Migration direction and accumulation characteristics of paleo-oil in the Changxing Formation in Yuanba Gas Field, Sichuan Basin

doi:10.13745/j.esf.sf.2023.2.22

Abstract

Abstract:

The natural gas in the Permian Changxing Formation (P₂ch) in the Yuanba gas field is mainly derived from oil cracking. Study on the migration direction and accumulation characteristics of paleo-oil can provide valuable insights into the reservoir evolution and fluid contact distribution in the P₂ch reservoirs to guide natural gas prospecting and exploration in the region. In this paper, the paleo-oil charging period was first determined using fluid inclusions data, and the reservoir cap structure during oil charging was restored. Based on the reservoir cap structure the oil migration pathway was simulated, and the oil migration direction was traced using alkyl dibenzothiophene markers from solid reservoir bitumen. Finally, the paleo-oil accumulation characteristics and oil/gas reservoir evolution in the P₂ch reservoirs were analyzed. The main findings were (1) bitumen inclusions were widespread, and the associated brine inclusions had a homogenization temperature of 110-130 ℃. Combined with the burial history and thermal evolution data, the estimated paleo-oil charging period spaned from the Late Triassic to Early Jurassic. (2) The reservoir cap structure during reservoir deposition was retained during paleo-oil charging. (3) Paleo-oil migrated laterally from the platform margin slope to the reef-shoal traps, while the alkyl dibenzothiophene ratio for solid reservoir bitumen also gradually decreased along the migration direction consistent with lateral oil migration. (4) The degree of paleo-oil filling was higher in reef-shoal traps in the platform margin compared to lower positions. On the whole, the paleo-oil in the Yuanba gas field was characterized by near-source charging and oil accumulation in reef-shoal traps in the elevated platform margin. For paleo-reservoirs that had undergone high temperature thermal evolution, alkyl dibenzothiophenes may be used to trace the paleo-oil migration direction.

Key words: alkyl dibenzothiophene, solid bitumen, migration direction of paleo-oil, Changxing Formation, Yuanba gas field, Sichuan Basin

CLC Number:

P618.13

LI Pingping, WEI Guanglu, XU Zuxin, LI Bisong, PENG Yuqing, ZOU Huayao. Migration direction and accumulation characteristics of paleo-oil in the Changxing Formation in Yuanba Gas Field, Sichuan Basin[J]. Earth Science Frontiers, 2023, 30(6): 277-288.

Figures/Tables 11

Fig.1 Structural units in the northern Sichuan, location of the Yuanba gas field and general stratigraphic column. Modified after [19,25].

Fig.2 Top structural map, reef shoal traps and gas reservoir distribution of the Changxing Formation in the Yuanba gas field. Modified after [21].

Fig.3 Thickness map of source rocks of the Wujiaping Formation (a) and Dalong Formation (b) in the northern Sichuan Basin. a modified after [19,30];b modified after [30].

Fig.4 Microscopic characteristics of fluid inclusions in the Changxing Formation of the Yuanba gas field.

Fig.5 Homogenization temperature of fluid inclusions (a) and charging time of paleo-oil (b) in Changxing Formation of the Yuanba gas field. b adapted from [21].

Fig.6 Top structural map of the Changxing Formation at the deposition of the Late Feixianguan Formation (a) and paleo-oil charing in the Early Jurassic (b) in the Yuanba gas field.

Fig.7 Secondary migration pathways and accumulation areas of simulated paleo-oil in the Changxing Formation of the Yuanba gas field.

Table 1 Analysis results of aromatic hydrocarbons and nitrogenous compounds in solid bitumen samples of the Changxing Formation in the Yuanba gas field

编号	井位	深度/m	分子标志物比值
			4-/1-	2,4-/1,4-	4,6-/1,4-	4-/1-	1,8-/2,4-	1,8-/2,5	[a]/[c]
			MDBT	DMDBT	DMDBT	MDBF	DMC	DMC	[a]/[c]
1	元坝102	6 725.40	4.68	1.06	1.62	0.51	4.89	4.11	3.19
2	元坝11	6 773.50	4.06	0.87	1.36	0.35	8.79	5.53	8.98
3	元坝104	6 710.20	2.78	0.75	1.05	0.21	3.23	2.90	1.73
4	元坝204	6 548.50	5.30	1.08	1.71	0.28	4.16	4.52	2.39
5	元坝205	6 459.38	4.27	0.92	1.49	0.32	5.38	3.71	5.51
6	元坝2	6 580.50	4.17	0.87	1.46	0.29	3.69	3.59	2.04
7	元坝27	6 301.60	7.06	1.09	1.64	0.17	2.31	2.26	2.77
8	元坝27	6 294.02	4.60	1.19	1.70	0.25	3.24	2.78	2.22
9	元坝271	6 324.10	2.67	0.99	0.98	0.30	—	—	—
10	元坝271	6 328.01	3.15	0.84	1.20	0.28	0.61	4.26	—
11	元坝271	6 339.90	3.21	0.74	1.20	0.39	—	—	—
12	元坝12	6 692.45	4.55	0.86	1.44	0.34	4.31	3.38	—
13	元坝16	6 966.72	2.47	1.05	0.84	0.36	2.42	2.99	2.48
14	元坝224	6 635.46	4.70	0.88	1.45	0.35	4.21	4.57	3.55
15	元坝224	6 638.40	4.25	0.91	1.46	0.38	—	—	—
16	元坝224	6 648.87	6.23	1.06	1.54	0.03	1.62	2.41	—
17	元坝273	6 827.35	4.12	0.89	1.54	0.33	3.86	3.88	3.92
18	元坝28	6 807.10	4.01	0.70	1.39	0.38	6.51	2.58	2.71
19	元坝29	6 640.00	3.94	0.82	1.33	0.37	5.64	4.76	2.96
20	元坝9	6 907.87	5.10	0.88	1.59	0.66	4.43	2.00	—
21	元坝9	6 908.47	3.12	0.74	1.20	0.45	6.40	4.97	—

Fig.8 The simulated and traced migration pathway by carbazole (a) and dibenzothiophene molecular markers (b) in the Changxing paleo-oil reservoir of the Yuanba gas field.

Fig.9 Relationship of source-reservoir-cap assemblage and charging process of the Changxing Formation paleo-oil reservoir in the Yuanba gas field (section line is shown in Fig.2)

Fig.10 Distribution of bitumen and paleo-oil layers in the wells Yuanba 2 (a), Yuanba 123 (b), Yuanba 224 (c) and well Yuanba 102 (d) of the Changxing Formation in the Yuanba gas field (a and b adapted from [20], and the bitumen content of Well Yuanba 102 is based on the systematic cuttings analysis of 6710-6850 m)

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