1. PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China 2. Key Laboratory of Continental Shale Hydrocarbon Accumulation and Efficient Development, Ministry of Education, Northeast Petroleum University, Daqing 163318, China 3. Departament de Mineralogia, Petrologia i Geologia Aplicada, Facultat de Ciències de la Terra, Universitat de Barcelona (UB), c/ Martí i Franquès s/n, Barcelona 08028, Spain 4. Departament de Geologia, Universitat Autònoma de Barcelona, Bellaterra 08193, Cerdanyola del Vallès, Spain 5. Hubei Geological Survey, Wuhan 430034, China 6. School of Geography Science and Geomatics Engineering, Suzhou University of Science and Technology, Suzhou 215009, China
Natural fractures are of crucial importance for oil and gas reservoirs, especially for those with ultralow permeability and porosity. The deep-marine shale gas reservoirs of the Wufeng and Longmaxi Formations are typical targets for the study of natural fracture characteristics. Detailed descriptions of full-diameter shale drill core, together with 3D Computed Tomography scans and Formation MicroScanner Image data acquisition, were carried out to characterize microfracture morphology in order to obtain the key parameters of natural fractures in such system. The fracture type, orientation, and their macroscopic and microscopic distribution features are evaluated. The results show that the natural fracture density appears to remarkably decrease in the Wufeng and Longmaxi Formations with increasing the burial depth. Similar trends have been observed for fracture length and aperture. Moreover, the natural fracture density diminishes as the formation thickness increases. There are three main types of natural fractures, which we interpret as (I) mineral-filled fractures (by pyrite and calcite), i.e., veins, (II) those induced by tectonic stress, and (III) those formed by other processes (including diagenetic shrinkage and fluid overpressure). Natural fracture orientations estimated from the studied natural fractures in the Luzhou block are not consistent with the present-day stress field. The difference in tortuosity between horizontally and vertically oriented fractures reveals their morphological complexity. In addition, natural fracture density, host rock formation thickness, average total organic carbon and effective porosity are found to be important factors for evaluating shale gas reservoirs. The study also reveals that the high density of natural fractures is decisive to evaluate the shale gas potential. The results may have significant implications for evaluating favorable exploration areas of shale gas reservoirs and can be applied to optimize hydraulic fracturing for permeability enhancement.
. [J]. Frontiers of Earth Science, 2023, 17(1): 337-350.
Shasha SUN, Saipeng HUANG, Enrique GOMEZ-RIVAS, Albert GRIERA, Βο LIU, Lulu XU, Yaru WEN, Dazhong DONG, Zhensheng SHI, Yan CHANG, Yin XING. Characterization of natural fractures in deep-marine shales: a case study of the Wufeng and Longmaxi shale in the Luzhou Block Sichuan Basin, China. Front. Earth Sci., 2023, 17(1): 337-350.
M O, Abouelresh L O Babalola (2020). 2D spatial analysis of the natural fractures in the organic-rich Qusaiba Shale outcrop, NW Saudi Arabia.J Petrol Sci Eng, 186: 106780 https://doi.org/10.1016/j.petrol.2019.106780
2
W C Belfield (1994). Multifractal characteristics of natural fracture apertures.Geophys Res Lett, 21(24): 2641–2644 https://doi.org/10.1029/94GL02409
3
P D, Bons M A, Elburg E Gomez-Rivas (2012). A review of the formation of tectonic veins and their microstructures.J Struct Geol, 43: 33–62 https://doi.org/10.1016/j.jsg.2012.07.005
4
M, Brudy M D Zoback (1999). Drilling-induced tensile wall-fractures: implications for determination of in-situ stress orientation and magnitude.Int J Rock Mech Min Sci, 36(2): 191–215 https://doi.org/10.1016/S0148-9062(98)00182-X
5
J C, Cai Z E, Zhang W, Wei D M, Guo S, Li P Q Zhao (2019). The critical factors for permeability-formation factor relation in reservoir rocks: pore-throat ratio, tortuosity and connectivity.Energy, 188: 116051 https://doi.org/10.1016/j.energy.2019.116051
6
S B, Chen Y M, Zhu Y, Qin H Y, Wang H L, Liu J H Fang (2014). Reservoir evaluation of the Lower Silurian Longmaxi Formation shale gas in the southern Sichuan Basin of China.Mar Pet Geol, 57: 619–630 https://doi.org/10.1016/j.marpetgeo.2014.07.008
7
S B, Chen Y M, Zhu H Y, Wang H L, Liu W, Wei J H Fang (2011). Shale gas reservoir characterisation: a typical case in the southern Sichuan Basin of China.Energy, 36(11): 6609–6616 https://doi.org/10.1016/j.energy.2011.09.001
8
A M Cook, L R Myer, N G W Cook, F M Doyle (1990). The effects of tortuosity on flow through a natural fracture. In: Rock Mechanics Contributions and Challenges: Proceedings of the 31st U S Symposium. New York: CRC Press
9
D, Cruset J, Vergés A, Benedicto E, Gomez-Rivas I, Cantarero C M, John A Travé (2021). Multiple fluid flow events from salt-related rifting to basin inversion (Upper Pedraforca thrust sheet, SE Pyrenees).Basin Res, 33(6): 3102–3136 https://doi.org/10.1111/bre.12596
10
J B Curtis (2002). Fractured shale-gas systems.AAPG Bull, 86(11): 1921–1938
11
W L, Ding C, Li C Y, Li C C, Xu K, Jiu W T, Zeng L M Wu (2012). Fracture development in shale and its relationship to gas accumulation.Geosci Front, 3(1): 97–105 https://doi.org/10.1016/j.gsf.2011.10.001
12
A Fall, P Eichhubl, S E Laubach, R J Bodnar (2012). Timing and duration of gas charge-driven fracturing in tight-gas sandstone reservoirs based on fluid inclusion observations: Piceance Basin, Colorado. In: AGU Fall Meeting
13
Q Q, Feng N S, Qiu T, Borjigin H, Wu J T, Zhang B J, Shen J S Wang (2022). Tectonic evolution revealed by thermo-kinematic and its effect on shale gas preservation.Energy, 240: 122781 https://doi.org/10.1016/j.energy.2021.122781
14
J M, Fan X F, Qu C, Wang Q, Lei L, Cheng Z Yang (2016). Natural fracture distribution and a new method predicting effective fractures in tight oil reservoirs of Ordos Basin, NW China.Pet Explor Dev, 43(5): 806–814 https://doi.org/10.1016/S1876-3804(16)30096-9
15
H Fossen (2010). Structural Geology. London: Cambridge University Press
16
J F W Gale, J Holder (2010). Natural fractures in some US Shales and their importance for gas production. In: Petroleum Geology Conference series. London: Geological Society: 1131–1140
17
J F W, Gale S E, Laubach J E, Olson P, Eichhubl A Fall (2014). Natural fractures in shale: a review and new observations.AAPG Bull, 98(11): 2165–2216 https://doi.org/10.1306/08121413151
18
J F W, Gale R M, Reed J Holder (2007). Natural fractures in the Barnett shale and their importance for hydraulic fracture treatments.AAPG Bull, 91(4): 603–622 https://doi.org/10.1306/11010606061
19
M, Gasparrini W, Sassi J F W Gale (2014). Natural sealed fractures in mudrocks: a case study tied to burial history from the Barnett Shale, Fort Worth Basin, Texas, USA.Mar Pet Geol, 55: 122–141 https://doi.org/10.1016/j.marpetgeo.2013.12.006
20
Y, Geng W, Liang J, Liu M, Cao Z Kang (2017). Evolution of pore and fracture structure of oil shale under high temperature and high pressure.Energy Fuels, 31(10): 10404–10413 https://doi.org/10.1021/acs.energyfuels.7b01071
21
O, Heidbach M, Rajabi X F, Cui K, Fuchs B, Müller J, Reinecker K, Reiter M, Tingay F, Wenzel F R, Xie M O, Ziegler M L, Zoback M Zoback (2018). The World Stress Map database release 2016: Crustal stress pattern across scales.Tectonophysics, 744: 484–498 https://doi.org/10.1016/j.tecto.2018.07.007
22
O Heidbach, M Rajabi, K Reiter, M Ziegler (2016a). World Stress Map 2016, GFZ Data Services
23
O Heidbach, M Rajabi, K Reiter, M Ziegler (2016b). World Stress Map Database Release 2016. GFZ Data Services
24
D G, Hill C R Nelson (2000). Gas productive fractured shales: an overview and update.Gas TIPS, 6(2): 4–13
25
S P, Huang D M, Liu Y D, Cai Q Gan (2019). In situ stress distribution and its impact on CBM reservoir properties in the Zhengzhuang area, Southern Qinshui Basin, North China.J Nat Gas Sci Eng, 61: 83–96 https://doi.org/10.1016/j.jngse.2018.11.005
26
P Q, Huy K, Sasaki Y, Sugai S Ichikawa (2010). Carbon dioxide gas permeability of coal core samples and estimation of fracture aperture width.Int J Coal Geol, 83(1): 1–10 https://doi.org/10.1016/j.coal.2010.03.002
27
T H Kim (2007). Fracture characterization and estimation of fracture porosity of naturally fractured reservoirs with no matrix porosity using stochastic fractal models. Dissertation for the doctoral Degree. College Station: Texas A&M University
28
A, Lakirouhani E, Detournay A P Bunger (2016). A reassessment of in-situ stress determination by hydraulic fracturing.Geophys J Int, 205(3): 1859–1873 https://doi.org/10.1093/gji/ggw132
29
S E Laubach (2003). Practical approaches to identifying sealed and open fractures.AAPG Bull, 87(4): 561–579 https://doi.org/10.1306/11060201106
30
S E, Laubach J E, Olson J F W Gale (2004). Are open fractures necessarily aligned with maximum horizontal stress?.Earth Planet Sci Lett, 222(1): 191–195 https://doi.org/10.1016/j.epsl.2004.02.019
31
H L, Liu J H, Zhang Y B, Ji X B Li (2022). The controlling effect of kerogen type of shale on asphaltene nanopore and its exploration significance.Unconventional oil Gas, 9(3): 1–10
32
H, Liu S, Sang J, Xue G, Wang H, Xu B, Ren C, Liu S Liu (2016). Characteristics of an in-situ stress field and its control on coal fractures and coal permeability in the Gucheng block, southern Qinshui Basin, China.J Nat Gas Sci Eng, 36: 1130–1139 https://doi.org/10.1016/j.jngse.2016.03.024
33
M, Mlella B, Surpless C, Beasley M K, Stewart L, Yazbeck La Rocha L De (2014). The effects of weathering on outcrop-based fracture characterization: a case study from the Stillwell Anticline, West Texas.GSA Abstracts with Programs, 46(1): 12
34
M J Mullen, J L Pitcher, D Hinz, M Everts, D Dunbar, G M Carlstrom, G R Brenize (2010). Does the presence of natural fractures have an impact on production? A case study from the middle Bakken Dolomite, North Dakota. In: Society of Petroleum Engineers
35
H K, Nie Z L, He R Y, Wang G R, Zhang Q, Chen D H, Li Z Y, Lu C X Sun (2020). Temperature and origin of fluid inclusions in shale veins of Wufeng–Longmaxi Formations, Sichuan Basin, south China: implications for shale gas preservation and enrichment.J Petrol Sci Eng, 193: 107329 https://doi.org/10.1016/j.petrol.2020.107329
36
J E, Olson S E, Laubach R H Lander (2007). Combining diagenesis and mechanics to quantity fracture aperture distributions and fracture pattern permeability.Spec Publ Geol Soc Lond, 270(1): 101–116 https://doi.org/10.1144/GSL.SP.2007.270.01.08
37
S, Paul R Chatterjee (2011a). Mapping of cleats and fractures as an indicator of in-situ stress orientation, Jharia Coalfield, India.Int J Coal Geol, 88(2–3): 113–122 https://doi.org/10.1016/j.coal.2011.09.006
38
S, Paul R Chatterjee (2011b). Determination of in-situ stress direction from cleat orientation mapping for coal bed methane exploration in south-eastern part of Jharia coalfield, India.Int J Coal Geol, 87(2): 87–96 https://doi.org/10.1016/j.coal.2011.05.003
39
L Pommer, J F W Gale, P Eichhubl, A Fall, S E Laubach (2012). Using structural diagenesis to infer the timing of natural fractures in the Marcellus shale. In: Unconventional Resources Technology Conference
40
Z, Qiu B, Liu D, Dong B, Lu Z, Yawar Z, Chen J Schieber (2020). Silica diagenesis in the Lower Paleozoic Wufeng and Longmaxi Formations in the Sichuan Basin, South China: implications for reservoir properties and paleoproductivity.Mar Pet Geol, 121: 104594 https://doi.org/10.1016/j.marpetgeo.2020.104594
41
P G, Ranjith W, Wanniarachchi M, Perera T D Rathnaweera (2018). Investigation of the effect of foam flow rate on foam-based hydraulic fracturing of shale reservoir rocks with natural fractures: an experimental study.J Petrol Sci Eng, 169: 518–531 https://doi.org/10.1016/j.petrol.2018.06.002
42
S, Sang W, Liu Z, Shen T Ma (2019). A method for extracting 3d fracture geometries and acquiring their mechanical properties from CT scanning images.J Porous Media, 22(10): 1305–1320 https://doi.org/10.1615/JPorMedia.2019025242
43
X L, Sun E, Gomez-Rivas J, Alcalde J D, Martín-Martín C F, Ma D, Muñoz-López D, Cruset I, Cantarero A, Griera A Travé (2021). Fracture distribution in a folded fluvial succession: the Puig-reig anticline (south-eastern Pyrenees).Mar Pet Geol, 132: 105169 https://doi.org/10.1016/j.marpetgeo.2021.105169
44
M, Tabatabaei Taleghani A, Dahi J N Hooker (2021). Debonding of cemented natural fractures during core recovery.J Struct Geol, 144: 104272 https://doi.org/10.1016/j.jsg.2020.104272
45
Y W Tsang (1984). The effect of tortuosity on fluid flow through a single fracture.Water Resour Res, 20(9): 1209–1215 https://doi.org/10.1029/WR020i009p01209
46
I, Walton J Mclennan (2013). The role in natural fractures in shale gas production. In: Bunger A P, Mclennan J, Jeffrey R, eds. Effective and Sustainable Hydraulic Fracturing.Intech Open, 327–356
47
R, Wang T, Pavlin M S, Rosen R W, Mair D G, Cory R L Walsworth (2005). Xenon NMR measurements of permeability and tortuosity in reservoir rocks.Magn Reson Imaging, 23(2): 329–331 https://doi.org/10.1016/j.mri.2004.11.044
pmid: 15833638
48
Y, Wang C H, Li Y Z, Hu T Q Mao (2017). Laboratory investigation of hydraulic fracture propagation using real-time ultrasonic measurement in shale formations with random natural fractures.Environ Earth Sci, 76(22): 768 https://doi.org/10.1007/s12665-017-7038-2
49
C, Zeeb E, Gomez-Rivas P D, Bons P Blum (2013). Evaluation of sampling methods for fracture network characterization using outcrops.AAPG Bull, 97(9): 1545–1566 https://doi.org/10.1306/02131312042
50
L B, Zeng W Y, Lyu J, Li L F, Zhu J Q, Weng F, Yue K W Zu (2016). Natural fractures and their influence on shale gas enrichment in Sichuan Basin, China.J Nat Gas Sci Eng, 30: 1–9 https://doi.org/10.1016/j.jngse.2015.11.048
51
Y S, Zhang J C, Zhang B, Yuan S X Yin (2018). In-situ stresses controlling hydraulic fracture propagation and fracture breakdown pressure.J Petrol Sci Eng, 164: 164–173 https://doi.org/10.1016/j.petrol.2018.01.050
52
Y Y, Zhang Z L, He S, Jiang S F, Lu D S, Xiao G H, Chen Y C Li (2019). Fracture types in the lower Cambrian shale and their effect on shale gas accumulation, Upper Yangtze.Mar Pet Geol, 99: 282–291 https://doi.org/10.1016/j.marpetgeo.2018.10.030
53
C J, Zhao J, Li G H, Liu X Zhang (2020a). Analysis of well stress with the effect of natural fracture nearby wellbore during hydraulic fracturing in shale gas wells.J Petrol Sci Eng, 188: 106885 https://doi.org/10.1016/j.petrol.2019.106885
54
G, Zhao W L, Ding Y X, Sun X H, Wang L, Tian J S, Liu S Y, Shi B C, Jiao L Cui (2020b). Fracture development characteristics and controlling factors for reservoirs in the Lower Silurian Longmaxi formation marine shale of the Sangzhi Block, Hunan Province, China.J Petrol Sci Eng, 184: 106470 https://doi.org/10.1016/j.petrol.2019.106470
55
S Q, Zheng X F, Xie L Y, Luo Y, Jing M, Tang R F, Yang G R, Zhong J, Wang Z Y Chen (2019). Fast and efficient drilling technologies for deep shale gas horizontal wells in the Sichuan Basin: a case study of Well Lu 203.Nat Gas Ind, 39(7): 88–93
56
W L, Zhu T F Wong (1996). Permeability reduction in a dilating rock: network modeling of damage and tortuosity.Geophys Res Lett, 23(22): 3099–3102 https://doi.org/10.1029/96GL03078
57
C N, Zou D Z, Dong S J, Wang J Z, Li X J, Li Y M, Wang D H, Li K M Cheng (2010). Geological characteristics and resource potential of shale gas in China.Pet Explor Dev, 37(6): 641–653 https://doi.org/10.1016/S1876-3804(11)60001-3
