Oil & Gas Geology ›› 2020, Vol. 41 ›› Issue (5): 1048-1059.doi: 10.11743/ogg20200515

• Petroleum Geology • Previous Articles     Next Articles

Identification and optimization of shale gas "sweet spots" in marine Niutitang Formation, South China

Shiqing Wu1,2(), Jianhua Guo1,2,*(), Zhiyu Li1,2, Mingyang Qin1,2, Yanran Huang1,2,3, Haonan He4   

  1. 1. School of Geosciences and Info-physics, Central South University, Changsha, Hunan 410083, China
    2. Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring(Central South University), Ministry of Education, Changsha, Hunan 410083, China
    3. Hunan Provincial Key Laboratory of Shale Gas Resource Exploitation, Hunan University of Science and Technology, Xiangtan, Hunan 411201, China
    4. China University of Petroleum(Beijing) at Karamay, Karamay, Xinjiang 834000, China
  • Received:2019-07-04 Online:2020-10-28 Published:2020-10-22
  • Contact: Jianhua Guo E-mail:sunshine0827@csu.edu.cn;gjh796@csu.edu.cn

Abstract:

Low-temperature adsorption/desorption of liquid nitrogen and isothermal adsorption of methane were used to characterize the shale reservoir capacity for adsorbed gas and free gas so as to describe the occurrence of shale gas in the original reservoirs of the Niutitang Formation in southern China.Combined with the gas-bearing properties of typical wells, the occurrence mechanism of the shale gas was studied and the "sweet spots" were identified and optimized in the study area.The results show that 1 to 2 layers of organic-rich shale dominated by black siliceous shale are developed in the Niutitang Formation (TOC>2.0%).Organic-matter pores (cylindrical pores with both ends open), interlayer pores of clay mineral (narrow parallel-plate pores with four sides open) and intergranular and intercrystalline pores (conical parallel-plate pores with four sides open) are developed with the organic-matter pores being the most important.The Niutitang Formation is generally poor in gas-bearing properties, except for one well (Well Ciye 1) measured with a residual gas volume fraction of as high as 69.7%.Both theoretical calculation and field experiments have shown that the occurrence of methane in Niutitang Formation shale reservoirs vary with TOC.Due to the relatively small organic-matter pore size (usually smaller than tens of nanometers), the original reservoirs in the formation store more adsorbed gas than free gas.With the increase of organic matter mass fraction, the elastic modulus of rocks decreases and the Poisson's ratio increases gradually.The "sweet spots" in the Niutitang Formation are characterized by "sweetness" and "brittleness", i.e.they have greater thickness (20-40 m) and high TOC values between 4% and 8%, high quartz mass fraction (>40%) and clay mineral mass fraction (30%-40%), low carbonate rock mass fraction (< 20%), a VBJH of (10-17)×10-3 cm3/g, a VL of 4-6 cm3/g, a free gas volume fraction of more than 40%, an elasticity modulus of 25~35 GPa and a Poisson's ratio of 0.20~0.25.In short, the "sweet spots" in the Niutitang Formation are mainly developed in deep shelf argillaceous facies with high free gas storage capacity.

Key words: shale gas, sweet spot, Niutitang Formation, marine formation, South China

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