Characteristics of Connected Pores and Evaluation of Shale Oil Mobility in the Qianjiang Formation, Qianjiang Sag, Jianghan Basin, China

Abstract

The connectivity of shale pores and the occurrence of movable oil in shales have long been the focus of research. In this study, samples from wells BX7 and BYY2 in the Eq3⁴-10 cyclothem of Qianjiang Formation in the Qianjiang depression, were analyzed. A double mercury injection method was used to distinguish between invalid and effective connected pores. The pore characteristics for occurrence of retained hydrocarbons and movable shale oil were identified by comparing pore changes in low temperature nitrogen adsorption and high pressure mercury injection experiments before and after extraction and the change in the mercury injection amounts in the pores between two separate mercury injections. The results show that less than 50% of the total connected pores in the Eq3⁴-10 cyclothem samples are effective. The development of effective connected pores affects the mobility of shale oil but varies with different lithofacies. The main factor limiting shale oil mobility in Well BX7 is the presence of pores with throat sizes less than 15 nm. In Well BYY2, residual mercury in injection testing of lamellar dolomitic mudstone facies was mainly concentrated in pores with throats of 10–200 nm, and in bulk argillaceous dolomite facies, it was mainly concentrated at 60–300 nm. The throats of hydrocarbon-retaining pores can be 5 nm or even smaller, but pores with movable shale oil in the well were found to have throat sizes greater than 40 nm. Excluding the influence of differences in wettability, the movability of shale oil is mainly affected by differences in lithofacies, the degree of pore deformation caused by diagenesis, the complexity of pore structures, and the connectivity of pore throats. Dissolution and reprecipitation of halite also inhibit the mobility of shale oil.

Keywords

Qianjiang depression / double mercury injection / effective connected pores / retained hydrocarbons / movable hydrocarbons / hydrocarbons

References

[1] BahadurJ, RadlinskiA P, MelnichenkoY B, et al.. Small-Angle and Ultrasmall-Angle Neutron Scattering (SANS/USANS) Study of New Albany Shale: A Treatise on Microporosity. Energy & Fuels, 2015, 29(2): 567-576

[2] BrunauerS, EmmettP H, TellerE. Adsorption of Gases in Multimolecular Layers. Journal of the American Chemical Society, 1938, 60(2): 309-319

[3] DongT, GaoJ, LiS J, et al.. Quartz Cementation in the Lower Paleozoic Shales, Middle Yangtze Region, South China: Implications for Shale Reservoir Properties. Journal of Earth Science, 2024, 35(6): 1918-1933

[4] FanC W, YouJ J, ZhouG. Shale Oil Mobility Evaluation and Multi-Scale Characterization of Oil Occurrence Space of Liushagang Formation in Weixinan Sag, Beibuwan Basin. Earth Science, 2025, 50(1): 127-143

[5] GaoZ Y, HuQ H, LiangH C. Gas Diffusivity in Porous Media: determination by Mercury Intrusion Porosimetry and Correlation to Porosity and Permeability. Journal of Porous Media, 2013, 16(7): 607-617

[6] García Del CuraM A, CalvoJ P, OrdóñezS, et al.. Petrographic and Geochemical Evidence for the Formation of Primary, Bacterially Induced Lacustrine dolomite: La Roda ‘White Earth’ (Pliocene, Central Spain). Sedimentology, 2001, 48(4): 897-915

[7] GiriA, TarafdarS, GouzeP, et al.. Fractal Pore Structure of Sedimentary rocks: Simulation in 2-D Using a Relaxed Bidisperse Ballistic Deposition Model. Journal of Applied Geophysics, 2012, 87: 40-45

[8] HuQ H, EwingR P, RoweH D. Low Nanopore Connectivity Limits Gas Production in Barnett Formation. Journal of Geophysical Research: Solid Earth, 2015, 120(12): 8073-8087

[9] HuQ H, LiuH M, LiM W, et al.. Wettability, Pore Connectivity and Fluid-Tracer Migration in Shale Oil Reservoirs of Paleogene Shahejie Formation in Dongying Sag of Bohai Bay Basin, East China. Acta Petrolei Sinica, 2018, 39(3): 278-289(in Chinese with English Abstract)

[10] HuQ H, ZhouW, HugginsP, et al.. Pore Structure and Fluid Uptake of the Springer/Goddard Shale Formation in Southeastern Oklahoma, USA. Geofluids, 2018, 20185381735

[11] JiangZ X, TangX L, LiZ, et al.. The Whole-Aperture Pore Structure Characteristics and Its Effect on Gas Content of the Longmaxi Formation Shale in the Southeastern Sichuan Basin. Earth Science Frontiers, 2016, 23(2): 126-134(in Chinese with English Abstract)

[12] KellyS, El-SobkyH, Torres-VerdínC, et al.. Assessing the Utility of FIB-SEM Images for Shale Digital Rock Physics. Advances in Water Resources, 2016, 95: 302-316

[13] KhaliliN R, PanM Z, SandíG. Determination of Fractal Dimensions of Solid Carbons from Gas and Liquid Phase Adsorption Isotherms. Carbon, 2000, 38(4): 573-588

[14] LiZ M, QianM H, LiM W, et al.. Favorable Conditions of Inter-Salt Shale Oil Formation and Key Parameters for Geological Sweet Spots Evaluation: a Case Study of Eq₃⁴-10 Rhythm of Qianjiang Formation in Qianjiang Sag, Jianghan Basin. Petroleum Geology & Experiment, 2020, 42(4): 513-523(in Chinese with English Abstract)

[15] LiZ Q, ShenX, QiZ Y, et al.. Study on the Pore Structure and Fractal Characteristics of Marine and Continental Shale Based on Mercury Porosimetry, N₂ Adsorption and NMR Methods. Journal of Natural Gas Science and Engineering, 2018, 53: 12-21

[16] LiuH M, ZhangS, BaoY S, et al.. Geological Characteristics and Effectiveness of the Shale Oil Reservoir in Dongying Sag. Oil & Gas Geology, 2019, 40(3): 512-523(in Chinese with English Abstract)

[17] LiuZ J, YangD, ShaoJ X, et al.. Evolution of Pore Connectivity in the Fushun Oil Shale by Low-Field Nuclear Magnetic Resonance Spectroscopy. Chinese Journal of Magnetic Resonance, 2019, 36(3): 309-318(in Chinese with English Abstract)

[18] MastandreaA, PerriE, RussoF, et al.. Microbial Primary Dolomite from a Norian Carbonate Platform: northern Calabria, Southern Italy. Sedimentology, 2006, 53(3): 465-480

[19] ShenJ JMechanism for Rormation of Lacustrine Dolomites of the Lower Xingouzui Formation in the Southern Part of the Qianjiang Depression and its Petroleum Geologic Implication, 2013, Wuhan. Yangtze University.

[20] SingK S W. Reporting Physisorption Data for Gas/Solid Systems with Special Reference to the Determination of Surface Area and Porosity (Recommendations 1984). Pure and Applied Chemistry, 1985, 57(4): 603-619

[21] SongD J, TuoJ C, WangY T, et al.. Research Advances on Characteristics of Nanopore Structure of Organic-Rich Shales. Acta Sedimentologica Sinica, 2019, 37(6): 1309-1324(in Chinese with English Abstract)

[22] SunM D, YuB S, HuQ H, et al.. Pore Characteristics of Longmaxi Shale Gas Reservoir in the Northwest of Guizhou, China: Investigations Using Small-Angle Neutron Scattering (SANS), Helium Pycnometry, and Gas Sorption Isotherm. International Journal of Coal Geology, 2017, 171: 61-68

[23] SunZ L, HeZ L, WangF R, et al.. Occurrence Characteristics of Saline-Lacustrine Shale-Oil in the Qianjiang Depression, Jianghan Basin, Central China. Journal of Earth Science, 2022, 33(4): 945-962

[24] SunZ L, WangF R, HanYJ, et al.. Evaluation of Fractal Characteristics of Intersalt Shale Oil Reservoirs in Qianjiang Depression. Bulletin of Geological Science and Technology, 2022, 41(4): 125-137(in Chinese with English Abstract)

[25] SunZ L, WangF R, HeS, et al.. The Pore Structures of the Shale about Typical Inter-Salt Rhythm in the Paleogene of Qianjiang Depression. Journal of Shenzhen University Science and Engineering, 2019, 36(3): 289-297(in Chinese with English Abstract)

[26] SunZ L, WangF R, HouY G, et al.. Main Controlling Factors and Modes of Organic Matter Enrichment in Salt Lake Shale. Earth Science, 2020, 45(4): 1375-1387(in Chinese with English Abstract)

[27] VegaB, DuttaA, KovscekA R. CT Imaging of Low-Permeability, Dual-Porosity Systems Using High X-Ray Contrast Gas. Transport in Porous Media, 2014, 101(1): 81-97

[28] WangF R, HeS, ZhengY H, et al.. Mineral Composition and Brittleness Characteristics of the Inter-Salt Shale Oil Reservoirs in the Qianjiang Formation, Qianjiang Sag. Petroleum Geology & Experiment, 2016, 38(2): 211-218(in Chinese with English Abstract)

[29] WangG C, JuY W, YanZ F, et al.. Pore Structure Characteristics of Coal-Bearing Shale Using Fluid Invasion methods: A Case Study in the Huainan-Huaibei Coalfield in China. Marine and Petroleum Geology, 2015, 62: 1-13

[30] WangM, MaR, LiJ B, et al.. Occurrence Mechanism of Lacustrine Shale Oil in the Paleogene Shahejie Formation of Jiyang Depression, Bohai Bay Basin, China. Petroleum Exploration and Development, 2019, 46(4): 833-846

[31] WangY, ZhuY M, WangH Y, et al.. Nanoscale Pore Morphology and Distribution of Lacustrine Shale Reservoirs: Examples from the Upper Triassic Yanchang Formation, Ordos Basin. Journal of Energy Chemistry, 2015, 24(4): 512-519

[32] WashburnE W. Note on a Method of Determining the Distribution of Pore Sizes in a Porous Material. Proceedings of the National Academy of Sciences of the United States of America, 1921, 7(4): 115-116

[33] WuS T, ZhuR K, LiX, et al.. Evaluation and Application of Porous Structure Characterization Technologies in Unconventional Tight Reservoirs. Earth Science Frontiers, 2018, 25(2): 191-203(in Chinese with English Abstract)

[34] XiaoFStudy on the Characteristics of Salt Shale Reservoir in Qianjiang Formation in Qianjiang Sag: A Case Study of the 10th Rhythm of Eq3⁴ and the 5th Rhythm of Middle Eq4⁰, 2018, Wuhan. Yangtze University.

[35] YangR, HaoF, HeS, et al.. Experimental Investigations on the Geometry and Connectivity of Pore Space in Organic-Rich Wufeng and Longmaxi Shales. Marine and Petroleum Geology, 2017, 84: 225-242

[36] YangR, HeS, HuQ H, et al.. Applying SANS Technique to Characterize Nano-Scale Pore Structure of Longmaxi Shale, Sichuan Basin (China). Fuel, 2017, 197: 91-99

[37] YangR, HuQ H, HeS, et al.. Wettability and Connectivity of Overmature Shales in the Fuling Gas Field, Sichuan Basin (China). AAPG Bulletin, 2019, 103(3): 653-689

[38] YangZ M, HuangH, LuoY T, et al.. New Measurement Method of Mixed Wettability in Tight Oil Reservoir and Its Application. Acta Petrolei Sinica, 2017, 38(3): 318-323(in Chinese with English Abstract)

[39] ZhangL Y, BaoY S, XiC W. Pore Structure Characteristics and Pore Connectivity of Paleogene Shales in Dongying Depression. Xinjiang Petroleum Geology, 2018, 39(2): 134-139(in Chinese with English Abstract)

[40] ZhangX, WangL L, CaiS Y, et al.. Effects of Hydrocarbon Generation on the Occurrence of Organic Nanopores during Thermal Maturity of Organic Matters. Earth Science, 2024, 49(9): 3292-3305(in Chinese with English Abstract)

[41] ZhaoJ H, JinZ J, HuQ H, et al.. Integrating SANS and Fluid-Invasion Methods to Characterize Pore Structure of Typical American Shale Oil Reservoirs. Scientific Reports, 2017, 715413

RIGHTS & PERMISSIONS

China University of Geosciences (Wuhan) and Springer-Verlag GmbH Germany, Part of Springer Nature