Introduction
1. Formation and evolution of paleo-tectonic and sedimentary environment in the South Atlantic and its adjacent regions
Fig. 1. Distribution map of passive continental margin basins and large oil and gas fields on both sides of the South Atlantic. |
Fig. 2. Stratigraphic correlation profile of the basins on both sides of the South Atlantic. |
1.1. Tectonic-sedimentary environment during the rift period in the Early Cretaceous
1.1.1. Berriasian-Barremian intracontinental rift stage
1.1.2. Aptian-Albian intercontinental rift-initial drift-related transition stage
1.2. Tectonic-sedimentary environment during the drift-related depression stage since the Late Cretaceous
1.2.1. Late Cretaceous-Paleogene drift-related marine transgressive depression stage
1.2.2. Neogene-Quaternary drift-related marine regressive depression stage
2. Types and geological characteristics of passive continental margin basins in the South Atlantic tectonic domain
Table 1. Classification and geological characteristics of passive continental margin basins on both sides of the South Atlantic |
| Basin types | Classification criteria | Sedimentary filling characteristics in drift period | Typical basins | ||||
|---|---|---|---|---|---|---|---|
| Main types | Sub-types | Basin architecture | Structure feature | Delta | Gravity flow deposits | Salt rock and carbonate rock | |
| Rift- continental margin depression composite | Salt-free rift-continental margin gravity flow composite type | Dual rift-depression layers, with sedimentary thicknesses greater than 3 500 m and 4 000 m, respectively, high geothermal gradient. | No salt structure | Mid- small | Medium channel- slope fan system | Not developed | Orange |
| Salt-free rift-continental margin delta composite type | Large | Large landslide/Channel- Submarine fan | Pelotas | ||||
| Salt-bearing rift-continental margin gravity flow composite type | Dual rift-depression layers, with sedimentary thicknesses greater than 3 500 m and 4 500 m, respectively. | Developed salt structures | Mid- small | Medium channel-slope fan system | Salt and carbonate rocks are developed in the lower part of the depression. | Santos | |
| Salt-bearing rift-continental margin delta composite type | Large | Large landslide/Channel- Submarine fans | Lower Congo | ||||
| Continental margin depression- dominated | Gravity flow-dominated continental margin depression type | The rift layers are not developed, and the sedimentary thickness of the depression layers are greater than 4 000 m. | Transform faults controlled basin formation, with narrow shelf, steep slope, salt-free structures. | Small | Mid-small skirt-shaped submarine fan | Not developed | Cote d'Ivoire, Guyana |
| Delta dominated continental margin depression type | Mainly composed of modified strata formed by highly constructive deltas since the Miocene. | From land to sea, four major annular tectonic zones: growth fault, plastic diapir, thrust fold, and foredeep gentle slope. | Large | Mid-large landslide/ Channel- Submarine fan | Not sure | Niger Delta, Foz do Amazonas | |
2.1. Salt-free rift-continental margin depression composite type
Fig. 4. 2D seismic interpretation section of passive continental margin basins of salt-free rift-continental margin depression composite type (see the location of the seismic section in Fig. 1). |
2.2. Salt-bearing rift-continental margin depression composite type
Fig. 5. Geological sections of passive continental margin basins of salt-bearing rift-continental margin depression composite type (see Fig. 1 for section locations). |
2.3. Gravity flow-dominated continental margin depression type
Fig. 6. 2D seismic interpretation sections of gravity flow-dominated passive continental margin basins in a continental margin depression (the locations of the seismic sections are shown in Fig. 1). |
2.4. Delta-dominated continental margin depression type
Fig. 7. Seismic-geological section and hydrocarbon accumulation pattern of large oil and gas fields in a delta-dominated passive continental margin basin in Niger. |
3. Orderly hydrocarbon accumulation with feature of being segmented along-strike, zoned across-strike, and layered vertically
3.1. Salt-free composite basins develop two interconnected plays
Fig. 9. Diagram of accumulation pattern of large oil and gas fields in a passive continental margin basin of salt-free rift-continental margin depression type. |
3.2. The salt-bearing composite basins contain two independent giant petroleum systems
Fig. 10. Diagram of accumulation pattern of large oil and gas fields in a passive continental margin basin of salt-bearing rift-continental margin depression type (modified from reference [35]). |
3.3. Large submarine fan oil and gas reservoirs are developed in gravity flow-dominated continental margin depression basins
Fig. 11. Accumulation pattern of large oil and gas fields in a passive continental margin basin of gravity flow-dominated continental margin depression type. |
3.4. Delta-fan complex hydrocarbon accumulation belts in delta-dominated continental margin depression basins
4. Exploration directions for large oil and gas fields
Table 2. Exploration directions for large oil and gas fields in the passive continental margins on both sides of the South Atlantic |
| Segments | Basin types | Main basins | Plays (lower/upper tectonic layers) | Major exploration targets | Evaluation on prospects |
|---|---|---|---|---|---|
| Southern segment | Salt-free rift- continental margin gravity flow composite type | Colorado, Valdes, and Falkland | Rift system volcanic rock formation/continental margin depression submarine fan | Fault blocks, buried hills, and fault-lithological complex traps within marine transgressive mud shale sections at the end of the rifting; slope fans/submarine fans during the drift-related depression period adjacent to the source rock center. | The shallow-water rift strata hold the greatest potential; the submarine fans surrounding the center of the Aptian- Cenomanian source rocks need further exploration. |
| Salt-free rift- continental margin delta composite type | Pelotas (deep-water area) | Rift system/continental margin depression delta-submarine fan | The rift system in lower tectonic layer of the deep-water area, and the deltas-submarine fans in upper tectonic layer. | The exploration level is limited, and both plays are worth exploring. | |
| Central segment | Salt-bearing rift- continental margin gravity flow composite type | Brazil's east coast: Santos, Espirito-Santo; West Africa coast: Kwanza, Gabon-Douala | Pre-salt lacustrine carbonate rocks/post-salt deep-water gravity channel-lobe | Large-scale pre-salt deep-water inherited structural traps and deep-water fans in the western post-salt depression of the Santos Basin; pre-salt carbonate rocks in the Kwanza Basin; post-salt and pre-salt channels and lobes in the Gabon-Douala Basin. | The preservation conditions are excellent, and large oil and gas fields can be formed in both pre- and post-salt layers; the pre-salt layers in Santos Basin and the Kwanza Basin, and the post-salt layers in the Gabon Basin are the keys. |
| Salt-bearing riftcontinental margin delta composite type | Lower Congo | Pre-salt carbonate rocks/post-salt fan delta + deep-water gravity flow fan | Pre-salt carbonate rock traps; deep-water gravity channel-fan in the central diapiric tectonic zone. | Numerous discoveries have been made, and the prospects are optimistic; the post-salt fan and the pre-salt carbonate rocks are equally important. | |
| Northern segment | Gravity flow- dominated continental margin depression type | Cote d'Ivoire, Guyana- Suriname sea area, and Piaui-Ceara | Upper Cretaceous skirt- shaped gravity flow submarine fans in drift-related depression period/isolated carbonate platforms in transitional period. | Upper Cretaceous submarine fans; isolated platform carbonate rock formations. | Skirt-shaped submarine fans are preferred; isolated platforms can form large oil and gas fields. |
| Delta-dominated continental margin depression type | Niger Delta | Delta sand bodies/deep- water to ultra-deep-water gravity flow channel-fan | Shallow-water deltaic sand bodies along growth fault zones; deep-water to ultra- deep-water lithological traps. | Oil and gas enrichment has been confirmed, with excellent prospects; refined potential mining in shallow-water area and lithological reservoirs in deep-water area are new targets. | |
| Foz do Amazonas (Northern Brazil) | Similar to the Niger Delta, gravity flow submarine fan | Undrilled deep-water areas: Upper Cretaceous, Paleogene, and Miocene gravity flow submarine fans. | The hydrocarbon accumulation conditions are similar to those of the Niger Delta Basin, with great potential in deep-water areas. |