Sedimentary characteristics and seismic geomorphology of the upper third member of Eocene Dongying Formation in double slope systems of Laoyemiao transverse anticline, Nanpu Sag, Bohai Bay Basin, China

Marine and Petroleum Geology 109 (2019) 36–55

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Research paper

Sedimentary characteristics and seismic geomorphology of the upper third member of Eocene Dongying Formation in double slope systems of Laoyemiao transverse anticline, Nanpu Sag, Bohai Bay Basin, China

T

Jiankun Zhanga, Jian Gaob, Jizhong Wua, Qiqi Lyuc,d,∗, Du Fanga a

Research Institute of Exploration and Development, PetroChina Jidong Oilfield, Hebei, Tangshan, 063004, China Sinopec Petroleum Exploration and Production Research Institute, Beijing, 100083, China c Hubei Cooperative Innovation Center of Unconventional Oil and Gas (Yangtze University), Wuhan, Hubei, 430100, China d Laboratory of Exploration Technologies for Oil and Gas Resources (Yangtze University), Ministry of Education, Wuhan, Hubei, 430100, China b

ARTICLE INFO

ABSTRACT

Keywords: Seismic geomorphology Stratal slice Sublacustrine fan Eocene upper Ed3 submember Double slope break zone Laoyemiao transverse anticline Nanpu sag Bohai Bay Basin

The upper third member of Eocene Dongying Formation (Ed3u) is considered to be one of the most important hydrocarbon-rich intervals in Nanpu Sag, Bohai Bay Basin, China. Through integrated analysis of the drilling cores, wire-line logs, and high-quality three-dimensional (3-D) seismic data, the sedimentary characteristics of Ed3u were analyzed by utilizing the seismic geomorphology approach in the double slope systems of Laoyemiao transverse anticline, Nanpu Sag. The Ed3u, a third-order sequence, can be subdivided into three fourth-order sequences (from base to top as SS1, SS2, and SS3). The fan delta front and sublacustrine fan have been identified from well-based analysis of the sedimentary facies and core examinations reveal that the sublacustrine fan deposits were dominated by six types of resedimented lithofacies, namely, slides, slumps, sandy debris flows, muddy debris flows, turbidity currents, and hyperpycnal flows. Stratal slices were obtained from 3-D singlefrequency seismic volumes to map the sediment dispersal characteristics and document the evolution of the depositional systems constrained by the double slope break zones of Laoyemiao transverse anticline. The unique structures of the double slope break zones and the variation in the lake level associated with the system tracts of the third-order sequence jointly control the dispersal patterns of the sand bodies and the spatial-temporal evolution of the depositional systems. Based on the above analysis, the depositional evolution model of Ed3u was established. The proposed facies-controlled model and sediment transport model can be used to accurately predict the favorable reservoir sandstones.

1. Introduction As a significant innovation in three-dimensional (3-D) seismic data interpretation and analysis, seismic geomorphology has proven to be a powerful and effective tool for hydrocarbon exploration that has attracted substantial interest from geologists and geophysicists during the last decade (Zeng et al., 1998a, 1998b; 2015; Sawyer et al., 2007; Dong et al., 2015; Liu et al., 2016a; Zhu et al., 2016, 2017). Seismic geomorphology, when used in combination with high-resolution sequence stratigraphy, represents the state of the art approach to extract geomorphic elements of high vertical resolution (2–3 m) and relative isochronism from predominantly 3-D seismic data (Zeng et al., 1998a, 1998b, 1998c; Zeng and Hentz., 2004; Zhu et al., 2011, 2014a, 2014b). It can potentially be more advantageous in identification of thin-bedded sand bodies, (quantitative) analysis of depositional systems (Wood,

∗

2007; Wood and Mize-Spansky, 2009; Wang et al., 2012; EI-Mowafy and Marfurt, 2016), depositional history or reservoir-scale depositional architecture for subsurface conditions with seismic stratal slices and interval attributes (root mean square amplitude or frequency division) employed (Liu et al., 2016a, 2016b). Furthermore, remarkable success has been achieved in the hydrocarbon exploration field since seismic geomorphology principles were applied in petroliferous basins. However, the rifted basin (Nanpu Sag in Bohai Bay Basin), compared with the depression basins and marine basins, is considered to be small scale, with multi-provenance supplies and intense multi-phase tectonic activities (Xian et al., 2013). It is, therefore, extremely difficult to image the sedimentary characteristics on isochronous stratigraphic interfaces by using seismic geomorphology owing to the complexities of the spatial-temporal distribution of the depositional systems and the sand body distribution.

Corresponding author. Hubei Cooperative Innovation Center of Unconventional Oil and Gas (Yangtze University), Wuhan, Hubei, 430100, China. E-mail address: [email protected] (Q. Lyu).

https://doi.org/10.1016/j.marpetgeo.2019.06.005 Received 28 December 2018; Received in revised form 28 May 2019; Accepted 4 June 2019 Available online 05 June 2019 0264-8172/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. (a) Location map of the study area showing the sub-tectonic units of the Bohai Bay Basin in China; (b) Distribution of main structural elements and normal faults in Nanpu Sag; (c) Structural contour map of Ed3u, showing the location of wells and the line of cross section.

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Slope break zones, a geomorphological concept in sequence stratigraphy, refer to the zones where the paleotopographic gradient displays a sudden change (Lin et al., 2000). During the tectono-sedimentary evolution of a basin, the slope break zones formed by syndepositional faults usually restrict the changes during accommodation and play an essential role in both the development of depositional system tracts and the distribution of sand bodies (Wang et al., 2002a,b; Lin et al., 2003; Ren et al., 2004; Hou et al., 2012), which tend to promote petroleum accumulation in the subtle traps and unconformity traps of a basin (Wang et al., 2002a,b; Feng and Xu, 2006; Hou et al., 2012). Hence, the theory of slope break zones is currently among the most important for the exploration of non-structural reservoirs and has become the hot spot in the exploration of the Bohai Bay Basin. Although a considerable amount of work on slope break zones has been carried out in recent years with respect to the genetic mechanism, types, accurate definition of geological models, and control of sand bodies (Lin et al., 2000, 2003; Wang et al., 2002a,b, 2003; Feng and Xu, 2006; Liu et al., 2006; Hou et al., 2012; Huang et al., 2012), intensive and systematic exploration for the spatial evolution model of depositional systems and lithofacies within slope break zones has still not been sufficiently performed. In particular, there are hardly any reports on the lithologic differences and facies patterns constrained by the distinctive structures of the double slope break zones in a transverse anticline, which is a rift basin. The study area for Laoyemiao structural belt is a transverse anticline, located in the northern part of Nanpu Sag of the Bohai Bay Basin, where high-yielding hydrocarbon fields with multi-layers and multitrap types have been discovered since 1996 (Wang et al., 2002a,b). The target layer, the upper third member of Eocene Dongying Formation (Ed3u), is one of the most important hydrocarbon-rich intervals in Laoyemiao area, Nanpu Sag (Wang and Dong, 2000; Liu et al., 2000; Zhang et al., 2009a, 2009b). Even though a substantial number of studies have been carried out for analyzing the depositional characteristics and gaining insights into the controls of the Laoyemiao transverse fold on the fan delta of Ed3, the geological conditions of the Ed3u submember, such as the sediment transport routes, lithofacies, distributions of the sedimentary systems and sand bodies, and depositional model, have not yet been well understood owing to the limitation of the antecedent 3-D seismic data in terms of a relatively low resolution and insufficient analysis of the core and logging data (Liu et al., 2000, 2008; Zhang et al., 2009a, 2009b; Jia et al., 2018), which limit future petroleum exploration in the Ed3u submember. Based on sequence stratigraphy and seismic geomorphology approaches, the primary objectives of this work, which utilizes drilling cores, wire-line logs, and recently acquired high-quality three-dimensional (3-D) seismic data with broad-band, wide-azimuth, and highdensity, are to (1) investigate the controls of the double slope break zones on the Ed3u submember; (2) characterize the lithofacies of different depositional facies; (3) take advantage of spectral decomposition technique to reveal the spatial-temporal distribution and evolution of the depositional systems at successive stratal slices in the systems tract; and (4) establish the depositional spatial-evolution model for Ed3u member.

Jizhong, Bozhong, Huanghua, and Liaohe depressions, and Cangxian and Chengning uplifts (Fig. 1a). With a range of slope systems controlling the development of the oil and gas accumulation zones, the Bohai Bay Basin is known as one of the most petroliferous basins in east China and accounts for almost one-third of the oil production in the country. Nanpu Sag, a small hydrocarbon-rich continental faulted sag in Bohai Bay Basin, is located in the northeast part of Huanghua Depression and covers a total area of nearly 1932 km2. The Nanpu Sag is bounded by Xinanzhuang–Baigezhuang faults from the northwest to the southeast and by Shaleitian high in the south. Structurally, the sag confined by two border faults (Xinanzhuang fault and Baigezhuang fault) is generally characterized by a duplex half-graben rift that displaces the northern part and overlaps with the southern part. Three hydrocarbon-generating sub-sags and eight petroliferous structural belts have been identified in Nanpu Sag, including the Shichang, Liuzan, Linque sub-sags, and Gaoliu, Laoyemiao, No. 1 through No. 5 structural belts (Fig. 1b). The Laoyemiao structure belt, with an exploration area of about 150 km2, is a transverse anticline on the downthrow side of Xinanzhuang fault, located in the northwestern part of the Nanpu Sag. Xinanzhuang fault, with NE strike, SW dip, a dip angle of 30°–50°, and a length of about 55 km in the study area, is a boundary fault in the northwest of Nanpu Sag that obviously controls the formation, evolution, and sedimentary filling of the Laoyemiao structure belt. The sedimentary stratigraphy of the Laoyemiao area has been extensively studied mostly through drilling for hydrocarbon exploration. The sedimentary successions include Member 1 of Eocene Shahejie Formation (Es1), Oligocene Dongying Formation (Ed), Miocene Guantao Formation (Ng), and Minghuazhen Formation (Nm). The Dongying Formation is divided into three intervals, Member 1 (Ed1), Member 2 (Ed2), and Member 3 (Ed3), from top to bottom. Member 3 (Ed3) is further divided into two submembers Ed3l and Ed3u. In the study area, the main tectonic activities have been reasonably explained with north-south extension model since the time of Es1 deposition, which is characterized by episodic tectonic movements (Jiang et al., 2010; Wang et al., 2012; Dong et al., 2013). Es1 corresponds to episode Ⅲ of the late Eocene-Oligocene synrifting stage (failed synrifting stage) when the extension strength of the Xinanzhuang fault decreased, resulting in reductions in the corresponding tectonic subsidence rate and total subsidence rate (Fig. 2). The strata of Es1 is thin, with a thickness of 150–300 m, and the depositional environment belongs to lacustrine and delta deposits. Ed corresponds to episode Ⅳ of the late Eocene-Oligocene synrifting stage and also to the reactivated synrifting stage when the boundary fault activity was strong again and the volcanisms in the early and late Ed were active. In this period, the maximum total subsidence rate was as high as 480 m/Ma, which then reduced to 260 m/Ma. The thickness of Ed varies from 860 m to 1300 m, and it is characterized by deltas and lacustrine deposits. NgNm corresponds to the postrifting stage (depression period). During the depositional stage of Ng, the study area had entered the tectonic quiescent period when most of the faults, including Xinanzhuang boundary fault, stopped being active, and the maximum total subsidence rate was only 50 m/Ma. In the early depositional stage of Ng, volcanic activity, however, was extremely intense and a set of basaltic volcanic rocks were widely distributed across the whole Nanpu Sag. The strata of Ng, with the thickness of 350–400 m, is characterized by equal thickness and complete depression subsidence, and the depositional environment is braided streams. The tectonic activity of the north-south extension continued during the depositional stage of Nm when its intensity was even greater than that of the depositional stage of Ed. The faults previously formed were still active and a large number of new faults developed. The maximum total subsidence rate of Nm is 78 m/Ma, ranging in thickness between 1100 m and 1500 m; it is characterized by meandering stream sediments (Fig. 2).

2. Geological settings The Bohai Bay Basin is located on the eastern coast of China, covering an area of approximately 200,000 km2. With the Taihangshan, Tanlu, southern Yanshan, Qihe-Guangrao, and Lankao-Liaocheng faults as the boundaries, the west, east, north, and south margins of the Bohai Bay Basin are adjacent to the Taihangshan uplift, Jiaolong uplift, Yanshan fold belt, and Luxi uplift, respectively. The Bohai Bay Basin experienced a subsidence stage in early Cenozoic (Paleogene) and a faulted stage in late Cenozoic (Neogene), and was ultimately formed at the end of the Neogene. The dominant basin-controlling faults in the NE, NEE, and NW strikes divided the basin into a series of depressions and tectonic ridges in the NE and NEE-EW strikes, namely, Jiyang, 38

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Fig. 2. Generalized stratigraphic column of Laoyemiao region in Nanpu Sag showing the lithologies, depositional environment, and tectonic evolution (modified from Dong et al., 2010; Dong et al., 2013; Guo et al., 2013; Chen et al., 2016). The study interval is the upper third member of the Dongying Formation. Sym. = System.

3. Data description and methods

data. An excellent well-to-seismic tie was realized by using a synthetic seismogram comprising 53 local wells and 11 vertical seismic profiles. The calculated seismic resolution limit (one-fourth the wavelength of the 18 Hz wavelet, with an average velocity of 4020 m/s) is 55 m. With a good amplitude-thickness relationship, a sandstone body as thin as 6 m (close to one-sixteenth the wavelength) can be detected on a horizontal seismic amplitude display. In this work, two techniques were used to improve the resolution of the seismic data. First, the seismic data in the dominant frequency band processed by spectrum decomposition technique can eliminate the noise of the seismic data in the low-frequency and high-frequency bands and improve the signal-tonoise ratio. Second, stratal slices can enhance the transverse resolution

The dataset utilized for this study included core samples from 13 wells, wire-line logs from 64 wells, and around 127 km2 of a post-stack 3-D seismic survey (Fig. 1c). The seismic data had been acquired and processed 6 times by Bureau of Geophysical Prospecting INC., CNPC from 1996 to 2015 with advances in the bin size from 15 m × 30 m–12.5 m × 12.5 m, in the full fold from 60 to 225, and in the azimuth angle from 60° to 90°. The post-stack 3-D seismic volumes are characterized by an effective frequency window of 4–60 Hz, with a dominant frequency of 18 Hz. The signal-to-noise ratio of the data is relatively high, and no multiples or coherent noise is apparent in the 39

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of the seismic data. When the lateral thickness changes due to formation, stratal slices can more accurately image the depositional elements. Spectrum decomposition is a common and useful technique for seismic interpretation. Its principle is to convert seismic data from the time domain to the frequency domain by mathematical transformation, which can be used to analyze the variations in amplitude and phase in the frequency domain and accurately describe the reservoir distribution, physical properties, and oil-gas properties. The common spectral decomposition methods consist of S transform (Stockwell et al., 1996), short-time Fourier transform (STFT) (Partyka et al., 1999), generalized S transform (Gao et al., 2003), and continuous wavelet transform (Morlet et al., 1982; Sinha et al., 2005). Each method has its own advantages and limitations, but STFT is a classical linear time-frequency analysis method, and the spectral characteristics represented by this transform are easier to understand than those of the wavelet transform. In this study, the STFT proposed by Cohen (1995) has been applied to the spectral decomposition of seismic data (Partyka et al., 1999) from 10 Hz to 40 Hz, and 16 single-frequency volumes were obtained in the decomposed form of a single frequency every 2 Hz. We utilize the fact that the varying thicknesses tune at varying frequencies (Marfurt and Kirlin, 2001; Sinha et al., 2005). Consequently, the geological body can be identified in the dominant frequency window through frequency division of the seismic data, which is unable to be identified when using the original seismic data (Zeng et al., 1998b; Zeng and Kerans, 2003; Zeng, 2013; Liu et al., 2016c; Meng et al., 2016). Single-frequency volumes of 18 Hz, with the strongest energy in tuned amplitude anomaly, were optimized to identify the geological bodies. Geoframe software was used to interpret the 3-D seismic volume, and GeoEast software was applied to generate frequency-divided stratal slices. The sedimentary characteristics and seismic geomorphology investigated in this study include (1) a fine sequence stratigraphic framework, the types of slope break zones, and its control on the analysis of the depositional facies based on paleogeomorphological restoration, 3-D seismic profiles, and well data; (2) an accurate description of the lithofacies within the slope break zones by core analysis; (3) conglomerate and sandstone content analysis for the planar distribution of the sedimentary facies; (4) stratal slices from 3-D single frequency seismic volumes to map the sediment dispersal patterns with good control in the systems tract; and (5) tectonic transfer zone analysis and palaeogeomorphologic reconstruction to rebuild the model of slope depositional systems from source to sink.

faults resulted from regional extensional stresses and tended to be parallel or mostly parallel to each other, presenting a multi-ladder-like profile (Fig. 6b and c). Multi-step fault slope break belts could be classified as either concordant or antithetic step-fault zones, according to the relationship between the fault dip and the sag center (Morley et al., 1990; Childs et al., 1995; Wang et al., 2003; Ren et al., 2004). In Nanpu Sag, the multi-step fault slope break belts, mainly controlled by the sag boundary fault, Xinanzhuang fault (Fig. 2b), developed the combination of concordant and antithetic step fault belts along the direction of the long axis of the transverse anticline. According to the abrupt slope position, the multi-step fault slope break zone can be subdivided into an upper slope break zone and a lower slope break zone from the Xinanzhuang fault to the sag center (Figs. 3 and 5). 4.1.2. Flexure slope break belt Flexural slope break belts are generated by the deformation of shallow strata as a result of simultaneous deep faulting, the flexural deflection of the anticline or the nose-shaped structures formed by synsedimentary folds, or draping onto the paleo-ridge, paleo fault-uplift and buried hill (Ren et al., 2004; Wang et al., 2003). The principal mark of flexural slope break belts is the abrupt change in the slope zone. Stratum erosion and unconformity interfaces can be formed on the upper slope break belt. Obvious stratum onlap and swelling can be observed on the lower slope break belt. In Laoyemiao structural belt of Nanpu Sag, the flexure slope break belts were mainly developed on two wings of the transverse anticline (Figs. 3 and 6b-c). Towards the source area of the sediments (Fig. 3), the flexure slope break belts were located towards the lower part of the antithetic step-fault belt. Geographically, it constituted the lower slope break zone (Fig. 5). On the other hand, in the direction perpendicular to the source area, the transverse anticline developed double slope break flexure belts, which were distributed on the upper and lower slope break zones in the geographical location (Fig. 6). 4.2. Sequence stratigraphy framework Based on the sequence-stratigraphic principles summarized in detail by Ilgar and Nemec (2005), Catuneanu (2006), Catuneanu et al. (2009), and Wang et al. (2017), interpretation of the 3-D seismic data, lithology and log data from wells and the sequence stratigraphy for the upper third member of Dongying Formation (Ed3u) are proposed. Ed3u is composed of one third-order sequence, which is bounded by T4 and T5 seismic reflectors. Sequence boundaries are depositional hiatus surfaces with sharp lithological contacts of thick glutenite or sandstone truncated underneath or above thick mudstone. The maximum flooding surface is characterized by widespread dark-gray or black thick mudstone (Figs. 5a and 6a). Recognition of the fourth-order sequences by using the seismic data of Laoyemiao structural belt as the higher-order sequences of other areas is also a challenge (Zhao et al., 2011). Numerous higher-order sequences consist of only one or several thick events; moreover, quite a few seismic events generate discontinuous and discordant reflection patterns, which disallow tracking without guessing (Figs. 5b and 6b), which is true for the cross-well seismic section in the direction perpendicular to the source area (Fig. 6b). Apparently, in Laoyemiao 3-D survey, picking of multiple higher-order sequences will be very timeconsuming. Therefore, higher-order sequence analysis for Ed3u follows the process from type-well analysis to cross-well correlation based on GR-log patterns and lithological combinations and successions. The correlation scheme of the well data is then mapped to the seismic sections for further adjustments. Following this approach, the Ed3u sequence from bottom to top can be further subdivided into three fourthorder sequences, namely, SS1, SS2, and SS3 (Fig. 4). The sequences comprise three genetically related relatively conformable sand units that are characterized by cyclic fining-upward and coarsening-upward. The sequences SS1, SS2, and SS3 are bounded by T5, T51, T52, and T4

4. Sequence stratigraphy and the depositional systems within the slope break belts 4.1. Slope break belt types On the basis of 3-D seismic data and palaeotopography analysis, the slope break belt in the Laoyemiao structure belt of Nanpu Sag has been investigated. Starting from identification of the features on the crosswell seismic and geological sections in the high-frequency (fourthorder) sequence units (Figs. 3–6), the palaeogeomorphology was reconstructed and the distribution pattern of the slope break belts in the Laoyemiao transverse anticline finally established. The Laoyamiao transverse anticline is typically characterized by double slope break zones. According to their natural geographical positions, double slope break zones can be divided into an upper slope break zone and a lower slope break zone. From the perspective of genetic types, they can be interpreted as multi-step fault slope break zones and flexure slope break zones. 4.1.1. Multi-step fault slope break belt Multi-step fault slope break belt is a complex palaeogeomorphology with the characteristic of a multi-grade fault-terrace jointly in control of a series of frequently active growth faults (Lin et al., 2000; Wang et al., 2003; Ren et al., 2004; Huang et al., 2012). All the fault planes of these 40

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Fig. 3. Paleo-geomorphological configuration and distribution of the slope break zones for the upper third member of the Dongying Formation in the Laoyemiao transverse anticline, Nanpu Sag.

Fig. 4. Stratigraphic framework of the study interval for the upper third member of the Dongying Formation. FMT = Formation; ST1 = the first sand unit, ST2 = the second sand unit, ST3 = the third sand unit. HST = highstand systems tract; TST = transgressive systems tract; LST = lowstand systems tract; MFS = maximum flooding surface.

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Fig. 5. Well- and seismic-log cross section AA' illustrating (a) the well-based sequence stratigraphic and depositional correlation section from Well-M40 to Well-XM1 constrained by the slope break zone (USBZ and LSBZ) along NE-SW; (b) the slope-break pattern and seismic sequence stratigraphy of Upper Ed3; (c) diagram for the genetic types of slope break belts from Xinanzhuang fault to the sag center. See the cross-section location in Fig. 1c. USBZ = upper slope break zone; LSBZ = lower slope break zone; MFS = maximum flooding surface.

that of sequence SS1 in the core of the transverse anticline. The sequence SS2 is also made up of a set of glutenite or coarse-grained sandstone in the LST. It becomes fine-grained in the TST, presenting a fining upward sequence. The HST comprises thick mudstone or mudstone and fine sandstone interbed. The thicknesses of the LST and TST in sequence SQ1 are similar to that of HST, whereas the percentage of sandstone is greater than that of HST (Figs. 5a and 6a). Sequence SS2 mainly develops two sets of thick glutenite or coarse-grained sandstone in the LST and HST and thick mudstone or multistory fine-grained sandstone in the TST (Figs. 5a and 6a). Generally, the three fourth-order sequences are primarily composed of four sets of sand bodies in the LST and HST of sequence SS3, corresponding to underwater distributary channel deposits in the core of the transverse anticline and sublacustrine fan deposits in the slope

seismic reflectors, and their thicknesses are 80–230 m, 90–180 m, and 85–170 m, respectively. Each sequence consists of the lowstand system tract (LST), transgressive system tract (TST), and highstand system tract (HST). The sequence boundaries are lithologic transformation surfaces and thick mudstone flooding surfaces. Among the three system tracts, the LST of sequence SS1 primarily consists of thick glutenite or multistory coarse-grained sandstone complex with thin mudstone interbedded. The lithology of the TST evolves into fine-grained sandstone or thick mudstone with thin coarsegrained sandstone. The thickness and percentage of sandstone of the LST and TST in sequence SQ1 are greater than those of HST (Figs. 5a and 6a). The HST is characterized by thick mudstone, with intercalation of relatively thin fine-grained sandstone. The combination of lithofacies of the three system tracts in sequence SS2 is basically consistent with 42

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Fig. 6. Well- and seismic-log cross section BB' illustrating (a) the well-based sequence stratigraphic and depositional correlation section from Well-M108-1 to WellM126-1 constrained by the slope break zone (USBZ and LSBZ) along E-W; (b) the slope-break pattern and seismic sequence stratigraphy of Upper Ed3; (c) diagram for the genetic types of slope break belts. See the cross-section location in Fig. 1c. USBZ = upper slope break zone; LSBZ = lower slope break zone; MFS = maximum flooding surface.

Ed3u were identified by using the well-logging data from 64 wells and the core samples from 13 wells. Furthermore, the sublacustrine fan is divided into six types of lithofacies according to their structures, textures, mud contents, and origins, namely, slides, slumps, sandy debris flows, muddy debris flows, turbidity currents, and hyperpycnal flows, which are associated with volcanic rocks (Fig. 7A8). It can be proved that slope break zones constrain the distribution of facies. The upper slope break zone, the boundary between the fan delta front and the sublacustrine fan, controls the development of the fan delta front, while the lower break zone controls the distribution of the sublacustrine fan.

break zone, whereas the TST predominantly comprises thick mudstone and argillaceous siltstone. Meanwhile, the LST of each sequence and the HST of SS3 exhibit the highest percentages of sandstone and therefore are regarded as a special lithological reservoir for hydrocarbon exploration. 4.3. Well-based analysis of sedimentary facies The common approaches applied to well-based sedimentological analysis generally consist of core description and interpretation, the study of lithofacies and log facies, and analyses of the sedimentary structure and texture, which can provide key evidence for deposition. In this study, the fan delta front (Fig. 7A1-8A7) and sublacustrine fan in

4.3.1. Fan delta front The fan delta front in the Ed3u of the Laoyemiao structure belt 43

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Fig. 7. Typical sedimentary structures of fan-delta front deposition in the upper third member of Dongying Formation. A1. Variegated, thick-bedded massive conglomerates supported by a coarse sandy matrix, 3034.2 m, Well M105-1; A2. brown-gray, thick-bedded massive conglomerates supported by a coarse-fine sandy matrix, 3043.8 m, Well M11-8; A3 and A4. thick-bedded massive conglomerates and gravels supported by a coarse-medium sandy matrix, 3112.8 m and 3247.6 m, Well M28-1; A5. brown-gray, thick-bedded pebbly unequally grained sandstone with 30%–40% oil-bearing area, 3007.9 m, Well-M30; light-gray fine sandstone containing an oil spot with cross bedding in the upper part and wavy bedding in the middle part, 3038.7 m, Well M11-8; A7. gray argillaceous siltstone containing argillaceous stripes with parallel bedding, 3036.4 m, Well M11-8; A8. dark gray basalt, 4007.02 m, Well M1.

coherent mass that glides downward along a planar glide plane under certain triggers (Shanmugam, 2013; Liu et al., 2015). The basal shear zone and the slide plane are the typical characteristics of slides. During the sliding process on a slope, the sandstone injections are generated by the shear stress component of the vertical mudstone (Yang et al., 2014, 2017a). The mixing of stripped, irregular, or massive sand bodies with the underlying mudstones is interpreted as a result of the dragging effect of sliding along the soft mudstone succession (Mohrig et al., 1998).

mainly consists of underwater distributary channel deposits and underwater distributary interchannel deposits (Fig. 7A5). Based on observations of the cores, the underwater distributary channel deposits are dominated by light gray or variegated conglomerates (Fig. 7A18A4) and brown-gray pebbly unequal grain sandstones (Fig. 7A5), and the sedimentary structures include cross bedding and wavy bedding (Fig. 7A6). The conglomerates are poorly sorted, whereas the sandstones are moderately sorted, with the roundness varying from subangular to subcircular; the gravels are 1–9 cm in diameter (Fig. 7A1A3). Wireline logs show that the underwater distributary channel deposits display low gamma-ray (GR) and high deep investigate double lateral resistivity log (RLLD) values and toothed box shapes. The seismic reflections are characterized by medium-high frequencies, moderate strengths, and well-continuity. The underwater distributary interchannel deposits are mainly composed of brown-gray medium-fine sandstone and dark gray argillaceous siltstone with corrugated and horizontal beddings (Fig. 7A7). The GR and RLLD curves exhibit lowwidth linear and medium-high amplitude finger shapes (Figs. 5 and 6). The seismic profile is characterized by a negative polarity (peak) and medium-high frequency reflections (Figs. 5 and 6).

4.3.2.2. Slumps. Description: Slumps in the study area are mainly distributed in the middle and upper parts of the slope break zone or in the transition zone between the slope break zones (Figs. 3 and 8B3–8B9). The lithology mainly includes light gray fine sandstone, argillaceous siltstone, and silty mudstone (Fig. 8B3-8B9). Micro faults (Fig. 8B3 and 8B4), chaotic sands with deformed muddy clasts (Fig. 8B5 and 8B6), soft-sediment deformation structure (Fig. 8B6 and 8B9), (sand and mud) corrugation texture (Fig. 8B7), and sandstone intrusions (Fig. 8B8) can be observed commonly in the cores. Interpretation: Fracture deformation, sedimentary plastic deformation, and sand injection are the main characteristics of slumps (Xian et al., 2012, 2013; Shanmugam, 2013; Yang et al., 2014; Liu et al., 2015, 2017a,b). Minor faults in deformed sandy and muddy layers indicate internal fractures caused by rotational movements. Sedimentary plastic deformation and reconstruction of sedimentary texture are considered to result from the forward inertial motion of slumps (Fig. 8B6 and 8B7). In this process, in-situ muddy sediments will be trapped and deformed, and mixed with the slumps to varying degrees. At the same time, deformed sandstone can intrude into the in-situ muddy sediments. Sedimentary structures in the cores represent evidences for sandy slumps in the Krishna–Godavari Basin of India (Shanmugam et al., 2009), the Eocene Dongying Depression of Bohai Bay Basin (Liu et al., 2017a,b), the Beibuwan Basin of South China Sea (Liu et al., 2014), and the southwestern Ordos Basin of NW China (Liu et al., 2017a,b).

4.3.2. Sublacustrine fan 4.3.2.1. Slides. Description: The sediments of this type mainly consist of light gray silt-fine sandstone and argillaceous siltstone. Centimeterscale features, such as basal primary glide planes (Fig. 8B1 and 8B3), basal shear zones (Fig. 8B2), and sand veins (injections; Fig. 8B1) can be identified in the cores of these sandstones. The slides in the study area are mainly developed in the upper part of the slope break zone or in the transition zone between the slope break zones (Figs. 3 and 8B1–8B3). Few slides are identified in the study area owing to the rapid transformation of the slides into slumps, and debris flows during the evolution of gravity flows. Interpretation: A slide, with little internal deformation, is a 44

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Fig. 8. Typical sedimentary structures of sublacustrine fan deposition in Ed3u, the location of the wells is shown in Fig. 1c. B1. a sandy slide with a glide plane and sand veins in dark thick mudstone, Well M4-1, depth 3725.60 m; B2. a shear glide plane on the top of light-gray fine sandstone, Well M38-1, depth 3570.96 m; B3. micro-faults structure, glide plane (GP), and stratified silty mudstone in the core of Well M38-1, depth 3574.29 m; B4. micro-faults structures, contorted and stretched sand clasts, and sand veins in dark thick mudstone, Well M14-1, depth 3396.92 m; B5. gray sandstone containing muddy debris and deformation structures, Well M108-1, depth 3304.80 m; B6. silty mudstone, which contains quartz granules, sand ball and vein, and mudstone clasts, Well M4-1, depth 3724.30 m; B7. slump folds (SF) in gray argillaceous siltstone, Well M381, depth 3572.13 m; B8. fluidization of sand causing the intrusion of sand veins (SV) in unstratified thick silty mudstone, Well M14-2, depth 3482.90 m; B9. soft-sediment deformation structure (SSDS) in dark, thick mudstone, Well M14-1, depth 3397.70 m; B10. mass sandstone with mudstone clast (MC) and sharp irregular bottom contact, Well M39-2, depth 3294.20 m; B11. Corrugated silty mudstone and mudstone clasts (MC) in light-gray block sandstone, Well M39-2, depth 3294.23 m; B12. chaotically floated muddy clasts in light gray, pebbly coarse sandstone, Well M4-1, depth 3656.40 m; B13. floating mudstone clasts (MC) in the upper part of the core, imbricated mudstone clasts and wavy bedding in the lower part of the core, Well M108-1, depth 3305.37 m; B14. floating mudstone clasts (MC) in the upper part of structureless blocky-shaped sandstone, Well M33-1, depth 3198.00 m; B15. floating mudstone clasts (MC) and quartz and flint gravel, and slump deformation structure in light gray, thick pebbly unequal grained sandstone, Well M33-1, depth 3407.10 m; B16. floating sandy clasts (SC) in dark gray thick mudstone, Well M33-1, depth 3404.90 m; B17. a large number of sandy clasts in dark gray sandy mudstones, Well XM-1, depth 3559.61 m; B18. superimposed Bouma Tae, Tae, Tbe, Tbe, Tbce sequence with ball- and pillow-structure (BPS) and flame structure (FS), Well M33-1, depth 3193.60 m; B19. Bouma Tae,Tabde sequence with flame structure (FS) and normal grading (NG), Well XM1, depth 3565.99 m; B20. Bouma Tabe sequence and sharp irregular bottom contact, Well XM1, depth 3801.90 m; B21. laminated silty mudstone, which contains multiple couplets of reversed and normally graded hyperpycnite layers in Well M38-1, depth 3574.51 m; B22. laminated bedding, parallel bedding, and intra-layer erosional surface (IES) in argillaceous siltstone, containing multiple couplets of reversed and normally graded hyperpycnite layers, Well M4-1, depth 3724.59 m; B23. argillaceous fine sandstone with laminated bedding, climbing ripple bedding (CRB), floating gravel (FG), tuffaceous mudstone lenses (TML), and intra-layer erosional surface (IES), which contains multiple reversed and normally graded rhythmites, Well M4-1, depth 3723.32 m. 45

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4.3.2.3. Sandy debris flows. Description: This type of deposit is composed of quartz and flint gravels and muddy-clasts bearing light gray, fine to medium massive sandstone (Fig. 8B10-8B15) The lithofacies are widespread over the middle of the slope, present structureless blocky shape, and display lower clay content, based on observation of the core. The gravels are elliptoid or semi-rounded, and 0.5–3 cm in diameter. The mudstone clasts are poorly sorted and moderately to poorly rounded, and 0.5–13 cm or larger in diameter. Both planar and random brecciated mudstone fragments with sharp margins and stretched deformations are common, which reveal that flows occur in laminar beds and materials deposit fast after shortdistance transport (Liu et al., 2015). These floating gravels and mudstone clasts can occur at an arbitrary point in the form of rafted clasts in massive sandstone beds (Xian et al., 2012; Liu et al., 2017a,b). Interpretation: A massive sandstone with emplaced chaotically floated clasts can be attributed to the typical sandy debrites first introduced by Hampton (1975) (Shanmugam, 1996, 2000; Stow and Johansson, 2000; Talling et al., 2012). Unstructured and blocky-shaped sedimentary textures suggest rapid accumulation through condensation in mass transport mode (Shanmugam, 1996; Lv et al., 2017). The origin of chaotic clasts has been related to channel margin collapse or headward slumping (Stow and Johansson, 2000). The low clay content of the sedimentary component indicates that this type of debris flow is neither grain flow nor muddy debris flow. The conclusion drawn from experimental studies is that as low as 0.5% clay content is sufficient to prevent frictional locking of grains through lubrication and provides strength to generate sandy debris flows (Shanmugam, 2000). Sandy debris flows are considered to represent a continuous spectrum of processes between cohesive and cohesionless debris flows (Shanmugam et al., 1995). Therefore, sandy debris flows are characterized by multiple sediment support mechanisms, based on cohesive strength, frictional strength, and buoyancy. From the perspective of hydrocarbon exploration, sandy debrites can be thick, extensive, and excellent reservoirs (Shanmugam and Zimbrick, 1996; Zou et al., 2012; Shanmugam 2006, 2012, 2013, 2016; Pu et al., 2014), which change the traditional knowledge that the center bottom of a lake is dominated by a mud deposit and a lack of effective reservoir rocks.

Tbe, Tabe, Tbc, Tbce, and Tabde sequence (Fig. 8B18-8B20). The contact at the bottom of the lithofacies is irregular because of flame, ball, and pillow structures (Fig. 8B18-8B20). Interpretation: Normally graded beddings and Bouma sequences are regarded as the typical features of turbidity currents (Mulder and Syvitski, 1995; Pan et al., 2017; Yang et al., 2017b). The superimposed multi-rhythmic layer (less than 0.2 m thick) with horizontal bedding and dark mudstones on the top is indicative of a low-energy lake environment. Each sequence of turbidity currents presents a tongue shape on the plane, and the fine-grained section is larger than the lower coarse-grained segment. Normally graded beddings result from gradual decreases in the velocity and particle size along the flow direction. The perfection of the Bouma sequence is determined by the deposition frequency and the intensity of the turbidity current. The scouring and erosion observed at the bottom of rhythmic layers is caused by subsequent sand deposition. 4.3.2.6. Hyperpycnal flows. Description: This type of deposit is composed of light-gray fine-grained sandstones, siltstones, and argillaceous siltstone interbedded with dark mudstones (Fig. 8B11, 8B13, and 8B21-8B23), which are distributed from the foot of the slope to the lake basin in the study area. Floating gravels and brownish lenses of tuffaceous mudstone can be observed in some cores (Fig. 8B22 and 8B23). Rhythmic beddings constituted by couplets of an upward coarsening interval (UCI) and an upward fining interval (UFI) are common (Pan et al., 2017; Yang et al., 2017a,b,c). These rhythmites are 2–35 mm thick (typically 2–6 mm). Intra-sequence (intra-layer) erosional surfaces (IES) usually separate a couplet of an upper upward-fining interval and a lower upward-coarsening interval. Sedimentary structures of flow genesis, such as laminated beddings, parallel beddings, waveform cross-beddings, and climbing beddings, are well developed. Additionally, mudstone clasts in blocky sandstone present imbricated orientation distribution and the maximum particle size is up to 10 cm, which indicates that the deposits were formed by bed-load transportation when the fluid energy was high. Interpretation: The sediments, with IES in the couplets of UCIs and UFIs and sedimentary structures of flow water origin, are commonly considered to be hyperpycnal flow deposits (Mulder et al., 2003; Soyinka and Slatt, 2008; Lamb and Mohrig, 2009; Pan et al., 2017; Yang et al., 2017b, 2017c). The couplets of UCIs and UFIs suggest a cycle of sedimentary events associated with flood-generated hyperpycnal flow. A UCI represents a sediment during the phase of waxing discharge, whereas a UFI implies that the deposits transported and settled under waning discharge. The IES are usually formed at the sudden beginning of a flood-generated hyperpycnal flow. Identification of hyperpycnites must therefore be based not only on the occurrence of couplets of UCIs and UFIs but also on the features of the IES (Mulder et al., 2003). The IES sometimes divide the two parts of a size-graded couplet resulting from waxing flows of sufficiently high velocities to erode the sediment previously deposited by the same flow. The IESs are, however, scarcely recognizable in muddy hyperpycnites (Fig. 8B11 and 8B21), which is possibly because of the loss of erosional capacity under low-turbulence conditions during flowage, or because the transition from waxing flows to waning flows is weak in the distal lobes of muddy hyperpycnites (Yang et al., 2017c). In addition, sedimentary structures of the flow genesis are significant indicators for distinguishing hyperpycnal flows from other gravity flows. Cross bedding and parallel bedding are relatively well-developed in hyperpycnites owing to the continuous water flow and wave effects during the formation of the hyperpycnal flow when the hydraulic energy of the flood first increases and then decreases.

4.3.2.4. Muddy debris flows. Description: This type of gravity-flow deposit consist of sandstone clasts bearing, dark gray silty mudstones or mudstones(Fig. 8B4, B16 and B17), which is frequently distributed from top to foot of slope and usually interbedded with massive sandstone intervals. The thickness of this lithofacies ranges from less than 10 cm to more than 5 m with clay content over 60 % by weight. Most sandy clasts are less than 15 cm, angular to subrounded. Planar, contorted, stretched, and irregular blocky sandy clasts are common, suggesting synsedimentary deformation(Fig. 8B4 and B17). Interpretation: Various sedimentary features observed in muddy matrix can be interpreted as muddy debris flows. High clay content is a favorable condition for the formation of muddy debris flow. A cohesive debris flow develops when the clay content exceeds a limit range from 22 wt% to 30 wt% (Grim, 1968). Moreover, various shaped sandy clasts observed in the cohesive matrix are supposed to unequivocal evidence of emplacement by cohesive debris flow. Planar and contorted sandy clasts suggest laminar state and plastic rheology, and elongated sand clasts demonstrate interfacial shear between the cohesive matrix and unconsolidated sands. 4.3.2.5. Turbidity currents. Description: The lithofacies is composed of fine-grained sandstones and siltstones interbedded with dark mudstones, which are widely dispersed from the slope belt to the lake basin. The thickness of the gravity-flow deposits varies from 0.015 m to 0.2 m in a single unit, and the deposits usually represent multi-rhythmic layers that are stacked together. Sedimentary structures, such as normally graded beddings, parallel beddings, cross beddings, and horizontal beddings, are common in sandstone, exhibiting Bouma Tae,

4.4. Planar distribution of sandstone and conglomerate contents Before seismic geomorphology study, the application of well data to geological analysis should be given priority. An area with high gravel 46

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Fig. 9. Distribution maps of sandstone and conglomerate contents of sand units in SS1 illustrating the lithological characteristics. (a) sandstone. (b) conglomerate. Generally, the area with a high gravel content can be considered as the main channel of the fan delta front, and the sand-rich parts are the main bodies of the fan delta front and the sublacustrine fan deposits. The names of wells are the same as those in Fig. 1c.

content can be considered to be mainly characteristic of the fan delta front. The sand-rich parts are the main bodies of the fan delta front and the sublacustrine fan deposits. The present geomorphology can basically reflect the relative fluctuations of the stratigraphic sedimentary period. According to the statistics for the conglomerate content in Ed3u (Figs. 9–11), the highest value is observed at the junction of the boundary fault and the Laoyemiao transverse anticline, and gradually decreases from the slope break belts towards the center of the basin. Specifically, the proportion of the conglomerate on both sides of the transverse anticline decreases sharply as a result of rapid transformation of the slope break zones. The distribution trends of the sand content are similar to those of the conglomerate content, which proves that the Laoyamiao anticline communicates with the source area of the basin margin. The intersection of the transverse anticline and the

boundary fault controls the entrance of the main water system (Figs. 3 and 9-11). The main channels are overall distributed along the long axis of the transverse anticline, and the sediments controlled by the double slope break zones exhibit corresponding evolutions when they are transported over a long distance to the center of the basin. In addition, the value of sand content is abnormally high in well-lin 2 area, due to the accumulation of sedimentary supplies from the NW and NE directions (Figs. 10b, 11b and 11d). 5. Analysis of sediment-dispersal patterns based on seismic geomorphology Much of recent research has been on analysis of facies architecture by means of outcrops, cores, seismic, well, and experimental data

Fig. 10. Distribution maps of sandstone and conglomerate contents of sand units in SS2 illustrating the lithological characteristics. (a) sandstone. (b) conglomerate. The names of wells are the same as those in Fig. 1c. 47

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Fig. 11. Distribution maps of sandstone and conglomerate contents of two sand units in SS3 illustrating the lithological characteristics. (a) sandstone content distribution of the LST. (b) conglomerate content distribution of the LST. (c) sandstone content distribution of the HST. (d) conglomerate content distribution of the HST. The names of wells are the same as those in Fig. 1c.

(Posamentier and Kolla, 2003; Sawyer et al., 2007; Sech et al., 2009; Reijenstein et al., 2011; Sumner et al., 2012; Abdel-Fattah et al., 2013; He et al., 2017). In this study, the percentages of conglomerate and sandstone obtained from well data were used to predict the main channel and sedimentary facies. Then, seismic geomorphology was employed to delineate in more detail the sediment-dispersal patterns controlled by the slope break zones. To extract as much information as possible, a total of 44 stratal slices were generated by proportional slicing within three fourth-order sequences to accurately reflect the geomorphology, sedimentary facies, and depositional history.

M36, XM1, Lin1, Lin2, and M33-1) evenly located in the study area were selected to obtain a statistical relationship between the argillaceous content and P-velocity of the Ed3u submember. In Fig. 12a, the different colored points represent different wells. The argillaceous content was computed from the GR curves, and the P-velocity was computed from the acoustic curve (AC). The statistics show that the argillaceous content and P-velocity show an inverse linear relationship (Fig. 12a). Sandstones correspond to high velocities, whereas mudstones correspond to low velocities. Lithology-decomposition analysis through seismic reflection is also necessary to define whether a single seismic event represents a single lithology or complex lithology (Zeng et al., 2012). In the study area, the sandstone and mudstone commonly exhibit interbedded deposition, and thick sandstone is usually intercalated with thin layers of mudstone (Figs. 5a and 6a). Consequently, no one-to-one correspondence exists between the single lithologic layer and a seismic event, and seismic events represent lithologic associations. Based on the synthetic seismic

5.1. Rock-physics analysis Analysis of rock-physics relationships is indispensable for establishing the corresponding relationship between seismic amplitude and lithology in order to predict the lithology in seismic sections or stratal slices based on seismic polarity or amplitude. Seven wells (M40, M30, 48

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Fig. 12. Rock-physics relationship analysis for sediments of Ed3u, the Laoyemiao transverse anticline in Nanpu Sag. (a) A cross-plot of P-velocity and argillaceous content extracted from wireline-logs (well-M40, M30, M36, XM1, Lin1, Lin2, and M33-1); (b–c) lithology correlated with the amplitude pattern and synthetic seismic tie of Ed3u for well-M36. The highly negative amplitude (red color) mostly suggests thick sandstones, whereas a less negative amplitude corresponds to thin or shaly sandstones; the positive amplitudes (black color) are indicative of mudstone. The well and cross-section locations are shown in Fig. 1c. (For interpretation of the references to color in this figure legend, the reader is asked to refer to the web version of this article.)

ties (Fig. 12b), it is inferred that a highly negative amplitude (red color) indicates thick sandstone (high sand-to-mud ratios), whereas a less negative amplitude (lighter-red color) corresponds to thin sandstone or argillaceous siltstone (Figs. 1, 5 and 62c). The positive amplitudes (black color) represent mudstone-prone units (low sand-to-mud ratios) (Figs. 1, 5 and 62c). As a result, the spectrum of seismic data can be confidently applied to the reliable prediction of sandstone in a study area in which few or no wells are possible through frequency division processing technology.

the slope break zones, the depositional setting, and the drilled data (lithology and well-logging features) for the Laoyemiao transverse anticline. The locations of the selected slices are shown in Figs. 13–15. In the 3-D working region, according to the characteristics of the palaeogeomorphology (transverse transfer zone and strike ramp) and stratal slices, the study area was divided into six zones (zone I, zone II, zone III, zone IV, zone V, and zone VI) (Figs. 13–15) to synthetically analyze the sediment-dispersal characteristics of Ed3u controlled by the double slope break zones of Laoyemiao transverse anticline.

5.2. Description and interpretation of stratal slices

5.2.1. SS1 Fig. 13a presents a stratal slice with the interpretation and Fig. 13a′ is the sedimentary facies map of SS1. The well data from previous research have been integrated to interpret the sedimentary facies of stratal slice 3. The results indicate that the provenance is from the north, and the transfer zones of the transverse anticline and strike ramp

To reveal the spatial-temporal evolution of fans in the double slope break zones, four stratal slices were selected from the sequences SS1, SS2, the LST of SS3, and the HST of SS3, namely, slices 3, 16, 31, and 42. The stratal slices were interpreted based on the tectonic patterns of 49

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Fig. 13. A representative stratal slice and the interpreted depositional system map of SS1 in Laoyemiao transverse anticline, Nanpu Sag, Bohai Bay Basin. Negative (red) amplitudes indicate sandstones and volcanic rocks, and positive (black) amplitudes refer to shales. Five sedimentary facies, namely, fan delta front, sublacustrine fan, volcanic, sub-lacustrine, and lake, are identified based on the morphological landforms of the stratal slice. See Figs. 5 and 6 to locate the stratal slices. The names of wells are the same as those in Fig. 1c. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

control the main drainage entry (Figs. 13 and 16). SS1 can be interpreted as five sedimentary facies: fan delta front, sublacustrine fan, volcanic, sublacustrine, and lake. The fan delta front in zones Ⅰ and Ⅱ with the areas of 19.91 km2 and 9.43 km2, respectively, close to the boundary fault is composed of multi-branch channel-shaped negative amplitude anomalies (red). The sublacustrine fan in zones Ⅲ, Ⅳ, and Ⅴ, covering a total area of 43.7 km2, is mainly triggered by volcanic events and is distributed between the upper and lower slope break zones, and

corresponds to roughly multiple lobe-shaped or bird foot shaped negative amplitude anomalies in slice 3. In zone Ⅴ, the sublacustrine fan experiences a long-distance transport of 5.19 km along the slope to the lower part of the lower slope break zone after sliding or slumping at the upper slope break zone, while the sublacustrine fan in zones Ⅲ and Ⅳ of the western and eastern areas of the anticline rapidly unloads and accumulates, resulting in a propulsive distance of only 2.03 km with an area of about 5.34 km2 in zone Ⅲ and less than 2 km in zone Ⅳ toward

Fig. 14. A representative stratal slice and the interpreted depositional system map of SS2 in Laoyemiao transverse anticline, Nanpu Sag, Bohai Bay Basin. Negative (red) amplitudes indicate sandstones, and positive (black) amplitudes refer to shales. Four sedimentary facies, namely, fan delta front, sublacustrine fan, sublacustrine, and lake, are identified based on the morphological landforms of the stratal slice. See Figs. 5 and 6 to locate the stratal slices. The names of wells are the same as those in Fig. 1c. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 50

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Fig. 15. Representative stratal slices and the interpreted depositional system maps of SS3 in Laoyemiao transverse anticline, Nanpu Sag, Bohai Bay Basin. (a) Slice 31; (b) Slice 42. Negative (red) amplitudes indicate sandstones, and positive (black) amplitudes refer to shales. Four sedimentary facies, namely, fan delta front, sublacustrine fan, sublacustrine, and lake, are identified based on the morphological landforms of the stratal slice. See Figs. 5 and 6 to locate the stratal slices. The names of wells are the same as those in Fig. 1c. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

the center of the lake. In addition, zone Ⅳ exhibits a highly negative amplitude, which is reflective of deposits of thicker sand bodies (with sandstone content of 60%), which are a result of joint accumulation of the sublacustrine fan sand bodies from the fan delta front of zones Ⅰ and Ⅱ. The shallow lacustrine facies presenting positive amplitudes (black) is located among the lobes of the sublacustrine fan. The distribution of volcanic facies in zone Ⅵ presents a fan-shaped zone with a highly negative amplitude in slice 3 that has an area of around 9.24 km2. The whole fan body consists primarily of the volcanic fan delta front and overlapped lobes of the sublacustrine fan, with an extension distance of approximately 8.89 km and an area of about 82.27 km2.

5.2.2. SS2 Fig. 14a presents a stratal slice with the interpretation and Fig. 14a′ is the sedimentary facies map of SS2. A comprehensive analysis of the lithology, well-logging data, and slice 16 indicates that the primary source area was also from the north and that the sediments were deposited mainly on the fan delta front, sublacustrine fan, sublacustrine, and lake facies of sequence SS2. Under the control of the upper and lower slope break zones, the sizes of the fans enlarged, due to the abundant supply of sediments and the deepening lake water. Compared with the case of SS1, the depositional area increased from 73.38 km2 to 83.51 km2 and the maximum transport distance from 8.56 km to 10.35 km. The fan delta front facies in zones Ⅰand Ⅱ expanded to 51

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Fig. 16. Transfer zones and their control on the distribution of depositional systems in Laoyemiao transverse anticline, Nanpu Sag, Bohai Bay Basin.

21.63 km2 and 9.22 km2, respectively. Particularly, the sublacustrine fans of zones Ⅲ and Ⅴ increased distinctly. The sublacustrine fan size in zone Ⅲ increased to the area of 5.72 km2 and the transported distance was 3.28 km, and the lobes of zone Ⅴ extended to the north of well XM1 and to the southwest of well M4-1, with the maximum length of 5.64 km and deposition range of 46.97 km2 in zone Ⅳ.

fact that the sublacustrine fan is superimposed by small lobes from different directions. The fan body in the HST continued to shrink (Fig. 15b and b′), which mainly indicated that the extended distance to the center of the lake decreased and the range further shrunk on the west side of the fan body and at the front end of the sublacustrine fan. The area of zone III reduced to 4.90 km2, and the extended distance of fan body in zone V retreated to the position of Xm1 well; the overall area reduced to 37.88 km2.

5.2.3. SS3 Fig. 15a and b represent the interpreted stratal slices and Fig. 15a′ and b′ are the sedimentary facies maps for the LST and the HST of SS3. Within the sedimentary period of SS3, the provenance is also from the northern Yanshanian fold belt, and the facies types are consistent with those of SS2. As the lake level fell and the sediment supply decreased, the fan body gradually shrunk from the LST to the HST, and the total area of the fan body and the range of each subfacies both decreased. In the deposition period of the LST, the fan area was 81.36 km2, which represented a reduction of 2.15 km2. The areas of zones I, II, III, and zones IV, Ⅴ reduced to 21.13 km2, 8.14 km2, 5.67 km2, and 46.42 km2 separately. The amplitude boundary between the fan delta front and the sublacustrine fan is more obvious in slices 31 and 42. More specifically, the highly negative amplitude anomaly in zone IV can clearly reflect the

6. Depositional model In the source-to-sink systems from the provenance to the basin, the tectonic paleogeomorphology formed by the syndepositional structures, together with the sediment supply sources, play a crucial role in controlling the sediment-dispersal routes, sedimentary structures, and distribution of the depositional systems (Ravnås and Streel, 1998; Lin et al., 2015). Restoration of paleoclimate and paleogeomorphology is a critical approach for reconstructing the depositional spatial-evolution model of Laoyemiao region. The sporopollen assemblages show that the vegetation of Dongying Formation in the study area is a mixed forest of conifer and deciduous 52

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broad-leaves, which now grow in the Yangtze River valley and its southern area and reflect the humid climate of a warm-subtropical zone with abundant precipitation (Zhang and Jiang, 2010; Xia et al., 2015). In this climate, the sediment supply sources are sufficient. The underwater distributary channel deposits of the fan delta front are well developed and the sand bodies exhibit the ability to be transported long distances along slopes. The flood can carry a large number of coarse debris into the slope zone of the transverse anticline, where overlapped lobes of the lacustrine fan are formed (Figs. 13–16). In a rift basin, the segmented growth process of the boundary fault controls the paleogeomorphological architecture near the upthrow and downthrow sides of the fault and constrains the drainage systems and the distribution of sand bodies in the basin (Jackson et al., 2005; Jiang et al., 2010; Sun et al., 2016, 2017). The continuous Xinanzhuang fault is formed in three stages by the interaction and connection of three independent secondary faults: isolation fault stage, interaction stage, and linkage stage (Dong et al., 2013; Sun et al., 2016). Each segment of Xinanzhuang fault is linked during the sedimentation period of Ed3u, but the middle and eastern sections are still segmented (Fig. 3), forming a series of semi-graben and transverse anticlinations on the downthrow side of Xinanzhuang fault (Sun et al., 2016). The transverse anticline is located at the junction of the middle and eastern sections, where the local slip rate of the fault is minimum (Sun et al., 2017). The overlapping area connecting the hanging wall of the eastern section and the footwall of the NNE fault within the basin is strike-ramp transfer zone, which controls the provenance from Gaoliu structural belt (Fig. 1). Thus, the topographical features in Laoyemiao region are dominated by transverse transfer zone (Fig. 16a), strikeramp transfer zone (Fig. 16b), and double slope break zones (Figs. 3, 5 and 6). The transverse anticline and strike-ramp transfer zones (Fig. 16a and b) restrain the sediment-transport pathways in the central and northeast transverse anticlines, respectively, while the unique structures of the double slope break zones, such as dip, extension scale, and gradient, control the plane distribution and spatial-temporal evolution of the depositional systems. Based on the above comprehensive analysis, the depositional model of Ed3u submember has been established to show the sediment dispersal patterns under the control of the transfer zones, double slope break zones, and lake level (Fig. 16).

double slope break zones, which controls the spatial-temporal evolution of depositional systems. During the deposition from SS1 to SS3 in the SQ1 of third-order sequence, Laoyemiao region experiences a complete sedimentary cycle and the lake level undergoes the transformation from transgression to regression. Meanwhile, three fourth-order sequences, SS1 to SS3, constrain the formation of four sets of sand bodies, which correspond to four stages of the sublacustrine fan deposits controlled by the double slope break zone. From the interpreted depositional systems shown in Figs. 13–15, the fan size and transport distance increase from SS1 to SS2 and then decrease from SS2 to SS3 (Figs. 13–15). Considering the narrowed distribution of the depositional system in the deposition of SS3, for instance, Fig. 16illustrates the depositional model constrained by transfer zones in the study area. Facies-controlled model, long-distance sediment-transport model, and lateral sand bodies transport model can be used in the double slope break zones to accurately predict the favorable reservoir sandstones according to the established depositional model. Moreover, it is worth noting that the sublacustrine fan, which exhibits the highest sandstone content in the lateral sand body transport area of the transverse anticline and strike ramp, may be the most favorable reservoir. 7. Conclusions By using high-quality 3-D seismic, well log, and cores data, this study demonstrated seismic geomorphology technique to delineate the sedimentary characteristics of the upper third member of Eocene Dongying Formation, which is controlled by the double slope systems of Laoyemiao transverse anticline in Nanpu Sag, Bohai Bay Basin. The following conclusions can be drawn from this study. (1) Laoyemiao transverse anticline can be divided into an upper slope break zone and a lower slope break zone, according to their natural geographical positions, while they can also be interpreted as a multi-step fault slope break zone and a flexure slope break zone from the perspective of genetic types. (2) The upper third member of the Dongying Formation (Ed3u), a thirdorder sequence, can be subdivided into three fourth-order sequences, namely, SS1, SS2, and SS3 from the base to the top, which correspond to the lowstand, transgressive, and highstand periods of Ed3u, respectively. (3) The fan delta front and sublacustrine fan are recognized by using well-logging data from 64 wells and the cores of 22 wells in Ed3u. Furthermore, the sublacustrine fan is subdivided into six types of lithofacies according to the structures, textures, mud contents, and their origins, namely, slides, slumps, sandy debris flows, muddy debris flows, turbidity currents, and hyperpycnal flows, which are closely associated with multiple volcanic eruption events and controlled by the double slope break zones. (4) The fan and its subfacies interpreted from stratal slices all increased in size and extended distance from SS1 to SS2, while the sizes of the fan and its subfacies decreased continuously from the LST of SS3 to the HST of SS3. The spatiotemporal evolution of the fan size within the deposits of SS1, SS2, and SS3 were determined by the sediment supply and the change in the lake level. (5) The influence of Laoyamiao transverse anticline on the sediment dispersal patterns of Edsu is as follows. First, the transverse and strike-ramp transfer zones constrain the drainage entry in the central and northeast transverse anticlines, respectively. Second, the upper slope break zone controls the distribution of the fan delta front, while the lower slope break zone controls the boundary between the sublacustrine fan and the lacustrine facies. The established facies-controlled, and long-distance and lateral sediment transport models can be used in the double slope break zones to accurately predict the favorable sand bodies, especially the sand bodies of the sublacustrine fan in zone Ⅴ. (6) Acquisition of high-quality seismic data is the key to conducting seismic

6.1. Facies distribution and sand body transport model The fan delta front constrained by the upper slope break zone is distributed in the core of Laoyemiao transverse anticline, and the lower slope break zone controls the boundary between the sublacustrine fan and the lacustrine facies. The sand bodies of the fan delta front in the slope break zones tend to be triggered by multiple volcanic eruption events (Xu and Guo, 1992; Dong et al., 2010; Du et al., 2014) and slide or slump to form the sublacustrine fan; the lithofacies formed during the transformation from the slides or slumps to sandy debris flows, muddy debris flows, or hyperpycnal flows between the upper break zone and the lower slope break zone. However, there are obvious differences in the sediment transport model between the front end (zone Ⅴ) and both sides of the transverse anticline where the double slope break zones present varied dips. At the front of the transverse anticline, the dips are 5.1° and 6.2° (Figs. 3 and 5b) and the friction between the sediments and water is small, so that the sand bodies benefitting from the water inertia being maintained can be transported over a long distance and an outward radiative fan is formed near the lower slope break zone. On the contrary, the double slope break zones on both sides of Laoyemiao transverse anticline are characterized by dips of 7.4°–14.6° (Figs. 3 and 6b), a deep water body, and large friction at the bottom, which leads to a fast decrease in the hydrodynamic force and the sediments being rapidly unloaded. 6.2. Spatial-temporal evolution of depositional systems The variation in the lake level changes in accommodation space in 53

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geomorphology studies. Particularly, in Nanpu Sag of a rift basin with complex structures, integrated analysis of broad-band, wide-azimuth, and high-density seismic data combined with core, thin section, well logging made it possible to map the sediment-dispersal patterns of Ed3u in the double slope system of Laoyemiao transverse anticline.

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