Organic Nano-pore Textural Characteristics of the Longmaxi Formation Shale in the Southeastern Sichuan Basin

doi:10.19657/j.geoscience.1000-8527.2023.054

Abstract

Abstract:

The widely developed nano-pores in organic-rich shale are important storage space for shale gas.To determine the pore development characteristics of organic matter in shale, the Lower Silurian Longmaxi Formation shale in Dingshan area, southeast Sichuan Basin is taken as the research object.The surface porosity of organic matter was evaluated and the nano-pore structure characteristics were characterized based on scanning electron microscopy (SEM) and low-pressure N₂ and CO₂ adsorption analysis.The results indicate that total organic carbon (TOC) content was the major factor influencing the specific surface area and pore volume of nanopores in the Longmaxi Formation shale.With increasing TOC contents, the specific surface area and volume of organic pores increase.With higher TOC contents, the specific surface area and volume of organic pores played a dominant role over that of inorganic pores.The pore volume contribution of the 2-10 nm-sized pores to the total pore volume is the largest in the Longmaxi shale samples, and the pyrobitumen contains larger numbers of mesopores than the other types of organic matter (e.g., kerogen).The pyrobitumen content increase would significantly increase the mesopore volumes of the shale.However, the oil expulsion efficiency can indirectly affect the nanopore development via the variation of pyrobitumen contents.The volume of mesopores and total pores in the shale would be reduced due to high oil expulsion efficiency.In the exploration and development of Longmaxi Formation shale, the influence of TOC content, oil expulsion efficiency and pyrobitumen content on the reservoir conditions of shale gas should be considered comprehensively.

Key words: organic pore, nano-pore structure, gas adsorption, pyrobitumen, Longmaxi Formation, Sichuan Basin

CLC Number:

LI Dongsheng, GAO Ping, GAI Haifeng, LIU Ruobing, CAI Yidong, LI Gang, ZHOU Qin, XIAO Xianming. Organic Nano-pore Textural Characteristics of the Longmaxi Formation Shale in the Southeastern Sichuan Basin[J]. Geoscience, 2023, 37(05): 1293-1305.

Figures/Tables 17

Fig.1 Stratigraphic profile (b) and sampling location (a) in the Dingshan area, southeastern Sichuan Basin (modified from Gao et al.[39])

Table 1 TOC content, pyrobitumen surface distribution rate and oil expulsion efficiency of shale samples from Well DY 5

样品	层位	深度 (m)	TOC (%)	焦沥青面分布率(%)	排油效率 (%)
DY5-43	龙马溪组	3743.0	0.77	0.38	28.6
DY5-39	龙马溪组	3749.8	1.24	0.38	60.3
DY5-34	龙马溪组	3759.4	0.81	0.39	30.6
DY5-23	龙马溪组	3776.8	0.92	0.42	35.2
DY5-22	龙马溪组	3778.7	1.11	0.40	51.9
DY5-19	龙马溪组	3784.3	1.49	0.66	38.0
DY5-12	龙马溪组	3796.8	2.38	0.57	70.3
DY5-09	龙马溪组	3802.4	2.81	0.64	71.9
DY5-07	龙马溪组	3806.4	2.70	0.64	70.6
DY5-06	龙马溪组	3808.5	3.69	0.78	74.1
DY5-05	龙马溪组	3809.7	4.56	0.91	75.7
DY5-02	五峰组	3814.6	4.39	1.31	61.7

Table 2 Pyrobitumen content and Cp/TOC ratio of the Longmaxi Formation shale samples in Well DY 5

样品	$W p$ (%)	C_p/TOC(%)
DY5-43	0.1939	25.1356
DY5-39	0.1939	15.6788
DY5-34	0.1991	24.6148
DY5-23	0.2144	23.3403
DY5-22	0.2042	18.4190
DY5-19	0.3373	22.5782
DY5-12	0.2912	12.2195
DY5-09	0.3271	11.6352
DY5-07	0.3271	12.1180
DY5-06	0.3989	10.8216
DY5-05	0.4657	10.2209
DY5-02	0.6717	14.9941

Table 2 Pyrobitumen content and Cp/TOC ratio of the Longmaxi Formation shale samples in Well DY 5

样品	$W p$ (%)	C_p/TOC(%)
DY5-43	0.1939	25.1356
DY5-39	0.1939	15.6788
DY5-34	0.1991	24.6148
DY5-23	0.2144	23.3403
DY5-22	0.2042	18.4190
DY5-19	0.3373	22.5782
DY5-12	0.2912	12.2195
DY5-09	0.3271	11.6352
DY5-07	0.3271	12.1180
DY5-06	0.3989	10.8216
DY5-05	0.4657	10.2209
DY5-02	0.6717	14.9941

Fig.2 Cross-plots of the TOC contents versus pyrobitumen content (a) and Cp/TOC ratio (b) of the Longmaxi Formation shale samples in Well DY5

Fig.3 Distribution of kerogen, pyrobitumen and organic matter-hosted pores in shale under SEM imaging

Fig.4 Frequency diagrams showing the distribution of surface porosity in various-sized pores of the kerogen (a) and pyrobitumen (b) in the shale samples

Fig.5 N2 adsorption desorption curves (a) and CO2 adsorption isotherms (b) of the typical Longmaxi Formation shale samples in Well DY5

Fig.6 Relationship between pore specific surface area and TOC values of the Longmaxi shale samples in Well DY5

Table 3 Nanopore specific surface area distribution of the Longmaxi shale samples in Well DY5

样品	TOC (%)	微孔比表面积(m²·g^-1)			非微孔比表面积(m²·g^-1)			总孔隙比表面积(m²·g^-1)
样品	TOC (%)	有机质微孔	无机质微孔	总微孔	有机质非微孔	无机质非微孔	总非微孔	总孔	有机质孔	无机质孔
DY5-43	0.77	1.5601	10.4310	11.9910	2.9198	9.2389	12.1587	4.4799	19.6699	24.1497
DY5-39	1.24	2.5011	11.3482	13.8493	4.6809	8.9460	13.6269	7.1820	20.2942	27.4762
DY5-34	0.81	1.6351	11.6503	13.2854	3.0602	8.5125	11.5727	4.6953	20.1628	24.8581
DY5-23	0.92	1.8573	10.0959	11.9532	3.4761	7.4745	10.9506	5.3333	17.5704	22.9038
DY5-22	1.11	2.2412	9.4912	11.7324	4.1946	7.0429	11.2375	6.4359	16.5340	22.9699
DY5-19	1.49	3.0207	10.1833	13.2040	5.6534	7.2446	12.8980	8.6741	17.4279	26.1020
DY5-12	2.38	4.8181	10.5126	15.3307	9.0174	7.8447	16.8621	13.8356	18.3572	32.1928
DY5-09	2.81	5.6835	11.4814	17.1649	10.6370	7.6172	18.2542	16.3205	19.0986	35.4191
DY5-07	2.70	5.4570	11.8370	17.2940	10.2132	7.8196	18.0328	15.6702	19.6566	35.3268
DY5-06	3.69	7.4526	10.5238	17.9765	13.9481	6.4992	20.4473	21.4007	17.0230	38.4238
DY5-05	4.56	9.2116	10.6541	19.8658	17.2402	8.4451	25.6853	26.4519	19.0992	45.5511
DY5-02	4.39	8.8760	9.9947	18.8708	16.6121	9.2204	25.8325	25.4881	19.2152	44.7033

Table 4 Nanopore volume distribution of the Longmaxi Formation shale samples in Well DY5

样品	TOC (%)	微孔体积(cm³·g^-1)			介孔体积(cm³·g^-1)			宏孔体积(cm³·g^-1)			总孔体积(cm³·g^-1)
样品	TOC (%)	有机质微孔	无机质微孔	总微孔	有机质介孔	无机质介孔	总介孔	有机质宏孔	无机质宏孔	总宏孔	有机质总孔	无机质总孔	总孔隙
DY5-43	0.77	0.0004	0.0012	0.0016	0.0010	0.0043	0.0053	0	0.0018	0.0018	0.0014	0.0073	0.0087
DY5-39	1.24	0.0006	0.0014	0.0020	0.0016	0.0039	0.0055	0	0.0012	0.0013	0.0023	0.0065	0.0088
DY5-34	0.81	0.0004	0.0014	0.0018	0.0011	0.0043	0.0054	0	0.0014	0.0014	0.0015	0.0071	0.0086
DY5-23	0.92	0.0005	0.0012	0.0017	0.0012	0.0037	0.0049	0	0.0013	0.0013	0.0017	0.0062	0.0079
DY5-22	1.11	0.0005	0.0010	0.0016	0.0015	0.0034	0.0049	0	0.0013	0.0013	0.0020	0.0057	0.0078
DY5-19	1.49	0.0007	0.0011	0.0018	0.0020	0.0033	0.0053	0	0.0011	0.0012	0.0028	0.0055	0.0083
DY5-12	2.38	0.0012	0.0013	0.0025	0.0032	0.0036	0.0067	0.0001	0.0015	0.0015	0.0044	0.0064	0.0108
DY5-09	2.81	0.0014	0.0014	0.0028	0.0037	0.0036	0.0073	0.0001	0.0013	0.0014	0.0052	0.0062	0.0114
DY5-07	2.70	0.0013	0.0013	0.0026	0.0036	0.0040	0.0076	0.0001	0.0014	0.0015	0.0050	0.0067	0.0117
DY5-06	3.69	0.0018	0.0011	0.0029	0.0049	0.0037	0.0085	0.0001	0.0013	0.0014	0.0068	0.0061	0.0129
DY5-05	4.56	0.0022	0.0013	0.0036	0.0060	0.0042	0.0102	0.0001	0.0019	0.0021	0.0084	0.0074	0.0159
DY5-02	4.39	0.0022	0.0012	0.0033	0.0058	0.0038	0.0096	0.0001	0.0009	0.0010	0.0081	0.0058	0.0139

Fig.7 Relationship between specific pore surface area (a) and TOC (b) content of the Longmaxi Formation shale samples in Well DY5

Fig.8 Relationship between TOC content and various-sized pore volume of the Longmaxi Formation shale samples in Well DY5

Fig.9 Relationship between organic and inorganic pore volume and TOC value of the Longmaxi Formation shale samples in Well DY5

Fig.10 Relationship between TOC and clay mineral contents of the Longmaxi Formation shale samples

Fig.11 Volume distribution characteristics of the various-sized pores of the Longmaxi Formation shale samples in Well DY5

Fig.12 Effect of pyrobitumen contents on nano-pore development in the Longmaxi shale samples of Well DY5

Fig.13 Relationship between oil expulsion efficiency and TOC-normalized pore volume of the Longmaxi Formation shale samples in Well DY5

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