加蓬盆地盐下裂谷期碎屑岩储层地球化学特征及物源分析

doi:10.19657/j.geoscience.1000-8527.2023.044

摘要/Abstract

摘要：

加蓬盆地盐下储层物源主要来自上地壳,以长英质岩石为主,主要为元古宙汇聚及俯冲构造背景下的变火山-沉积岩组成的古老变质岩系,同时具有一定量碱性玄武岩和花岗岩的混合。盐下Gamba组及Dentale组地层物源碎屑成分及微量元素与源区构造背景继承性好,不同地层差异不明显。主量元素、微量元素及稀土元素特征值分析表明盐下Gamba组及Dentale组物源主要来自具有沟-弧-盆体系的活动大陆边缘的汇聚及俯冲带,具有低SiO₂、Na₂O的特征,与刚果克拉通西缘元古宙大陆岛弧背景沉积的基麦赞超群、马永贝超群及西刚果超群等变沉积-火山岩系等有亲缘关系。盆地盐下Gamba组及Dentale组储层具有较高的稀土总量,稀土元素配分模式与刚果克拉通元古宙的西刚果造山带的外部带和中间褶皱带具有亲源性,但与南美圣弗朗西斯科克拉通元古宙—太古宙不同构造活动带差异较大,表明它们不是盆地的主要物源,说明盆地盐下Gamba组及Dentale组物源主要来自刚果克拉通。

关键词: 加蓬盆地, 盐下碎屑岩储层, 主量元素, 微量元素, REE配分模式

Abstract:

Provenance of the subsalt reservoir in the Gabon basin mainly comes from the upper crust, mainly felsic rocks, together with the ancient metavolcanic-sedimentary rocks under Proterozoic convergence and subduction tectonic setting. There is certain amount of mixing of alkaline basalt and granite, with good inheritance of provenance composition and tectonic background of the source region, and no obvious difference. Differences in major and trace element compositions in Gamba and Dentale formations are unobvious. Analyses of major elements, trace elements and rare earth elements show that the provenance mainly comes from the convergence and subduction zones of active continental margin, with trench-Arc basin system. They are featured by low SiO₂ and Na₂O, similar to the Katanga supergroup and Pore group deposited in Proterozoic back-Arc basin on the western margin of Congo Craton. The Delonggu group is related to each other. The sub-salt reservoirs in the basin have high total REE content. The REE distribution pattern is related to the Proterozoic Lufili orogenic belt on the western margin of Congo Craton, but differs greatly from the Proterozoic Archean tectonic belts of San Francisco Craton in South America. This indicates that they are not the main provenance of the basin, and that the provenance is mainly the Congo Craton.

Key words: Gabon basin, subsalt clastic reservoir, major element, trance element, REE pattern

中图分类号:

P588.21

陈全红, 阳怀忠, 赵红岩, 张科, 黄海滨, 郭家铭, 刘新颖, 袁野. 加蓬盆地盐下裂谷期碎屑岩储层地球化学特征及物源分析[J]. 现代地质, 2023, 37(05): 1385-1397.

CHEN Quanhong, YANG Huaizhong, ZHAO Hongyan, ZHANG Ke, HUANG Haibin, GUO Jiaming, LIU Xinying, YUAN Ye. Geochemical Characteristics and Provenance Analysis of Clastic Reservoirs in Sub-Salt Rift Period in Gabon Basin[J]. Geoscience, 2023, 37(05): 1385-1397.

图/表 15

图1 加蓬盆地构造单元划分及地层格架图

Fig.1 Structural unit division and stratigraphic framework of the Gabon basin

图2 刚果克拉通和南美圣弗朗西斯科克拉通碰撞造山及主要构造带示意图(据文献[6??-9]修改)

Fig.2 Collision orogeny and main tectonic belts of the Congo Craton and San Francisco Craton in South America(modified after references [6??-9])

图3 加蓬盆地Gamba组和Dentale组储层岩石组分三角图(L1、L2和N1分别为取自L1井、L2井和N1井的样品,钻井位置见图1;下文同)

Fig.3 Ternary petrological discrimination plot for the Gamba and Dentale Formation reservoir in the Gabon basin

图4 加蓬盆地Gamba组和Dentale组储层岩屑组分百分含量直方图

Fig.4 Debris composition histogram for the Gamba and Dentale Formation reservoir in the Gabon Basin

表1 加蓬盆地盐下碎屑岩储层主量组分统计结果

Table 1 Contents of major elements of subsalt clastic reservoir in the Gabon basin

地层及构造环境		SiO₂	TiO₂	Al₂O₃	TFe₂O₃	MnO	MgO	CaO	Na₂O	K₂O	P₂O₅	LOI	TFe₂O₃+ MgO	Al₂O₃/ SiO₂	K₂O/ Na₂O	Al₂O₃/ (CaO+Na₂O)	TFe₂O₃/ K₂O
Gamba 组	L1(6)	52.2	00.89	15.05	5.68	0.04	3.32	1.90	0.47	4.38	0.32	11.41	09.00	0.29	10.35	06.43	1.30
	L2(9)	47.03	00.68	12.34	4.77	0.07	3.07	5.00	0.59	3.35	0.28	16.40	07.84	0.27	05.84	02.91	1.42
	N1	52.94	00.90	15.05	5.27	0.08	2.63	2.09	1.23	4.12	0.29	10.71	07.90	0.28	03.34	04.53	1.28
	Tsi(2)	64.52	0	21.54	3.73	0	4.17	0.33	0.46	4.65	0	-	07.90	0.34	10.13	28.36	0.80
Dentale 组	L1	56.02	0.81	14.70	4.74	0.05	2.80	1.98	0.47	5.25	0.30	08.91	07.54	0.26	11.11	06.00	0.90
	L2	53.61	0.74	13.16	4.49	0.03	2.41	2.20	1.03	4.31	0.28	11.22	06.90	0.25	04.20	04.08	1.04
	N1(6)	53.86	0.86	13.90	5.78	0.06	2.67	2.26	1.61	4.11	0.32	09.99	08.45	0.26	02.57	03.60	1.41
	Oba(2)	68.09	0	13.10	3.55	0	7.18	0	0.46	5.29	0	-	10.73	0.22	09.40	26.59	0.67
OIA	58.83	01.06	17.11	01.95	0.15	3.65	5.83	4.10	1.60	0.26	5.52	11.73	00.29	0.39	01.72	01.22
CIA	70.69	00.64	14.04	01.43	0.10	1.97	2.68	3.12	1.89	0.16	3.05	06.79	00.20	0.61	02.42	00.76
ACM	73.86	00.46	12.89	01.30	0.10	1.23	2.48	2.77	2.90	0.09	1.58	04.63	00.17	1.05	02.46	00.45
PCM	81.95	00.49	08.41	01.32	0.05	1.39	1.89	1.07	1.71	0.12	1.76	02.89	00.10	1.60	02.84	00.77
上地壳	66	00.64	15.20	04.50	0.08	2.20	4.20	3.90	3.40	0.15	-	07.15	00.23	0.87	01.88	01.32

图5 加蓬盆地Gamba组和Dentale组储层Hark图解

Fig.5 Hark-type diagram of Gamba and Dentale Formation reservoir in the Gabon basin

图6 加蓬盆地Gamba组和Dentale组砂岩样品风化特征A-CN-K图(CIA=[Al2O3/(Al2O3+K2O+Na2O+CaO*)]×100,其中CaO*=CaO-(10/3P2O5);底图据文献[15])

Fig.6 Ternary A-CN-K diagram for evaluating sandstone weathering process of Gamba and Dentale formations in the Gabon Basin (basemap after reference [15])

图7 加蓬盆地Gamba组和Dentale组砂岩样品物源判别图解(F1=-1.773(TiO2)N+0.607(Al2O3)N+0.76(Fe2O3)N-1.5(MgO)N+0.616(CaO)N+0.509(Na2O)-1.224(K2O)N-9.09,F2=0.445(TiO2)N+0.07(Al2O3)N-0.25(Fe2O3)N-1.142(MgO)N+0.438(CaO)N+1.475(Na2O)N+1.426(K2O)N-6.681;底图据文献[16])

Fig. 7 Provenance discrimination diagram for sandstone of Gamba and Dentalein Formations in the Gabon basin (basemap after reference [16])

图8 加蓬盆地Gamba组和Dentale组砂岩样品Zr/Sc-Th/Sc图解

Fig.8 Zr/Sc-Th/Sc diagram of sandstone samples from Gamba and Dentale formations in the Gabon basin

图9 加蓬盆地Gamba组和Dentale组砂岩样品La/Sc-Co/Th图解

Fig.9 La/Sc-Co/Th diagram of sandstone samples from Gamba and Dentale formations in the Gabon basin

图10 加蓬盆地Gamba和Dentale组砂岩La-Th-Sc (a)、Th-Co-Zr/10 (b)、Th-Sc-Zr/10 (c)及Hf-La/Th (d)判别图解(底图据文献[11])

Fig.10 La-Th-Sc (a),Th-Co-Zr/10 (b),Th-Sc -Zr/10 (c) and La/Th-Hf (d) distinction diagrams of Gamba and Dentale sandstones(basemap after reference[11])

表2 不同构造环境微量元素及稀土元素参数特征

Table 2 Parameter characteristics of trace elements and REE in different tectonic environments

构造环境及层段		La	Ce	Nd	∑REE	LREE/HREE	δEu	(La/Yb)_N	La/Y	La/Yb
大洋岛弧	杂砂岩^*	8.72±2.5	22.5±35.9	11.36±2.9	58±10	3.8±0.9	1.04±0.11	2.8±0.9	0.48±0.12	4.2±1.3
大陆岛弧	杂砂岩^*	24.4±2.3	50.5±4.3	20.8±1.6	146±20	7.7±1.7	0.79±0.13	7.5±2.5	1.02±0.07	7.5±2.5
活动陆缘	杂砂岩^*	33.0±4.5	72.7±9.8	25.4±3.4	186	9.1	0.6	8.5	1.33±0.09	8.5
被动陆缘	杂砂岩^*	44.5±5.8	71.9±11.5	29.0±5.03	210	8.5	0.56	10.8	1.31±0.26	10.8
Gamba组	L1(7)	35.3	64.1	29.9	159	9.2	0.61	10.2	1.6	15.8
	L2(8)	43.1	82.2	36.6	210	9.2	0.72	9.4	1.7	14.5
	N1(1)	78.2	139	50	319	12	0.97	15.7	2.8	24.2
	Tsi(23)	6.2	10.2	4.6	26.1	8.4	0.81	8.9	1.6	13.8
Dentale组	L1(2)	37.3	68.5	32	169.2	9.4	0.67	10.2	1.6	15.8
	L2(2)	54.7	106	46.2	273.2	10.9	0.67	10.9	1.9	6.9
	N1(6)	58.9	110.7	43.5	263	9.7	0.95	11.3	2	17.4
	Oba(43)	9.4	15.7	7.3	39.9	9.4	0.79	11.1	1.8	17.1
构造环境及层段		Zr	Sc	V	Co	Ti	La/Sc	Ti/Zr	Sc/Cr
大洋岛弧	杂砂岩^*	96±20	19.5±5.2	131±40	186.3	4800±1200	0.55±0.22	56.8±2.14	0.57±0.16
大陆岛弧	杂砂岩^*	229±27	14.8±1.7	89±13.7	122.7	3900±600	1.82±0.3	19.7±4.3	0.32±0.06
活动陆缘	杂砂岩^*	179±33	8.0±1.1	48±5.9	101.7	2600±200	4.55±0.8	15.3±2.4	0.30±0.02
被动陆缘	杂砂岩^*	298±80	6.0±1.4	31±9.9	52.4	2200±600	6.25±1.35	6.74±0.9	0.16±0.02
Gamba组	L1(7)	176	11.7	74	15.7	0.48	3.1	-	0.16
	L2(8)	299	15.4	125	17.8	0.83	2.9	-	0.18
	N1(1)	301	19.8	157	25.5	-	3.9	-	0.17
	Tsi(23)	100.2	1.9	17.6	-	-	4	-	0.61
Dentale组	L1(2)	186	12.4	76	17.8	-	3	-	0.17
	L2(2)	354	16.3	133	15.4	-	3.4	-	0.17
	N1(6)	334.5	17.4	133.7	19.4	-	3.4	-	0.18
	Oba(43)	115.7	2.2	30.8	-	-	4.8	-	0.79

图11 加蓬盆地Gamba(a)和Dentale(b)组砂岩碎屑锆石年龄分布概率图

Fig.11 Probability plots of detrital zircon age of Gamba (a) and Dentale Formation sandstones (b) in the Gabon basin

图12 加蓬盆地Gamba和Dentale组砂岩样品在La/Yb-∑REE图解中的分布(底图据文献[23])

Fig.12 La/Yb- ∑REE diagram for Gamba and Dentale Formation sandstone samples in the Gabon basin (basemap after reference [23]))

图13 加蓬盆地与刚果克拉通及南美圣弗朗西斯科克拉通稀土元素配分模式((c)和(d)稀土元素数据分别来自文献[25]和[26]) (a)Gamba组L1井砂岩储层; (b)西非刚果卡拉通元古宙地层; (c)南美圣弗朗西斯科克拉通太古宙结晶基底; (d)南美圣弗朗西斯科克拉通太古宙弧后地层

Fig.13 Chondrite-normalized REE patterns of Gabon basin, Congo Craton and San Francisco Craton in South America (REE data of Fig.(c) and (d) are from references [25-26])

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