Insights into the Formation Mechanism of Low Water Saturation in Longmaxi Shale in the Jiaoshiba Area, Eastern Sichuan Basin

Abstract

Investigating the variation of water content in shale reservoir is important to understand shale gas enrichment and evaluate shale gas resource potential. Low water saturation is widely spread in Longmaxi marine organic-rich shale. To illustrate the formation mechanism of low water saturation, this paper analyzed water saturation of Longmaxi shale reservoir, restored the history of natural gas carrying water capacity combining homogenization temperature and trapping pressure of fluid inclusion with simulated thermal history, and established a model to explain pore water displaced by natural gas during the thermal evolution. Results show that the gas-rich Longmaxi shale reservoir is characterized by low water saturation with measured values ranging from 9.81% to 48.21% and an average value of 28.22%. TOC in high-mature to over-high-mature Longmaxi organic-rich shale is negatively correlated with water saturation, indicating that well-connected organic pores are not available for water. However, quartz and clay mineral content are positively correlated with water saturation, which suggests that inorganic-matter-hosted pores are the main storage space for water formation. The water carrying capacity of natural gas varies as a function of gas generation and expulsion history, which displaces bound and movable water in organic pores that are part of bound and movable water from inorganic pores. The process can be divided into two phases. The first phase occurred due to the kerogen degradation into gas at Ro of 1.2%–1.6% with a water carrying capacity of natural gas ranging from 5 632.57–7 838.73 g/km³. The second phase occurred during the crude oil cracking into gas at Ro>1.6% with a water carrying capacity of natural gas ranging from 10 620.04 and 19 480.18 g/km³. The water displacement associated with natural gas generation and migration resulted in gas filling organic pores and gas-water coexisting in the brittle-mineral-hosted pores and clay-mineral-hosted pores.

Keywords

water occurrence / shale enrichment / shale reservoir / Longmaxi shale

References

[1] Bennion D W, Shaw D R, Thomas F B, . Compositional Numerical Modelling in Naturally Fractured Reservoirs. Journal of Canadian Petroleum Technology, 1999, 38(7): 235-248.

[2] Bryndzia, L. T., 2012. Origin of High Salinities in Hydraulic Fracture Flow Back Fluids—An Example from the Haynesville Shale Gas Play, USA. 3rd EAGE Shale Workshop—Shale Physics and Shale Chemistry, January 23–25, 2012, Barcelona. https://doi.org/10.3997/2214-4609.20143943

[3] Burnham A K, Sweeney J J. A Chemical Kinetic Model of Vitrinite Maturation and Reflectance. Geochimica et Cosmochimica Acta, 1989, 53(10): 2649-2657.

[4] Chalmers G R, Bustin R M, Power I M. Characterization of Gas Shale Pore Systems by Porosimetry, Pycnometry, Surface Area, and Field Emission Scanning Electron Microscopy/Transmission Electron Microscopy Image Analyses: Examples from the Barnett, Woodford, Haynesville, Marcellus, and Doig Units. AAPG Bulletin, 2012, 96(6): 1099-1119.

[5] Chalmers G R L, Ross D J K, Bustin R M. Geological Controls on Matrix Permeability of Devonian Gas Shales in the Horn River and Liard Basins, Northeastern British Columbia, Canada. International Journal of Coal Geology, 2012, 103: 120-131.

[6] Chen L, Lu Y C, Jiang S, . Heterogeneity of the Lower Silurian Longmaxi Marine Shale in the Southeast Sichuan Basin of China. Marine and Petroleum Geology, 2015, 65: 232-246.

[7] Chung F H. Quantitative Interpretation of X-Ray Diffraction Patterns of Mixtures. I. Matrix-Flushing Method for Quantitative Multicomponent Analysis. Journal of Applied Crystallography, 1974, 7(6): 519-525.

[8] Dodson C R, Standing M B. Pressure-Volume-Temperature and Solubility Relations for Natural-Gas-Water Mixtures. Drilling and Production Practice, 1944, 44: 173-179.

[9] Duan Z H, Møller N, Greenberg J, . The Prediction of Methane Solubility in Natural Waters to High Ionic Strength from 0 to 250 °C and from 0 to 1 600 Bar. Geochimica et Cosmochimica Acta, 1992, 56(4): 1451-1460.

[10] Fang C H, Huang Z L, Wang Q Z. Cause and Significance of the Ultra-Low Water Saturation in Gas-Enriched Shale Reservoir. Natural Gas Geoscience, 2014, 25(3): 471-476. (in Chinese with English Abstract)

[11] Gasparik M, Bertier P, Gensterblum Y, . Geological Controls on the Methane Storage Capacity in Organic-Rich Shales. International Journal of Coal Geology, 2014, 123: 34-51.

[12] Gou Q, Xu S. Quantitative Evaluation of Free Gas and Adsorbed Gas Content of Wufeng-Longmaxi Shales in the Jiaoshiba Area, Sichuan Basin, China. Advances in Geo-Energy Research, 2019, 3(3): 258-267.

[13] Hu Y N, Devegowda D, Sigal R. A Microscopic Characterization of Wettability in Shale Kerogen with Varying Maturity Levels. Journal of Natural Gas Science and Engineering, 2016, 33: 1078-1086.

[14] Ji L M, Qiu J L, Zhang T W, . Experiments on Methane Adsorption of Common Clay Minerals in Shale. Earth Science, 2012, 37(5): 1043-1050. (in Chinese with English Abstract)

[15] Jiang L, Deng B, Liu S G, . Paleo-Fluid Migration and Conservation Conditions of Shale Gas in Jiaoshiba-Wulong Area. Earth Science, 2019, 44(2): 524-538. (in Chinese with English Abstract)

[16] Korb J P, Nicot B, Louis-Joseph A, . Dynamics and Wettability of Oil and Water in Oil Shales. The Journal of Physical Chemistry C, 2014, 118(40): 23212-23218.

[17] Lan Q, Xu M X, Binazadeh M, . A Comparative Investigation of Shale Wettability: The Significance of Pore Connectivity. Journal of Natural Gas Science and Engineering, 2015, 27: 1174-1188.

[18] Lewan M D. Experiments on the Role of Water in Petroleum Formation. Geochimica et Cosmochimica Acta, 1997, 61(17): 3691-3723.

[19] Li J, Lu J, Li Z, . ‘Four-Pore’ Modeling and Its Quantitative Logging Description of Shale Gas Reservoir. Oil & Gas Geology, 2014, 35(2): 266-271. (in Chinese with English Abstract)

[20] Li P P, Hao F, Guo X S, . Processes Involved in the Origin and Accumulation of Hydrocarbon Gases in the Yuanba Gas Field, Sichuan Basin, Southwest China. Marine and Petroleum Geology, 2015, 59: 150-165.

[21] Liu H L, Wang H Y. Ultra-Low Water Saturation Characteristics and the Identification of Over-Pressured Play Fairways of Marine Shales in South China. Natural Gas Industry, 2013, 33(7): 140-144. (in Chinese with English Abstract)

[22] Lu J M, Ruppel S C, Rowe H D. Organic Matter Pores and Oil Generation in the Tuscaloosa Marine Shale. AAPG Bulletin, 2015, 99(2): 333-357.

[23] Modica C J, Lapierre S G. Estimation of Kerogen Porosity in Source Rocks as a Function of Thermal Transformation: Example from the Mowry Shale in the Powder River Basin of Wyoming. AAPG Bulletin, 2012, 96(1): 87-108.

[24] Nie H, Jin Z, Zhang J. Characteristics of Three Organic Matter Pore Types in the Wufeng-Longmaxi Shale of the Sichuan Basin, Southwest China. Scientific Reports, 2018, 8(1): 7014

[25] Passey Q R, Creaney S, Kulla J B, . A Practical Model for Organic Richness from Porosity and Resistivity Logs. AAPG Bulletin, 1990, 74: 1777-1794

[26] Pommer M, Milliken K. Pore Types and Pore-Size Distributions across Thermal Maturity, Eagle Ford Formation, Southern Texas. AAPG Bulletin, 2015, 99(9): 1713-1744.

[27] Ross D J K, Bustin R M. Shale Gas Potential of the Lower Jurassic Gordondale Member, Northeastern British Columbia, Canada. Bulletin of Canadian Petroleum Geology, 2007, 55(1): 51-75.

[28] Ross D J K, Bustin R M. The Importance of Shale Composition and Pore Structure Upon Gas Storage Potential of Shale Gas Reservoirs. Marine and Petroleum Geology, 2009, 26(6): 916-927.

[29] Ruppert L F, Sakurovs R, Blach T P, . AUSANS/SANS Study of the Accessibility of Pores in the Barnett Shale to Methane and Water. Energy & Fuels, 2013, 27(2): 772-779.

[30] Schimmelmann A, Boudou J P, Lewan M D, . Experimental Controls on D/H and ¹³C/¹²C Ratios of Kerogen, Bitumen and Oil during Hydrous Pyrolysis. Organic Geochemistry, 2001, 32(8): 1009-1018.

[31] Schimmelmann A, Lewan M D, Wintsch R P. D/H Isotope Ratios of Kerogen, Bitumen, Oil, and Water in Hydrous Pyrolysis of Source Rocks Containing Kerogen Types I, II, IIS, and III. Geochimica et Cosmochimica Acta, 1999, 63(22): 3751-3766.

[32] Shen C B, Hu D, Min K, . Post-Orogenic Tectonic Evolution of the Jiangnan-Xuefeng Orogenic Belt: Insights from Multiple Geochronometric Dating of the Mufushan Massif, South China. Journal of Earth Science, 2020, 31(5): 905-918.

[33] Song D J, Wu C J, Chen K, . Gas Generation from Marine and Terrestrial Shales by Semi-Closed Pyrolysis Experiments. Earth Science, 2019, 44(11): 3639-3652. (in Chinese with English Abstract)

[34] Shen W, Li X, Cihan A, . Experimental and Numerical Simulation of Water Adsorption and Diffusion in Shale Gas Reservoir Rocks. Advances in Geo-Energy Research, 2019, 3(2): 165-174.

[35] Tian Y D, Li N. Affecting Factors of the Coal Adsorbting Methane Capability. Journal of Xi’an University of Science and Technology, 2007, 27(2): 247-250. (in Chinese with English Abstract)

[36] Wang X F, Liu W H, Xu Y C. Thermal Simulation of the Role of Water in the Evolution of Gaseous Hydrocarbons from Organic Matter. Progress in Natural Science, 2006, 16(10): 1275-1281

[37] Wang P W, Liu Z B, Jin Z J, . Main Control Factors of Shale Gas Differential Vertical Enrichment in Lower Cambrian Qiongzhusi Formation, Southwest Sichuan Basin, China. Earth Science, 2019, 44(11): 3628-3638. (in Chinese with English Abstract)

[38] Yang R, He S, Yi J Z, . Nano-Scale Pore Structure and Fractal Dimension of Organic-Rich Wufeng-Longmaxi Shale from Jiaoshiba Area, Sichuan Basin: Investigations Using FE-SEM, Gas Adsorption and Helium Pycnometry. Marine and Petroleum Geology, 2016, 70 27-45.

[39] Yang R, He S, Hu Q H, . Pore Characterization and Methane Sorption Capacity of Over-Mature Organic-Rich Wufeng and Longmaxi Shales in the Southeast Sichuan Basin, China. Marine and Petroleum Geology, 2016, 77: 247-261.

[40] Yao J, Wang H, Pei G, . The Formation Mechanism of Upper Paleozoic Tight Sand Gas Reservoirs with Ultra-Low Water Saturation in Eastern Ordos Basin. Natural Gas Industry, 2014, 34(1): 37-43. (in Chinese with English Abstract)

[41] Zhang H, Kang Y, Chen Y, . The Study of Geology Course and Experiment Simulation for Forming Ulter-Low Water Saturation in Tight Sandstones Gas Reservoir. Natural Gas Geoscience, 2005, 16(2): 186-189. (in Chinese with English Abstract)