Large-scale conical and saucer-shaped sand injectites have been identified in the Upper Miocene sediments of the Lower Congo Basin. These structures are evidenced on the 3D high-resolution seismic data at about 600 ms TWT (two-way traveltime) beneath the seabed. The conical and saucer-shaped anomalies range from 20 to 80 m in height, 50 to 300 m in diameter, and 10 to 20 ms TWT in thickness. They are located within a sedimentary interval of about 100 m in thickness and are aligned over 20 km in dip direction (NE-SW), above the NW margin of an underlying Upper Miocene submarine fan. We have interpreted the conical and saucer-shaped anomalies as upward-emplaced sand injectites sourced from the Upper Miocene fan because of their discordant character, the postsedimentary uplifting of the sediments overlying the cones and saucer-shaped bodies, the alignment with the lateral fringe of the Upper Miocene submarine fan, and the geological context. Sand injection dates from the Miocene–Pliocene transition (approximately 5.3 Ma). The prerequisite overpressure to the sand injection process may be due to the buoyancy effect of hydrocarbons accumulated in the margins of the fan. Additionally, overpressure could have been enhanced by the lateral transfer of fluids operating in the inclined margins of the lobe. The short duration of sand injection and the presence of many sandstone intrusions suggested that the process of injection was triggered by an event, likely due to a nearby fault displacement related to diapiric movements. This is the first time that sand injectites of seismic scale have been described from the Lower Congo Basin. The localized nature of these injectites has led to a change in the migration path of fluids through the sedimentary cover. Consequently, the sand intrusions are both evidence and vectors of fluid migration within the basin fill.
Sand injectites (sandstone intrusions) have been described in geological literature as far back as the early 19th century (Strangways 1821; Murchison 1827) but their significance and interest as hydrocarbon reservoirs were only recently recognized (Newsom 1903; Jenkins 1930; Dixon et al. 1995; Hurst & Cartwright 2007), in particular in the Paleogene of the North Sea basin (Dixon et al. 1995; Lonergan et al. 2001; Molyneux et al. 2002; Løseth et al. 2003; Huuse & Mickelson 2004; Huuse et al. 2004; Jackson 2007; Jackson et al. 2011), due to improvements in coring and seismic data quality (Huuse et al. 2007). Seismically imaged sand injectites can be divided into two main types: winglike sandstone intrusions typically emanate upward from the margins of closed sand bodies (Lonergan et al. 2001; Huuse et al. 2004, 2007; De Boer et al. 2007; Jackson 2007) and conical or saucer-shaped sandstone intrusions emanating upward from deeper sand bodies (Molyneux et al. 2002; Løseth et al. 2003; Huuse & Mickelson 2004; Shoulders & Cartwright 2004; Huuse et al. 2007; Shoulders et al. 2007; Cartwright et al. 2008).
Today, km-scale sandstone intrusions occur widely in the North Sea where they are associated with hydrocarbon reservoirs (Dixon et al. 1995; MacLeod et al. 1999; Lonergan et al. 2001; Hurst & Cartwright 2007; Shoulders et al. 2007; Szarawarska et al. 2010) or constitute reservoir (De Boer et al. 2007) and migration paths (Duranti & Hurst 2004; Duranti & Mazzini 2005; Huuse et al. 2007, 2010). In the studied area, discordant amplitude anomalies are suspected to correspond to conical and saucer-shaped sandstone intrusions.
In this article, we document the occurrence, scale, and geometry of conical and saucer-shaped sand injectites, their relation with surrounding shales and associated depositional systems. Based on the well-known framework and sedimentary setting of the study area, we will discuss their origin, the parent sand, the timing of injection, the supposed processes, and trigger mechanisms of injection and the reasons of their location. We will then discuss the implications of the scale of sand injectites for oil and gas exploration and production in the Lower Congo basin.
The West African margin formed during breakup of Gondwana during Late Jurassic and Early Cretaceous times (Brice et al. 1982; Teisserenc & Villemin 1989; Guiraud & Maurin 1992; Karner & Driscoll 1999). During rifting, the Kwanza, Gabon, and Lower Congo basins have developed along the West African margin (Broucke et al. 2004) since mid-Aptian (Brice et al. 1982; Reyre 1984; Walgenwitz et al. 1990).
The anomalies studied are located in the Lower Congo Basin (Fig. 1), limited to the north and south by basement highs (Standlee et al. 1992). Following prerift continental deposition during the Jurassic, syn-rift lacustrine sediments overlapped tilted blocks of the basement (Marton et al. 2000; Broucke et al. 2004). The transition to a passive margin was initiated during postrift thermal subsidence (Broucke et al. 2004; Brownfield & Charpentier 2006) and is recorded by marine sediments interbedded with a thick salt layer of Middle Aptian age (Brice et al. 1982; Uchupi 1992; Marton et al. 2000). Climatic and sea level changes induced the formation of a carbonate to siliciclastic ramp in the Albian overlying salt layers (Eichenseer et al. 1999; Séranne 1999). During the Late Cretaceous, sedimentation was dominated by open marine and pelagic sediments of the Iabe Fm (Broucke et al. 2004), which later generated thermogenic hydrocarbons in the area (Burwood 1999; Cole et al. 2000; Brownfield & Charpentier 2006). From Paleocene to Eocene, condensed pelagic sediments filled the basin (Broucke et al. 2004). The Eocene–Oligocene transition is characterized by a major erosional event (Séranne et al. 1992; McGinnis et al. 1993; Broucke et al. 2004), likely linked to a global sea level fall (lowstand icehouse-induced conditions) and a coeval uplift of the inner margin (Haq et al. 1988; Marton et al. 2000; Miller et al. 2005). The resulting unconformity is overlain by siliciclastic sediments that prograded from the East (Malembo Fm) (Teisserenc & Villemin 1989; Séranne et al. 1992; Broucke et al. 2004). The Oligocene–Miocene interval is composed of submarine channels encased in the Malembo Formation (Fig. 2). Oligocene and Miocene channel complexes are, typically and respectively, approximately 2 km wide (up to 10 km including external levees) and 100–150 m thick, and approximately 2 km wide and 150–200 m thick (Broucke et al. 2004). They are the first components of the Congo fan, in which sand deposits are induced by high-frequency climatic variations (Brice et al. 1982; Uchupi 1992; Droz et al. 1996), and/or sea level changes (Broucke et al. 2004), and/or autocyclic processes (Dott 1988; Einsele et al. 1991). Growth structures, faults, turtle-back anticlines, and salt diapirs, affected the paths of submarine channels (Fig. 3) (Broucke et al. 2004). From the Pliocene onward, submarine deposits were delivered beyond the salt escarpment directly onto the abyssal plain, so that the study area only undergoes hemipelagic mud sedimentation.
Database and Methodology
In this article, we present the interpretation of two 3D HR (Three-Dimensional High-Resolution) seismic surveys covering an area of about 20 000 km2 in the Lower Congo basin. Approximately 300 km2 of this data set is used in this study (Fig. 1). The surveys have a bin spacing of 12.5 m, and they were acquired with a dominant frequency of 60 Hz in the interval of study. Surveys are zero-phase processed and normal polarity, meaning that a negative amplitude (displayed in red on seismic profiles) corresponds to a downward decrease in acoustic impedance with depth, while positive amplitudes (displayed in black) represent a downward increase in acoustic impedance. Both amplitude and coherency volumes were available for the 3D seismic survey.
Tertiary clay-rich sediments in recent deep-sea fans have a velocity range around 1800–2200 m s−1 (Hamilton 1976). Based on the data from the nearest well to the study area state distance, the interval velocity of the studied interval is around 2 km s−1 (with which 1 ms converts to 1 m). The maximum vertical resolution can be considered as the quarter of the dominant wavelength (λ/4), while the maximum horizontal resolution can be approximated as half of the dominant wavelength (Brown 1999, 2004). Consequently, with an interval velocity of 2 km s−1 in the study area, the maximum vertical resolution is about 8 m and the maximum horizontal resolution is about 16 m.
Conical and saucer-shaped amplitude anomalies are present in a restricted interval on 3D seismic data, defined by mapping of the two reflections constituting its lower (B) and upper (T) boundary on seismic sections. The 3D geometry of the anomalies was investigated by horizons and cross sections.
Seismic attribute maps, including amplitude, dip, and coherency extractions, were used around and within the studied interval during the analysis of the relation between anomalies and surrounding sediments. Mapping was carried out through horizon automatic tracking and isoproportional slices between horizons. Time thickness maps were also used to evaluate the topography at different stages of sedimentation.
Conical and saucer-shaped amplitude anomalies are located approximately 10 km from Well-1 (Fig. 3), and their sedimentology and age are partially inferred from this well and other wells located in the study area.
Seismic Observations on the Occurrence and Morphology of the Anomalies and their Relationships of the Encasing Rocks and the Submarine Channels
The study area is located between two salt diapirs. A system of conjugate normal faults of several kilometers in length, with principal directions NE–SW and NW–SE, developed as a result of diapir growth (Fig. 3). The studied conical and saucer-shaped anomalies are located between these two salt diapirs and are not cut by the fault system. They are aligned in the dip direction (NE–SW) over stripe 20 km long and 1 km wide on the southeast edge of the 3D seismic survey area studied (Fig. 3). The investigated interval (Upper Miocene–Pliocene) is mainly composed of low-amplitude, parallel, and locally draping facies interpreted as argillaceous deposits. Their monotony is locally interrupted by two channel-shaped and a few lobe-shaped seismic anomalies identified between the two salt diapirs (Figs 2 and 4). In plan view, the uppermost Miocene channel-shaped seismic anomaly (Channel 1) is straight to slightly sinuous, 1–1.5 km wide, and trends northeast – southwest. Conversely, the Pliocene channel-shaped seismic anomaly (Channel 2) is slightly sinuous, 500–1 km wide, and trends parallel to Channel 1 with a local change in direction near the southeastern salt diapir (Fig. 3). In section view, both channels are approximately 60–80 m thick, and the lobes are a few meters thick (Fig. 4). Mass transport complexes (MTC) were identified on the seismic sections by their chaotic seismic signature (Fig. 4). In addition to the salt diapirs and the MTC, which remobilize sedimentary deposits in the study area, we find pockmarks of approximately 1 km in diameter aligned above Channel 2 (Pliocene), also argillaceous successions of up to 100 or 200 m that have been deformed by a system of polygonal faults (Figs 3 and 4). This type of postdepositional deformation is very common in the Lower Congo Basin (Gay et al. 2006a,b; Gay et al. 2007; Andresen & Huuse 2011).
Stratigraphic correlation provided by Well-1 (Fig. 3) and other nearby wells shows that the conical and saucer-shaped reflections occur in the uppermost Miocene age. It is about 600 ms TWT beneath the sea bottom (Fig. 4). They are identified within a hemipelagic succession affected by a polygonal system of normal faults (Fig. 4). The bottom and top of this polygonal fault system (PFS, Cartwright 2011) are, respectively, identified as Horizons B (uppermost Miocene) and T (Top Miocene) in the figures accompanying this article (Fig. 4). This interval has an average thickness of 100 m in the vicinity of the conical and saucer-shaped reflections. Its precompaction thickness can be estimated at 160 m using the porosity-depth profiles of Hamilton (1976). This sedimentary interval thins southwestwards (downslope) and thickens to the southeast where it fills a roughly circular local topographic depression, caused by the rising salt diapir southeast of the study area (Figs 3 and 4) (Broucke et al. 2004).
Geometry and scale
Mapping of the conical and saucer-shaped reflections in the study area has allowed their overall geometry to be estimate. Our discussion of their cross-sectional geometry is based on three representative seismic sections (Fig. 5). The anomalies are typically U- or V-shaped on seismic sections, with a maximum thickness of about 20 m (Fig. 5). In three dimensions, the studied reflections have a circular to elliptical base whose diameter may range from 10 m (V-shaped) to several hundreds of meters (U-shaped), from which lateral wings are developed with slopes of 25 to 50o, forming conical or saucer-shaped structures (Fig. 6). Anomalies with conical or saucer-shaped geometry have been identified locally in this interval throughout the studied area (Fig. 3), but it is even more common to see simple concentric amplitude anomalies which do not develop visible lateral wings. In addition, it appears that in certain cases, the sediments located directly above the sides of cones are seismically chaotic and form smaller cones (two to three times smaller), here called chaotic conical anomalies, which develop along the continuations of the flanks of the underlying cones (Fig. 5A,B).
The studied reflections have heights between 20 and 80 m and diameters between 50 and 300 m; their apices are not invariably on the same seismic horizon, but they are never lower than Horizon B (Figs 4-7). The basal concordant beds of the U-shaped anomalies are located along a more reflective bed of the surrounding sediments (Fig. 5C). The apices of the V-shaped anomalies are systematically deeper than the basal concordant beds of the U-shaped anomalies. The conical and saucer-shaped reflections define volumes of about 1–5 × 104 m3. The apices of the conical reflections point downwards; consequently, the cones diameters increase upwards through the stratigraphy (Fig. 6). The conical anomaly in Fig. 6 is rooted in Horizon B (Fig. 6.1) and reaches its maximum diameter of 250 m 30 m higher up (Fig. 6.3). This cone opens into a second cone 10 m above its apex (Fig. 6.2), along the continuation of its southeastern edge, forming a new cone with a maximum diameter of 150 m (Fig. 6.4) and a maximum height of 50 m on its south side (TWT map).
Relationships with the argillaceous host rocks and the submarine channels
The first part of this study has shown that the anomalies are aligned in the dip direction in a limited interval and also that in three dimensions, they sometimes form cones several tens of meters high. The aim here is to define the relationships and interactions between the anomalies and the host strata in which they formed.
Conical and saucer-shaped anomalies versus encasing series
The dipping reflections of the conical and saucer-shaped anomalies are clearly discordant with the enclosing strata: They cut and deform the surrounding argillaceous deposits, and disturb their lateral continuity (Figs 5 and 6). Deposits beyond the sides of the cone appear unaffected, whereas internal deposits are uplifted by several tens of meters from their regional trend (Fig. 5). The uplift of the argillaceous host succession is expressed at the top of the interval affected by the conical and saucer-shaped anomalies (Horizon T). This horizon is given by the sediments that postdate the deformation and show onlap onto these uplifts (Fig. 5B,C). These uplifts have a circular shape and range from 10 to 30 m thick and 100 to 300 m in diameter. On seismic sections, the uplift at the top of the interval does not always match the uplift of the underlying sediments. In this case, the uplifted sediments seem to have collapsed into the zone represented by the chaotic conical anomalies (Fig. 5A,B).
Lastly, beneath the apices of the conical reflections or beneath the concordant bases of the saucer-shaped reflections, we see a disturbance of the seismic signal as follows: pull-up of the horizons of about 10 ms TWT visible over more than a hundred ms (TWT) vertically (maximum 150 ms), and high-amplitude columns above the conical and saucer-shaped reflections, passing through Horizon T (Fig. 7) with a circular geometry in map view.
Conical and saucer-shaped anomalies versus submarine channels
The conical and saucer-shaped anomalies are preferentially aligned in a NE–SW direction, like the submarine channel and lobe complexes in the study area (Fig. 3) located 100 ms TWT below. The submarine channel complexes comprise a system of 200–300 m wide individual sinuous channels. These individual channels develop within a major erosion zone 2 km wide and 150–200 m thick (Fig. 4) (Broucke et al. 2004; Labourdette & Bez 2010). The channel complexes also have very poorly developed external levees (sensu Kane and Hodgson, 2011) and therefore are considered to be dominantly erosional channels. The lobe complexes are made up of massive sands in the area (Broucke et al. 2004). They show the characteristics of confined terminal lobes by the depocenter encountered between the salt diapirs (e.g., Adeogba et al. 2005; Shultz & Hubbard 2005; Deptuck et al. 2008), fed by elementary channels of the channel complexes.
The interval of conical and saucer-shaped anomalies interval is bounded at its base by a lobe-shaped body (Lobe 1) crossed by an Upper Miocene channel complex (Channel 1) southwest of the anomalies and is bounded at its top by a succession of lobe-shaped bodies (Figs 3 and 4). The upper boundary (Horizon T) is also intersected southwest of the anomalies by a highly erosive Pliocene channel (Channel 2). A series of pockmarks ranging in diameter from a few hundred m to 1 km are aligned above its axis (Figs 3 and 4). We note the presence of a 20-to-40-m-thick MTC above Horizon B (Fig. 4). This mass flow possibly traveled in the direction of the circular depocenter developed during the late Miocene – Pliocene period (Fig. 8).
Amplitude maps allow the definition of lithological contrasts between coarse-grained lithologies (high amplitudes) mainly deposited in topographic lows, and fine-grained lithologies (low amplitudes). The amplitude map of Horizon B (Fig. 9) enables silty to sandy lithologies to be distinguished from argillaceous lithologies and thereby reveals the morphology of Lobe 1, which is elongated along the slope of the basin (NE–SW), with an average width of 6 km. The geographic correspondence between the northwest edge of the lobe and the alignment of the conical and saucer-shaped amplitude anomalies (Figs 3 and 7) will be one of the subjects of our discussion.
Over the last 10 years, several examples of seismic scale sandstone intrusions have been described in literature (Molyneux et al. 2002; Shoulders & Cartwright 2004; Huuse et al. 2007). Some of these studies show that sand injection can produce conical and saucer-shaped structures of a scale even larger than those examined in this study.
Because of the discordant geometry and the postsedimentary uplift above the cones and saucer-shaped bodies, we interpret these bodies as intrusions of material from deeper strata. The timing of intrusions is well constrained, mainly based on geometric evidence (Shoulders & Cartwright 2004). The uplifted beds above the cones and saucer-shaped bodies have the same thickness as those nonaffected, so their formation predates or is time equivalent to the last uplifted horizon. The sediments that onlap these uplifts (Fig. 5B,C) postdate the formation of the conical and saucer-shaped bodies. This means that (i) Horizon T represents the seabed at the time of sand intrusions and (ii) that the window of activity was very short and occurred around Miocene – Pliocene boundary (horizon T). The velocity of formation of the conical and saucer-shaped bodies was therefore faster than the sedimentation rate.
Lithology of the cones and saucer-shaped intrusions
As no well has intersected the observed conical and saucer-shaped anomalies, their lithology must be determined indirectly. The presence of argillaceous sediments in the deposits surrounding the conical and saucer-shaped anomalies is suggested by their very continuous and parallel character, and the interpretation of well logs from nearby wells. From the Oligocene to the present day, the sedimentary succession in the study area consists mainly of hemipelagic clays, locally alternating with submarine sandstones. Albian salt diapirs have deformed and intruded the sedimentary succession.
The high-amplitude conical and saucer-shaped anomalies contrast sharply with the low-amplitude argillaceous host. This difference in amplitude indicates a strong impedance contrast, but does not exclude the possibility that there are two mudstones with different compaction state or composition. The visible disturbance of seismic horizons beneath the studied structures (Fig. 7) can be explained by a pull-up effect induced by higher velocities in the sediments composing the intrusions, compared with the velocities of the clay-rich host rock sediments (Andresen et al. 2009). Sediments with higher impedances than the clays may be sand, carbonates, magma, or salt. No igneous material is described from the Cenozoic succession in the Lower Congo basin, and therefore, an igneous origin is excluded. Conical and saucer-shaped carbonate and salt intrusions are unknown in the literature. The intrusions are encased in a shaly host rock and are bounded by sandy lobes. Accordingly, we interpreted the reflections as sandstone intrusions. Commonly in the North Sea, conical and saucer-shaped injectites are carbonate cemented possibly related to methane-related microbial carbonate precipitation (Løseth et al. 2003) or due to long-term fluid seepage through the intrusions (Jonk et al. 2003, 2005).
Comparison with North Sea and Faeroe–Shetland seismic scale sand injectites
The sandstone intrusions studied have either a conical (Fig. 5A,B) or saucer-shaped (Fig. 5C) geometry. Sand injectites either propagate vertically before feeding 20–50° dipping dykes (conical shape) or form a basal laccolith before feeding 20–50° dipping dykes (saucer shape) (Fig. 10). Conical and saucer-shaped sandstone intrusions have been widely documented in the Paleogene sediments of the North Sea (e.g., Molyneux et al. 2002; Løseth et al. 2003; Huuse & Mickelson 2004; Shoulders & Cartwright 2004; Huuse et al. 2007; Shoulders et al. 2007; Cartwright et al. 2008). These intrusions have a similar shape to these studied here but with larger dimensions, that is, ranging from 100 to 2000 m in diameter, 50–300 m in height, and 1–80 m in thickness (Cartwright et al. 2008). Extrudites have been suggested to occur when theses intrusions reach the seabed (Huuse et al. 2004, 2005; Shoulders & Cartwright 2004; Hurst et al. 2007). There is no direct evidence of extrudites in the study area to redefine the paleo-seabed during injection, but the upward intrusion of material in the sediments induced the uplift of the overlying sediments up to the paleo-seabed (Huuse et al. 2004; Shoulders & Cartwright 2004). The uplift induced fractures that intersected the paleo-seabed (Horizon T) during the process of sand injection and also induced domal forced folds on the paleo-seabed (Fig. 5A–C) that are probably enhanced by the differential compaction between sand and surrounding shaly sediments as observed above submarine sandbodies (e.g., Jackson 2007). Also, the actual dip of the limbs of conical and saucer-shaped sandstone intrusions is likely smaller than the original emplacement angle due to the later compaction of surrounding sediments (Huuse et al. 2004; Shoulders et al. 2007). Conical intrusions of the Faroe-Shetland Basin can develop a succession of other sandstone cones at the end of their flanks until they reach the paleo-seabed (Shoulders & Cartwright 2004; Shoulders et al. 2007). These features potentially occur in our study area (Fig. 6) as small chaotic cones observed above the conical sandstone injectites of the study induced a collapse of the overlying paleo-seabed (Fig. 5A,B); as a result, we suggested that they are not sand cones above sand cones as observed in the Faroe-Shetland Basin (their origin will be discussed later in this article).
The presence of sandstone intrusions of seismic scale indicates the existence of a parent sand body and therefore that a flow of sand from this sand body took place over several tens or hundreds of meters (Hurst et al. 2003; Cartwright et al. 2008). As mentioned earlier, the conical and saucer-shaped sand intrusions are aligned with the northwest edge of Lobe 1, which is the highest point of this lobe (Fig. 4). We therefore suggest that vertical migration of fluids through the argillaceous sediments transported sand from the silt-sand pinchout of the lobe toward the paleosurface. It means that the sand in the lobe fringe is presumably ‘clean’, in contrast to lobe fringes that are rich in hybrid beds (e.g., Ito 2008; Hodgson 2009). The seismic acquisition does not allow vertical reflectors to be imaged, and so, it is not possible to see whether there are conduits that could connect the cone and saucer-shaped bodies to the lobe. Therefore, we suggest two possible connections: (i) Pipes or columnar intrusions (e.g., Huuse et al. 2004; Chan et al. 2007) and (ii) Planar dykes as wing structures developed on the margins of depositional sand bodies (e.g., Lonergan & Cartwright 1999; Jackson 2007). Large-scale sand injection requires high volumes of sand and their transport through the argillaceous cover, which necessitates more than 50 % of fluids to carry the sediments (Maltman 1994; Hurst et al. 2003; Duranti 2007; Cartwright et al. 2008).
The forced ascending intrusion of sand into the sedimentary column is usually attributed to hydraulic fracturing (Lonergan et al. 2001; Hurst et al. 2011; Mourgues et al. 2012). Hydraulic fracturing may occur in response to an increase in pore pressure if the fluid pressure (Pf) in the sand exceeds the minimum principal stress (S3) plus the tensile strength (T) of the host rock (Price & Cosgrove 1990; Cosgrove 2001). Both tensile and shear hydrofracturing of host strata can occur (Hurst et al. 2011). Hydraulic fractures propagate parallel to the maximum compressive stress direction (S1) and perpendicular to the minimum compressive stress direction (S3) (Anderson 1951; Delaney et al. 1986). This mode of fracturing occurs if Pf > S3 + T and S3 + T < S2 < S1 (Hurst et al. 2011). The creation of this space, which was necessary for the emplacement of the sand intrusions studied in the study area, caused pronounced uplifts in the paleosurface T (Figs 4-7). These uplifts are commonly called forced folds or domal forced folds (Cosgrove & Hillier 2000; Hansen & Cartwright 2006; Cartwright et al. 2008).
Process of formation of the sand injectites
The forceful intrusion of remobilized clastic sediment form by injection of fluidized sand (Duranti & Hurst 2004; Ross et al. 2011) from an overpressured sand unit into hydraulically fractured low-permeability sediments (Cosgrove 2001; Jolly & Lonergan 2002). Overpressures in deep-sea environments can occur for a large range of reasons but are mainly due to the disequilibrium compaction and hydrocarbon (gas) generation and the lateral transfer of pressure (Osborne & Swarbrick 1997; Swarbrick & Osborne 1998; Grauls 1999; Swarbrick et al. 2002). For the injection and fluidization of unconsolidated sand to occur, a trigger mechanism is commonly required (Jolly & Lonergan 2002; Oliveira et al. 2009) such as (i) earthquake (e.g., Obermeier 1996, 1998; Boehm & Moore 2002; Huuse & Mickelson 2004; Levi et al. 2011), (ii) tectonic stress (e.g., Vitanage 1954; Harms 1965; Scholz et al. 2009), (iii) localized excess pore fluid pressures generated by deposition-related processes (e.g., Truswell 1972; Taylor 1982; Rijsdijk et al. 1999; Rowe et al. 2002; Callot et al. 2008), and (iv) the influx of an overpressured fluid from deeper within the basin into a shallow sand body (e.g., Jenkins 1930; Brooke et al. 1995; Jolly & Lonergan 2002; Molyneux et al. 2002; Duranti & Mazzini 2005; Jonk et al. 2005; Andresen et al. 2009). A less common trigger process cited in the literature is the mechanical failure of hydrocarbon reservoirs in the shallow subsurface caused by the buoyancy effect of hydrocarbons (Sales 1993; Jonk 2010).
In the study area, the onset of the maturation and migration of hydrocarbons (oil and gas) took place during the late Early Miocene (approximately 18 Ma), while filling of the Miocene reservoirs occurred during the Late Miocene to Pleistocene (approximately 5 Ma to the present day; based on a 3D basin modeling study carried out by Total, 2000), which coincides with the timing of the sand injection. Consequently, the Upper Miocene Lobe 1 was able to accumulate hydrocarbons rapidly after its burial. This hydrocarbon column was trapped in the northwest portion of Lobe 1 following local subsidence beneath the axis of the lobe, which caused its topographic inversion (distal portion higher than the central portion) (Figs 4 and 8). In view of the tectono-sedimentary analysis and on the hydrocarbon migration and filling history, the most likely process for the generation of overpressures in the shallow submarine fan of the study area, that is, in the parent unit, is attributed to the hydrocarbon buoyancy associated with the lateral transfer of (overpressured) fluids. The lateral transfer is a mechanism that can locally further enhance pore pressures at structural crests due to the transmission of pore fluid overpressure by water flow along laterally inclined sand reservoirs encased in a seal lithology (Mann & Mackenzie 1990; Yardley & Swarbrick 2000; Mourgues et al. 2011). The buoyancy effect is the pressure difference between two immiscible phases, that is, formation water and hydrocarbons, generated by density contrast (Swarbrick et al. 2002). The overpressure due to hydrocarbon buoyancy is in addition to the overpressure induced by the lateral transfer of fluids (Jonk 2010).
The seismic scale sand injectites studied are evidence of hydraulic fracturing of the cover rocks; it means that the leaks of fluids from the Miocene reservoirs took place under a seal of ‘hydraulic’ type (poorly permeable), as defined by Watts (1987). Cover rocks called ‘hydraulic’ have a very high capillary entry pressure Pe (very fine-grained clays, anhydrite, halite, etc.) such that capillary leaks of hydrocarbons cannot occur. Capillary leaking is the normal mode of seal failure under hydrostatic conditions or moderate overpressures (Clayton & Hay 1994). Rupture by fracturing generally takes place only in environments under high overpressures but is possible at shallow depths (Fig. 11). The conical and saucer-shaped intrusions in the study area were very probably initiated from the pinchout northwest of Lobe 1, at a depth of 160 m (assumptions of decompacted thickness) beneath an effective argillaceous cover. At this burial depth, the overpressure in the parent sand body could not have been induced by the disequilibrium compaction. We therefore suggest that the overpressure began at shallow depth and mainly under the effect of buoyancy pressure Phc caused by a column of hydrocarbons with a minor contribution from lateral transfer (Fig. 11) (Sales 1993; Grauls 1997, 1999; Osborne & Swarbrick 1997). The validity of this hypothesis is supported by a series of calculations based on theoretical equations.
Based on Pascal's principle, that is, the principle of transmission of fluid pressure, we calculated the necessary thickness of a hydrocarbon column to fracture its seal below 160 m of marine sediments, that is, the estimated burial of the source body of sand (NW margin of Lobe 1) during the formation of the injectites and under a 1000-m column of seawater. In our study area, the hydrocarbons may consist of oil and gas during sand injection. Therefore, the results of calculations will be given for oil and gas to define a range of thickness from a maximum value (only oil) to a minimum value (only gas). Pascal's principle is defined in the following equation:
where Δp is the pressure difference (Pa); ρ is the density of the fluid (kg m−3); g is the gravitational acceleration (m s−2); Δh is the height of fluid above the point of measurement (m).
The major horizontal stress S3 is determined graphically on Fig. 11A by the following equations:
where ρw is the density of the seawater (1030 kg m−3); Zsf the depth of the sea bottom below sea level (m); K0 is the neutral earth pressure coefficient or the ratio of the horizontal stress to the vertical stress (K0 ≈ 0.85); ρlitho is the bulk density of sediment (1800 kg m−3); b is the height of the overburden (Z – Zsf); ρhc is the density of the hydrocarbon (ρgas ≈ 200 kg m−3, and ρoil ≈ 800 kg m−3); and h is the thickness of the hydrocarbon column. All density values are extracted from Cathles et al. (2010). From equations 2 and 3, we deduced the value of the thickness of the hydrocarbon column (h) during the fracture initiated in the overburden:
In conclusion, at the time when the studied sand intrusions were being formed, Lobe 1 was buried under 160 m of argillaceous sediment. These sediments, which had a high capillary entry pressure (Pe), could have been hydraulically fractured by the buoyancy effect (Phc) produced by (i) a column of oil at least 350 m high, (ii) a column of gas at least 100 m high, and (iii) a column of oil and gas ranging from 100 m to 350 m high, trapped in the northwest margin of Lobe 1. The maximum case of hydrocarbon filling in the lobe is geometrically estimated at up to about 100 m during sand injection. This estimate corresponds to the actual thickness at the time when Horizon T was deposited (Fig. 8), or half the thickness currently observed on seismic. Therefore, unless we consider this maximum case and filling by gas only, the values of hydrocarbon thickness obtained seem too high to consider the buoyancy effect as the trigger mechanism of sand injection. In addition, the presence of many sandstone intrusions means that the fracture pressure (Pf > σ3 + T) was reached simultaneously in many points of the northwest margin of the lobe.
This situation is thought to be conceivable only if a sudden trigger event occurred just before sand injection. As a result, we estimate that the simpler trigger event of sand injection in our study area is the activity of a nearby fault in relation to diapiric movements, which can have induced seismic shaking (e.g., Boehm & Moore 2002) or allowed the rapid flow of overpressured fluid into Lobe 1 from a deeper geobody (e.g., Sibson 1981; Grauls & Baleix 1994).
Propagation mechanisms for sand injectites
The initiation of fracturing creates a hydraulic gradient between the tip of the fracture and the source body of sand, which can entrain particles of sand if the flow velocity (υ) exceeds the fluidization velocity (υfl) of the injected granular material (see fig. 11 in Vigorito & Hurst 2010). The low density and viscosity of gas are not favorable for fluidization; therefore, it may be possible that the driving force for the fluidization is the movement of the aqueous fluids accompanied by significant quantities of dissolved hydrocarbon gas (Jonk 2010). The role of hydrocarbon gas as a support to sand fluidization has been widely mentioned in the literature (Brooke et al. 1995; Hubbard et al. 2007).
As mentioned earlier, hydraulic fracturing follows planes perpendicular to the minor compressive stress plus the tensile strength (Pp = S3 + T). As a result, injectites usually have simple geometries like dykes (cutting across the stratigraphy) or sills (parallel to the stratigraphy), which may locally be reoriented by planes of weakness (Delaney et al. 1986; Grauls 1999). Conical and saucer-shaped geometries are less common and known only at large scale (Huuse & Mickelson 2004; Shoulders & Cartwright 2004; Huuse et al. 2007; Shoulders et al. 2007). They result from the interaction between the propagation of a fracture and the proximity of a free surface, which is deformed. These conical and saucer-shaped structures are also known from magmatic intrusions (Hansen & Cartwright 2006) or from fluid-escape structures of pockmark type (Gay et al. 2006b, 2012). The formation of the cones and the controlling parameters had recently been studied by Cartwright et al. (2008), and Mourgues et al. (2012). Cartwright et al. (2008) have proposed a simple model of apical cones formation and suggested that a small laccolith of sand forming at the top of a feeder dyke induced the rotation of σ1, thereby allowing the development of low-angle dykes. Mourgues et al. (2012) have used analog and numerical modeling to show in 2D that the formation of vertical fracturing (from a body of sand) requires a sufficiently strong effective vertical stress (fairly thick cover and low level of overpressure in the cover). At a critical depth, vertical propagation stops and the fracture splits into two dilatant branches, forming a ‘V’ shape (Mourgues et al. 2012). These two dilatant branches are taken over by shear zones which extend to the surface and strongly accentuate the amplitude of the forced folds (Fig. 5B). The depth at which the cones initiate is mainly controlled by the mechanical parameters of the host rocks, for example, their cohesion, and the level of fluid overpressure. Mourgues et al. (2012) have also demonstrated that the presence of distributed overpressures in the host rocks (related to under-compaction phenomena, for example) promotes the formation of cones at greater depth. In addition, the same authors have shown that the pressure field induced by the diffusion of overpressures around the sand body was the source of a stress rotation (Mourgues & Cobbold 2003), which also favored the formation of inclined fractures. The geometrical configuration of conical and saucer-shaped intrusions was previously discussed by Pollard & Holzhausen (1979), for igneous intrusions. These authors predicted that once the dimensions of the sill reached a critical value, the fracture interacted with the free surface, and the sill turned upward toward the surface. The formation of saucer-shaped sandstone intrusions may be explained by a greater competence contrast at a boundary (Cartwright et al. 2008). It is consistent with our own observations regarding the laccoliths of saucer-shaped intrusions, which are always located at the same stratigraphic level in the affected interval of sediments (Fig. 5C).
At the top of the conical injectite branches, smaller cones may form (Fig. 5A,B). Their passage deforms and remobilizes the sediments of the surrounding rocks, and the overlying domal forced fold is collapsed. Consequently, they are not interpreted as sand intrusions (e.g., Shoulders et al. 2007; Cartwright et al. 2008) but as deformation cones resulting from the migration of fluids (e.g., Gay et al. 2012). Thus, the sand intrusions are evidence of a propagation of fluids localized in the host rocks, whereas at shallower depths, deformation cones may form as a continuation of the sand cones, indicating that the migration is transforming into a flow of fluid distributed throughout the unconsolidated sediments (Gay et al. 2012). The expulsion cones of fluid formed at the top of the injectite branches lead to a collapse of the sediments up to the paleosurface (Horizon T), which indicates that they are coeval with emplacement of the injectites. Lastly, fluid-expulsion chimneys located above some of the sand injectites revealed a new phase of leakage after burial of Horizon T (Fig. 7). These hydrocarbon (gas) leaks are evidenced by the presence of pockmarks above Channel 2 (Figs 3 and 4). We therefore suggest that hydrocarbons continued to migrate within the reservoirs and along the injectites after additional burial. Some authors have already shown from fluid inclusion and stable isotope data that large-scale sand injectites act as long-term fluid conduits (Hurst et al. 2003; Jonk et al. 2003, 2005).
Implications and possible misinterpretations
Interpreting the conical and saucer-shaped reflectors as sand injectites implies a major remobilization event between the deposition of the Upper Miocene channel complexes and that of the overlying Pliocene sediments. Previous studies on the sediment remobilization show that the Pliocene is a period of large-scale remobilization of fluids in the Lower Congo Basin (Gay et al. 2006a,b, 2007; Nöthen & Kasten 2011). Thus, it is likely that other seismic scale injectites of this age will be found in the future in this basin.
Large-scale sand intrusions not only have major reservoir implications (connections, changes in geometry, etc.), they are also both evidence and vectors of the migration of fluids in the Lower Congo Basin. It is possible to reconstruct the history of fluid migration in the study area from the Miocene to the Pleistocene (Fig. 12). Hydrocarbons have filled the sands contained in Channel 1 and Lobe 1 during the Late Miocene. During this filling (Mio-Pliocene transition), the buoyancy effect of hydrocarbons induced build up of overpressure in the sandbodies. Subsequent diapiric movements induced nearby fault displacements responsible for the critical overpressure reached and subsequently hydraulic fracturing of at the overburden above the northwest fringe of Lobe 1 (Fig. 12B). Fracturing propagated up to the seabed, allowing a mixture of sand, hydrocarbons, and water to escape from the reservoirs. The hydrocarbon probably dissipated at the surface, and the sand accumulated in the open fractures, forming the conical and saucer-shaped injectites. Hemipelagites and younger submarine lobes onlapped the seabed deformation by the injectites, while Channel 1 was partially eroded by Channel 2 (Fig. 3). Any hydrocarbon which continued to migrate accumulated in the conical and saucer-shaped injectites and within Channel 2 (km-scale pockmarks). The hydrocarbons continued to flow in sand injectites a long-time after their emplacement and escape via expulsion chimneys in Pleistocene cover rocks (Fig. 12C).
Sandstone intrusions, like those observed in this study, cannot constitute significant petroleum reservoirs because they are too small (1–5 × 104 m3). However, similar objects of larger scale, drilled in the North Sea, reveal good lithologic characteristics (clean sand, washed of all fine particles, and well sorted), which can represent true reservoirs (MacLeod et al. 1999; Duranti & Hurst 2004; Huuse & Mickelson 2004; Huuse et al. 2004, 2005; Briedis et al. 2007; De Boer et al. 2007).
The characteristic ‘V’ or ‘U’ shapes on the seismic sections (2-D view) can result from a variety of geologic processes. These include (i) erosional submarine channels (Broucke et al. 2004; Labourdette & Bez 2010), (ii) indicators of expulsion, such as pockmarks, pipes (Gay et al. 2006a,b, 2007, 2012), and (iii) intrusions of mud, sand, or igneous material (Huuse et al. 2007; Shoulders et al. 2007; Cartwright et al. 2008). The preceding discussion on lithology has shown that intrusions cannot be considered as mud or igneous intrusions. Similarly, we can exclude submarine channels because the reflections studied are subcircular in map view.
Fluid-escape features generally appear in fine-grained, unconsolidated sediments, as cone-shaped depressions, called pockmarks. Their diameters can vary from a few meters to 300 m or more and their depths from 1 m to 80 m (Hovland et al. 1984; Gay et al. 2006b). These objects can thus have the same morphology and be of about the same size as the conical reflectors in the study area (Figs 3 and 4). Typically, pockmarks are concentrated in fields several km2, and their arrangement is a direct reflection of underlying features, such as submarine paleochannels, or suggesting structural control on the flows of fluids (Gay et al. 2006a, 2007; Nosike et al. 2010). However, unlike the sand intrusions, the sediments overlying the pockmarks are evacuated and thus not present above the surface (Fig. 4). In conclusion, the uplift or subsidence of the overlying sediments is the primary criterion to discriminate pockmarks from conical sand intrusions.
Our study in the Lower Congo Basin has revealed the presence of large-scale conical and saucer-shaped objects (1–5 × 104 m3). We have interpreted these objects as intrusions of sand in argillaceous cover rocks. We have shown that these intrusions are aligned above the fringe of an Upper Miocene lobe this is interpreted to be the parent sand. This forced penetration of sand toward the surface created changes in the cover, in particular by uplifting the sediments above the injectites, recorded all the way to the paleosurface. This study has shown that
The formation of sand injectites represents a very short duration event (faster than the sedimentation rate) around the Miocene-Pliocene boundary.
Sand injection was initiated at the updip fringe of a submarine lobe. Local subsidence due to diapiric salt movements induced the topographic reversal of this lobe before injection.
Hydrocarbon filling of Miocene reservoirs took place during the Late Miocene in the study area. Therefore, prerequisite overpressure to the sand injection process may be due to the buoyancy effect of hydrocarbons accumulated in the lobe. Additionally, overpressure could have been enhanced by the lateral transfer operating in the inclined margins of the lobe.
The very short duration of sand injection and the presence of many sandstone intrusions suggest that the process of injection was triggered by a sudden event. A likely trigger is nearby fault displacement related to diapiric movements. This event can induce an earthquake or allowed the rapid overpressured fluid flow into the lobe from a deeper geobody.
At a critical depth, fluid-expulsion cones can form at the tips of the sandstone intrusions, deforming the shallow unconsolidated sediments.
In conclusion, sandstone intrusions are most likely present elsewhere in the basin fill and play the role of vertical conduits, through which fluids are drained from submarine reservoirs to the top of the sand injectites. They are thus evidence of fluid propagation located in the host rocks. A new stage of fluid migration took place from the tips of the injectites after their formation (expulsion chimneys), also above Channel 2 (pockmarks). This study shows that sand injectites of seismic scale are evidence for and vectors of fluid migration; consequently, they show the importance of identifying and understanding the post-sedimentary fluid processes which develop in a basin.
This study was directed by the Faculty of Geosciences of the Université de Montpellier 2. We thank Total S.A. and their partners for providing the seismic data and also for having agreed to the publication of the results. We also thank D. Grauls for rewarding discussions on the effectiveness of the cover rocks and hydrocarbon migrations. We would like to thank the reviewers, Mads Huuse and David Hodgson, for their comments that improved the manuscript.