Source Rock Characteristics and Its Accumulation Contribution in the Lower Congo Basin, South Atlantic

doi:10.19657/j.geoscience.1000-8527.2023.064

Abstract

Abstract:

The Lower Congo Basin, located along the African coast of the South Atlantic, is a petroliferous basin characterized by the salt tectonic development, which is composed of the Mesozoic rift basin and the Cenozoic passive continental margin basin.Based on two-dimensional seismic, well logging and geochemical data, combined with previous exploration and research results, we defined the types and sedimentary environments of source rocks in this basin, and clarified the source rock development characteristics and revealed their contributions to hydrocarbon accumulation.The study was conducted through thermal evolution simulation of source rocks, anatomy of typical hydrocarbon reservoirs, and analysis of the temporal-spatial distribution characteristics of hydrocarbon reservoirs.The results show that lacustrine source rocks are developed in the Bucomazi Formation under the salt layer, and marine source rocks are developed in the Iabe and Landana Formations above the salt layer.Lacustrine source rocks are developed in dry and reducing lacustrine basin environment with high organic matter content.Marine source rocks have large cumulative thickness, and developed in (semi-)abyssal environment.After the Miocene, the Bucomazi Formation source rocks entered the gas generation stage, and the Iabe Formation source rocks entered the oil generation stage, providing the main hydrocarbons in the region.The type and distribution of hydrocarbon reservoirs in the Lower Congo Basin are affected by multiple factors, such as the source rocks, faults and salt tectonics, among which the maturity and distribution of source rocks are the key ones.The subsalt oil and gas reservoirs are mostly formed by the late supply of lacustrine source rocks near the shore, while the soil and gas reservoirs above the salt were formed by the multi-stage supply of hydrocarbon from multiple source rocks.

Key words: Lower Congo Basin, salt rock, source rock, thermal evolution simulation, accumulation charac-teristics

CLC Number:

P618.13

YANG Xiongbing, WANG Hongyu, SU Yushan, GUAN Chao. Source Rock Characteristics and Its Accumulation Contribution in the Lower Congo Basin, South Atlantic[J]. Geoscience, 2023, 37(05): 1369-1384.

Figures/Tables 14

Fig.1 Regional location and major faults distribution of the Lower Congo Basin from IHS database

Fig.2 Comprehensive stratigraphic column of the Lower Congo Basin [27-28]

Fig.3 Tectonic evolution model of Lower Congo Basin (seeing A-A’ in Fig.1 for location)

Fig.4 Climate conditions in the separation of African and South American plates [40]

Fig.5 Sedimentary characteristics of each period in the central region of South Atlantic[41]

Table 1 Geochemical characteristics of main source rocks in the Lower Congo Basin [34]

深度(m)	层位	总有机碳 TOC(%)	生烃潜力 S₂(mg/g)	氢指数	氧指数	T_max均值 (℃)	生烃指数 (S₁/S₁+S₂)
1200	Lanada	1.5~2.4	1.8~6.8	100~235	23~118	429	0.03~0.06
1400	Lanada	1.4~2.2	2.4~5.0	116~199	30~48	430	0.04~0.08
1600	Lanada	1.6~2.2	2.5~8.2	122~303	21~55	428	0.05~0.07
1800	Iabe	3.4~5.0	39.3~46.5	>400	8~30	424	0.03~0.05
2000	Iabe	3.2~4.8	13.2~28.8	343~513	23~38	427	0.03~0.05
2200	Iabe	2.1~3.9	7.5~21.4	275~432	33~51	433	0.05~0.10
2300	Iabe	2.0~2.8	6.0~15.3	253~378	33~48	435	0.04~0.11
2400	Bucomazi	0.8~1.8	2.7~9.8	277~428	57~108	430	0.10~0.12
2500	Bucomazi	1.2~2.8	7.5~17.2	>480	42~94	431	0.09~0.12
2600
2800	Bucomazi	3.2~4.8	>12.3	>400	48~105	433	0.09~0.11
3000
3200	Bucomazi	1.3~2.7	4.2~10	317~482	62~103	436	0.09~0.17
3400	Bucomazi	1.5~4.1	5~13.1	126~285	30~76	445	0.05~0.09
3600	Bucomazi	2.0~4.1	2.5~10	131~320	38~101	441	0.05~0.10
3800

Fig.6 Intersection diagram of hydrogen index (HI) and pyro-lysis temperature peak (Tmax) of source rocks in the Lower Congo Basin [42]

Fig.7 Depth vs.geothermal intersection diagram of land and sea area in the Lower Congo Basin

Fig.8 Thermal evolution profile of the northern Lower Congo Basin (seeing B-B’ in Fig.1 for location)

Fig.9 Thermal evolution profile of the southern Lower Congo Basin (seeing C-C’ in Fig.1 for location)

Fig.10 Maturity distribution of main source rocks in the Lower Congo Basin at present

Fig.11 Plane distribution characteristics of oil and gas reservoirs in the Lower Congo Basin [41]

Fig.12 “Source rock and reservoir" combination relationship of the main reservoirs in the study area

Fig.13 Hydrocarbon accumulation model of the Lower Congo Basin

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