The Paleogene multi-phase tectono-sedimentary evolution of the syn-rift stage in the Nanpu Sag,Bohai Bay Basin,East China

Abstract

The Bohai Bay Basin is the second largest oil-producing basin in China located on the east Asian margin. The Bohai Bay Basin contains numerous depressions, sub-basins, and sags. One of these, the Nanpu Sag, has played a particularly important role in oil and gas exploration in recent years. Four depositional systems are recognized in the Nanpu Sag, fan-delta, braided-river delta, turbidite deposits, and lacustrine systems. In the Paleogene, the Nanpu Sag underwent complex and multi-phased rifting evolution. Two evolutionary phases have been identified: the syn-rift phase and the post-rift phase, the syn-rift stage can be further sub-divided into four episodes. This study reveals the considerable faulting activity and associated strong subsidence that occurred during the deposition of the Dongying Formation in the fourth episode of the syn-rift stage. The depositional systems and the tectonic activity during the fourth episode in the Nanpu Sag have very different characteristics compared to those of other depressions or sub-basins in the Bohai Bay Basin. Boundary fault activity was extremely intense during the deposition of the Dongying Formation, especially the east to west trending faults, including the Xinanzhuang Fault and the Gaoliu Fault. Moreover, the migration of subsidence centers from the Shahejie Formation to the Dongying Formation is a result of the strong down-warping that occurred during the fourth episode of the syn-rift stage. In the Nanpu Sag, the Dongying Formation is of great significance to hydrocarbon exploration, which is affected by both the intensity of fault activity and magnitude of basement subsidence.

Keywords

Depositional system fault activity subsidence Nanpu Sag Dongying Formation

Introduction

Tectono-sedimentary analysis is a critical component of basin analysis, contributing to our understanding of the relationship between the tectonic regime and sedimentary infill in continental rift basins (Deng et al., 2008; Hou et al., 2012; Strecker et al., 1999). Most continental rift basins have undergone a complicated evolution, and the observed sequence stratigraphy, patterns of sedimentation, sea/lake accommodation space, and sediment supply are all controlled by syn-sedimentary tectonism (Devlin et al., 1993; Gawthorpe and Leeder, 2000; Pascucci et al., 2006; Williams, 1993).

The Mesozoic–Cenozoic Bohai Bay Basin is the second largest petroliferous basin in China and consists of nearly 50 sub-basins or depressions. The Nanpu Sag has an area of only 1930 km², it is one of the many small sags within the Bohai Bay Basin (Dong et al., 2010, Wang et al., 1983; Xu et al., 2008). The Dongying Formation of the fourth episode in the syn-rift stage shows a high potential of hydrocarbon in the Nanpu Sag (Cong and Zhou, 1998; Li et al., 1997; Tourba et al., 2005, 2006). Multitude studies document a systematic tectonic analysis including structures, tectonic evolution, and geodynamic processes (Liu et al., 2006, 2007; Xu et al., 2004; Zhou et al., 2000, 2004). With a close relation to the tectonic activities including fault activity and basement subsidence, numerous investigation focused on the sedimentary infill and spatial distribution that are controlled by dynamic tectonic and sedimentation in the Paleogene of the Nanpu Sag, Bohai Bay Basin (Dong et al., 2014; Chen et al., 2016; Cong and Zhou 1998; Zhang et al., 2009). The syn-rift succession of the Dongying Formation shows a high potential of hydrocarbon (Dong et al., 2010; Xu et al., 2004). However, the basin structure and sedimentary filling of rift stage, particularly the Dongying Formation, which are characterized by the intense fault activity and basement subsidence have not been analyzed in detail. In addition, study of the intense fault activity and basement subsidence that occurred in the fourth episode of the syn-rift stage should take into consideration not only the tectonic evolution but also the development of source, reservoir, and cap rocks.

In this study, we analyzed sedimentary infill history, regional fault activity, and subsidence based on wells and seismic data from the Nanpu Sag. The influences of tectonic factors on the development of depositional systems have been demonstrated in the Dongying Formation of the Nanpu Sag, particularly the depositional systems and the tectonic activity. Our study will make a significant contribution to the body of work on the tectono-sedimentary evolution and hydrocarbon exploration in the Nanpu Sag, Bohai Bay Basin.

Geological setting

Structural setting

The Bohai Bay Basin is a very large, complex, Mesozoic, and Cenozoic continental rift basin located in the eastern North China Craton and has the form of a north-east trending rhombic shape on the regional geologic map (Dong et al., 2010; Guo et al., 2016) (Figure 1). It is a petroliferous basin on the western margin of the Taihang Mountains, east of the Jiaoliao Uplift, south of the Luxi Uplift, and north of the Yanshan Mountains, with an area of approximately 200,000 km² (Allen et al., 1997; Li et al., 2010; Liang et al., 2016). The contemporary stress orientations are dominated by crustal stretching, shear, and compression taken up by uplift of horst blocks. The structural framework shows NE trending tensional faults which are cross-cut by NW striking shear faults in the Eocene-Oligocene of the Bohai Bay Basin (Tian et al., 1992). Four major strike-slip zones (Teng et al., 2014) are developed in the Bohai Bay Basin (Figure 1): the active right-lateral Tan-Lu fault zone to the east, the active oblique-slip Taihang fault zone to the west, the Yanshan Mountain fold belt to the north, and the Qihe-Guangrao and Lankao-Liaocheng fault zones to the south (Figure 1) (Huang et al., 2015; Yao et al., 1994; Zhang, 2009). From northeast to the southwest, the Bohai Bay Basin can be divided into six major sub-basins or depressions, including Liaohe, Bozhong, Jiyang, Jizhong, Huanghua, and Dongpu sub-basins or depressions and five major uplifted blocks including Chengning, Shaleitian (or Bozhong), Cangxian, Xingheng, and Neihuang (Figure 1) (Gong, 1997). Each sub-basin or depression contains numerous sags or half-grabens. Most of the sub-basins or depressions display the geometry of half-grabens whereby depocenters are controlled by master faults, and the infilling sedimentary sequences are interrupted by unconformities like the Nanpu Sag (Peng et al., 2013) (Figure 2(a)).

Figure 1.

Schematic map of the structure units of the Bohai Bay Basin and the locations of the Nanpu Sag and Qikou Sag.

Figure 2.

(a) Geologic and structural map of the Nanpu Sag, planer faults filled with red present the master faults, planer faults filled with purple present the second-order faults. (b) and (c) The simplified geologic cross-sections that show basin structural framE-Work across Nanpu Sag. Nm: Minghuazhen Formation; Ng: Guantao Formation; Ed₁: Member 1 of deposition of the Dongying Formation, Ed₂: Member 2 of deposition of the Dongying Formation, Ed₃: Member 3 of deposition of the Dongying Formation, Es₁: Member 1 of Shahejie Formation, Es₂: Member 2 of Shahejie Formation, Es3: Member 3 of Shahejie Formation.

The Nanpu Sag is located in the northeast part of the Huanghua Depression (Figure 1). Three neighboring uplifted blocks bound the Nanpu Sag including the southern Shaleitian Uplift, northern Laowangzhuang Uplift, and northeastern Baigezhuang Uplift. Each of these highs provided source material for sediments in the Nanpu Sag and were controlled by boundary faults such as the Shaleitian Fault in the south, the Xinanzhuang Fault (XNZF) in the north, and the Baigezhuang Fault (BGZF) in the northeast (Figure 2(a)). Eight major structural features in the Nanpu Sag show a northeast trend, including the Gaoshangpu, Liuzan, Laoyemiao, Beipu, Nanpu No.1, Nanpu No.2, Nanpu No.3, Nanpu No.4, and Nanpu No.5 structures. These boundary faults are regarded as master faults dipping steeply toward the south and southwest (Figure 2(b) and (c)), with a long history of activity and large displacement. The second-order faults, including the No. 1 Fault, No. 2 Fault, No. 3 Fault, No. 4 Fault, No. 5 Fault, and the Gaoliu Fault control the formation, distribution, and development of the major structures in the Nanpu Sag (Figure 2(a)).

Stratigraphy

The Cenozoic succession includes the Eocene Shahejie (Es), Oligocene Dongying (Ed) Formations, and the Miocene Guantao (Ng) and Minghuazheng (Nm) Formations (Chen et al., 2016; Jiang et al., 2009; Liu and Zhang, 2011). The Paleocene Kongdian Formation (Ek) and the Eocene Member 4 of the Shahejie Formation are absent within the Nanpu Sag (Figure 3). The Shahejie (Es) Formation contains the main source rocks and consists of three lithostratigraphic members, from top to bottom, Es₁, Es₂, and Es₃, and the lower member, Es₃, is further divided into five sub-members including Es₃¹, Es₃², Es₃³, Es₃⁴, Es₃⁵ (Dong, 2002; Zhou et al., 2000) and the lithologic description of each member is provided in Figure 3 (Jiang et al., 2009; Wang et al., 2002a). The Dongying (Ed) Formation is the most prospective for hydrocarbons and can be divided into three members from top to bottom; Ed₁, Ed₂, and Ed₃, which together constitute a complete depositional sequence. The Guantao and Minghuazhen Formations (Ng and Nm) show similar depositional sequences and stratigraphy across the whole Bohai Bay Basin.

Figure 3.

Generalized Cenozoic stratigraphy of the Nanpu Sag, showing depositional environment, tectonic evolution stages, and the Cenozoic evolutionary of the Tan-Lu Fault Zone (modified from Guo et al., 2016; Huang et al., 2015; Wang et al., 2002b).

Tectonic history

The Nanpu Sag underwent both a syn-rift and a post-rift subsidence stage like the other sub-basins/depressions in the Bohai Bay Basin, and it is filled with formations deposited in continental environments including alluvial, fluvial, and lacustrine (Kang et al., 2016; Zhang et al., 2016). The syn-rift stage can be further subdivided into four rifting episodes (Jiang, 2009; Wang et al., 2011) (Figure 3): (1) Episode I: Initial rifting beginning in Es₃⁵ and ending in Es₃⁴, these two sub-members are characterized by red sandstones and gray-greenish mudstones interbedded with coarse clastic deposits that represent alluvial fan depositional systems. The fault system is characterized by E-W striking large-scale faults and it is at this time that subsidence along the boundary faults begins. The Nanpu Sag began to take shape during Es₃⁵, and the Shichang sub-sag to the north was the focus of both the deposition and subsidence; (2) Episode II: Main period of rifting begins in Es₃³ and ends in Es₂, during which time the region of intense fault activity and the depocenter migrated to the Linque sub-sag. Similar with the whole Bohai Bay Basin, a suite of dark sandstones and mudstones were deposited during Es₃ in the Nanpu Sag reflecting various lacustrine, alluvial, and fan-delta depositional systems. The Es₂ member consists of red mudstones in the Shichang sub-sag and gray siltstones and fine-grained sandstones in the Beipu anticlinal structure zone, representing alluvial fan depositional systems; (3) Episode III: Weakened rifting episode during Es₁, the master fault of the Gaoliu Fault, becomes active and divides the Nanpu Sag into two separate sedimentation regions. The development of pale-gray fine sandstone, gray sandy conglomerate, siltstone, and dark-gray mudstone during this episode represents a lacustrine fan-delta depositional environment; (4) Episode IV: The rift-depression transition stage during the deposition of the Dongying (Ed) Formation. Intense activity on the Gaoliu Fault results in depocenter migration and development of the Liunan sub-sag. The majority of hydrocarbons discovered in the south part of the Nanpu Sag are all in the Dongying Formation especially in Ed₁.

Dataset and methods

The datasets used in this investigation comprised core data, logging data, lithological data, and three-dimensional (3D) seismic data. Seven wells (B35, B7, M30, G86, NP208, LNP1, and L2X1) focused on different sedimentary environments in the Nanpu Sag were described and interpreted with sedimentary microfacies analysis (Figures 4 to 6, see Figure 2(a) for the location of wells). Combined with the lithological data on lithofacies, grain size, and sedimentary structures, the log data from seven wells assist in the description of the sedimentary environment. The high-resolution 3D seismic coverage with an approximate area of 78 km² was acquired by the petroChina Jidong Oilfield (Figures 4, 5, and 7). A series of seismic sections are used to describe the structure and how reflection character varies through the sag. Seventy-one wells data including grain size and lithology percentage (Figure 8) were used in determining the depositional system types present. Based on seismic interpretations, the development of the BGZF, XNZF, and Gaoliu Faults were described. The interpretation of fault throw data and the geologic ages shown in Figure 3 were used to estimate fault activity during the deposition of each Paleogene formation (Figures 9 and 10). Using the isochronous sequence stratigraphic framework, the subsidence history of the Paleogene in the Nanpu Sag has been simulated quantitatively and dynamically based on erosion recovery basin modeling (Figures 11 to 13).

Figure 4.

Core, well log, and seismic reflection characteristics of fan-delta deposits (Well G87, B35, and M30). Note the various seismic facies of fan-delta plain deposits and fan-delta front deposits (see more details in text; the site of the wells are marked in Figure 2(a)).

Result

Sedimentary systems in the Nanpu Sag as observed in cores

Based on core analysis, well logs, and seismic profiles, four types of depositional system can be recognized in the Nanpu Sag, including fan-delta, braid-river delta, lacustrine systems, and turbidite deposits. Similar depositional systems have been reported in the other sub-basins of Paleogene age in the Bohai Bay Basin (e.g. Chen et al., 2014; Feng et al., 2013, 2016; Hou et al., 2012; Liu et al., 2015; Muhammad et al., 2016).

Fan-deltas

Fan-deltas are mainly located adjacent to the boundary faults in the Nanpu Sag including the XNZF and the BGZF faults. Rapid gradient increases play a significant role in fan-delta development (Jin et al., 2013; Postma, 1984; Sohn, 2000). The internal lithologies of the fan-deltas usually consist of conglomerates and multiple-stage sandstones. The grain size of the deposits is coarser than that noted in braided-river delta deposits (Figure 4). Generally, fan-delta deposits can be classified into subaqueous (prodelta and delta front) or subaerial (delta plain) (McPherson et al., 1987, 1988; Postma 1984). According to the core and logging data, three main subfacies are identified: fan-delta plain, fan-delta front, and pro-delta. Two microfacies are identified: subaqueous distributary channels and mouth bars in the fan-delta plain penetrated by Well B35 (Figure 4). The internal lithology of the subaqueous distributary channel deposits is characterized by fine sandstones and pebbly sandstones, a fining-upward sequence with basal conglomeritic sandstones overlying a basal erosional surface. The resistivity (resisitivity log curve [RILD]) log curve shows the classic bell-shape and high-frequency serrated features indicating a high-energy background. Coarsening-upward sequences of medium-coarse sandstones, mudstones, and siltstones with wavy rippled laminations and cross-bedding, and an associated funnel-shaped resistivity log curve seen in the RILD data are interpreted as mouth bar deposits. The internal lithology of the delta plain deposits is dominated by fine, pebbly, and conglomeritic sandstones with a larger sandstone content than that of the delta front deposits (Well G87) (Figure 4). The distributary channel deposits show a fining-upward character and the crevasse splay deposits show a coarsening-upward character. Looking at the log data, the gamma ray (GR) curves through these deposits show a bell-shaped response, the self-potential curves a funnel-shaped response. Foreset reflection geometries are observed in seismic profiles across the fan-delta front, and fan-delta plain deposits have a chaotic internal seismic reflection character. (Figure 4).

Braided-river delta

In contrast to fan-delta deposits, braided-river delta deposits mainly developed in regions of relatively gentle gradients and the grains in these deposits are described as well sorted, well rounded, and fine grained (Jin et al., 2013). Based on the core description, the observed coarse-grained sandstones and conglomeratic sandstones with cross bedding which correlate to a bell-shaped response on the (GR) log are interpreted as representing braided-river channel deposits of the braided-delta plain (Well LNP1) (Figure 5). Two sedimentary sub-environments are interpreted within the braided-river delta front in the Nanpu Sag: underwater distributary channels and mouth bars (Well NP208) (Figure 5). The upward-fining, cross-bedded sandstones, with associated bell-shaped response on the (GR) log are interpreted as being underwater distributary channel deposits. The fine- to medium-grained sandstones with horizontal bedding and muddy pebbles, associated with a funnel-shaped response on the (GR) logs, are recognized as mouth bar deposits. The braided-river deltas are formed by the progradation of a braided fluvial system into a standing body of water, and internally they show oblique foreset seismic reflections with medium-low frequency, and medium continuity (McPherson et al., 1987) (Figure 5). The pro-deltaic deposits are characterized by massive gray-brown mudstone with horizontal bedding and fine sandstones with wavy ripple laminations (Well B13) (Figure 5).

Figure 5.

Cores, well logs, and seismic reflection characteristics of braided-delta deposits (Well LNP1, NP208, and B13). Note the foreset seismic reflection configurations, indicating the multistage deposition of braided-delta. See more details in text (the site of the well is marked in Figure 2(a)).

Lacustrine system

In the Nanpu Sag, the lacustrine system is widely developed in Member 2 of the Dongying Formation (Ed₂). It can be divided into shallow-lacustrine and deep-lacustrine deposits. The shallow-lacustrine lithofacies consist of gray mudstones and interbedded thinly laminated siltstones showing wavy, rippled laminations, and horizontal bedding (Figure 6). The deep-lacustrine lithofacies consist of massive dark mudstones, showing high-frequency, high-amplitude parallel reflections in seismic section.

Figure 6.

Cores and well logs characteristics of lacustrine and turbidite deposits (Well B7 and L2X1). See more details in text (the site of the well is marked in Figure 2(a)).

Turbidite deposits

From Well B7 in Ed₃ (Figure 6), the cores show a thick sequence of gray-black mudstones with silty mudstones and homogenous, gray, medium-grained sandstones. Lenticular bedding, muddy laminated beds, wavy ripple laminations, and soft sediment deformation related to loading structures are shown in Figure 6. The associated acoustic curve has a high-frequency serrated shape, which is interpreted as being associated with high-energy lake deposits. The depositional style is interpreted as turbiditic gravity flows. The turbidite deposits are located in the distal parts of subaqueous fans (Dasgupta, 2002) and show a lenticular geometry on seismic sections.

Spatial and temporal distribution of the depositional systems

This study selected four layers, Es₃³, Es₂, Es₁ and Ed₁, and give insights into the spatial distribution of the depositional systems during different rifting episodes (Figure 8). Formations Es₃³ and Es₂ were deposited during Episode II, but deposition of Es₃³ is rapidly followed by deposition of Es₃⁴⁺⁵ with a high subsidence rate (Figures 3 and 11), this high subsidence rate is interpreted as being associated with the intense initial rifting stage of the Nanpu Sag.

The sedimentary distribution of the Nanpu Sag in the Paleogene strata was defined following a comprehensive analysis of the core lithology, log curves (Figures 4 to 6), and sand content statistics (Figure 14). Three main sources of sediment supply into the Nanpu Sag were controlled by boundary faults, forming the northern, northeastern, and southern delta systems (See the purple arrow in Figure 14 indicating the influx direction). There are several second-order faults in the sag that controlled the distribution of subfacies including the No. 1 Fault, No. 2 Fault, No. 3 Fault, No. 4 Fault, No. 5 Fault, and the Gaoliu Fault (Figures 2 and 14). Fan-delta facies are widely developed in the northern and northeastern regions of the Nanpu Sag, whereas braided-river delta facies are located proximal to the southern margin fault (Shaleitian Fault) of the Nanpu Sag. The spatial distribution and stratal architecture of the turbidite deposits are controlled by the active, syn-rift, second-order faults.

During the deposition age of Es₃³ and Es₂ (Figure 14(c) and (d)), the spatial distribution of the northeastern fan-delta plain is controlled by the No. 4 Fault, the spatial distribution of the northern fan-delta plain is controlled by the No. 2 Fault and the No. 5 Fault, small-scale turbidite deposits developed in the sag due to the small extensions across the second-order faults.

Extension across the Gaoliu Fault began during the deposition of Es₁. From Es₂ to Es₁, decreased sandstone percentages observed in the fan-deltas in the areas near the foot-wall of the BGZF, indicating a transgressive process (Figure 14(b)).

During the deposition of the Dongying Formation, a large-scale fan-delta and braided-river delta developed across the whole Nanpu Sag (Figure 14(a)). Second-order faults controlled the distribution of subfacies, the No. 3 Fault, No. 4 Fault, and No. 5 Fault controlled the delta front and the delta plain extents. The turbidite deposits developed away from the delta front and controlled by the second-order faults, No. 1 Fault, No. 2 Fault, and No. 3 Fault (Figure 14(a)).

Main Faults activity and its sedimentary responses

XNZF, BGZF, and Gaoliu Fault activity

The faults growth rate was calculated based on fault heave analysis. The boundary faults such as the XNZF and the BGZF were all active during the deposition of Es₃⁴⁺⁵ to Ng, and the Gaoliu Fault was active since the deposition of Es₁. The average offsets across the XNZF, BGZF, and the Gaoliu Fault in each period are shown in Table 1. Offset across XNZF peaks is observed in both Episodes II and IV and offset across the BGZF peaks is observed in both Episodes I and IV.

Table 1.

The activity of three boundary fault in the Nanpu Sag in different periods.

Depositional periods/Faults	Es₃⁴⁺⁵	Es₃¹⁺²⁺³	Es₂	Es₁	Ed₃	Ed₂	Ed₁	Ng
Xinanzhuang Fault	25.12	141.63	113.59	101.22	364.67	159.5	180.00	39.46
Baigezhuang Fault	175.39	165.83	93.7	120.55	308.35	65.22	78.49	11.34
Gaoliu Fault	–	–	–	67.88	255.62	122.55	193.37	13.92

The largest offsets across the three faults occur during the deposition of the Dongying Formation (Episode IV). The fault activity curves show two significant peaks during the deposition of the Dongying Formation of the all the points except the Point 3 at the XNZF and the Points 4 and 5 at the BGZF. During the deposition of Ed₃, the average offsets across the XNZF, the BGZF and the Gaoliu Fault are 364.67 m/Ma, 308.35 m/Ma, and 255.62 m/Ma respectively (See Figure 2(a) for the location of the points). The Points 4 and 5 (northern part of the BGZF) show the fault stop activity and the Point 3 (eastern part of the XNZF) shows a low activity feature during Ed₂ (Figure 8). These were due to the influence of stronger dextral shear associated with the extensional deformation of the Tan-Lu Fault Zone (Teng et al., 2014). The Gaoliu Fault integrated with the XNZF and the BGZF and developed into a whole boundary fault during the deposition of Ed₂ (Figure 9).

Stratal thickness response to the tectonic activity

The maximum thicknesses of the syn-depositional strata in the Nanpu Sag are approximately 600 m, 700 m, 700 m, and 800 m in Es₃³, Es₂, Es₁, and Ed₁ respectively (Figure 10). The syn-depositional strata thickness continued to increase from the time of the deposition of the Shahejie Formation to the deposition of the Dongying Formation. During the deposition of Es₃³, sequence thicknesses were relatively homogeneous during the initial rifting period (Figure 10(d)). During the 2deposition of Es₂, the maximum thicknesses are located near the hanging wall of the XNZF (Figure 10(c)). During the deposition of Es₁, the depocenter is located near the hanging wall of the Gaoliu Fault which became active during this period. The main depocenter was in the Linque sub-sag with a maximum thickness of 800 m during the deposition of Ed₁ (Figure 10(a)). The sediment thicknesses distribution during the deposition of Ed₁ shows a thick interval across the whole sag, main depocenters developed in the hanging wall of the XNZF (Laoyemiao anticlinal structure), Linque sub-sag, and the hanging wall of the BGZF (Nanpu No. 4 nose structure). Generally, it can be seen that, the XNZF, the BGZF, and the Gaoliu Fault have had an important influence on the migration of depocenters. The orientation of the depocenters is almost parallel to the boundary faults. Before the development of the Gaoliu Fault, the XNZF played the primary role in the development of the depocenters in the Nanpu Sag. The average sedimentation rates of units Es₃³, Es₂, Es₁, and Ed₁ are 200 m/Ma, 259 m/Ma, 200 m/Ma, and 533 m/Ma, respectively. These values were calculated using the depositional times of Es₃³, Es₂, Es₁, and Ed₁, which are 3.0 Ma, 2.7 Ma, and 1.5 Ma, respectively (Figure 3). Not only Ed₁ but also Ed₂ and Ed₃ in Ed have higher depositional rates than Es. The stratal thickness is about 2100 m in Ed and 3500 m in Es. While the ratio of the stratal thickness of Ed versus Es is about 2:3(Figure 15), the duration of deposition of unit Ed is only 4.7 Ma and the duration of deposition of Es is about 17 Ma. Therefore, it can be seen that the thick strata and high sedimentation rates observed during the deposition of the Dongying Formation are related not only to the strong sedimentation supply rate but also to high fault activity (Figures 8 and 9) and high subsidence rates (details see the section Spatial and temporal distribution of the depositional systems).

Discussion

Subsidence history and sedimentation evolution

Recovery of the subsidence history of the Nanpu Sag was studied quantitatively using the back-stripping analysis technique, which calculates the subsidence rate and reproduces the sedimentary evolution process (Figures 7 and 11). Seismic profile CC’ used in the back-stripping subsidence history study is shown in Figure 7. The XNZF is the northern section of the boundary fault in the Nanpu Sag, which has a half-graben geometry with the north side faulted and south side overlapped by prograding sediments. Prior to the deposition of Es₁, strata was much thicker in the northern part of the Nanpu Sag than the southern part and overlapped the southern slope of the sag (Figure 7(g) and (h)). An anticline crestal zone developed proximal to the XNZF, small-scale synthetic faults developed on the southern slope during the deposition of units Es₃ to Es₂ (Figure 7(e) to (h)). During the deposition of Es₁ and the Dongying Formation, significant extension occurred; however, the Nanpu Sag retained its half graben architecture. The strike-slip faults developed continuously during the deposition of the Dongying Formation (Figure 7(a) to (c)) in the Beipu anticlinal structural and the Nanpu No. 1 nose structure zone (Figure 15). The observed flower structures mainly developed during the deposition of units Ed₂ and Ed₃. These are composed of strike-slip faults and strike-slip drag folds (Figure 7(a) and (b)) and are widely distributed in other sub-basins and depressions in the Bohai Bay Basin (Xu et al., 2006; Zhou et al., 2009).

Figure 7.

Subsidence history recovery profiles of the Paleogene in the western part of the Nanpu Sag. See blue lines CC’ in Figure 2 to locate this survey line.

Figure 8.

Boundary faults (Xinanzhuang Fault and Baigezhuang Fault) and Gaoliu Fault activities within the different Episode of the Nanpu Sag. (a) Xinanzhuang Fault, (b) Baigezhuang Fault, and (c) Gaoliu Fault. See Figure 2(a) for the location of the point.

The subsidence rates during the deposition of units Es₃³, Es₂, Es₁, and Ed₁ are shown in Figure 11. The subsidence centers in the Shahejie Formation (Es₃³, Es₂, and Es1) are dispersive and small, and almost all the subsidence centers are located near the hanging wall of the XNZF (Figure 11(b) to (d)). The maximum subsidence rate in the Shahejie Formation during Es₃³ is about 600 m/Ma (Figure 11(d)). The maximum subsidence rate in Ed₁ is about 450 m/Ma (Figure 11(a)), and the maximum subsidence rate reached 1000 m/Ma in Ed₃ (Figure 11(a)). The subsidence rate during the deposition of the Dongying Formation is considerably higher than during the deposition of the Shahejie Formation. The main subsidence centers always correspond to the regions of greatest stratal thickness of each stage (Figure 10(a) to (d)). The sedimentary thicknesses were large when the subsidence rate was high. The migration of subsidence centers lead to the migration of depocenters. The subsidence centers with maximum subsidence rate during the deposition of Ed₁ developed both in the Linque sub-sag and near the hanging wall of the XNZF, and the subsidence centers with maximum subsidence rate during the deposition of Ed₃ developed both in the Caofeidian sub-sag and near the hanging wall of the XNZF (Figures 11(a) and 12(a)). In short, subsidence effects are the main factor controlling the development of depocenters during the deposition of unit Ed, and the boundary faults also play an important role in controlling the sedimentation of the Nanpu Sag.

The Qikou Sag is located adjacent to the Nanpu Sag in the Huanghua Depression (Chen et al., 2011; Huang et al., 2012a; Zhang et al., 2008;) (Figure 1). The sedimentation rate of Ed₃ in the Qikou Sag is much higher than the subsidence rate (Figure 12(b)), while the sedimentation rate of Ed₃ in the Nanpu Sag is equal to or much lower than the subsidence rate (Figure 12(a)). This observation indicates that intense subsidence occurred in the Nanpu Sag during the deposition of the Dongying Formation (Chen et al., 2012), which is not seen in the other sub-basins or depressions in the Bohai Bay Basin. The Linqing sub-basin, the Jiyang sub-basin, and the Jizhong sub-basin are located around the Huanghua Depression of the Bohai Bay Basin (Figure 1). Compared to the other sub-basins and depressions in the Bohai Bay Basin (e.g. Gong et al., 2007, 2010; Huang et al., 2012a; Huang and Wang, 2012; Tang et al., 2008; Ye et al., 1985), the tectonic subsidence that took place during four Paleogene rifting episodes in the Nanpu Sag is considered to be an outlier (Figure 13): In addition to the intense tectonic subsidence that occurred during Es₃ (the initial rifting stage), tectonic subsidence rates were very high during the deposition of the deposition of the Dongying Formation (Ed) in the Nanpu Sag (Figure 13(d)), while strong subsidence only occurred during the deposition of the Shahejie Formation in the other sub-basins (Figure 13(a) to (c)).

Petroleum geology significance

The Nanpu Sag has already come under the spotlight in recent years due to the discovery in 2014 of significant oil and gas pools with 0.55 million tons crude oil equivalent (Chen et al., 2016). The geochemical analysis of potential oil-sources indicates that a large amount of the oil and gas was derived from the source rocks of Ed₃ strata (Wang et al., 2008; Xu et al., 2008; Zhao et al., 2015). The source rocks of Es₁ and Es₃ are widely developed across the Bohai Bay Basin, while the Ed₃ source rock is only found in the Nanpu Sag (Yang and Li, 2012). Geochemical analysis of the mudstones in Ed₃ in the Nanpu Sag shows that they can be classed as good–excellent source rocks (Chen et al., 2016; Liu et al., 2009; Zhang et al., 2010). During a period of intense fault activity and regional subsidence, a set of thick mudstones interbedded with siltstone developed in a semi-lacustrine system at the top of Ed₂, the thickness of individual beds is up to 50 m and total thickness is more than 250 m This unit is the effective cap rock in unit Ed₂ of the Nanpu Sag (Figure 16).

The Tan-Lu Fault Zone to the east and Lankao-Liaocheng Faults to the south are the most important strike-slip zones that influence the tectonic evolution of the Nanpu Sag in the Bohai Bay Basin (Liang et al., 2016; Ren et al., 2002; 2010) (Figure 1). The role of Pacific Plate subduction cannot be ignored in the regional tectonic stress analysis. From the time of deposition of units Ek (The Eocene Kongdian Formation) to unit Es₄, the Pacific Plate developed an NWW movement, and the Bohai Bay Basin formed as an extensional basin. From the time of deposition of units Es₃ to Es₂, the subduction direction changed from NW–SE to E-W, while the North China Craton extended to the NW, this resulted in dextral strike-slip tectonic activity in the Bohai Bay Basin. From the time of deposition of units Es₁ to Ed, the subduction direction was unchanged, but the subduction rate increased rapidly. The Tan-Lu Fault formed in response to intense dextral strike-slip stress, and NS extension occurred in the northeast of the Huanghua sub-basin (Ren et al., 2010). The principle regional tectonic stress orientation in the Huanghua sub-basin changed from NW–SE (Es₃ to Es₂) to N-S (Es₁ to Ed) and this led the migration of the depocenter from the boundary fault (Cangdong Fault) to internal parts of the sub-basin (Qi et al., 2010; Ren et al., 2010). The migration of depocenters in the Nanpu Sag (Figure 15) was influenced by dextral strike-slip on the Tan-Lu Fault Zone and the subduction of the Pacific Plate. Additional support for this theory is given by the formation and evolution of the Gaoliu Fault (Figure 9).

Figure 9.

The formation and the evolution of the Gaoliu Fault from Es₁ to Ed₂. The red arrow shows the stress direction of the Nanpu Sag. See Figure 2(a) for the location of the point. XNZF: Xinanzhuang Fault; BGZF: Baigezhuang Fault.

Figure 10.

Isopach maps showing the thickness of syndepositional strata of (a) Member 1 of thedeposition of the Dongying Formation (Ed₁), (b) Member 1 of the Shahejie Formation (Es1), (c) Member 2 of the Shahejie Formation (Es₂), and (d) Third sub-member of Member 3 of the Shahejie Formation (Es₃³). The red two-way arrows present the extended trending of the thick strata value.

Figure 11.

Subsidence rate plan of (a) Member 1 of the deposition of the Dongying Formation (Ed₁), (b) Member 1 of the Shahejie Formation (Es₁), (c) Member 2 of the Shahejie Formation (Es₂), and (d) third submember of Member 3 of the Shahejie Formation (Es₃³). The red two-way arrows present the extended trending of the high rate value.

Figure 12.

Subsidence rate and sedimentation rate of Member 3 of the deposition of the Dongying Formation (Ed₃) in (a) the Nanpu Sag and (b) the Qikou Sag in the Bohai Bay Basin. The location of the Qikou Sag is shown in Figure 1.

Figure 13.

Tectonic subsidence rate in Cenozoic of (a) the Linqing sub-basin, (b) Jiyang sub-basin, (c) Jizhong sub-basin, and (d) Nanpu Sag. The location of the Linqing sub-basin, Jiyang sub-basin, and the Jizhong sub-basin is shown in Figure 1.

Figure 14.

Lacustrine sedimentary distribution map in the Nanpu Sag. (a) Ed₁ sedimentary system distributions, (b) Es₁ sedimentary system distributions, (c) Es₂ sedimentary system distributions, (d) Es₃³ sedimentary system distributions. The provenances came from the northeastern, northern and northwestern fan-delta, and the southern braided-delta.

Figure 15.

Interpreted seismic section C-C’ in depth domain that showing seismic reflection characteristics and the thickness of the deposition of the Dongying Formation (with pink color) and the Shahejie Formation (with yellow color). The location of the seismic section CC’ is shown in Figure 2(a).

Figure 16.

Hydrocarbon accumulation model in the Nanpu Sag. The location of profile is shown in Figure 2(a).

The influence of Pacific Plate subduction was not limited to changes in the primary regional stress orientation but included changing the deep dynamic mechanisms of the Nanpu Sag. Most of the sub-basins or depressions in the Bohai Bay Basin underwent the faulted-depressed diversionary tectonic phase from Es₁ to Ed (Gong et al., 2007; Tang et al., 2008). The Bozhong sub-basin has undergone the strongest subsidence and has an associated depth to Moho of 28 km (Tong et al., 2010; Xin et al., 2015). The Nanpu Sag, located adjacent to the Bozhong sub-basin, has also undergone extensive subsidence during the deposition of the Dongying Formation.

Most of the tectonic and geological history of the Nanpu Sag has occurred since the late Mesozoic. The Nanpu Sag has undergone geodynamic and tectonic history strongly influenced by the dynamic background of the Bohai Bay Basin. Study of continental dynamics and the tectonic evolution of the Bohai Bay Basin would help understanding the fault activity and subsidence history during the deposition of the Ed unit in the Nanpu Sag.

Conclusion

The following conclusions can be drawn from the present study:

Four types of depositional systems have been identified in the Es and the Ed of the Nanpu Sag: fan-delta sedimentary systems (including fan-delta plains and delta fronts), braided-river delta systems (including braided-river delta plains and delta fronts), and turbidite deposits in a lacustrine environment.

The fourth rifting episode during the deposition of unit Ed, both fault activity and subsidence rates, which increased to reach maximum values, shows significant influence on the topography of the Nanpu Sag. The topography in turn influenced the accommodation, sediment supply, and sedimentary architecture. The boundary faults XNZF, BGZF, and the Gaoliu Fault controlled the distribution and location of depocenters and the subsidence centers. Intense basement subsidence provided enough accommodation space, and when combined with abundant sediment supply, this meant that huge thicknesses of the Dongying Formation were deposited in a short time.

The widely distributed thick Ed₃ mudstones are interpreted as the likely source rocks, and three thick mudstones developed in Ed₂ are the effective cap rock. The Dongying Formation, which was affected by intense faulting and rapid basement subsidence, has great significance for hydrocarbon exploration.

Footnotes

Acknowledgements

Siding Jin acknowledges the support of the Chinese Scholarship Council’s overseas student scholarship that allowed her to visit the University of Vienna for 24 months. The authors appreciate the anonymous reviewers, whose comments and suggestions have greatly helped to improve the original manuscript. Michael Wagreich whose constructive comments and suggestions have been of great help and enhanced the quality of this work.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This paper was supported by the Research Institute Exploration and Development, PetroChina Jidong Oilfield Company. The research is financially supported by the “China National Key Research Project” (Grant No. 2016ZX05006–006-002, 2017ZX0536–004-006) and the “National Natural Science Foundation of China” (Grant No. 41702114).

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