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Article

Deformation and Evolution of Akata Formation Mudstone in the Niger Delta Basin: Insights from Analogue Models

by
Shuaiyu Shi
1,2,
Wenlong Ding
1,2,
Yixin Yu
3,4,* and
Jixin Zhang
5
1
Key Laboratory for Marine Reservoir Evolution and Hydrocarbon Abundance Mechanism, Ministry of Education, China University of Geosciences, Beijing 100083, China
2
School of Energy Resources, China University of Geosciences, Beijing 100083, China
3
State Key Laboratory of Petroleum Resources and Engineering, China University of Petroleum (Beijing), Beijing 102249, China
4
College of Geosciences, China University of Petroleum, Beijing 102249, China
5
Shandong Institute of Petroleum and Chemical Technology, Dongying 257061, China
*
Author to whom correspondence should be addressed.
J. Mar. Sci. Eng. 2025, 13(3), 590; https://doi.org/10.3390/jmse13030590
Submission received: 13 February 2025 / Revised: 12 March 2025 / Accepted: 14 March 2025 / Published: 17 March 2025
(This article belongs to the Topic Basin Analysis and Modelling)
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Abstract

:
The Niger Basin is a typical marginal basin with complex internal structures and abundant oil and gas resources, exhibiting unique marine geological characteristics and processes. The distribution and deformation characteristics of Akata Formation mudstone in the basin significantly influence hydrocarbon accumulation. In this study, four analogue models were used to analyze the main factors affecting mudstone tectonics and establish an evolution model of mudstone structures. The results show that the tectonic features in the basin reflect the combined influence of gravity sliding and spreading. The main mechanism driving mudstone deformation is gravity spreading caused by differential loading. The basement morphology is the decisive factor in the development of zonation involving extension, translation, and contraction zones. The development of mudstone structures is also affected by the inclination of the basement slope and the thicknesses of both the mudstone layer and overlying layers. A relatively large basement slope inclination is conducive to the rapid flow of mudstone, leading to the rapid development of mudstone formations. A thin mudstone layer with weak plastic mobility is favorable for the full development of mudstone formations. A relatively thick overburden leads to enhanced gravity spreading, which in turn leads to the formation of larger and more numerous mudstone structures. The formation and evolution of mudstone structures in the Niger Basin involved through three stages: (1) during the Paleocene–Middle Oligocene, thick marine mudstone was deposited; (2) in the Middle Oligocene–Late Oligocene, the mudstone and overlying layers were strongly deformed, and numerous mudstone structures developed with tectonic zonation; and (3) since the Pliocene, the tectonic activity in the basin weakened. The simulation of the evolutionary process and evolutionary model established in this study improves the understanding of mudstone tectonics and provides a reference for analyzing the genetic mechanism and hydrocarbon exploration in the basin.

1. Introduction

The Niger Delta Basin, as a typical passive continental margin basin, has developed a Tertiary deltaic sedimentary system with a thickness of approximately 12 km [1]. Among these deposits, the Akata Formation shale serves as an excellent source rock, characterized by high organic matter abundance (TOC content generally exceeding 2%) and significant hydrocarbon generation potential (dominated by Type II–III kerogen) [2]. Its sedimentary deformation provides favorable dynamic conditions for hydrocarbon migration and accumulation. Delta front facies and turbidite channel sandstones constitute high-porosity and high-permeability reservoirs, while the overlying thick interbedded shales provide effective regional seals [3]. This high-quality source–reservoir–seal combination, along with its extensive spatial distribution, endows the basin with abundant hydrocarbon reserves, making it one of the world’s most significant petroleum exploration regions [4,5,6]. In recent years, with advancements in drilling technology, exploration has gradually shifted from onshore and shallow-water areas to deep-water regions. In onshore and shallow-water areas, oil and gas fields are primarily associated with rollover anticline traps related to growth faults. In contrast, in deep-water areas, major oil fields such as Alba and Erha, as well as Agbami, Bonga, and Nnwa, are predominantly characterized by structural traps formed by mud diapirs and compressional folds [7,8].
The gravity decollement structure is a common tectonic pattern in passive continental margin basins on both sides of the South Atlantic Ocean. This structure is the sum of the structures formed by the sliding and deforming of the geologic body along slip surfaces under the action of gravity [9,10,11,12]. Due to its complex geologic characteristics and controlling effect on the accumulation and distribution of oil and gas, gravity-driven deformation is important for oil and gas exploration in many basins, including the mudstone structures in the Niger Basin, magmatic diapirs in the Pearl River Mouth Basin, and the salt structures in the Kwanza Basin [13,14,15,16]. The structural characteristics and formation conditions of gravity-slip tectonic features have been extensively studied using three-dimensional seismic data, equilibrium profile restoration, and simulations [17,18,19,20]. Hudec and Jackson (2007) [18] reported that gravity-driven deformation on passive margins is generally controlled by gravity sliding and spreading. Simulations have shown that gravity-slip tectonics are controlled by many factors, including the viscosity of ductile layers, the sealing ability of cap layers, denudation, tectonic stress, and basement inclination and uplift [20,21,22]. However, these past works mainly focused on salt structures; research on the genetic mechanism of mudstone structures remains insufficient. Although the deformation process of mudstone structures has many similarities with that of salt structures, there are obvious differences in the properties of the plastic detachment layers [23,24]. These differences may lead to different basin tectonic patterns and oil and gas accumulation characteristics.
Existing studies in the Niger Delta have focused on the characterization of mudstone structure, analysis of the sedimentary system, the effects of geologic variables on basin structures, and the influence of mudstone structures on oil and gas reservoirs [2,8,25,26,27,28,29,30,31]; 2D seismic interpretations have revealed the zonal characteristics of mudstone structures in the Niger Delta Basin [1,8,27,29]. By analyzing parameters such as fault displacement and sedimentation rates, the evolutionary processes of the offshore tectono-stratigraphic and the fold-thrust belt in the deep-water region have been clarified [26,30]. The development of the fold-thrust belt significantly influences the faulting of reservoir sand bodies, thereby creating effective lateral sealing conditions [30]. Integrating well logging and 3D seismic data, the depositional models of the reservoirs have been further elucidated, highlighting the control of sedimentary dynamics factors such as waves, fluvial, and sea-level changes on the heterogeneity and complexity of sedimentary strata and reservoirs [2,25,28,31]. Few studies have focused on the factors controlling mudstone structure formation and tectonic evolution, which limits the in-depth analysis of the factors that influence mudstone tectonic deformation. Despite the conduction of numerical and physical simulations pertaining to the basin [32,33], the morphology of the rifted basement has been neglected in these considerations. Wu et al. (2015) [33] utilized the progradational style and the underlying oceanic fault zone as experimental variables to investigate the controlling factors of mud diapirs. However, the basin basement was simplified as a flat horizontal surface, with the influence of the basement dip angle overlooked.
In this study, physical modeling of tectonic processes was established based on previous works and the basement morphology of an actual stratigraphic section in the Niger Delta. The effects of gravity sliding and spreading on the development of mudstone tectonics were clarified. In addition, the effects of mudstone thickness, overburden thickness, and basement morphology on the evolution of mudstone structures were comparatively analyzed. An evolution model for mudstone structures was established to provide a reference for mudstone structures analysis and evaluation of favorable zones for oil and gas exploration in the Niger Basin.

2. Geologic Setting

The Niger Delta Basin is located in the Gulf of Guinea on the margin of West Africa, with an area of approximately 360,000 km2 [4]. The Niger Basin lies at the triple point of the oceanic crust–land crust interface, the Pan-African Rift, and the Central African Transform Fault. The basin is bounded by the Benue Trough to the northeast, the Benin Basin to the west, the Atlantic Ocean to the south, and the Cameroon Volcanic Belt to the southeast (Figure 1) [34].
As for the other basins lining the West African coast, the tectonic history of the Niger Basin is closely related to rifting processes [36]. The formation and evolution of the basin can be divided into two stages: (1) the Rift stage (Early Cretaceous Aptian to Late Cretaceous Santonian). In the early Cretaceous, with the expansion of the South Atlantic, a graben-type basin dominated by extensional normal faults was formed, and the southern part of the basin was connected with the Cameroon Volcanic Belt. During this period, a series of thermal deposition events occurred, forming the Andean uplift that provided the heat source for the maturation of organic matter [37,38]. (2) Drift stage (Late Cretaceous Campanian to Holocene). By the Oligocene, the Atlantic Ocean had evolved from an early rift valley to a vast ocean, forming a mature passive continental margin. The basement of the ocean was uplifted, forming a basement pattern with a concave center and two uplifted ends. Large-scale transgression and regression occurred alternately, and large-scale sedimentation deltas began to be deposited [39]. With the continued thickening of sedimentary strata and the thermal effects provided by Cenozoic magmatic activities (e.g., the Cameroon Volcanic Line), the plastic mobility of shale is significantly enhanced. Consequently, under the driving forces of gravitational spreading and differential loading, shale structures begin to develop extensively.
The Niger Delta can be divided into three lithostratigraphic units, that range in age from Paleocene to recent. From bottom to top, these units are the Akata, Agbada, and Benin Formations, which are all diachronous stratigraphic units (Figure 2). The Akata Formation is mainly composed of pre-delta and shallow marine mud shales with thicknesses reaching 6000 m. This formation is the dominant hydrocarbon source rock in the basin [40]; with the continuous accumulation of the delta, the mudstone layer developed overpressure and became the main detachment layer of the delta [41]. The Agbada Formation, which has been developing continuously since the Eocene, is the main body of the delta sedimentary system and the main layer of oil and gas enrichment [42]. The Agbada Formation is an offshore deltaic clastic deposit that was mainly developed in the deltaic frontal margin and pre-deltaic environments with an overall stratigraphic thickness of 300–4500 m [43]. The upper and middle parts of the formation are dominated by sandstone, and the lower part is dominated by dark gray shale. The content of mud shale increases with burial depth. The Benin Formation consists of Eocene–Holocene continental deposits, including alluvial plain and coastal plain deposits, with a thickness of up to 2000 m. The Benin Formation, which is mainly developed in the area above the coast and under the water, is dominated by sandstone. The lower and middle parts of the Benin Formation are laterally interbedded with gray shale. This formation can be used as a regional cover, and a small amount of oil and gas reservoirs can be found in the sandstone layer in some areas [44].
The most significant tectonic deformation in the Niger basin is represented by the formation of rich gravity decollement structures under the influence of the plastic flow of mudstone in the Akata Formation. These structures show obvious tectonic zonation. In the map view, land-to-sea development is characterized by the development of an extensional zone at the trailing zone, a central transitional mud diapir zone, and a leading-edge extrusion tectonic zone (Figure 1 and Figure 3a). The leading-edge extrusion tectonic zone can be subdivided into an inner fold and thrust zone, a detachment fold transition zone, and an outer fold and thrust zone (Figure 3b) [1,4,5,30,32]. The mudstone layer in the trailing extension zone is generally thin, and the primarily developed structures are mud rolls, normal faults, and rollover anticline structures. The upper wall of these faults generally develops associated normal faults, also called reverse-regulating faults (Figure 3a). The main tectonic structures in the transition zone are high-amplitude mud diapirs and draped-anticline, roof-collapse structures formed at the tops of the diapirs (Figure 3). The extrusional zone is located at the delta front. The tectonic structures of the zone are dominated by fault propagation folds, fault-turning folds, and slip folds. Imbricate faults along the leading edge disrupt the Akata and Agbada formations all the way to the seafloor (Figure 3b).

3. Methodology

Analyses of the structural evolution of the Niger Basin and the geologic characteristics of the mudstone structures summarized in the preceding section provided important constraints for establishing of the initial and boundary conditions for analogue modeling.

3.1. Experimental Material

Referring to previous analogue experiments simulating gravitational detachment structures under the gravitational conditions of the passive margin, we used silicone polymer with a higher molecular weight to simulate mudstone and quartz sand for overlying sandstone [46,47,48,49,50]. The silicone polymer had a molecular weight of 5 million, a viscosity coefficient of 5 × 104 Pa·s, and a density of 987 kg/m3, and it had Newtonian fluid properties at low strain rates. The use of this viscosity in analogue modeling is well-documented [33,51]. Mudstone is plastic and deforms only after the deviatoric stress overcomes its strength [52]; in contrast, the silicone polymer used in the experiment deformed immediately after loading. In our experiments, the plastic behavior of overpressured mudstone was approximated by instantaneously loading the polymer with the initial differential loading. The deformation of dry quartz sand follows the Mohr–Coulomb theory, which has been widely used to simulate the brittle deformation of shallow crustal sedimentary rocks (<10–15 km) [53]. The quartz sand used in the experiment had a particle size of 100–400 μm, a density of 1500 kg/m3, and a cohesion of approximately 200 Pa. The angle of internal friction was approximately 31°. The physical properties of colored quartz sand do not change with color [54]; thus, the quartz sand was dyed using various colors to facilitate the observation of structural deformation. The main scaling between the analogue model and the geologic prototype was as follows:
Gravity similarity ratio (deformation of both the model and geologic prototype occur in the natural gravity field):
g* = 1
Geometric similarity ratio (1 cm in the model represents 500 m of the geologic prototype):
λ* = 2 × 10−5
Density similarity ratio (the density of the experimental material is approximately half that of the geologic prototype):
ρ* = 0.5
Viscosity similarity ratio (the viscosity coefficient of silicone polymer is much lower than that of natural mudstone rock):
μ* = 2 × 10−14
Time similarity ratio (this timescale was appropriate to simulate instantaneous loading by very rapid sedimentary progradation) [55]:
t* = μ*/(g*·λ*·ρ*) = 2 × 10−9

3.2. Model Design

In this study, we analyzed the basin-scale mudstone deformation process rather than the development of local or single mudstone structures. The experimental design was based on the real geologic conditions, including the initial basin structure and the initial shape of the mudstone layer. Due to the significant variations in thickness of the Akata Formation shale and the overlying Agbada and Benin Formations at different locations within the Niger Delta Basin, accurately defining their initial depositional thickness poses considerable challenges. Based on previous simulation experiments, it has been demonstrated that models can effectively reflect actual geological conditions when the shale layer thickness is less than 2 km and the overlying strata thickness is approximately 4 km [32,33]. Accordingly, integrating the regional geological cross-section (Figure 3a), we designed the model with a shale layer thickness of 2 cm and an overlying layer thickness of 4.5 cm to simulate the depositional sequence and mechanical behavior under realistic geological conditions. Based on the seismic profiles and previous studies, the continental slope inclination in the Niger Delta Basin is 1–5° [33,56]. In most profiles across the basin, the slope angle in the extension area is slightly greater than 3°, with a maximum of 5° and a minimum of approximately 2°. The slope angle in the extrusion zone is 1–3°. The experiments were designed based on the actual geologic profile (Figure 3a), and the simplified model is shown in Figure 4.
The specific settings for experiment I were as follows (Figure 4a; Table 1): (1) The model had dimensions of 75 × 20 cm with fixed baffles on the left and right sides. A basement slope with a length of approximately 30 cm and an inclination of 3° was placed on the left side of the model to simulate the continental slope. A basement slope with a length of approximately 20 cm and an inclination of 1° was placed on the right side of the model to simulate the landward slope (oceanic crust) of the basement in the deep sea area. The length of the middle horizontal base was 25 cm. (2) A thick silicone polymer with a thickness of 2 cm was placed in the experimental device and left standing for 10 h before removing the air bubbles in the gel, resulting in a smooth surface. (3) Quartz sand was spread from the left side of the model to the right side in a form similar to delta progradation. The first layer of sand, which had a thickness of 1.5 cm and length of 15 cm, was laid on the left side of the model. (4) Four hours later, the second layer, which terminated 10 cm ahead of the first layer, was added, with a thickness of 1.5 cm. (5) After 8 h, the third layer was deposited with a thickness of 1.5 cm, which terminated 10 cm ahead of the second layer. (6) Subsequently, the fourth, fifth, and sixth layers of quartz sand were, respectively, deposited at 12 h, 16 h, and 18 h, without any additional increase in thickness.
To reveal the effect of mudstone layer thickness on the development of mudstone structures, experiment II was established for comparison. The thickness of the silicone polymer in experiment II was 1.5 cm; all other experimental conditions were the same as in experiment I (Figure 4b; Table 1). Comparison experiment III was established to analyze the influence of overburden thickness on the development of mudstone structures. The angle of the basement slope, the thickness of the silicone polymer layer, and the process for depositing the sand layers were the same as in experiment I; however, the thickness of each layer of quartz sand was decreased to 1 cm. Comparison experiment IV was established to reveal the effect of basement slope inclinations on the development of mudstone structures. The only change with respect to the experiment I was the slope inclination, which was 5° for the left side of the model and 3° for the right side (Figure 4d; Table 1). Due to the increase in slope inclination, the thickness of the silicone polymer on the slope was slightly reduced in experiment IV. However, compared to the overall silicone polymer content of the model, this change had little effect on the experimental results.

4. Results

4.1. Experiment I

After the first layer of quartz sand was deposited, mudstone analogue began to flow and deform (Figure 5a). Because of the differential load of the overlying layer and the effect of gravity, the polymer on the left side of the model began to flow downslope. The overlying layer began to extend and subside, and several small normal faults developed at the leading edge of the layer (Figure 5b,b’). After approximately 8 h, localized thickening and thinning of the silicone polymer in the upward-dipping direction of the slope were evident. The thickened areas were generally located in the footwall of the fault, forming mud roll structures (Figure 5c,c’). Twelve hours later, the polymer continued to flow in the downward slope direction, and more obvious thickening occurred in the central transition zone. Faults continued to form and develop, and reverse-regulating, left-dipping (continental direction) faults also developed (Figure 5d,d’). It should be noted that the present basin zone is divided into the extensional, translational, and extrusional zones, reflecting the properties of the mudstone structures. However, when discussing the experiment process, these zones only represent the position corresponding to the current region; they do not reflect the tectonic properties.
After 16–20 h, with the accumulation of overlying sand layers, early faults were covered by sand layers and gradually ceased to be active. Several mud diapirs developed in the central transition area, and the size of the micro-basin between the mud diapirs increased gradually. In the extrusion zone, the mudstone analogue was significantly thickened (Figure 5e,e’,f,f’). By the end of the experiment, several small mud roll structures and rollover anticline structures developed in the extension zone of the left slope, and mud diapirs developed in the central transition zone with a large uplift. The silicone polymer was significantly thickened in the extrusion zone on the right side, with a small thrust fault developed locally (Figure 5g,g’).
More obvious deformation characteristics and differences between different tectonic zones can be observed in the internal profile (Figure 5h,h’). Numerous normal faults were identified in the extensional zone, and small mud rolls, welded structures, rollover anticlines, and other structures were also noted. The transitional zone developed mud diapirs with large uplifts and different shapes, and normal faults developed at the top of the diapirs, which combined to form small graben structures. A small number of thrust faults developed in the extrusion zone. Although the silicone polymer thickened, its top surface was not horizontal, and it showed some properties of an extrusion thrust.

4.2. Experiment II

The silicone polymer in experiment II was thinner than that in experiment I, which obviously delayed the time at which the polymer started to flow (Figure 6a,a’). After 12 h, the early faults were no longer active because the plastic layer in the extension zone was almost exhausted and could no longer drag the overlying strata downslope (Figure 6b,b’). The polymer thickened significantly in the middle transition zone. By the end of the experiment, the model had developed distinct mud roll and mud diapir structures (Figure 6c,c’). In the profile, the extension zone developed normal faults and mud roll structures. The transition zone developed mud diapirs, and the mudstone in the extrusion zone showed some extrusional nature, although no obvious thrust structures developed (Figure 6d,d’). From land to sea, the mudstone formations progressively enlarged, and thick mudstone was formed in the extrusion zone (Figure 6c’,d’).

4.3. Experiment III

Due to the low thickness of the overlying sand layers, the plastic flow characteristics of the silicone polymer were not significant in experiment III. After 8 h, a slight subsidence was observed in the overlying sand layer, and one small positive fault developed at the front edge of the layer (Figure 7a,a’). After 12 h, early developed faults continued to develop, and a new fault was formed in the front of the layer (Figure 7b,b’). A slight thinning of the polymer occurred between two faults. At the end of the experiment, the silicone polymer layer did not show any obvious localized thickening or thinning. A small number of normal faults developed in the extensional zone, small graben structures developed in the transition zone, and a small thrust fault developed in the extrusion zone (Figure 7c,c’). In the profile (Figure 7d,d’), the overall tectonic activity of the model was weak, and only a few mudstone formations developed. Two small triangular mud roll structures and normal faults developed in the extension zone. Three low-amplitude mud diapirs developed in the middle transition zone, and small graben structures formed on top of them. In the extrusion zone, a mud diapir was formed, but no thrust fault was formed, indicating weak extrusion thrusting (Figure 7d’).

4.4. Experiment IV

Due to the higher inclination of the basement slope in the extension area, the degree of plastic flow of silicone polymer in the downhill direction was significantly stronger in experiment IV than in experiment I. Under the differential load of the overlying sand layer and the action of gravity, the polymer on the left side of the model began to flow downhill, and several small normal faults developed at the front of the layer (Figure 8a,a’). After 8 h, the polymer showed obvious local thickening and thinning in the upward-dipping direction of the slope, and mud roll structures were formed. The faults continued to be active, and some reverse-regulating faults developed locally (Figure 8b,b’).
After 12 h, the silicone polymer thickened significantly in the transition zone and spilled over the overlying strata in some areas (Figure 8c,c’). The overlying sand layer partially subsided close to the basement slope, indicating the loss of the silicone polymer on the left slope of the model, which means that the polymer was unable to drag the overlying strata toward the downslope direction. This caused the faults above the slope to become inactive as they were covered by sand (Figure 8b,c). With the continuous accumulation of the overlying sand layers, mud diapirs developed in the transition zone (Figure 8d,e). At the end of the experiment, several small mud roll structures and rollover anticlines developed in the extension zone. Furthermore, mud diapirs with large uplifts developed in the transition zone, and small grabens developed at the top of the diapirs. The silicone polymer was obviously thickened in the right extrusion area, and a thrust fault developed locally (Figure 8f,f’). In the profile, the model exhibited distinctive tectonic features of differential deformation. The extensional zone mainly developed rollover anticlines, normal faults, reverse-regulating faults, and mud roll structures. The transition zone contained many mud diapirs with high uplift, and the sand layer between the mud diapirs showed obvious thickening. A few thrust faults formed in the extrusion zone, and polymer thickening was evident (Figure 8g,g’).

5. Discussion

5.1. Validity and Limitations of the Model

The simulation device was bordered by glass walls. Due to the friction of glass and the slight non-uniformity of the sand layer near the glass walls during the experiments, some experimental error was introduced. For example, the deformation features observed on the model surface and in the profile were not completely consistent (Figure 7). Due to the adjustment of the shooting position (the distance between the cut profile and the camera is greater than that of the experimental surface), the cut profile appears slightly smaller in scale compared to the experimental surface photos in the images. This may result in minor discrepancies between the positions of the mudstone structures displayed in the internal profile and those observed on the experimental surface. Overall, however, the results of our analogue model experiments are meaningful. Comparing these results with the actual geologic profile of the Niger Basin (Figure 9), the following two aspects are in particularly good agreement: (1) the mudstone tectonics of the entire basin show zonal characteristics, with continuous translation from extensional structures to contractional structures; (2) different structural patterns develop in different zones. Generally, the extension zone contained the thin mudstone layer, along with welded structures, rollover anticlines, small-scale mud roll structures, and numerous normal faults and reverse-regulating faults. In contrast, large-scale and large-amplitude mud diapirs appeared in the middle diapir zone. Small graben structures developed on top of the diapirs, with micro-basins developing between diapirs. Thrust structures developed in the leading-edge extrusion zone.
By comparing the experimental results with the real geologic profiles, we identified some shortcomings of the model experiments. First, a small number of thrust faults and decollement folds formed in the extrusion zone, in contrast to the imbricated thrust faults formed in the Niger Basin. The detachment fold styles observed in the experiments are typical above salt or relatively weak mudstone decollements; in contrast, the imbricate thrust arrays in the Niger Basin are more typical of strong, frictional detachment [24,33]. This discrepancy may be because the right baffle obstructed the flow of silicone polymer, reducing the compressive stress on the polymer and the overburden in the extrusion zone. Second, some welded structures, which are rare in the Niger Basin, developed in the extension zone during the experiments. This discrepancy suggests that the mudstone formations in the Niger Basin are more cohesive or less plastically mobile than the silicone polymer used in this study. Third, in the basin, mud diapirs are typically larger in scale and closer to the surface, whereas the mud diapirs formed in analogue experiments are relatively smaller in scale. This discrepancy may be primarily attributed to the greater thickness of overburden layers and the shorter time intervals between successive depositions in the experimental design. The thicker overburden enhances the mobility of shale under gravitational forces, causing the shale to accumulate more in the extrusion zone, while the shorter deposition intervals inhibit the full development of mud diapirs. Fourth, in our experiments, we used dry quartz sand to simulate the overlying strata of the Niger Basin. However, the proximal Niger Basin is subaerial, whereas the distal delta is in a submerged environment. The horizontal buttress force provided by the water stabilizes the overlying strata, reduces deformation, and decreases the pressure difference between the overburden and mudstone layer [57]. This horizontal buttress force has a relatively large effect on local structures but a small influence on larger regional scales [57]. Based on these shortcomings, several aspects of our experiments can be improved in the future: (1) the silicone polymer should be selected with stronger cohesion or weaker fluidity to more accurately simulate mudstone; (2) the overburden thickness should be reduced appropriately to minimize the mobility of the silicone polymer; (3) the depositional intervals of the overburden should be appropriately increased to allow for the full development of mudstone structures; (4) the length of the right slope (simulated oceanic crust) should be extended so that the relevant structures of the extrusion zone can develop fully; (5) an initial seaward-free boundary should be set to provide the polymer with space to move in the extrusion zone; and (6) analogue models should be established in subaerial and subaqueous settings.

5.2. Controlling Factors

5.2.1. Gravity Gliding and Spreading

Published analogue models based on horizontal substrates have shown that the experiment only formed paired regional and counter-regional growth fault grabens in the extensional zone, while large-scale mud diapirs did not develop in the transition zone, resulting in an overall lack of distinct structural zonation [33]. In contrast, experiments with inclined substrates developed more growth faults and rollover structures in the extensional zone, as well as multiple mud diapirs with significant uplift amplitudes in the transition zone (Figure 9). These differences indicate that the morphology of the substrate plays a significant controlling role in the structural zonation of the basin. In light of previous reports on gravitational sliding and expansion [58,59], the mudstone in the Niger Delta Basin, under the combined effects of gravitational sliding and gravitational spreading, has formed structural deformation features associated with the extensional zone, transition zone, and compressional zone. Gravity sliding is mainly controlled by the morphology of the basement. The plastic mudstone will slide along the slope in the seaward direction under the action of gravity sliding, and the overlying strata will be fractured during the sliding process. Gravity spreading is controlled by differential sedimentary loading. Due to the basement undulation and the thickness, density, uneven deposition of the overburden, and differential loading will cause the mudstone to flow plastically from regions of higher load to those of lower load, with a larger pressure difference resulting in faster flow. The accumulation of the plastic mudstone will result in the formation of mud diapirs. High pressure will cause the diapirs to rupture or even lead to fault formation in the overlying brittle strata. The faults will provide the plastic mudstone with an outlet, accelerating diapir formation and increasing diapir size. Taking experiment I as an example, the deformation of mudstone was controlled by gravity sliding and gravity spreading in the early stage. Growth faults began to develop in the extension zone, and the mudstone migrated downslope to form mud roll structures. In the transition and extrusion zones, the deformation of mudstone was mainly controlled by gravity spreading, and various mud diapirs and thrust structures were formed.

5.2.2. Thickness of the Mudstone Layer

According to the results of experiments I and II, the thickness of the mudstone layer influenced the formation and evolution of mudstone structures. Decreasing the thickness of the silicone polymer significantly delayed the time at which the polymer began to flow (Figure 5 and Figure 6). Comparing profiles from experiments I and II shows that the mudstone structures in experiment II were more mature and fully developed (Figure 9b,c). In the extension zone, six symmetrical triangular mud roll structures developed in experiment II, and the silicone polymer between the mud roll structures nearly drained to form welded structures. Furthermore, reverse-regulating faults were almost absent in the overlying strata (Figure 9c). In contrast, five mud roll structures developed in experiment I, but their morphologies were asymmetric. Silicone polymer remained between some of the mud roll structures, and a small number of reverse-regulating faults developed in the overlying strata (Figure 9b). In the transition zone, the mud diapirs developed in experiment II were significantly larger than those in experiment I (Figure 9c). In the extrusion zone, a small number of thrust faults formed in both experiments; however, the thickness of the mudstone layer was larger in experiment I (Figure 9b). These comparative experiments show that thickening the mudstone layer enhances the plastic fluidity of the mudstone, leading to more rapid development of mudstone structures but on a smaller scale. Moreover, in the model with the thicker mudstone layer, the overlying layers were more tectonically active as they were dragged by mudstone, leading to the development of numerous normal and reverse-regulating faults. The thinner mudstone layer was conducive to the formation of fully developed mudstone structures, which were large in both size and number. Welded structures were also more likely to develop between the mudstone structures.

5.2.3. Thickness of the Overlying Strata

Comparing the results of experiments I and III shows that the thickness of the overlying sand layers obviously affected the formation and evolution of mudstone structures. In model III, with a small overlying layer thickness, the plastic flow of the silicone polymer was not obvious, and few faults developed in the overburden (Figure 7). In experiment I, with a thicker overburden layer, the plastic mobility and deformation of the polymer, along with the deformation of the overlying layers, was more obvious (Figure 5). In the profile, the mudstone in experiment III showed slight thinning in the extension zone. Two small mud roll structures developed at the foot of the slope, and only three fault layers formed in the overlying strata (Figure 9d). In contrast, in experiment I, the thinning of the mudstone layer was more obvious, welded structures formed in some areas, and more faults formed in the overlying strata (Figure 9b). Thus, although mudstone deformation was controlled by gravity sliding and spreading, the gravity spreading caused by the differential load of the overburden was predominant. In the transition zone, the mud diapirs developed in experiment III were also small in size. In the extrusion zone, the effect of gravity expansion was weaker in experiment III than in experiment I, leading to weaker horizontal extrusion stress at the foot of the slope (oceanic crust); thus, thrust faults were not developed in experiment III (Figure 9d). The comparative experimental findings demonstrate that a thicker overlying strata corresponds to greater differential loading, larger and more numerous mudstone structures, and more obvious overburden deformation.

5.2.4. Basement Slope Inclination

As the dip of the basement slope increases, the forces exerted by the overlying strata and mudstone layer on the lower slope will also increase, causing the mudstone to move faster down the continental slope. In the extension zone, the mud roll structures developed in experiment IV were fewer in number and smaller in size than those developed in experiment I (Figure 9b,e). Moreover, the mud diapirs developed in the transition zone were larger. In the extrusion zone, the dip of the reverse fault formed in experiment IV and the thickness of the mudstone layer were larger (Figure 9e). The comparative experimental results demonstrate that a larger inclination of the basement slope in the extensional zone is conducive to the rapid flow of mudstone, leading to the rapid development of mud structures; thus, mud roll structures with lower maturity and mud diapirs with a larger scale developed, and more faults developed in the overlying strata. The larger inclination of the slope in the extrusion zone leads to the compressive stress of mudstone and the overlying strata, leading to the formation of thrust faults with larger dip angles.
Based on the above four comparative experiments (Figure 9), although the inclination of the basement slope and the thickness of the mudstone layer and overlying strata affected the scale of the developed mudstone structures and the number of faults formed in the overlying strata, they did not affect the zonation of the models. Thus, the tectonic zonation of the Niger Basin is controlled by the basement morphology.

5.3. Evolutionary Model of Mudstone Structures

Based on the tectono-sedimentary background of the basin, previous studies on the evolution of mudstone structures [24,30,45], and the results of simulation experiments, the tectonic evolution of the mudstone in the Niger Delta Basin can be divided into three stages: an initial depositional weakly active stage (Paleocene–Middle Oligocene); a strongly active uplift stage (Late Oligocene–Middle Oligocene); and the weakly active overall burial stage (Pliocene to present).
During the Cretaceous, along with the expansion of the South Atlantic, the basin gradually formed the rifted-basement shape with low middle and high sides (Figure 10). The rifting of the basin gradually stopped, and the tectonic region stabilized. In the Paleocene–Eocene, a thick plastic mudstone layer was deposited in the basin, which provided the material basis for the formation of mudstone structures. At the same time, under the influence of the basement slope, the mudstone layer was thinner in the upward-dipping direction of the slope and thicker in the downward-dipping direction (Figure 10a). Before the Middle Oligocene, the mudstone began to flow plastically in the downward-dipping direction (Atlantic direction) under the influence of gravity sliding and weak gravity spreading, and a small number of detachment faults began to develop on the top surface of the mudstone layer in the extension zone. The Late Oligocene–Middle Miocene was a period of intense activity in terms of mudstone tectonics [60]. Under the effect of gravity spreading, mudstone flowed plastically, and numerous growth faults developed in the extension zone. At the same time, the difference in gravity between the upper and lower walls induced the upward arching of the mudstone layer in the lower wall, resulting in local thickening (Figure 10b). Mud diapirs developed in the transition zone, and several faults developed at the top, forming small grabens (Figure 10b). In the Late Miocene, a large amount of plastic mudstone was continuously extruded toward the delta front and was blocked by the stable strata in the extrusion zone; the plastic mudstone began to accumulate and arch upward, forming a series of thrust nappe structures (Figure 10b). Since the Pliocene, the tectonic activity in the basin has generally weakened [1]. Under the action of gravity spreading, the mud diapirs were slightly uplifted, and some new normal faults developed on top of the diapirs. The mudstone continued to flow toward the Atlantic under the effect of gravity. Small triangular mud roll structures were formed in the lower walls of the faults in the extension zone, and welding structures also developed locally (Figure 10c). The imbricated faults in the extrusion zone continued to develop.

6. Conclusions

By comparing analogue model experimental results to the actual geologic profiles of the Niger Delta Basin, we have provided the following insights regarding mudstone structures:
  • The tectonic deformation of the Niger Basin results from the combined influence of gravity sliding and spreading. The main driver of mudstone deformation is gravity spreading caused by the differential load of overlying layers. The basement morphology is the decisive factor in tectonic zonation in the Niger Basin;
  • The development of mudstone structures is also affected by the inclination of the basement slope and the thicknesses of the mudstone and overlying layers. A larger basement slope inclination favors the rapid flow of mudstone, which promotes the development of low-maturity mud roll structures, large-scale mudstone diapirs, and high-angle thrust nappe structures. Thinner shale layers, characterized by lower plastic mobility, facilitate the full development of mudstone structures, resulting in symmetrical mud diapirs and mud-roll structures. The effects of differential loading and gravity spreading were enhanced as the overburden thickness increased, leading to the development of large and numerous mudstone structures and stronger overburden deformation;
  • The formation and evolution of mudstone structures in the Niger Basin mainly involve three stages: initial sediment weakly active stage; strongly active uplift stage; and weakly active overall burial stage. Before the Middle Oligocene, a thick layer of marine mudstone was deposited in the basin, and a few detachment faults were formed. In the Late Oligocene to Miocene, numerous mudstone structures developed and showed obvious tectonic zonation. Since the Pliocene, the tectonic activity of the basin has weakened, welding structures have developed in some areas of the extension zone, and the mud diapirs in the transition zone have been slightly uplifted.

Author Contributions

Conceptualization, S.S. and Y.Y.; methodology, S.S. and Y.Y.; formal analysis, S.S. and J.Z.; investigation, S.S. and J.Z.; resources, W.D. and Y.Y.; data curation, Y.Y.; writing—original draft preparation, S.S.; writing—review and editing, Y.Y and W.D.; supervision, Y.Y.; funding acquisition, Y.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the National Natural Science Foundation of China, grant number 42372145.

Data Availability Statement

All data and materials are available upon request from the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Tectonic map of the Niger Basin (adapted from Krueger and Grant, 2011) [35]. The background colors represent the different basin zones: the extensional zone (pink), the mud diapir transition zone (blue), the inner fold thrust belt (dark yellow), the detachment zone (light yellow), and the outer fold and thrust zone (light green). The dashed blue lines indicate basin shorelines and water depth lines. Dark blue lines indicate regional growth faults. Red lines represent counter-regional faults. Brown lines indicate oblique extensional faults. Green lines show delta toe fold-faults.
Figure 1. Tectonic map of the Niger Basin (adapted from Krueger and Grant, 2011) [35]. The background colors represent the different basin zones: the extensional zone (pink), the mud diapir transition zone (blue), the inner fold thrust belt (dark yellow), the detachment zone (light yellow), and the outer fold and thrust zone (light green). The dashed blue lines indicate basin shorelines and water depth lines. Dark blue lines indicate regional growth faults. Red lines represent counter-regional faults. Brown lines indicate oblique extensional faults. Green lines show delta toe fold-faults.
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Figure 2. (a) The Strata, lithology, sedimentary setting, regional tectonic movement, and sea level change in the Niger Basin (adapted from Corredor et al., 2005; Gao et al., 2024) [5]; (b) depositional sequence and well log curves of Well W1 (see Figure 1 for location) (adapted from Gao et al., 2024) [2].
Figure 2. (a) The Strata, lithology, sedimentary setting, regional tectonic movement, and sea level change in the Niger Basin (adapted from Corredor et al., 2005; Gao et al., 2024) [5]; (b) depositional sequence and well log curves of Well W1 (see Figure 1 for location) (adapted from Gao et al., 2024) [2].
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Figure 3. Profiles of the Niger Delta (see Figure 1 for locations): (a) the entire basin (adapted from Rouby et al., 2011) [45]; (b) the mud diapir zone and the extrusion zone (adapted from Corredor et al., 2005) [5].
Figure 3. Profiles of the Niger Delta (see Figure 1 for locations): (a) the entire basin (adapted from Rouby et al., 2011) [45]; (b) the mud diapir zone and the extrusion zone (adapted from Corredor et al., 2005) [5].
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Figure 4. Simulation models (side view) of mudstone structures in the Niger Basin: (a) Experiment I (general model); (b) experiment II (mold with decreased silicone polymer thickness); (c) experiment III (model with decreased overburden thickness); and (d) experiment IV (model with larger slope inclination). The notation “15 cm, 4 h” indicates that the sand layer with a length of 15 cm was deposited, and the next sand layer was deposited 4 h later.
Figure 4. Simulation models (side view) of mudstone structures in the Niger Basin: (a) Experiment I (general model); (b) experiment II (mold with decreased silicone polymer thickness); (c) experiment III (model with decreased overburden thickness); and (d) experiment IV (model with larger slope inclination). The notation “15 cm, 4 h” indicates that the sand layer with a length of 15 cm was deposited, and the next sand layer was deposited 4 h later.
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Figure 5. Process and profile of experiment I obtained at different times after initial deposition: 0 h (a), 4 h (b), 8 h (c), 12 h (d), 16 h (e), 20 h (f), and 24 h (g). (h) Profile of the deformation features of experiment I, showing obvious zonal characteristics. (b’–h’) are line-drawn interpretations of (b–h). The light and dark blue indicate overlying sand layers, while the pink indicates the silicone polymer.
Figure 5. Process and profile of experiment I obtained at different times after initial deposition: 0 h (a), 4 h (b), 8 h (c), 12 h (d), 16 h (e), 20 h (f), and 24 h (g). (h) Profile of the deformation features of experiment I, showing obvious zonal characteristics. (b’–h’) are line-drawn interpretations of (b–h). The light and dark blue indicate overlying sand layers, while the pink indicates the silicone polymer.
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Figure 6. Process and profile of experiment II obtained at different times after initial deposition: 8 h (a), 12 h (b), and 24 h (c). (d) Profile showing the deformation features in experiment II. The scale of mudstone structures gradually increased moving from land to sea. (a’–d’) are line-drawn interpretations of (a–d). The light and dark blue indicate overlying sand layers, while the pink indicates the silicone polymer.
Figure 6. Process and profile of experiment II obtained at different times after initial deposition: 8 h (a), 12 h (b), and 24 h (c). (d) Profile showing the deformation features in experiment II. The scale of mudstone structures gradually increased moving from land to sea. (a’–d’) are line-drawn interpretations of (a–d). The light and dark blue indicate overlying sand layers, while the pink indicates the silicone polymer.
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Figure 7. Process and profile of experiment III obtained at different times after initial deposition: 8 h (a), 12 h (b), and 24 h (c). (d) Profile showing the deformation features of experiment III. Due to the low thickness of overlying sand layers, the experiment showed weak structural activity. (a’–d’) are line-drawn interpretations of (a–d). The light and dark blue indicate overlying sand layers, while the pink indicates the silicone polymer.
Figure 7. Process and profile of experiment III obtained at different times after initial deposition: 8 h (a), 12 h (b), and 24 h (c). (d) Profile showing the deformation features of experiment III. Due to the low thickness of overlying sand layers, the experiment showed weak structural activity. (a’–d’) are line-drawn interpretations of (a–d). The light and dark blue indicate overlying sand layers, while the pink indicates the silicone polymer.
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Figure 8. Process and profile of experiment IV obtained at different times after initial deposition: 4 h (a), 8 h (b), 12 h (c), 16 h (d), 20 h (e), and 24 h (f). (g) Profile showing the deformation features of experiment IV. (a’–g’) are line-drawn interpretations of (a–g). The light and dark blue represent overlying sand layers, while the pink represents the silicone polymer.
Figure 8. Process and profile of experiment IV obtained at different times after initial deposition: 4 h (a), 8 h (b), 12 h (c), 16 h (d), 20 h (e), and 24 h (f). (g) Profile showing the deformation features of experiment IV. (a’–g’) are line-drawn interpretations of (a–g). The light and dark blue represent overlying sand layers, while the pink represents the silicone polymer.
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Figure 9. Comparison of the experimental profiles with the actual profile of the Niger Basin. (a) Actual geologic profile of the basin. (b–e) Profiles of experiments I, II, III, and IV, respectively. The simulated tectonic zonation showed good agreement with the actual profile.
Figure 9. Comparison of the experimental profiles with the actual profile of the Niger Basin. (a) Actual geologic profile of the basin. (b–e) Profiles of experiments I, II, III, and IV, respectively. The simulated tectonic zonation showed good agreement with the actual profile.
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Figure 10. Evolutionary model of mudstone structures in the Niger basin. (a) Initial depositional weakly active period: the mudstone layer was deposited, and a small number of detachment faults developed above the slope. (b) Strong uplift activity: normal faults, mud diapirs, thrust faults, and mud roll structures developed, forming the extension, transition, and extrusion zones. (c) Weakly active overall burial period: the tectonic activity of the basin weakened, welded structures appeared in some areas, slight uplift occurred in the mud diapirs, and imbricated thrust faults continued to develop.
Figure 10. Evolutionary model of mudstone structures in the Niger basin. (a) Initial depositional weakly active period: the mudstone layer was deposited, and a small number of detachment faults developed above the slope. (b) Strong uplift activity: normal faults, mud diapirs, thrust faults, and mud roll structures developed, forming the extension, transition, and extrusion zones. (c) Weakly active overall burial period: the tectonic activity of the basin weakened, welded structures appeared in some areas, slight uplift occurred in the mud diapirs, and imbricated thrust faults continued to develop.
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Table 1. Summary of the main experimental parameters for the four analogue models.
Table 1. Summary of the main experimental parameters for the four analogue models.
Model NumberExperiment IExperiment IIExperiment IIIExperiment IV
Model summaryGeneral modelModel with thinner mudstone layerModel with thinner overburden layerModel with larger slope inclination
Inclination of the slope on the landward side3°3°3°5°
Inclination of the slope on the ocean side1°1°1°3°
Mudstone thickness2 cm1.5 cm2 cm2 cm
Overburden thickness4.5 cm4.5 cm3 cm4.5 cm
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MDPI and ACS Style

Shi, S.; Ding, W.; Yu, Y.; Zhang, J. Deformation and Evolution of Akata Formation Mudstone in the Niger Delta Basin: Insights from Analogue Models. J. Mar. Sci. Eng. 2025, 13, 590. https://doi.org/10.3390/jmse13030590

AMA Style

Shi S, Ding W, Yu Y, Zhang J. Deformation and Evolution of Akata Formation Mudstone in the Niger Delta Basin: Insights from Analogue Models. Journal of Marine Science and Engineering. 2025; 13(3):590. https://doi.org/10.3390/jmse13030590

Chicago/Turabian Style

Shi, Shuaiyu, Wenlong Ding, Yixin Yu, and Jixin Zhang. 2025. "Deformation and Evolution of Akata Formation Mudstone in the Niger Delta Basin: Insights from Analogue Models" Journal of Marine Science and Engineering 13, no. 3: 590. https://doi.org/10.3390/jmse13030590

APA Style

Shi, S., Ding, W., Yu, Y., & Zhang, J. (2025). Deformation and Evolution of Akata Formation Mudstone in the Niger Delta Basin: Insights from Analogue Models. Journal of Marine Science and Engineering, 13(3), 590. https://doi.org/10.3390/jmse13030590

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