(1. China Petroleum Exploration and Development Research Institute, Beijing100083; 2. Postdoctoral Mobile Station of China Geo University (Beijing), Beijing 100083)

The coastal basin in West Africa is a typical superimposed basin of rift and passive continental margin, which has experienced four stages of evolution: pre-rift stage, synrift stage, transitional stage and passive continental margin stage. Two sets of main high-quality source rocks, lacustrine mudstone and salt-Shanghai shale, are developed, and oil and gas are mainly concentrated in the overturned anticline delta and underwater fan clastic rock reservoirs and salt-related structural traps widely developed in Paleogene. Salt-rock related structures, overturned anticline structures and subsalt rift structures are the main exploration directions and fields in West Africa, and the areas with low exploration degree in the north and south sections of West Africa have certain exploration potential.

Keywords exploration potential; Main controlling factors of reservoir formation; West African basin

Reservoir-forming constraints and oil and gas exploration potential in coastal basins of West Africa

Dong 1, 2, Xue 1, Ping 1, Qing 1

(1. China Petrochemical Exploration and Development Research Institute, Beijing100083; 2. China Geo University, Beijing 100083)

West African coastal basins are typical rift and passive marginal basins, which have experienced four stages of structural evolution: pre-rift, synrift, transition and passive marginal. Pre-salt lacustrine shale and post-salt shallow marine shale are the main source rocks in the west coast basin. Oil and gas are mainly concentrated in delta and subfan clastic rocks of tertiary overturned anticlines and salt structural traps, which are exploration areas in West Africa. The low exploration basins along the coast of northern and southwestern Africa have certain oil and gas potential.

Keywords: The evolution of reservoirs in West Africa basin restricts the oil and gas potential

Africa's Atlantic marginal zone is more than 10000 kilometers long, and there are more than 60 basins with a total area of 763.8× 104km2. Rich in oil and gas resources, the proven recoverable reserves of oil are 105653 MMbbl, accounting for 42.5% of Africa's total reserves, and the natural gas reserves are 265.5TCF, accounting for 30% of Africa's total reserves [65433 West African countries have good diplomatic relations with China. At present, China's three major oil companies have exploration and development blocks in West Africa, which are currently concentrated in the Niger Delta, Gabon coast and Lower Congo basin in the north-central part of the southern section. Deepening the study on the main controlling factors and exploration potential of reservoir formation in West Africa basin will help to further broaden the field of exploration and development in West Africa and gain more investment opportunities.

1 regional geological background

The coastal basins in West Africa are similar in structure and evolution. They are all superimposed basins of rift and passive continental margin, and have experienced the evolution process of pre-rift stage, synrift stage, transitional stage and passive continental margin stage. Due to the difference in time series and tectonic background of the split between the North and South Atlantic, there are great differences in basin formation scale and sedimentary filling characteristics between the north and south stages of West Africa, which fundamentally determines the disparity in oil and gas enrichment in West Africa basins.

1. 1 presplitting stage

Residual sediments in the pre-rift stage are mainly distributed in northern West Africa. Northwest Africa experienced the pre-Hercynian extensional movement in the pre-rift stage, and the compression, transformation and destruction of Caledonian and Hercynian orogeny in the later stage. Pre-rift deposits include Precambrian-Devonian. According to seismic data interpretation, Pre-Mesozoic [2] developed in the deep sea of Senegal Basin, in which Cambrian is about 1200m thick, Ordovician sandstone is about 1400m thick, Silurian contains 400m high-quality shale source rocks distributed in North Africa, and Devonian is about 400m sandy mudstone stratum.

1.2 Synchronism

The development of the synrift stage in the coastal basins of West Africa is mainly controlled by the formation and evolution of the Mesozoic and Cenozoic Atlantic Ocean. From the end of Permian to the early Cretaceous, Pan-continent was affected by geothermal events and formed multi-stage rift in Africa. Among them, due to three thermal events, two rifts were formed in today's Atlantic Ocean, which eventually evolved into the Atlantic Ocean. The first thermal event was the eruption of the Camp mantle plume in the mid-Atlantic magmatic province 2 100 Ma ago, which formed a part of the Newark rift system at that time, and the rift basin group in northern West Africa began to deposit in rift stage on this basis [3]. The second thermal event is the explosion of the Karoo mantle plume. At 183Ma, the explosion of Karroo mantle plume along the southeast coast of Africa led to the division of Gondwana continent. During 180 ~ 133 Ma, rifts, including the South Atlantic Rift, were widely developed on the African continent and its margin. The third thermal event is the eruption of Tristan mantle plume in the southern South Atlantic (Figure 1). The eruption of Tristan mantle plume at 13 1Ma caused the South American plate to break away from the Africa-Arabia plate, and the South Atlantic began to expand from the eruption point of Tristan mantle plume, and the South American plate rapidly rotated away from the Africa-Arabia plate, while the African plate rotated 45 counterclockwise around the shallow mantle plume at 7° N,11e, and the rift valley along the west Africa coast.

Controlled by the formation and development of the Atlantic Ocean, the scale and sedimentary filling characteristics of rift basins in the north and south of West Africa are quite different. The exploration degree in the northern part of the synrift period is low. According to the USGS data, Triassic continental clastic rocks with a certain thickness were deposited during the non-rift period in northwest China, which were relatively thick at sea and thinned towards the land [2], and lacustrine hydrocarbon source rocks rich in organic matter may be developed. The formation of most rift basins in southern West Africa mainly began in Cretaceous, and they are a series of NW-trending faults and barriers restricted by NNW-trending and NEE-trending basement faults, which are arranged alternately in the east and west [5]. Influenced by NEE to transform fault, the fault depression staggered from north to south and uplifted laterally, forming a secondary basin. In the same rift stage, a set of non-marine strata was deposited in southern West Africa. The lithology at the bottom is conglomerate, feldspar, timely sandstone and mudstone, and gradually transitions to the interbedded interval of limestone, sandstone and mudstone. In the late stage, due to the strong extension of the newcomb floor, deep lake mudstone deposits developed, forming the most important set of source rocks under salt in the coastal basins of West Africa.

Figure 1 Mesozoic Pan-Palaeocontinental Mantle Plume Eruption and the Development and Evolution of Atlantic Coastal Basins.

(modified according to reference [4])

1.3 transition stage

Rifting in the northern and southern coastal basins of West Africa ended at the end of Triassic and the beginning of Late Cretaceous respectively. After that, the southern segment began to uplift, reverse and suffer denudation, forming a regional unconformity between rift sequence and transition sequence. Then, a set of marine conglomerate strata was deposited at the bottom of the flattened non-marine filled rift basin. On this transitional conglomerate layer, a large area of evaporite deposits began on the coast of West Africa. The transitional stage of the northern rift basin group is completely different from that of the southern rift basin group, that is, there is no regional uplift and erosion after the synrift period, and there is no obvious regional unconformity between salt rocks and synrift deposits. Salt rock is a set of sequences widely developed in the transitional stage of West Africa. It is not only a high-quality caprock of the subsalt reservoir-forming assemblage, but also an important factor for the development of the suprasalt assemblage structure, which is of great significance to oil and gas accumulation and enrichment in West Africa. The distribution of salt sequences in West Africa is quite different between the north and the south basins: ① the northern part was deposited in the Late Triassic and the southern part in the apter period at the end of the Early Cretaceous; (2) The southern section has a wide range of deposits, and the apter Stage deposits are in the range of about 100× 104km2 between Walvis Ridge and Cameroon [4], while there is a small salt basin in the northern section, and different depressions are isolated from each other; (3) The thickness of salt sequence in the southern section is thin, generally tens to hundreds of meters, while the thickness in the northern section is generally thick, with the maximum thickness reaching several kilometers.

1.4 passive continental margin stage

The tectonic background of the northern and southern basins in West Africa in the passive continental stage is similar, and the sediments are mainly in wide and gentle depressions, but the development time is different. The northern segment was in the passive continental stage in Jurassic, while the southern segment began to accept marine deposits in the late Cretaceous and began to evolve synchronously with the northern segment. The structure of passive continental margin stage is relatively stable. Due to the rapid deposition of the thick Upper Cretaceous and Paleogene, overturned anticlines and salt-rock related structures related to contemporaneous faults are widely developed in the basin [6]. The deposition of passive continental basins in West Africa is mainly controlled by the relative change of global sea level, the tectonic background of rift period and the supply of provenance. The sedimentary sequence of passive continental margin on salt can be divided into carbonate rock and clastic rock sequence from bottom to top. The carbonate sequence in the north of West Africa is well developed, with a sedimentary thickness of several kilometers, which is mainly related to the Jurassic relative sea level rise, while the carbonate strata in the south are undeveloped and relatively small in thickness. The clastic rock sequence above carbonate rocks is dominated by large transitional facies deposits such as deltas and underwater fans, which gather most of the oil in West Africa. The rift system formed by Cretaceous thermal events in West Africa is the main controlling factor for the development of large delta and underwater fan sedimentary system in West Africa coast. These rifts developed into some rivers in Paleogene and Neogene, and terrigenous debris entered the basin along these rivers to form huge transitional facies deposits, such as Niger Delta related to Niger River, Congo Fan related to Congo River and Ourunzhi Basin related to Ourunzhi River.

2. Main controlling factors of reservoir formation

2. 1 There are many sets of source rocks, which have great hydrocarbon generation potential.

There are many sets of source rocks such as Silurian, Jurassic, Cretaceous and Paleogene in the coastal basins of West Africa, among which Cretaceous and Paleogene source rocks are the main source rocks in West Africa (Figure 2).

Distribution and combination map of source rocks, reservoirs and caprocks in main basins along the coast of West Africa.

Cretaceous source rocks are developed in coastal basins of West Africa and are the most widely distributed source rocks in West Africa. Cretaceous source rocks are mainly open marine shale in the northern section of Aaiun Tarfaya Basin and Senegal Basin, and mainly lacustrine dark shale in the southern section. In southern West Africa, Cretaceous source rocks are usually very abundant. The organic carbon of lacustrine dark shale in Lower Congo Basin, Bukomaqi Formation in Neocombian, Melania Formation in Gabon Basin and A Bing Formation in Kuanza Basin can reach 30%, 20% and 6.3% respectively, and the type of organic matter is mainly Type II, which is now in the oil generation stage [7].

Paleogene source rocks are widely distributed in Niger Delta Basin, Lower Congo Basin and Kuanza Basin in the Gulf of Guinea, which is another set of main source rocks in West Africa. Compared with Cretaceous source rocks, Paleogene source rocks are poor in abundance and organic matter types. For example, prodelta facies of Akata Formation and open marine shale in Niger Delta Basin have TOC of 0.5% ~ 4.4%, with an average value of 1.7%, belonging to Class II-III organic matter. The huge sedimentary scale of Paleogene makes up for the shortage of source rocks and forms the largest oil reservoir in West Africa.

Silurian source rocks are mainly developed in Aaiun Tarfaya basin and Senegal basin in the north of West Africa, and this set of source rocks belongs to pre-rift period. According to the outcrop data, the source rock of Silurian black shale is about 40m thick, with organic carbon content of 1% ~ 5.5%, mainly amorphous kerogen, and Ro value of 0.9% ~ 1.3%. Silurian source rocks in the northern coastal basin of West Africa are similar to those in Gudamis basin and Tanezzuft basin of North Africa in origin, environment, quality and abundance.

Jurassic source rocks are mainly distributed in Aaiun Tarfaya basin, belonging to open sea shale deposits on the passive continental margin of the post-rift. The content of organic carbon is 1.47% ~ 2.49%, and the type of organic matter is type II. The Cretaceous Campeni period has matured and is currently in the angry stage.

2.2 Oil and gas enrichment of Mesozoic-Cenozoic reservoir-cap assemblage

There are more than 40 Mesozoic Jurassic-Neogene oil-producing layers in West Africa, which are divided into three sets of reservoir-cap combinations: subsalt, intersalt and suprasalt, with salt rocks and corresponding strata as the interface. Oil and gas are mainly concentrated in Cretaceous and Paleogene clastic reservoirs (table 1). Lower Cretaceous sandstone is the main reservoir in the subsalt reservoir-cap assemblage, which belongs to the continental river-lake sedimentary formation in rift period. Widely developed in the Lower Congo Basin, Gabon Basin, Kwanza Basin, Namibia Basin and coastal basins in southwest Africa, the reservoir has good physical properties, with porosity of 10% ~ 30% and permeability of (100 ~ 5000 )×. The Lower Cretaceous reservoir and its overlying regional salt rocks constitute a good reservoir-cap combination. Cretaceous-Neogene is the main salt reservoir-cap assemblage in the upper part of southern West Africa, and delta and underwater fan sandbodies in the passive continental margin stage of Niger Delta Basin, Lower Congo Basin and Kuanza Basin in the middle are the most developed. The Paleogene reservoirs in these basins are huge in scale and have good physical properties. For example, the Paleogene Agbada Formation in Niger Delta is 3500m thick, with porosity 15% ~ 40% and permeability (100 ~ 5000) ×10-3μ m2, which is the main oil-producing area in West Africa. The suprasalt assemblage in northern West Africa is mainly composed of carbonate reservoirs, which are small in scale, mainly distributed in Aaiun Tarfaya basin and Senegal basin in northern West Africa, and mainly composed of reefs. At present, the reserves found in carbonate reservoirs are very limited [9].

Table 1 distribution of proven recoverable oil and gas in west Africa

The distribution of inter-salt reservoir-cap assemblage is limited, only distributed in Lower Congo Basin and Kwanza Basin in central West Africa (table 1).

2.3 trap and hydrocarbon generation match

West Africa is dominated by structural traps. The characteristics of traps in different stages are quite different. The sub-salt structural layer is mainly controlled by the extensional structure in the rift period, mainly by normal faults, forming tilting fault blocks and fold structural traps. Due to denudation, the uplift area of rift system forms carbonate rock development area after denudation, which becomes the drape structure of overlying strata. The scale of traps in rift period is generally small, and no large-scale reserves have been found.

Salt-rock related structures and overturned anticline structures are the two most developed traps in the passive continental salt sequence in West Africa. Salt-related structures are mainly controlled by salt movement. In the process of salt dome piercing, the surrounding rock is strongly deformed, the strata around the salt core are tilted upward, and the strata at the top of the salt core are lifted to form anticlines, accompanied by complex graben fault systems. There are many types of traps related to salt rocks, mainly including compound anticline structure on salt dome, turtle back anticline trap, fault anticline on salt dome, compound graben system fault trap on salt dome, sand body updip pinchout formation trap and fault shielding trap, and lithologic pinchout compound trap (Figure 3). The salt structure moves from east to west, and the salt rock has little deformation at the outcrop of the basement adjacent to the land. The structure extends into the sea, and then it is a transitional structure to the west (related to the vertical movement of salt). Finally, complex structures related to salt extrusion deformation are developed in the western boundary of the basin.

Fig. 3 Salt-rock related structures in Lower Congo Basin

Reservoir-forming model of overturned anticline reservoir in Niger Delta.

Overturning anticline trap is another important trap type in salt, which is mainly developed in Paleogene of Niger Delta Basin. The formation of overturning anticline is related to the plastic flow of clay rock of Akata Formation deposited in prodelta facies, and it is a reverse traction structure formed by the growth fault bending the falling bedrock under the action of gravity or differential compaction. Generally, the structural amplitude is not large, and the trap area is small, mostly below 10 km2, distributed in the south of contemporaneous faults, such as simple overturning anticlines without dislocation, overturning anticlines formed by multiple faults and major contemporaneous faults. The latter is widely distributed in Niger Delta Basin, with about 70 oil fields (Figure 4).

Lithologic trap is a trap type with less reserves found at present, which is mainly divided into sandstone pinchout type and carbonate reef. The former is mainly developed in passive continental margin deltas and underwater fans in southern West Africa, while the latter is mainly developed in Mesozoic and Cenozoic carbonate rocks in northern West Africa.

There is a good matching relationship between the formation of traps in oil and gas areas in West Africa and the hydrocarbon generation period. The traps in West Africa mainly began to form in the Late Cretaceous and formed in the Paleogene-Neogene. In most basins, source rocks and traps enter the oil generation stage at the same time, and then continue to sink and bury. The oil generation period and oil and gas filling period are long and uninterrupted. Due to the structural stability of West Africa in the passive continental margin stage, oil and gas entered the overturned anticline and salt-rock related structures through faults and sediment transport systems, and many large primary oil fields were formed without reconstruction or adjustment.

3 Exploration potential analysis

The geological characteristics of oil and gas in the coastal basins of West Africa are similar, with superior reservoir-forming conditions, banded distribution of oil and gas, low overall exploration degree and huge exploration prospects. At present, China's three major oil companies have obtained several exploration and development blocks in West Africa.

3. 1 exploration potential of basins with high exploration degree in the Gulf of Guinea area

The Gulf of Guinea region in central West Africa has excellent petroleum geological conditions, and although it has a high degree of exploration, it still has high exploration potential.

Niger Delta Basin is the most abundant basin in West Africa. So far, 97,849 million barrels of proven recoverable oil and gas equivalent have been discovered. The basin has excellent oil source conditions and good reservoir performance, and the Paleogene-Neogene Mesozoic reservoir-cap assemblage is well developed and well matched. The main body of Niger Delta Basin has a high degree of exploration, but the relatively new deep-sea sedimentary belt has a low degree of exploration, and its petroleum geological conditions are similar to those of adjacent areas, so it is the first choice for further exploration.

Lower Congo basin is one of the oil and gas-rich areas in West Africa, and its proven resources are second only to Niger Delta basin. There are 232 oil and gas discoveries in the basin (offshore 198 and onshore 34), with a total proven oil and gas equivalent of 12 103MMbbl (offshore 165438+ onshore 974MMbbl). There are good reservoir-forming conditions under and above the salt in the basin, but at present, the main discoveries are concentrated in the passive continental margin sequence above the salt, and the exploration degree under the salt is relatively low, which has great exploration potential.

Gabon basin is divided into North Gabon sub-basin and South Gabon sub-basin. At present, oil and gas discoveries are mainly concentrated in the North Gabon sub-basin, with a relatively high degree of exploration, most of which are in the hands of western oil companies, and there are few opportunities for exploration and development. However, the offshore part of Nanjiapeng sub-basin and the offshore part of North Gabon sub-basin are relatively low in exploration degree due to years of war, but they have the basic conditions for oil and gas accumulation and have certain exploration potential, which are the key target areas for further evaluation and tracking.

3.2 Exploration potential of basins with low oil and gas discovery degree in the north and south of West Africa

3.2. 1 exploration potential of low oil and gas discovery basin in northern section

There are two basins in the north of West Africa, namely Laayoune-Tarfaya Basin and Senegal Basin, with basin areas of 33.28× 104km2 and104x0/04km2 respectively. The overall exploration degree of the two basins is very low. As of June 65438+February, 2006, 79 wells have been drilled in this basin, including 2 oil wells, 3 oil and gas display wells, and the rest are dry wells, so the success rate of exploration is very low. Only limited exploration activities have been carried out in most areas of Senegal Basin, and the drilling density is about one wildcat well per 10000km2 new area.

There are many reservoir-forming assemblages in northern West Africa, which have great exploration value with Mesozoic source rocks. In addition, according to the data of drilling and ground investigation, radioactive shale source rocks of the pre-rift Silurian Tanezzuft Formation are also developed in northern West Africa, which are widely distributed in North Africa and are the main source rocks of Muzuk Basin and Gudamis Basin, with an abundance of 2% ~ 20%. Therefore, it is speculated that the reservoir-forming assemblage with radioactive shale of Tanezzuft Formation as the source and Triassic salt rock as the regional cap rock may be another major reservoir-forming assemblage in North Africa.

3.2.2 Exploration potential of basins with low oil and gas discovery degree in the southern section.

The southwest coastal basin and Namibe basin in southern West Africa have a low degree of exploration. Only 80 wells have been drilled in the southwest coastal basin in the area of nearly 50× 104km2, and the Namibe basin is one of the few unexplored areas in the world. These two basins have similar formation and evolution characteristics with other oil-rich basins in West Africa. They all experienced pre-rift, synrift and late thermal subsidence stages, and developed lower Cretaceous source rocks to varying degrees, with similar reservoir-forming conditions. Especially in the coastal basins of southwest Africa, the sedimentary caprock in Orange sub-basin is tHIck (> 7000m), and the early Apudian anoxic shale of Lower Cretaceous is deposited in the South Atlantic, with a thickness of 40 ~ 146m, an organic carbon content of 1.6 1% ~ 2.6%, with a maximum of 25%, and a hydrogen index (hi The study of buried depth history shows that the apter (K 1) source rocks in Orange sub-basin are located in the oil source window in a large area, which has good source conditions. Existing discoveries and existing seismic data have identified many types of traps, including inclined fault blocks, stratigraphic pinchout and drape structures, especially the compaction and gravity slump structures widely developed in this area, so the Orange Sub-basin has the ability to form oil and gas fields.

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