Search

Carbonate rocks in the first member of the Middle Permian Maokou Formation in the Sichuan Basin (hereinafter “Mao 1 Member” for short) have been taken as a set of carbonate source rocks, and they have not been specifically studied from the aspects of reservoir evaluation and testing. By referring the exploration ideas of unconventional natural gas, the Mao 1 Member in Fuling area of southeastern Sichuan Basin has obtained industrial gas flow in development wells in recent years. In order to further clarify the natural gas exploration potential of the Mao 1 Member in this area, it is necessary to study its sedimentary characteristics, natural gas accumulation conditions and main control factors based on the data of field section measurement, systematic coring in drilling and laboratory testing. The following research results were obtained. First, the gas reservoir in the Mao 1 Member in the Fuling area is of source–reservoir integration, and its natural gas is mainly enriched in blackish gray marlite and nodular marlite. Second, its reservoir spaces are dominated by grain margin pores (fractures), diagenetic shrinkage pores (fractures), organic pores and fractures. Third, the pores are mostly in nanometer scale, and the main pore diameter is in the range of 5–50 nm, which is between shale reservoir and conventional reservoir, with strong heterogeneity. Fourth, the gas reservoir is characterized by source–reservoir coexistence, lithology controlling reservoir and extensive layered distribution, presenting two-stage differential hydrocarbon enrichment, namely intraformational near-source enrichment in the early stage and interformational blowdown adjustment in the late stage. Fifth, the development of blackish gray organic-rich fine marlite which is deposited with the episodic upwelling in the outer ramp facies belt is the foundation for the natural gas accumulation, the transformation of clay minerals controls the development of higher-quality reservoirs, good preservation conditions are the key to the natural gas accumulation, and fracture development is favorable for the enrichment and high yield of natural gas. In conclusion, the Mao 1 Member in this area is a special type of gas reservoir, i.e., carbonate source rock gas reservoir, which has greater potential of natural gas exploration and industrial gas flow have been obtained in several wells. The discovery of such type of gas reservoirs not only expands the domain of natural gas exploration in the Sichuan Basin, but provides the reference for the natural gas exploration in other areas of South China.

The pore structure of gas shale reservoirs strongly influences gas occurrence, migration and enrichment, which has become an important fundamental issue for shale gas exploration and production. To...

Marine–continent transitional facies shale of the Longtan Fm, Upper Permian is an important source rock stratum in the Sichuan Basin. The previous researches on it mainly focus more on source rock evaluation, but less on shale gas accumulation. In this paper, the test and analysis results of Well DYS1, the coring well of Longtan Fm were dissected. Combined with the drilling results of its adjacent area, the organic rich shale in the Longtan Fm were studied from the aspects of distribution, geochemical, reservoir, gas-bearing characteristics and roof and floor conditions, and then compared with the shale strata with similar sedimentary background and good shale gas shows. And accordingly, its geological conditions for the formation of Longtan shale gas and its exploration potential were made clear. And the following research results were obtained. First, in the southeastern Sichuan Basin, the organic rich shale of transitional facies of Longtan Fm is developed with a thickness of more than 40 m. It is characterized by high brittle mineral content, high porosity, high total organic carbon (TOC), moderate thermal evolution (Ro) and good gas-bearing property. Therefore, it is geologically favorable for the formation of shale gas. Second, compared with the existing transitional facies shales at home and abroad, the Longtan shale is better in terms of porosity, TOC, Ro, gas content and other key parameters. Third, the type of organic matters is the main reason for the low development degree of organic pores in the Longtan shale. And during its shale gas exploration and selection, the coal seam enriched sections shall be avoided. In conclusion, considering the development degree, interlayer thickness, depth and preservation conditions of organic rich shale comprehensively, the Qijiang–Chishui area is the best exploration area for the transitional facies shale gas of Longtan Fm in the Sichuan Basin. Keywords: Sichuan basin, Late Permian, Longtan Fm, Marine–continental transitional facies, Shale, Total organic carbon, Thermal evolution maturity, Gas content, Shale gas exploration potential

The Carboniferous shale in the Guizhong Depression of central Guangxi presents good exploration potential of shale gas, but its exploration effectiveness is restricted by the unclear distribution, quality and potential of high-quality shale. In this paper, the Lower Carboniferous Luzhai shale was taken as the research object in order to guide the exploration of Carboniferous shale gas in the Guizhong Depression. Based on the field outcrops, well drilling, well logging and test data of the Guizhong Depression and its surrounding areas, the shale formation was studied systematically based on the characteristics of distribution, organic geochemistry and reservoir, and the preservation conditions. Then, the development characteristics of black shale were confirmed and the distribution laws of high-quality shale were summarized. Finally, shale gas exploration was analyzed and the favorable exploration areas were defined. And the following research results were obtained. First, the Luzhai shale in the Guizhong Depression is mainly carbonaceous shale and siliceous shale of deepwater continental shelf facies. Second, the high-quality Luzhai shale is 20–60 m thick and it is characterized by high total organic carbon, good type of organic matters, moderate maturity of organic matter, high content of brittle minerals, good reservoir property and developed pores of organic matters. Third, regional caprocks and roofs & floors are well developed in the Liucheng Slope and the Yishan Sag. They are weakly deformed, suffer from less uplift and denudation, and are far away from the strong strike–slip reverse fault, so they have good preservation conditions. Fourth, shallow wells which were drilled in the surface structures of the Guizhong Depression in the early stage present abundant gas shows, and most of them produce gas flow, which reveals a promising prospect of shale gas exploration in this area. In conclusion, the wide and gentle synclines with large burial depth in the Liucheng Slope and the Yishan Sag are the favorable areas for shale gas exploration in the Guizhong Depression. Keywords: Guizhong depression, Luzhai Fm, Lower Carboniferous, Shale, Thickness, Organic geochemical characteristic, Preservation conditions, Shale gas exploration potential, Favorable exploration area

The marine organic-rich shale of the Longmaxi Formation in the Dingshan area, southeast Sichuan, was studied using scanning electron microscopy (SEM), nitrogen adsorption/desorption and methane iso...

In addition to marine and marine-continental transitional strata, the continental ones are also widely distributed in various oil and gas-bearing basins in China. The continental shale generally provides favorable material bases for hydrocarbon generation, such as wide distribution, large thickness, multiple series of strata, high TOC content, nice organic matter type, and moderate thermal evolution. Part of such shale contains shale oil, but the pore space characteristics for the occurrence of this oil are not thoroughly studied. In order to accurately and quantitatively characterize the pore space where the continental shale oil in different types of lithofacies occurs, we sampled the rock cores from the Middle Jurassic Lianggaoshan Formation in the southeastern Sichuan Basin of the Upper Yangtze Area. The TOC content and mineral composition were analyzed, and we also carried out experiments on CO2 and N2 adsorptions, high-pressure mercury injection, and wash oil. Results show significant differences in pore space characteristics for the occurrence of shale oil in different types of lithofacies. In organic-rich mixed and clayey mudstones with the highest TOC content, the free shale oil, occupying the largest reservoir space, mainly occurs in macropores and mesopores, and the adsorbed shale oil, occupying the largest reservoir space, mainly occurs in mesopores. In the organic-bearing clayey mudstone, which has a higher TOC content, the free shale oil takes a larger reservoir space and mainly occurs in macropores, followed by mesopores, and the absorbed one, occupying a larger reservoir space, mostly occurs in micropores and then the mesopores. The organic-bearing mixed mudstone has a moderate TOC content, in which the free shale oil occupies a smaller reservoir space and primarily occurs in mesopores, followed by macropores, and the absorbed one, which takes a larger reservoir space, all occurs in mesopores. In the fine sandstone, the free shale oil occupies a smaller reservoir space and primarily occurs in mesopores, while the absorbed one occupies a smaller reservoir space and all occurs in mesopores.

Based on the understandings on enrichment rules of marine shale gas in southern China and data obtained from exploration and development in Fuling shale gas field, this article discusses the key controlling factors on shale gas enrichment and their relationships, it also discusses further the theory of Two-Factor Enrichment of marine shale gas in southern China. The bases for shale gas enrichment are shale gas generation and accumulation, the shale gas reservoirs of deep-water shelf are characterized by high TOC, high porosity, high gas contents and high siliceous contents, with high hydrocarbon-generation intensity. The organic pores are rich in shales in favorable for stimulation, so they are the bases for large scale hydrocarbon accumulation. Preservation conditions are vital to the formation and enrichment of shale gas reservoirs, good top and base layers can effectively prevent hydrocarbon from escaping vertically at the beginning of hydrocarbon generation. Shale gas preservation conditions depend on the intensity and duration of tectonic movements, good preservation conditions are key geological factors for shale gas accumulation, shale reservoirs have high gas contents, high porosity and high pressure and are likely to form high yield area of shale gas. Key words: Sichuan Basin, Fuling shale gas field, Lower Silurian, Longmaxi Formation, shale gas, enrichment and high production, controlling factors

Longmaxi Formation is the leading target play of shale gas development in Sichuan Basin (China) with a large amount of natural gas produced from this organic-rich overmature shale since 2014. In order to investigate the nano-scale pore system, small-angle neutron scattering (SANS) technique was applied on four Longmaxi shale samples (JY-01 to -04) with a range of different lithology. The scattering length density (SLD) for each sample is calculated as a volume average using the chemical composition of different mineral components, with values from 3.58 × 10−10 cm−2 in JY-02 to 3.81 × 10−10 cm−2 in JY-04. Scattering results and analyses reveal Longmaxi shales are mass fractal with an exponent D less than 3. Pore size distribution, specific surface area and porosity are calculated using Porod invariant method and Polydisperse spherical pore (PDSP) model, and are comparable to mercury intrusion porosimetry (MIP) results, suggesting SANS is valuable to study nano-scale pore space from microscopic to nearly micrometer scale. In addition, with the increasing depth and the scattering vector (Q) values, the scattering intensity decrease over five orders of magnitude, reflecting the differences in the certain scale structure of scatters at various burial depths of Longmaxi shale. For a certain Q value, with an increasing depth, the scattering intensity values initially increase slowly first, and then increase rapidly. The fractions of closed pore also increase with depth and reach the maximum in JY-04, which is related to the TOC content and mineral composition and is partially developed during the development of very complicated pore network.

Currently, the Upper Ordovician Wufeng (O3w) and Lower Silurian Longmaxi (S1l) Formations in southeast Sichuan Basin have been regarded as one of the most important target plays of shale gas in China. In this work, using a combination of low-pressure gas adsorption (N2 and CO2), mercury injection porosimetry (MIP) and high-pressure CH4 adsorption, we investigate the pore characteristics and methane sorption capacity of the over-mature shales, and discuss the main controlling factors for methane sorption capacity and distribution of methane gas in pore spaces. Low pressure CO2 gas adsorption shows that micropore volumes are characterized by three volumetric maxima (at about 0.35, 0.5 and 0.85 nm). The reversed S-shaped N2 adsorption isotherms are type Ⅱ with hysteresis being noticeable in all the samples. The shapes of hysteresis loop are similar to the H3 type, indicating the pores are slit- or plate-like. Mesopore size distributions are unimodal and pores with diameters of 2–16 nm account for the majority of mesopore volume, which is generally consistent with MIP results. The methane sorption capacities of O3w-S1l shales are in a range of 1.63–3.66 m3/t at 30 °C and 10 MPa. Methane sorption capacity increase with the TOC content, surface area and micropore volume, suggesting organic matter might provide abundant adsorption site and enhance the strong methane sorption capacity. Samples with higher quartz content and lower clay content have larger sorption capacity. Our data confirmed that the effects of temperature and pressure on methane sorption capacity of shale formation are opposite to some extent, suggesting that, during the burial or uplift stage, the gas sorption capacity of hydrocarbon reservoirs can be expressed as a function of burial depth. Based on the adsorption energy theory, when the pore diameter is larger than 2 nm, much methane molecular will be adsorbed in pores space with distance to pore wall less than 2 nm; while free gas is mainly stored in the pore space with distance to pore wall larger than 2 nm. Distributions of adsorption space decrease with the increasing pore size, while free gas volume increase gradually, assuming the pore are cylindrical or sphere. Particularly, when the pore size is larger than 30 nm, the content of adsorbed gas space volume is very low and its contribution to the all gas content is negligible.

The lithofacies types of Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation shale, the main producing layer in Fuling gas field, are classified in detail using the modified ternary diagram of siliceous minerals-carbonates minerals-clay minerals. There develop eight lithofacies in the Wufeng-Longmaxi shale: siliceous shale lithofacies (S), mixed siliceous shale lithofacies (S-2), clay-rich siliceous shale lithofacies (S-3), calcareous/siliceous mixed shale lithofacies (M-1), argillaceous/siliceous mixed shale lithofacies (M-2), mixed shale lithofacies (M), silica-rich argillaceous shale lithofacies (CM-1), and argillaceous/calcareous mixed shale lithofacies (M-3). The advantageous shale lithofacies is defined as lithofacies with gas content reaching a specific industrial standard. Based on the current development status of the study area, advantageous shale lithofacies is divided into two classes, namely, Class I with gas content of more than 4.0 m3/t (also known as extra superior), Class II with gas content of 2.0−4.0 m3/t (also known as superior). The correlation between the abundance of organic matter, the content of siliceous mineral, clay content and gas content has been analyzed to establish the classification criteria for advantageous shale lithofacies in the Wufeng-Longmaxi shale. The mixed siliceous shale lithofacies (S-2) and clay-rich siliceous shale lithofacies (S-3) have been identified as Class I advantageous shale lithofacies, and argillaceous/siliceous mixed shale lithofacies (M-2) as Class II. The classification criteria of advantageous shale lithofacies can provide reference for shale gas evaluation in other exploration areas Key words: shale, advantageous shale lithofacies, Ordovician Wufeng Formation, Silurian Longmaxi Formation, Fuling gas field, Sichuan Basin

A study of paleo-ocean redox environments is important for understanding the deposition of black shale and has practical implications for shale gas exploration. Here, we selected a total of 52 shale samples from JY1 Well, the first shale gas well of commercial exploitation in China, to analyze the redox conditions of Upper Ordovician Wufeng (O3w) and the first member in lower Silurian Longmaxi shale (S1l1) in the Jiaoshiba area. Abundant pyrite framboids are observed in these units, with average framboid diameters ranging from 3.1 to 4.7 μm and maximum diameter about 10 μm. Analyses of redox-sensitive trace elements suggest the redox environment has evolved from an anoxic condition in the bottom of Member A to a dysoxic condition in the top of Member A, and to a dysoxic–oxic environment in Members B and C. Graptolite and radiolarian are discovered in these shale samples, indicating an oxygenated seafloor, which seems to be contradictory to the results from pyrite framboids and redox-sensitive trace elements. This contradiction is explained as follows: when the shale was deposited, the sedimentation was dominated by euxinic conditions; however, some oxygen may also occasionally migrate to the bottom water due to oxygen-deficient environment, deglaciation, and (or) strong upwelling of nutrient-enriched seawater, while the limited oxygen and anoxic environment is favorable for the preservation and accumulation of organic matter. Combined with the geological characteristics and redox conditions, Member A, especially the shales at the bottom of Member A, is expected to be the most favorable shale gas reservoir in the Jiaoshiba area.

Marine shale gas in South China is widely distributed and demonstrates an enriched resource. Compared with the North American commercial shale gas field, the shale formation in South China is featured by old age, multiple stages tectonic movements, and high thermal evolution degree and complex reservoir forming conditions. As a result, the existing theories and technical methods of exploration in North America cannot be simply applied to South China. Since 2007, based on the in-depth study on a large quantity of analytical test data, we have conducted the theory and technology research for the southern marine shale gas, found Wufeng-Longmaxi Group deep water shelf high-quality shale gas critical parameters coupling laws, proposed the “Binary Enrichment” theory for highly evolved marine shale gas in southern complex tectonic zone, and established a zone selection and evaluation criteria, which are based on the quality of shale, for the key of preservation condition, and for the purpose of economy. Taking this as a guide, we selected Longmaxi formation Lower Silurian of Fuling area in southeastern Sichuan Province as the preferred breakthrough field for the shale gas exploration. The Jiaoye 1 well has the daily gas production of 20.3 × 104 m3, which embarked the found of China’s first large-scale business development gas field, the Fuling shale gas field. And we submit the first domestic shale gas proven reserves of 1067.5 × 108 m3. The Fuling shale gas reservoirs are deep water shelf marine high-quality shale, with favorable thickness and even distribution, without dissection in the middle. They are typical self-generation and self-storage shale gas reservoirs. The gas fields have high production gas well; high pressure gas reservoir, good gas components and good exploit results, and there for the Fuling shale gas field is characterized by mid-depth, high pressure, high reservoir pressure, and high quality gas accumulation. Up to March 24, 2014, all of the 101 fracture-gas testing wells have obtained middle and upper shale airflow, with the average single well test production of 32.6 × 104 m3/d. In the exploration and development process, technology series in geological evaluation, horizontal well, drilling, well completion, piecewise fracturing techniques have been gradually formed. It is of great importance to commercially develop the domestic shale gas and promote the restructuring of China’s energy structure.

Complex tectonic movements and high thermal maturity of marine shale dominate South China, where preservation conditions are critical for shale gas enrichment and productivity. Based on the exploration practices of the Silurian shale gas at the southeastern margin of the Sichuan Basin in recent years, conventional gas and shale gas were compared in terms of their preservation conditions. The results revealed that superior roof and floor conditions are indispensable to shale gas preservation. Moreover, the self-sealing ability and the huge gap of up to 2–8 times between vertical and lateral permeability of shale gas reservoirs determine the lateral diffusion as the basic pattern of shale gas migration. The unconformity at the bottom of Lower Cambrian leads to worse preservation conditions in the system, and cutting by faults may accelerate the diffusion of shale gas. Major controlling factors for shale gas preservation and their criteria of discrimination were also investigated. It is suggested that: (1) the strength of tectonic modification is the major factor controlling shale gas preservation. Broad and gentle structures with continuous seals and an anticlinal setting are more favorable for the enrichment of shale gas, and a closed evolutionary environment with late uplifting is more favorable for the preservation of shale gas; (2) shale gas can be preserved well in downdip areas without faults or effectively closed or shielded by faults and areas far away from outcrops or zones with stratigraphic hiatus; (3) pressure coefficient is a comprehensive indicator for discriminating preservation conditions. In the study area, the pressure coefficient is in positive correlation with shale gas production and the high or super-high pressure of reservoir is a signal of good preservation condition for shale gas; and (4) in the areas within the southeastern Sichuan Basin, other than those close to erosion zones or hiatus, the Wufeng Fm. of Upper Ordovician and the Longmaxi Fm. of Lower Silurian present high pressure coefficient (up to 2.25) generally, demonstrating good preservation conditions for shale gas, while the pressure coefficient reduces progressively toward or outside the margin of the basin, corresponding to downgrading preservation conditions.