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Formation and evolution of oblique anticline in eastern Niger Delta
PETROLEUM EXPLORATION AND DEVELOPMENT Volume 45, Issue 1, February 2018 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2018, 45(1): 55–67.
RESEARCH PAPER
Formation and evolution of oblique anticline in eastern Niger Delta SUN Yonghe1, 2, 3, *, LIU Zhida1, 2, 3, LI Xuesong4, HU Guangyi5, FAN Tingen5, GAO Yunfeng5 1. Innovation Team of Science and Technology for Fault Deformation, Sealing and Fluid Migration, Northeast Petroleum University, Daqing 163318, China; 2. Laboratory of CNPC for Fault Controlling Reservoir, Northeast Petroleum University, Daqing 163318, China; 3. Earth Science College, Northeast Petroleum University, Daqing 163318, China; 4. Exploration and Development Research Institute of Daqing Oilfield Co Ltd., Daqing 163712, China; 5. Research Institute of China National Offshore Oil Corporation, Beijing 100027, China
Abstract: Based on the three-dimensional seismic interpretation data, this paper analyzed the formation mechanism and the growth process of the oblique anticline AE of the M region of the eastern Niger Delta, as well as the evolution process of the associated fault systems. The study results show that the stratigraphic sedimentary period between reflector H4-H6 of the middle and late Miocene was the initial fold-thrust stage, the anticline AE was a half-graben controlled by oblique extensional faults derived from the oblique extensional transfer structure formed by local initial differential fold-thrusting. At the same time the tear faults developed as a result of the differential sliding. During the stratigraphic sedimentary period between reflector H1-H4 of the late Miocene to Pliocene, the large-scale folding and thrusting occurred, differential contractional deformation resulted in the pre-existing extensional half-graben became AE anticline by oblique tectonic inversion, then the anticline grew continually and the crest of the anticline migrated gradually. The newly formed fault systems consist of a small number of associated tear-normal faults caused by differential thrusting and gravity-driven domino normal faults predominantly induced by the slope inclination of the anticline limb. During the stratigraphic sedimentary period between reflector H0-H1 of the Pleistocene to Holocene, as the growth of the anticline ceased, the area entered post-fold thrusting stage. The formation and distribution of conjugated faults were controlled by the local gravity return collapse, local differential sliding and reactivation of pre-existing positive inversion faults jointly. The research results of genetic mechanism of the oblique inversion anticline and evolution of associated faults are helpful to reveal the key factors controlling the accumulation and distribution of oil and gas. Key words: Niger Delta; gravity sliding; differential contractional deformation; oblique anticline; fault system; tectonic inversion; Paleogene; Neogene
Introduction Structural inversion refers to the structural deformation resulted from the combined effect of extensional deformation and contractional deformation, and the resultant geological structure is called an inversion structure[1]. Positive inversion corresponds to extensional deformation followed by contractional deformation, while reverse inversion corresponds to contractional deformation followed by extensional deformation[2]. Basin inversion is usually used to describe the process of post-superimposition and shortening deformation of extensional basin[3]. There exist a large number of petroliferous basins around the world that have experienced inversion deformation[45], and the oil and gas accumulation in these basins
is usually closely related to the positive inversion deformation[68]. However, it is often difficult to clearly identify complex structures formed as a result of reverse inversion in basins, mostly because the shortening of basin is mainly deformation adjusted by reactivating preexisting faults, during which a large number of new contractional structures and secondary extensional structures, such as folds and associated extensional faults, are formed, so the structures change significantly after the inversion deformation compared with the corresponding extensional basins prior to the inversion deformation[9]. Thus, it is of great significance to study the mechanisms of inversion deformation and the formation process of associated new structures for understanding the
Received date: 12 Jul. 2017; Revised date: 15 Dec. 2017. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the National Natural Science Foundation of China (41572127, U1562214); China National Science and Technology Major Project (2016ZX05006-005, 2016ZX05054-009). Copyright © 2018, Research Institute of Petroleum Exploration & Development, PetroChina. Publishing Services provided by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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rules governing the enrichment of oil and gas as well as their migration, formation and storage processes. Previous studies showed that during the inversion process, the compressive stress usually originated from the horizontal stress field produced by plate interactions within a basin[10]. Under such compressive stress, the main branches of preexisting extensional faults usually inversed and reactivated. Meanwhile, the preexisting extensional basins experienced uplifting and erosion, and these processes were accompanied by concomitant depositional refilling. This combination consequently results in significantly thinned local strata and development of local unconformities or local formation of progradation strata[11].
Fig. 1.
For gravity slip structures, e.g., the Niger Delta, the major stress responsible for their structural deformation is the gravity slip effect from delta progradation bodies, which can lead to formation of fold-thrust structures in contractional tectonic domains. Whether such contractional tectonic domains can transform into inversion structures as a result of varied regional stress field polarity is the focus of this study. We chose a typical oblique anticline in the M block of the east Niger Delta as our research object (Fig. 1), and by considering the differential fold-thrust process induced by gravity slip, we discuss the inversion deformation mechanism of this oblique anticline and the formation process of the associated fracture system.
Structural outline, fold thrust pattern, and main fracture characteristics of the M block.
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1.
Regional geological background
Located in central West Africa (Fig. 1a), the Niger Delta is one of the delta basins with rich petroleum resource in the world[1214]. Given that the Niger Delta is located at a passive continental margin, it is characterized by development of gravity slip structures[1517] and can be divided into extensional tectonic region, translational tectonic region, and contractional tectonic region from the above-water basin region toward the deep-water basin region[1819]. The exploratory drilling in the past ten years has confirmed that the contractional tectonic region possesses high petroleum resource potential[20]. The M block in the east Niger Delta lies in the transitional zone between the translational tectonic region and the contractional tectonic region (Fig. 1a). This block contains seven oil-enriched anticlines, mostly in WNW-ESE direction (Fig. 1b), suggesting the regional delta progradation direction
Fig. 2.
or gravity slip direction is NE-NNE to SW-SSW. Observation of the cross-sectional profile reveals that most of these anticlines are fault propagation folds predominantly controlled by shovel-like thrust faults (Fig. 1c). There is also an AE anticline in NE-SW strike in the M block (Fig. 1b), which is nearly parallel to the gravity slip direction and almost perpendicular to the strike of the main anticlines. The AE anticline is therefore defined as an oblique anticline. The drilling data in recent years reveals that the oblique anticline has better oil and gas prospects than the other main anticlines[21]. The strata in the AE oblique anticline are mainly composed of a Cenozoic delta depositional sequence[2223], and the multi-periodic delta progradation led to formation structure with good vertical stratification and diachronism (Fig. 2). At the delta bottom, deposited mainly marine mud shale named the Akata Formation. This sequence of mud shale is a main hydrocarbon
Stratigraphic structure and seismic reflection characteristics of typical interfaces in the Niger Delta.
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source rock sequence[24], and a main set of detachment layer caused by the gravity slip of the delta induced by excessive pressure formed within the strata due to continuous progradation of the delta[22]. Above the Akata Formation lies the Agbada Formation. This stratigraphic sequence has been continuously growing from the Eocene to the present and is the main body of the delta deposition sequences as well as the main sequence for petroleum enrichment[25]. It is mostly composed of offshore delta detrital deposits at the delta front and the pre-delta[26], with water channels and basin fan deposits being the main accumulation bodies[2728]. Above the Agbada Formation lies the Benin Formation, which is composed of continental deposits of Eocene-Holocene age, including both alluvial plain and coastal plain deposits. This stratigraphic sequence mainly developed above coastal or water areas. We focus on strata above the Miocene series of the Agbada Formation in this study, which include the H0, H1, H2, H3, H4, H5, H6, and R; of them, the R stratum corresponds to the top boundary of the reservoir with the richest petroleum in the AE anticline.
2. General characteristics and growth periods of the oblique anticline 2.1.
Geometric characteristics
Most anticlines in the M block strike WNW-ESE, including the AP, AES1, AES2, AEE1, AEE2, and AA anticlines (Fig. 1b). It can be seen from the cross-sectional profile that these anticlines are broad, the top endpoints for fold-related thrust faults within this block usually disappear in strata above the H4 interface stratum, and these folds can therefore be classified as fault propagation folds (Fig. 1c). The AE oblique anticline in this block strikes NE-SW, nearly perpendicular to the main anticlines and almost parallel to the delta gravity slip direction. The AE anticline and the main anticlines, AES1 and AEE1, are all located in the upper wall of F3 thrust fault (Fig. 1b). The AE anticline is in the northeast segment at the joint of F3a and F3b, but the anticline
Fig. 3.
extends northeasterly far beyond the length range of the F3a-F3b segment, and the deformation range controlled by this segment; in addition, the anticline top near the F3a-F3b connecting segment does not lie close to the top endpoint (Fig. 1b). Another thrust fault related to the AE anticline is the F6 fault in WNW-ESE strike, which is located in the central part of the anticline. From the cross-sectional profile, we can see that the top endpoints of these two thrust faults both disappear in the H3-H4 strata and cannot be observed in any of the stratigraphic sequences on both side of the faults because of thickness differences developed during the anticline growth (Fig. 3a). This relationship implies that these two faults are not the dominant factors controlling the anticline growth. From the cross-sectional profile, one can barely distinguish the short and long axes of the anticline, and the anticline is therefore close to an equiaxial anticline (Fig. 3a, 3b). However, in plan view, the anticline appears as a short axial anticline trending NE-SW (Figs. 1b and 4a). Further analysis reveals that the AE anticline appears as an equiaxial anticline on the cross-sectional profile mainly because in the H1-H4 strata, the top segment (hinge) of the anticline gradually migrates northwesterly from bottom to top (Fig. 3b), thus leading to an enlargement along the NW-SE direction. It can be preliminarily inferred that there may exist a fault of northeast trend that controls the distribution of this anticline. And in-depth seismic analysis indeed found that within the AE anticline, there is a thrust fault striking NE-SW and dipping northwest, F0, which controls the anticline deformation (Figs. 3b and 4). In addition, there is a distinct difference between the thickness variation of strata above and below the H4 interface in the AE anticline. The strata above H4 in the anticline core are thicker than those in the limbs, representing the syndeposition during anticline growth. On the other hand, the two sets of strata below H4 are slightly thicker in the anticline core than in the limbs, especially in the syncline region east of the anticline the two layers are clearly thinner (Fig. 3b). This phenomenon illustrates that prior to the controlling effect of thrust deformation
Typical seismic cross-sectional profile and planar shape of the AE anticline (see Fig. 1b for the location of the cross-sections).
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Fig. 4.
Distribution and characteristics of fracture system related to the AE anticline.
on the anticline growth (at least before the H4 deposition), F0 experienced extensional deformation and as a result, a hanging half-graben basin formed. Later, with the superimposition effect of inversion deposition, the anticline formed. 2.2.
Syndeposition of sequences in the anticline
During the fold growth process, topographic difference forms between the core and the limbs. If the area receives deposits and erosion occurs only locally in regions with relatively high elevation, the depositional sequence thin at the core and thicker in the limbs (the syndepositional characteristics) will be formed. In this case, the thickness difference of the syndepositional sequence represents the growth process and the magnitude of the fold[29]. The larger the fold growth magnitude, the larger the thickness difference between the strata in the core and limbs of the anticline is. In addition, regarding delta gravity slip structures at passive continental margins, folds with relatively large growth magnitude usually lead to stratum erosion at the anticline cores and collapse and migration of deposits in the steep limb, which can explain the overlap and truncation phenomena above or below local unconformities. Thus, these phenomena can help identify fold growth periods more accurately. The AES1, AEE1, AEE2, and AE anticlines related to the F3 thrust fault were taken to count the formation thicknesses at their limbs and cores, and calculated the corresponding ratios (Fig. 5). The results show that during the H4-H6 stratigraphic depositional period, the growing anticlines mainly included the AEE1 anticline corresponding to the F3c fault and the AEE2 anticline corresponding to the F4 fault. But the stratigraphic syndepositional thickness ratio is quite small, representing a low level of deformation as with the initial gravity slip deformation characteristics during the delta progradation process. In the AE anticline region, because the
thickness of the H4-H6 strata at the core is larger than those in the limbs, the corresponding thickness ratios are both less than one, and the thicker strata are located in the upper wall of the F0 fault dipping southwest, so it is inferred that during this period the AE anticline region was an extensional half-graben basin. Above the H4 reflection layer, the AES1, AEE1, AEE2, and AE anticlines all exhibit growing characteristics. However, the growth of the AEE1 and AEE2 anticlines continued only to the H2-H3 stratigraphic depositional period, and their growth intensities were relatively weak. In contrast, the AES1 and AE anticlines had been continuously growing at high intensities until the H1-H2 stratigraphic depositional period, and during the H0-H1 stratigraphic depositional period, they grew at much lower intensities, and finally stopped growing.
Fig. 5. Histogram of thickness ratios of the anticline limb to core of different sets of stratigraphic units.
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Moreover, from the seismic cross-sectional profiles, one can see that at the H4, H3, and H2 interfaces in the paleo-anticline core region, there exist unconformable interfaces and associated overlap and truncation phenomena (Figs. 3b, and 4b), implying that the AE anticline continuously grew at relatively large magnitudes during the H3-H4, H2-H3, and H1-H2 stratigraphic depositional periods. Comparison of the growth trajectories of these four adjacent anticlines (Fig. 5) reveals that prior to H4, the AE anticline region was an extensional half-graben, and its formation was related to the initial growth of the AEE1 and AEE2 main anticlines on its east. The subsequent formation of the AE anticline after H4 and its continuous growing process, are related to the strong growth of the AES1 main anticline.
3. Characteristics and activity rules of fault systems in the oblique anticline region 3.1.
Types and development of fault systems
In the AE anticline region, developed a large number of faults of various nature, including thrust faults (F3a-F3b and F6), a positive inversion fault (F0), strike-slip faults (approximately eight faults), and normal faults (Fig. 4a). The thrust faults, F3a-F3b and F6, are located at the south end and central part of the anticline, respectively. These two faults appear shovel-like on their cross-sectional profiles: their bottoms smoothly converge into the deep mud-shale detachment layer of the Akata Formation, and their top endpoints disappear in the H1-H2 strata. The positive inversion fault, F0, dips northwesterly and strikes NE-SW (Figs. 3b and 4). Straight on profile, it cuts down to the mud-shale detachment layer of the Akata Formation, disappears in the H1-H2 strata to the top; in the R reflection layer, only the trajectory of its central segment can be seen (Fig. 4). The strike-slip faults mainly occur close to the south-dipping end of the anticline, approximately eight in total (Fig. 4a). Striking S-N, and near upright on cross-sectional profiles, they disappear in the lower Agbada Formation or the Akata Formation at the bottom end, and in the H4-H6 strata at the top end (Fig. 4b). Most in number in the AE anticline region, normal faults are smaller in scale, with a typical fault distance of 40 m (5080 m in some individual strata), extensional lengths of 2-9 km, and dip angles of mostly 5070. First, in the north part of the anticline, three positive inversion faults have developed dipping southeasterly with NE-SW strikes (Fig. 4a). They are larger in vertical extension, and the two faults in the east cut down to the mud-shale detachment layer of the Akata Formation and reach up to the H0-H1 strata respectively (Fig. 4b). Those two faults and the straight normal faults of the shallower segment form part of the late conjugate fault combination. Second, the most well developed normal faults belong to a multiple domino fault combination mainly composed of normal faults dipping northwesterly (Fig. 4). Most of the faults are less than 40 m in length and 7080 m individually. The upper segments of the faults appear straight or slightly
bent on their cross-sectional profiles and tilt towards the down-dip direction of the west limb of the anticline. The top and bottom ends of the faults both disappear within strata of the Agbada Formation. According to the disappearance location of the top of domino faults and the fault combination characteristics in the anticline region, they are further divided into four groups, DFS1, DFS2-1, DFS-2, and DFS-3 (Fig. 3b). The faults in each domino fault group have top endpoints disappearing in the same stratum. Specifically, the endpoints of DFS1 disappear in the H3-H4 strata, those of DFS2-1 below the H2-H3 strata, those of DFS2-2 above the H2-H3 strata, and those of DFS3 in the H1-H2 strata. Apparently, the top endpoint disappearance locations of these four groups of domino faults gradually become shallower, corresponding to gradually younger formation times. The bottom disappearance locations of faults in one fault group are different, but follow certain rules: towards the west limb of the anticline, the disappearance locations of the faults gradually become shallower, forming a triangular cross-sectional distribution pattern. Previous studies showed that although these domino faults currently exist at the anticline top, in fact, all the four groups of faults were formed close to the anticline hinge in the west limb and then migrated westerly as a result of the changing location of the hinge line location during the H3-H4, H2-H3, and H1-H2 stratigraphic depositional periods, which led to the sequential distribution of the domino fault combination described above[30]. Third, the northeast-striking faults (two faults dipping southeasterly and partially northwesterly) and nearly S-N-striking faults (dipping easterly) constituting conjugate fault combination, are the most well developed normal faults besides domino faults. Similar to the domino faults, the conjugate faults are not large, but they are governed by different distribution rules. Overall, the conjugate faults are mainly developed in the H0-H1 strata with their top endpoints disappearing above the H0-H1 strata or close to the marine bottom and their bottom endpoints generally disappearing below the H0-H1 strata. In addition, the conjugate faults are mostly distributed at the anticline tops or the protruding terrains at marine bottoms (Figs. 1c and 3). However, in the AE anticline region, conjugate faults are concentrated in the east limb of the anticline, and their distribution overlaps with the trajectory of the F0 fault (Fig. 4). The top endpoints of these faults are indeed close to the marine bottom, but the bottom endpoints usually disappear in different H1-H4 strata, and some of the faults are hard connected to the domino faults at the base, and the two faults dipping southeasterly noted above. This geometry reveals that these two kinds of faults (the domino fault and the southeasterly dipping fault) were formed as a result of reactivation of preexisting faults during the formation of conjugate faults. Along the planar direction, these faults also show a conjugate combined relationship, but the nearly S-N faults are usually limited or terminated by the northeasterly faults.
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3.2.
Formation and reactivation rules of faults
The faults in the AE anticline region differ in activity period and reactivation rules. According to the analysis above, we know that the thrust faults, F3a-F3b and F6, were active mainly in the H3-H4 stratigraphic depositional period, then weakened or ceased in activity. On the other hand, the positive inversion fault, F0, experienced continuous extensional deformation during the H4-H6 stratigraphic depositional period and positive inversion deformation during the H4-H1 stratigraphic depositional period and became reactivated during the H0-H1 stratigraphic depositional period; its top endpoint propagates upward to the H1-H2 strata. In conclusion, the large number of caprock faults in the AE anticline region have top and bottom endpoints disappear within the basin caprock layers. For this type of fault, their fault throws, dip directions were taken to compile their dislocation profiles, i.e., fault throw-burial depth curves, to represent the fault growth and nucleation and reactivation rules[31]. First, one to two typical faults of every fault group were selected to compile their fault throw-burial depth curves (Fig. 6). Then, by considering the unconformable development characteristics of different strata, the fault activity periods were identified. Our results show that F1, F3a, F4a, F4b, F5a, F5b, and F6 are faults with the same
Fig. 6.
period of activity. Among them, F1 represents the near-vertical strike-slip faults, which were active mainly in the H4-H5 stratigraphic depositional period; F3a represents DFS1 faults, which were active mainly in the H3-H4 stratigraphic depositional period; F4a and F4b represent the DFS2-1 faults which were active in the early H2-H3 period; F5a and F5b represent the DFS2-2 faults which were active in the late depositional period of H2-H3; and F6 represents the nearly S-N-striking conjugate fault which was active in the depositional period of H0-H1. On the other hand, F2 and F3b are reactivated faults with different activity periods. Specifically, F2 represents three normal faults dipping southeasterly, while F3b represents the normal conjugate fault connected to DFS1. Their reactivation periods correspond to the depositional periods of H3-H4 and H0-H1, respectively. Apparently, among the large number of faults developed in the AE anticline region, the near-vertical strike-slip faults formed first. Since the three normal faults dipping southeasterly constrained the DFS faults according to the cross-sectional profiles (Fig. 4b), the normal faults should have formed no later than the domino faults. The domino faults formed prior to the conjugate faults. The reactivated faults include the F0 faults, the two faults dipping southeasterly and several individual DFS1 faults. It can be seen that the reactivation processes of these faults are coupled.
Vertical fault throw-burial depth curves of typical faults.
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4. Formation mechanisms and evolution of oblique anticline and associated fault systems 4.1. Formation of the oblique extensional half-graben and associated fracture system The H4-H6 depositional period corresponds to the initial gravity slip stage of delta sedimentary bodies, during which small folds associated with faults formed, i.e., the AEE1 anticline controlled by the F3b fault and the AEE2 anticline controlled by the F4 fault (Fig. 7). During this period, the AE anticline region was an extensional half-graben mainly controlled by the F0 normal fault (Fig. 8). In addition to F3b, F4, and F0, the active faults also include near-vertical strike-slip faults (Figs. 8 and 9a). As the initial strike of the AEE1 and AEE2 anticlines was WNW-ESE, the thrust direction of the gravity slip was NNE, in small angle with the F0 fault and the half-graben in the hanging wall of F0 fault in northeast strike. Therefore, when the progradation body slipped towards SSW, thrust-fold deformation occurred in the east region along the F0 fault trajectory, leading to the formation of the AEE1 and AEE2 anticlines. In comparison, in the west region along the
Fig. 7.
F0 fault trajectory, no significant thrust-fold process occurred, and as a result, the terrain in the east side of the F0 fault trajectory uplifted. Thus, the differential contractional deformation resulted in an oblique extensional effect along the F0 fault trajectory (Fig. 10a) and the formation of the F0 oblique extensional fault with a steep dip angle and the extensional half-graben in its hanging wall. Moreover, this type of differential gravity slip deformation induced tearing effect on the interior of the progradation body, giving rise to near-vertical strike-slip faults. 4.2. Inversion mechanism and formation process of oblique anticline and formation process of its associated fault system During the H3-H4 depositional period, the gravity slip deformation entered strong fold-thrust deformation stage (Fig. 7). All the main anticlines in Block M formed successively. Comparison shows that different segments of F3 fault differ in the degree of contractional deformation (Fig. 5), and that the deformation strength of the AES1 anticline related to the F3a fault is clearly greater than that of the AEE1 anticline related
Distribution of residual paleo-thicknesses and main active faults of Block M during different periods
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Fig. 8. Formation and evolutionary process of AE anticline and development history of faults.
to the F3b fault. Consequently, under the contraction-thrust effect along the NNE direction, oblique contraction-thrust deformation occurred along the preexisting F0 fault trajectory as a result of differential thrust (Fig. 10b), and the preexisting F0 extensional fault and the half-graben in its hanging experienced tectonic inversion, thus forming the initial shape of the AE anticline (Figs. 7 and 8). During the oblique thrust process, the F0 fault acted as an oblique slope, and the hanging half-graben experienced folding inversion and transformed into an anticline[4]. Affected by stratigraphic distortion and rotation during the formation process of the anticline, the dip angle of the F0 fault became smaller. During this period, two types of faults were formed (Fig. 9a). The first type is fault dipping southeasterly, there are three of them located in the north part of the anticline, which are probably tear normal faults formed as a result of the differential thrust and anticline uplift process. The other type is the domino normal fault (DFS1), these faults are in the west flank of the anticline close
to the hinge line, and dip towards the flank; the driving force for their formation was the differential gravity resulting from a gradually steepened dip angle in the flank region[30]. At the east flank of the AE anticline, the strata are steeper than in the west flank. The study by Morley[30] suggested that when the slope angle reached 12°–13°, the domino fault system driven by gravity would not form, rather migration movement of sedimentary material occurred. Thus, the steeper east limb region is mainly characterized by a unconformable interface resulted from erosion, with strata depositing later overlapping on it (Figs. 3b and 4b). During the H2-H3 depositional period, the continuous fold-thrust deformation caused by gravity slip mainly affected the F3a, F5, and F3c faults and associated folds (Fig. 7), and of them, F3a suffered the most severe thrust deformation. Meanwhile, along with the strong fold-thrust effect on F3a, the AE anticline grew on and extended in range, so its top and hinge line gradually migrated northwesterly (Fig. 8). In more detail, during the early H2-H3 depositional period, the DFS2-1 domino faults formed in the H2-H3 strata in the west limb close to the hinge line (Figs. 8 and 9a); during the late H2-H3 depositional period, the dip angle of the lower strata in the west limb further steepened, and DFS2-2 domino faults consequently formed in the lower dipping limb region close to DFS2-1. During this period, the east limb was still mainly characterized by an unconformable interface, e.g., truncation and overlapping, as a result of erosion. During the H1-H2 depositional period, the contractional deformation strength of the thrust faults weakened significantly, but the AE anticline expanded in range constantly (Fig. 7), and the anticline top also migrated farther towards the west. Similar to the formation mechanism of DFS1 and DFS2, the DFS3 domino faults related to the anticline hinge line formed in the H1-H2 strata in the anticline west limb (Fig. 8). During the anticline growth process, affected by the gradual decrease of dip angle, the distances between DFS1, DFS2-1, DFS2-2, and DFS3 faults became larger and the fault density became lower gradually[30]. 4.3. Formation and evolution of associated fault systems after the end of oblique anticline growth During the H0-H1 depositional period, the fold-thrust deformation gradually ceased in Block M, and in turn, the AE anticline stopped growth and entered the post fold-thrust evolutionary stage. The contraction effect of gravity sliding also weakened, and as a result, the previously strongly uplifted material shrank and returned, and thus conjugate normal faults came about due to gravity collapse and extension[30] (Fig. 3), which are mainly located in the H0-H1 strata. In contrast, the conjugate faults in the AE anticline region are concentrated in the east limb of the anticline (Figs. 8 and 9) and conjugate on cross-section and plane. More specifically, the nearly S-N-striking faults are new normal faults, and besides new normal faults, the northeast-strike faults also include two re-
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Fig. 9.
Distribution of active faults in the AE anticline region in different periods (R reflection layer).
Fig. 10. Formation models of extensional half-graben and oblique lateral slope induced by differential contractional deformation during the gravity slip process.
activated preexisting normal faults dipping southeasterly formed as a result of tearing effect, and a small number of reactive DFS1 faults of gravity genesis. From the perspective of fault distribution region, the faults are mainly distributed along the trajectory and extensional range of the F0 fault. Our study shows that in addition to collapse-related to local gravity return, the formation of the conjugate faults is also related to slip deformation of the preexisting F0 fault. During this period, the F0 fault was reactivated, and the fault displacement propagated upwards, with the top endpoint finally disappearing in the H1-H2 strata. Meanwhile, the reactivated F0 fault controlled the formation and distribution of the conjugate fault system. Therefore, the fault deformation processes were all coupled from the perspective of kinematics. This relationship occurred because during the post fold-thrust evolutionary
period, although there existed little contractional deformation on a large scale, with the continuous progradation of delta sedimentary bodies, material still experienced local differential slip deformation. As a preexisting structure, F0 was a weak zone for deformation, and was prone to re-activation. Thus, with the progradation body in NNE strike acting on the northeast-striking F0 fault, the F0 fault and some of the preexisting northeast-striking faults experienced slip re-activation, which also controlled the formation of the nearly S-N-striking faults conjugating with the F0 fault. As a result, the nearly S-N-striking faults are mostly restricted or terminated at the northeast-striking faults. Thus, the gravity collapse caused by material return and the local gravity slip effect induced slipping movement worked together to control the formation and distribution of the conjugate faults during this late period in
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the AE anticline region.
5.
Petroleum geological significance
The fact that oblique inversion anticlines are quite promising petroleum enrichment geological structures is because they possess optimal geological conditions and effective accumulation processes. Regarding the accumulation geological condition, the oblique anticline region has larger accumulation bodies. The reservoirs in the Niger Delta are mainly in the H4-H5 and H5-H6 strata, and the higher quality reservoirs are largely in H4-H5 strata. According to the analysis above, the H4-H6 depositional period was prior to the occurrence of fold-thrust deformation on a large scale, and the relatively weak differential thrust deformation led to the formation of the oblique extensional fault, F0, and the half-graben in its hanging wall. Because the strikes of F0 and the half-graben are nearly parallel to the progradation direction of delta sedimentary bodies and the half-graben was in a structural segment of low elevation, deltas, basin fans, and water channels in NNE-NE direction (confirmed by exploration practices) influenced by the topography, were formed along the half-graben of in the hanging wall of F0 fault. Given that the east region of basin fan partially crosses over the main F0 fault, it can be inferred that although the F0 fault was not large in scale at this time, the fault at least reached the H4-H6 strata in its center. In addition, although the fault exhibits the characteristics of blind faults towards its ends, the half-graben at the central part of the fault and the forcing folds at its ends were sufficient to induce topographic differences at the paleo-surface (sea bottom). Consequently, the water system mainly flowed along the hanging wall region of the F0 fault. In addition, the NNE-NE water system formed an accumulation body, and after the formation of the anticline, the width of the water system was usually smaller than the range of the anticline. This resulted in the high-angular or near-vertical relationship between the extensional direction of the accumulation body and the anticline strike in the main anticline region, while the extensional direction of the accumulation body and the anticline strike in the oblique anticline region are in small angle or near-parallel (AE anticline), so the reservoirs in the oblique anticline region are clearly larger in range than those in the anticline region. Thus, the oblique anticline region has better reservoir strata[32]. In addition, from the petroleum accumulation process, the F0 positive inversion fault running across the anticline controlled the distribution of reactivated faults during the late period. The reactivated faults usually connect hydrocarbon source rocks with trap structures, and thus acting as effective hydrocarbon-supply channels. Furthermore, although the AE anticline is a completely closed anticline structure, from the aspect of the oil and water distribution relationship, large faults are more likely to stagger sand bodies to form effective lateral sealings, thus dividing the entire anticline trap into multiple complex blocks with independent oil and water system.
To date, exploration practices have confirmed that there exist at least four to five fault block traps with independent oil and water systems in the AE anticline region. In this region, the F6 thrust fault and the reactivated normal faults are larger in scale. The F6 fault divides the whole anticline into south and north two parts. The fact that the growth strength of the south part (DD’ cross-section) is much larger than that of the north part of the anticline (CC’ cross-section) also corroborates that the F6 fault has some dividing effect on the anticline (Fig. 5). Thus, from the perspective of oil and gas division, one needs to pay attention to the relationship between the fault throw and sand-body thickness of large-scale faults, and by also considering the lateral sealing property of the faults and the horizontal regularity of sand-body properties, the blocks with independent oil and water systems can be identified or delineated. Moreover, during the water flooding development, as the pressure increases, the faults could be reactivated, causing accidents. Thus, for faults dividing blocks, especially reactivated faults controlled by F0 fault, assessing their stability is the key to ensure safe exploitation of oil and gas.
6.
Conclusions
Against the background of a contracted delta resulted from gravity slip, the AE oblique anticline experienced three major evolutionary stages: (1) The development stage of the extensional half-graben prior to the anticline formation (H4-H6 depositional period), the main boundary fault of the half-graben, F0, formed as a result of oblique extension induced by differential thrust transition; during this period, the differential contractional deformation caused the formation of near-vertical strike-slip tear faults. (2) During the half-graben inversion-induced anticline formation and its growth stage (H1-H4 stratigraphic depositional period), tectonic inversion occurred, which transferred the preexisting structures (F0 and the hanging half-graben) via oblique contraction due to the differential thrust strength. During this period, the differential thrust effect led to the formation of local tear normal faults and multi-periodic domino normal faults caused by the gravity effect of the slope dip angle at the limb. The domino faults were controlled by the paleo anticline top (or hinge) and distributed in an orderly manner with the western migration of the anticline top. (3) The post fold-thrust stage when the growth ceased (H0-H1 depositional period), local structural segments with high elevations on the ground surface collapsed as a result of returned gravity, which led to the formation of sparsely distributed small-scale conjugate normal faults. In the east AE anticline region, the conjugate normal faults were jointly affected by local gravity return and collapse and oblique distortion of the F0 fault post folding. The coupling of the oblique reactivated F0 fault with the conjugate faults controlled the planar combination pattern and distribution of the conjugate faults. Oblique inversion anticlines are broad promising petroleum structures because during the extensional half-graben period,
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the fault hanging walls at the main interfaces control the paths of delta water channels and basin fans. In turn, the distributions of water channels and basin fans are later parallel to the anticline distribution, which is favorable for development of high-quality large-scale reservoirs. The reactivated faults after the anticline formation (F0 fault and the coupled reactivated normal faults) usually link hydrocarbon source rock with the reservoir. The reactivated faults are mostly located in the anticline main body and distributed along the long axis of the anticline, which is conducive to the migration of hydrocarbon toward the trap and accumulation therein. The relatively large faults in the anticline region include the thrust faults, F6 and the reactivated normal faults controlled by F0. They can offset sand-bodies, forming effective lateral sealing, and dividing the entire anticline trap into multiple complex block traps with independent oil and water systems. The stability of these faults separating the oil and water systems is crucial to the petroleum production safety.
COOPER M A, WILLIAMS G D. Inversion tectonics: A disWILLIAMS G D, POWELL C M, COOPER M A. Geometry and kinematics of inversion tectonics. Geological Society, 1989, 44(1): 3–15.
[3]
BUCHANAN J G, BUCHANAN P G. Basin inversion. Geological Society Special Publications, 1995, 88(3): 596.
[4]
BURLIGA S, KOYI H A, KRZYWIEC P. Modelling cover deformation and decoupling during inversion, using the Mid-Polish Trough as a case study. Journal of Structural Geology, 2012, 42(2): 62–73.
[5]
MANSY J L, MANBY G M, AVERBUCH O, et al. Dynamics and inversion of the Mesozoic Basin of the Weald–Boulonnais area: Role of basement reactivation. Tectonophysics, 2003, 373(1): 161–179.
[6]
MCCLAY K R. The geometries and kinematics of inverted fault systems: A review of analogue model studies. Geological Society, London, Special Publications, 1995, 88(1): 97–118.
[7]
[9]
role of deformation in controlling depositional patterns in the south-central Niger Delta, West Africa. Journal of Structural Geology, 2002, 24(4): 847–859. [13] CORREDOR F, SHAW J H, BILOTTI F. Structural styles in the deep-water fold and thrust belts of the Niger Delta. AAPG Bulletin, 2005, 89(6): 753–780. [14] BRIGGS S E, DAVIES R J, CARTWRIGHT J A, et al. Multiple detachment levels and their control on fold styles in the compressional domain of the deepwater west Niger Delta. Basin Research, 2006, 18(4): 435–450. [15] MORLEY C K. Mobile shale related deformation in large deltas developed on passive and active margins. Geological Society, London, Special Publications, 2003, 216(1): 335–357. [16] ROWAN M G, PEEL F J, VENDEVILLE B C. Gravity-driven
thrust belt classification, tectonics, structure and hydrocarbon prospectivity: A review. Earth-Science Reviews, 2011, 104(1): 41–91. [18] MAGBAGBEOLA O A, WILLIS B J. Sequence stratigraphy and syndepositional deformation of the Agbada Formation, Robertkiri field, Niger Delta, Nigeria. AAPG Bulletin, 2007, 91(7): 945–958. [19] CONNORS C D, RADOVICH B, DANFORTH A, et al. The structure of the offshore Niger Delta. Trabajos De Geología, 2009, 29(29): 182–188. [20] BILOTTI F, SHAW J H. Deep-water Niger Delta fold and thrust belt modeled as a critical-taper wedge: The influence of elevated basal fluid pressure on structural styles. AAPG Bulletin, 2005, 89(11): 1475–1491. [21] BEGLINGER S E, DOUST H, CLOETINGH S. Relating petroleum system and play development to basin evolution: West African South Atlantic basins. Marine and Petroleum Geology, 2012, 30(1): 1–25.
TURNER J P, WILLIAMS G A. Sedimentary basin inversion
[22] COHEN H A, MCCLAY K. Sedimentation and shale tectonics
and intra-plate shortening. Earth-Science Reviews, 2004,
of the northwestern Niger Delta front. Marine and Petroleum Geology, 1996, 13(3): 313–328.
65(3): 277–304. [8]
[12] HOOPER R J, FITZSIMMONS R J, GRANT N, et al. The
[17] MORLEY C K, KING R, HILLIS R, et al. Deepwater fold and
cussion. Geological Society, 1989, 44(1): 335–347. [2]
teristics of inversion. Geological Society, 1989, 44(1): 17–39.
fold belts on passive margins. Tulsa: AAPG, 2004: 157–182.
References [1]
Journal of Earth Sciences, 2005, 94(5/6): 782–798. [11] HAYWARD A B, GRAHAM R H. Some geometrical charac-
BONINI M, SANI F, ANTONIELLI B. Basin inversion and
[23] ROUBY D, NALPAS T, JERMANNAUD P, et al. Gravity
contractional reactivation of inherited normal faults: A review
driven deformation controlled by the migration of the delta
based on previous and new experimental models. Tectono-
front: The Plio-Pleistocene of the Eastern Niger Delta. Tec-
physics, 2012, 523(3): 55-88.
tonophysics, 2011, 513(1): 54–67.
AMILIBIA A, SABAT F, MCCLAY K R, et al. The role of
[24] AIZEBEOKHAI A P, OLAYINKA I. Structural and strati-
inherited tectono-sedimentary architecture in the development
graphic mapping of Emi field, offshore Niger Delta. Journal of
of the central Andean mountain belt: Insights from the Cordil-
Geology, 2011, 3(2): 25–38.
lera de Domeyko. Journal of Structural Geology, 2008, 30(12):
[25] JERMANNAUD P, ROUBY D, ROBIN C, et al. Plio-Pleistocene sequence stratigraphic architecture of the eastern Niger
1520–1539. [10] MAZUR S, SCHECK-WENDEROTH M, KRZYWIEC P. Different modes of the Late Cretaceous–Early Tertiary inversion in the North German and Polish basins. International
66
Delta: A record of eustasy and aridification of Africa. Marine and Petroleum Geology, 2010, 27(4): 810–821. [26] RIBOULOT V, CATTANEO A, BERNÉ S, et al. Geometry
SUN Yonghe et al. / Petroleum Exploration and Development, 2018, 45(1): 55–67
and chronology of late Quaternary depositional sequences in the Eastern Niger Submarine Delta. Marine Geology, 2012,
to decollement folds and fault-propagation folds. Inferences on fold kinematics. Tectonophysics, 1997, 282(1/2/3/4): 353–373. [30] MORLEY C K. Development of crestal normal faults associ-
319(2): 1–20. [27] KOLLA V, POSAMENTIER H W, WOOD L J. Deep-water and fluvial sinuous channels: Characteristics, similarities and dissimilarities, and modes of formation. Marine and Petro-
ated with deepwater fold growth. Journal of Structural Geology, 2007, 29(7): 1148–1163. [31] BAUDON C, CARTWRIGHT J. Early stage evolution of growth faults: 3D seismic insights from the Levant Basin,
leum Geology, 2007, 24(6): 388–405. [28] SYLVESTER Z, PIRMEZ C, CANTELLI A. A model of submarine channel-levee evolution based on channel trajectories: Implications for stratigraphic architecture. Marine and
Eastern Mediterranean. Journal of Structural Geology, 2008, 30(7): 888–898. [32] JIANG F J, PANG X Q, BAI J, et al. Comprehensive assessment of source rocks in the Bohai Sea area, eastern China.
Petroleum Geology, 2011, 28(3): 716–727. [29] STORTI F, POBLET J. Growth stratal architectures associated
67
AAPG Bulletin, 2016, 100(6): 969–1002.
RESEARCH PAPER
Formation and evolution of oblique anticline in eastern Niger Delta SUN Yonghe1, 2, 3, *, LIU Zhida1, 2, 3, LI Xuesong4, HU Guangyi5, FAN Tingen5, GAO Yunfeng5 1. Innovation Team of Science and Technology for Fault Deformation, Sealing and Fluid Migration, Northeast Petroleum University, Daqing 163318, China; 2. Laboratory of CNPC for Fault Controlling Reservoir, Northeast Petroleum University, Daqing 163318, China; 3. Earth Science College, Northeast Petroleum University, Daqing 163318, China; 4. Exploration and Development Research Institute of Daqing Oilfield Co Ltd., Daqing 163712, China; 5. Research Institute of China National Offshore Oil Corporation, Beijing 100027, China
Abstract: Based on the three-dimensional seismic interpretation data, this paper analyzed the formation mechanism and the growth process of the oblique anticline AE of the M region of the eastern Niger Delta, as well as the evolution process of the associated fault systems. The study results show that the stratigraphic sedimentary period between reflector H4-H6 of the middle and late Miocene was the initial fold-thrust stage, the anticline AE was a half-graben controlled by oblique extensional faults derived from the oblique extensional transfer structure formed by local initial differential fold-thrusting. At the same time the tear faults developed as a result of the differential sliding. During the stratigraphic sedimentary period between reflector H1-H4 of the late Miocene to Pliocene, the large-scale folding and thrusting occurred, differential contractional deformation resulted in the pre-existing extensional half-graben became AE anticline by oblique tectonic inversion, then the anticline grew continually and the crest of the anticline migrated gradually. The newly formed fault systems consist of a small number of associated tear-normal faults caused by differential thrusting and gravity-driven domino normal faults predominantly induced by the slope inclination of the anticline limb. During the stratigraphic sedimentary period between reflector H0-H1 of the Pleistocene to Holocene, as the growth of the anticline ceased, the area entered post-fold thrusting stage. The formation and distribution of conjugated faults were controlled by the local gravity return collapse, local differential sliding and reactivation of pre-existing positive inversion faults jointly. The research results of genetic mechanism of the oblique inversion anticline and evolution of associated faults are helpful to reveal the key factors controlling the accumulation and distribution of oil and gas. Key words: Niger Delta; gravity sliding; differential contractional deformation; oblique anticline; fault system; tectonic inversion; Paleogene; Neogene
Introduction Structural inversion refers to the structural deformation resulted from the combined effect of extensional deformation and contractional deformation, and the resultant geological structure is called an inversion structure[1]. Positive inversion corresponds to extensional deformation followed by contractional deformation, while reverse inversion corresponds to contractional deformation followed by extensional deformation[2]. Basin inversion is usually used to describe the process of post-superimposition and shortening deformation of extensional basin[3]. There exist a large number of petroliferous basins around the world that have experienced inversion deformation[45], and the oil and gas accumulation in these basins
is usually closely related to the positive inversion deformation[68]. However, it is often difficult to clearly identify complex structures formed as a result of reverse inversion in basins, mostly because the shortening of basin is mainly deformation adjusted by reactivating preexisting faults, during which a large number of new contractional structures and secondary extensional structures, such as folds and associated extensional faults, are formed, so the structures change significantly after the inversion deformation compared with the corresponding extensional basins prior to the inversion deformation[9]. Thus, it is of great significance to study the mechanisms of inversion deformation and the formation process of associated new structures for understanding the
Received date: 12 Jul. 2017; Revised date: 15 Dec. 2017. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the National Natural Science Foundation of China (41572127, U1562214); China National Science and Technology Major Project (2016ZX05006-005, 2016ZX05054-009). Copyright © 2018, Research Institute of Petroleum Exploration & Development, PetroChina. Publishing Services provided by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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rules governing the enrichment of oil and gas as well as their migration, formation and storage processes. Previous studies showed that during the inversion process, the compressive stress usually originated from the horizontal stress field produced by plate interactions within a basin[10]. Under such compressive stress, the main branches of preexisting extensional faults usually inversed and reactivated. Meanwhile, the preexisting extensional basins experienced uplifting and erosion, and these processes were accompanied by concomitant depositional refilling. This combination consequently results in significantly thinned local strata and development of local unconformities or local formation of progradation strata[11].
Fig. 1.
For gravity slip structures, e.g., the Niger Delta, the major stress responsible for their structural deformation is the gravity slip effect from delta progradation bodies, which can lead to formation of fold-thrust structures in contractional tectonic domains. Whether such contractional tectonic domains can transform into inversion structures as a result of varied regional stress field polarity is the focus of this study. We chose a typical oblique anticline in the M block of the east Niger Delta as our research object (Fig. 1), and by considering the differential fold-thrust process induced by gravity slip, we discuss the inversion deformation mechanism of this oblique anticline and the formation process of the associated fracture system.
Structural outline, fold thrust pattern, and main fracture characteristics of the M block.
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1.
Regional geological background
Located in central West Africa (Fig. 1a), the Niger Delta is one of the delta basins with rich petroleum resource in the world[1214]. Given that the Niger Delta is located at a passive continental margin, it is characterized by development of gravity slip structures[1517] and can be divided into extensional tectonic region, translational tectonic region, and contractional tectonic region from the above-water basin region toward the deep-water basin region[1819]. The exploratory drilling in the past ten years has confirmed that the contractional tectonic region possesses high petroleum resource potential[20]. The M block in the east Niger Delta lies in the transitional zone between the translational tectonic region and the contractional tectonic region (Fig. 1a). This block contains seven oil-enriched anticlines, mostly in WNW-ESE direction (Fig. 1b), suggesting the regional delta progradation direction
Fig. 2.
or gravity slip direction is NE-NNE to SW-SSW. Observation of the cross-sectional profile reveals that most of these anticlines are fault propagation folds predominantly controlled by shovel-like thrust faults (Fig. 1c). There is also an AE anticline in NE-SW strike in the M block (Fig. 1b), which is nearly parallel to the gravity slip direction and almost perpendicular to the strike of the main anticlines. The AE anticline is therefore defined as an oblique anticline. The drilling data in recent years reveals that the oblique anticline has better oil and gas prospects than the other main anticlines[21]. The strata in the AE oblique anticline are mainly composed of a Cenozoic delta depositional sequence[2223], and the multi-periodic delta progradation led to formation structure with good vertical stratification and diachronism (Fig. 2). At the delta bottom, deposited mainly marine mud shale named the Akata Formation. This sequence of mud shale is a main hydrocarbon
Stratigraphic structure and seismic reflection characteristics of typical interfaces in the Niger Delta.
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source rock sequence[24], and a main set of detachment layer caused by the gravity slip of the delta induced by excessive pressure formed within the strata due to continuous progradation of the delta[22]. Above the Akata Formation lies the Agbada Formation. This stratigraphic sequence has been continuously growing from the Eocene to the present and is the main body of the delta deposition sequences as well as the main sequence for petroleum enrichment[25]. It is mostly composed of offshore delta detrital deposits at the delta front and the pre-delta[26], with water channels and basin fan deposits being the main accumulation bodies[2728]. Above the Agbada Formation lies the Benin Formation, which is composed of continental deposits of Eocene-Holocene age, including both alluvial plain and coastal plain deposits. This stratigraphic sequence mainly developed above coastal or water areas. We focus on strata above the Miocene series of the Agbada Formation in this study, which include the H0, H1, H2, H3, H4, H5, H6, and R; of them, the R stratum corresponds to the top boundary of the reservoir with the richest petroleum in the AE anticline.
2. General characteristics and growth periods of the oblique anticline 2.1.
Geometric characteristics
Most anticlines in the M block strike WNW-ESE, including the AP, AES1, AES2, AEE1, AEE2, and AA anticlines (Fig. 1b). It can be seen from the cross-sectional profile that these anticlines are broad, the top endpoints for fold-related thrust faults within this block usually disappear in strata above the H4 interface stratum, and these folds can therefore be classified as fault propagation folds (Fig. 1c). The AE oblique anticline in this block strikes NE-SW, nearly perpendicular to the main anticlines and almost parallel to the delta gravity slip direction. The AE anticline and the main anticlines, AES1 and AEE1, are all located in the upper wall of F3 thrust fault (Fig. 1b). The AE anticline is in the northeast segment at the joint of F3a and F3b, but the anticline
Fig. 3.
extends northeasterly far beyond the length range of the F3a-F3b segment, and the deformation range controlled by this segment; in addition, the anticline top near the F3a-F3b connecting segment does not lie close to the top endpoint (Fig. 1b). Another thrust fault related to the AE anticline is the F6 fault in WNW-ESE strike, which is located in the central part of the anticline. From the cross-sectional profile, we can see that the top endpoints of these two thrust faults both disappear in the H3-H4 strata and cannot be observed in any of the stratigraphic sequences on both side of the faults because of thickness differences developed during the anticline growth (Fig. 3a). This relationship implies that these two faults are not the dominant factors controlling the anticline growth. From the cross-sectional profile, one can barely distinguish the short and long axes of the anticline, and the anticline is therefore close to an equiaxial anticline (Fig. 3a, 3b). However, in plan view, the anticline appears as a short axial anticline trending NE-SW (Figs. 1b and 4a). Further analysis reveals that the AE anticline appears as an equiaxial anticline on the cross-sectional profile mainly because in the H1-H4 strata, the top segment (hinge) of the anticline gradually migrates northwesterly from bottom to top (Fig. 3b), thus leading to an enlargement along the NW-SE direction. It can be preliminarily inferred that there may exist a fault of northeast trend that controls the distribution of this anticline. And in-depth seismic analysis indeed found that within the AE anticline, there is a thrust fault striking NE-SW and dipping northwest, F0, which controls the anticline deformation (Figs. 3b and 4). In addition, there is a distinct difference between the thickness variation of strata above and below the H4 interface in the AE anticline. The strata above H4 in the anticline core are thicker than those in the limbs, representing the syndeposition during anticline growth. On the other hand, the two sets of strata below H4 are slightly thicker in the anticline core than in the limbs, especially in the syncline region east of the anticline the two layers are clearly thinner (Fig. 3b). This phenomenon illustrates that prior to the controlling effect of thrust deformation
Typical seismic cross-sectional profile and planar shape of the AE anticline (see Fig. 1b for the location of the cross-sections).
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Fig. 4.
Distribution and characteristics of fracture system related to the AE anticline.
on the anticline growth (at least before the H4 deposition), F0 experienced extensional deformation and as a result, a hanging half-graben basin formed. Later, with the superimposition effect of inversion deposition, the anticline formed. 2.2.
Syndeposition of sequences in the anticline
During the fold growth process, topographic difference forms between the core and the limbs. If the area receives deposits and erosion occurs only locally in regions with relatively high elevation, the depositional sequence thin at the core and thicker in the limbs (the syndepositional characteristics) will be formed. In this case, the thickness difference of the syndepositional sequence represents the growth process and the magnitude of the fold[29]. The larger the fold growth magnitude, the larger the thickness difference between the strata in the core and limbs of the anticline is. In addition, regarding delta gravity slip structures at passive continental margins, folds with relatively large growth magnitude usually lead to stratum erosion at the anticline cores and collapse and migration of deposits in the steep limb, which can explain the overlap and truncation phenomena above or below local unconformities. Thus, these phenomena can help identify fold growth periods more accurately. The AES1, AEE1, AEE2, and AE anticlines related to the F3 thrust fault were taken to count the formation thicknesses at their limbs and cores, and calculated the corresponding ratios (Fig. 5). The results show that during the H4-H6 stratigraphic depositional period, the growing anticlines mainly included the AEE1 anticline corresponding to the F3c fault and the AEE2 anticline corresponding to the F4 fault. But the stratigraphic syndepositional thickness ratio is quite small, representing a low level of deformation as with the initial gravity slip deformation characteristics during the delta progradation process. In the AE anticline region, because the
thickness of the H4-H6 strata at the core is larger than those in the limbs, the corresponding thickness ratios are both less than one, and the thicker strata are located in the upper wall of the F0 fault dipping southwest, so it is inferred that during this period the AE anticline region was an extensional half-graben basin. Above the H4 reflection layer, the AES1, AEE1, AEE2, and AE anticlines all exhibit growing characteristics. However, the growth of the AEE1 and AEE2 anticlines continued only to the H2-H3 stratigraphic depositional period, and their growth intensities were relatively weak. In contrast, the AES1 and AE anticlines had been continuously growing at high intensities until the H1-H2 stratigraphic depositional period, and during the H0-H1 stratigraphic depositional period, they grew at much lower intensities, and finally stopped growing.
Fig. 5. Histogram of thickness ratios of the anticline limb to core of different sets of stratigraphic units.
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Moreover, from the seismic cross-sectional profiles, one can see that at the H4, H3, and H2 interfaces in the paleo-anticline core region, there exist unconformable interfaces and associated overlap and truncation phenomena (Figs. 3b, and 4b), implying that the AE anticline continuously grew at relatively large magnitudes during the H3-H4, H2-H3, and H1-H2 stratigraphic depositional periods. Comparison of the growth trajectories of these four adjacent anticlines (Fig. 5) reveals that prior to H4, the AE anticline region was an extensional half-graben, and its formation was related to the initial growth of the AEE1 and AEE2 main anticlines on its east. The subsequent formation of the AE anticline after H4 and its continuous growing process, are related to the strong growth of the AES1 main anticline.
3. Characteristics and activity rules of fault systems in the oblique anticline region 3.1.
Types and development of fault systems
In the AE anticline region, developed a large number of faults of various nature, including thrust faults (F3a-F3b and F6), a positive inversion fault (F0), strike-slip faults (approximately eight faults), and normal faults (Fig. 4a). The thrust faults, F3a-F3b and F6, are located at the south end and central part of the anticline, respectively. These two faults appear shovel-like on their cross-sectional profiles: their bottoms smoothly converge into the deep mud-shale detachment layer of the Akata Formation, and their top endpoints disappear in the H1-H2 strata. The positive inversion fault, F0, dips northwesterly and strikes NE-SW (Figs. 3b and 4). Straight on profile, it cuts down to the mud-shale detachment layer of the Akata Formation, disappears in the H1-H2 strata to the top; in the R reflection layer, only the trajectory of its central segment can be seen (Fig. 4). The strike-slip faults mainly occur close to the south-dipping end of the anticline, approximately eight in total (Fig. 4a). Striking S-N, and near upright on cross-sectional profiles, they disappear in the lower Agbada Formation or the Akata Formation at the bottom end, and in the H4-H6 strata at the top end (Fig. 4b). Most in number in the AE anticline region, normal faults are smaller in scale, with a typical fault distance of 40 m (5080 m in some individual strata), extensional lengths of 2-9 km, and dip angles of mostly 5070. First, in the north part of the anticline, three positive inversion faults have developed dipping southeasterly with NE-SW strikes (Fig. 4a). They are larger in vertical extension, and the two faults in the east cut down to the mud-shale detachment layer of the Akata Formation and reach up to the H0-H1 strata respectively (Fig. 4b). Those two faults and the straight normal faults of the shallower segment form part of the late conjugate fault combination. Second, the most well developed normal faults belong to a multiple domino fault combination mainly composed of normal faults dipping northwesterly (Fig. 4). Most of the faults are less than 40 m in length and 7080 m individually. The upper segments of the faults appear straight or slightly
bent on their cross-sectional profiles and tilt towards the down-dip direction of the west limb of the anticline. The top and bottom ends of the faults both disappear within strata of the Agbada Formation. According to the disappearance location of the top of domino faults and the fault combination characteristics in the anticline region, they are further divided into four groups, DFS1, DFS2-1, DFS-2, and DFS-3 (Fig. 3b). The faults in each domino fault group have top endpoints disappearing in the same stratum. Specifically, the endpoints of DFS1 disappear in the H3-H4 strata, those of DFS2-1 below the H2-H3 strata, those of DFS2-2 above the H2-H3 strata, and those of DFS3 in the H1-H2 strata. Apparently, the top endpoint disappearance locations of these four groups of domino faults gradually become shallower, corresponding to gradually younger formation times. The bottom disappearance locations of faults in one fault group are different, but follow certain rules: towards the west limb of the anticline, the disappearance locations of the faults gradually become shallower, forming a triangular cross-sectional distribution pattern. Previous studies showed that although these domino faults currently exist at the anticline top, in fact, all the four groups of faults were formed close to the anticline hinge in the west limb and then migrated westerly as a result of the changing location of the hinge line location during the H3-H4, H2-H3, and H1-H2 stratigraphic depositional periods, which led to the sequential distribution of the domino fault combination described above[30]. Third, the northeast-striking faults (two faults dipping southeasterly and partially northwesterly) and nearly S-N-striking faults (dipping easterly) constituting conjugate fault combination, are the most well developed normal faults besides domino faults. Similar to the domino faults, the conjugate faults are not large, but they are governed by different distribution rules. Overall, the conjugate faults are mainly developed in the H0-H1 strata with their top endpoints disappearing above the H0-H1 strata or close to the marine bottom and their bottom endpoints generally disappearing below the H0-H1 strata. In addition, the conjugate faults are mostly distributed at the anticline tops or the protruding terrains at marine bottoms (Figs. 1c and 3). However, in the AE anticline region, conjugate faults are concentrated in the east limb of the anticline, and their distribution overlaps with the trajectory of the F0 fault (Fig. 4). The top endpoints of these faults are indeed close to the marine bottom, but the bottom endpoints usually disappear in different H1-H4 strata, and some of the faults are hard connected to the domino faults at the base, and the two faults dipping southeasterly noted above. This geometry reveals that these two kinds of faults (the domino fault and the southeasterly dipping fault) were formed as a result of reactivation of preexisting faults during the formation of conjugate faults. Along the planar direction, these faults also show a conjugate combined relationship, but the nearly S-N faults are usually limited or terminated by the northeasterly faults.
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3.2.
Formation and reactivation rules of faults
The faults in the AE anticline region differ in activity period and reactivation rules. According to the analysis above, we know that the thrust faults, F3a-F3b and F6, were active mainly in the H3-H4 stratigraphic depositional period, then weakened or ceased in activity. On the other hand, the positive inversion fault, F0, experienced continuous extensional deformation during the H4-H6 stratigraphic depositional period and positive inversion deformation during the H4-H1 stratigraphic depositional period and became reactivated during the H0-H1 stratigraphic depositional period; its top endpoint propagates upward to the H1-H2 strata. In conclusion, the large number of caprock faults in the AE anticline region have top and bottom endpoints disappear within the basin caprock layers. For this type of fault, their fault throws, dip directions were taken to compile their dislocation profiles, i.e., fault throw-burial depth curves, to represent the fault growth and nucleation and reactivation rules[31]. First, one to two typical faults of every fault group were selected to compile their fault throw-burial depth curves (Fig. 6). Then, by considering the unconformable development characteristics of different strata, the fault activity periods were identified. Our results show that F1, F3a, F4a, F4b, F5a, F5b, and F6 are faults with the same
Fig. 6.
period of activity. Among them, F1 represents the near-vertical strike-slip faults, which were active mainly in the H4-H5 stratigraphic depositional period; F3a represents DFS1 faults, which were active mainly in the H3-H4 stratigraphic depositional period; F4a and F4b represent the DFS2-1 faults which were active in the early H2-H3 period; F5a and F5b represent the DFS2-2 faults which were active in the late depositional period of H2-H3; and F6 represents the nearly S-N-striking conjugate fault which was active in the depositional period of H0-H1. On the other hand, F2 and F3b are reactivated faults with different activity periods. Specifically, F2 represents three normal faults dipping southeasterly, while F3b represents the normal conjugate fault connected to DFS1. Their reactivation periods correspond to the depositional periods of H3-H4 and H0-H1, respectively. Apparently, among the large number of faults developed in the AE anticline region, the near-vertical strike-slip faults formed first. Since the three normal faults dipping southeasterly constrained the DFS faults according to the cross-sectional profiles (Fig. 4b), the normal faults should have formed no later than the domino faults. The domino faults formed prior to the conjugate faults. The reactivated faults include the F0 faults, the two faults dipping southeasterly and several individual DFS1 faults. It can be seen that the reactivation processes of these faults are coupled.
Vertical fault throw-burial depth curves of typical faults.
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4. Formation mechanisms and evolution of oblique anticline and associated fault systems 4.1. Formation of the oblique extensional half-graben and associated fracture system The H4-H6 depositional period corresponds to the initial gravity slip stage of delta sedimentary bodies, during which small folds associated with faults formed, i.e., the AEE1 anticline controlled by the F3b fault and the AEE2 anticline controlled by the F4 fault (Fig. 7). During this period, the AE anticline region was an extensional half-graben mainly controlled by the F0 normal fault (Fig. 8). In addition to F3b, F4, and F0, the active faults also include near-vertical strike-slip faults (Figs. 8 and 9a). As the initial strike of the AEE1 and AEE2 anticlines was WNW-ESE, the thrust direction of the gravity slip was NNE, in small angle with the F0 fault and the half-graben in the hanging wall of F0 fault in northeast strike. Therefore, when the progradation body slipped towards SSW, thrust-fold deformation occurred in the east region along the F0 fault trajectory, leading to the formation of the AEE1 and AEE2 anticlines. In comparison, in the west region along the
Fig. 7.
F0 fault trajectory, no significant thrust-fold process occurred, and as a result, the terrain in the east side of the F0 fault trajectory uplifted. Thus, the differential contractional deformation resulted in an oblique extensional effect along the F0 fault trajectory (Fig. 10a) and the formation of the F0 oblique extensional fault with a steep dip angle and the extensional half-graben in its hanging wall. Moreover, this type of differential gravity slip deformation induced tearing effect on the interior of the progradation body, giving rise to near-vertical strike-slip faults. 4.2. Inversion mechanism and formation process of oblique anticline and formation process of its associated fault system During the H3-H4 depositional period, the gravity slip deformation entered strong fold-thrust deformation stage (Fig. 7). All the main anticlines in Block M formed successively. Comparison shows that different segments of F3 fault differ in the degree of contractional deformation (Fig. 5), and that the deformation strength of the AES1 anticline related to the F3a fault is clearly greater than that of the AEE1 anticline related
Distribution of residual paleo-thicknesses and main active faults of Block M during different periods
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Fig. 8. Formation and evolutionary process of AE anticline and development history of faults.
to the F3b fault. Consequently, under the contraction-thrust effect along the NNE direction, oblique contraction-thrust deformation occurred along the preexisting F0 fault trajectory as a result of differential thrust (Fig. 10b), and the preexisting F0 extensional fault and the half-graben in its hanging experienced tectonic inversion, thus forming the initial shape of the AE anticline (Figs. 7 and 8). During the oblique thrust process, the F0 fault acted as an oblique slope, and the hanging half-graben experienced folding inversion and transformed into an anticline[4]. Affected by stratigraphic distortion and rotation during the formation process of the anticline, the dip angle of the F0 fault became smaller. During this period, two types of faults were formed (Fig. 9a). The first type is fault dipping southeasterly, there are three of them located in the north part of the anticline, which are probably tear normal faults formed as a result of the differential thrust and anticline uplift process. The other type is the domino normal fault (DFS1), these faults are in the west flank of the anticline close
to the hinge line, and dip towards the flank; the driving force for their formation was the differential gravity resulting from a gradually steepened dip angle in the flank region[30]. At the east flank of the AE anticline, the strata are steeper than in the west flank. The study by Morley[30] suggested that when the slope angle reached 12°–13°, the domino fault system driven by gravity would not form, rather migration movement of sedimentary material occurred. Thus, the steeper east limb region is mainly characterized by a unconformable interface resulted from erosion, with strata depositing later overlapping on it (Figs. 3b and 4b). During the H2-H3 depositional period, the continuous fold-thrust deformation caused by gravity slip mainly affected the F3a, F5, and F3c faults and associated folds (Fig. 7), and of them, F3a suffered the most severe thrust deformation. Meanwhile, along with the strong fold-thrust effect on F3a, the AE anticline grew on and extended in range, so its top and hinge line gradually migrated northwesterly (Fig. 8). In more detail, during the early H2-H3 depositional period, the DFS2-1 domino faults formed in the H2-H3 strata in the west limb close to the hinge line (Figs. 8 and 9a); during the late H2-H3 depositional period, the dip angle of the lower strata in the west limb further steepened, and DFS2-2 domino faults consequently formed in the lower dipping limb region close to DFS2-1. During this period, the east limb was still mainly characterized by an unconformable interface, e.g., truncation and overlapping, as a result of erosion. During the H1-H2 depositional period, the contractional deformation strength of the thrust faults weakened significantly, but the AE anticline expanded in range constantly (Fig. 7), and the anticline top also migrated farther towards the west. Similar to the formation mechanism of DFS1 and DFS2, the DFS3 domino faults related to the anticline hinge line formed in the H1-H2 strata in the anticline west limb (Fig. 8). During the anticline growth process, affected by the gradual decrease of dip angle, the distances between DFS1, DFS2-1, DFS2-2, and DFS3 faults became larger and the fault density became lower gradually[30]. 4.3. Formation and evolution of associated fault systems after the end of oblique anticline growth During the H0-H1 depositional period, the fold-thrust deformation gradually ceased in Block M, and in turn, the AE anticline stopped growth and entered the post fold-thrust evolutionary stage. The contraction effect of gravity sliding also weakened, and as a result, the previously strongly uplifted material shrank and returned, and thus conjugate normal faults came about due to gravity collapse and extension[30] (Fig. 3), which are mainly located in the H0-H1 strata. In contrast, the conjugate faults in the AE anticline region are concentrated in the east limb of the anticline (Figs. 8 and 9) and conjugate on cross-section and plane. More specifically, the nearly S-N-striking faults are new normal faults, and besides new normal faults, the northeast-strike faults also include two re-
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Fig. 9.
Distribution of active faults in the AE anticline region in different periods (R reflection layer).
Fig. 10. Formation models of extensional half-graben and oblique lateral slope induced by differential contractional deformation during the gravity slip process.
activated preexisting normal faults dipping southeasterly formed as a result of tearing effect, and a small number of reactive DFS1 faults of gravity genesis. From the perspective of fault distribution region, the faults are mainly distributed along the trajectory and extensional range of the F0 fault. Our study shows that in addition to collapse-related to local gravity return, the formation of the conjugate faults is also related to slip deformation of the preexisting F0 fault. During this period, the F0 fault was reactivated, and the fault displacement propagated upwards, with the top endpoint finally disappearing in the H1-H2 strata. Meanwhile, the reactivated F0 fault controlled the formation and distribution of the conjugate fault system. Therefore, the fault deformation processes were all coupled from the perspective of kinematics. This relationship occurred because during the post fold-thrust evolutionary
period, although there existed little contractional deformation on a large scale, with the continuous progradation of delta sedimentary bodies, material still experienced local differential slip deformation. As a preexisting structure, F0 was a weak zone for deformation, and was prone to re-activation. Thus, with the progradation body in NNE strike acting on the northeast-striking F0 fault, the F0 fault and some of the preexisting northeast-striking faults experienced slip re-activation, which also controlled the formation of the nearly S-N-striking faults conjugating with the F0 fault. As a result, the nearly S-N-striking faults are mostly restricted or terminated at the northeast-striking faults. Thus, the gravity collapse caused by material return and the local gravity slip effect induced slipping movement worked together to control the formation and distribution of the conjugate faults during this late period in
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the AE anticline region.
5.
Petroleum geological significance
The fact that oblique inversion anticlines are quite promising petroleum enrichment geological structures is because they possess optimal geological conditions and effective accumulation processes. Regarding the accumulation geological condition, the oblique anticline region has larger accumulation bodies. The reservoirs in the Niger Delta are mainly in the H4-H5 and H5-H6 strata, and the higher quality reservoirs are largely in H4-H5 strata. According to the analysis above, the H4-H6 depositional period was prior to the occurrence of fold-thrust deformation on a large scale, and the relatively weak differential thrust deformation led to the formation of the oblique extensional fault, F0, and the half-graben in its hanging wall. Because the strikes of F0 and the half-graben are nearly parallel to the progradation direction of delta sedimentary bodies and the half-graben was in a structural segment of low elevation, deltas, basin fans, and water channels in NNE-NE direction (confirmed by exploration practices) influenced by the topography, were formed along the half-graben of in the hanging wall of F0 fault. Given that the east region of basin fan partially crosses over the main F0 fault, it can be inferred that although the F0 fault was not large in scale at this time, the fault at least reached the H4-H6 strata in its center. In addition, although the fault exhibits the characteristics of blind faults towards its ends, the half-graben at the central part of the fault and the forcing folds at its ends were sufficient to induce topographic differences at the paleo-surface (sea bottom). Consequently, the water system mainly flowed along the hanging wall region of the F0 fault. In addition, the NNE-NE water system formed an accumulation body, and after the formation of the anticline, the width of the water system was usually smaller than the range of the anticline. This resulted in the high-angular or near-vertical relationship between the extensional direction of the accumulation body and the anticline strike in the main anticline region, while the extensional direction of the accumulation body and the anticline strike in the oblique anticline region are in small angle or near-parallel (AE anticline), so the reservoirs in the oblique anticline region are clearly larger in range than those in the anticline region. Thus, the oblique anticline region has better reservoir strata[32]. In addition, from the petroleum accumulation process, the F0 positive inversion fault running across the anticline controlled the distribution of reactivated faults during the late period. The reactivated faults usually connect hydrocarbon source rocks with trap structures, and thus acting as effective hydrocarbon-supply channels. Furthermore, although the AE anticline is a completely closed anticline structure, from the aspect of the oil and water distribution relationship, large faults are more likely to stagger sand bodies to form effective lateral sealings, thus dividing the entire anticline trap into multiple complex blocks with independent oil and water system.
To date, exploration practices have confirmed that there exist at least four to five fault block traps with independent oil and water systems in the AE anticline region. In this region, the F6 thrust fault and the reactivated normal faults are larger in scale. The F6 fault divides the whole anticline into south and north two parts. The fact that the growth strength of the south part (DD’ cross-section) is much larger than that of the north part of the anticline (CC’ cross-section) also corroborates that the F6 fault has some dividing effect on the anticline (Fig. 5). Thus, from the perspective of oil and gas division, one needs to pay attention to the relationship between the fault throw and sand-body thickness of large-scale faults, and by also considering the lateral sealing property of the faults and the horizontal regularity of sand-body properties, the blocks with independent oil and water systems can be identified or delineated. Moreover, during the water flooding development, as the pressure increases, the faults could be reactivated, causing accidents. Thus, for faults dividing blocks, especially reactivated faults controlled by F0 fault, assessing their stability is the key to ensure safe exploitation of oil and gas.
6.
Conclusions
Against the background of a contracted delta resulted from gravity slip, the AE oblique anticline experienced three major evolutionary stages: (1) The development stage of the extensional half-graben prior to the anticline formation (H4-H6 depositional period), the main boundary fault of the half-graben, F0, formed as a result of oblique extension induced by differential thrust transition; during this period, the differential contractional deformation caused the formation of near-vertical strike-slip tear faults. (2) During the half-graben inversion-induced anticline formation and its growth stage (H1-H4 stratigraphic depositional period), tectonic inversion occurred, which transferred the preexisting structures (F0 and the hanging half-graben) via oblique contraction due to the differential thrust strength. During this period, the differential thrust effect led to the formation of local tear normal faults and multi-periodic domino normal faults caused by the gravity effect of the slope dip angle at the limb. The domino faults were controlled by the paleo anticline top (or hinge) and distributed in an orderly manner with the western migration of the anticline top. (3) The post fold-thrust stage when the growth ceased (H0-H1 depositional period), local structural segments with high elevations on the ground surface collapsed as a result of returned gravity, which led to the formation of sparsely distributed small-scale conjugate normal faults. In the east AE anticline region, the conjugate normal faults were jointly affected by local gravity return and collapse and oblique distortion of the F0 fault post folding. The coupling of the oblique reactivated F0 fault with the conjugate faults controlled the planar combination pattern and distribution of the conjugate faults. Oblique inversion anticlines are broad promising petroleum structures because during the extensional half-graben period,
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the fault hanging walls at the main interfaces control the paths of delta water channels and basin fans. In turn, the distributions of water channels and basin fans are later parallel to the anticline distribution, which is favorable for development of high-quality large-scale reservoirs. The reactivated faults after the anticline formation (F0 fault and the coupled reactivated normal faults) usually link hydrocarbon source rock with the reservoir. The reactivated faults are mostly located in the anticline main body and distributed along the long axis of the anticline, which is conducive to the migration of hydrocarbon toward the trap and accumulation therein. The relatively large faults in the anticline region include the thrust faults, F6 and the reactivated normal faults controlled by F0. They can offset sand-bodies, forming effective lateral sealing, and dividing the entire anticline trap into multiple complex block traps with independent oil and water systems. The stability of these faults separating the oil and water systems is crucial to the petroleum production safety.
COOPER M A, WILLIAMS G D. Inversion tectonics: A disWILLIAMS G D, POWELL C M, COOPER M A. Geometry and kinematics of inversion tectonics. Geological Society, 1989, 44(1): 3–15.
[3]
BUCHANAN J G, BUCHANAN P G. Basin inversion. Geological Society Special Publications, 1995, 88(3): 596.
[4]
BURLIGA S, KOYI H A, KRZYWIEC P. Modelling cover deformation and decoupling during inversion, using the Mid-Polish Trough as a case study. Journal of Structural Geology, 2012, 42(2): 62–73.
[5]
MANSY J L, MANBY G M, AVERBUCH O, et al. Dynamics and inversion of the Mesozoic Basin of the Weald–Boulonnais area: Role of basement reactivation. Tectonophysics, 2003, 373(1): 161–179.
[6]
MCCLAY K R. The geometries and kinematics of inverted fault systems: A review of analogue model studies. Geological Society, London, Special Publications, 1995, 88(1): 97–118.
[7]
[9]
role of deformation in controlling depositional patterns in the south-central Niger Delta, West Africa. Journal of Structural Geology, 2002, 24(4): 847–859. [13] CORREDOR F, SHAW J H, BILOTTI F. Structural styles in the deep-water fold and thrust belts of the Niger Delta. AAPG Bulletin, 2005, 89(6): 753–780. [14] BRIGGS S E, DAVIES R J, CARTWRIGHT J A, et al. Multiple detachment levels and their control on fold styles in the compressional domain of the deepwater west Niger Delta. Basin Research, 2006, 18(4): 435–450. [15] MORLEY C K. Mobile shale related deformation in large deltas developed on passive and active margins. Geological Society, London, Special Publications, 2003, 216(1): 335–357. [16] ROWAN M G, PEEL F J, VENDEVILLE B C. Gravity-driven
thrust belt classification, tectonics, structure and hydrocarbon prospectivity: A review. Earth-Science Reviews, 2011, 104(1): 41–91. [18] MAGBAGBEOLA O A, WILLIS B J. Sequence stratigraphy and syndepositional deformation of the Agbada Formation, Robertkiri field, Niger Delta, Nigeria. AAPG Bulletin, 2007, 91(7): 945–958. [19] CONNORS C D, RADOVICH B, DANFORTH A, et al. The structure of the offshore Niger Delta. Trabajos De Geología, 2009, 29(29): 182–188. [20] BILOTTI F, SHAW J H. Deep-water Niger Delta fold and thrust belt modeled as a critical-taper wedge: The influence of elevated basal fluid pressure on structural styles. AAPG Bulletin, 2005, 89(11): 1475–1491. [21] BEGLINGER S E, DOUST H, CLOETINGH S. Relating petroleum system and play development to basin evolution: West African South Atlantic basins. Marine and Petroleum Geology, 2012, 30(1): 1–25.
TURNER J P, WILLIAMS G A. Sedimentary basin inversion
[22] COHEN H A, MCCLAY K. Sedimentation and shale tectonics
and intra-plate shortening. Earth-Science Reviews, 2004,
of the northwestern Niger Delta front. Marine and Petroleum Geology, 1996, 13(3): 313–328.
65(3): 277–304. [8]
[12] HOOPER R J, FITZSIMMONS R J, GRANT N, et al. The
[17] MORLEY C K, KING R, HILLIS R, et al. Deepwater fold and
cussion. Geological Society, 1989, 44(1): 335–347. [2]
teristics of inversion. Geological Society, 1989, 44(1): 17–39.
fold belts on passive margins. Tulsa: AAPG, 2004: 157–182.
References [1]
Journal of Earth Sciences, 2005, 94(5/6): 782–798. [11] HAYWARD A B, GRAHAM R H. Some geometrical charac-
BONINI M, SANI F, ANTONIELLI B. Basin inversion and
[23] ROUBY D, NALPAS T, JERMANNAUD P, et al. Gravity
contractional reactivation of inherited normal faults: A review
driven deformation controlled by the migration of the delta
based on previous and new experimental models. Tectono-
front: The Plio-Pleistocene of the Eastern Niger Delta. Tec-
physics, 2012, 523(3): 55-88.
tonophysics, 2011, 513(1): 54–67.
AMILIBIA A, SABAT F, MCCLAY K R, et al. The role of
[24] AIZEBEOKHAI A P, OLAYINKA I. Structural and strati-
inherited tectono-sedimentary architecture in the development
graphic mapping of Emi field, offshore Niger Delta. Journal of
of the central Andean mountain belt: Insights from the Cordil-
Geology, 2011, 3(2): 25–38.
lera de Domeyko. Journal of Structural Geology, 2008, 30(12):
[25] JERMANNAUD P, ROUBY D, ROBIN C, et al. Plio-Pleistocene sequence stratigraphic architecture of the eastern Niger
1520–1539. [10] MAZUR S, SCHECK-WENDEROTH M, KRZYWIEC P. Different modes of the Late Cretaceous–Early Tertiary inversion in the North German and Polish basins. International
66
Delta: A record of eustasy and aridification of Africa. Marine and Petroleum Geology, 2010, 27(4): 810–821. [26] RIBOULOT V, CATTANEO A, BERNÉ S, et al. Geometry
SUN Yonghe et al. / Petroleum Exploration and Development, 2018, 45(1): 55–67
and chronology of late Quaternary depositional sequences in the Eastern Niger Submarine Delta. Marine Geology, 2012,
to decollement folds and fault-propagation folds. Inferences on fold kinematics. Tectonophysics, 1997, 282(1/2/3/4): 353–373. [30] MORLEY C K. Development of crestal normal faults associ-
319(2): 1–20. [27] KOLLA V, POSAMENTIER H W, WOOD L J. Deep-water and fluvial sinuous channels: Characteristics, similarities and dissimilarities, and modes of formation. Marine and Petro-
ated with deepwater fold growth. Journal of Structural Geology, 2007, 29(7): 1148–1163. [31] BAUDON C, CARTWRIGHT J. Early stage evolution of growth faults: 3D seismic insights from the Levant Basin,
leum Geology, 2007, 24(6): 388–405. [28] SYLVESTER Z, PIRMEZ C, CANTELLI A. A model of submarine channel-levee evolution based on channel trajectories: Implications for stratigraphic architecture. Marine and
Eastern Mediterranean. Journal of Structural Geology, 2008, 30(7): 888–898. [32] JIANG F J, PANG X Q, BAI J, et al. Comprehensive assessment of source rocks in the Bohai Sea area, eastern China.
Petroleum Geology, 2011, 28(3): 716–727. [29] STORTI F, POBLET J. Growth stratal architectures associated
67
AAPG Bulletin, 2016, 100(6): 969–1002.