Preliminary study on nanopores, nanofissures, and in situ accumulation of Gulong shale oil

doi:10.13745/j.esf.sf.2022.8.32-en

摘要/Abstract

摘要：

The Qingshankou Formation shale oil in the Gulong Sag is an important oil and gas reservoir in the Daqing oilfield, with geological resources of 15.1 billion tons. The fabric of shale can reflect not only its genesis but also the nature of the reservoir space, its physical properties, oil content, and development value. Here, the characteristics of clay minerals in the Gulong shale oil reservoir were studied via electron microscopy, with the primary focus on the microfabrics and reservoir space; thereafter, the in situ accumulation was studied and discussed. Electron backscattering patterns revealed that nanometer pores and fissures were well developed in the Gulong shale oil reservoir. The nano pores were mostly 20-50 nm in diameter (median 20-30 nm), irregularly shaped, mostly, polygonal, and connected with nanofissures. The widths of nanofissures ranged mostly between 10-50 nm (median 20-30 nm); moreover, these fissures were mainly formed by F-F condensation of clay sheets (clay domains). The coagulation of clays was closely related to organic matter, especially algae. The clay colloids were negatively charged due to isocrystalline replacement; hence, metal cations were absorbed around the clay, forming a positive clay group. The positively charged clays subsequently adsorbed negatively charged humic acid (organic matter) and initially degraded algae to form an organic clay flocculant. When the organic clay flocculates reached the threshold for hydrocarbon generation and expulsion, the volume of organic matter decreased by 87%; thereafter, the generated and expelled hydrocarbon filled the nearby pores formed by this contraction. Moreover, the discharged hydrocarbon could not migrate due to capillary resistance (~12 MPa) of the nanopores; hence, the nanopores formed a unique continuous in situ reservoir within the Gulong shale oil. This study demonstrated that the Gulong shale oil reservoir is an actual clay-type shale reservoir with numerous nanopore and fissures. During coagulation, a large amount of organic matter (including layered algae) was absorbed by the clay, forming an organic clay condensate that could have provided the material foundation for hydrocarbon generation at a later stage. Thermal simulation experiments revealed that the volume of organic matter decreased sharply after hydrocarbon generation and expulsion.

关键词: shale, clay, organic matter, nanopores, nanofissures, in situ accumulation, Gulong Sag

HE Wenyuan. Preliminary study on nanopores, nanofissures, and in situ accumulation of Gulong shale oil[J]. 地学前缘, 2023, 30(1): 260-280.

HE Wenyuan. Preliminary study on nanopores, nanofissures, and in situ accumulation of Gulong shale oil[J]. Earth Science Frontiers, 2023, 30(1): 260-280.

图/表 19

Fig.1 Map showing the location and structural zoning of the study area. A-A' represents the location of seismic profile shown in Fig.2.

Fig.2 (a) EW-trending seismic profile and (b) its interpretation in the Songliao Basin (T2-T11 are the Qingshankou Group)

Fig.3 Generalized stratigraphic column of the Songliao Basin

Fig.4 Microstructure of the fabric of Gulong shale from the Qingshankou Formation

Fig.5 Arrangement of the clay-organic aggregate and its particles

Fig.6 Algal debris-clay aggregates

Fig.7 Organic carbon distribution of 4 Wells in the Qingshankou Formation of Gulong Sag

Fig.8 Backscattering diagram and energy spectrum diagram of the Qing 1 section of the Guye2HC well

Fig.9 Histogram of the energy spectrum analysis

Table 1 Analysis results of the energy spectrum of the Qing 1 section of Well Guye 8

No.	Mass fraction (%)
No.	C	O	Na	Mg	Al	Si	K	Fe
70	41.12	21.54	0.31	0.61	9.31	17.56	5.15	2.31
71	35.66	23.55	0.30	0.58	10.95	20.03	5.79	2.55
72	40.83	13.53	0.33	0.58	11.13	22.84	7.19	3.02
73	50.95	20.26	0.83	0.35	5.56	15.22	2.42	1.66
74	56.44	18.07	0.89	0.25	4.19	12.99	1.72	2.18
75	26.41	32.72	4.84		7.50	23.59	0.26	3.63
76	16.99	33.35	4.11		10.53	32.09	2.08	0.85
77	60.47	12.55	1.11	0.12	3.12	13.24	0.98	5.39
78	41.42	16.36			2.08	37.15	2.34	0.65
79	31.26	19.41	0.54	0.55	8.05	34.64	3.78	1.77
82	72.24	15.66	0.46	0.13	1.70	6.56	0.82	0.74
80	31.84	19.42	0.28	0.81	11.65	27.72	5.78	2.50
81	82.69	10.77	0.68		0.63	2.17	0.24	0.36
83	44.19	21.45	0.51	1.21	6.72	13.73	3.43	3.68
84	53.87	21.30	0.39	0.39	5.33	13.99	2.78	1.31
85	80.18	7.19	0.29	0.14	2.39	5.72	1.29	1.26
86	91.46	0.21	1.51	3.70	0.44	0.05	0.76	1.36
87	89.24		0.28	0.12	1.80	4.42	0.90	1.21
Average	52.63	18.08	1.04	0.68	5.73	16.87	2.65	2.02

Fig.10 Nanofissures and organic material in clay

Fig.11 Image analysis of nano fissures between clay domains

Fig.12 Correlation of the area shrinkage rate of algae and vitrinite with temperature and R o [ 1 ]

Fig.13 Field emission scanning electron microscopic characteristics of the shale from member 1 of the Qingshankou Formation in the Gulong area in different hydrocarbon generation stages[28] (a) Stratified shale, face rate 1.7%. Ro=0.58%; (b) Stratified algea, face rate 5.1%. Ro=0.83%; (c) Stratified algea, face rate 14.5%. Ro=1.32%; (d) Stratified shale, face rate 18.4%. Ro=1.40%; (e) Contraction fractures formed by hydrocarbon generation from organic matter; (f) Image analysis of the pores.

Fig.14 Nanopores and micropores in kerogen (a) A few circular and nearly circular nanopores, which could have been formed by filling of liquid hydrocarbon, were developed in the center of dry collateral roots. (b) Numerous circular and nearly circular nanopores developed in the center of kerogen, which made the kerogen appear spongy, and they were likely formed by the filling of liquid hydrocarbon. (c) A small number of nearly round pores developed in the vitrinite, which could also have been formed by liquid hydrocarbon charging. (d) A small number of nearly round pores developed in the structure vitrinite, which could also have been formed by the filling of liquid hydrocarbon.

Fig.15 Schematic diagram of wormlike microfissures and organic matter filling the fissures

Fig.16 Development of a large number of dewatering desiccation pores and desiccation fissures

Fig.17 (Solid) bitumen in the large (nano) pores (a) Authigenic pyrite, authigenic clay (chlorite), and (solid) bitumen in intergranular pores. An evident fissureless connection was observed between the bitumen and mineral particles, indicating that the bitumen was liquid during filling and subsequently converted into a solid. The existence of a few isolated pores (blue arrows) was also observed. Two/three well-crystallized dolomite particles (orange dotted line) were suspended in the bitumen. (b) Intragranular nanopores (yellow arrows in authigenic albite (Al) filled with asphalt.

Fig.18 Correlation between pressurization (a) or resistances (b, c) of shale oil and gas and organic matter Ro in the Gulong area[28]

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