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Keywords:

  • Huanghekou Sag;
  • Bohai Bay Basin;
  • China: Paleogene;
  • erosion;
  • deposition;
  • slope break system

Abstract

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Geological Background
  5. 3 Data and Methods
  6. 4 Sequence Stratigraphy
  7. 5 Depositional Systems
  8. 6 Types of Slope Break Belt and Subdivision of Slope Break System
  9. 7 Slope Break System and Erosion–Deposition Response
  10. 8 Discussion: Origin of the Reservoir Sandbody Accumulating in a Slope Break System
  11. 9 Conclusions
  12. Acknowledgements
  13. References

The sequence architecture and depositional systems of the Paleogene lacustrine rift succession in the Huanghekou Sag, Bohai Bay Basin, NE China were investigated based on seismic profiles, combined with well log and core data. Four second-order or composite sequences and seven third-order sequences were identified. The depositional systems identified in the basin include: fan delta, braid delta, meander fluvial delta, lacustrine and sublacustrine fan. Identification of the slope break was conducted combining the interpretation of faults of each sequence and the identification of syndepositional faults, based on the subdivision of sequence stratigraphy and analysis of depositional systems. Multiple geomorphologic units were recognized in the Paleogene of the Huanghekou Sag including faults, flexures, depositional slope break belts, ditch-valleys and sub-uplifts in the central sag. Using genetic division principles and taking into consideration tectonic features of the Paleogene of the Huanghekou Sag, the study area was divided into the Northern Steep Slope/Fault Slope Break System, the Southern Gentle Slope Break System and T10 Tectonic Slope Break System/T10 Tectonic Belt.

Responses of slope break systems to deposition–erosion are shown as: (1) basin marginal slope break is the boundary of the eroded area and provenance area; (2) ditch-valley formed by different kinds of slope break belts is a good transport bypass for source materials; (3) shape of the slope break belt of the slope break system controls sediments types; (4) the ditch-valley and sub-sag of a slope break system is an unloading area for sediments; and (5) due to their different origins, association characteristics and developing patterns, the Paleogene slope break belt systems in the Huanghekou Sag show different controls on depositional systems. The Northern Fault Slope Break system controls the deposition of a fan delta-lacustrine-subaqueous fan, the Southern Gentle Slope Break system controls the deposition of a fluvial–deltaic–shallow lacustrine and sublacustrine fan, and the T10 Tectonic Slope Break System controls the deposition of shallow lacustrine beach bar sandbodies. The existence of a slope break system is a necessary but not a sufficient condition for studying sandbody development. The formation of effective sandbodies along the slope break depends on the reasonable coupling of effective provenance, necessary association patterns of slope break belt, adequate unloading space and creation of definite accommodation space. Copyright © 2013 John Wiley & Sons, Ltd.

1 Introduction

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Geological Background
  5. 3 Data and Methods
  6. 4 Sequence Stratigraphy
  7. 5 Depositional Systems
  8. 6 Types of Slope Break Belt and Subdivision of Slope Break System
  9. 7 Slope Break System and Erosion–Deposition Response
  10. 8 Discussion: Origin of the Reservoir Sandbody Accumulating in a Slope Break System
  11. 9 Conclusions
  12. Acknowledgements
  13. References

Sequence-stratigraphic conceptual models (e.g. Vail et al., 1977; Posamentier et al., 1988, Posamentier and Vail, 1988; Van Wagoner et al., 1990; Posamentier and Morris, 2000) have been widely applied, particularly in the hydrocarbon industry, where they have been used to predict reservoir distributions and geometries in continental basins (e.g. Lin et al., 2000; Lin et al., 2001; Li et al., 2002; Wang et al., 2002; Prather, 2003; Paton et al., 2008; Zagrarni et al., 2008; Catuneanu et al., 2009; Fyhn et al., 2009). Currently, many researchers proposed the concept of the lacustrine basin slope break belt through studies of continental lacustrine sequence stratigraphy (Fan and Li, 1999; Lin et al., 2000; Wang et al., 2004; Ren et al., 2004; Liu et al., 2006; Xu 2006; Huang et al., 2012) and divided it into a tectonic slope break belt, erosional slope break belt and depositional slope break belt according to their origin. They also proposed that the slope break belt controls the developing pattern of the sequence, deposition and even traps, which in turn controls the development of reservoir sandbodies and more subtle traps.

More research is still needed to be conducted regarding the following aspects: (1) the comprehensive studies of multi-order slope break belt association with a similar origin mechanism, since earlier researchers mostly studied a single or some kind of slope break belt or multi-order slope break belt, and few studies were related with a slope break system; (2) Many previous researchers have referred to the slope break belt and its association in planar view, and its obvious control on depositional systems and reservoir sandbodies, but few researchers have mentioned how the slope break belt controls effective reservoir sandbodies.

The Huanghekou Sag of the Bohai Sea is a hydrocarbon-rich and hydrocarbon-producing sag and also an area of significant petroleum accumulation. Several tens of large- and medium-sized oil and gas fields and a series of petroleum-bearing structural traps have been discovered in the sag during its 30 years of exploration. Despite all the fruitful exploration achievements, how to accurately predict a reservoir sandbody has always been an obstacle to expanding the exploration domain in the Paleogene of the study area.

In fact, different kinds of slope break systems in lacustrine basins are closely related with the deep basal (basement) tectonic framework of a basin and its regional tectonic dynamic setting. On a basin scale, these slope break belts actually reflect the characteristics of a prototype basin for some period. Slope break belts in the same slope break system obviously have original paragenetic relationships. The slope break system reflects the geomorphology in the same tectonic dynamic setting. Thus, not only does the slope break system control the development of the depositional systems, but also, in these slope break systems, the type, developing pattern, difference of slope break belt associations and consequential change of accommodation will probably successfully control the development of an effective reservoir.

This paper attempts to: via studies of the Paleogene sequence stratigraphy and depositional systems of the Huanghekou Sag, Bohai Bay Basin, and taking a single type of slope break belt in the key area of the western Sag as a case study, and combined with the tectonic differences of the study area, subdivide the slope break belt into different types and, furthermore, discuss how the coupling of the slope break systems controls the origin of favourable reservoir sandbodies.

2 Geological Background

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Geological Background
  5. 3 Data and Methods
  6. 4 Sequence Stratigraphy
  7. 5 Depositional Systems
  8. 6 Types of Slope Break Belt and Subdivision of Slope Break System
  9. 7 Slope Break System and Erosion–Deposition Response
  10. 8 Discussion: Origin of the Reservoir Sandbody Accumulating in a Slope Break System
  11. 9 Conclusions
  12. Acknowledgements
  13. References

The Huanghekou Sag is located in the southeastern Bohai Bay Basin, NE China, with an area of about 3300 km2. To the south there is the North Laizhouwan low uplift and to the north there is the South Bohai low uplift (Gong et al., 2000; Tian et al., 2009; Zhou et al., 2010). In the Paleogene, two sets of faults developed respectively, in an approximately E–W direction and NNE–SSW direction. The Huanghekou Sag developed as a dustpan-shape with a fault in the north and overlap in the south (Figs. 1 and 2). The two sets of faults trending NNE are western branches of the Tan–Lu Fault Belt which, characterized by its strike–slip feature, penetrates the entire Bohai Bay Basin, and controls the formation of multiple sub-basins (or sub-sags) in the Bohai Bay Basin (Li, 1984; Chao et al., 1999; Brendan et al., 2000; Cai et al., 2001; Hsiao et al., 2004; Qi et al., 2008; Li et al., 2013).

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Figure 1. Location of the Huanghekou Sag in the Bohai Bay Basin. Tectonic unit characters are illustrated according to the interpretation of the bottom boundary of the Paleogene. The figure is available in colour at wileyonlinelibrary.com/journal/gj

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Figure 2. Schematic structure profiles across the Huanghekou Sag, Bohai Bay Basin. Locations of the profiles are shown in Figure 1. The figure is available in colour at wileyonlinelibrary.com/journal/gj

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The Paleogene and Neogene developed completely in the Huanghekou Sag. In ascending order, the Paleogene strata includes the Kongdian, Shahejie and Dongying formations. The Shahejie Formation is subdivided into Members 1, 2, 3 and 4; the Dongying Formation is subdivided into Members 1, 2 and 3. The Member 3 of the Shahejie Formation is also subdivided into the upper, middle and lower submembers, but the upper submember is lacking in the sag due to intensive erosion. Tectonic evolution witnessed a transition from a synrift development stage to a postrift depression stage in the entire Bohai Bay Basin (Fig. 3). The Paleogene tectonic evolution of the Huanghekou Sag can be divided into four rift-stretch periods: (i) the Kongdian Formation, (ii) Member 4 of the Shahejie Formation, (iii) Member 3 of the Shahejie Formation, and (iv) Members 2 and 1 of the Shahejie Formation and the Dongying Formation, respectively, developed during these four episodes. In the inner basin, regional uplifting occurred after each rift-stretch period which led to the formerly deposited strata being eroded and thus formed several micro-angular unconformities or parallel unconformities that can be correlated regionally (Fig. 3; Cai et al., 2001; Tian et al., 2009).

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Figure 3. Lithostratigraphy and sequence classification for the Huanghekou Sag (sb—sequence boundary; csb—composite sequence boundary).

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3 Data and Methods

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Geological Background
  5. 3 Data and Methods
  6. 4 Sequence Stratigraphy
  7. 5 Depositional Systems
  8. 6 Types of Slope Break Belt and Subdivision of Slope Break System
  9. 7 Slope Break System and Erosion–Deposition Response
  10. 8 Discussion: Origin of the Reservoir Sandbody Accumulating in a Slope Break System
  11. 9 Conclusions
  12. Acknowledgements
  13. References

Regional correlations, facies and slope break belt maps were conducted by identifying and correlating major sequence boundaries and maximum flooding surfaces in 56 wells (including 12 core descriptions) and 1000 km-long 2-D seismic and 3-D seismic surveys of the 3000 km2 study area.

Well logs and seismic data were combined and correlated to generate a detailed sequence stratigraphic framework. All sequence stratigraphic terminology in this paper follows the definitions from Vail et al. (1977, 1991).

On the basis of the established regional third-order sequence stratigraphic framework, investigations of slope break belt were made. Then, by the identification of features of the seismic and geological profiles, reconstruction of palaeotectonics in sequence units, coupled with the strata isopach maps, we finally established the distribution pattern of the slope break belts within the palaeo-tectonic framework (Wang et al., 2004; Liu et al., 2006).

4 Sequence Stratigraphy

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Geological Background
  5. 3 Data and Methods
  6. 4 Sequence Stratigraphy
  7. 5 Depositional Systems
  8. 6 Types of Slope Break Belt and Subdivision of Slope Break System
  9. 7 Slope Break System and Erosion–Deposition Response
  10. 8 Discussion: Origin of the Reservoir Sandbody Accumulating in a Slope Break System
  11. 9 Conclusions
  12. Acknowledgements
  13. References

4.1 Sequence boundary and maximum flooding surface

Identification of sequence boundaries in the Huanghekou Sag is based on analyses of seismic profiles complemented by well logs and core data. We used the following criteria to identify sequence boundaries in the basin.

  1. Unconformable stratigraphic contacts are reflected on seismic profiles as truncations, surfaces of onlap, toplap and downlap of lowstand systems tracts (Fig. 4). Composite (second-order) sequence boundaries are commonly angular unconformities which have strong reflection amplitude and typically extend into the central part of the basin. Third-order sequence boundaries develop most commonly along basin margins, particularly along the hinged margins of half-grabens. They are identified as local erosional truncations with strong reflection amplitude and towards the central part of the basin grade into conformable contacts, shown as strong parallel reflection zones.
  2. Sequence boundaries are also shown as abrupt changes of physical characteristics such as lithology and sedimentary facies. Such boundaries are identified using synthetic seismograms, interval velocities, and shapes of well logs. A synthetic seismogram is the principal basis for determining the depth of the sequence boundaries on seismic profiles, but is used in combination with well log and lithofacies data for a more accurate identification of the positions of sequence boundaries (Fig. 5).
  3. Sequence boundaries are defined by incised valleys or channels up to hundreds of metres wide and tens of metres deep and weathering or denudation surfaces. Large-scale incised valleys can be observed along sequence boundaries (Fig. 6). The incised valleys are typically filled with thick, multi-storied sandstones.
  4. The maximum flooding surface refers to the depositional surface formed when lake level rises to the maximum and the lacustrine basin reaches the widest area. It is a key surface in a sequence, above which the lacustrine area spreads and below which the lacustrine area shrinks. Seismic reflection of the maximum flooding surface is mainly shown as high continuity, strong intensity reflections, and downlap is represented by lacustrine dark mudstones or bioturbated, fine-grained beds on well logs and core data (Figs. 4 and 5).
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Figure 4. Annotated 2-D and 3-D seismic profiles across the southern sub-sag of the Huanghekou Sag, showing composite sequence boundary (csb) and sequence boundary (sb). Both boundary types are characterized by truncation followed by onlap and toplap. LTST—lowstand and transgressive systems tract; HST—highstand systems tract. See Figure 1 for location of seismic profile. The figure is available in colour at wileyonlinelibrary.com/journal/gj

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Figure 5. Log of borehole T6 in the Huanghekou Sag with interpretation of lithofacies, sequence boundaries, depositional systems, and systems tracts. The log is based on a wire-line log supplemented by cores of selected intervals. Wire-line log is gamma ray (GR). Location of the well is shown in Figure 1. Abbreviations are as follows: sb—sequence boundary; LTST—lowstand and transgressive systems tracts; HST—highstand systems tract; mfs—maximum flooding surface; FD—fan delta; SF—sublacustrine fan; BD—braid delta; MD—meander delta; BS—beach sandbar.

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Figure 6. Incised valley fills above the sequence boundaries of the Huanghekou Sag. These incised valleys, which perpendicularly overlay each other, mainly developed near the southern marginal slope belt of the Sag and are the main passageways of sediment supply. See Figure 1 for location of seismic profile. The figure is available in colour at wileyonlinelibrary.com/journal/gj

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4.2 Sequence division result

Using the above-mentioned four criteria, three orders of unconformity-bounded sequences were recognized within the Huanghekou Sag. The Paleogene succession is considered to represent a first-order sequence (or supersequence), and comprises four second-order or composite sequences. The subdivision of the Paleogene sequence of the Bohai Bay Basin indicates that the Member 4 of the Shahejie Formation and the Kongdian Formation belong to two different supersequences. Although they do not develop well in this area, and the internal second-order and third-order unconformities are difficult to identify, herein, they are taken as a set of strata. Identification and subdivision of the third-order sequence mainly refers to the interval from Member 3 of the Shahejie Formation to the top of the Dongying Formation and seven sequences were recognized (Fig. 3).

5 Depositional Systems

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Geological Background
  5. 3 Data and Methods
  6. 4 Sequence Stratigraphy
  7. 5 Depositional Systems
  8. 6 Types of Slope Break Belt and Subdivision of Slope Break System
  9. 7 Slope Break System and Erosion–Deposition Response
  10. 8 Discussion: Origin of the Reservoir Sandbody Accumulating in a Slope Break System
  11. 9 Conclusions
  12. Acknowledgements
  13. References

Depositional systems identified within the Huanghekou Sag include fan-delta, braid delta, meander delta, lacustrine and sub-lacustrine fan deposits. Similar depositional systems have been recognized in other Paleogene rift basins in eastern China (Li, 1996; Katz and Liu, 1998; Lin et al., 2000).

5.1 Fan delta

The fan delta mainly developed during the early depositional period of the sag (depositional period of CSQ4–CSQ2; Fig. 3). On seismic profile the fan delta is characterized by low-medium frequency, low amplitude, low-medium continuity and wedge-shape foreset reflections (Fig. 7a).

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Figure 7. Seismic reflection characters of different kinds of sedimentary facies. (a) fan delta; (b) meander delta; (c) shore-shallow lacustrine and beach sandbar; (e) deeper lacustrine; (f) sublacustrine fan. The figure is available in colour at wileyonlinelibrary.com/journal/gj

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Fan-delta systems are mostly located adjacent to basin margin faults. Similar to river deltas, fan deltas are characterized by grossly coarsening-upward successions which consist of subaqueous (prodelta and delta front) and subaerial (delta plain) facies (Fig. 8a; McPherson et al., 1987, 1988).

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Figure 8. Vertical successions of (a) fan-delta, (b) braid delta, (c) meander delta, (d) lacustrine and (e) sublacustrine fan deposits in the Huanghekou Sag. Wire-line logs are spontaneous-potential (SP), gamma (GR) and acoustic (AC). The figure is available in colour at wileyonlinelibrary.com/journal/gj

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Proximal fan-delta facies are mainly composed of (1) thick-bedded conglomerates and pebbly to coarse-grained sandstones associated with fining-upward intervals (Fig. 9a), (2) interbedded sandstones and sandy mudstones. The coarse-grained facies is interpreted as channel fills on the delta plain, whereas facies 2 is considered to represent intra-channel deposits (cf. McPherson et al., 1988).

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Figure 9. Photographs of representative various types of sedimentary facies. See Figure 8 for location of cores. (a) light grey sandy conglomerate, conglomerate is a well-sorted, angular-subangular, disorderly arranged, debris-flow deposit (T1, 3455.8-3456 m); (b) conglomerate, sandy conglomerate, conglomerate-bearing coarse-grained sandstone.(T1, 3400.5-3400.8 m); (c) laminated siltstone, (T1, 3483–3483.2 m); (d) conglomerate-bearing coarse-grained sandstone with cross-bedding (T12, 2620.5–2620.6 m); (e) purple mottled grey green mudstone, bioturbation structure, organism burrow occurring (T12, 2619.5 m); (f) mottle coloured fine-grained conglomerate, subangular-subrounded, with the largest grain size of 2 cm (T12, 2650.5 m); (g) grey fine-grained sandstone intercalated with muddy belt, more carbonaceous seams, wedge-shaped cross-bedding, and loading structure (T12, 2668.2 m); (h) grey siltstone, gentle wavy cross-bedding intercalated with muddy laminae (T12, 2668.8 m); (i) grey white fine-grained sandstone, gentle wavy bedding, containing oil stains (T2, 2950.1 m); (j) grey white fine-grained sandstone, large-scale inclined bedding (T2, 2950.2 m); (k) grey siltstone intercalated with brown mudstone, showing strong bioturbation (T7, 3140 m); (l) grey white fine-grained sandstone, massive bedding (T7, 3204.5 m); (m) grey siltstone, low-angle cross-bedding (T7, 3213.2 m); (n) sandstone and dark grey muddy siltstone developed as relatively independent blocks, collapse rolling deformation structure occurring (T3, 3830.1 m). Scale: coin is 2 cm in diameter; core is 10 cm in diameter. The figure is available in colour at wileyonlinelibrary.com/journal/gj

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The fan delta front is composed of a distal bar and proximal bar. The proximal bar is dominated by conglomerate, sandy conglomerate, conglomerate-bearing coarse-grained sandstone, coarse-grained sandstone intercalated with thin bedded dark silty and muddy sediments. Large-scale inclined bedding, large-scale cross-bedding, imbricated or oriented pebbles are also developed (Fig. 9b). The distal bar is composed of medium-grained sandstone and fine-grained sandstone intercalated with dark muddy sediments. Large-scale inclined bedding, large-scale cross-bedding, wavy structure and wavy cross-bedding (Fig. 9c) and oriented pebbles intercalated with slump deposit developed.

The pro-fan delta is mainly composed of dark siltstone, muddy siltstone, silty mudstone and mudstone with blocky bedding, horizontal bedding and gentle wavy bedding intercalated with subaqueous lacustrine slumped deposits.

5.2 Braid delta

The Braid delta is characterized by high amplitude, medium–high frequency, medium–high continuity foreset seismic reflections (Fig. 7b). It is the most commonly developed sedimentary facies in CSQ1–CSQ4 of the Huanghekou Sag and can be subdivided into delta plain, delta front and prodelta subfacies (Fig. 8b).

The braid delta plain is mainly composed of braid channel and intra-channel deposits. The lithology of the braid channel includes sandy conglomerate, conglomerate-bearing coarse-grained sandstone, coarse-grained sandstone and medium-grained sandstone, with pebbles arranging in a definite direction, and large-scale cross-bedding develops (Fig. 9d) in a thinning-upward succession. Intra-channel is thin bedded, and its lithology is dominated by greyish green, purplish siltstone, tuffaceous siltstone, muddy siltstone, silty mudstone, mudstone and mainly blocky bedding develops (Fig. 9e). Logging curves are characterized by high gamma, low-medium negative abnormity on saline solution mud fluid spontaneous potential curve (Fig. 8b).

Mainly mouth bar and distant sand bar were identified from the braid delta front. The lithology of the mouth bar includes conglomerate, sandy conglomerate, conglomerate-bearing coarse-grained sandstone, coarse-grained sandstone, medium-grained sandstone, fine-grained sandstone and tuffaceous sandstone. Sedimentary structures include large-scale cross-bedding, blocky bedding and oriented pebbles in conglomerate-bearing coarse-grained sandstone (Fig. 9f). The lithology of the distal sand bar includes fine-grained sandstone, dark coloured siltstone and mudstone interbedded with siltstone. Sedimentary structures include wedge-shaped cross-bedding, load structures, gentle wavy bedding, blocky bedding and pseudo-lateral bedding (Fig. 9g, h).

The braided prodelta is dominated by dark coloured siltstone, tuffaceous siltstone, muddy siltstone, silty mudstone and mudstone, and blocky bedding, lateral bedding intercalated with subaqueous lacustrine deposits developed.

5.3 Meander delta

The meander delta is characterized by medium amplitude, medium–high continuity, S-shaped and imbricated foreset seismic reflections (Fig. 7c).

The meander delta developed during the depositional period of SQ1–SQ3. Emphasis was placed on the analysis of the meander delta front and pro-delta subfacies, since drilling wells in the study area focused mainly on examining the delta front and prodelta deposits (Fig. 8c).

The delta front is subdivided into mouth bar and distal sand bar microfacies. The mouth bar is dominated by thick- to medium-grained sandstone, medium- to fine-grained sandstone and fine-grained sandstone intercalated with thin mudstone. The distal sand bar is characterized by silty fine-grained sandstone, siltstone and muddy siltstone interbedded with mudstone. Sedimentary structures include gentle wavy bedding and large-scale inclined bedding (Fig. 9i, j).

The pro-delta is dominated by dark grey and grey mudstone, silty mudstone intercalated with thin siltstone and muddy siltstone. The logging curve (gamma ray curve) is characterized by gentle low amplitude peaks.

5.4 Lacustrine facies

The lacustrine system in this study is subdivided into shore to shallow lacustrine and deep to semi-deep lacustrine types (Fig. 8d).

Shore to shallow lacustrine facies is characterized by medium–low amplitude, medium–low continuity sub-parallel or wedge-shaped divergent seismic reflections (Fig. 7d).

The shore to shallow lacustrine facies is subdivided into mud beach, mud and sand-mixed beach and sandy beach bar microfacies according to their developing environments and sediment characters. The mud beach is dominated by grey–green and grey mudstone and silty mudstone (Fig. 9k). Sand and mud-mixed beach is dominated by mudstone, silty mudstone, tuffaceous mudstone intercalated with thin-bedded siltstone and fine-grained sandstone, with gentle wavy cross-bedding, lenticular bedding, wavy bedding, wavy cross-bedding, and common bioturbation structures. The sandy beach bar was well developed during the depositional interval from the submember 2 of the Shahejie Formation to the Dongying Formation (Deng et al., 2011). It is dominated by medium- and fine-grained sandstone, silty sandstone, and conglomerate-bearing coarse-grained sandstone. Pebbles are arranged in a certain direction, low-angle cross-bedding and blocky bedding are developed (Fig. 9l, m). The composition of clastic grains is dominated by quartz (over 75%), and mainly as quartz sandstones.

Semi-deep to deep lacustrine deposits are dominated by dark mudstone, with lateral bedding and blocky bedding. On seismic profiles, they are characterized by high amplitude, high continuity parallel reflections (Fig. 7e).

5.5 Sublacustrine fan

The sublacustrine fan refers to the depositional system composed of subaqueous gravity-flow deposits which are formed in a sublacustrine environment. Fans may be fed by subaqueous channels in front of the deltas or directly from the basin margins. Sublacustrine fan deposits have been described in many modern lake environments and identified from ancient lacustrine basin fills (Scholz and Rosendahl, 1990; Nelson et al., 1999; Wang et al., 2013). The sublacustrine fan is characterized by seismic reflections of medium–high amplitude, low–medium continuity, bidirectional down overlap and mound, with wavy-disordered shape (Fig. 7a, f).

The sublacustrine fan of the Huanghekou Sag mainly developed during the depositional period of CSQ2 and is subdivided into proximal sublacustrine fan, middle sublacustrine fan and distal sublacustrine fan (Fig. 8e). The proximal sublacustrine fan is composed of collapsing and clastic debris deposits (Fig. 9n); the middle sublacustrine fan is composed of clastic debris and turbidite deposits; and the distal sublacustrine fan is composed of turbidite deposits (Wang et al., 2013).

6 Types of Slope Break Belt and Subdivision of Slope Break System

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Geological Background
  5. 3 Data and Methods
  6. 4 Sequence Stratigraphy
  7. 5 Depositional Systems
  8. 6 Types of Slope Break Belt and Subdivision of Slope Break System
  9. 7 Slope Break System and Erosion–Deposition Response
  10. 8 Discussion: Origin of the Reservoir Sandbody Accumulating in a Slope Break System
  11. 9 Conclusions
  12. Acknowledgements
  13. References

6.1 Types of slope break belt

Utilizing the identification methods of the slope break belt and combined with fault interpretation within each sequence, the following structures: syndepositional faults, faults, flexures, depositional slope break belts, ditch-valleys and local sub-uplifts, were identified within the Huanghekou Sag.

Faults (Fig. 10), flexures (Fig. 11) and depositional slope break belts (Fig. 12) agree well with the concepts and features proposed by earlier researchers (Fan and Li, 1999; Lin et al., 2000; Wang et al., 2004; Ren et al., 2004; Liu et al., 2006; Xu, 2006; Huang et al., 2012).

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Figure 10. Seismic characteristics of the Northern Fault Slope Break Belt in the Huanghekou Sag. Fault slope break belt is composed of multi-syndepositional faults, which are characterized by fault terraces in the same direction. See Figure 1 for location of seismic profile. The figure is available in colour at wileyonlinelibrary.com/journal/gj

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Figure 11. Seismic characteristics of the western flexure break belt of the Huanghekou Sag. Flexure slope break developed on the broken termination of the buried faults of the early period showing as a monoclinal fold widening upwards. See Figure 1 for location of seismic profile. The figure is available in colour at wileyonlinelibrary.com/journal/gj

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Figure 12. Seismic characteristics of depositional slope break of the Huanghekou Sag. The formation of this slope break is closely related with delta deposition. A geomorphologic slope was formed within the delta front and prodelta-lacustrine facies due to the great difference in sedimentary rate between the delta front and prodelta. Lacustrine to subaqueous fan facies developed below the slope. See Figure 1 for location of seismic profile. The figure is available in colour at wileyonlinelibrary.com/journal/gj

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Ditch-valleys mainly developed in the southern and northern uplifts and slope belts in the Huanghekou Sag. They are divided into eroded and tectonic types according to their origins. An eroded ditch-valley mainly develops above the uplift (Fig. 6). A tectonic ditch-valley is mainly composed of the association of one to two fault slope break belts, including the ‘trumpet’ shape association of a single fault slope break belt (Fig. 13a) and a ditch-valley formed by a fault-adjusting belt. The fault adjusting belt refers to the association of syndepositional faults of superimposition-type in the same direction, and the ditch-valley develops at the superimposed section of superimposed faults in the same inclined direction (Fig. 13c, d). Since the transitional slope joins the hanging wall of a fault with the downthrown wall of another fault, which leads to the gentle transition between them; the downthrown wall represents a low tectonic unit. Thus the trend-slope transmitting the tectonic slope break belt is an entrance of mountain rivers into the basin and it is easy to form a ditch-valley. Two ditch-valley geomorphologic units were formed adjacent to the South Bohai low uplift and the North Laizhouwan low uplift due to the dextral strike slip movement of the Tan–Lu Fault Zone and is quite clear on the seismic profile (Fig. 13b).

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Figure 13. Developing characters of the Paleogene valley of the Huanghekou Sag. Base map of section location is a thickness map of CSQ2–CSQ1 in time-domain, and faults are the seismic interpretation of csb5. The figure is available in colour at wileyonlinelibrary.com/journal/gj

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A local uplift within the central sag is the secondary tectonic belt of the basin, which, in the Huanghekou Sag, is referred to as the T10 Tectonic Belt (Fig. 14).

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Figure 14. Subdivision of the Paleogene slope break system of the Huanghekou Sag. Three-dimensional block map is from the analysis of the Paleogene geomorphologic features. (1) The Northern Steep Slope/Fault Slope Break System; (2) The Southern Gentle Slope/Fault–Flexure Slope Break System and (3) T10 Tectonic Belt/Slope Break System in the central sag, flanked by the northern and southern sub-sags. The figure is available in colour at wileyonlinelibrary.com/journal/gj

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6.2 Division of slope break system

The slope break system is the association of a single or multi-kinds of slope break belt with correlated origins and is controlled by the same tectonic stress in a basin (or a sag). The slope break system of the Huanghekou Sag is actually the direct representation of different dynamic settings and their different geomorphologic responses. In the entire Neogene time, the Bohai Bay Basin was influenced by a thermodynamic (diapir, expansion) system from the bottom of the lithosphere and the NE–ENE lateral compression dynamic system resulted from the plate tectonic movement of the West Pacific Ocean margin (Glider et al., 1999; Chi and Zhao, 2000; Zhang et al., 2001; Zhu et al., 2003; Schellart and Lister, 2005; Yu et al., 2006; Qi et al., 2008). But during the Paleogene rift stage, the extensional stress perpendicular to the axis of mantle uplift maybe offset by the compressional pressure. Thus, the stress field of the Huanghekou Sag was characterized by crustal extension and dextral strike slip of the Tan–Lu Fault Belt (Cai et al., 2001; Deng, 2001; Hsiao et al., 2004; Yu et al., 2006; Qi et al., 2008, 2010).

Combining the Paleogene tectonic features of the Huanghekou Sag and subdivision principles of the slope break system, three slope break systems were recognized: the Northern Steep Slope/Fault, Southern Gentle Slope/Fault–Flexure and T10 Tectonic Belt/Slope Break System.

6.2.1 The northern steep slope/fault slope break system

The Northern Steep Slope System is mainly distributed in the northern sag and it includes the South Bohai low uplift and the northern sub-sag. Since intense thermal expansion and diapirism occur at the bottom of the lithosphere and extensional stress is the most intensive in the northern Huanghekou Sag, thus a series of E–W trending boundary faults and their descendant fault terrace zones developed. Seismic interpretation of faults indicates that multiple sets of syndepositional fault belts developed from the South Bohai low uplift to the northern sub-sag (Fig. 10). These fault belts present an en echelon association on seismic profiles. Multiple slope breaks are subdivided into basin-margin and inner-basin slope break, according to the location of the slope breaks. The basin-margin fault slope break, which is located near the boundary of a sag, is a boundary-controlling large fault of a sag. This kind of fault is active for a long period, the elevation difference between the hanging wall and the foot wall is obvious and herein forms a continuous sagging basin boundary (Figs. 10 and 15).

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Figure 15. Associated features of multiple fault slope breaks of the Northern Steep Slope System in the Huanghekou Sag. Base map is a palaeogeomorphologic reconstruction of CSQ2, and profile features of multiple fault slope breaks are shown in Figure 10. The figure is available in colour at wileyonlinelibrary.com/journal/gj

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6.2.2 The southern gentle slope/fault–flexure slope break system

The Southern Gentle Slope System is distributed in the southern and western gentle slope belts, and also includes the southern sub-sag. The types of palaeogeomorphologic unit present include: the ditch-valley above the North Laizhouwan low uplift (Fig. 6), fault slope break, flexure slope break (Fig. 11), and locally distributed depositional slope breaks (Fig. 12). Similar to the northern steep slope break, multiple slope breaks are subdivided into basin-margin and inner-basin types according to the tectonic location where the slope break develops (Fig. 16).

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Figure 16. Associated features of multiple slope breaks of the Southern Gentle Slope/Break System in the Huanghekou Sag. Abundant types of slope break develop. Basin-margin slope break is dominated by syndepositional slope break, and inner-basin slope break includes faults, flexures and depositional slope breaks. Base map is the geomorphologic map of CSQ2. The figure is available in colour at wileyonlinelibrary.com/journal/gj

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Due to the tilting of the basement fault blocks and combined with the dextral slip of the Tan–Lu Fault Belt, a series of syndepositional normal faults developed during the Paleocene–Eocene time which are of relatively small-scale and present an en echelon planar arrangement in the southern and western Huanghekou Sag. These normal faults are mainly distributed along the northern flank of the North Laizhouwan low uplift in the southern sag and constitute a fault slope break controlling the basin boundaries (Fig. 12).

Simultaneously, faults do not develop well in the Late Paleogene (Oligocene). When fundamental fractures began to act in the early period, the termination of the fault began to expand upwards, resulting in the fall of the hanging wall and elevation of the foot wall. This led to passive bending of strata in the later period and thus formed a flexure-fold, named as a ‘growth fold’ by some scholars (Gupta et al., 1999) or ‘forced fold’ (Nancye et al., 2000) (Fig. 11).

In contrast, the depositional slope break, formed by a sudden change of geomorphology due to different sedimentation rates, develops in the southern sub-sag (Fig. 12).

Thus, the Southern Gentle Slope System is an association of multiple kinds of slope break, extremely similar to the research results of most rift lacustrine basins in eastern China (Fan and Li, 1999; Lin et al., 2000; Wang et al., 2004; Ren et al., 2004; Xu, 2006; Huang et al., 2012).

6.2.3 T10 tectonic belt/slope break system

This tectonic belt includes the entire T10 Tectonic Belt. It is distributed in an E–W direction, the western part borders on the Southern Gentle Slope Break System and is separated by a ‘ditch-valley’ with relatively shallow water (Fig. 17). The eastern part of the T10 Tectonic Belt directly connects with the central tectonic ridge of the Huanghekou Sag and is cut by the Tan–Lu strike–slip fault, the southern and northern parts are limited by a series of horst associations formed by normal faults (Fig. 2). The southern–northern horst association of faults is distributed as an en echelon arrangement, which is obviously influenced by extension and dextral sliding of the Tan–Lu Fault Belt (Fig. 14). The vertical development of the T10 Tectonic Slope Break System is different, and is of an obvious inheritance (Fig. 17).

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Figure 17. Development and evolution features of the T10 Tectonic Belt. It was an aquatic uplift before deposition of CSQ2 which provided sediments for the northern sub-sag and evolved into a subaqueous uplift during the deposition of CSQ1. The figure is available in colour at wileyonlinelibrary.com/journal/gj

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7 Slope Break System and Erosion–Deposition Response

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Geological Background
  5. 3 Data and Methods
  6. 4 Sequence Stratigraphy
  7. 5 Depositional Systems
  8. 6 Types of Slope Break Belt and Subdivision of Slope Break System
  9. 7 Slope Break System and Erosion–Deposition Response
  10. 8 Discussion: Origin of the Reservoir Sandbody Accumulating in a Slope Break System
  11. 9 Conclusions
  12. Acknowledgements
  13. References

The steep slope belt in a dustpan-shaped rift lacustrine basin is characterized by its increasing accommodation space and deepening water. The nearshore depositional system, i.e. fan delta, usually develops during a base level rise period and forms an asymmetrical depositional superimposing succession. Along the gentle slope belt, due to the tilting of the basement fault block, the increase in the sedimentation rate is relatively low, and the characteristics of the depositional cycle are controlled by the A/S value (ratio of accommodation to sediment supply rate). Along the gentle slope, it is dominated by the deposition during base level falling period when sedimentary provenance is sufficient and A/S < 1 which forms a typical shoaling-upwards depositional cycle and an unconformity developing along the erosional area of the Earth's surface (Figs. 18 and 21).

image

Figure 18. Palaeogeomorphologic characteristics of the Paleogene and the erosion-converging-depositional response pattern of depositional systems in the Huanghekou Sag. The figure is available in colour at wileyonlinelibrary.com/journal/gj

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7.1 Basin marginal slope break is the boundary of denuded and source area

The basin marginal slope break is generally related with the stress field transition during different stages of basin tectonic evolution and periodic change of subordinate tectonic movement intensity. As shown on the profiles, the basin marginal slope break generally coincides with the boundary of an erosional area of a basin or a depression, thus directly controls the provenance boundary (Figs. 15 and 16, base map of Fig. 13). Large-scale structural truncation, depositional overlap, angular unconformity and fault elevation and erosion usually develops near the basin marginal slope break on seismic profiles (Figs. 4, 10 and 11).

7.2 Different kinds of ditch-valley constitute main transport passageway for provenance

The ditch-valley above the uplift is usually vertical to the basin marginal slope break and through which eroded sediments in the provenance area are transported into the basin. The trumpet-shape association of a single fault slope break is usually the joining area of all kinds of stress fields, where the local low-lying topography can be formed as the main transport of provenance from a denuded area along the basin margin (Fig. 13a). The drainage relatively concentrates in these areas, general-scale sediments develop whether vertically or on the plane.

Superimposition of the slope break association in the same direction belongs to a trend slope tectonic pattern and is an entrance where a mountain river flows into the basin. Distributary fluvial systems flowing into the basin are easily connected with each other along the transitional slope and then flow into the basin. Thus, sand-abundant clastic sediments develop well. This kind of adjusting belt develops well along the border of the uplift in the Huanghekou Sag (Fig. 13c, d).

The provenance-controlling ‘ditch-valley’ related with strike–slip faults mainly develops adjacent to the Tan–Lu Slide Belt in the eastern part of the sag. This strike–slip fault penetrates the whole sag. The trend of the fault is oblique with the structural trend of the Huanghekou Sag, showing an approximately vertical contact along the southern gentle slope belt and directly connected with the provenance area of the southern and northern uplift. Sediments along the denuded belt can be directly transported into the interior of the basin through the ‘ditch-valley’, formed by a strike–slip system (Fig. 13b).

7.3 Shape of slope break controls sediments types

Different structural subsidence leads to a different gradient of subsidence profile along the slope break and influences the strength of water dynamics and thus controls the rate of sediment supply. The elevation difference of the slope break influences the topography of the provenance area and thus controls the static energy of the provenance area and catchment area, and the degree of erosion of strata above the slope break. As a result, it obviously controls sediments types. The basin marginal slope break, with relatively larger slope gradient and scale, mainly controlled by the first-order tectonic movements, can directly control the amount of sediment supply, sediment grain size and characteristics. Simultaneously, the gradient and subsidence amplitude are different along the gentle slope and steep slope, which leads to obvious differences of sediment supply rate and sediment grain size. Thus, the northern slope break system of the Huanghekou Sag controls multiple depositional systems dominated by the fan delta with immense thickness and the braid fluvial delta. Whereas, the braid fluvial delta deposit is dominant in the southern gentle slope break system (Fig. 19), and fluvial facies may develop above the basin margin slope break.

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Figure 19. Relationship between the slope break system and sequence depositional system of the Paleogene in the Huanghekou Sag. Based on the 3-D analysis of the Huanghekou Sag fill, it is possible to recognize three types of sequence- depositional system. Type A is alluvial fan, fluvial, braid fluvial delta and shallow lacustrine facies during the deposition of CSQ3–CSQ4; Type B is fan delta, braid fluvial delta and semi-deep lacustrine facies during the deposition of CSQ2; Type C is meander fluvial, braid fluvial and shallow lacustrine facies during the deposition of CSQ1. The figure is available in colour at wileyonlinelibrary.com/journal/gj

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7.4 Ditch-valley and sub-sag are the unloading areas of sediments

Multiple slope breaks are both a passageway and an unloading place for sediments. Unloading of large-scale sediments usually occurs at a low-lying belt, namely, as all sub-sags of the main sag. The sub-sags of the western Huanghekou Sag include the southern and northern secondary sub-sags (Figs. 14-16). Since a great deal of ditch-valley developed along all kinds of multiple slope break belts, parts of the sedimentary succession will be preserved with the base level rising and falling. The northern slope break system, dominated by its steep and large-scale slope, can form a great amount of the ‘ditch-valley’ system. However, it is unfavourable for the preservation of sediments due to the large gradient of the ditch-valley itself and the strong hydrodynamic force (Fig. 15). In the gentle slope break system, except for the basin marginal slope break, different slope breaks are characterized by a relatively lower gradient and smaller scale, and thus a great deal of the ditch-valley systems developed which is favourable for the preservation of sediments (Fig. 16).

7.5 Slope break system presents different controls on the depositional system

Due to the different origin, association characteristics and developing pattern of the three kinds of slope break system in the Huanghekou Sag, they show different controls on the depositional system.

The Northern Steep Slope Break System usually controls the development of extremely thick, near-source, coarse-grained sediments during the deposition of CSQ4 (Fig. 19). At this stage, the rapid tectonic subsidence, relatively higher lake level and a great volume of sediments are necessary conditions for the formation of a deep water fan delta. Decrease of sediment supply or increase of tectonic subsidence is favourable for the formation of a nearshore sublacustrine fan or subaqueous fan. When CSQ3 started its deposition, the range of uplift decreased with the decrease of subsidence of the South Bohai low uplift and weakening of syndepositional faults. The scale and gradient of the slope break at this period decreased, and the depositional system changed to a braid fluvial–meander fluvial delta (Fig. 19).

In contrast to the Northern Slope Break System, the Southern Slope Break System has an obvious control on the braid fluvial delta due to the small scale, gentle gradient and large distance between the slope breaks (Fig. 19). The basin marginal slope break in this system controls the boundary of the fluvial delta above and below the slope break or the boundary between the aquatic delta plain and subaqueous delta plain. Towards the basin, multiple inner-basin slope breaks probably control the subfacies boundaries of the same facies, or directly separate different sedimentary facies. For example, during the deposition of SQ6, multiple slope breaks developed in the basin, the tectonic slope break nearest to the inner basin obviously separated the braid fluvial delta above the slope and the sublacustrine fan below the slope (Fig. 20).

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Figure 20. Control of multiple slope breaks on the depositional system during the deposition of SQ6 in the Southern Gentle Slope Break System of the Huanghekou Sag. (a) is a palaeogeomorphology map of the Southern Slope Break System during the deposition of SQ6; sedimentary facies interpretation of this map is from drilling well and 3-D seismic data; (b) is a seismic interpretation of slope break and interpretation of its sedimentary facies. The figure is available in colour at wileyonlinelibrary.com/journal/gj

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Bounded by SQ5, the T10 Slope Break System was mainly an aquatic denuded area in the early period, and began to be flooded by water and evolved into a subaqueous low uplift which led to different control on the depositional system. Before the deposition of SQ5, the slope break system was characterized by a steep slope in the south and a gentle slope in the north. The syndepositional fault developed in the south is of large-scale and directly controls the deposition of the fan delta, whereas in the north, a mainly shore-shallow lacustrine facies developed (Fig. 13d). The entire sag was characterized by shallow water during the deposition of CSQ1 and the sediment supply was less. The tectonic belt was distributed widely and in a relatively higher location of the lacustrine basin, reconstructed by lake water, multi-period beach bar sandbodies were deposited nearby (Fig. 19).

8 Discussion: Origin of the Reservoir Sandbody Accumulating in a Slope Break System

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Geological Background
  5. 3 Data and Methods
  6. 4 Sequence Stratigraphy
  7. 5 Depositional Systems
  8. 6 Types of Slope Break Belt and Subdivision of Slope Break System
  9. 7 Slope Break System and Erosion–Deposition Response
  10. 8 Discussion: Origin of the Reservoir Sandbody Accumulating in a Slope Break System
  11. 9 Conclusions
  12. Acknowledgements
  13. References

Many research results show that the formation, development and evolution of continental basins are controlled by tectonic movement with different orders. Tectonic movement, through patterns like changing geomorphologic features, indirectly influences erosional rate, sediment types, sediment supply rate and even local climatic conditions. Herein, the tectonic factors exert far more important control on the development of continental sequences than in the marine environment (Vail et al., 1991; Shanley and McCabe, 1993; Ravnas and Steel, 1998; Posamentier and Allen, 1999; Strecker et al., 1999; Lin et al., 2000; Limarino et al., 2001; Egger et al., 2002; Eyles and Januszczak, 2007; Martins-Neto and Catuneanu, 2010; Freitas et al., 2011). Accommodation space versus sedimentation rate decides the superimposing pattern of a depositional system. In other cases geologists pay more attention to the influence from geomorphology on sequence stratigraphy (Nichols and Watchorn, 1998; Lemons and Chan, 1999; Gupta and Allen, 2000; Brault et al., 2004; Liu et al., 2006; Frouin et al., 2007). Geomorphology is mainly shown as the distribution of tectonic patterns in different tectonic stress field settings which, to a large degree, will control the evolution and development of the depositional system and sedimentary sequence.

The slope break is a main area for clastic material accumulating in a continental rift basin, whereas it does not mean that favourable reservoir sandbodies will be explored along the slope break. The existence of a slope break system is a necessary condition, but not a complete one for analyzing sandbody development. The formation of effective reservoir sandbodies along the slope break depends on the coupling of effective provenance, necessary association pattern of slope break, sufficient downloading area and creating definite accommodation space (Fig. 21).

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Figure 21. 3-D Block diagram reconstruction showing the pattern of slope break systems controlling strata, sedimentary facies and sandbodies of the Palaeogene in the Huanghekou Sag. The figure is available in colour at wileyonlinelibrary.com/journal/gj

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Firstly, for an effective provenance there must be substantial sediment supply. Whether the sediment supply is abundant decides on the developing degree of sandbodies in a sag. Secondly, an effective provenance must contain suitable material. Only those rocks which are controlled by large-scale basin marginal slope break, that already existed, the larger uplifting area being eroded for a long period (which is not able to act as an effective provenance in the later periods), and the mother rock of the provenance area is weathered to form clastic particles, such as metamorphic and magmatic rocks. In contrast, carbonate rocks and muddy sedimentary rocks of the Mesozoic may not become an effective provenance. The southern and northern uplift of the Huanghekou Sag were eroded for a long period with a large erosional area and the lithology of the basement was dominated by granite and andesite, thus demonstrating the provenance is effective.

The association pattern of the slope break refers to those geomorphologic units with large-scale ditch-valley characteristics. Only these large-scale ditch-valley groups are the main passageway for sandbody transportation.

Sufficient unloading areas must be those low-lying areas in depositional periods, such as deep water areas (sub-sag), large-scale gentle channels etc.

Accommodation space is another very important factor controlling the development of effective sandbodies. Possessing some unloading area is not definitely favourable for the accumulation of sandbodies. When the accommodation space is too small, it is not possible for the preservation of a great number of sandbodies. On the other hand, if there is too large an accommodation space, sandbodies do not develop. Only when the sediment supply balances with accommodation space, is the best condition for sandbody accumulation achieved. For example, the formation of a nearshore subaqueous fan in a depressional lacustrine basin depends on the following conditions: (1) rapid tectonic subsidence setting (rapid tectonic subsidence leads to a gradual increase of accommodation space and forms a deep-water basin environment); (2) relatively higher lake level; (3) the increasing accommodation space exceeds or equals the sediment supply. If a great volume of sediments are supplied, they will quickly accumulate and form aquatic areas to develop a fan delta. Herein, a nearshore subaqueous fan or a sublacustrine fan usually develops along the steep rift belt in a deep lacustrine setting and mainly during a rapid subsiding period of a basin. The depositional period of CSQ2–CSQ3 in the Huanghekou Sag is another example. In this period, due to lake level rise, shrinkage of the provenance area, and obvious decrease of sediment supply, the accommodation space of the deep water area of the sag is too large, and thus it is dominated by starved deposition; whereas, those low uplifting subaqueous areas, with relatively smaller accommodation space, are favourable areas for the formation of some beach bar sandbodies.

In general, the effective provenance, necessary association patterns of slope break, enough unloading area and definite accommodation space are comprehensive controlling factors for sandbody development. As for practical sandbody prediction, not only the geometric shape and main controlling factors of slope break should be taken into consideration, but also the slope break as a system, combined with the provenance–accumulation–depositional system should be considered from multiple angles and multiple factors, which should be paid more attention.

9 Conclusions

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Geological Background
  5. 3 Data and Methods
  6. 4 Sequence Stratigraphy
  7. 5 Depositional Systems
  8. 6 Types of Slope Break Belt and Subdivision of Slope Break System
  9. 7 Slope Break System and Erosion–Deposition Response
  10. 8 Discussion: Origin of the Reservoir Sandbody Accumulating in a Slope Break System
  11. 9 Conclusions
  12. Acknowledgements
  13. References
  1. Based on regional 2-D and 3-D seismic, drilling and logging and core data, comprehensive analyses have been conducted on the sequence stratigraphy and sedimentology of the Palaeogene in the Huanghekou Sag. Four second-order sequences or supersequences and seven third-order sequences are recognized, and fan delta, braid fluvial delta, meander fluvial delta, lacustrine and sublacustrine fan depositional systems were identified.
  2. Through the identification of a slope break combined with the interpretation of faults and identification of syndepositional faults, faults, flexures, depositional slope breaks, ditch-valleys and local sub-uplifts were identified in the Palaeogene of the Huanghekou Sag. According to the tectonic feature of the Palaeogene of the western Huanghekou Sag and division principles of similar tectonic dynamics, the study area was divided into the Northern Steep Slope, the Southern Gentle Slope and the T10 Tectonic Belt Slope Break Systems.
  3. The slope break system is closely related with erosion–deposition. The slope break system obviously controls sediment provenance and sediment types, and simultaneously the ditch-valley and sub-sag in a slope break system are sediment downloading areas. Different kinds of slope break systems control the development of different depositional systems.
  4. The existence of a slope break system is a necessary but not a vital condition for analyzing sandbody development. The formation of effective reservoir sandbodies along slope breaks depends on the coupling of effective provenance, necessary association patterns of slope break, sufficient unloading area and creation of definite accommodation space.

Acknowledgements

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Geological Background
  5. 3 Data and Methods
  6. 4 Sequence Stratigraphy
  7. 5 Depositional Systems
  8. 6 Types of Slope Break Belt and Subdivision of Slope Break System
  9. 7 Slope Break System and Erosion–Deposition Response
  10. 8 Discussion: Origin of the Reservoir Sandbody Accumulating in a Slope Break System
  11. 9 Conclusions
  12. Acknowledgements
  13. References

We would like to thank the National Science and Technology Major Project (Exploration Technologies for Offshore Hidden Oil/Gas) (project no.: 2011ZX05023-002-05) and the Fundamental Research Funds for the Central Universities for support.

References

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Geological Background
  5. 3 Data and Methods
  6. 4 Sequence Stratigraphy
  7. 5 Depositional Systems
  8. 6 Types of Slope Break Belt and Subdivision of Slope Break System
  9. 7 Slope Break System and Erosion–Deposition Response
  10. 8 Discussion: Origin of the Reservoir Sandbody Accumulating in a Slope Break System
  11. 9 Conclusions
  12. Acknowledgements
  13. References
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