Pet.Sci.(2013)10:139-148 139 DOI 10.1007/s12182-013-0261-x

Sedimentary architecture models of deepwater turbidite channel systems in the Niger Delta continental slope, West Africa

Liu Li1, 2, Zhang Tingshan1, Zhao Xiaoming1, 2 , Wu Shenghe3, Hu Jialiang4, Wang Xing5 and Zhang Yikai5 1 School of Resource and Environment, Southwest Petroleum University, Chengdu, Sichuan 610500, China 2 State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu, Sichuan 610500, China 3 College of Geosciences, China University of Petroleum, Beijing 102249, China 4 The Petroleum Institute, Abu Dhabi 2533, The United Arab Emirates 5 Research Institute, China National Offshore Oil Corp, Beijing 100027, China

© China University of Petroleum (Beijing) and Springer-Verlag Berlin Heidelberg 2013

Abstract: This paper studied an architecture model of turbidite channel systems based on the shallow- layer high resolution 3D seismic information in the deepwater area in the Niger Delta continental slope, West Africa as a prototype model. Different types of channel systems were identified and the corresponding architecture models were established. The controlling factors, evaluation criteria and spatial distribution of different channel systems were analyzed. This study shows that turbidite channel systems RI:HVW$IULFDFRXOGEHFODVVL¿HGLQWRWKUHHW\SHVFRQ¿QHGVHPLFRQ¿QHGDQGXQFRQ¿QHGDFFRUGLQJWR the condition of canyon and the levees on both sides. On one hand, along the transport direction, channel system evolves from confined to unconfined. Within channel systems, channel complexes, including two types of incised and enveloped, are the most important reservoir bodies. On the other hand, there is a channel complex evolution from incised to enveloped vertically. The geological factors exert impacts of different levels on the architecture of the turbidite channels in different sedimentary systems or even within the same system.

Key words: Niger Delta continental slope, deepwater deposits, turbidite channel systems, architecture models

1 Introduction Amazon fan for example, has a canyon breadth of up to 15 NLORPHWHUV 6KHSDUGDQG(PHU\+HVVHHWDO ZLWK Deepwater sedimentation is an important factor in current thickness from tens to hundreds meters. In short, deepwater RLODQGJDVH[SORUDWLRQ &KHQ-LDQJHWDO&KHQ VHGLPHQWDWLRQEHORQJVWRODUJHVFDOHXQLWVKRZHYHURXWFURSV =KXHWDO:XHWDO +RZHYHUWKHVWXG\ can only provide limited architectural information of small of its reservoir architecture model has lagged behind that of scale units. Consequently, it is difficult to characterize an ÀXYLDODOOXYLDODQGGHOWDLFGHSRVLWV 6ODWW 7KHUHDVRQV entire system only using information from outcrops. are as follows. First, deepwater depositional systems cannot Therefore, 2D or 3D seismic-reflection technologies are EHHDVLO\REVHUYHGLQWKHPRGHUQHQYLURQPHQWFRQWUDU\ needed to characterize the architecture model of turbidite to non-marine sedimentation, they deposit in the marine channel systems. Especially in recent years, there has been environment where water depth usually exceeds 300 m, rapid development of seismic acquisition and processing as a consequence, it is difficult to use remote-observation technologies leading to a continuous improvement in the techniques to study their sedimentary patterns. Second, vertical resolution of seismic data. For this reason, many outcrops of deepwater sedimentation can barely provide scholars have begun to apply seismic information to describe LQIRUPDWLRQRQDIUDFWLRQRIWKHHQWLUHV\VWHPVXEPDULQH and measure deepwater turbidite channels and there have deposits are characterized by large scale of generally several EHHQDVHULHVRIDFKLHYHPHQWV &RUEHDQXHWDO9LDQD kilometers or even more than ten kilometers in width. The HWDO6DPXHOHWDO'UR]HWDO6DOOHUHWDO :RRGDQG0L]H6SDQVN\6KDRHWDO'RQJ *Corresponding author. email: [email protected] HWDO&KHQHWDO  Received August 26, 2012 140 Pet.Sci.(2013)10:139-148

However, previous and present studies of deepwater are 1) the upper extensional zone of listric growth faults architecture models mainly focused on middle and small scale EHQHDWKWKHRXWHUVKHOI WKHWUDQVODWLRQDO]RQHRIGLDSLUV sedimentary units, such as channel complexes and single DQGVKDOHULGJHVEHQHDWKWKHXSSHUVORSHDQG WKHORZHU FKDQQHOV .ROODHWDO%HDXERXHI/DERXUGHWWH compressional zone of imbricate thrust structures (toe thrusts) DQG-RQHV3\OHVHWDO DQGWKHUHDUHIHZVWXGLHV beneath the lower slope and doming area. The study area RIODUJHVFDOHIHDWXUHVVXFKDVFKDQQHOV\VWHPV 0DUWLQHW lies within the translational zone between extensional and al, 2011). In addition, systematic and hierarchical studies are FRPSUHVVLRQDO]RQHV &RKHQDQG0F&OD\&RUUHGRUHW greatly needed. We studied the turbidite channel architecture al, 2005), and belongs to the diaper-tectonic zone (Fig. 1). model within passive basins based on shallow-layer high resolution 3D seismic information in deepwater Niger 2.2 Depositional setting and stratigraphy Delta of West Africa as a prototype model. Different types The stratigraphy succession of Niger Delta mainly of channel systems were identified and the corresponding consists of three regressive lithostratigraphic units, as shown architecture models were established and the controlling in Fig. 2. From the oldest to the youngest these are the Akata, factors and spatial distribution of different channel Agbada, and Benin formations (Short and Stauble, 1967). systems were analyzed. This study may provide theoretical The Akata Formation of marine deposits, whose lithology is foundation, from a geological perspective, for the evaluation predominantly dark grey shale with turbidite or slope canyon and development of deepwater reservoirs. sediments, provides the most important source rocks in the Niger Delta Basin, and its thickness is 2,000 m to 7,000 2 Geological background m (Doust and Omatsola, 1990). The Agbada Formation The study area covers 350 km2 and is situated in the Niger of fluvial delta and marine facies, mainly comprised of Delta of West Africa, roughly 200 km to the south of Port sandstone interbedded with mudstone and marine shale, is the Harcourt with water depths of about 1,300-1,500 m (Fig. 1). major oil producing interval in weakly cemented sandstone The source area is the Niger River system to the north (Lü accumulated from until with a thickness et al, 2008). This subsection provides a brief introduction of over 3,500 m (Corredor et al, 2005). The Benin Formation, to the background of tectonics, depositional settings and whose lithology is mainly sandstone with thin mudstone or stratigraphy. shale, represents a set of continental delta plain sediments. The possible microfacies of sand bodies include mouth bar, branch channel and levee. This formation was deposited 4°E 6° 8° mainly in continental and neritic environments from the Lagos Legend Late Eocene to Holocene, and its thickness is up to 2,000 m Extensional zone (Corredor et al, 2005). 6° Translational N Niger zone The target interval is located in the Agbada Formation. Compressional River zone Lü et al (2008), from the perspective of sequence Cross stratigraphy, pointed that the study area had been in a Niger Delta River Calabar deepwater sedimentary environment, although there were Harcourt several obvious regression events. In addition, it is likely to have gravity slumping and submarine turbidites since 2° -100m the study area lies near the lower slope. Actually, seismic -500m Study sedimentology analysis shows that deepwater gravity flow N -1500m-1000m area which deposits under LST (lowstand systems tract) is the -2000m -3500m -3000m 2500m Area main sedimentary system for the study area, and the facies 100 km enlarged predominately consist of mass transport deposits and turbidite channels, etc (Li et al, 2008). Fig. 1 The Niger Delta continental margin showing bathymetry, zones of JUDYLW\WHFWRQLFVW\OH 0RGL¿HGIURP$GHRJEDHWDO  3 Seismic database and methodology The data set used for this research is a conventional 3D 2.1 Tectonic setting seismic volume with sampling interval of 3 ms and line The Niger Delta Basin developed in the spacing of 12.5 m in both crossline and inline directions. The and is a passive continental margin basin whose evolution can study interval is from the seafloor to the fourth condensed be divided into rifting and drifting phases. Development of section. There are four major sedimentary cycles in the this basin is attributed to the collapse of Gondwanaland and LQWHUYDOZLWKDQH[WHQVLYHDQGODWHUDOO\FRQWLQXRXVUHÀHFWLRQ the splitting of the south and equatorial Atlantic (Karner et al, at the top of each one (Fig. 3(a)). This interval ranges in 1997). thickness (two way travel time, TWT) from 0.6 s to 1.5 s, and Since the Eocene, long-term regression had prompted spectrum analysis shows that it has a frequency bandwidth the formation of the present Niger Delta, and during this from 15 Hz to 90 Hz with a dominant frequency of 70 Hz progradational process, the Niger Delta, affected by gravity, (Fig. 3(a)). If we take the interval velocity at 1,900 m/s, the formed a series of tectonic zones. Damuth (1994) divided resolution of thickness is approximately 6 m, which can WKHGHOWDLQWRWKUHHPDMRU]RQHVIURPQRUWKWRVRXWKWKHVH basically meet the need of architecture characterization of Pet.Sci.(2013)10:139-148 141

Southwest Northeast Quaternary Pliocene Benin Fm.

Late

Afram clay

Soku clay

Miocene Middle Opuana Fm. Buguma clay

Agbada clay Early

Akata Fm. Oligocene Agbada Fm. Alluvial deposits Late Deltaic deposits Channel complex Middle Carbonates Eocene Marine shales Early Synrift clastics Oceanic crust Cretaceous Late Erosional truncation

Fig. 2 Schematic diagram of the regional stratigraphy of the Niger Delta in the outer fold and thrust belt (After Corredor et al, 2005) different single channel bodies. and moderate-bright amplitudes (Fig. 3(b)). In most cases Because insufficient borehole data have been collected they have conformable contacts with the overlying and in the study interval to calibrate seismic interpretation with the underlying strata, but sometimes they slightly onlap lithology, sedimentary types are inferred from the external the syndepositional terrain at the bottom suggesting a morphology, the internal amplitude characteristics of seismic homogeneous sedimentation with no contribution from the facies, and stratigraphic position as well as indirectly by undulating ground. analogy with seismic and sedimentary facies interpretation from other areas where borehole data are available (Weimer, 4 Results and observation %HDXERXHIDQG)ULHGPDQQ'HSWXFNHWDO Reservoir architectures of different depositional sets 0RVFDUGHOOLHWDO;XHWDO 7KUHHW\SHV even within the same deep-water turbidite channel system of sedimentary facies, including massive transport, channel VKRZGLYHUVLW\WRVRPHH[WHQWLQÀXHQFHGE\GLIIHUHQWOHYHOV system and deepwater drape deposits, are interpreted. of incisional ability, which indicates that channel systems 0DVVLYHWUDQVSRUWHGGHSRVLWVZLWKZHDNHYHQWFRQWLQXLW\DUH have diverse types and that one specific channel system PDLQO\FKDUDFWHUL]HGE\PDVVLYHDQGWXPEOHUHÀHFWLRQWH[WXUH corresponds to a unique evolutionary process. with dusky or translucent amplitudes (Fig. 3(b)). Commonly, they are preserved with large thickness and lateral extension 4.1 Channel system types as a result of sediment slumping from shelf margins or slopes in LST during regression. Channel deposits, with Based on seismic facies features from the shallow seismic V-shape or U-shape in seismic sections, are predominantly data, channel systems, with different geomorphologic features, characterized by rough or imbricated reflection texture of FDQEHFODVVLILHGLQWRWKUHHW\SHVFRQILQHGVHPLFRQILQHG poor-moderate continuity and moderate-bright (strong) DQGXQFRQ¿QHG&RQ¿QHGFKDQQHOV\VWHPVZLWK98VKDSH amplitudes (Fig. 3(b)). Sometimes, on sides of channels, in seismic sections, develop an obvious incised surface (Fig. there are wedge-shape levees developed with reflectors of 4(a)), which is the boundary of large scale incised valleys high-continuity and amplitudes of weak to moderate-bright (or canyons). Although semi-confined channel systems also (Fig. 3(b)). Theoretically, channels have diverse origins with GHYHORSDQREYLRXVLQFLVHGVXUIDFHFRPSDUHGZLWKFRQ¿QHG FRPSOH[LQWHUQDOSDWWHUQVDQGXVXDOO\H[HUWVWURQJLQÀXHQFH ones, they have wedge seismic facies developed on both sides of incision on the underlying sediments. This will be covered (Fig. 4(b)), which correspond to large-scale levee deposits. in this paper. Submarine drape facies are characterized by They are generally seagull morphology in seismic sections parallel seismic reflections with highly continuous events (as the sloping levees at the sides of the channels look like 142 Pet.Sci.(2013)10:139-148

N S (a) 2 km 0.008

0.7s 0.006

0.004 Amplitude Major depositional 0.002 cycles 0.9s 10 3050 70 90 Frequency

1.1s

1.3s

1.5s

(b) N S 2 km

0.7s

Hemipelagic draping

0.9s Levee deposits Channel deposits

1.1s

1.3s

Block transport complex 1.5s

Fig. 3 (a) The major depositional cycles and the frequency dependence of the shallow seismic data. (b) The external morphology and the internal amplitude characteristics of seismic facies seagull’s wings). Within unconfined channel systems, the energies. most important feature is that no large-scale incised valley is According to topography and the corresponding types of developed as an obvious boundary (Fig. 4(c)) and they have the inner channel complexes, channel systems can be further hummocky morphology in seismic sections. FODVVL¿HGLQWRWKUHHW\SHVRIFRQ¿QHGFKDQQHOV\VWHPVHPL Channel complexes develop in the channel system. As FRQ¿QHGFKDQQHOV\VWHPDQGXQFRQ¿QHGFKDQQHOV\VWHPZLWK shown in Fig. 5, a channel complex can be classified into two subgroups for each of them (Table 1). The nomenclature two categories on the basis of the external morphology and of each subgroup follows the name of channel system and the internal amplitude characteristics of seismic facies. One takes the type of inner channel complexes into account. A is the incised channel complex developed in circumstances confined channel system, for example, is called confined of strong erosion with characteristic small-scale incised incised channel system when incised channel complexes valleys. The break points of seismic events mark the complex GRPLQDWHRURQWKHRWKHUKDQGDFRQ¿QHGHQYHORSHGFKDQQHO boundary. An incised valley contains multiple single channels system when enveloped channel complexes dominate. of hummocky-parallel reflections with strong amplitude as seismic features. The other category is the enveloped 4.2 Architecture patterns of different channel channel complex, developed in conditions of weak erosion, systems with no incised submarine canyons but with a boundary as A distinctive architecture corresponds to a unique channel an envelope-line of multiple single channels, which have an system. Based on the seismic response features, architecture LPEULFDWHUHÀHFWLRQFRQ¿JXUDWLRQRQVHLVPLFSUR¿OHV,QFLVHG models for different channel systems (Fig. 6) have been and enveloped channel complexes could develop in the same established, as described below. channel system but they often have different proportions $FRQ¿QHGFKDQQHOV\VWHPLVFKDUDFWHUL]HGE\ODUJHVFDOH within different channel systems of different sediment incised valley with negligible or no natural levees. According Pet.Sci.(2013)10:139-148 143

(a) P500 m WRGLIIHUHQWSDWWHUQVRILQWHUQDOFKDQQHOFRPSOH[HVFRQ¿QHG

50 m 50 channel systems can be further classified into incised and enveloped subgroups. In confined incised channel systems, LQFLVHGYDOOH\VFRPPRQO\DUH¿OOHGZLWKDODUJHQXPEHURI massive loads, collapsed and passing sediments with local a1 incised channel complexes. On the other hand, confined enveloped channel systems are dominated by enveloped P500 m channel complexes with local distribution of collapsed, 50 m 50 passing and draping sediments.

Confined channel system Incised valleys could also develop in semi-confined channel systems, but compared with those in confined a2 systems, migration of these incised valleys is probably (b) much more common and there are large scale natural levees P 500 m (incised-valley natural levees) on both sides. The incised- 50 m 50 YDOOH\QDWXUDOOHYHHVKDYHDZHGJHVKDSHLQSUR¿OHWDNLQJXS two thirds of the width of the entire channel system. These b1 large-scale natural levees also have two categories of near-

P channel levees and off-channel levees according to their P500 m

distances from the channels. The near-channel levees are 50 m 50 characterized by thin interbedded sands and shales, while the off-channel levees are dominated by mud with locally thin sand layers. On the other hand, semi-confined channel V\VWHPVFDQEHFODVVL¿HGLQWRWZRVXEJURXSVRILQFLVHGDQG b2

Moderately confined channel system enveloped based on channel-complex internal patterns. The P (c) P500 m P FKDQQHOFRPSOH[HVLQVHPLFRQ¿QHGLQFLVHGFKDQQHOV\VWHPV

50 m 50 are basically filled with collapsed sediments while the FKDQQHOFRPSOH[HVZLWKLQVHPLFRQ¿QHGHQYHORSHGFKDQQHO systems are mainly filled with relative fine sediments from c1 draping and passing. This kind of channel systems could also be classified into two categories of strongly-confined- P P P500P m FKDQQHOVDQGZHDNO\FRQ¿QHGFKDQQHOV7KHW\SLFDOIHDWXUH 50 m 50 of the former is that enveloped channel complexes develop solely in the incised valleys, and the latter is that enveloped

Unconfined channel system channel complexes not only develop in the incised valleys but c2 also cover the top of incised valleys. Canyon The levee of canyon No incised valley exists in unconfined channel systems, Incised channel complex Single channel which differs from confined and semi-confined systems.

Legend Enveloped channel complex

of boundary According to the type of internal channel complexes, an unconfined channel system can also be classified into two Fig. 4 Seismic profile showing the three major channel systems LGHQWL¿HGLQWKHVWXG\DUHDEDVHGRQWKHLQWHUQDOFRQ¿JXUDWLRQDQGWH[WXUH categories of incised channel system and enveloped channel DQGH[WHUQDOJHRPHWU\RIVHLVPLFUHÀHFWLRQV system. The vertical and lateral migration of incised channel

P500 m 500 m 50 m 50 m characteristics Seismic response Incised Enveloped channel complex channel complex

500 m

50 m pattern Architecture

Fig. 5,QWHUSUHWHG XSSHU VHLVPLFSUR¿OHVKRZLQJWKHDUFKLWHFWXUHSDWWHUQVRIGHHSZDWHUFKDQQHOFRPSOH[HV ORZHU 144 Pet.Sci.(2013)10:139-148

Table 1 &ODVVL¿FDWLRQVFKHGXOHRIFKDQQHOV\VWHPVLQWKHVWXG\DUHDLQWKH1LJHU'HOWD%DVLQ:HVW$IULFD

Characteristics Channel system types Geomorphologic features Channel complex types Channel system subtypes RIVHLVPLFUHÀHFWLRQ Incised channel complex &RQ¿QHGLQFLVHGFKDQQHOV\VWHP Hummocky/parallel &RQ¿QHGFKDQQHOV\VWHP V-shaped or U-shaped Enveloped channel complex &RQ¿QHGHQYHORSHGFKDQQHOV\VWHP Imbricate

Incised channel complex 6HPLFRQ¿QHG Hummocky/parallel 6HPLFRQ¿QHG incised channel system Seagull morphology channel system 6HPLFRQ¿QHG Enveloped channel complex enveloped channel system Imbricate Incised channel complex 8QFRQ¿QHGLQFLVHGFKDQQHOV\VWHP Hummocky/parallel 8QFRQ¿QHGFKDQQHOV\VWHP Hummocky morphology 8QFRQ¿QHGHQYHORSHG Enveloped channel complex channel system Imbricate

Channel Channel Channel system architecture pattern system types complex types 1000 m Incised

50 m

1000 m Confined channel system

Enveloped 50 m

1000 m

50 m

1000 m

50 m

1000 m Enveloped Incised Moderately confined channel system

50 m

1000 m

50 m

1000 m

50 m Enveloped Incised Unconfined channel system

Incised Enveloped Single Canyon channel complex channel complex channel

Crevasse splay Slump Debris, passing draping Basal lags deposit deposit Legend Proximal Distal Proximal Distal canyon levee canyon levee channel levee channel levee Fig. 6 Schematics showing the architecture models of different channel systems in the study area of Niger Delta Basin, West Africa Pet.Sci.(2013)10:139-148 145 complexes contribute to the development of incised channel where, E is the energy of sediment, m is loading mass, v is the systems, while enveloped channel systems are derived from ÀRZUDWHRIORDGȡ is the density of load, V is the volume of vertical and lateral migration of a single channel. In this load. kind of channel system, small scale natural levees, referred According to the functions above, the physical parameters as channel levees, are developed along both sides of the that control the erosive ability of sediments include the channel complexes. This kind of channel levees could also be GHQVLW\YROXPHDQGÀRZUDWHRIWKHORDGZKLFKDUHWKHUHVXOW FODVVL¿HGLQWRWZRFDWHJRULHVRIQHDUFKDQQHODQGRIIFKDQQHO of a series of geological factors (Table 2). In terms of density, based on the distance from channels. it is mainly controlled by the grain size of the sediments, which itself is determined by distance to the provenance and 5 Discussion the type of source rocks. Normally, high density corresponds to coarse loads. In terms of load volume, it is essentially 5.1 Analysis of controlling factors GHWHUPLQHGE\WKHDPRXQWRIVHGLPHQWVXSSO\WKHLQÀXHQFLQJ The types and features of deepwater detrital deposition factors of which include sea-level fluctuation, geological are results of the interaction of a series of auto-cycles and events namely tsunamis, hurricanes and earthquakes, as well KHWHURF\FOHV (PHU\DQG0\HUV:DQJHWDO  DVFOLPDWLFFKDQJHVLQVHGLPHQWVRXUFHV,QWHUPVRIÀRZUDWH 7KHFRQWUROOLQJIDFWRUVLQFOXGHVHDOHYHOÀXFWXDWLRQUHJLRQDO it is mainly controlled by topography, distance to the source tectonics, sediment types, the rate of sediment supply as well DUHDDQGWKHZLGWKGHSWKUDWLR,QPRVWVLWXDWLRQVÀRZUDWH DVWKHW\SHRIWUDQVSRUWDWLRQ 6WRZHWDO6WRZDQG increases as the distance from the source decreases, slope -RKDQVVRQ3LSHUDQG1RUPDUN:DQJHWDO increases and width-depth ratio decreases. )XJHOOLDQG2OVHQ/LHWDO ,QDGGLWLRQ These geological factors exert impacts at different levels frequent events such as hurricanes and earthquakes with high on the architecture of the turbidite channels in different energy could also have triggered the transportation of debris sedimentary systems or even within the same system. In grains, down shelf and slope, to form deep-water sediments different turbidite channel systems, the source area plays a (Shanmugam, 2008). more important role in reservoir architectures than eustasy, It is hard to compare the importance of each of these slope, and climate. For example, confined channel systems controlling factors. However, it is certain the different erosive are more likely to develop with source areas rich in gravels DELOLWLHVRIVHGLPHQWVGHWHUPLQHWKHLQ¿OOLQJSDWWHUQVZLWKLQ and sands, on the contrary, semi-confined and unconfined different channel systems or within the same channel system, channel systems tend to develop with source areas rich in which could be reflected in the kinetic energy and mass sand-mud mixtures or mud. An individual channel system’s functions as follows, type is controlled by interaction of various geological factors. However, primary factors play the most important role. For instance, sediment flow rate is high in steep-slope areas Emv u (1) ZLWKLQWHQVHHURVLRQUHVXOWLQJLQFRQ¿QHGDQGVHPLFRQ¿QHG mV U u (2) FKDQQHOV\VWHPVZKLOHVHPLFRQ¿QHGDQGXQFRQ¿QHGFKDQQHO systems mainly develop in gentle-slope areas. Similarly, all

Table 2 Factors affecting erosive abilities of different sediments

Sedimentary Physical parameters Geological factors factors Distance to the source area Load density Grain size Source types, for example, rich conglomerate, rich sand, mixture of sand and mud, rich mud Load quality 6HDOHYHOÀXFWXDWLRQIRUH[DPSOHORZVWDQGV\VWHPWUDFWWUDQVJUHVVLYHV\VWHPWUDFW highstand system tract Load volume Sediment supply Geological events, for example, tsunami, hurricane, earthquake Sediment energy Climate changes in the source area, for example, drought, wet

Topography, for example, steep slope area, gentle slope area

/RDGÀRZUDWH Distance to the source area

Width-depth ratio kinds of channel systems could exist in a lowstand system spite of the fact that they are controlled by multiple factors. tract during regression, but theoretically, turbidite systems In short, channel systems evolve laterally and channel would not develop in highstand system tracts without complexes evolve vertically within channel systems. LQÀXHQFHVIURPRWKHUJHRORJLFDOIDFWRUV In the plane, channel systems influenced by the GLIIHUHQWHURVLYHDELOLW\RIVHGLPHQWJUDYLW\ÀRZVDORQJWKH 5.2 Spacing evolution trend transporting direction could show some kind of evolution (Fig. Channel systems possess spatial evolutionary trends in  1HDUVRXUFHVHGLPHQWVDUHRIODUJHJUDLQVL]HKLJKÀRZ 146 Pet.Sci.(2013)10:139-148 rate and strong erosive ability at the proximal upper slope, the semi-confined enveloped channel systems that will be where canyons or large scale incised valleys could evolve further discussed below. During the early stage of channel LQWRFRQ¿QHGFKDQQHOV\VWHPV6HGLPHQWWUDQVSRUWDWLRQLVWKH GHYHORSPHQWDODUJHDPRXQWRIVHGLPHQWVDWKLJKÀRZUDWH major process in this kind of channel system, resulting in large have strong erosion ability, tending to form incised channel amounts of debris flow, turbidite and collapsed sediments complexes. Channel systems, as a transportation path, are with some locally developed straight or braided channels. primarily filled with debris flow, collapsed and passing On the middle slope, sedimentation gradually supersedes sediments with subsidiary incised channel complexes. HURVLRQDVVHGLPHQWVEHFRPH¿QHUDQGÀRZUDWHVVORZGRZQ During the middle stage, sediment supply is still sufficient In this situation, the most common channel system is a semi- EXWWKHÀRZUDWHGHFUHDVHVUHVXOWLQJLQDGHFUHDVHRILQFLVLRQ FRQ¿QHGFKDQQHOV\VWHPLQZKLFKERWKLQFLVHGDQGHQYHORSHG exceeded by an increase of sedimentation. Enveloped channel channel complexes are developed with some debris, collapsed complexes are the primary sedimentary units in this stage. DQGSDVVLQJVHGLPHQWVLQ¿OOLQJDQGODUJHVFDOHQDWXUDOOHYHHV During the late stage, sediment supply decreases as sea-level on both sides. Noticeably, channels in this area are sinuous rises under non-sudden-event conditions. Consequently, there to some degree. At the distal lower slope, sediments are only develop a small number of enveloped channel complexes FKDUDFWHUL]HGE\VPDOOJUDLQVL]HDQGORZÀRZUDWHVKRZLQJ RUHYHQLVRODWHGPXG¿OOHGVLQJOHFKDQQHOV stronger ability of aggregation and lateral migration but less It is important to notice that the evolutions cited above are HURVLRQ8QGHUWKLVFLUFXPVWDQFHXQFRQ¿QHGFKDQQHOV\VWHPV not absolute processes, because internal patterns of channel would dominate. These channel systems are characterized by systems are determined by many factors. For example, highly meandering channels which become less sinuous or FRQ¿QHGFKDQQHOV\VWHPVFRXOGDOVRH[LVWPLGIDQLIWKHVORSH even straight as sediment transportation carries on and lateral is large enough. Similarly, large scale channels could also migration ability decreases. This phenomenon has been develop within highstand system tracts if there is sufficient VKRZQLQPRGHUQVHDIORRUIDQV)RUH[DPSOHLQWKH=DLUH sediment supply caused by events such as earthquakes, fan, the average curvature decreases from 1.7 to less than 1.1 tropical cyclones and convulsive tsunamis, etc (Shanmugam, as the water depth increases from 3,300-4,100 m to 4,100- 2008). 4,800 m (Babonneau et al, 2002). In the vertical direction, a single channel system contains 6 Conclusions a set of evolutional channel complexes as a result of sea- By using 3-D high frequency seismic data from the level fluctuation and sediment supply rate changes (Fig. 8). shallow layers in a deep-water area of the Niger Delta, the This vertical evolution is much more obvious especially in reservoir architectures of deep-water turbidite channels

1000 m decreasing Lower sinuous single channel

50 m

1000 m Along flow direction

50 m Lower fan 1000 m

Higher sinuous single channel

50 m Along this direction, grain size fining, channel aggradation and lateral migration increasing 1000 m

1000 m 50 m Lower or higher sinuous single channel

Middle fan 50 m

1000 m

1000 m 50 m Straight or lower sinuous single channel

Upper fan 50 m

Strong Middle Weak Erosional competency Fig. 7 Plane evolutionary trend along channel systems Pet.Sci.(2013)10:139-148 147

decreasing 1000 m

Developmental period 50 m Along this direction,

1000 m

channel aggradation and lateral migration increasing Middle Late 50 m

1000 m

Early 50 m

Strong Middle Weak Erosional competency Fig. 89HUWLFDO RULQWHUQDO HYROXWLRQDU\WUHQGRIVHPLFRQ¿QHGHQYHORSHGFKDQQHOV\VWHPV in West Africa are established. Channel systems can be Acknowledgements FODVVL¿HGLQWRWKUHHPDMRUFDWHJRULHVFRQ¿QHGVHPLFRQ¿QHG DQGXQFRQ¿QHG7KHERXQGDULHVRIERWKFRQ¿QHGDQGVHPL The authors would like to acknowledge with whom we FRQ¿QHGV\VWHPVDUHREYLRXVLQWHUIDFHVRIODUJHVFDOHLQFLVHG had the honor and pleasure to work with, and learn from, valleys, but large scale wedge-shape natural levees only during our research process. These people include, but are develop along the valley flanks of semi-confined systems. QRWOLPLWHGWR+X*XDQJ\L6XQ/LFKXQ=KDR3HQJIHLDQG There are two kinds of channel complexes developed in Li Yan from China National Offshore Oil Company. This channel systems, namely incised channel complexes and work is supported by Open Fund (PLC201203) of State enveloped channel complexes. Incised channel complexes Key Laboratory of Oil and Gas Reservoir Geology and are characterized by small-scale incised valleys containing Exploitation (Chengdu University of Technology), National multiple single channels. Enveloped channel complexes are 6FLHQFHDQG7HFKQRORJ\0DMRU3URMHFW =; without incised valleys bounded by an enveloping line of  DQG0DMRU3URMHFWRI(GXFDWLRQ'HSDUWPHQWLQ6LFKXDQ multiple single channels. Accordingly, channel systems could 3URYLQFH =$  further be classified into six subgroups. Geological factors exert impacts of different levels on the architecture of the References turbidite channels in different sedimentary systems or even $GHRJED$$0F+DUJXH75DQG*UDKDP6$7UDQVLHQWIDQ within the same system. architecture and depositional controls from near-surface 3-D seismic Although the internal patterns of channel systems are data, Niger Delta continental slope. AAPG Bulletin. 2005. 89(5): controlled by multiple factors, they still exhibit a spatial 627-643 evolutionary trend. In planar view, the upper-fan is dominated %DERQQHDX16DYR\H%&UHPHU0HWDO0RUSKRORJ\DQGDUFKLWHFWXUH E\FRQ¿QHGFKDQQHOV\VWHPVDVDWUDQVSRUWSDWKRIVHGLPHQWV RIWKHSUHVHQWFDQ\RQDQGFKDQQHOV\VWHPRIWKH=DLUHGHHSVHDIDQ 0DULQHDQG3HWUROHXP*HRORJ\   ¿OOHGZLWKODUJHDPRXQWRIGHEULVÀRZFROODSVHGVHGLPHQWV Bea ubouef R T. Deep-water leveed-channel complexes of the Cerro Toro and locally developed straight or braided channels. The Formation, Upper Cretaceous, southern Chile. AAPG Bulletin. 2004. middle-fan is mainly composed of semi-confined channel 88(11): 1471-1500 systems containing a large number of curved incised or %HDXERXHI57DQG)ULHGPDQQ6-+LJKUHVROXWLRQVHLVPLFVHTXHQFH enveloped channel complexes with local distribution of debris stratigraphic framework for the evolution of Pleistocene intra slope flow, collapsed and passing sediments. Unconfined channel EDVLQVZHVWHUQ*XOIRI0H[LFR'HSRVLWLRQDOPRGHOVDQGUHVHUYRLU systems mainly exist in the lower-fan areas, characterized DQDORJV,Q:HLPHU3-6ODWW50-&ROHPDQ1&5HWDOHGV by meandering channels which become less curved or even 'HHSZDWHU5HVHUYRLUVRIWKH:RUOG*XOI&RDVW6HFWLRQ6(30WK straight as sediment transportation continues. Vertically, Annual Research Conference. 2000. 40-60 incision is the primary process during the early development &KHQ+4=KX;0=KDQJ*&HWDO6HLVPLFIDFLHVLQDGHHSZDWHU stage of channel systems, resulting in large amount of debris area of a marine faulted basin: Deepwater area of the Paleogene Lingshui Formation in the Qiongdongnan Basin. Acta Geologica flow, collapsed and passing sediments and incised channel Sinica (English Edition). 2012. 86(2): 473-483 complexes. During the middle stage, enveloped channel &KHQ-:1HZH[SORUDWLRQGRPDLQIRURLODQGJDVLQGHHSZDWHUEDVLQV complexes develop because deposition exceeds erosion. 0DULQH*HRORJ\/HWWHUV   LQ&KLQHVH Sediment supply decreases dramatically in the late stage, Che n W. Status and challenges of Chinese deepwater oil and gas leading to the development of a small number of enveloped development. Petroleum Science. 2011. 8(4): 477-484 FKDQQHOFRPSOH[HVRULVRODWHGPXG¿OOHGVLQJOHFKDQQHOV &RKHQ+$DQG0F&OD\.6HGLPHQWDWLRQDQGVKDOHWHFWRQLFVRIWKH 148 Pet.Sci.(2013)10:139-148

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