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Control of salt structures on hydrocarbons in the passive continental margin of West Africa
PETROLEUM EXPLORATION AND DEVELOPMENT Volume 38, Issue 2, April 2011 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2011, 38(2): 196–202.
RESEARCH PAPER
Control of salt structures on hydrocarbons in the passive continental margin of West Africa Liu Zuodong1,2,*, Li Jianghai2 1. PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China; 2. School of Earth and Space Science, Peking University, Beijing 100871, China
Abstract: Salt basins along the passive continental margin of West Africa are becoming one of the most attractive areas for hydrocarbon exploration. The oil and gas discovered recently are related to salt structures. The salt structures are widespread in Anglo-Cameroon, the thickness of salt is up to 1 500 m and the average width is 300 km. These basins can be divided into two parts in horizontal direction: extensional zone and compressional zone, which extend 100-150 km and 100-200 km respectively. The extensional zone includes sealed titled zone, growth fault zone and diapir zone, and is characterized by tilted block, rollover structure and turtle structure anticline. The compressional zone is characterized by salt sheet, salt tongue, thrust fault and small folds. Oil and gas are distributed in both zones, oil and gas reserves in the extensional zone are a little larger than reserves in the compressional zone. Several types of salt related traps can be recognized, such as salt anticline, salt diapir and fault related trap, unconformity and structural-lithologic traps. Salt anticlines are the most potential exploration targets. Key words: West Africa; passive continental margin; salt structure; extensional zone; compressional zone
Introduction Salt structures or salt diapirs are defined as a kind of geological deformation body from the flow of salt rock, mudstone or other rocks whose densities are lower than overlying rock under the control of gravity/buoyancy and regional stress[13]. Most of salt structures in the world were developed in passive continental margins, rifts, aulacogens and rift basins[4]. However, few salt structures were developed in active continental margin basins. Salt structures are widely distributed in South America, North America, Africa, Europe and the Middle East. In recent ten years, there are many giant fields discovered in salt basins, such as Mexico Gulf, Lower Congo Basin, Gabon Basin, Campos Basin, Santos Basin, Persian Gulf, North Sea, which arouse extensive attention of petroleum geologists. Therefore, some giant salt basins especially salt basins in passive continental margins become the research focuses. Among which, passive continental margins in West Africa are considered as the most potential area in the future. The structural settings of the passive continental margins in West Africa have some similarities. The comparison of salt structures in each salt basin in discovered oil and gas fields and the discussion on the relationship between salt rock and hydrocarbon accumulation are beneficial to deeply understand the hydro-
carbon accumulation rule of salt basins in passive continental margins of West Africa, which is favorable for oil and gas exploration in these areas and providing guidance for exploration in salt basins in China.
1 Regional geological structure setting and evolution The salt basins along West Africa passive continental margins are concentrated in the Angola–Cameroon section including Duala Basin, South Gabon Basin, North Gabon Basin, Kwanza Basin. Bounded by Walvis Ridge, separated by a series of basins such as Walvis Basin and Luderitz Basin in the southern Namibia–South Africa, the salt basins include Duala Basin, South Gabon Basin, North Gabon Basin, Kwanza Basin, Lower Congo Basin, Namibia Basin[5] (Fig. 1). In this paper, we focus on the North Gabon Basin, South Gabon Basin, Kwanza Basin and Lower Congo Basin. The development of salt basins sourced from the Atlantic Ocean opening. The Walvis Ridge which blocked off sea water and the favorable climate are key factors for the formation of salt basins. In the early Cretaceous, rifting of Gondwana land resulted in opening of the South Atlantic from south to north[6], so rift basins developed along the West Africa passive continental margin. During the Neocomian-Barremian, a suit
Received date: 31 May 2009; Revised date: 13 Nov. 2010. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the State Major Basic Research Development Planning (973) Program (2009CB1219302). Copyright © 2011, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.
Liu Zuodong et al. / Petroleum Exploration and Development, 2011, 38(2): 196–202
Fig. 1
Salt basin section in the West Africa passive continental margin and the structural zonation
of lake source rock deposited in the rift basins. In the Aptian, the South Atlantic Ocean totally opened, water began to invade. The Walvis Ridge held back sea water. Sea water coming from the south invaded Angola–Cameroon periodically. In the Mid-Late Aptian, evaporation capacity exceeds rainfall capacity due to high temperature in the study area, so a suit of thick evaporation salt deposit distributed regionally was formed above continental facies and marine-continental transitional facies strata in the basins to the north of Walvis Ridge. Since the Cretaceous to Quaternary, the basins experienced intense salt structure movements, and different salt body has different thickness. The average thickness of salt body
reached 1 km and the average width was 250 km[7], with the maximum up to over 400 km. Gabon Basin, Lower Congo Basin and Kwanza Basin are three important oil and gas bearing salt basins in West Africa. The Aptian salt rock is the key factor for formation of large-scale salt structures in the three salt basins during the Late Cretaceous to the Cenozoic. In the Albian Stage of Late Cretaceous, a suit of shallow water carbonate rock deposited above the thick salt rock, then above which, marine shale deposited due to transgression in the Campanian–Maastrichtian stage of Late Cretaceous, resulting in pinching out of sediments seawards. In the Paleocene, sea level decreased, and continental sedimentary wedge mainly
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formed. In the Oligocene, due to large-scale regression and uplift of coast region in Tertiary, Africa continent had undergone intense erosion; terrigenous materials were carried to continental shelf and deep water basins. Basins began to deposit fast. Passive continental margin in West Africa extended once again to form a series of listric faults. In the Neogene, a large suit of shallow sea–deep sea facies clastic rocks of drift episode deposited. The maximal thickness of post-salt deposit reached over ten thousand meters.
2
Structural belt division in salt basins
In terms of regional stress, oil and gas bearing salt basins may be divided into two types: one is foreland basin formed in a compression environment, such as the Persian Gulf salt basin and Kuqa Basin[8]; the other is passive continental margin basin formed in an extension environment, such as the Mexico Gulf salt basin[9,10], passive margin basins in eastern coast of Brazil[11] and West Africa passive continental margin basins[12]. Basins between Cameroon and Angola in West Africa are characterized by developed thick salt layer of the Aptian stage. From the continental shelf toward the sea, it extended about 300 km long with thickness over one thousand meters. The thickness of overlying terrigenous sediments decreases continuously toward the sea. Salt layers in Lower Congo Basin, North Gabon Basin, South Gabon Basin and outer Kwanza Basin are mainly distributed in the continental shelf except the inner Kwanza basin’s salt layer distributed in the continent. Various salt structures formed under the effect of differential load and tensile stress, such as salt dome, salt stock canopy, salt-overhang, salt stock, salt roller, salt wall, salt anticline, salt-nappe, salt weld and mini-basin[4]. Salt basins can be divided into extensional zone and compressional zone in terms of salt tectonic type[13]. There is a transitional zone between two zones above with width of 20-30 km. But there is no evident boundary. So the transitional zone is discussed as a part of compressional zone in this paper. The average width of extensional zone is 100150 km and the width of compressional zone is 100250 km[14]. 2.1
Uphill extensional zone
The extensional zone is characterized by the development of tilted blocks, rollover structures and extensional diapirs. Most of developed salt structures are non-piercement type, such as turtle anticline and salt pillow with low maturity. But in the area with intense compressional stress, there are salt structures with high maturity such as salt wall and salt stock. In general, the extensional zone can be further divided into slope structural belt, growth fault belt and diapir structural belt[15,16]. The uphill structural belt is mainly tilted block. Normal faults in dense distribution downward converged to the detachment layer of evaporate rocks of Aptian Stage. Most of tilted blocks generated in the Aptian–Cenomanian Stage was sealed by the Upper Cretaceous strata, and only a few blocks
were still active in the Neogene. Active normal faults in this area led to salt roller structures. The growth fault belt experienced strong thin-skinned extension. Due to detachment of salt layer, this area developed many drift blocks, which appeared alternately with the Tertiary grabens. In the Lower Congo Basin, the spacing between listric faults (or drift blocks) is 540 km along the up-dip margin. The combined action of those drift blocks and salt rocks formed many complex structures[17]. Most of exogenic inclusions are foreign rocks which migrated along the slope to the present position and were captured. They not only destroyed the lateral continuity of deposit but also invaded into salt walls and split large salt walls into several small separate salt walls or salt diapers. Sometimes, they even subsided to the basement or near the fault block and contributed to the development of salt structures. In the diapir structural belt, extensional diapirs, turtle anticlines, salt pillows, a few columned diapirs, long wave folds, large salt walls and relative complex salt domes are developed. Although extensional diapirs situate in the extensional zone, they had undergone compression. Some scholars believed that almost all extensional diapirs undergone compression[1820]. Among which, some thought the stress in compressional area was parallel to the slope trend and transferred upwards along the slope to compress the extensional diapers; the other thought that, the extensional diapers resulted from the compression of upwards and downwards movement of raft structures which were formed under extension. 2.2
Downhill compressional zone
The downhill compressional zone was influenced by the compressional stress significantly. In general, the downhill compressional zone has a wider coverage than the extensional zone. The compressional zone is characterized by development of compressional diapers. In addition, multi-folds and reverse thrusts resulted from compressional stress were also developed. Generally we considered the compressional zone was formed firstly at the front edge of salt wedge, and it has two features compared to the extensional zone: salt layers become thicker suddenly and extensional diapers with compressional nature occur. So, this section is considered as the transitional zone between extensional zone and compressional zone. The scope and distribution of this transitional zone is closely related to the Atlantic hinge zone (Fig. 2). Controlled by the hinge zone, about 1525 km to the west of the hinge zone is usually the transitional zone from extensional zone to compressional zone. Under the control of compressional stress, salt slippage could form many different salt piercement structures, such as salt stock, salt wall, salt tongue and salt sheet. The salt stock is a kind of neck-shape salt diapir representing round or elliptic in plane; salt wall is a long and narrow salt diapir. Both salt tongue and salt sheet are formed when salt pierces the overlying rocks and extends laterally to form sheet-shape structures. Whether it is called salt tongue or salt sheet depends on the
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Fig. 2
Cross section and structure division of the salt basin
salt stem. If the sheet-shape structures joint together, we can call them salt canopies[2]. Folds of salt sheet or some small and deep basins[21] separated by a series of large salt wall are called as mini-basins. Compressional zone developed folds, thrust structures and compressional diapirs. It can be divided into three secondary zones in terms of the characteristics: compressional diapir zone, which is adjacent to the extensional diapir in slope extensional zone, where compressional diapirs and many long-wave folds (up to 30 km) are developed; middle zone, with intense compression, where thrust structures and short-wave (with wave length of 36 km) folds are developed. In the area close to thrust front, salt canopy, salt sheet and salt tongue emerged in turn. In addition, this area is possible to develop large salt walls associated with salt suspension; the third area covers a small range, which is the transitional zone between the compressional zone and the undeformed area, with moderate deformation, where mainly folds are developed. In general, the division of structural belts in the compressional zone is decided by transmission time and magnitude of compressional stress. However, there is no definite boundary but a nature transition. For example, salt sheets, salt canopies and salt tongues are not definitely limited within a certain area but they generally complied with the following rule: salt body is extended from uphill to downhill, and it is possible to joint the whole salt sheet into salt canopies finally.
3
Formation of salt structures
The development of salt structures in the passive continental margin salt basins of West Africa can be divided into three stages. Stage 1: Before the opening of the South Atlantic, the Aptian thick salt rocks were developed overlying early rift basins. In the Cretaceous, after the South Atlantic opened, the transitional continental crust cooled and subsided, basins received thermal subsidence deposit, with thick deposit overlying. The deposition activity was lasting until now[22]. Stage 2: The sediment was thinning seawards, so load of offshore salt rock was much lower than that of nearshore salt rock, salt migrated slowly down the slope[21], when extensional tectonics was developing. Downhill extraction and up-
hill extension progressed almost simultaneously. According to the records of some thrust faults[20], the compression of salt rock occurred in the Aptian or Cenomanian. Fault activities of basement and extensional thinning of upper-salt deposit can accelerate the salt flow[2325]. Stage 3: Sediment was migrating downwards along the slope, the extensional zone continued expanding downwards, and compressional diapirs appeared in the extensional zone. The compressional zone is shown as shortening and bidirectional development towards the uphill and downhill. In Middle Cretaceous, extension of the basin led to formation of salt swells and salt diapers[26]. In the extensional zone, the pattern and distribution of salt structures were controlled by detachment. As a result, the distribution of plastic and brittle rocks determines the structural deformation in the extensional zone. In early stage of extension, brittle strata formed grabens. Under a high strain rate, the extensional zone developed tilted blocks and reverse drag structures; under a low strain rate, grabens continued expanding, forming grabens and extensional diapers[27]. The gradient, sedimentation rate, salt thickness are the key factors of influencing salt structures in the extensional zone. Salt thickness and the gradient are positively correlated with the development of tilted blocks, and negatively correlated with the development of diapers[4,28]. The overlying sediments folded and thickened, the contraction of salt layers and faulting would cause strongly contraction and upwelling of salt diapirs. With extension continuing, plastic rocks thinned, which sealed normal faults and formed sealed tilted blocks. Transferring of compressional stress is related to the time and space. In the early stage of deformation, the compressional zone was shrinking; then the compressional zone developed towards both two sides. On one hand, near the region where compression began to form, downhill compressional diapers and extensional diapers resulted from compressional stress transferring upwards were developed. Diapir shape evolved into salt pillow–salt wall–salt suspension–pop-up structures with time[29,30]. Due to the relationship between sedimentation rate and gradient, the volume and height of downhill diapers were larger than those of uphill diapirs. At the downhill, more diapirs are developed at the point closer to
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the sea surface and most of them are suspension diapers. However, at the uphill, the diapir’s wave crest was covered by old rocks. On the other hand, compressional stress was transferred downwards, so salt rocks were folded, leading to thickening and thrusting of overlying rocks. The thicker the salt rock, the longer it deposited, the more intense the thrusting and folding. However, folds were formed after compression lasted for some time. When compressional stress was relatively moderate, multi-folds, thrusts, salt anticlines or narrow box folds were developed in the compressional zone; in the area with higher compressional stress, short-wave folds were developed, which were also controlled by overlying sediments. Under high compressional stress, sediment is easy to be captured by plastic salt layers with compressional syncline and form pop-up structures and lentoid growth synclines. Until now, there are no examples of identifying pop-up structure in seismic profiles, but we can infer that it should occur in the highly compressional zone. Thrust faults are the sigh of highest compressional stress, we considered the followed transitional zone between the compressional zone and the undeformed zone is the extension of the compressional zone.
4 Control of salt structures on hydrocarbon accumulation Growth faults and structural combination patterns generated from salt rocks and salt structures deposited in the Aptian–early Albian stage are key factors for controlling the distribution and type of post-salt turbidites. They played a very important role in the sedimentary type, distribution, trap and oil/gas accumulation in West Africa passive margin basins[30]. Salt movement led to different salt detachment deformations, which had impact on sedimentary facies and sand distribution and formed different structural traps and lithologic-stratigraphic traps. The thicker the salt layer and its overlying sediments, the more possible the salt movement. For example, salt deposit is 1 0002 000 m thick in Gabon Basin and has thick overlying sediments, so salt structures were easy to develop in this basin and they are very favorable for forming structural traps[28]. Among the discovered fields, salt-related fields are unevenly distributed in the salt basins. For example,
Fig. 3
salt related fields are distributed in the extensional zone in Gabon Basin; but in the Lower Congo basin, the compressional zone contains more salt related fields; the distribution of salt related fields is even in Kwanza Basin. In terms of oil and gas reserves, reserves in the extensional zone in salt-related fields are slightly more than those in the compressional zone, but without significant difference. Therefore, both extensional zone and compressional zone are favorable for hydrocarbon accumulation. Along the West Africa continental margin, structural traps formed from salt structures can be classified as follows: (1) Dome traps generated from salt pillow, salt anticline, turtle stucture and rollover anticline[2]. Due to salt piercement or thrust folding, overlying strata are also possible to form dome traps (Fig. 3a-d). Salt piercement enables oil and gas under the salt rock to migrate upwards to reservoir rocks along faults or fractures, resulting in oil and gas accumulation in a large scale. In recent years, most of oil and gas reservoirs discovered in deep water of the Lower Congo basin sourced from rollover anticlines. Dome traps have large scale and high oil and gas reserves, which are the most potential traps in West Africa. (2) Traps formed by salt piercement on two flanks. This kind of trap is also formed due to lateral sealing. Its difference from the conventional fault sealing lies in that it is sealed by dense salt rock (Fig. 3e). (3) Traps bounded by faults. The fault could be the normal fault generated by regional or local extension caused by salt arch. Sand-mud interbed led to lateral sealing and form traps. Hanging walls are reverse drag structures, which form rollover structures; lower walls are sealed laterally, mostly forming small traps. In addition, the hanging walls of growth faults usually developed secondary traps, or formed good fault sealing traps or complex traps (Fig. 3f and g). (4) Traps pinching out to either two sides or single side in the same tectonic movement. They could form lenticular sand reservoirs, lithological reservoirs sealed by salt plug or lithological-structural reservoirs[28] (Fig. 3h). (5) Unconformity traps. Salt diapirism is easy to form unconformity structures which provide space for hydrocarbon
Salt structures and traps
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accumulation (Fig. 3i). (6) When diapir is further developed, it will collapse and turn into coarse clastic deposit, which is good reservoir rock. If there is dense cap rock above, it could be a good trap (Fig 3j). (7) Mini-basins. Salt structures are favorable for the formation of sub-basins. This kind of sub-basins is also called as mini-basins, which are distributed in both the extensional zone and compressional zone and are favorable for sedimentation of sandstone. The oil and gas reservoirs discovered in the Mexico Gulf are mostly distributed in mini-basins. Salt flow not only forms many structures, lithological-stratigraphic traps and complex traps but also generates many complex fault systems which provide good passages for vertical short-distance migration of hydrocarbons. Salt rock is usually very dense and soluble in acid, and it has high viscoplasticity and good heat conductivity. It is good cap rock to seal syn-rift hydrocarbons under the salt rock. Moreover, it enables the oil window of pre-salt source rock to extend downwards, so the source rock is not over-mature or low-mature. Due to the heat conductivity of salt rock, the temperature of pre-salt strata is lower than that of the strata without salt rock overlying at the same depth. Low temperature prohibited the diagenesis of pre-salt reservoir rock. Therefore, the strata even at the depth over 6000 m have favorable porosity and permeability. This is also an important inspiration for salt structure exploration. The giant oilfields discovered in pre-salt reservoir rock in deep water of Mexico and Brazil are closely related to the salt rocks[27].
5
Conclusions
The opening of the Atlantic Ocean led to the development of a series of rift basins in the passive continental margin of West Africa. Due to the relatively sealed environment and dry climate, the periodical influx of seawater precipitated a thick layer of salt in these basins. The salt rock is over 1 000 m thick, distributed in 300 km on average. According to the differences of salt structural styles and stress mechanism, the salt basins can be divided into two tectonic zones: the uphill extensional zone and the downhill compressional zone. The latter has a wider range than the former. The uphill extensional zone can be subdivided into slope structural belt, growth fault and diapir structural belt, where tilted blocks and turtle structures are mainly developed. While the downhill compressional zone mainly developed folds, thrust structures and compressional diapers. The compressional zone is characterized by extensional diapers with compressional nature. And the extensional zone is characterized by sudden thickening of salt rock. The maximum stress in the compressional zone appears in the middle of compressional zone, where thrust faults are located. The development of salt structures can be divided into three stages. After the deposition of thick salt layer, due to differential load, salt detachment occurs under gravity, forming various salt structures. Controlled by extensional stress, the extensional zone was developed and expanded downwards
along slopes. In the compressional zone, the compressive stress transmitted to both sides respectively. The structural features in the extensional zone are mainly controlled by the gradient, sedimentation rate and salt thickness. The compressional zone transfer is related to the time and space, and the gradient and sedimentation rate is the key to influence the salt structures. Salt structures in the passive continental margin of West Africa formed a series of good traps, including dome traps generated from the development of salt anticline and salt pillow or salt piercing overlying salt rock; traps generated from lateral sealing of salt rocks or faults; unconformity traps resulted from salt diapers. Mini-basins and salt diapers resulted in collapse of overlying rocks, producing coarse clastic deposit, which provides a good place for hydrocarbon accumulation. In terms of oil and gas reserves, oil and gas reserves related to salt in the passive continental margin of West Africa are slightly higher in the extensional zone than in the compressional zone. Dome traps developed in the extensional zone are the most potential traps.
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RESEARCH PAPER
Control of salt structures on hydrocarbons in the passive continental margin of West Africa Liu Zuodong1,2,*, Li Jianghai2 1. PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China; 2. School of Earth and Space Science, Peking University, Beijing 100871, China
Abstract: Salt basins along the passive continental margin of West Africa are becoming one of the most attractive areas for hydrocarbon exploration. The oil and gas discovered recently are related to salt structures. The salt structures are widespread in Anglo-Cameroon, the thickness of salt is up to 1 500 m and the average width is 300 km. These basins can be divided into two parts in horizontal direction: extensional zone and compressional zone, which extend 100-150 km and 100-200 km respectively. The extensional zone includes sealed titled zone, growth fault zone and diapir zone, and is characterized by tilted block, rollover structure and turtle structure anticline. The compressional zone is characterized by salt sheet, salt tongue, thrust fault and small folds. Oil and gas are distributed in both zones, oil and gas reserves in the extensional zone are a little larger than reserves in the compressional zone. Several types of salt related traps can be recognized, such as salt anticline, salt diapir and fault related trap, unconformity and structural-lithologic traps. Salt anticlines are the most potential exploration targets. Key words: West Africa; passive continental margin; salt structure; extensional zone; compressional zone
Introduction Salt structures or salt diapirs are defined as a kind of geological deformation body from the flow of salt rock, mudstone or other rocks whose densities are lower than overlying rock under the control of gravity/buoyancy and regional stress[13]. Most of salt structures in the world were developed in passive continental margins, rifts, aulacogens and rift basins[4]. However, few salt structures were developed in active continental margin basins. Salt structures are widely distributed in South America, North America, Africa, Europe and the Middle East. In recent ten years, there are many giant fields discovered in salt basins, such as Mexico Gulf, Lower Congo Basin, Gabon Basin, Campos Basin, Santos Basin, Persian Gulf, North Sea, which arouse extensive attention of petroleum geologists. Therefore, some giant salt basins especially salt basins in passive continental margins become the research focuses. Among which, passive continental margins in West Africa are considered as the most potential area in the future. The structural settings of the passive continental margins in West Africa have some similarities. The comparison of salt structures in each salt basin in discovered oil and gas fields and the discussion on the relationship between salt rock and hydrocarbon accumulation are beneficial to deeply understand the hydro-
carbon accumulation rule of salt basins in passive continental margins of West Africa, which is favorable for oil and gas exploration in these areas and providing guidance for exploration in salt basins in China.
1 Regional geological structure setting and evolution The salt basins along West Africa passive continental margins are concentrated in the Angola–Cameroon section including Duala Basin, South Gabon Basin, North Gabon Basin, Kwanza Basin. Bounded by Walvis Ridge, separated by a series of basins such as Walvis Basin and Luderitz Basin in the southern Namibia–South Africa, the salt basins include Duala Basin, South Gabon Basin, North Gabon Basin, Kwanza Basin, Lower Congo Basin, Namibia Basin[5] (Fig. 1). In this paper, we focus on the North Gabon Basin, South Gabon Basin, Kwanza Basin and Lower Congo Basin. The development of salt basins sourced from the Atlantic Ocean opening. The Walvis Ridge which blocked off sea water and the favorable climate are key factors for the formation of salt basins. In the early Cretaceous, rifting of Gondwana land resulted in opening of the South Atlantic from south to north[6], so rift basins developed along the West Africa passive continental margin. During the Neocomian-Barremian, a suit
Received date: 31 May 2009; Revised date: 13 Nov. 2010. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the State Major Basic Research Development Planning (973) Program (2009CB1219302). Copyright © 2011, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.
Liu Zuodong et al. / Petroleum Exploration and Development, 2011, 38(2): 196–202
Fig. 1
Salt basin section in the West Africa passive continental margin and the structural zonation
of lake source rock deposited in the rift basins. In the Aptian, the South Atlantic Ocean totally opened, water began to invade. The Walvis Ridge held back sea water. Sea water coming from the south invaded Angola–Cameroon periodically. In the Mid-Late Aptian, evaporation capacity exceeds rainfall capacity due to high temperature in the study area, so a suit of thick evaporation salt deposit distributed regionally was formed above continental facies and marine-continental transitional facies strata in the basins to the north of Walvis Ridge. Since the Cretaceous to Quaternary, the basins experienced intense salt structure movements, and different salt body has different thickness. The average thickness of salt body
reached 1 km and the average width was 250 km[7], with the maximum up to over 400 km. Gabon Basin, Lower Congo Basin and Kwanza Basin are three important oil and gas bearing salt basins in West Africa. The Aptian salt rock is the key factor for formation of large-scale salt structures in the three salt basins during the Late Cretaceous to the Cenozoic. In the Albian Stage of Late Cretaceous, a suit of shallow water carbonate rock deposited above the thick salt rock, then above which, marine shale deposited due to transgression in the Campanian–Maastrichtian stage of Late Cretaceous, resulting in pinching out of sediments seawards. In the Paleocene, sea level decreased, and continental sedimentary wedge mainly
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formed. In the Oligocene, due to large-scale regression and uplift of coast region in Tertiary, Africa continent had undergone intense erosion; terrigenous materials were carried to continental shelf and deep water basins. Basins began to deposit fast. Passive continental margin in West Africa extended once again to form a series of listric faults. In the Neogene, a large suit of shallow sea–deep sea facies clastic rocks of drift episode deposited. The maximal thickness of post-salt deposit reached over ten thousand meters.
2
Structural belt division in salt basins
In terms of regional stress, oil and gas bearing salt basins may be divided into two types: one is foreland basin formed in a compression environment, such as the Persian Gulf salt basin and Kuqa Basin[8]; the other is passive continental margin basin formed in an extension environment, such as the Mexico Gulf salt basin[9,10], passive margin basins in eastern coast of Brazil[11] and West Africa passive continental margin basins[12]. Basins between Cameroon and Angola in West Africa are characterized by developed thick salt layer of the Aptian stage. From the continental shelf toward the sea, it extended about 300 km long with thickness over one thousand meters. The thickness of overlying terrigenous sediments decreases continuously toward the sea. Salt layers in Lower Congo Basin, North Gabon Basin, South Gabon Basin and outer Kwanza Basin are mainly distributed in the continental shelf except the inner Kwanza basin’s salt layer distributed in the continent. Various salt structures formed under the effect of differential load and tensile stress, such as salt dome, salt stock canopy, salt-overhang, salt stock, salt roller, salt wall, salt anticline, salt-nappe, salt weld and mini-basin[4]. Salt basins can be divided into extensional zone and compressional zone in terms of salt tectonic type[13]. There is a transitional zone between two zones above with width of 20-30 km. But there is no evident boundary. So the transitional zone is discussed as a part of compressional zone in this paper. The average width of extensional zone is 100150 km and the width of compressional zone is 100250 km[14]. 2.1
Uphill extensional zone
The extensional zone is characterized by the development of tilted blocks, rollover structures and extensional diapirs. Most of developed salt structures are non-piercement type, such as turtle anticline and salt pillow with low maturity. But in the area with intense compressional stress, there are salt structures with high maturity such as salt wall and salt stock. In general, the extensional zone can be further divided into slope structural belt, growth fault belt and diapir structural belt[15,16]. The uphill structural belt is mainly tilted block. Normal faults in dense distribution downward converged to the detachment layer of evaporate rocks of Aptian Stage. Most of tilted blocks generated in the Aptian–Cenomanian Stage was sealed by the Upper Cretaceous strata, and only a few blocks
were still active in the Neogene. Active normal faults in this area led to salt roller structures. The growth fault belt experienced strong thin-skinned extension. Due to detachment of salt layer, this area developed many drift blocks, which appeared alternately with the Tertiary grabens. In the Lower Congo Basin, the spacing between listric faults (or drift blocks) is 540 km along the up-dip margin. The combined action of those drift blocks and salt rocks formed many complex structures[17]. Most of exogenic inclusions are foreign rocks which migrated along the slope to the present position and were captured. They not only destroyed the lateral continuity of deposit but also invaded into salt walls and split large salt walls into several small separate salt walls or salt diapers. Sometimes, they even subsided to the basement or near the fault block and contributed to the development of salt structures. In the diapir structural belt, extensional diapirs, turtle anticlines, salt pillows, a few columned diapirs, long wave folds, large salt walls and relative complex salt domes are developed. Although extensional diapirs situate in the extensional zone, they had undergone compression. Some scholars believed that almost all extensional diapirs undergone compression[1820]. Among which, some thought the stress in compressional area was parallel to the slope trend and transferred upwards along the slope to compress the extensional diapers; the other thought that, the extensional diapers resulted from the compression of upwards and downwards movement of raft structures which were formed under extension. 2.2
Downhill compressional zone
The downhill compressional zone was influenced by the compressional stress significantly. In general, the downhill compressional zone has a wider coverage than the extensional zone. The compressional zone is characterized by development of compressional diapers. In addition, multi-folds and reverse thrusts resulted from compressional stress were also developed. Generally we considered the compressional zone was formed firstly at the front edge of salt wedge, and it has two features compared to the extensional zone: salt layers become thicker suddenly and extensional diapers with compressional nature occur. So, this section is considered as the transitional zone between extensional zone and compressional zone. The scope and distribution of this transitional zone is closely related to the Atlantic hinge zone (Fig. 2). Controlled by the hinge zone, about 1525 km to the west of the hinge zone is usually the transitional zone from extensional zone to compressional zone. Under the control of compressional stress, salt slippage could form many different salt piercement structures, such as salt stock, salt wall, salt tongue and salt sheet. The salt stock is a kind of neck-shape salt diapir representing round or elliptic in plane; salt wall is a long and narrow salt diapir. Both salt tongue and salt sheet are formed when salt pierces the overlying rocks and extends laterally to form sheet-shape structures. Whether it is called salt tongue or salt sheet depends on the
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Fig. 2
Cross section and structure division of the salt basin
salt stem. If the sheet-shape structures joint together, we can call them salt canopies[2]. Folds of salt sheet or some small and deep basins[21] separated by a series of large salt wall are called as mini-basins. Compressional zone developed folds, thrust structures and compressional diapirs. It can be divided into three secondary zones in terms of the characteristics: compressional diapir zone, which is adjacent to the extensional diapir in slope extensional zone, where compressional diapirs and many long-wave folds (up to 30 km) are developed; middle zone, with intense compression, where thrust structures and short-wave (with wave length of 36 km) folds are developed. In the area close to thrust front, salt canopy, salt sheet and salt tongue emerged in turn. In addition, this area is possible to develop large salt walls associated with salt suspension; the third area covers a small range, which is the transitional zone between the compressional zone and the undeformed area, with moderate deformation, where mainly folds are developed. In general, the division of structural belts in the compressional zone is decided by transmission time and magnitude of compressional stress. However, there is no definite boundary but a nature transition. For example, salt sheets, salt canopies and salt tongues are not definitely limited within a certain area but they generally complied with the following rule: salt body is extended from uphill to downhill, and it is possible to joint the whole salt sheet into salt canopies finally.
3
Formation of salt structures
The development of salt structures in the passive continental margin salt basins of West Africa can be divided into three stages. Stage 1: Before the opening of the South Atlantic, the Aptian thick salt rocks were developed overlying early rift basins. In the Cretaceous, after the South Atlantic opened, the transitional continental crust cooled and subsided, basins received thermal subsidence deposit, with thick deposit overlying. The deposition activity was lasting until now[22]. Stage 2: The sediment was thinning seawards, so load of offshore salt rock was much lower than that of nearshore salt rock, salt migrated slowly down the slope[21], when extensional tectonics was developing. Downhill extraction and up-
hill extension progressed almost simultaneously. According to the records of some thrust faults[20], the compression of salt rock occurred in the Aptian or Cenomanian. Fault activities of basement and extensional thinning of upper-salt deposit can accelerate the salt flow[2325]. Stage 3: Sediment was migrating downwards along the slope, the extensional zone continued expanding downwards, and compressional diapirs appeared in the extensional zone. The compressional zone is shown as shortening and bidirectional development towards the uphill and downhill. In Middle Cretaceous, extension of the basin led to formation of salt swells and salt diapers[26]. In the extensional zone, the pattern and distribution of salt structures were controlled by detachment. As a result, the distribution of plastic and brittle rocks determines the structural deformation in the extensional zone. In early stage of extension, brittle strata formed grabens. Under a high strain rate, the extensional zone developed tilted blocks and reverse drag structures; under a low strain rate, grabens continued expanding, forming grabens and extensional diapers[27]. The gradient, sedimentation rate, salt thickness are the key factors of influencing salt structures in the extensional zone. Salt thickness and the gradient are positively correlated with the development of tilted blocks, and negatively correlated with the development of diapers[4,28]. The overlying sediments folded and thickened, the contraction of salt layers and faulting would cause strongly contraction and upwelling of salt diapirs. With extension continuing, plastic rocks thinned, which sealed normal faults and formed sealed tilted blocks. Transferring of compressional stress is related to the time and space. In the early stage of deformation, the compressional zone was shrinking; then the compressional zone developed towards both two sides. On one hand, near the region where compression began to form, downhill compressional diapers and extensional diapers resulted from compressional stress transferring upwards were developed. Diapir shape evolved into salt pillow–salt wall–salt suspension–pop-up structures with time[29,30]. Due to the relationship between sedimentation rate and gradient, the volume and height of downhill diapers were larger than those of uphill diapirs. At the downhill, more diapirs are developed at the point closer to
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the sea surface and most of them are suspension diapers. However, at the uphill, the diapir’s wave crest was covered by old rocks. On the other hand, compressional stress was transferred downwards, so salt rocks were folded, leading to thickening and thrusting of overlying rocks. The thicker the salt rock, the longer it deposited, the more intense the thrusting and folding. However, folds were formed after compression lasted for some time. When compressional stress was relatively moderate, multi-folds, thrusts, salt anticlines or narrow box folds were developed in the compressional zone; in the area with higher compressional stress, short-wave folds were developed, which were also controlled by overlying sediments. Under high compressional stress, sediment is easy to be captured by plastic salt layers with compressional syncline and form pop-up structures and lentoid growth synclines. Until now, there are no examples of identifying pop-up structure in seismic profiles, but we can infer that it should occur in the highly compressional zone. Thrust faults are the sigh of highest compressional stress, we considered the followed transitional zone between the compressional zone and the undeformed zone is the extension of the compressional zone.
4 Control of salt structures on hydrocarbon accumulation Growth faults and structural combination patterns generated from salt rocks and salt structures deposited in the Aptian–early Albian stage are key factors for controlling the distribution and type of post-salt turbidites. They played a very important role in the sedimentary type, distribution, trap and oil/gas accumulation in West Africa passive margin basins[30]. Salt movement led to different salt detachment deformations, which had impact on sedimentary facies and sand distribution and formed different structural traps and lithologic-stratigraphic traps. The thicker the salt layer and its overlying sediments, the more possible the salt movement. For example, salt deposit is 1 0002 000 m thick in Gabon Basin and has thick overlying sediments, so salt structures were easy to develop in this basin and they are very favorable for forming structural traps[28]. Among the discovered fields, salt-related fields are unevenly distributed in the salt basins. For example,
Fig. 3
salt related fields are distributed in the extensional zone in Gabon Basin; but in the Lower Congo basin, the compressional zone contains more salt related fields; the distribution of salt related fields is even in Kwanza Basin. In terms of oil and gas reserves, reserves in the extensional zone in salt-related fields are slightly more than those in the compressional zone, but without significant difference. Therefore, both extensional zone and compressional zone are favorable for hydrocarbon accumulation. Along the West Africa continental margin, structural traps formed from salt structures can be classified as follows: (1) Dome traps generated from salt pillow, salt anticline, turtle stucture and rollover anticline[2]. Due to salt piercement or thrust folding, overlying strata are also possible to form dome traps (Fig. 3a-d). Salt piercement enables oil and gas under the salt rock to migrate upwards to reservoir rocks along faults or fractures, resulting in oil and gas accumulation in a large scale. In recent years, most of oil and gas reservoirs discovered in deep water of the Lower Congo basin sourced from rollover anticlines. Dome traps have large scale and high oil and gas reserves, which are the most potential traps in West Africa. (2) Traps formed by salt piercement on two flanks. This kind of trap is also formed due to lateral sealing. Its difference from the conventional fault sealing lies in that it is sealed by dense salt rock (Fig. 3e). (3) Traps bounded by faults. The fault could be the normal fault generated by regional or local extension caused by salt arch. Sand-mud interbed led to lateral sealing and form traps. Hanging walls are reverse drag structures, which form rollover structures; lower walls are sealed laterally, mostly forming small traps. In addition, the hanging walls of growth faults usually developed secondary traps, or formed good fault sealing traps or complex traps (Fig. 3f and g). (4) Traps pinching out to either two sides or single side in the same tectonic movement. They could form lenticular sand reservoirs, lithological reservoirs sealed by salt plug or lithological-structural reservoirs[28] (Fig. 3h). (5) Unconformity traps. Salt diapirism is easy to form unconformity structures which provide space for hydrocarbon
Salt structures and traps
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accumulation (Fig. 3i). (6) When diapir is further developed, it will collapse and turn into coarse clastic deposit, which is good reservoir rock. If there is dense cap rock above, it could be a good trap (Fig 3j). (7) Mini-basins. Salt structures are favorable for the formation of sub-basins. This kind of sub-basins is also called as mini-basins, which are distributed in both the extensional zone and compressional zone and are favorable for sedimentation of sandstone. The oil and gas reservoirs discovered in the Mexico Gulf are mostly distributed in mini-basins. Salt flow not only forms many structures, lithological-stratigraphic traps and complex traps but also generates many complex fault systems which provide good passages for vertical short-distance migration of hydrocarbons. Salt rock is usually very dense and soluble in acid, and it has high viscoplasticity and good heat conductivity. It is good cap rock to seal syn-rift hydrocarbons under the salt rock. Moreover, it enables the oil window of pre-salt source rock to extend downwards, so the source rock is not over-mature or low-mature. Due to the heat conductivity of salt rock, the temperature of pre-salt strata is lower than that of the strata without salt rock overlying at the same depth. Low temperature prohibited the diagenesis of pre-salt reservoir rock. Therefore, the strata even at the depth over 6000 m have favorable porosity and permeability. This is also an important inspiration for salt structure exploration. The giant oilfields discovered in pre-salt reservoir rock in deep water of Mexico and Brazil are closely related to the salt rocks[27].
5
Conclusions
The opening of the Atlantic Ocean led to the development of a series of rift basins in the passive continental margin of West Africa. Due to the relatively sealed environment and dry climate, the periodical influx of seawater precipitated a thick layer of salt in these basins. The salt rock is over 1 000 m thick, distributed in 300 km on average. According to the differences of salt structural styles and stress mechanism, the salt basins can be divided into two tectonic zones: the uphill extensional zone and the downhill compressional zone. The latter has a wider range than the former. The uphill extensional zone can be subdivided into slope structural belt, growth fault and diapir structural belt, where tilted blocks and turtle structures are mainly developed. While the downhill compressional zone mainly developed folds, thrust structures and compressional diapers. The compressional zone is characterized by extensional diapers with compressional nature. And the extensional zone is characterized by sudden thickening of salt rock. The maximum stress in the compressional zone appears in the middle of compressional zone, where thrust faults are located. The development of salt structures can be divided into three stages. After the deposition of thick salt layer, due to differential load, salt detachment occurs under gravity, forming various salt structures. Controlled by extensional stress, the extensional zone was developed and expanded downwards
along slopes. In the compressional zone, the compressive stress transmitted to both sides respectively. The structural features in the extensional zone are mainly controlled by the gradient, sedimentation rate and salt thickness. The compressional zone transfer is related to the time and space, and the gradient and sedimentation rate is the key to influence the salt structures. Salt structures in the passive continental margin of West Africa formed a series of good traps, including dome traps generated from the development of salt anticline and salt pillow or salt piercing overlying salt rock; traps generated from lateral sealing of salt rocks or faults; unconformity traps resulted from salt diapers. Mini-basins and salt diapers resulted in collapse of overlying rocks, producing coarse clastic deposit, which provides a good place for hydrocarbon accumulation. In terms of oil and gas reserves, oil and gas reserves related to salt in the passive continental margin of West Africa are slightly higher in the extensional zone than in the compressional zone. Dome traps developed in the extensional zone are the most potential traps.
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