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Gravity-flow deposits and their exploration prospects in the Oligocene Dongying Formation, northwestern Bozhong Subbasin, Bohai Bay Basin, China
Marine and Petroleum Geology 96 (2018) 179–189
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Research paper
Gravity-flow deposits and their exploration prospects in the Oligocene Dongying Formation, northwestern Bozhong Subbasin, Bohai Bay Basin, China
T
Shang Xua,∗, Fang Haob,a,∗∗, Changgui Xuc, Huayao Zoud, Xintao Zhangc, Yuanyin Zhange, Baishui Gaof, Qi Wangb a
Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan 430074, China School of Geosciences, China University of Petroleum, Qingdao 266580, China Tianjin Branch of China National Offshore Oil Company Ltd, Tianjin 300452, China d State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China e Oil and Gas Survey, China Geological Survey, Beijing 100037, China f Strategic Research Center of Oil and Gas Resources, MLR, Beijing 100034, China b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Gravity-flow deposits Mass-transport deposits Channel-lobe complexes Exploration prospects Lacustrine basin
Gravity-flow deposits are important hydrocarbon exploration targets in most basins. In this study, an integration of geological, geophysical, and geochemical analysis is used to investigate the gravity-flow deposits and their petroleum prospects in the Oligocene Dongying Formation (E3d), Bozhong Subbasin, Bohai Bay Basin. Masstransport deposits (MTDs) and channel-lobe complexes are the main gravity-flow depositional units. The masstransport deposits (MTDs) show hummocky and chaotic reflections with poor continuity and variable amplitude. The channels display linear geomorphology with weak root-mean-square (RMS) amplitude, and the lobes are dominated by high-continuous and high-amplitude reflection at the terminal channels. The formation of gravityflow deposits is primarily controlled by high sedimentation rate during a relative lowstand of lake level. The E3d3 source rock has TOC contents in the range of 0.6–3.1%, Rock-Eval S2 values in the range of 0.7–19.8 mg HC/g rock, and Hydrogen indices (HI) in the range of 100–600 mg/g TOC. The E3d2L source rock has TOC contents ranging from 0.5 to 1.9%, Rock-Eval S2 values from 0.4 to 7.8 mg HC/g rock, and HI from 40 to 480 mg/g TOC. Both E3d3 and E3d2L source rocks have good hydrocarbon-generation potential. The E3d3 and E3d2L source rocks have entered the oil-generation peak window with the vitrinite reflectance (Ro) higher than 1.0%. Oils generated from the underlying source rocks shortly migrated to the overlying gravity-flow lithologic traps. The gravity-flow reservoirs could serve as potential and effective hydrocarbon exploration targets.
1. Introduction The Bohai Bay Basin is one of China's most petroliferous basins located on its east coast (Fig. 1), accounting for nearly one-third of the total oil production of the country (Hao et al., 2009). The Bozhong subbasin is the largest generative kitchen in the offshore area (Hao et al., 2009) (Fig. 1). Previous petroleum exploration has mainly focused on the shallow Neogene reservoirs, and more than 25 × 108 tons (18.3 × 109 bbl) reserves have been proven in the Bozhong sub-basin (Hao et al., 2009; Gong et al., 2010). With exploration entering the mature phase, deep reservoirs, such as the Oligocene Dongying Formation (E3d), will be important exploration targets in the future.
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Deepwater gravity-flow deposits have received considerable attention in the petroleum industry. Most hydrocarbon reserves are within deepwater gravity-flow reservoirs, such as channel-fill, levee, sheets, and mass-transport deposits (Weimer et al., 2006; Shanmugam et al., 2009; Gong et al., 2014). Approximately 60% of the oil production in the northern deep Gulf of Mexico is from sheet sands, 25% is from channel-fill deposits, and 15% is from thin beds in levees (Lawrence and Bosman-Smits, 2000). Porosity and permeability in deepwater gravityflow reservoirs are generally high, because they are commonly fed from mature river systems (Weimer et al., 2006). Lacustrine gravity-flow deposits are also effective hydrocarbon reservoirs (Feng et al., 2010; Zhang et al., 2016; Liu et al., 2016; Corella et al., 2016), although
Corresponding author. Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Lumo Road 388, Wuhan 430074, China. Corresponding author. School of Geosciences, China University of Petroleum, Changjiangxi Road No. 66, Qingdao 266580, China. E-mail addresses: [email protected] (S. Xu), [email protected] (F. Hao).
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https://doi.org/10.1016/j.marpetgeo.2018.06.001 Received 9 February 2017; Received in revised form 1 June 2018; Accepted 1 June 2018 Available online 02 June 2018 0264-8172/ © 2018 Elsevier Ltd. All rights reserved.
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Fig. 1. (A) Location map showing the Bohai Bay Basin. (B) Location of the study area in the Bozhong subbasin, Bohai Bay Basin.
fluvial and delta sandstones are major reservoir types in lacustrine basin (Gong, 1997). For instance, Liu et al. (2016) recently recognized a channel complex in the Huanghua Depression, Bohai Bay Basin, and the channel sandstone has proven to be important hydrocarbon reservoirs. Zhang et al. (2016) described four sets of lacustrine mass-transport complexes, which could serve as potential exploration targets in the Songliao Basin, northeastern China. In the Bozhong Subbasin, Bohai Bay Basin, the Dongying Formation (E3d) was deposited in the deep to shallow lacustrine environments, containing a series of gravity-flow depositional systems (Gong, 1997; Dong et al., 2011). The purpose of this paper is to investigate the gravity-flow deposits and their petroleum prospects in the E3d formations by integrated geological, geophysical, and geochemical data.
2. Geological setting The Bohai Bay Basin has an area of approximately 200,000 km2. Above the Cenozoic basement, this basin filling can be sub-divided into two tectono-stratigraphic units (Fig. 2): a rifting stage during the Paleogene and a thermal subsidence stage during the Neogene and Quaternary (Huang and Pearson, 1999; Yang and Xu, 2004; Qi and Yang, 2010; Huang et al., 2012). During the synrift stage (60.5–24.6 Ma), a series of grabens and half grabens developed along major NW and NE trending fault sets (Huang and Pearson, 1999). These grabens and half grabens progressively became one large basin during the late Oligocene, and the Bohai Bay Basin entered the thermal subsidence stage (24.6 Ma to the present). 180
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Fig. 2. Generalized stratigraphy of the Bohai Bay basin (modified from Hao et al., 2009). The target stratum is showing in the black dashed box. Form = Formation; PY = Pingyuan; Rs = Reflectors. Fig. 3. The general sequence stratigraphic chart of the Oligocene Dongying Formation (E3d), Bohai Bay Basin (modified from Hu et al., 2000; Dong et al., 2011; Li et al., 2012; Zhu et al., 2014; Feng et al., 2016). Rs = Reflectors; LDW = Liaodongwan subbasin; JY = Jiyang subbasin; HH = Huanghua subbasin; BZ = Bozhong subbasin. The relative lake level of the Dongying Formation is modified from Zhu et al. (2014). In this chart, the relative lake level or sequence stratigraphic (Sq1, Sq1, etc.) is 3rd order.
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E3d2U and E3d1 (Hu et al., 2000; Zhu et al., 2014) (Fig. 3). In this study, we used the sequence stratigraphy scheme proposed by Zhu et al. (2014) (Fig. 3). 3. Data and methodology The 3D seismic reflection data, well logs and core data used in this study are provided by the Tianjin Branch of the China National Offshore Oil Corporation Ltd. The seismic data (Fig. 4), with grid spacing being 12.5 m × 25 m, covers the entire Shijiutuo Uplift and northwestern Bozhong subbasin. The target stratum spans the intervals between 2.5 and 3.0s two-way travel time (TWT). The dominant frequency is about 30 Hz in the interval of interest. Gamma-ray (GR), self-potential (SP) and VSP (Vertical Seismic Profiling) logs of key wells are used for stratigraphic and lithological analysis, and for correlation of the time to depth conversion. Regional unconformities and stratal terminations were recognized on seismic profiles, which provide key information of the sequence stratigraphic framework and systems tracts. Coherence and amplitude attributes are used to analyze the plan-view pattern of depositional systems. The coherence technique gives apparent continuity to discontinuous features such as faults and edges, and is sensitive to discontinuous bedding (Hart, 1999). Coherence cube slicing was conducted using Landmark seismic interpretation software. Root-meansquare (RMS) amplitude was extracted across certain time windows using Landmark software. Rock-Eval pyrolysis was conducted on more than 90 shale samples collected from drill cuttings from five wells (W01, W02, W03, W04 and W05) in the Bozhong subbasin (Fig. 1) using Rock-Eval 6 analyser. All the rock samples were cleaned prior to crushing and powdering. The analytical program of Rock-Eval pyrolysis was described by Espitalié et al. (1977).
Fig. 4. Formation thickness of the upper segment of the second member of the Dongying Formation (E3d2U). The location of this 3-D seismic data is shown in Fig. 1.
The Bohai Bay Basin consists of several subbasins. The Bozhong subbasin is one of the largest subbasins and is currently the offshore area of the Bohai Bay Basin (Fig. 1). In ascending order, the syn-rift sediments in the Bozhong subbasin are composed of the Kongdian (E1k), Shahejie (E2s4, E2s3, E2s2 and E2s1), and Dongying (E3d3, E3d2 and E3d1) Formations (Fig. 2). The post-rift sediments consist of the Guantao (N1g), Minghuazhen (N1mL, N2mU), and Pingyuan (Qp) formations (Fig. 2). These formations mainly contain fluival-lacustrine sediments (Gong, 1997). The exploration target stratigraphic interval is the Oligocene Dongying Formation (E3d). Previous studies suggested that the E3d formations have different sequence-stratigraphic architecture in each subbasin (Fig. 3, the hierarchical order is according to Vail et al. (1977) and Catuneanu (2006)). For instance, seven third-order sequences were recognized in the Liaodongwan subbasin (LDW), Bohai Bay Basin (Dong et al., 2011). Three and four third-order sequences were indentified in the Jiyang subbasin (JY) and Huanghua subbasin (HH) of Bohai Bay Basin, respectively (Feng et al., 2016) (Fig. 3). In the Bozhong subbasin (BZ), the E3d formations contain two thick sedimentary cycles (Li et al., 2012), i.e. the third member of the Dongying Formation (E3d3) and the lower segment of the second member of the Dongying Formation (E3d2L), and the upper segment of the second member of the Dongying Formation (E3d2U) and the first member of the Dongying Formation (E3d1) (Fig. 3). Based on the high-resolution sequence stratigraphy, they can be further sub-divided into four third-order sequences, namely Sq1, Sq2, Sq3, and Sq4, corresponding to four members E3d3, E3d2L,
4. Stratigraphic framework of the Dongying Formation 4.1. Stratal terminations and sequence stratigraphy In this study, we focused on the sequence stratigraphic architecture of the E3d2U, because the underlying E3d2L and E3d3 are thick mudstones and potential source rocks which will discuss in detail in the following section. Stratal terminations were originally defined by Mitchum et al. (1977) when interpreting seismic reflection profiles. Four stratal terminations can be used to identify sequence stratigraphic surfaces, i.e. onlap, downlap, toplap and truncation. Fig. 5 displays the key seismic reflectors (T3, T3m, T3a, T3u and T2) and the regional stratigraphic architecture (E3d3, E3d2L, E3d2U and E3d1) of the Dongying Formation (E3d). In order to establish the sequence-stratigraphic framework of the E3d2U, the detailed stratal terminations were observed (Fig. 6). The T3a interface (yellow dotted line) is the basal boundary of E3d2U. At the basin margin, there are distinct toplap and downlap terminations occurring below and above this surface, respectively (Fig. 6A). In the
Fig. 5. Seismic profile displaying the stratigraphic framework of the Dongying Formation (E3d). The location of seismic profile is shown in Fig. 4. 182
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Fig. 6. Stratal terminations and sequence-stratigraphic framework of the upper segment of the second member of the Dongying Formation (E3d2U). SB = sequence boundary, LST = lowstand system tract, MFS = maximum flooding surface, HST = highstand system tract. The location of seismic profile is shown in Fig. 5.
4.2. System tracts
basin center, this surface displays kame and kettle topography, which may be due to the action of erosion (Fig. 6B and C). Calibrated by VSP logs, the T3a interface separates E3d2U sandstone (high self-potential and low gamma-ray) above underlying E3d2L thick mudstone (low selfpotential and high gamma-ray) (Fig. 7). The T3u interface (green dotted line) is the top boundary of E3d2U. Obvious toplap and onlap terminations are present above and below this surface (Fig. 6A and B). Well logs also show a clear contrast in lithological association and lithological sequence (Fig. 7). Therefore, we consider the T3a and T3u interfaces as sequence boundaries (SB), and the E3d2U is a third-order sequence (according to its duration about 2 Ma (Vail et al., 1977; Catuneanu, 2006)). A highly continuous and high-amplitude reflection (blue dotted line), which pinches out between T3a and T3u, occurs in the E3d2U sequence (Fig. 6). There are obvious downlap terminations on this reflection interface (Fig. 6A and B). So, we consider this interface as maximum flooding surface (MFS).
System tracts were firstly defined on the basis of stratal stacking patterns interpreted from the architecture and lapout terminations of seismic reflections (Brown and Fisher, 1977; Catuneanu et al., 2009). Based on the stratal geometries and terminations, the E3d2U third-order sequence can be divided into three systems tracts: lowstand systems tract (LST), transgressive systems tract (TST), and highstand systems tract (HST) (Fig. 6). The LST is located in the lower part of the sequence (Fig. 6). At the basin margin, it is characterized by relatively high-continuous, highamplitude and prograding reflections (Fig. 6A). These reflections are indicative of a deltaic depositional system. In the basin center, the formations display discontinuous, high-amplitude and chaotic seismic reflections with strong incision (Fig. 6B and C), which may represent gravity flow deposits. The TST is below the maximum flooding surface (MFS) and is composed of highly continuous and high-amplitude reflections (Fig. 6). The thickness of the TST is relatively thin in the E3d2U sequence cycle. 183
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Fig. 7. Well logs and lithology characteristics of the Dongying Formation (E3d). GR = gamma-ray log, SP = self-potential log, SB = sequence boundary. Note the three inverted rhythmic units in E3d2U segment. The location of well 01 is shown in Fig. 6.
5. Gravity-flow deposits Within the sequence stratigraphic framework, we further discuss the depositional architecture of the Dongying Formation. Seismic geomorphology has been proven to be a powerful tool in the interpretation of subsurface geological conditions (Bertoni and Cartwright, 2005; Jackson et al., 2010; Back et al., 2011; Jackson and Lewis, 2012). The RMS amplitude (along the T3a) map could show the major seismic geomorphology and depositional system in the lowstand system tract (LST) of the E3d2U third-order sequence (Fig. 8). Mass-transport deposits (MTDs) and channel-lobe complexes are the main depositional units.
5.1. Mass-transport deposits (MTDs) Mass-transport deposits (MTDs) are defined as stratigraphic intervals that have been moved from their original deposition, for instance slides, slumps, debris flows, mass flows, and slope failure complexes (Moscardelli et al., 2006; Weimer et al., 2006; Moscardelli and Wood, 2008; Gamboa et al., 2010; Gamberi et al., 2011; Olafiranye et al., 2013; Zhang et al., 2016). The MTDs commonly have huge volumes and could easily be imaged on seismic surveys. As displayed in Fig. 9, the shapes of MTDs are variable and irregular, and they tend to be elongated downslope. For instance, the MTD 1 is about 8–10 km in the basinward direction and about 5 km in the depositional strike direction. Generally, the shapes reflect the transportation direction and distance (Weimer et al., 2006). The thickness of MTDs is about 200 m, calculated by an average velocity of 2700 m/s (Fig. 10). The seismic facies of MTDs are characterized by hummocky and chaotic reflections with poor reflector continuity and variable amplitude (Fig. 10). The basal surface of MTDs has considerable erosional, mounded relief resulting from removal of the underlying sediment (Weimer et al., 2006). A series of normal (extensional) to reverse (contractional) faults occur on the basal surface (Fig. 10). These faults are characterized by lineations with various widths and lengths (Fig. 9). Those curvilinear lines often mark the trace
Fig. 8. Root mean square (RMS) attribute of the T3a interface. The location of RMS map is shown in Fig. 4.
The HST is bounded by the maximum flooding surface (MFS) and sequence boundary (SB) (Fig. 6). Well logs show that the HST can be divided into three upward-coarsening reverse graded cycles (Figs. 6C and 7). Upward-coarsening can be indicative of prograding shoreface systems or deltaic depositional systems. Besides, the HST is characterized by large-scale prograding reflections (Fig. 6A and B). Therefore, the HST is represented by a deltaic depositional system.
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confirmed to be effective source rocks in the Bozhong subbasin (Hao et al., 2009, 2011). In this study, we evaluate the quality of the E3d3 and E3d2L source rocks, which have potential contribution to the E3d2U gravity-flow reservoirs. Rock-Eval pyrolysis is a commonly used method to define organic matter (OM) types and assess the potential of hydrocarbon generation. The E3d3 and E3d2L source rocks show wide variations in total organic carbon (TOC) contents, Rock-Eval S2 peaks, and hydrogen indices (HI) (Fig. 13). The E3d3 source rock has TOC contents between 0.6 and 3.1% and Rock-Eval S2 values between 0.7 and 19.8 mg HC/g rock (Fig. 13). Hydrogen indices of E3d3 samples range from 100 to 600 mg/g TOC. The E3d2L source rock has TOC contents ranging from 0.5 to 1.9%, Rock-Eval S2 values ranging from 0.4 to 7.8 mg HC/g rock, and HI from 40 to 480 mg/g TOC (Fig. 13). As shown in Fig. 1, all the sampling wells are located at the basin margin, and we infer that source rocks in the study area should have better quality than pyrolysis-tested samples. Thus, both E3d3 and E3d2L source rocks have good hydrocarbon-generation potential. 7. Discussion 7.1. Genesis of the gravity-flow deposits Mass-transport deposits (MTDs) are commonly observed along deepwater margins, constituting more than 50% of the depositional sequences in some deepwater areas (Beaubouef et al., 2003; McGilvery and Cook, 2003; Newton et al., 2004). Weimer et al. (2006) summarized the triggers for MTDs, including high sedimentation rate, submarine canyon formation, gas-hydrate decompression, deepwater currents, earthquakes, and meteorite impacts. In lacustrine basins, we mainly discuss the lake level changes and sediment supply that control the development of the MTDs. Allocyclic (allogenic) factors, such as eustasy, tectonics and climate, control the relative sea level changes and sediment supply (Einsele et al., 1991; Catuneanu and Zecchin, 2013). As discussed in the previous section, the MTDs developed within the lowstand systems tract (LST) of the E3d2U third-order sequence. Some key features are observed: (1) the MTDs overlie an erosional unconformable surface (T3a), (2) the MTDs erode the underlying E3d2L thick mudstone which was deposited during relative highstand in the lake level, and (3) the MTDs are associated with incising channels. Previous studies also suggest that only a few MTDs can develop during transgressive and highstand times, and most MTDs occur primarily at the base of a depositional sequence as part of the earliest lowstand systems tract (Weimer et al., 2006; Gong et al., 2014). Thus, the lake level changes play an important role on the development of the MTDs. High sedimentation rate has been proposed as a contributing factor to the formation of MTDs in many different settings (Coleman et al., 1983). Based on the formation thickness, we reconstructed the palaeogeomorphology of E3d2U, such as the uplifts, slopes and depocenters (Fig. 4). The Shijiutuo Uplift was a long-term structural high which had topographic relief until the Oligocene, and it was the major sediment source to the study area (Fig. 4). As displayed in Fig. 12, delta deposits developed at the basin margin and prograded from north to south (Fig. 12). The delta could offer abundant sediment to the slope. MTDs commonly develop from the failure of the delta front and slope (Weimer et al., 2006; Zhang et al., 2016). Larger and thicker complexes are associated with large head scarps at the slope margins (Weimer et al., 2006). In our study, the MTDs mainly occurred on the slope associated with high sediment supply (Fig. 12). Channels could serve as significant conduits for terrigenous clastics transported to the basin floor. The main gravity-flow types in channels include slides, slumps, debris flows, and turbidity currents (Shanmugam, 2000; Hasiotis et al., 2005; Shanmugam et al., 2009). Some channels can develop during highstand of the sea level, while most channels are interpreted to have formed during periods of relative
Fig. 9. Coherence slice and root mean square (RMS) attribute of the masstransport deposits (MTDs). The locations of slices are shown in Fig. 8.
of thrust faults within the MTD and often point in the direction of transport. The upper surface of the MTDs is irregular (Fig. 10), since it is often altered by channel systems (Weimer et al., 2006). 5.2. Channel-lobe complexes Channels are geomorphic units maintained by turbidity flows and serve as long-term pathways for sediment transport (Mutti and Normark, 1991). In plan view, the channels show linear geomorphic features with weak root-mean-square (RMS) amplitude signatures (Figs. 8 and 9). On seismic profiles, the channels incise into the underlying E3d2L thick mudstone which are dominated by low-frequency and low-amplitude reflections (Fig. 11A). Lobes are formed as the sediments that have bypassed through channels and are deposited in an unconfined setting at the terminus of channels (Mutti and Normark, 1987, 1991; Weimer et al., 2006). In plan view, the lobes occur at the terminal channel and display strong sheet-like RMS amplitude (Figs. 8 and 12). On seismic profiles, the lobes appear as highly continuous and high-amplitude reflections and pinch out laterally (Fig. 11B and C). 6. Source rocks Three hydrogen-rich, oil-prone source rocks have developed in the E2s3, E2s1, and E3d3 formations (Fig. 2). All of them have been 185
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Fig. 10. Seismic profile showing internal architecture of the mass-transport deposits (MTDs). The location of seismic profile is shown in Fig. 9.
as the lake level rise (e.g. the transgressive system tract). Lobes or sheet sandstone are considered to be excellent reservoirs (Saller et al., 2004), accounting for approximately 60%–85% of the production in the northern deep Gulf of Mexico (Pacht et al., 1992; Lawrence and Bosman-Smits, 2000). This is due to their good lateral continuity, good vertical connectivity, high aspect ratio, narrow range in grain size, and few erosional features (Weimer et al., 2006). As displayed in Fig. 11, the lobes show highly-continuous, high-amplitude reflection. They display reverse reflection characteristics with the underlying mudstone (Fig. 11). Thus, the lobes are mainly dominated by sandstone.
sea-level lowstand, when large volumes of coarse-grained sediment could erode the slope and pass through to the basin floor (Weimer et al., 2006). The MTDs occur together with the channels (Fig. 12), and the surfaces of MTDs are partly channelized (Fig. 10). We propose the channels formed in the same gravity-flow background (low lake level and high sediment supply) as MTDs.
7.2. Exploration prospects of the gravity-flow deposits 7.2.1. Sandstone reservoirs Most MTDs are shale-rich and considered to be poor reservoirs (Moscardelli et al., 2006; Weimer et al., 2006; Gong et al., 2014), because they formed from the failures of deepwater mud-dominated slopes. Only a few examples of sand-prone MTDs have been reported (Shanmugam et al., 2009; Loucks et al., 2011). Zhang et al. (2016) recognized four sets of lacustrine mass-transport complexes in the Songliao Basin, which formed due to the progradation and slumping of the pro-delta. In our study area, the Shijiutuo Uplift supplied sufficient sediments into the slope and basin. Large-scale deltas have been recognized in the lowstand system tract (LST) of the E3d2U sequence (Fig. 6). Thus, the MTDs are likely sand-dominated and could serve as significant petroleum reservoirs if the sorting is not very poor. Gravity-flow channels (channel fills) are one of the major sand-rich units in deepwater environments. They are generally considered to be good reservoirs and primary exploration targets. However, in our study area, the channel fills display similar low-amplitude reflection characteristics as the underlying thick mudstone (Fig. 11). Therefore, they are likely mud-dominated. It is possible that the channels served as conduits for sediment transport during the low lake level stage, and the sediment supply was limited and the channels were filled by mudstone
7.2.2. Petroleum prospects Hao et al. (2007, 2010) measured the vitrinite reflectance (Ro) of more than 200 samples (Fig. 14). Although there is a relatively wide variation of measured Ro values at the same burial depth, the measured Ro values increase regularly with burial depth down to about 4200 m (Fig. 14). The Bozhong sub-basin has a relatively high thermal gradient of approximately 33 °C/km (Hao et al., 2010). Measured Ro reaches 0.6%, and Tmax reaches 435 °C at about 2700 m, suggesting that source rocks below 2700 m are mature for oil generation (Hao et al., 2010). However, the burial depths of E3d3 and E3d2L source rocks are deeper than 4170 m (Fig. 15). Based on the trend of thermal maturity with depth, the E3d3 and E3d2L source rocks have a high maturity that vitrinite reflectance (Ro) reaches 1.0–1.3%. Thus, E3d3 and E3d2L source rocks have entered the peak of oil generation, and they could provide abundant oil sources for the overlying E3d2U gravity-flow reservoirs (Fig. 15). Fine-grained deposits in the transgressive system tract (TST) could serve as effective seal for the gravity-flow reservoirs in the lowstand 186
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Fig. 11. Seismic profiles showing the channels and lobes. The locations of seismic profiles are shown in Fig. 8.
however, sealing formations with thickness less than 10 m are still able to reserve a significant volume of hydrocarbons (Jiang, 1998). The pressure coefficient of the E3d2L source rocks ranges from 1.52 to 1.75 with a mean value of 1.67 (Xu et al., 2015). We basically defined overpressure as the pressure coefficient is higher than 1.06. Pressure coefficient between 1.06 and 1.27, 1.27 and 1.73 and higher than 1.73 are defined as weak overpressure, overpressure and strong overpressure, respectively. In the study area, the formation pressure is from overpressure to strong overpressure. The hyperpressure can promote the discharge and migration of generated hydrocarbon. Therefore, oils generated from underlying rocks experience primary migration and short-distance secondary migration into the overlying traps (Fig. 15). Recent studies show that the gravity-flow sandstone have proven to be important hydrocarbon reservoirs on the Qinan Slope, Huanghua Depression of the Bohai Bay Basin (Liu et al., 2016). In our study, the gravity-flow reservoirs could serve as potential and effective hydrocarbon exploration targets, and the Oligocene Dongying Formation (E3d) could be significant deep targets in the future. 8. Conclusions (1) Mass-transport deposits (MTDs) and channel-lobe complexes are the main gravity-flow depositional units in the E3d2U formations. The mass-transport deposits (MTDs) show hummocky and chaotic reflections with poor continuity and variable amplitude. Their basal surfaces have considerable erosional relief. The channels are dominated by linear geomorphology with weak root-mean-square (RMS) amplitude and incise into the underlying thick mudstone. The lobes display high-continuous and high-amplitude reflection at the terminal channels. (2) The genesis of gravity-flow deposits is primarily controlled by high sedimentation rate during a relative lake-level lowstand. The
Fig. 12. The depositional systems of lowstand system tract (LST) of the E3d2U sequence. Note the slumps, channels and lobes.
system tract (LST) (Catuneanu, 2006; Zecchin and Catuneanu, 2015). The TST mudstone is not very thick in the E3d2U sequence cycle (Fig. 6). Basically, thick seals are beneficial to oil and gas preservation,
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Fig. 13. (A) Variation of Rock-Eval hydrogen index with Tmax for samples from E3d3 and E3d2L source rocks in the Bozhong subbasin, Bohai Bay Basin. The Tmax is the temperature at which the maximum amount of S2 hydrocarbons (HC3) is generated. Ro = vitrinite reflectance. (B) Total organic carbon (TOC) contents versus Rock-Eval S2 peaks for samples from the Bozhong subbasin, Bohai Bay Basin, showing the hydrocarbon (HC)-generation potentials.
lacustrine sand-rich MTDs were dominated by abundant sediment supply from proximal deltas. Channels serve as significant conduits for terrigenous clastics transport, and sheet-sandstone lobes formed in an unconfined depositional setting at the end of the channels. (3) The E3d3 source rock has TOC contents between 0.6 and 3.1%, Rock-Eval S2 values between 0.7 and 19.8 mg HC/g rock, and Hydrogen indices between 100 and 600 mg/g TOC. The E3d2L source rock has TOC contents ranging from 0.5 to 1.9%, Rock-Eval S2 values from 0.4 to 7.8 mg HC/g rock, and HI from 40 to 480 mg/ g TOC. Both E3d3 and E3d2L source rocks have good hydrocarbongeneration potential. (4) Gravity-flow reservoirs are favorable lithologic traps. The underlying E3d3 and E3d2L source rocks have entered the peak of oil generation, therefore, they could provide abundant oil sources. Oils generated from source rocks migrate shortly into the overlying traps. Thus, the gravity-flow reservoirs could serve as potential and effective hydrocarbon exploration targets in our study area.
Acknowledgements This research was financially funded by the National Natural Science Foundation of China (41690131, 41690134, 41702155), National Science & Technology Specific Project (2016ZX05024-003008) and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (CUG160605). We thank MPG Editor Dr. Massimo Zecchin, Associate Editor Dr. Sven O. Egenhoff and anonymous reviewers for their thorough and critical reviews and suggestions to improve the manuscript. We appreciate the enthusiastic support of Dr. Tian Dong at University of Geosciences (Wuhan).
Fig. 14. Variation of measured vitrinite reflectance with depth in the Bozhong sub-basin, Bohai Bay basin (modified from Hao et al., 2007, 2010).
Fig. 15. Gravity-flow deposits and their petroleum prospects in the Dongying Formation (E3d), Bozhong Subbasin, Bohai Bay Basin. These lines come from the depth of well W01, and the %Ro is estimated by the relationship of depth and vitrinite reflectance (see Fig. 14). Note the thermal maturity of E3d3 and E3d2L source rocks (Ro > 1.0%).
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Research paper
Gravity-flow deposits and their exploration prospects in the Oligocene Dongying Formation, northwestern Bozhong Subbasin, Bohai Bay Basin, China
T
Shang Xua,∗, Fang Haob,a,∗∗, Changgui Xuc, Huayao Zoud, Xintao Zhangc, Yuanyin Zhange, Baishui Gaof, Qi Wangb a
Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan 430074, China School of Geosciences, China University of Petroleum, Qingdao 266580, China Tianjin Branch of China National Offshore Oil Company Ltd, Tianjin 300452, China d State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China e Oil and Gas Survey, China Geological Survey, Beijing 100037, China f Strategic Research Center of Oil and Gas Resources, MLR, Beijing 100034, China b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Gravity-flow deposits Mass-transport deposits Channel-lobe complexes Exploration prospects Lacustrine basin
Gravity-flow deposits are important hydrocarbon exploration targets in most basins. In this study, an integration of geological, geophysical, and geochemical analysis is used to investigate the gravity-flow deposits and their petroleum prospects in the Oligocene Dongying Formation (E3d), Bozhong Subbasin, Bohai Bay Basin. Masstransport deposits (MTDs) and channel-lobe complexes are the main gravity-flow depositional units. The masstransport deposits (MTDs) show hummocky and chaotic reflections with poor continuity and variable amplitude. The channels display linear geomorphology with weak root-mean-square (RMS) amplitude, and the lobes are dominated by high-continuous and high-amplitude reflection at the terminal channels. The formation of gravityflow deposits is primarily controlled by high sedimentation rate during a relative lowstand of lake level. The E3d3 source rock has TOC contents in the range of 0.6–3.1%, Rock-Eval S2 values in the range of 0.7–19.8 mg HC/g rock, and Hydrogen indices (HI) in the range of 100–600 mg/g TOC. The E3d2L source rock has TOC contents ranging from 0.5 to 1.9%, Rock-Eval S2 values from 0.4 to 7.8 mg HC/g rock, and HI from 40 to 480 mg/g TOC. Both E3d3 and E3d2L source rocks have good hydrocarbon-generation potential. The E3d3 and E3d2L source rocks have entered the oil-generation peak window with the vitrinite reflectance (Ro) higher than 1.0%. Oils generated from the underlying source rocks shortly migrated to the overlying gravity-flow lithologic traps. The gravity-flow reservoirs could serve as potential and effective hydrocarbon exploration targets.
1. Introduction The Bohai Bay Basin is one of China's most petroliferous basins located on its east coast (Fig. 1), accounting for nearly one-third of the total oil production of the country (Hao et al., 2009). The Bozhong subbasin is the largest generative kitchen in the offshore area (Hao et al., 2009) (Fig. 1). Previous petroleum exploration has mainly focused on the shallow Neogene reservoirs, and more than 25 × 108 tons (18.3 × 109 bbl) reserves have been proven in the Bozhong sub-basin (Hao et al., 2009; Gong et al., 2010). With exploration entering the mature phase, deep reservoirs, such as the Oligocene Dongying Formation (E3d), will be important exploration targets in the future.
∗
Deepwater gravity-flow deposits have received considerable attention in the petroleum industry. Most hydrocarbon reserves are within deepwater gravity-flow reservoirs, such as channel-fill, levee, sheets, and mass-transport deposits (Weimer et al., 2006; Shanmugam et al., 2009; Gong et al., 2014). Approximately 60% of the oil production in the northern deep Gulf of Mexico is from sheet sands, 25% is from channel-fill deposits, and 15% is from thin beds in levees (Lawrence and Bosman-Smits, 2000). Porosity and permeability in deepwater gravityflow reservoirs are generally high, because they are commonly fed from mature river systems (Weimer et al., 2006). Lacustrine gravity-flow deposits are also effective hydrocarbon reservoirs (Feng et al., 2010; Zhang et al., 2016; Liu et al., 2016; Corella et al., 2016), although
Corresponding author. Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Lumo Road 388, Wuhan 430074, China. Corresponding author. School of Geosciences, China University of Petroleum, Changjiangxi Road No. 66, Qingdao 266580, China. E-mail addresses: [email protected] (S. Xu), [email protected] (F. Hao).
∗∗
https://doi.org/10.1016/j.marpetgeo.2018.06.001 Received 9 February 2017; Received in revised form 1 June 2018; Accepted 1 June 2018 Available online 02 June 2018 0264-8172/ © 2018 Elsevier Ltd. All rights reserved.
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Fig. 1. (A) Location map showing the Bohai Bay Basin. (B) Location of the study area in the Bozhong subbasin, Bohai Bay Basin.
fluvial and delta sandstones are major reservoir types in lacustrine basin (Gong, 1997). For instance, Liu et al. (2016) recently recognized a channel complex in the Huanghua Depression, Bohai Bay Basin, and the channel sandstone has proven to be important hydrocarbon reservoirs. Zhang et al. (2016) described four sets of lacustrine mass-transport complexes, which could serve as potential exploration targets in the Songliao Basin, northeastern China. In the Bozhong Subbasin, Bohai Bay Basin, the Dongying Formation (E3d) was deposited in the deep to shallow lacustrine environments, containing a series of gravity-flow depositional systems (Gong, 1997; Dong et al., 2011). The purpose of this paper is to investigate the gravity-flow deposits and their petroleum prospects in the E3d formations by integrated geological, geophysical, and geochemical data.
2. Geological setting The Bohai Bay Basin has an area of approximately 200,000 km2. Above the Cenozoic basement, this basin filling can be sub-divided into two tectono-stratigraphic units (Fig. 2): a rifting stage during the Paleogene and a thermal subsidence stage during the Neogene and Quaternary (Huang and Pearson, 1999; Yang and Xu, 2004; Qi and Yang, 2010; Huang et al., 2012). During the synrift stage (60.5–24.6 Ma), a series of grabens and half grabens developed along major NW and NE trending fault sets (Huang and Pearson, 1999). These grabens and half grabens progressively became one large basin during the late Oligocene, and the Bohai Bay Basin entered the thermal subsidence stage (24.6 Ma to the present). 180
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Fig. 2. Generalized stratigraphy of the Bohai Bay basin (modified from Hao et al., 2009). The target stratum is showing in the black dashed box. Form = Formation; PY = Pingyuan; Rs = Reflectors. Fig. 3. The general sequence stratigraphic chart of the Oligocene Dongying Formation (E3d), Bohai Bay Basin (modified from Hu et al., 2000; Dong et al., 2011; Li et al., 2012; Zhu et al., 2014; Feng et al., 2016). Rs = Reflectors; LDW = Liaodongwan subbasin; JY = Jiyang subbasin; HH = Huanghua subbasin; BZ = Bozhong subbasin. The relative lake level of the Dongying Formation is modified from Zhu et al. (2014). In this chart, the relative lake level or sequence stratigraphic (Sq1, Sq1, etc.) is 3rd order.
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E3d2U and E3d1 (Hu et al., 2000; Zhu et al., 2014) (Fig. 3). In this study, we used the sequence stratigraphy scheme proposed by Zhu et al. (2014) (Fig. 3). 3. Data and methodology The 3D seismic reflection data, well logs and core data used in this study are provided by the Tianjin Branch of the China National Offshore Oil Corporation Ltd. The seismic data (Fig. 4), with grid spacing being 12.5 m × 25 m, covers the entire Shijiutuo Uplift and northwestern Bozhong subbasin. The target stratum spans the intervals between 2.5 and 3.0s two-way travel time (TWT). The dominant frequency is about 30 Hz in the interval of interest. Gamma-ray (GR), self-potential (SP) and VSP (Vertical Seismic Profiling) logs of key wells are used for stratigraphic and lithological analysis, and for correlation of the time to depth conversion. Regional unconformities and stratal terminations were recognized on seismic profiles, which provide key information of the sequence stratigraphic framework and systems tracts. Coherence and amplitude attributes are used to analyze the plan-view pattern of depositional systems. The coherence technique gives apparent continuity to discontinuous features such as faults and edges, and is sensitive to discontinuous bedding (Hart, 1999). Coherence cube slicing was conducted using Landmark seismic interpretation software. Root-meansquare (RMS) amplitude was extracted across certain time windows using Landmark software. Rock-Eval pyrolysis was conducted on more than 90 shale samples collected from drill cuttings from five wells (W01, W02, W03, W04 and W05) in the Bozhong subbasin (Fig. 1) using Rock-Eval 6 analyser. All the rock samples were cleaned prior to crushing and powdering. The analytical program of Rock-Eval pyrolysis was described by Espitalié et al. (1977).
Fig. 4. Formation thickness of the upper segment of the second member of the Dongying Formation (E3d2U). The location of this 3-D seismic data is shown in Fig. 1.
The Bohai Bay Basin consists of several subbasins. The Bozhong subbasin is one of the largest subbasins and is currently the offshore area of the Bohai Bay Basin (Fig. 1). In ascending order, the syn-rift sediments in the Bozhong subbasin are composed of the Kongdian (E1k), Shahejie (E2s4, E2s3, E2s2 and E2s1), and Dongying (E3d3, E3d2 and E3d1) Formations (Fig. 2). The post-rift sediments consist of the Guantao (N1g), Minghuazhen (N1mL, N2mU), and Pingyuan (Qp) formations (Fig. 2). These formations mainly contain fluival-lacustrine sediments (Gong, 1997). The exploration target stratigraphic interval is the Oligocene Dongying Formation (E3d). Previous studies suggested that the E3d formations have different sequence-stratigraphic architecture in each subbasin (Fig. 3, the hierarchical order is according to Vail et al. (1977) and Catuneanu (2006)). For instance, seven third-order sequences were recognized in the Liaodongwan subbasin (LDW), Bohai Bay Basin (Dong et al., 2011). Three and four third-order sequences were indentified in the Jiyang subbasin (JY) and Huanghua subbasin (HH) of Bohai Bay Basin, respectively (Feng et al., 2016) (Fig. 3). In the Bozhong subbasin (BZ), the E3d formations contain two thick sedimentary cycles (Li et al., 2012), i.e. the third member of the Dongying Formation (E3d3) and the lower segment of the second member of the Dongying Formation (E3d2L), and the upper segment of the second member of the Dongying Formation (E3d2U) and the first member of the Dongying Formation (E3d1) (Fig. 3). Based on the high-resolution sequence stratigraphy, they can be further sub-divided into four third-order sequences, namely Sq1, Sq2, Sq3, and Sq4, corresponding to four members E3d3, E3d2L,
4. Stratigraphic framework of the Dongying Formation 4.1. Stratal terminations and sequence stratigraphy In this study, we focused on the sequence stratigraphic architecture of the E3d2U, because the underlying E3d2L and E3d3 are thick mudstones and potential source rocks which will discuss in detail in the following section. Stratal terminations were originally defined by Mitchum et al. (1977) when interpreting seismic reflection profiles. Four stratal terminations can be used to identify sequence stratigraphic surfaces, i.e. onlap, downlap, toplap and truncation. Fig. 5 displays the key seismic reflectors (T3, T3m, T3a, T3u and T2) and the regional stratigraphic architecture (E3d3, E3d2L, E3d2U and E3d1) of the Dongying Formation (E3d). In order to establish the sequence-stratigraphic framework of the E3d2U, the detailed stratal terminations were observed (Fig. 6). The T3a interface (yellow dotted line) is the basal boundary of E3d2U. At the basin margin, there are distinct toplap and downlap terminations occurring below and above this surface, respectively (Fig. 6A). In the
Fig. 5. Seismic profile displaying the stratigraphic framework of the Dongying Formation (E3d). The location of seismic profile is shown in Fig. 4. 182
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Fig. 6. Stratal terminations and sequence-stratigraphic framework of the upper segment of the second member of the Dongying Formation (E3d2U). SB = sequence boundary, LST = lowstand system tract, MFS = maximum flooding surface, HST = highstand system tract. The location of seismic profile is shown in Fig. 5.
4.2. System tracts
basin center, this surface displays kame and kettle topography, which may be due to the action of erosion (Fig. 6B and C). Calibrated by VSP logs, the T3a interface separates E3d2U sandstone (high self-potential and low gamma-ray) above underlying E3d2L thick mudstone (low selfpotential and high gamma-ray) (Fig. 7). The T3u interface (green dotted line) is the top boundary of E3d2U. Obvious toplap and onlap terminations are present above and below this surface (Fig. 6A and B). Well logs also show a clear contrast in lithological association and lithological sequence (Fig. 7). Therefore, we consider the T3a and T3u interfaces as sequence boundaries (SB), and the E3d2U is a third-order sequence (according to its duration about 2 Ma (Vail et al., 1977; Catuneanu, 2006)). A highly continuous and high-amplitude reflection (blue dotted line), which pinches out between T3a and T3u, occurs in the E3d2U sequence (Fig. 6). There are obvious downlap terminations on this reflection interface (Fig. 6A and B). So, we consider this interface as maximum flooding surface (MFS).
System tracts were firstly defined on the basis of stratal stacking patterns interpreted from the architecture and lapout terminations of seismic reflections (Brown and Fisher, 1977; Catuneanu et al., 2009). Based on the stratal geometries and terminations, the E3d2U third-order sequence can be divided into three systems tracts: lowstand systems tract (LST), transgressive systems tract (TST), and highstand systems tract (HST) (Fig. 6). The LST is located in the lower part of the sequence (Fig. 6). At the basin margin, it is characterized by relatively high-continuous, highamplitude and prograding reflections (Fig. 6A). These reflections are indicative of a deltaic depositional system. In the basin center, the formations display discontinuous, high-amplitude and chaotic seismic reflections with strong incision (Fig. 6B and C), which may represent gravity flow deposits. The TST is below the maximum flooding surface (MFS) and is composed of highly continuous and high-amplitude reflections (Fig. 6). The thickness of the TST is relatively thin in the E3d2U sequence cycle. 183
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Fig. 7. Well logs and lithology characteristics of the Dongying Formation (E3d). GR = gamma-ray log, SP = self-potential log, SB = sequence boundary. Note the three inverted rhythmic units in E3d2U segment. The location of well 01 is shown in Fig. 6.
5. Gravity-flow deposits Within the sequence stratigraphic framework, we further discuss the depositional architecture of the Dongying Formation. Seismic geomorphology has been proven to be a powerful tool in the interpretation of subsurface geological conditions (Bertoni and Cartwright, 2005; Jackson et al., 2010; Back et al., 2011; Jackson and Lewis, 2012). The RMS amplitude (along the T3a) map could show the major seismic geomorphology and depositional system in the lowstand system tract (LST) of the E3d2U third-order sequence (Fig. 8). Mass-transport deposits (MTDs) and channel-lobe complexes are the main depositional units.
5.1. Mass-transport deposits (MTDs) Mass-transport deposits (MTDs) are defined as stratigraphic intervals that have been moved from their original deposition, for instance slides, slumps, debris flows, mass flows, and slope failure complexes (Moscardelli et al., 2006; Weimer et al., 2006; Moscardelli and Wood, 2008; Gamboa et al., 2010; Gamberi et al., 2011; Olafiranye et al., 2013; Zhang et al., 2016). The MTDs commonly have huge volumes and could easily be imaged on seismic surveys. As displayed in Fig. 9, the shapes of MTDs are variable and irregular, and they tend to be elongated downslope. For instance, the MTD 1 is about 8–10 km in the basinward direction and about 5 km in the depositional strike direction. Generally, the shapes reflect the transportation direction and distance (Weimer et al., 2006). The thickness of MTDs is about 200 m, calculated by an average velocity of 2700 m/s (Fig. 10). The seismic facies of MTDs are characterized by hummocky and chaotic reflections with poor reflector continuity and variable amplitude (Fig. 10). The basal surface of MTDs has considerable erosional, mounded relief resulting from removal of the underlying sediment (Weimer et al., 2006). A series of normal (extensional) to reverse (contractional) faults occur on the basal surface (Fig. 10). These faults are characterized by lineations with various widths and lengths (Fig. 9). Those curvilinear lines often mark the trace
Fig. 8. Root mean square (RMS) attribute of the T3a interface. The location of RMS map is shown in Fig. 4.
The HST is bounded by the maximum flooding surface (MFS) and sequence boundary (SB) (Fig. 6). Well logs show that the HST can be divided into three upward-coarsening reverse graded cycles (Figs. 6C and 7). Upward-coarsening can be indicative of prograding shoreface systems or deltaic depositional systems. Besides, the HST is characterized by large-scale prograding reflections (Fig. 6A and B). Therefore, the HST is represented by a deltaic depositional system.
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confirmed to be effective source rocks in the Bozhong subbasin (Hao et al., 2009, 2011). In this study, we evaluate the quality of the E3d3 and E3d2L source rocks, which have potential contribution to the E3d2U gravity-flow reservoirs. Rock-Eval pyrolysis is a commonly used method to define organic matter (OM) types and assess the potential of hydrocarbon generation. The E3d3 and E3d2L source rocks show wide variations in total organic carbon (TOC) contents, Rock-Eval S2 peaks, and hydrogen indices (HI) (Fig. 13). The E3d3 source rock has TOC contents between 0.6 and 3.1% and Rock-Eval S2 values between 0.7 and 19.8 mg HC/g rock (Fig. 13). Hydrogen indices of E3d3 samples range from 100 to 600 mg/g TOC. The E3d2L source rock has TOC contents ranging from 0.5 to 1.9%, Rock-Eval S2 values ranging from 0.4 to 7.8 mg HC/g rock, and HI from 40 to 480 mg/g TOC (Fig. 13). As shown in Fig. 1, all the sampling wells are located at the basin margin, and we infer that source rocks in the study area should have better quality than pyrolysis-tested samples. Thus, both E3d3 and E3d2L source rocks have good hydrocarbon-generation potential. 7. Discussion 7.1. Genesis of the gravity-flow deposits Mass-transport deposits (MTDs) are commonly observed along deepwater margins, constituting more than 50% of the depositional sequences in some deepwater areas (Beaubouef et al., 2003; McGilvery and Cook, 2003; Newton et al., 2004). Weimer et al. (2006) summarized the triggers for MTDs, including high sedimentation rate, submarine canyon formation, gas-hydrate decompression, deepwater currents, earthquakes, and meteorite impacts. In lacustrine basins, we mainly discuss the lake level changes and sediment supply that control the development of the MTDs. Allocyclic (allogenic) factors, such as eustasy, tectonics and climate, control the relative sea level changes and sediment supply (Einsele et al., 1991; Catuneanu and Zecchin, 2013). As discussed in the previous section, the MTDs developed within the lowstand systems tract (LST) of the E3d2U third-order sequence. Some key features are observed: (1) the MTDs overlie an erosional unconformable surface (T3a), (2) the MTDs erode the underlying E3d2L thick mudstone which was deposited during relative highstand in the lake level, and (3) the MTDs are associated with incising channels. Previous studies also suggest that only a few MTDs can develop during transgressive and highstand times, and most MTDs occur primarily at the base of a depositional sequence as part of the earliest lowstand systems tract (Weimer et al., 2006; Gong et al., 2014). Thus, the lake level changes play an important role on the development of the MTDs. High sedimentation rate has been proposed as a contributing factor to the formation of MTDs in many different settings (Coleman et al., 1983). Based on the formation thickness, we reconstructed the palaeogeomorphology of E3d2U, such as the uplifts, slopes and depocenters (Fig. 4). The Shijiutuo Uplift was a long-term structural high which had topographic relief until the Oligocene, and it was the major sediment source to the study area (Fig. 4). As displayed in Fig. 12, delta deposits developed at the basin margin and prograded from north to south (Fig. 12). The delta could offer abundant sediment to the slope. MTDs commonly develop from the failure of the delta front and slope (Weimer et al., 2006; Zhang et al., 2016). Larger and thicker complexes are associated with large head scarps at the slope margins (Weimer et al., 2006). In our study, the MTDs mainly occurred on the slope associated with high sediment supply (Fig. 12). Channels could serve as significant conduits for terrigenous clastics transported to the basin floor. The main gravity-flow types in channels include slides, slumps, debris flows, and turbidity currents (Shanmugam, 2000; Hasiotis et al., 2005; Shanmugam et al., 2009). Some channels can develop during highstand of the sea level, while most channels are interpreted to have formed during periods of relative
Fig. 9. Coherence slice and root mean square (RMS) attribute of the masstransport deposits (MTDs). The locations of slices are shown in Fig. 8.
of thrust faults within the MTD and often point in the direction of transport. The upper surface of the MTDs is irregular (Fig. 10), since it is often altered by channel systems (Weimer et al., 2006). 5.2. Channel-lobe complexes Channels are geomorphic units maintained by turbidity flows and serve as long-term pathways for sediment transport (Mutti and Normark, 1991). In plan view, the channels show linear geomorphic features with weak root-mean-square (RMS) amplitude signatures (Figs. 8 and 9). On seismic profiles, the channels incise into the underlying E3d2L thick mudstone which are dominated by low-frequency and low-amplitude reflections (Fig. 11A). Lobes are formed as the sediments that have bypassed through channels and are deposited in an unconfined setting at the terminus of channels (Mutti and Normark, 1987, 1991; Weimer et al., 2006). In plan view, the lobes occur at the terminal channel and display strong sheet-like RMS amplitude (Figs. 8 and 12). On seismic profiles, the lobes appear as highly continuous and high-amplitude reflections and pinch out laterally (Fig. 11B and C). 6. Source rocks Three hydrogen-rich, oil-prone source rocks have developed in the E2s3, E2s1, and E3d3 formations (Fig. 2). All of them have been 185
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Fig. 10. Seismic profile showing internal architecture of the mass-transport deposits (MTDs). The location of seismic profile is shown in Fig. 9.
as the lake level rise (e.g. the transgressive system tract). Lobes or sheet sandstone are considered to be excellent reservoirs (Saller et al., 2004), accounting for approximately 60%–85% of the production in the northern deep Gulf of Mexico (Pacht et al., 1992; Lawrence and Bosman-Smits, 2000). This is due to their good lateral continuity, good vertical connectivity, high aspect ratio, narrow range in grain size, and few erosional features (Weimer et al., 2006). As displayed in Fig. 11, the lobes show highly-continuous, high-amplitude reflection. They display reverse reflection characteristics with the underlying mudstone (Fig. 11). Thus, the lobes are mainly dominated by sandstone.
sea-level lowstand, when large volumes of coarse-grained sediment could erode the slope and pass through to the basin floor (Weimer et al., 2006). The MTDs occur together with the channels (Fig. 12), and the surfaces of MTDs are partly channelized (Fig. 10). We propose the channels formed in the same gravity-flow background (low lake level and high sediment supply) as MTDs.
7.2. Exploration prospects of the gravity-flow deposits 7.2.1. Sandstone reservoirs Most MTDs are shale-rich and considered to be poor reservoirs (Moscardelli et al., 2006; Weimer et al., 2006; Gong et al., 2014), because they formed from the failures of deepwater mud-dominated slopes. Only a few examples of sand-prone MTDs have been reported (Shanmugam et al., 2009; Loucks et al., 2011). Zhang et al. (2016) recognized four sets of lacustrine mass-transport complexes in the Songliao Basin, which formed due to the progradation and slumping of the pro-delta. In our study area, the Shijiutuo Uplift supplied sufficient sediments into the slope and basin. Large-scale deltas have been recognized in the lowstand system tract (LST) of the E3d2U sequence (Fig. 6). Thus, the MTDs are likely sand-dominated and could serve as significant petroleum reservoirs if the sorting is not very poor. Gravity-flow channels (channel fills) are one of the major sand-rich units in deepwater environments. They are generally considered to be good reservoirs and primary exploration targets. However, in our study area, the channel fills display similar low-amplitude reflection characteristics as the underlying thick mudstone (Fig. 11). Therefore, they are likely mud-dominated. It is possible that the channels served as conduits for sediment transport during the low lake level stage, and the sediment supply was limited and the channels were filled by mudstone
7.2.2. Petroleum prospects Hao et al. (2007, 2010) measured the vitrinite reflectance (Ro) of more than 200 samples (Fig. 14). Although there is a relatively wide variation of measured Ro values at the same burial depth, the measured Ro values increase regularly with burial depth down to about 4200 m (Fig. 14). The Bozhong sub-basin has a relatively high thermal gradient of approximately 33 °C/km (Hao et al., 2010). Measured Ro reaches 0.6%, and Tmax reaches 435 °C at about 2700 m, suggesting that source rocks below 2700 m are mature for oil generation (Hao et al., 2010). However, the burial depths of E3d3 and E3d2L source rocks are deeper than 4170 m (Fig. 15). Based on the trend of thermal maturity with depth, the E3d3 and E3d2L source rocks have a high maturity that vitrinite reflectance (Ro) reaches 1.0–1.3%. Thus, E3d3 and E3d2L source rocks have entered the peak of oil generation, and they could provide abundant oil sources for the overlying E3d2U gravity-flow reservoirs (Fig. 15). Fine-grained deposits in the transgressive system tract (TST) could serve as effective seal for the gravity-flow reservoirs in the lowstand 186
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Fig. 11. Seismic profiles showing the channels and lobes. The locations of seismic profiles are shown in Fig. 8.
however, sealing formations with thickness less than 10 m are still able to reserve a significant volume of hydrocarbons (Jiang, 1998). The pressure coefficient of the E3d2L source rocks ranges from 1.52 to 1.75 with a mean value of 1.67 (Xu et al., 2015). We basically defined overpressure as the pressure coefficient is higher than 1.06. Pressure coefficient between 1.06 and 1.27, 1.27 and 1.73 and higher than 1.73 are defined as weak overpressure, overpressure and strong overpressure, respectively. In the study area, the formation pressure is from overpressure to strong overpressure. The hyperpressure can promote the discharge and migration of generated hydrocarbon. Therefore, oils generated from underlying rocks experience primary migration and short-distance secondary migration into the overlying traps (Fig. 15). Recent studies show that the gravity-flow sandstone have proven to be important hydrocarbon reservoirs on the Qinan Slope, Huanghua Depression of the Bohai Bay Basin (Liu et al., 2016). In our study, the gravity-flow reservoirs could serve as potential and effective hydrocarbon exploration targets, and the Oligocene Dongying Formation (E3d) could be significant deep targets in the future. 8. Conclusions (1) Mass-transport deposits (MTDs) and channel-lobe complexes are the main gravity-flow depositional units in the E3d2U formations. The mass-transport deposits (MTDs) show hummocky and chaotic reflections with poor continuity and variable amplitude. Their basal surfaces have considerable erosional relief. The channels are dominated by linear geomorphology with weak root-mean-square (RMS) amplitude and incise into the underlying thick mudstone. The lobes display high-continuous and high-amplitude reflection at the terminal channels. (2) The genesis of gravity-flow deposits is primarily controlled by high sedimentation rate during a relative lake-level lowstand. The
Fig. 12. The depositional systems of lowstand system tract (LST) of the E3d2U sequence. Note the slumps, channels and lobes.
system tract (LST) (Catuneanu, 2006; Zecchin and Catuneanu, 2015). The TST mudstone is not very thick in the E3d2U sequence cycle (Fig. 6). Basically, thick seals are beneficial to oil and gas preservation,
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Fig. 13. (A) Variation of Rock-Eval hydrogen index with Tmax for samples from E3d3 and E3d2L source rocks in the Bozhong subbasin, Bohai Bay Basin. The Tmax is the temperature at which the maximum amount of S2 hydrocarbons (HC3) is generated. Ro = vitrinite reflectance. (B) Total organic carbon (TOC) contents versus Rock-Eval S2 peaks for samples from the Bozhong subbasin, Bohai Bay Basin, showing the hydrocarbon (HC)-generation potentials.
lacustrine sand-rich MTDs were dominated by abundant sediment supply from proximal deltas. Channels serve as significant conduits for terrigenous clastics transport, and sheet-sandstone lobes formed in an unconfined depositional setting at the end of the channels. (3) The E3d3 source rock has TOC contents between 0.6 and 3.1%, Rock-Eval S2 values between 0.7 and 19.8 mg HC/g rock, and Hydrogen indices between 100 and 600 mg/g TOC. The E3d2L source rock has TOC contents ranging from 0.5 to 1.9%, Rock-Eval S2 values from 0.4 to 7.8 mg HC/g rock, and HI from 40 to 480 mg/ g TOC. Both E3d3 and E3d2L source rocks have good hydrocarbongeneration potential. (4) Gravity-flow reservoirs are favorable lithologic traps. The underlying E3d3 and E3d2L source rocks have entered the peak of oil generation, therefore, they could provide abundant oil sources. Oils generated from source rocks migrate shortly into the overlying traps. Thus, the gravity-flow reservoirs could serve as potential and effective hydrocarbon exploration targets in our study area.
Acknowledgements This research was financially funded by the National Natural Science Foundation of China (41690131, 41690134, 41702155), National Science & Technology Specific Project (2016ZX05024-003008) and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (CUG160605). We thank MPG Editor Dr. Massimo Zecchin, Associate Editor Dr. Sven O. Egenhoff and anonymous reviewers for their thorough and critical reviews and suggestions to improve the manuscript. We appreciate the enthusiastic support of Dr. Tian Dong at University of Geosciences (Wuhan).
Fig. 14. Variation of measured vitrinite reflectance with depth in the Bozhong sub-basin, Bohai Bay basin (modified from Hao et al., 2007, 2010).
Fig. 15. Gravity-flow deposits and their petroleum prospects in the Dongying Formation (E3d), Bozhong Subbasin, Bohai Bay Basin. These lines come from the depth of well W01, and the %Ro is estimated by the relationship of depth and vitrinite reflectance (see Fig. 14). Note the thermal maturity of E3d3 and E3d2L source rocks (Ro > 1.0%).
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