Discovery of Fuling Shale Gas Field and its prospects

Abstract

A series of breakthroughs have been made in the understanding, evaluation, and exploration of shale gas from discovery, environmental protection to efficient exploration in the discovering of Fuling Gas Field. By revealing the positive correlation between organic carbon content and siliceous mineral content of shale deposited in deep shelf, dynamic preservation mechanism of “early retention and late deformation,” it is clarified that the shales deposited in deep shelf are the most favorable for shale gas generation, storage and fracturing. The preserving conditions determine the levels of shale gas accumulation, thus the evaluation concept of taking the quality of the shale as the base and the preserving conditions as key is proposed, the evaluation system for strategic selection of favorable zones is established for marine shale gas exploration in Southern China. Moreover, the “sweet point” seismic forecasting technologies for marine shale gas, the “six properties” logging technologies for evaluating shale gas layers, the technologies for quick and efficient drilling of horizontal well groups, and the fracturing technologies for composite fractures for horizontal wells are invented. The paper discussed the exploration prospect of shale gas in the shales of Wufeng-Longmaxi Formation in great depth in Sichuan Basin, shale gas exploration in the outer region of the south, and continental shale gas exploration in China.

Keywords

shale gas / accumulation laws / exploration technologies / Longmaxi Formation / Fuling Shale Gas Field / Sichuan Basin

Introduction

The Assessment Office of Oil and Gas Reserves of the Ministry of Land and Resources examined and proved the first proven shale gas reserves in China in Chengdu in July 2014, officially announcing the birth of the Fuling Shale Gas Field. A series of breakthroughs have been made in the understanding and assessment of gas exploration theories and exploration technologies in the discovering of Fuling Shale Gas Field. The gas field has a productivity of 90 × 10⁸ m³ and an accumulation production of more than 140 × 10⁸ m³. The discovery and development of the gas field promotes domestic shale gas exploration, and the usage of shale gas resources has entered a new phase.

Discovery of Fuling Shale Gas Field

Well JY1, the discovery well of Fuling Shale Gas Field, was deployed in September 2011, started drilling on February 14, 2012 and completed on May 18, at a depth of 2450 m, at the Shizipu Formation of Middle Ordovician. The well drilled a 89 m shale gas formation of the Wufeng-Longmaxi Formation, of which, 38 m are of the high-quality shale gas formations with a total organic carbon (TOC) of ≥2.0%.

After the drilling of Well JY1 was completed, the vertical well fracturing and gas testing were not conducted, the horizontal well was drilled and completed on September 16, 2012. The drilling depth was 3653.99 m and the horizontal length was 1007.90 m. In November 2012, the horizontal well of Well JY1HF (2646.09‒3653.99 m) was divided into 15 sections and hydraulic fracturing was performed. On November 28, an industrial gas flow of 20.3 × 10⁴ m³/d was tested and the discovery of Fuling Shale Gas Field was announced.

The success of Well JY1 confirmed the understandings of the evaluations of previous shale gas constituencies. To further implement the resources, clarify the characteristics of shale gas reservoirs, develop and improve the understanding of marine shale gas enrichment laws, and evaluate exploration technologies, in November 2012, a comprehensive evaluation of exploration and production capacity test was made. Three evaluation wells of Well JY2, Well JY3, and Well JY4 were planned in Jiaoshiba Formation, and at the same time, a 594.50 km² of 3D seismic exploration was planned. An area of 28 km² around Well JY1 was chosen to perform productivity experiments. The experiments proved that all of the above wells reached the expected targets. In 2014, Well JY5, Well JY6, Well JY7, and Well JY8 were planned for shale gas exploration in various formation patterns and greater depths, which enlarged exploration and development areas.

Since the first parameter Well JY1 was deployed, technologies for environment assessments and protection have been studied, the concepts of environmental protection and technical researches have always been emphasized from the discovery of Fuling Shale Gas Field to the controlling of whole shale gas field, and technologies for water resources protection and waste disposal have been developed. In three years, the whole project was planned as two stages and carried out step by step. By September, 2015, the total proven shale gas reserves in place is 3805.98 × 10⁸ m³ and the exploration goals of environment protection and efficient exploration are realized.

Theoretical progresses

The positive correlation between TOC and siliceous minerals of shales deposited on deep water shelf

In Southern China, shale gas resources have the highest potentiality, and eight major shale formations are developed, of which, 7 shale strata have been developed in the upper Yangtze and Hunan-Guangxi Area from Cambrian to Jurassic. An evaluation of the basic geological conditions of the shale gas for these layers indicate that they all have the potential for shale gas exploration from the composition and origin of shale, because they have a great thickness, a high TOC, and silica contents [¹].

Further studies find that the deep water shelf shales in the Wufeng-Longmaxi Formation not only have an TOC but also have a high content of endogenous siliceous minerals. Moreover, the organic pores will be developed when the equivalent Ro is 2%‒3%, and there is a good positive correlation between TOC, silica contents, and pores in organic matters.

Usually, the TOC of shales deposited on the deep water shelf from typical outcrops such as Qilong and Qiliao profile in South-eastern Sichuan province is 4%, sometimes can even be as high as 6%, and the silica content is 30%‒70%. The shales deposited on the deep water shelf in Fuling Area mainly develop in the 1st interval of the 1st member of Wufeng-Longmaxi Formation, in which organic matters are rich and the average TOC is up to 3.6%, which is much higher than the average TOC of 1.69% [²] of shales deposited on the shallow water shelf in the 1st member of Longmaxi Formation. In addition, the silica contents of the shales deposited on the deep water shelf is 22.9%‒80.5% with an average content of 49.0%, while the average silica content of shales deposited on the shallow water shelf is only 35.8%. The relationship between TOC and silica contents of shales from the same profile shows that there is a positive correlation between TOC and siliceous contents in deep water shelf shales (Fig. 1(a)), but there is no positive correlation between TOC and siliceous contents in shallow water (Fig. 1(b)), nor there is a positive correlation between TOC and siliceous content in Terrestrial shales [³].

Depositional environments are the most important controlling factor for the accumulation of organic matters in sediments. The high paleo-productivity of organic matters, the destructive power of inorganic oxides on organic matters, and the dilution rate of organic matters by clasts are the three most important direct control variables. Only in sedimentary environments with a rich source of organic matter, with the smallest oxidation, and with a moderate deposition of clasts can higher abundance of organic deposits be formed [¹]. The enrichments of Ni, Cu, and Zn imply that a large number of elements were brought in sediments by organic matters with higher contents, the contents can be the index of paleo-productivity, but Ba and Mo elements from biogenetic derivation can directly reflect the paleo-productivity level. The Qiliao profile of Wufeng-Longmaxi Formation reveals that the paleo-productivity of shales deposited on deep water shelf is higher than that of shales deposited on shallow water shelf. The features of rock composition show that terrigenous debris are the main components of silica in the upper siliceous content, while there are few land-based silica in the lower deep water shelf. The reason for endogenous silica is that there are prosperous living creatures, which are mainly form radiolarian. This is proved by the great differences of radiolarian in the upper part and lower part (Fig. 2).

The exploration achievements of Fuling Shale Gas Field and peripherals reveal that there exists a positive correlation between TOC and porosity of shales deposited on the deep water shelf in Wufeng-Longmaxi Formation [²,³]. The organic matter pores are the main storage space for shale gas [³,⁴]. There are more organic matter pores in the shales deposited on deep shelf than those in the shales deposited on shallow water shelf and lacustrine facies. Therefore, the gas contents also increase with increasing TOC.

So shales deposited on deep water shelf are the most favorable shale type for gas generation, accumulation and fracturing stimulation. They not only have a high TOC, possessing basic conditions to provide high porosity and gas contents, but also have endogenous brittle silica. There exists a positive correlation between silica and TOC, providing material conditions for “artificial gas reservoirs,” which is the base of “hydrocarbon generation and reserves controlling” [⁵]. Compared with several major shale formations in North America, the deep water shale facies shale in Wufeng-Longmaxi Formation is comparable to or better than that of the North America [²].

The “early retention and late deformation” dynamic preservation mechanism of shale gas

Fuling Shale Gas Field experienced two major stages, namely the stage of continuous deepening before the Early Cretaceous and the stage of uplift and erosion after the Late Cretaceous. During the stage of deep-buried period, the shales in Wufeng-Longmaxi Formation had completed hydrocarbon generation. At the end of the Middle Triassic, the Ro value increased to 0.7%‒1.3% and shales entered the peak of oil generation. After the Middle Jurassic, the Ro value was at 1.3%‒2.0%, organic matter evolution reached high maturity, and a great amount of wet gas and oil cracked gas were generated. After the Early Cretaceous, the gas generated was dominated by dry gas. At the end of Cretaceous, the depth reached the maximum, and the Ro value increased to 2.65%.

Characteristics of carbon isotope and gas components of other strata in Fuling Area and Sichuan Basin indicate that the shale gas of Fuling Shale Gas comes from the source rocks themselves. The reason for inversion of carbon isotope is most likely to be a mixture of kerogen pyrolysis gas and cracked gas of crude oils [⁶,⁷]. The organic matter pores, being the main storage space, consist of kerogen pores and bitumen pores, which implies that there exist two causes for the gas. In particular, the bitumen is one of the major organic matters of shales in Wufeng-Longmaxi Formation, which is mainly filled in minerals or in the pores of minerals, and the bitumen reflectivity is in agreement with the conversion maturity history, indicating that the gas is thermogenic gas (Fig. 3). In other words, there existed a large scale paleao-reservoir.

During the process of deep gas cracking and gas formation, overpressure develops in the reservoir, and the pressure coefficient at the main part of the gas reservoir still remains at about 1.55. Barker argued that in an ideal closed system, 1% of the volume of crude oil cracked into gas which might cause the reservoir pressure to reach the rock pressure. Then gas would be expelled and injected into the strata nearby [⁸]. Due to the tight lithology of gas caprock overlying limestone and underlying limestone in the Wufeng-Longmaxi Formation, the breakthrough pressure reached about 70 MPa, nearly twice as high as that of the gas layer, forming a good sealing condition. From the beginning of generating of shale gas by shales, they could effectively prevent hydrocarbons from diffusion and then make the hydrocarbons stay in the shales. They are pre-conditions for gas accumulation and the overpressure of the gas pool can also be kept.

Gas reservoir overpressure is a good manifestation of shale gas preservation conditions, and is also the implication of gas enrichment and high production. Late tectonic uplift and fault cause damage to shale roof and floor, self-sealing and regional cap rock, resulting in the loss and release of shale gas, the intensity and time of shale gas determine the remaining abundant.

Uplifting and erosion can make gas layers and their caprocks crack brittlely or make the faults open to reduce the sealing capacity of shales and caprocks. The triaxial physical simulation of shale sample from Longmaxi Formation in Well JY1 displays that when the shales are elevated to 1000‒1500 m, the confining pressure will reduce from 50 MPa to about 16.2 MPa, and as a result, shear fractures occur in rocks and produce micro fractures (Fig. 4). When the confining pressure of samples from Longmaxi Formation in Well JY2 reduces to about 15 MPa, micro fractures begin to open on a large scale, porosity and permeability rise substantially, and the sealing capacity of shales becomes poor as the burial depth become shallow rapidly (Fig. 5).

The enrichment of shale gas mainly depends on maintenance of pore pressure. As the pressure coefficient increases, the gas content increases, there existing a good positive correlation (Fig. 6). When the pressure coefficient exceeds 1.2, generally, high production of shale gas can be obtained [⁹]. Once the pressure leaks, porosity decreases as well as gas contents, the gas reservoir becomes bad and the production decline, which has been proven by the wells near the faults in Fuling Shale Gas Field. Thus, the preservation conditions determine the degree of enrichment of shale gas, which is the key to “hydrocarbon accumulation and production controlling” [⁵].

Evaluation system form arine shale gas strategic area in Southern China

The evaluation of shale gas concentrating areas by foreign major oil companies such as BP, Nitta Company, Exxon Mobil, Chevron Corporation and HESS, etc.) is mainly focused on a single geological unit with a high degree of exploration, and the evaluation parameters and standards adopted takes into consideration all kinds of geological factors. For example, the method of comprehensive analysis on risks for shale gas exploration of BP primarily takes into consideration 7 parameters, and the threshold values for evaluations are Ro>1.2%, the TOC of target intervals is more than 4% with a thickness of 75‒150 m and a large distribution areas, the matrix porosity is 4%‒6%, and the strata are overpressured with silicate rocks which are favorable for fracturing. Harding-Shelton Energy Company considers not only geological factors, but also environments and drilling, etc.

At the beginning of this century, China started to pay attention to and discuss the issue of shale gas resources and prospect area evaluation. Initially, the methods for shale gas resources assessments and evaluations on prospect areas were proposed by referring to the evaluation parameters used in North America, and the geological conditions were also emphasized [¹⁰‒¹³]. With the gradual development of shale gas exploration, the features of superimposed basin were clarified step by step in China, and the impacts of multi-cycle tectonic movements in Southern China on shale gas reservoirs were especially clarified. Therefore, researches on preservation conditions should be emphasized during the course of shale gas resources assessment and evaluating on prospect areas [¹⁴‒¹⁷].

The method for evaluations of strategic prospect areas for shale gas exploration in marine shales in Southern China is based on comparisons of typical wells at home and abroad and understandings on enrichment rules of shale gas. The concept of evaluation of prospect areas is proposed based on the idea that quality shale is the base and preservation condition is the key. Besides, economy is also a factor to be considered because the target to evaluate prospect areas is to choose the areas that can have economic values. The standards for evaluation of prospect areas include 18 parameters, which can be divided into three types (Table 1). The establishment of standards not only emphasizes preservation conditions, but also decides the standards of quality areas of shale gas resources. That is, when the strata or areas reach the standards of 18 parameters, they are the targets to be chosen, the scope of prospect areas can also be determined according to these parameters.

Evaluation and exploration technologies

The seismic technologies for forecasting the “sweet point” of marine shale gas

The commercial development of shale gas needs to identify the “sweet point” of the reservoir, but the marine shales in southern China are featured by old times, strong transforms and great differences in gas contents, which poses great challenges to seismic evaluations and predictions. Based on enrichment rules of shale gas in marine areas in southern China with complex structural deformations and high thermal evolution, key seismic technologies for seismic prediction of TOC, the brittleness index, and the gas contents with high precise are developed, which are all parameters for evaluating shale the “sweet point” of gas, providing strong technical supports to shale gas exploration in marine areas.

TOC is an important index for evaluating hydrocarbon-generating capacity and shale gas reservoirs. The wave impedance inversion technique widely used in the early prediction has a low accuracy [¹⁸]. Lots of lithological analyses and digital studies on cores show that there exist a good negative correlations between the density and the TOC of shales, thus a model to show the relations between the TOC and the density of shales is established, and a seismic quantitative forecasting technology with prestack density inversion for forecasting TOC is developed, whose relative error is less than 2%, which greatly increases the accuracy. Brittleness index is an important parameter to show the fracturing efficiency of shale reservoirs. The shales in Longmaxi Formation in Sichuan Basin have experienced intensive tectonic transforms with multi-phase and multi-direction, which are dominated by over-thrusting and extrusion. Based on the Rickman formula, Lame coefficient, which describes the shrinkage deformation of shales by extrusion, and shear modulus, which describes deformation by sliding shearing, are used in the model for predicting the brittleness index [¹⁹], thus new correlations among Young modulus, Poisson’s ratio, shear modulus × density, Lame coefficient × density, and shale brittleness index are established, and the technology for predicting the brittle index of shales with multi-parameter is developed, which is adaptable for complex structural environments. The error of prediction is reduced from 13% to 3% (Fig. 7).

The pressure coefficient is a comprehensive indictor of shale gas preservation conditions. A comparative analysis of the well with a lower pressure coefficient in the complex structural area of Jiaoshiba Area indicates that faults and fractures are relatively developed. Further statistics on pressure ratio predicted by using the classic Fillippone model show that there exist a positive correlation between the predicted deviation and the density of fractures with a high angle in the top and floor layers of shale reservoirs, thus a new model for predicting the pressure ratio is established and relatively accurate pressure ratios can be obtained. Therefore a new relationship is established among gas contents, TOC, and pressure ratio, based on which, the success rate of exploration wells is 100%. Of the 216 wells which have been developed, 94.4% can have an output of over 10 × 10⁴ m³/d.

The “six properties” logging technology for evaluating shale gas layers

Passey et al. pointed out that the DlgR method is limited to LOM= 6.0 to 10.5 (Ro is 0.5%‒0.9%) and should be exercised beyond the limits. When the LOM is beyond the limit, the method should be used cautiously [²⁰]. Non-Passey rocks, which is beyond the LOM limit, will lead to wrong results when the DlgR method is used [²¹]. The shale reservoirs in Fuling Gas Field have a Ro of 2.42%‒3.13%, with an average value 2.65%, which are highly evolved. Therefore, when the DlgR method is used to decide shale reservoirs and to calculate TOC, certain defects will occur.

The superposition method, in combination with logging curves can better recognize shale reservoirs. Logging curves of TH-K can recognize clay mineral qualitatively. When the space between the two curves of TH and K is wide, it is suggested that the contents of clay minerals in strata are higher. The ratio of TH and U is less than 2, which suggests that the sedimentary environment is strongly deoxidized. When the ratio is 2‒4, it is suggested that the sedimentary environments are between strong deoxidized and semi-deoxidized; when the values of U in shale intervals is more than 6 ppm, it is suggested that the intervals are rich in organic matters (Fig. 8).

Based on the identification of shale reservoirs, a method for calculating the TOC of shale reservoirs in the area studied is formed using the lithology density logging information.

\mathrm{T}\mathrm{O}\mathrm{C}=A\cdot \mathrm{D}\mathrm{E}\mathrm{N}+B,

where DEN is the density of rocks, g/cm³, and A and B are the regional empirical coefficients, which are decided by the TOC in the area studied.

The minerals in shale reservoirs are featured by complexity, diversity, and multiplicity. The density of framework rocks varies longitudinally. Therefore, it is difficult to get the porosity. The model to calculate the density of framework rocks by using ECS is set up.

{{\displaystyle \rho }}_{\mathrm{m}\mathrm{a}}={{\displaystyle A}}_{\mathrm{1}}(\mathrm{D}\mathrm{W}\mathrm{A}\mathrm{L})+{{\displaystyle B}}_{\mathrm{1}}*(\mathrm{D}\mathrm{W}\mathrm{C}\mathrm{A})+{{\displaystyle C}}_{\mathrm{1}}*(\mathrm{D}\mathrm{W}\mathrm{F}\mathrm{E})+{{\displaystyle D}}_{\mathrm{1}}*(\mathrm{D}\mathrm{W}\mathrm{S}\mathrm{I})+{{\displaystyle E}}_{\mathrm{1}}*(\mathrm{D}\mathrm{W}\mathrm{S}\mathrm{U})+{{\displaystyle P}}_{\mathrm{1}},

where r_ma is the density of framework rocks; DWAL, DWSI, DWCA, DWFE, and DWSU are the contents of dry weight of A1, Si, Ca, Fe, and S respectively, which can be obtained by ECS; and A₁, B₁, C₁, D₁, E₁, and P₁ are empirical parameters. Due to the differences of sedimentary environments in different areas, the parameter are different.

The Archie formula and its derivative formulas used to calculate the water saturation of reservoirs are based on the rock conductivity mechanism and are suitable for pure sandstone formations with medium-high porosity and high permeability [²²‒²⁴]. The a, b, m, and n parameters in the Archie formula are generally determined by rock conductive experiments, which will directly influence the accuracy of saturation calculation. At present, it is difficult to make shale samples with low porosity and permeability saturated by water completely in rock conductive experiments. Therefore, it is difficult to decide the values of m and n accurately. Because there is no water during the course of shale gas development, the RW of underground water cannot be obtained. At the same time, the presence of pyrites in shales leads to an abnormally low RW in the resistance curve, taking the shape of spine. These factors influence the accuracy of water saturation calculated by using the Archie’s formula and its derivative formulas.

To avoid the shortage of Arichie formula in calculating the water saturation of shale reservoirs, the idea of using non-electric logging data to calculate the water saturation of shale reservoirs is proposed, and the method for density quadratic polynomial fitting is developed to calculate the water saturation of shale reservoirs. Take Well JY5 as an example, there exists a good agreement between the water saturation calculated by using logging data and that obtained by measuring cores. The absolute error is 0.79% and relative error is 2.55% (Fig. 9).

Technologies for quick-and-efficient drilling horizontal well groups

To meet the needs of commercial development, technologies for well drilling and completion with low costs have been developed in North America such as the technologies for perfect quick-and-efficient horizontal well drilling, “well factories” and so on.

At present, technologies for quick-and-efficient drilling horizontal well have been developed in Fuling Shale Gas Field. Aimed at the complex characteristics of the ground surface and underground conditions in Fuling Shale Gas Field, a completion model of well structure with three levels has been formed (Fig. 10) [²⁵]. The upper strata are drilled by air drilling or water drilling with quick velocity. Tools for oil resistance drilling stem with large torque and equal wall thickness specially used in drilling shales, super-short integrated PDC bit are developed, thus inclined and horizontal drilling can be finished efficiently one-time. Based on the researches on instability mechanism of bole wall, an oil-based drilling fluid system is developed, having the features of “four lows and three highs,” in which the amount of main treating agents are reduced by 28%. Besides, an elastic cement mud and an efficient clearing fluid system have been developed. Therefore, the ratio of quality cementing is 89%.

To meet the special requirements of the development plan, the mode of “well factories” in mountainous regions is innovated (Fig. 11) [²⁶], the techniques to design incremental five sections and 3D hook-shape tracks of parallel cluster horizontal wells, and the techniques to control well tracks based on efficient curved directional stems are developed. Therefore, the drilling efficiency increases by 35.71% and the ratio to drill quality reservoirs reaches 97%.

Fracturing technologies for composite fractures for horizontal wells

Studies suggest that the high-quality shale in Fuling Shale Gas Field possesses high brittle mineral contents, high Young’s modulus, low Poisson’s ratio, little difference between horizontal stresses, developed beddings and natural fractures, and a good fracability. Through large-scale hydraulic fracturing physical model experiments (Fig. 12) [²⁷] and numerical simulations, the cracking initiation and extension mechanism of crack network and complex cracks in shales in various intervals (Longmaxi Formation) are revealed. Based on practices, the new idea of “controlling the nearby and extending to the far, mixed fracturing and supporting by levels” for fracture network stimulation is proposed. A friction-reducing water system with a reduction rate of 78% is developed [²⁸]. The technology for paving proppant with a low density is developed by integrating three kinds of specifications and three kinds of grains. The mixed mode of fracturing by slick water and glue fluids comes into being. The techniques for fracturing with large internal diameter are developed by integrating pumping bridge plug, multistage perforating and testing, well head combinations. Thus the fracturing technology for complex fracture network adaptable for shales in Fuling Area is invented. The success rate is 98%, and the average open flows per well is 39.3 × 10⁴ m³/d.

Prospects

The construction of the first project with a productivity of 50 × 10⁸ m³ in Fuling Shale Gas Field was completed, and the construction of the second project with a productivity of 50 × 10⁸ m³ was also completed in 2017. Fuling Shale Gas Field is the first large-scale, high-quality commercial shale gas field developed in China. Its discovery, exploration and development have achieved significant innovation in geological theory and technology. According to the “Evaluation of the Construction Fuling National Shale Gas Demonstration Area” by Chongqing Municipal Government, Fuling Shale Gas Field is a model of theoretical innovation, technological innovation and management innovation in shale gas exploration and development in China. It plays an important role in guiding shale gas exploration and development in China and greatly promotes the confidence of shale gas industrial development and demonstrates good prospects of shale gas exploration and development.

Just as the course of development in shale gas in North America, the key to the successful and rapid exploration and development in shale gas is the progress of engineering technology [²⁹‒³⁵]. The development of shale gas and its prospects will become better with the progresses made in independent engineering technology in China.

Sichuan Basin generally has similar depositional conditions to the shale gas formation in Fuling Gas Field. The favorable facies of the Wufeng-Longmaxi Formation deep water shale covers most of the Sichuan Basin. The shale has a large thickness with a stable distribution and high-quality shale thickness. The overall pressure coefficient is higher, and the preservation conditions are better. In 2016, the annual production of shale gas in Sichuan Basin reached 7.88 billion cubic meters, showing a good prospect for shale gas exploration.

Most of the shales of the deep depth of Wufeng-Longmaxi Formation in the Sichuan Basin are deposited in the deep shelf. The organic-rich shales have a great thickness, a wide distribution range, a high TOC content, and favorable preservation conditions, thus having a good prospect for shale gas exploration. However, with the increase of burial depth, the stress of shale increases, so does the fracture pressure. As a result, fracturing is difficult. At present, Sinopec has achieved initial success in its engineering technology research and development in shale gas exploration at a depth of nearly 4500 m in Dingshan and Yongchuan Areas. Once the technologies and equipment for shale gas exploration at great depth are ready, the costs on shale gas exploration and development will be reduced, and the domestic shale gas exploration areas and resources will be greatly increased in China.

The continental basins in China mainly developed after the late Paleozoic. The continental organic-rich shales have multiple layers, with a wide distribution range and a great potential for shale gas resources. The continental shale gas resources in China amounts to 35.26 × 10¹² m³, and the recoverable resources is 7.92 × 10¹² m³, accounting for 31.6% of the total. However, little work for exploring continental shale gas are performed, few specialized exploratory well are drilled, the research and understandings of continental shale gas exploration is insufficient, and less studies have been performed on conditions for shale gas formation, storage mechanism, enrichment rules and evaluation on choosing prospecting area. Sinopec has obtained shale gas flows in many wells drilled in 2 strata in 3 areas in Sichuan Basin, Shaanxi Yanchang Oil (Group) Co., Ltd. has obtained commercial shale gas flows in many wells in Yanchang Formation in Yanan, Shaanxi province. All those show that continental shales also have values for shale gas exploration and development. Usually, limestone or sandstone interlayers are developed in continental shale layers, if special evaluations and technological studies are performed according to the idea of “shale gas layers” based on the features of continental shales [³], there will be good prospects for continental shale gas exploration.

Shale gas exploration in peripheral areas of Sichuan Basin also has some potential, with a recoverable resource of 6.18 × 10¹² m³, accounting for 24% of the total. However, due to the strong tectonic transformation in the periphery, the formation and enrichment of shale gas is complicated. At present, based on the study of shale gas enrichment and the evaluation of the area, the emphasis should be laid on Pengshui and Wulong synclines on the relatively stable basin outer structural transformation. The drilling technology should be strengthened to improve single well productivity and reduce development cost and strive for the early realization of shale gas commercial development in the complex areas in the south.

References

[1] Guo X S. Enrichment Mechanism and Exploration Technologies in Jiaoshiba Area in Fuling Shale Gas Field. Beijing: Science Press, 2014 (in Chinese)

[2] Guo X S, Hu D F, Wen Z D, Major factors controlling the shale gas accumulation and high productivity in marine shales in the Lower Paleozoic of Sichuan Basin and its periphery: taking Wufeng-Longmaxi Formation as an example. Geology in China, 2014, 41(3): 893–901 (in Chinese)

[3] Guo X S, Hu D F, Li Y P, Analyses and thoughts on accumulation mechanism of marine and lacustrine shale gas: taking Longmaxi Formation and Da’anzhai Member of Ziliujing Formation in Sichuan Basin as examples. Earth Science Frontiers, 2016, 23(2): 18–28 (in Chinese)

[4] Nie H K, Bian R K, Zhang P X, Micro-types and characteristics of shale reservoirs of the Lower Paleozoic in southeastern part of Sichuan Basin and their effects on the gas contents. Earth Science Frontiers, 2014, 21(4): 331–343 (in Chinese)

[5] Guo X S. Rules of two-factor enrichment for marine shale gas in Southern China: understandings from explorations and practices for shale gas in Longmaxi Formation in Sichuan Basin and its surrounding area. Acta Geologica Sinice, 2014, 8(7): 1209–1218 (in Chinese)

[6] Liu R B. Typical features of the first giant shale gas field in China. Natural Gas Geoscience, 2015, 26(8): 1488–1498 (in Chinese)

[7] Feng Z Q, Liu D, Huang S P, Carbon isotopic compositions of shale gas in Longmaxi Formation of Silurian in the Changning Area, Sichuan Basin. Petroleum Exploration and Development, 2016, 43(5): 705–713in Chinese)

[8] Barker C. Calculated volume and pressure changes during the thermal cracking of oil to gas in reservoirs. AAPG Bulletin, 1990, 74(8): 1254–1261

[9] Hu D F, Zhang H R, Ni K, Preservation conditions and main controlling factors on shale gas in marine south-eastern part of Sichuan Basin. Natural Gas Industry, 2014, 34(6): 17–23 (in Chinese)

[10] Li T J, Liu H, Liu J X, Geological selection for prospect areas and methods for evaluations on resource Potentials. Journal of Southwest Petroleum University (Science& Technology Edition), 2011, 33(2): 28–34 (in Chinese)

[11] Dong D Z, Cheng K M, Wang S Q, Methods for evaluations on shale gas resources and application in Sichuan Basin. Natural Gas Industry, 2009, 29(5): 33–39 (in Chinese)

[12] Li Y X, Nie H K, You P Y. Features of organic-rich shales and strategic selection of prospect areas in China. Natural Gas Industry, 2009, 29(12): 115–118 (in Chinese)

[13] Fan B J, Shi L, Pang X Q. Characteristics of shale gas reservoirs and conditions for prospecting areas. Petroleum Geology and Recovery Efficiency, 2011, 18(6): 9–13 (in Chinese)

[14] Guo X S, Guo T L, Wei Z H, Thoughts on shale gas exploration and evaluations in southern China. Engineering and Science, 2012, 14(6): 101–105 (in Chinese)

[15] Zhao J Z, Fang C Q, Zhang J, Selection and evaluations on prospecting areas for shale gas exploration in China on the views of shale gas exploration and development in North America. Journal of Xi’an Shiyou University (Natural Science Edition), 2011, 26(2): 1–7 (in Chinese)

[16] Wang S Q, Wang S Y, Man L, Method for selection and evaluations on prospect areas and key parameters. Journal of Chengdu University of Technology (Science & Technology Edition), 2013, 40(6): 609–620 (in Chinese)

[17] Li J, Gao Y, Hua C, Enlightenments for building evaluation methods for selecting prospects areas for marine shale gas in South China by experiences of shale gas exploration in North America. Petroleum Geology and Recovery Efficiency, 2014, 21(4): 23–27 (in Chinese)

[18] Løseth H L, Wensaas M, Gading K, Duffaut K, Springer M. Can hydrocarbon source rocks be identified on seismic data? Geology, 2011, 39(12): 1167–1170

[19] Rickman R, Mullen M J, Petre J E, A practical use of shale petrophysics for stimulation design optimization: all shale plays are not clones of the Barnett Shale. Society of Petroleum Engineers, 2008

[20] Passey Q R, Creaney S, Kulla J B. A practical model for organic richness from porosity and resistivity logs. AAPG Bulletin, 1990, 74(5): 1777–1794

[21] Mallick R K, Raju S V. Thermal maturity evaluation by parts of Upper Assam Basin, India. Organic Geochemistry, 1995, 23(10): 871–879

[22] Sun J G. Archie’s formula: historical background and earlier debates. Progress in Geophysics, 2007, 22(2): 472–486 (in Chinese)

[23] Li X Y, Qin R B, Liu C C. Analyses on effects on calculating reservoir saturation by rock electrical parameters. Journal of Southwest Petroleum University (Science & Technology Edition), 2014, 36(3): 68‒195 (in Chinese)

[24] Li Y G, Lu Z Q, Shi L H, Studies saturation interpretation model based on variable rock-electro parameters. Well Logging Technology, 2015, 39(2): 181–195 (in Chinese)

[25] Zhou X H. Drilling &completion techniques used in shale gas horizontal wells in Jiaoshiba block of Fuling Area. Petroleum Drilling Techniques, 2013, 41(5): 26–30 (in Chinese)

[26] Zhou X H, Zang Y B. Application of “well factory” drilling technology in Fuling shale gas field. Petroleum Drilling Techniques, 2015, 43(3): 46–49 (in Chinese)

[27] Zhang X, Jiang T X, Jia C G, Studies on physical simulation of hydraulic fracturing of shale gas reservoirs. Petroleum Drilling Techniques, 2013, 41(2): 70–74 (in Chinese)

[28] Zhou D H, Jiao F Z, Jia C G, Large-scale multi-stage hydraulic fracturing technology for shale gas horizontal in Well JY1HF. Petroleum Drilling Techniques, 2014, 42(1): 75–80 (in Chinese)

[29] Schettler Jr P D, Parmely C R, Juniata C. Contributions to total storage capacity in Devonian shales. In: SPE Eastern Regional Meeting. Lexington, Kentucky, 1991

[30] Bowker K A. Recent development of the Barnett Shale play, Fort Worth Basin. West Texas Geological Society Bulletin, 2003, 42(6): 1–11

[31] Montgomery S L, Jarvie D M, Bowker K A, Missiissippian Barnett Shale, Fort Worth Basin, north-central Texas: gas shale play with Multitrillion Cubic Foot Potential. AAPG Bulletin, 2005, 89(2): 155–175

[32] Curtis J B. Fractured shale-gas systems. AAPG Bulletin, 2002, 86(11): 1921–1938

[33] Huang Y Z, Huang J L, Ge C M, Technical progresses are key factor promoting rapid development of shale gas exploration in United States. Natural Gas Industry, 2009, 29(5): 7–10 (in Chinese)

[34] Dong D Z, Zou C N, Li J Z, Potentials of shale gas resources and its exploration and development prospects. Geological Bulletin of China, 2011, 30(2‒3): 324–336 (in Chinese)

[35] Zhao W Z, Dong D Z, Li J Z, Potentials of shale gas resources and its future place in natural gas development in China. Engineering and Science, 2012, 14(7): 46–52 (in Chinese)

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