing good exploration prospect of Niutitang shale gas. But the complex structure and the lower aquifer provide a challenge to the later reservoir reconstruction. In order to further evaluate the gas bearing and production capacity of well TX1, we obtain the gas flame reaching 2 to 3 meters high after fracturing and postfrac flowback. But the flowback water is higher than 100%. This paper analyzes the cause of water and fracturing effect through evaluating the reservoir geological characteristics and fracturing fitting. The evaluation shows that the high water flowback resulted as the fracture linked up the bottom high water aquifers during fracturing process. No complex fracture network has been formed by the fracturing fitting result, which limits the reservoir reconstruction effect. From this we can see that vertical well fracturing effect is limited by fracturing size, it can only be used for reservoir and production evaluation. Horizontal wells and large scale staged fracturing are needed for commercial development. The drilling and fracturing of well TX1 can facilitate further exploration in this area.
Well TX1 is the first well aimed at Niutitang Formation that was fractured outside of Sichuan basin. The field gas content of well TX1 core in Guizhou Cengong area is high at 1.00 to 3.06m3/t, showing good exploration prospect of Niutitang shale gas. But the complex structure and the lower aquifer provide a challenge to the later reservoir reconstruction. In order to further evaluate the gas bearing and production capacity of well TX1, we obtain the gas flame reaching 2 to 3 meters high after fracturing and postfrac flowback. But the flowback water is higher than 100%. This paper analyzes the cause of water and fracturing effect through evaluating the reservoir geological characteristics and fracturing fitting. The evaluation shows that the high water flowback resulted as the fracture linked up the bottom high water aquifers during fracturing process. No complex fracture network has been formed by the fracturing fitting result, which limits the reservoir reconstruction effect. From this we can see that vertical well fracturing effect is limited by fracturing size, it can only be used for reservoir and production evaluation. Horizontal wells and large scale staged fracturing are needed for commercial development. The drilling and fracturing of well TX1 can facilitate further exploration in this area.
2017, 42(7): 1116-1123.
doi: 10.3799/dqkx.2017.090
Abstract:
The favorable facies classification for shale has not been established so far, and it is a key issue in shale gas evaluation to characterize the full scale pore size distribution by reasonable relating of different results. In this study, lithofacies classification is established based on TOC and XRD experiments on cores. There are 9 types of lithofacies, namely the Organic-rich siliceous shale (ORS), Organic-rich mixed shale (ORM), Organic-rich argillaceous shale (ORA), Organic-fair siliceous shale (OMS), Organic-fair mixed shale (OMM), Organic-fair argillaceous shale (OMA), Organic-poor siliceous shale (OPS), Organic-poor mixed shale (OPM), and Organic-poor argillaceous shale (OPA) develop in southeastern Chongqing. Low pressure nitrogen adsorption and high pressure mercury intrusion experiments are conducted to quantitatively characterize the full scale pore size distribution. It is found that spectrum of pore size distribution for ORS have multiple peaks at 2-3nm, 70-90nm and 200-300nm, and the peak shifts to the small pore size when the TOC value increases. Mesopores and macropores have the largest proportion in pore volumes, whereas mesopores and micropores take the largest part of the pore surface areas. The pore volume percentage of micropore, mesopore and marcropore is 12%, 53%, and 52% respectively, and the pore surface area percentage of micro pore, mesopore and marcropore is 47%, 57%, and 11% respectively. The volume of 200-400nm pore obviously increases when clay mineral content increases. The contribution to the surface area is over 90% for micropores and mesopores. The contribution to the pore volume is over 90% for mesopores and marcropores. The organic-rich siliceous shale, which has the largest pore volume and surface area, is the most favourable lithofacies for shale gas enrichment.
The favorable facies classification for shale has not been established so far, and it is a key issue in shale gas evaluation to characterize the full scale pore size distribution by reasonable relating of different results. In this study, lithofacies classification is established based on TOC and XRD experiments on cores. There are 9 types of lithofacies, namely the Organic-rich siliceous shale (ORS), Organic-rich mixed shale (ORM), Organic-rich argillaceous shale (ORA), Organic-fair siliceous shale (OMS), Organic-fair mixed shale (OMM), Organic-fair argillaceous shale (OMA), Organic-poor siliceous shale (OPS), Organic-poor mixed shale (OPM), and Organic-poor argillaceous shale (OPA) develop in southeastern Chongqing. Low pressure nitrogen adsorption and high pressure mercury intrusion experiments are conducted to quantitatively characterize the full scale pore size distribution. It is found that spectrum of pore size distribution for ORS have multiple peaks at 2-3nm, 70-90nm and 200-300nm, and the peak shifts to the small pore size when the TOC value increases. Mesopores and macropores have the largest proportion in pore volumes, whereas mesopores and micropores take the largest part of the pore surface areas. The pore volume percentage of micropore, mesopore and marcropore is 12%, 53%, and 52% respectively, and the pore surface area percentage of micro pore, mesopore and marcropore is 47%, 57%, and 11% respectively. The volume of 200-400nm pore obviously increases when clay mineral content increases. The contribution to the surface area is over 90% for micropores and mesopores. The contribution to the pore volume is over 90% for mesopores and marcropores. The organic-rich siliceous shale, which has the largest pore volume and surface area, is the most favourable lithofacies for shale gas enrichment.
2017, 42(7): 1124-1133.
doi: 10.3799/dqkx.2017.091
Abstract:
The Wufeng-Longmaxi shales in Jiaoshiba is rich in natural gas. However, the research on the geochemical characteristic of the shale gas is weak, and its geological significance is still unclear. This article discusses the origin type and the "full reversal" carbon isotopic composition of shale gas from Wufeng Formation-Longmaxi Formation based on the analysis of the shale gas component and its isotope. The high hydrocarbon generation potential of Wufeng-Longmaxi shales, the positive correlation of the abundance of organic carbon and the content of gas, the similar carbon isotope distribution between methane and kerogen and the overpressure in Wufeng Formation-Longmaxi Formation suggest the shale gas should be generated from and preserved in the Wufeng Formation-Longmaxi Formation. The content of methane and its isotope indicate the stage of thermal maturity for shale gas has reached the overmature phase and the shale gas is mainly dry gas. The shale gas is mainly formed from the cracking of oil generated early based on the ratio of ln(C1/C2) and ln(C2/C3). The "full reversal" carbon isotope distribution of shale gas resulted from the oil cracking in a relatively closed system and it also reflects a favorable preservation and accumulation conditions for shale gas.
The Wufeng-Longmaxi shales in Jiaoshiba is rich in natural gas. However, the research on the geochemical characteristic of the shale gas is weak, and its geological significance is still unclear. This article discusses the origin type and the "full reversal" carbon isotopic composition of shale gas from Wufeng Formation-Longmaxi Formation based on the analysis of the shale gas component and its isotope. The high hydrocarbon generation potential of Wufeng-Longmaxi shales, the positive correlation of the abundance of organic carbon and the content of gas, the similar carbon isotope distribution between methane and kerogen and the overpressure in Wufeng Formation-Longmaxi Formation suggest the shale gas should be generated from and preserved in the Wufeng Formation-Longmaxi Formation. The content of methane and its isotope indicate the stage of thermal maturity for shale gas has reached the overmature phase and the shale gas is mainly dry gas. The shale gas is mainly formed from the cracking of oil generated early based on the ratio of ln(C1/C2) and ln(C2/C3). The "full reversal" carbon isotope distribution of shale gas resulted from the oil cracking in a relatively closed system and it also reflects a favorable preservation and accumulation conditions for shale gas.
2017, 42(7): 1134-1146.
doi: 10.3799/dqkx.2017.092
Abstract:
Pore structure and its difference is a basic issue in shale gas content and capacity evaluation. A number of studies have been made on the pore structure of the shale in the Wufeng Formation and Longmaxi Formation in Pengshui area, southeastern Sichuan. However, there is a lack of research on shale pore structure difference and organic pore quantitative characteristics. Using low-temperature and low-pressure N2 adsorption and Ar ion milling field emission scanning electron microscopy (FE-SEM), for Wufeng and Longmaxi marine shales in Pengshui area of southeastern Sichuan, this paper investigated the pore structure of shale samples with the pore size ranging from 3nm to hundreds nm, and its difference between Wufeng and Longmaxi formations was also analyzed by observing and counting nano-pores in two-dimension, and computing fractal dimension. Results indicate that organic pores from Wufeng and Longmaxi shales in Pengshui area are very developed. Results of N2 adsorption show the shale samples contain open cylindrical-like pores, layered slit-like pores and ink-bottle pores. And observation of scanning electron microscopy reveals organic pore morphology mainly is nearly circular, elliptic and polygon. The obvious difference of pore structure between Wufeng and Longmaxi shales is mainly reflected on the pore size, shape and quantity. N2 adsorption results show that Wufeng shales have higher specific surface area and total pore volume than that of Longmaxi shales, and micropore within Wufeng shales accounts for higher proportion of total pore; pore size of Wufeng shales is narrower than that of Longmaxi shales. Observation and statistics of FE-SEM two-dimensional images indicate organic pores in Wufeng Formation are mainly pores with diameter less than 35nm and have irregular shapes. While Longmaxi organic pores are mainly less than 50nm in diameter and pore shapes mainly are nearly circular or elliptic. Based on the pore fractal dimension calculation of N2 adsorption and FE-SEM, Wufeng Formation has higher pore fractal dimension values than Longmaxi Formation, which indicates that the pore structure of Wufeng shales is more complex than that of Longmaxi shales.
Pore structure and its difference is a basic issue in shale gas content and capacity evaluation. A number of studies have been made on the pore structure of the shale in the Wufeng Formation and Longmaxi Formation in Pengshui area, southeastern Sichuan. However, there is a lack of research on shale pore structure difference and organic pore quantitative characteristics. Using low-temperature and low-pressure N2 adsorption and Ar ion milling field emission scanning electron microscopy (FE-SEM), for Wufeng and Longmaxi marine shales in Pengshui area of southeastern Sichuan, this paper investigated the pore structure of shale samples with the pore size ranging from 3nm to hundreds nm, and its difference between Wufeng and Longmaxi formations was also analyzed by observing and counting nano-pores in two-dimension, and computing fractal dimension. Results indicate that organic pores from Wufeng and Longmaxi shales in Pengshui area are very developed. Results of N2 adsorption show the shale samples contain open cylindrical-like pores, layered slit-like pores and ink-bottle pores. And observation of scanning electron microscopy reveals organic pore morphology mainly is nearly circular, elliptic and polygon. The obvious difference of pore structure between Wufeng and Longmaxi shales is mainly reflected on the pore size, shape and quantity. N2 adsorption results show that Wufeng shales have higher specific surface area and total pore volume than that of Longmaxi shales, and micropore within Wufeng shales accounts for higher proportion of total pore; pore size of Wufeng shales is narrower than that of Longmaxi shales. Observation and statistics of FE-SEM two-dimensional images indicate organic pores in Wufeng Formation are mainly pores with diameter less than 35nm and have irregular shapes. While Longmaxi organic pores are mainly less than 50nm in diameter and pore shapes mainly are nearly circular or elliptic. Based on the pore fractal dimension calculation of N2 adsorption and FE-SEM, Wufeng Formation has higher pore fractal dimension values than Longmaxi Formation, which indicates that the pore structure of Wufeng shales is more complex than that of Longmaxi shales.
Micro-Nano Pore Structure Characteristics in the Lower Cambrian Niutitang Shale, Northeast Chongqing
2017, 42(7): 1147-1156.
doi: 10.3799/dqkx.2017.093
Abstract:
Lower Cambrian Niututang Formation shale in south China is one of the main target zones for shale gas exploration. However, the exploration of shale in northeastern Chongqing is not satisfactory as the pore structure of the shale is unclear. This study examined pores in shale samples from the Niutitang Formation using focused ion-beam scanning electron microscopy, nano-CT, and gas adsorption analysis. Results show that pores in Niutitang shale are rare and have small diameters and poor connectivity. Most Niutitang-shale pores are inorganic pores, including intraparticle and intergranular pores. N2 adsorption hysteresis loop of Niutitang shale belongs to type H4 corresponding to narrow slit pore. The average total pore volume of the Niutitang shale is 0.0317mL/g, and the average total surface area is 34.57m2/g. Over-high thermal evolution degree resulting small number of organic pores and poor connectivity of micro-nano pores in Niutitang shale. Mesopores contributed most of the pore volume and micropores contributed more pore surface area than that of mesopores.
Lower Cambrian Niututang Formation shale in south China is one of the main target zones for shale gas exploration. However, the exploration of shale in northeastern Chongqing is not satisfactory as the pore structure of the shale is unclear. This study examined pores in shale samples from the Niutitang Formation using focused ion-beam scanning electron microscopy, nano-CT, and gas adsorption analysis. Results show that pores in Niutitang shale are rare and have small diameters and poor connectivity. Most Niutitang-shale pores are inorganic pores, including intraparticle and intergranular pores. N2 adsorption hysteresis loop of Niutitang shale belongs to type H4 corresponding to narrow slit pore. The average total pore volume of the Niutitang shale is 0.0317mL/g, and the average total surface area is 34.57m2/g. Over-high thermal evolution degree resulting small number of organic pores and poor connectivity of micro-nano pores in Niutitang shale. Mesopores contributed most of the pore volume and micropores contributed more pore surface area than that of mesopores.
2017, 42(7): 1157-1168.
doi: 10.3799/dqkx.2017.094
Abstract:
The existing prediction methods of shale gas content include in situ desorption, logging interpretation, isothermal adsorption, linear fitting, seismic inversion and so on, but each method is deficient in some aspects. A new calculation model of shale gas content in Fuling area is established in this study, laying the foundation for the evaluation of shale resources. Based on the core experiment, the main controlling factors were selected through analyzing the influencing factors of shale free gas content and adsorbed gas content in Fuling area. Then, calculation models of free gas content and adsorbed gas content were established respectively with the integration of the theoretical calculation model and experimental formula to analyze the main controlling factors. Finally, the evolution charts of shale gas content versus porosity, TOC and depth and distribution characteristics of shale gas content in single well in studied area were obtained utilizing the calculation models proposed above. The shale gas content increases with depth when porosity and TOC are constant, but the increment decreases progressively; when the depth is certain, the shale gas content increases with porosity and TOC. The shale gas content of Wufeng Formation and Longmaxi Formation of well A increases with layer depth; the total gas content is up to 7.76m3/t under lower shale reservoir of Member 1, the content of free gas accounted for 60.57%, which indicates high-quality member for shale gas reservoir.
The existing prediction methods of shale gas content include in situ desorption, logging interpretation, isothermal adsorption, linear fitting, seismic inversion and so on, but each method is deficient in some aspects. A new calculation model of shale gas content in Fuling area is established in this study, laying the foundation for the evaluation of shale resources. Based on the core experiment, the main controlling factors were selected through analyzing the influencing factors of shale free gas content and adsorbed gas content in Fuling area. Then, calculation models of free gas content and adsorbed gas content were established respectively with the integration of the theoretical calculation model and experimental formula to analyze the main controlling factors. Finally, the evolution charts of shale gas content versus porosity, TOC and depth and distribution characteristics of shale gas content in single well in studied area were obtained utilizing the calculation models proposed above. The shale gas content increases with depth when porosity and TOC are constant, but the increment decreases progressively; when the depth is certain, the shale gas content increases with porosity and TOC. The shale gas content of Wufeng Formation and Longmaxi Formation of well A increases with layer depth; the total gas content is up to 7.76m3/t under lower shale reservoir of Member 1, the content of free gas accounted for 60.57%, which indicates high-quality member for shale gas reservoir.
2017, 42(7): 1169-1184.
doi: 10.3799/dqkx.2017.095
Abstract:
The organic rich marine black shale of the upper Ordovician Wufeng Formation-lower Silurian Longmaxi Formation is widely developed in the upper Yangtze area.However, there are few studies on lithofacies types and on the response of the deposition process to the main geological events.From the Late Ordovician to the end of sedimentary filling period of the first member of Longmaxi Formation, the upper Yangtze area had experienced three major geological events.Based on the high resolution sequence stratigraphy study on the fine-grained sediments, two 3rd order sequences Wufeng (Sq1) and Long1(Sq2) including 12 para-sequences and 13 graptolite zones are identified during this interval.Within the isochronal stratigraphic framework which was constrained by the biostratigraphy, we have demonstrated the characteristics and genesis mechanism of different shale lithofacies.The siliceous shale of high TOC is likely attributed to volcanic eruption.The mixed shale of low TOC is likely attributed to the bottom flow intrusion and the argillaceous shale of low TOC is likely due to the enhanced terrigenous input though turbidity current.Thus a genetic model has been established.We suggest that the deposition of the Wufeng Formation and the first member of Longmaxi Formation heterogeneous shale is a sedimentary response to the altered paleo-climate and ocean environment which are attributed to three geological events:volcanic eruption (including glacial and biological extinction), bottom flow intrusion and terrigenous input.
The organic rich marine black shale of the upper Ordovician Wufeng Formation-lower Silurian Longmaxi Formation is widely developed in the upper Yangtze area.However, there are few studies on lithofacies types and on the response of the deposition process to the main geological events.From the Late Ordovician to the end of sedimentary filling period of the first member of Longmaxi Formation, the upper Yangtze area had experienced three major geological events.Based on the high resolution sequence stratigraphy study on the fine-grained sediments, two 3rd order sequences Wufeng (Sq1) and Long1(Sq2) including 12 para-sequences and 13 graptolite zones are identified during this interval.Within the isochronal stratigraphic framework which was constrained by the biostratigraphy, we have demonstrated the characteristics and genesis mechanism of different shale lithofacies.The siliceous shale of high TOC is likely attributed to volcanic eruption.The mixed shale of low TOC is likely attributed to the bottom flow intrusion and the argillaceous shale of low TOC is likely due to the enhanced terrigenous input though turbidity current.Thus a genetic model has been established.We suggest that the deposition of the Wufeng Formation and the first member of Longmaxi Formation heterogeneous shale is a sedimentary response to the altered paleo-climate and ocean environment which are attributed to three geological events:volcanic eruption (including glacial and biological extinction), bottom flow intrusion and terrigenous input.
Occurrence Mechanism and Key Controlling Factors of Wufeng-Longmaxi Shale Gas, Eastern Sichuan Basin
2017, 42(7): 1185-1194.
doi: 10.3799/dqkx.2017.096
Abstract:
Shale gas is one important kind of natural resources, which occurs in reservoir by free state and absorbing state. In order to reveal its accumulation mechanism, it is significant to study and illuminate the TOC content, main controlling factors, evolution rule of shale gas. In this study, we analyze the main controlling factor of free gas and absorbed gas in reservoir comprehensively by high temperature and high pressure adsorption isothermal experiment, FE-SEM, CO2 adsorption experiment, N2 adsorption experiment and mercury injection experiment. The results show that free gas is controlled by pore types and pore structure of shale, temperature and pressure in reservoir; while absorbed gas is controlled by TOC, maturity and moisture. In different stages of hydrocarbon generation, the content of free gas and absorbed gas changes regularly. Free gas occurs in the highly mature stage, and increases the process of hydrocarbon generation and the evolution of pores. It transforms into absorbed gas in the lifting stage because of the changes of temperature and pressure, which results in free gas decrease and absorbed gas increase.
Shale gas is one important kind of natural resources, which occurs in reservoir by free state and absorbing state. In order to reveal its accumulation mechanism, it is significant to study and illuminate the TOC content, main controlling factors, evolution rule of shale gas. In this study, we analyze the main controlling factor of free gas and absorbed gas in reservoir comprehensively by high temperature and high pressure adsorption isothermal experiment, FE-SEM, CO2 adsorption experiment, N2 adsorption experiment and mercury injection experiment. The results show that free gas is controlled by pore types and pore structure of shale, temperature and pressure in reservoir; while absorbed gas is controlled by TOC, maturity and moisture. In different stages of hydrocarbon generation, the content of free gas and absorbed gas changes regularly. Free gas occurs in the highly mature stage, and increases the process of hydrocarbon generation and the evolution of pores. It transforms into absorbed gas in the lifting stage because of the changes of temperature and pressure, which results in free gas decrease and absorbed gas increase.
2017, 42(7): 1195-1208.
doi: 10.3799/dqkx.2017.097
Abstract:
Organic-rich lacustrine shales have high hydrocarbon potential, but relevant studies did not start until recently, suffering from many deficiencies including relatively few studies on lithofaceis, depositional process and origin and evolution of lacustrine shales. The lacustrine shale succession of the upper submember of Es4 in the Dongying depression, Bohai Bay basin, eastern China, is the most important hydrocarbon source rock and an unconventional reservoir. In this study, we conduct detailed lithofacies characterization to the upper submember of Es4 shale succession in the NY1 drilling core by combining core descriptions, microscopic observations, mineralogy, major and minor elements and log data. Based on the theories of petrography, sedimentology, element geochemistry and T-R sequence stratigraphy, lithofacies features, depositional process of lithofacies, coupling relationship between lithofacies and sequence stratigraphy, and evolutions of paleoclimate and paleolake are investigated in detail. Four major lithofacies associations are identified, including the interbedded evaporite and shale lithofacies association, silt-bearing clay-rich shale lithofacies association, the massive calcareous shale lithofacies association, and the laminated calcareous shale lithofacies association from bottom to top. These lithofacies associations are interpreted to be deposited in small salt lake, a pro-delta environment, shallow lacustrine environment and deep, stratified, anoxic lake, respectively. By contrast, there is coupling between dominant lithofacies and system tracts through time. The changes of dominant lithofacies in the upper submember of Es4 shale succession through time suggest that paleoclimate changed from arid to humid and paleolake changed from small and shallow lake to large and deep lake, and it is inferred that paleolake evolution was controlled by paleoclimate changes and the tectonic subsidence associated with the development of depression-bounding faults.
Organic-rich lacustrine shales have high hydrocarbon potential, but relevant studies did not start until recently, suffering from many deficiencies including relatively few studies on lithofaceis, depositional process and origin and evolution of lacustrine shales. The lacustrine shale succession of the upper submember of Es4 in the Dongying depression, Bohai Bay basin, eastern China, is the most important hydrocarbon source rock and an unconventional reservoir. In this study, we conduct detailed lithofacies characterization to the upper submember of Es4 shale succession in the NY1 drilling core by combining core descriptions, microscopic observations, mineralogy, major and minor elements and log data. Based on the theories of petrography, sedimentology, element geochemistry and T-R sequence stratigraphy, lithofacies features, depositional process of lithofacies, coupling relationship between lithofacies and sequence stratigraphy, and evolutions of paleoclimate and paleolake are investigated in detail. Four major lithofacies associations are identified, including the interbedded evaporite and shale lithofacies association, silt-bearing clay-rich shale lithofacies association, the massive calcareous shale lithofacies association, and the laminated calcareous shale lithofacies association from bottom to top. These lithofacies associations are interpreted to be deposited in small salt lake, a pro-delta environment, shallow lacustrine environment and deep, stratified, anoxic lake, respectively. By contrast, there is coupling between dominant lithofacies and system tracts through time. The changes of dominant lithofacies in the upper submember of Es4 shale succession through time suggest that paleoclimate changed from arid to humid and paleolake changed from small and shallow lake to large and deep lake, and it is inferred that paleolake evolution was controlled by paleoclimate changes and the tectonic subsidence associated with the development of depression-bounding faults.
2017, 42(7): 1209-1222.
doi: 10.3799/dqkx.2017.098
Abstract:
Despite successful studies on source rocks of Raoyang sag, existing studies on the ancient lake environment during lower submember of Sha1 source rocks development period are not systematic, an important factor controlling the geochemical characteristics and temporal and spatial distribution of source rocks, is often ignored.In this study, samples of source rocks in the lower submember of Sha1 were analyzed with palaeontological material, organic geochemistry, trace element and isotopic geochemistry to reveal the paleolake environment including the paleoclimate, water depth, salinity, organic matter preservation and productivity, and to construct depositional model for high-quality source rocks of the lower submember of Sha1.Results show that the high-quality source rocks mainly formed in the transgressive system tracts period.Meantime, the climate was in the transition from arid to humid, and the water depth of the paleolake was about 2-14 meters, wave base depth was 7.5 meters.Paleosalinity parameters such as Sr/Ba and B/Ga indicate the lower submember of Sha1 belonged to the saline environment due to the effect of water evaporation.High salinity was favorable to stable halocline formation, resulting in the bottom water in an anoxic condition for the preservation of organic matter.And the stable carbon isotope and nutrient elements distribution characteristics display that the surface water had high initial productivity in the stage of the lower submember of Sha1.Such paleolake environment caused wide distribution of strong reducing salt water semi-deep lake organic facies, beneficial to the growth of high-quality hydrocarbon source rocks.
Despite successful studies on source rocks of Raoyang sag, existing studies on the ancient lake environment during lower submember of Sha1 source rocks development period are not systematic, an important factor controlling the geochemical characteristics and temporal and spatial distribution of source rocks, is often ignored.In this study, samples of source rocks in the lower submember of Sha1 were analyzed with palaeontological material, organic geochemistry, trace element and isotopic geochemistry to reveal the paleolake environment including the paleoclimate, water depth, salinity, organic matter preservation and productivity, and to construct depositional model for high-quality source rocks of the lower submember of Sha1.Results show that the high-quality source rocks mainly formed in the transgressive system tracts period.Meantime, the climate was in the transition from arid to humid, and the water depth of the paleolake was about 2-14 meters, wave base depth was 7.5 meters.Paleosalinity parameters such as Sr/Ba and B/Ga indicate the lower submember of Sha1 belonged to the saline environment due to the effect of water evaporation.High salinity was favorable to stable halocline formation, resulting in the bottom water in an anoxic condition for the preservation of organic matter.And the stable carbon isotope and nutrient elements distribution characteristics display that the surface water had high initial productivity in the stage of the lower submember of Sha1.Such paleolake environment caused wide distribution of strong reducing salt water semi-deep lake organic facies, beneficial to the growth of high-quality hydrocarbon source rocks.
2017, 42(7): 1223-1234.
doi: 10.3799/dqkx.2017.099
Abstract:
The black shales in lower Cambrian Niutitang and lower Silurian Longmaxi develop well in Yangtze area, which are the key horizons for shale gas exploration because of its wide distribution, large thickness, high organic matter abundance. The porosity of Niutitang Formation shows a distinct difference from Longmaxi Formation. There are also significant differences in the pore structure, organic carbon content and density of the two sets of shales. Based on the logging data, buried depth (overlying pressure), porosity, organic carbon and maturity test data, comparative analysis is conducted. Results show that pore structure has been affected by the high degree of thermal evolution and organic carbon of Niutitang Formation. Organic matter is carbonized, organic porosity is collapsed and filled. The depth of Niutitang Formation is deep and the overburden pressure is also large, causing the inorganic porosity to be compacted. These factors make the porosity of Niutitang Formation obviously less than Longmaxi Formation.
The black shales in lower Cambrian Niutitang and lower Silurian Longmaxi develop well in Yangtze area, which are the key horizons for shale gas exploration because of its wide distribution, large thickness, high organic matter abundance. The porosity of Niutitang Formation shows a distinct difference from Longmaxi Formation. There are also significant differences in the pore structure, organic carbon content and density of the two sets of shales. Based on the logging data, buried depth (overlying pressure), porosity, organic carbon and maturity test data, comparative analysis is conducted. Results show that pore structure has been affected by the high degree of thermal evolution and organic carbon of Niutitang Formation. Organic matter is carbonized, organic porosity is collapsed and filled. The depth of Niutitang Formation is deep and the overburden pressure is also large, causing the inorganic porosity to be compacted. These factors make the porosity of Niutitang Formation obviously less than Longmaxi Formation.
2017, 42(7): 1235-1246.
doi: 10.3799/dqkx.2017.100
Abstract:
The shale heterogeneity in different regions of China is the basic geological issue to be addressed urgently. However, the study of continental shale gas in the Sichuan basin is very weak. Based on analysis of the petrology and geochemistry of the Dongyuemiao member of lower Jurassic Ziliujing Formation in western Hubei-eastern Chongqing area, and combined with the logging curve, the vertical variation of geochemical characteristics and organic matter enrichment have been discussed. The results show that the Dongyuemiao member has high brittle mineral content and low clay mineral content, and higher organic matter content at the lower part than that at the upper part. Geochemical index, such as paleoclimate, paleoproductivity, paleoredox, paleosalinity has obvious vertical differences in the Dongyuemiao member. The lower part of Dongyuemiao member shows dry paleoclimate, dysoxic water column, saline water and high paleoproductivity, but the upper part of Dongyuemiao member shows warm and humid paleoclimate, low salination water and low paleoproductivity. It is concluded that the lower part of Dongyuemiao member has been more favourable for organic matter enrichment.
The shale heterogeneity in different regions of China is the basic geological issue to be addressed urgently. However, the study of continental shale gas in the Sichuan basin is very weak. Based on analysis of the petrology and geochemistry of the Dongyuemiao member of lower Jurassic Ziliujing Formation in western Hubei-eastern Chongqing area, and combined with the logging curve, the vertical variation of geochemical characteristics and organic matter enrichment have been discussed. The results show that the Dongyuemiao member has high brittle mineral content and low clay mineral content, and higher organic matter content at the lower part than that at the upper part. Geochemical index, such as paleoclimate, paleoproductivity, paleoredox, paleosalinity has obvious vertical differences in the Dongyuemiao member. The lower part of Dongyuemiao member shows dry paleoclimate, dysoxic water column, saline water and high paleoproductivity, but the upper part of Dongyuemiao member shows warm and humid paleoclimate, low salination water and low paleoproductivity. It is concluded that the lower part of Dongyuemiao member has been more favourable for organic matter enrichment.
