Introduction
Isolated carbonate platforms are dominated by carbonate buildups and surrounded by water at great depths. They are gentle and circumscribed by transitional zones from steep scarps (or slopes) to deep sea (or lakes). The Bahamas Platform in the Caribbean Sea is a typical example of isolated carbonate platforms composed of modern sediments [1⇓-3]. Isolated carbonate platforms were surrounded by source rocks during their formation and would then be buried at great depths for a long period. For these reasons, they can easily develop into large and integrated reef-shoal reservoirs and thus have attracted more attention for oil and gas exploration. Since 2006, 14 large oil fields, including Lula, Buzios and Aram fields, have been discovered in deep-water pre-salt carbonates in passive continental marginal basin of the Santos Basin in Brazil. In these oilfields, the average recoverable reserves are 2.13×108 t for a field. The recoverable reserves in Buzios Field, the largest one, amount to 14.3×108 t. And the commercial exploration success rate is higher than 40% [4], indicating that deep-water carbonate platforms in passive continental marginal basins have great prospects for oil and gas exploration. In other deep-water passive marginal basins around which major oil and gas discoveries have been made, such as the Gulf of Mexico Basin, the Guyana Basin, the Lower Congo Basin, and the Rovuma Basin, hydrocarbon is mainly concentrated in the deep-water gravity-flow clastic depositional systems formed in the late drift and depression stage. Because these basins have been highly explored, where many oil and gas fields have been discovered, their geological conditions and hydrocarbon accumulation patterns have been clearly described [5⇓⇓⇓-9]. The pre-salt carbonate platforms in the Santos Basin were formed in the early intra-continental and inter-continental rifting stages. Affected by the low extent of exploration and the limited amount of available data in deep-water pre-salt sediments, previous studies on the Santos Basin were focused primarily on the regional geology [6⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓-23] and a small number of discoveries [4-5], the geological setting for the formation of deep-water pre-salt isolated carbonate platforms in the basin, sedimentary build-ups and distribution patterns of these platforms, and the main factors controlling the formation of large oilfields have not been adequately studied, so the future targets for oil and gas exploration in the basin still remain unclear. Based on available 2D/3D seismic, core and drilling data, this paper analyzes the superposition and evolution processes of the Santos Basin’s prototypes and the characteristics of sedimentary fillings in these prototype basins, reveals the geological setting for the formation of pre-salt carbonate platforms, establishes the sedimentary build-up models and describes the seismic response of two types of isolated carbon platforms, identifies the main factors controlling the formation of large pre-salt carbonate reservoirs through the dissection of large oilfields discovered in the pre-salt play. It also predicts the favorable zones of hydrocarbon accumulation based on seismic facies, and finally proposes the favorable zones and targets for future exploration to provide reference for the strategic selection of exploration blocks/zones, the evaluation of new exploration projects, and arrangement of exploration wells in passive margin basins.
1. Geological setting for the formation and evolution of prototype basins and the formation of carbonate platforms since the Early Cretaceous
The Santos Basin is located entirely offshore in the western South Atlantic Ocean, eastern sea of Brazil, and covers a total area of 32.69×104 km2. It is one of the passive continental marginal basins associated with the breakup of Gondwanaland and the formation of the South Atlantic Ocean from the Mesozoic to the Cenozoic [16⇓-18] (Fig. 1a ). Based on the structure of the bottom of the present-day evaporite sequence (i.e., the top of the Cretaceous BVE Formation), the Santos Basin can be divided into the following six primary structural units: Western Slope, Western Depression, Western Uplift, Central Depression, Eastern Uplift and Eastern Depression (Fig. 1a , 1c). The basin has been created by the superposition and evolution of three prototype basins, namely, the intra-continental rift basin formed in the rifting period, the inter-continental rift basin formed in the transitional period, and the passive continental marginal basin formed in the drifting and depression period, which were filled with lacustrine, lagoonal and marine deposits, respectively (Fig. 1b ). The transition from intra-continental rifting to inter-continental rifting provided favorable tectonic and paleogeographic settings for carbonate build-up and evaporite deposition.
Fig. 1. Comprehensive geological setting of the Santos Basin. |
1.1. Intra-continental rifting from the Early Cretaceous Berriasian to the Early Aptian
In the Early Cretaceous, under the influence of the Tristan mantle plume event, extension that caused the breakup of Gondwana began from the southern end of Gondwana, the present-day West Coast of South Africa, and rifting occurred from south to north, creating a nearly S-N trending, long and narrow rift system that is relatively closed at the northern end and controlled by the nearly S-N trending extensional fault zone [20-21]. At that time, the Santos Basin was a part of this large S-N trending rift system [24⇓-26], where a structural pattern consisting of nearly E-W trending alternating horsts and grabens was created (Fig. 1c ), with Rio Grande transform fault zone as southern boundary. Borehole and outcrop data [4] show that, during the period from Berriasian to Hauterivian, the Santos Basin was mainly filled with basaltic and pyroclastic sediments. During the Early-Middle Barremian, the basin was dominated by continental fluvial, deltaic and lacustrine sediments of the Cretaceous PIC Formation (Fig. 2a ) deposited in semi-deep to deep lacustrine environments, with developed thick argillaceous shales. During the deposition of the ITP Formation from the Late Barremian to Early Aptian, under the influence of a relatively closed rifting environment and strong evaporation near the equator, the lake water became shallower, and the nearshore area was still a fluvial-deltaic environment. However, a coquina sequence was developed in the shallow lacustrine environment above the paleo-uplift, and the semi-deep to deep lacustrine depressions were dominated by argillaceous limestone and mudstone deposits [15⇓⇓-18]. In the final stage of the ITP Formation deposition, as mantle plume activities were intensified, a narrow segment of oceanic crust appeared between the South American and African plates. Thermal uplift occurred, and a fracture plane throughout the entire rift system was created. The facture plane is manifested as a large-scale sedimentary hiatus and represents the end of intra-continental rifting and the beginning of inter-continental rifting [16⇓⇓⇓⇓-21].
Fig. 2. Formation, evolution, and reconstructed lithofacies and paleogeography of passive margin basins on both sides of the South Atlantic Ocean (modified from Reference [20]). |
1.2. Inter-continental rifting from the Middle-Late Aptian of the Early Cretaceous
The emergence of the fracture plane in the final stage of the ITP Formation deposition indicates the formation of an S-N trending narrow segment of oceanic crust in the middle of the large S-N trending rift system between the South American and African plates. As this initial narrow segment of oceanic crust formed from south to north [19⇓⇓⇓⇓-24], the southern seawater invaded on a large scale, heralding the beginning of inter-continental rifting [25⇓⇓-28], and intense magmatic activities occurred, creating volcanic highlands such as the nearly E-W trending Rio Grande Rise and Walvis Ridge along southern margins of the Santos Basin and the Angola Basin in West Africa. During the Middle Aptian, due to the presence of the transverse barrier formed by the Rio Grande Rise, a lagoon depositional environment with restricted seawater circulation was formed in the long, narrow rift zone at north of the southern margin of the Santos Basin. In addition, affected by the newly formed narrow segment of oceanic crust, the geothermal gradient increased, the boundary fault blocks on both sides of the inter-continental rift underwent thermal expansion and tilting, and the amounts of clastic sediments at the continental margin decreased. Under the jointing effects of these factors and continuous evaporation near the equator, a widespread carbonate sediments in the BVE Formation were formed, which are widely distributed across the basin, except the basin margin dominated by clastic rocks [24⇓⇓-27] (Fig. 2b ). Meanwhile, large shellfish species in the paleo-uplift zone became extinct with the increase in salinity, and microbial limestones such as grainstone, pelletal limestone and stromatolite were deposited, and the dominant sedimentary facies in depression zones transitioned from argillaceous limestone to mudstone.
During the Late Aptian, as salinity increased continuously, the Ariri Formation evaporite sequence composed of thin layers of anhydrite and thick layers of halite was formed atop carbonate build-ups (Fig. 2c ), with a total area of nearly 20×104 km2 and maximum thickness of 2 500 m [29⇓⇓⇓⇓⇓⇓-36]. Throughout the whole inter-continental rifting stage, the episodic activity of the mid-ocean ridge on the middle oceanic crust caused continental crust segments on both sides to drift and expand outwards, the activity of the faults formed in the early intra-continental rifting stage was weakened significantly, and depression-type disc-shaped sedimentary units were dominant.
1.3. Passive continental margin from the Early Cretaceous Albian to the present
Since the Early Albian, as mantle plume activities continued, the oceanic crust became increasingly wider, and the seafloor spread continuously, which drove the lithosphere to move toward both sides, and thus creating a new open ocean. The oceanic crust segments on both sides cooled down and underwent symmetric thermal subsidence toward the mid-ocean ridge. Due to the intrusion of large amounts of seawater, evaporite deposition ceased. With eustatic sea level changes in the drift stage on passive continental margin, a large-scale marine transgression-regression cycle occurred from bottom to top, creating sedimentary wedges on the continental crust and transition zone [30⇓⇓-33]. The transgressive strata include the Albian succession (formed during the Early Albian), which is composed primarily of littoral and shallow-marine carbonates and gradually transitions to the Turonian succession dominated by deep-marine clastic sediments (Fig. 2d ). During the period from the Early Albian to Turonian, a black shale sequence was deposited in an anoxic environment [6]. The regressive strata include the formations from the Coniacian to present. Due to the continuous falling of the global sea level, abundant sediments were supplied from both sides, sandstone bodies prograded seaward, and medium to large seaward-opening fluvial- deltaic-marine depositional systems were formed (Fig. 2e ).
2. Formation and distribution of Lower Cretaceous isolated carbonate platforms
Exploration practice shows that marine carbonate platforms are mainly composed of rimmed continental shelves and gently sloping sedimentary structures [1⇓-3]. Although the present-day Santos Basin is dominated by marine deposits, all the pre-salt carbonate platforms in the basin were formed in the Early Cretaceous intra- continental rifting and inter-continental rifting stages. In these two stages, the prototype basins were situated in lacustrine and lagoonal depositional environments, respectively. Comprehensive analysis of the oil and gas fields discovered in pre-salt carbonates shows that they are typical isolated carbonate platforms. Based on the tectonic setting, the isolated carbonate platforms are classified into two types, namely, inter-depression fault-uplifts (IrDFU) and intra-depression fault-highs (IaDFH) (Fig. 3 ). This paper analyzes the seismic responses of the mound-shoal complexes in these two types of platforms, and defines the distribution of favorable pre-salt carbonate reservoirs and facies zones.
Fig. 3. Sedimentary build-up models of two types of Lower Cretaceous isolated carbonate platforms in the Santos Basin. |
2.1. Build-ups of isolated carbonate platforms
2.1.1. Inter-depression fault-uplifts
Large oil fields discovered in IrDFU isolated carbonate platforms, such as Buzios, Lula and Mero, are almost distributed in the eastern uplift zone which is a large, inherited inter-depression paleo-uplift formed in the rifting stage. This NE-trending paleo-uplift has been complicated by multi-episodic rifting, and alternating secondary horsts and grabens are formed on the paleo-uplift. Most horst blocks are controlled by a single fault, with a steep scarp on one side and a gentle slope formed by multiple small fault steps on the other side (Fig. 4 ). A few horsts are controlled by two faults, with steep scarps on both sides, and paleo-landform changing from steep to gentle along the faults. Platforms formed by single-fault controlled horsts and double-fault controlled horsts often differ in sedimentary build-ups and seismic responses.
Fig. 4. Seismic responses of pre-salt IrDFU isolated carbonate platforms (single-fault-controlled) in the Santos Basin (see |
This study takes Lula Oil Field which covers an area of 1 050 km2 and has a trap closure amplitude of 450 m as a case to characterize single-fault-controlled IrDFU isolated carbonate platforms. During the deposition of the ITP Formation in the late intra-continental rifting stage, inherited horst-type fault blocks were situated in a high- energy shallow lacustrine environment, where coquinas and grainstones composed primarily of shells and bioclasts were formed. The formation of these carbonate rocks is related to the in-situ deposition and short-distance transport. Coquinas are widely distributed along the flanks, but they are absent near the top, possibly due to thermal uplift and erosion at the end of the intra-continental rifting period. The coquina sequence becomes thick in downdip direction, till 189 m to the maximum, and displays mound-shaped seismic reflections with low amplitude, low frequency, discontinuity and disorder. The BVE Formation in the inter-continental rift is constructed of a microbial mound-shoal complex consisting of 79-187 m thick interbeds of laminated algal limestone and oolitic limestone. It is difficult to distinguish mounds from shoals on seismic sections since their seismic responses are subparallel and relatively continuous reflections with low to medium amplitude and low to medium frequency. The microbial mound-shoal complexes prograde from the structural high to the flanks of the paleo-uplift, and display notable mound-shaped reflections at local highs in the slope zone, and dominated by laminated algal limestone. However, in the relatively gentle slope zone, oolitic limestone accounts for a higher proportion. Generally, the mound-shoals are characterized by sediment progradation towards structural lows, and their seismic reflections gradually tend to be parallel and continuous reflections with medium to high frequency and high amplitude towards low-energy environments.
Mero Oil Field where the trap covers an area of about 189 km2 and has 650 m closure amplitude is an example from northern double-fault-controlled IrDFU isolated carbonate platforms. Compared with Lula Oil Field, the coquinas of the ITP Formation above fault blocks in Mero Oil Field are more developed (Fig. 5a , 5b), with thickness ranging from 20 m to 220 m, and BVE Formation microbial limestones are also well-developed and widely distributed, with thickness ranging from 50 m to 400 m. Similarly, there are laminated algal limestone (Fig. 5c -5e) and oolitic limestone (Fig. 5f , 5g) interbeds of varying thickness in Mero Oil Field. The seismic reflections recorded in Mero Oil Field are similar to those in Lula Oil Field. However, due to the presence of steep scarps on both sides, the external structure is situated in a low- energy deep-water environment, and in terms of geometric form, it is a remarkable mound-like structure showing the characteristics of sediment aggradation (Fig. 6 ).
Fig. 5. Images and microstructures of core samples of pre-salt coquina, stromatolite and oolitic limestone in Mero Oil Field of the Santos Basin (Well A-1 modified after Reference [15]). (a) Coquina, ITP Formation, 5 641 m, Well A-2; (b) Coquina, ITP Formation, 5 629.85 m, Well A-2, plane-polarized light; (c) Tree-like stromatolites, BVE Formation, 5 518 m, Well A-1; (d) Shrub-like stromatolites, BVE Formation, 5 312 m, Well A-1; (e) Dendritic stromatolites, BVE Formation, 5 320 m, Well A-1; (f) Oolitic limestone, BVE Formation, 5 502 m, Well A-1; (g) Oolitic limestone, BVE Formation, 5 502 m, Well A-1, cross-polarized light, green arrows pointing to extinction crosses. |
Fig. 6. Seismic responses of pre-salt IrDFU isolated carbonate platforms (double-fault-controlled) in the Santos Basin (modified from Reference [18]; see |
2.1.2. Intra-depression fault-highs
Oil and gas fields discovered in IaDFH isolated carbonate platforms in the Santos Basin, such as Aram and Carcara oil fields, are mainly distributed in the central depression zone. This study selects Carcara Oil Field which covers an area of 144.32 km2 as an example, and the trap has a 758 m closure height (Fig. 7 ). In the Santos Basin, IaDFH isolated carbonate platforms have developed on horst blocks, and the platform margin is obviously controlled by two primary extensional faults whose dip directions are opposite, and there are a large number of secondary faults. The ITP Formation was deposited in a low-energy deep-water environment where shell-dominated bioclastic shoals were not developed. Before the deposition of the BVE Formation, the basaltic lavas produced by episodic volcanic eruptions during the intra-continental rifting period overflowed along fault systems, creating uplifts. Meanwhile, the water level declined due to salinization [37⇓⇓-40], resulting in a shallow-water environment where microbial limestone of the BVE Formation started to build up. Similar to the carbonate build-ups in the IrDFU isolated carbonate platforms formed during the inter-continental rifting period, the BVE limestone is also constructed of a microbial mound-shoal complex comprising laminated algal limestone and oolitic limestone interbeds of varying thickness. This mound-shoal complex has seismic responses that are basically similar to those of the BVE Formation in Libra Oil Field (a double-fault-controlled IrDFU isolated carbonate platform), and it is dominated by upward aggradation. Due to sediment aggradation and build-up along the upper parts of primary faults, its thickening is very obvious, and it has changed from a mound-like structure to a tower-like structure. Such a tower-like structure may be linked to the travertine mound formed by subsequent hydrothermal alteration. Note that not all IaDFH isolated carbonate platforms have a spire-like top.
Fig. 7. Seismic responses of pre-salt IaDFH isolated carbonate platforms in the Santos Basin (see the location of section in |
2.2. Distribution of isolated carbonate platforms
The distribution of IrDFU and IaDFH isolated carbonate platforms were identified through 2D/3D seismic interpretation based on their geological settings and seismic response characteristics.
Both single-fault- and double-fault-controlled IrDFU isolated carbonate platforms are found in the inherited paleo-uplift that starts in the intra-continental rift. In addition in the eastern uplift zone which has been explored to a great extent, they are widely distributed in the western uplift zone according to 2D seismic data. It is obvious that the inter-depression paleo-uplift controls the extension of these platforms. A single IrDFU isolated carbonate platform is approximately elliptical, and its long axis is consistent with the orientation of the paleo-uplift. Single-fault-controlled IrDFU isolated carbonate platforms are mainly distributed along the axis of the paleo-uplift, which are large and generally 300-1 000 km2 in scale each. In contrast, double-fault-controlled IrDFU isolated carbonate platforms are mainly distributed on the flanks of the paleo-uplift and relatively small, generally 100-300 km2 in scale each. Two types of hydrocarbon reservoirs, namely, ITP coquina reservoirs and BVE microbial limestone reservoirs, are well developed in single-fault-controlled and double-fault-controlled horsts. The BVE microbial limestone reservoirs developed on the top of the ITP coquina reservoirs, and the latter are generally larger than the former (Figs. 8 and 9 ).
Fig. 8. Distribution of BVE microbial limestone developed in early inter-continental rifting in the Santos Basin. |
Fig. 9. Distribution of ITP coquinas developed in late intra-continental rifting in the Santos Basin. |
IaDFH isolated carbonate platforms are primarily controlled by uplifts created by intra-depression basaltic lava overflows produced by volcanic eruptions during intra-continental and inter-continental rifting. Through seismic interpretation, it has been found that, among the present-day pre-salt eastern, central and western depression zones of the Santos Basin, the central depression zone is the place where IaDFH isolated carbonate platforms are most developed. The reason is that, during the intra-continental rifting period, the central depression zone had the largest sediment thickness and was located in the center of extension, where it could be easily connected with the asthenosphere by fractures. Therefore, volcanic activity was intense there, resulting in multiple uplifts formed by basaltic lava overflows. Among IaDFH isolated carbonate platforms, the Aram platform is the largest, over 700 km2, while the other platforms are generally smaller than 200 km2. The orientation of these isolated carbonate platforms (NE trending), regardless of their scale, is controlled by extensional faults. In the eastern depression zone, IaDFH isolated carbonate platforms are sparsely distributed and generally small. In the Western depression zone which is relatively narrow and adjacent to the western slope zone, carbonate platforms have not developed under the influence of terrigenous clastic sediments. Among the three depression zones, only Aram and Carcara platforms, etc. in central depression zone have been explored through drilling wells, and the drilling results have demonstrated that the BVE microbial mound-shoal complex is well-developed and widely distributed (Fig. 8 ).
3. Formation of giant pre-salt carbonate reservoirs in the Santos Basin
3.1. Source rocks
The high-quality lacustrine source rocks deposited in the middle and lower sections of the Barremian during the middle and late stages of intra-continental rifting provide a solid material basis for the formation of giant pre-salt oil and gas reservoirs in the Santos Basin. In early intra-continental rifting, the basin was dominated by igneous and pyroclastic rocks, and there were no well-developed source rocks. In middle and late intra-continental rifting, the basin was dominated by lacustrine deposits and subjected to intermittent volcanic activities (Fig. 10 ). Meanwhile, the basin was situated in a low-latitude warm and humid climate zone where the hydrothermal fluids formed by volcanic activities carried large amounts of minerals, providing plenty of nutrients for microbial growth. As a result, organic-rich lacustrine source rocks were formed in the middle and lower sections of the Barremian [34⇓⇓-37]. These source rocks contain Type I kerogen which consists primarily of algal and bacterial organic matter. The PIC source rocks formed in the intra-continental middle rifting stage have an average TOC value of 5.7%, an average HI (hydrogen index) of 832 mg/g, and an average hydrocarbon generation potential (S1+S2) of 48.1 mg/g. In comparison, the ITP source rocks formed in the late rifting stage have an average TOC value of 4.4%, an average HI of 738 mg/g, and an average hydrocarbon generation potential (S1+S2) of 37.8 mg/g [36]. These source rocks were formed during the early intra-continental rifting stage in the prototype thermal basin, but they suffered from a high heat flow during the middle intra-continental rifting stage and were overlaid by sediments during the late passive continental marginal stage. For these reasons, the present-day source rocks are buried at depths greater than 4 000 m. Among 38 pre-salt oil and gas fields discovered in the eastern uplift and central depression zones, only two small- to medium-sized discoveries (Gato do Mato and Temisto) are gas fields. The main reason is that thick halite layers are well-developed and widely distributed across the central depression zone, eastern uplift zone and eastern depression zone. Influenced by the overlying thick halite layer with high thermal conductivity, hydrocarbon generation and expulsion by the source rocks in the depression zones did not reach the peak until the Miocene. At present, these source rocks are still in an oil window, and with Ro values ranging from 0.8% to 1.2%. Take the large pre-salt Alam Oil Field discovered in the center of the central depression zone as an example, all surrounding source rocks are buried below 5 000 m (excluding water depth) and generate light oil.
Fig. 10. Hydrocarbon accumulation model in the Santos Basin. |
3.2. Reservoirs
Both IrDFU and IaDFH isolated carbonate platforms can develop into large mound-shoal reservoirs of high quality. The pre-salt layer in the western uplift zone has not been drilled yet, and IrDFU isolated carbonate platforms have only been discovered in the eastern uplift zone. In both single-fault-controlled and double-fault- controlled IrDFU isolated carbonate platforms, the coquinas of the ITP Formation and microbial limestones of the BVE Formation are well-developed and widespread. Since the western and eastern depression zones have not been drilled yet, the existence of IaDFH isolated carbonate platforms has only been confirmed by drilling data in the central depression zone where the microbial limestones of the BVE Formation are well-developed and widely distributed.
The coquina reservoirs of the ITP Formation have developed only in the 34 pre-salt IrDFU isolated carbonate platforms discovered in the eastern uplift zone. These reservoirs are generally 50-180 m thick, and 248 m to the maximum. Descriptive and statistical analysis of 190 coquina samples has revealed that the main types of pores in coquina reservoirs are moldic pores, dissolution pores and cavities, intercrystalline pores and intergranular pores. In terms of origin, secondary pores were mainly formed by diagenetic alteration, moldic pores were mainly formed by meteoric water dissolution during early diagenesis, and the formation of dissolution pores and cavities is related to burial dissolution during diagenesis. Most of the pores and cavities formed by burial dissolution are irregular in geometry because the dissolution process is primarily dominated by non-fabric-selective dissolution. The reservoirs described above have minimum porosity of 6.3%, maximum porosity of 30.0%, average porosity of 16.9%, minimum permeability of 1.10×10−3 μm2, maximum permeability of 1 180.0×10−3 μm2, and average permeability of 101.80×10−3 μm2. In general, these reservoirs are limestone reservoirs with medium to high porosity and medium permeability [17⇓-19].
The microbial limestone reservoirs of the BVE Formation are well-developed in both 34 pre-salt IrDFU isolated carbonate platforms discovered in the eastern uplift zone and four IaDFH isolated carbonate platforms located in the central depression zone. These reservoirs are generally 60-200 m thick, and 252 m to the maximum. Descriptive and statistical analysis of 216 microbial limestone samples shows that the main types of pores in microbial limestone reservoirs are dissolution pores, intergranular pores, organic-framework pores, intragranular pores and intracrystalline pores, and the primary pores formed by diagenetic alteration are dominant. These reservoirs have minimum porosity of 5.0%, maximum porosity of 26.5%, average porosity of 13.4%, minimum permeability of 1.00×10−3 μm2, maximum permeability of 3 234.00×10−3 μm2, and average permeability of 183.70× 10−3 μm2. In general, they are also limestone reservoirs with medium to high porosity and medium permeability [17⇓-19].
3.3. Seals
The ultra-thick evaporite sequence of the Ariri Formation deposited in the late inter-continental rifting stage forms a high-quality regional seal. Developed in a lagoon environment involving strong evaporation during the late inter-continental rifting stage, and controlled by the tectonic and depositional settings, the evaporate sequence became a disc-shaped sedimentary unit generally thinning from the basin center to the western slope and the eastern slope (now it is the eastern slope of the Angola Basin in West Africa). After marine sediments were deposited in the passive continental margin of the drifting period, the evaporite sequence was covered by large amounts of clastic sediments from the west of the basin. Due to their high flowability and the action of gravity, the evaporite sequence was “driven” by the overlying sediments to flow toward the center of subsidence in the east of the basin, resulting in the present-day distribution. In general, this evaporite sequence is thick from west to east, but not developed in the Western Depression Zone. In the western uplift zone and the northwest of central depression zone, the halite sequence is 0-200 m thick, with many salt windows. In the southeast of the central depression zone and the eastern uplift zone, the thickness of the halite sequence generally ranges from 200 m to 1 000 m and its lateral variation is great. In the eastern depression zone, the thickness of the halite sequence generally ranges from 1 000 m to 2 000 m and is relatively stable laterally [41⇓⇓⇓⇓⇓⇓-48]. It is worth noting that there is a well-developed, stable, 3-10 m thick anhydrite layer at the bottom of the halite sequence. The pre-salt reservoirs discovered in the northern Campos Basin are overlaid by an evaporite sequence that serves as a seal, and well-developed salt windows can be clearly observed on seismic sections. Drilling operation encountered thin gypsum layers near the salt windows, indicating that the anhydrite layer and thick halite sequence described above are effective regional seals. This is further verified by the fact that all of the 14 giant oil and gas fields discovered in the Santos Basin are located in the eastern uplift zone and the central depression zone where these regional seals are relatively well-developed.
3.4. Traps and hydrocarbon accumulation
Large structural-stratigraphic composite oil-gas reservoirs may form in IrDFU and IaDFH isolated carbonate platforms developed during intra-continental and inter- continental rifting.
IrDFU isolated carbonate platforms are horst blocks inherited from the intra-continental and inter-continental rifts, and can be classified by tectonic setting into two types: single-fault-controlled and double-fault-controlled. Both types of IrDFU isolated carbonate platforms have favorable tectonic conditions for the formation of fault traps. Therefore, two types of high-quality reservoirs, namely, coquina reservoirs of the ITP Formation and microbial mound-shoal reservoirs of the BVE Formation, and multiple isolated, medium to large structural-stratigraphic composite traps have been formed. Before the deposition of the ITP Formation in the early rifting stage, IaDFH isolated carbonate platforms were mainly low- relief fault blocks. During the deposition of the ITP Formation, water was relatively deep. During the deposition of the BVE Formation, underwater highs were created by basaltic lava overflows produced by volcanic eruptions, providing favorable conditions for the formation of composite traps by the microbial mound-shoal complexes of the BVE Formation situated in a high-energy shallow- water environment.
The structural-stratigraphic traps formed by IrDFU isolated carbonate platforms are mainly distributed in the eastern and western uplift zones of the Santos Basin. These traps took their final forms after the deposition of the BVE Formation during inter-continental rifting in the Aptian Age of the Early Cretaceous, and then the high- quality evaporite sequence of the Ariri Formation was formed, providing an effective seal. Before these traps took their final forms, the following two suites of high- quality source rocks had already been formed: the source rocks of the PIC Formation formed in the middle rifting stage and the source rocks of the ITP Formation formed in the late rifting stage. However, it was not until the Miocene Epoch of the Neogene that hydrocarbon generation and expulsion of these source rocks reached the peak. Hydrocarbons migrated vertically and laterally along faults and strata into isolated mound-shoal complexes, and the optimal spatiotemporal relationship composed of “source rocks at the bottom, reservoirs in the middle, and caprocks at the top” was formed.
The structural-stratigraphic traps formed by IaDFH isolated carbonate platforms are mainly distributed in the central and eastern depression zones of the Santos Basin. No effective traps were formed in the ITP Formation during the early rifting stage, but the traps formed in the BVE Formation have the best source-reservoir-seal combination which is the same as the case of IrDFU isolated carbonate platforms, providing good conditions for the formation of large, integrated, structural-stratigraphic composite reservoirs. This has been confirmed by the discoveries made during hydrocarbon exploration. Among the 14 giant oil and gas reservoirs discovered in the pre- salt play of the Santos Basin, Aram and Carcara platforms are of the IaDFH type, and the other carbonate platforms are of the IrDFU type. Discovered reservoirs on both types of platforms have been completely filled with hydrocarbon, and the maximum closure is 758 m high (Fig. 10 ).
4. Future targets for oil and gas exploration in the Santos Basin
The systematic analysis of the geological setting, sedimentary build-up, spatiotemporal distribution, and formation conditions of the 14 large carbonate oil and gas fields discovered in the Santos Basin suggests that the pre-salt play in the Santos Basin still has great prospects for exploration in the future. Hydrocarbon exploration should focus primarily on the eastern uplift zone, the western uplift zone, the central depression zone, and the eastern depression zone. The structural-stratigraphic combination traps formed by IrDFU and IaDFH isolated carbonate platforms should be selected as exploration targets. Based on the findings above, China National Petroleum Corporation (CNPC) acquired equity share in two large exploration blocks, namely, Libra and Aram, in 2013 and 2020, respectively, and made major oil and gas discoveries in both blocks [11]. The Santos Basin will still be a hotspot for future exploration and discovery of large oil and gas fields.
The eastern uplift zone differs greatly from the western one in terms of the extent of exploration. The eastern uplift zone has been explored to a relatively large extent, where the discovered oil and gas fields are mainly concentrated in the large IrDFU isolated carbonate platforms in the middle and northern parts. In addition to the IrDFU isolated carbonate platforms in the southern part, which are subsequent exploration targets, the small and medium-sized isolated carbonate platforms around the oil and gas fields discovered in the middle and northern parts should be considered, too. These small and medium-sized platforms have limited hydrocarbon reserves, but with favorable factors such as thick pre-salt carbonate pay and high initial oil production per well, they can bring considerable economic benefits. In the western uplift zone with sediments at great buried depths, only 2D seismic data is available, and the seismic data for deep pre-salt layers is of low quality. Through the interpretation of available seismic data, it has been found that the evaporite sequence is thin, and there are well-developed salt windows. However, the inherited paleo-uplift in the pre-salt play has distinct characteristics, showing the typical seismic reflections of IrDFU isolated carbonate platforms. Even if evaporites (caprocks) are not developed, the ultra-thick marine argillaceous shale can provide an effective seal, which is favorable for hydrocarbon accumulation and worthy be explored in the future.
Among the three (central, eastern, and western) major depression zones, the central depression zone is still the key exploration target. Currently, hydrocarbon exploration focuses primarily on the relatively large hydrocarbon accumulation zones in the middle, and two large oilfields, Aram and Carcara, have been discovered. 2D seismic data collected in the southern and northern parts of the central depression zone (the only available seismic data) shows there are multiple small- and medium-sized isolated carbonate platforms which are worthy of evaluation and exploration through 3D seismic survey. In the eastern depression zone, sporadically distributed IaDFH isolated carbonate platforms have been identified based on available 2D seismic data, which are also worthy of exploration. In the western depression zone, carbonates have not developed under the influence of terrigenous clastic sediments from the western slope zone. The exploration targets in this zone should change accordingly.
5. Conclusions
The deep-water pre-salt carbonate sedimentary build- ups in the Santos Basin (a passive continental marginal basin) were not formed in a marine depositional environment, instead they were formed during the early intra-continental and inter-continental rifting stages within two prototype basins situated in lacustrine and lagoonal depositional environments, respectively. During the intra-continental rifting period, strong extension and intense volcanic activity created a structural pattern consisting of alternating horsts and grabens. During the inter-continental rifting period, fault activity was weakened significantly, creating a structural pattern consisting of alternating uplifts and depressions on the top of horsts and grabens. All the structural patterns evolved into shallow-water environments and provided favorable tectonic settings for the formation of isolated carbonate platforms.
Under different paleo-tectonic settings, two types of isolated carbonate platforms, namely, IrDFU and IaDFH, were developed in the Santos Basin. IrDFU isolated carbonate platforms are built on the inherited fault blocks formed by single-fault- or double-fault-controlled horst blocks between depressions and mainly distributed in some secondary structural highs in the eastern uplift zone and the west uplift zone. The following two types of reservoirs have developed from bottom to top: ITP coquina reservoirs formed during the late intra-continental rifting stage and BVE microbial limestone reservoirs formed during the inter-continental rifting period. Both single-fault-controlled and double-fault-controlled IrDFU isolated carbonate platforms are dominated by carbonate sedimentary build-ups resulted from progradation. IaDFH isolated carbonate platforms have been formed on the structural highs caused by intra-depression double-fault-controlled horsts due to volcanic uplift in the rifting phase. These carbonate platforms are distributed primarily at some secondary structural highs in the central depression zone and the eastern depression zone. Only the BVE microbial limestone reservoirs formed during the inter-continental rifting period are well-developed. IaDFH isolated carbonate platforms are characterized by carbonate build-ups resulted from aggradation.
Exploration practice has shown that both IrDFU and IaDFH isolated carbonate platforms can form large, integrated, structural-stratigraphic combination reservoirs. ITP coquina reservoirs and BVE microbial limestone reservoirs formed on these two types of isolated carbonate platforms are characterized by large cumulative thickness and good petrophysical properties, and they are circumscribed by high-quality lacustrine source rocks and overlaid by a thick evaporite sequence that serves as an effective regional seal. Large oilfields discovered in the pre-salt play of the Santos Basin are completely filled with hydrocarbon, and with high degree of hydrocarbon enrichment. The western uplift zone, the southern part of the eastern uplift zone, and the northern part of the central depression zone have great prospects, but are less explored. The IrDFU and IaDFH isolated carbonate platforms in these zones can be accurately explored through large-scale 3D seismic survey to enhance the exploration success rate.