Received: 1 April 2017
Revised: 10 August 2017
Accepted: 10 November 2017
DOI: 10.1002/gj.3117
RESEARCH ARTICLE
Sequence architecture and depositional evolution in Lufeng Depression, Pearl River Mouth Basin, South China Sea Part A: The braided delta deposits of Cenozoic Enping Formation Shiqiang Xia1 Lianfeng Gao1
Zhen Liu2 Guiyu Dong1
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Donghong Luo3 | Liangfeng Shu3 Linting Zhang1 | Chunpeng Leng1
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Zhenguo Zhang1 Bingkun Zhang1
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1
The College of Mining Engineering, North China University of Science and Technology, Tangshan, Hebei Province, China
2
China University of Petroleum (Beijing), Beijing, China
The seismic merging dataset, wireline logs, core photographs, thin sections, and other geological information were employed to investigate sequence architecture and reveal depositional evolution of Enping Formation in Lufeng Depression, Pearl River Mouth Basin. The results showed that one second‐order sequence and four third‐order sequences were identified based
3
Shenzhen Oil Research Institute, China National Offshore Oilfield Corporation Limited – Shenzhen, Guangzhou, China Correspondence Zhen Liu, The College of Geosciences, China University of Petroleum (Beijing), No.18 Fuxue Road, Changping District, Beijing 102249, China. Email:
[email protected]
on the unconformities along basin margin/slope and correlative conformities in the basin centre. In addition, several associated sedimentary facies or microfacies (including braided channel deposits, subaqueous distributary channel deposits, beach‐bar deposits, and so on) that were recognized within sequence stratigraphic framework and depositional evolution were revealed by well tie–seismic correlations with seismic attributes extracted from the stratal slicing in terms of sand sections/units in Enping Formation. It was characterized by progradational process from seismic attributes in general that the delta deposits gradually retrograded. Through comprehen-
Funding information National Key Basic Research Project, Grant/ Award Numbers: 2011ZX05025‐002‐05, 2011ZX05001‐001‐04 and 201105002‐006; Natural Science Foundation of China, Grant/ Award Numbers: 91328201, 40972081, 40372056 and U1262205; Fundamental Research Funds for the Central Universities, Grant/Award Number: 2010ZD07; Natural Science Project for Hebei Province, Grant/ Award Numbers: D2015209075, 2017209236 and ZD2016077
sive analysis, it can be concluded that episodic tectonic movements and subsequent uplift played
Handling Editor: S. Li
KEY W ORDS
an important role in controlling sequence architecture. Sediment supply and lacustrine level fluctuations derived from accommodation space changes exerted an essential effect on depositional evolution. This study provides new and robust insights into understanding the sequence architecture, depositional evolution, and hydrocarbon exploration in Enping Formation. In addition to the significance for academia, study of sequence architecture and depositional evolution are of great importance for hosting potential hydrocarbon reserves, because they are frequently related to important and potential reservoirs. Consequently, investigating sequence architecture and depositional evolution are significant economically as well.
depositional evolution, Enping Formation, Lufeng Depression, Pearl River Mouth Basin, seismic merging dataset, sequence architecture
1
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I N T RO D U CT I O N
and settings of basin evolution provide us the basis for determining sequence architecture and depositional evolution. Because the interac-
Sequence stratigraphy, depositional evolution, and their controlling
tion of controlling factors during basin development and evolution in dif-
factors in sedimentary basin analysis have long been of great scientific
ferent stages generates different sequence architecture and sedimentary
and practical interest to oil industries and academics both in China
facies in different positions, all these will assist in achieving a new under-
and abroad (Allen, 2005, 2007, 2008a, 2008b; Allen & Allen, 2013;
standing of sequence stratigraphic framework and depositional evolution
Gawthorpe, Leeder, Collier, et al., 2006; Li, 2015; Li, Xu, & Gao, 2013;
within it in rock records and also will assist in making further decisions in
Lin, 2009; Posamentier & Kolla, 2003; Wood & Ethridge, 2012).The con-
terms of petroleum exploration and development in sedimentary basins.
trolling factors consist of tectonic movements, sea‐level/lake‐level fluc-
In the last few decades, improvements and progress in 3‐D seismic
tuations, sediment supply, and climate (Catuneanu, 2006). The stages
data acquisition have enhanced our understanding on how to improve
Geological Journal. 2018;1–16.
wileyonlinelibrary.com/journal/gj
Copyright © 2018 John Wiley & Sons, Ltd.
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seismic processing techniques perfectly, especially seismic merging
et al., 2015; Zhu et al., 2015), but little progress has been made on
processing technique to meet the needs of seismic‐related research
sequence architecture and depositional evolution of the late Paleo-
day by day. Seismic merging processing technique, referring to special
gene in Lufeng Depression due to high exploration cost and compli-
seismic process technique in the last few years, not only deals with
cated basin architecture. However, the newly reprocessed seismic
seismic data derived from different years, various azimuth, and multi-
merging datasets and widely accepted sequence stratigraphic and sed-
ple acquisition of the same survey but also acts as the extension and
imentary principles provide the basis for further study on sequence
development of seismic data merging (Wang, Zeng, Yi, et al., 2010). It
architecture and depositional evolution in Enping Formation.
follows the principle of calibrating seismic energy and phase character-
This paper presents preliminary and effective results of a compre-
istics, eliminating differences of different surveys caused by
hensive study employing 3‐D high‐resolution seismic merging data,
nongeological influences and establishing unified bin spacing for seis-
wireline logs, core photographs, and other related materials collected
mic datasets of different period based on original frequency and azi-
in the study area to establish sequence stratigraphic framework, reveal
muth. All these procedures can ensure the kinematic and dynamic
sequence architecture, identify sedimentary facie types, and recon-
consistency of original seismic datasets (He, Zhang, Yao, et al., 2013;
struct depositional systems evolution. Understanding these can make
Shi, Huang, Wang, et al., 2015; Wang et al., 2010).Through information
great contributions for prospect prediction and evaluation and provide
complementary resulted from seismic merging processing technique, it
new insights into petroleum potential of the study area.
can largely enrich the azimuth information of seismic data that makes it genuine wide‐azimuth seismic datasets. It is designed to better image the structure and analyse the sequence architecture and depositional
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R E G I O N A L G E O LO G I C A L S ET T I N G S
evolution due to wide‐azimuth seismic reflections. The Pearl River Mouth Basin, one of the most important
The Pearl River Mouth Basin, a petroliferous basin in Southeast China,
petroliferous basins in Southeast China, developed in response to
is a Cenozoic‐dominated continental‐margin basin located in the cen-
movements of plates and subsequent tectonic events around it (Briais,
tral part of northern continental margin of the South China Sea. It
Patriat, & Tapponier, 1993; Cullen, Reemst, Henstra, Gozzard, & Ray,
covers an area of 17.5 × 104 km2. The maximum buried depth is over
2010; Hayes & Nissen, 2005; Lin, Watts, & Hesselbo, 2003; Xie, Ren,
10,000 m and located in Baiyun Depression. The whole basin is
& Wang, 2015; Zhang, Wang, & Wu, 2015; Zhu et al., 2015) and
surrounded by Wanshan Uplift to the north, Hainan Uplift to the west,
served as an essential passive continental margin analogue for the
Xisha and Dongsha Uplift to the south, and Dongsha Uplift to the east.
study of sequence architecture and depositional evolution. Lufeng
Due to tectonic movements of adjacent plates, it is composed of seven
Depression, as one of the most petroleum‐rich areas in the northeast
substructural units with northeast strike: Northern Depression, Central
of Pearl River Mouth Basin, had gone through episodic rifting and
Depression, Southern Depression, Shenhuansha Uplift, Hainan Uplift,
depression stages in the Paleogene period, and as a result, had been
Dongsha Uplift, and Northern Fault Belts (Figure 1). The Pearl River
very complicated in sequence architecture and depositional evolution.
Mouth Basin had undergone long‐term geological evolution with multi-
Numerous studies have been conducted on the sedimentology and
stages of tectonic deformations due to conjunct influence of the Eur-
sequence stratigraphy of Mesozoic and Cenozoic deposits in the last
asian, Pacific, and Indian plates since the Cenozoic, which shared
few years (Lin et al., 2003; Wang & Li, 2009; Xie et al., 2015; Zhang
similarities to many other passive‐margin basins. It mainly underwent
Location and tectonic units of Pearl River Mouth Basin, South China Sea (integrated and modified from H. S. Shi, 2013; Xie et al., 2015; Zhu et al., 2015) [Colour figure can be viewed at wileyonlinelibrary.com]
FIGURE 1
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rifting stage, transition stage, early drifting, and late drifting stages sep-
caused an increase in basin area and a decrease in water depth. During
arated by many regional angular or minor‐angular unconformities
this period, it was dominated by lacustrine‐deltaic deposits that gradu-
(Briais et al., 1993; Cullen et al., 2010; Dong, Zhang, Zhong, Yuan, &
ally evolved into source rocks. In contrast, the latter occurred in late Oli-
Wu, 2009; Hayes & Nissen, 2005; Li, 1993; Lin, Zheng, Ren, et al.,
gocene (Since 32 Ma) and lasted to early Miocene (18.5 Ma). The
2003; Pang et al., 2009; Ru, 1988; Xie et al., 2015; Zhang et al., 2015;
tectonic movement was characterized by inherited uplifts and erosions.
Zhu et al., 2015). Based on the characteristics of fault activities and
However, the water moved landward in the meantime.
magmatic information, there were five essential tectonic movements
Due to tectonic deformations and associated stratigraphic
during the Cenozoic in the Pearl River Mouth Basin: Shenhu Move-
responses, the Eogene strata keeps a record of Shenhu Formation,
ment, the first episode of Zhuqiong Movement, the second episode
Wenchang Formation, Enping Formation, Zhuhai Formation, Zhujiang
of Zhuqiong Movement, Nanhai Movement, and Dongsha Movement
Formation, Hanjiang Formation, Yuehai Formation, and Wanshan For-
(Dong et al., 2009; Ru, 1988; Zhu et al., 2015). All these movements
mation in ascending order (Figure 2). The target interval in the research
exerted an important influence in controlling the palaeo‐geomorphol-
is Enping Formation, and the detailed descriptions of Enping Formation
ogy, sequence architecture, and depositional evolution of Cenozoic
are as follows: The Enping Formation unconformably overlies
strata. The deposition of Enping Formation (38–32 Ma) developed in
Wenchang Formation and separated by T80 seismic reflection surface
response to the second episode of Zhuqiong Movement and early
(Figure 2). The strata overlie basements directly adjacent to or around
Nanhai Movement. The former occurred between middle‐late Eocene
local uplifts. The lithology is mainly composed of offwhite sandstone,
and early Oligocene (39–32 Ma), which was characterized by long‐
sandy conglomerate, and black shale interbedded. The buried depth is
lasted and intensive tectonic activity. It resulted in big‐scale uplifts, ero-
about several hundreds or thousands of metres. Occasionally, it
sions, associated fault movements, and magma eruptions. All these
develops thin coal beds that may be interpreted as flooding plain
FIGURE 2 Comprehensive stratigraphic column in Lufeng Depression, Pearl River Mouth Basin, South China Sea. SQ = sequence, the column showing sequence classification, depositional evolution, and different sedimentary facies by different colours with legend below [Colour figure can be viewed at wileyonlinelibrary.com]
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deposits. During this deposition period, it is characterized by river‐del-
last few years. Data quality in the survey at the target stratigraphic
taic deposits that appear as fining‐upward characteristics in general with
interval is excellent, with 35 Hz in the main frequency width regionally
carboniferous phytoclast and coarsening‐upward characteristics locally.
that theoretically can be used to identify 10‐m sandstones more or
As the target interval of this study, Enping Formation has become an
less. In addition, the frequency width is sufficient for reconstruction
essential high‐quality reservoir in Lufeng Depression demonstrated by
of second‐order and third‐order sequence stratigraphic framework in
hydrocarbon exploration in recent years. However, the sequence
the region of seismic coverage outside well control. The seismic
architecture and depositional evolution are still poorly understood.
datasets include pure wave data and result data.
The study area, one of the most essential hydrocarbon migration and accumulation zone, is located in southern Huilu Low Uplift of sion to the west and Lufeng Depression to the east. There is a distance
3.2 | Well data (wireline logs, core photos, and thin sections)
of 240 km to southern Hongkong with water depth of 145 m on aver-
For the time being, there are 45 wells in the study area, many of which
age (Figure 3).
are production wells. Only 13 wells were drilled to target interval.
Northern Depression, Pearl River Mouth Basin with Huizhou depres-
Wireline logs tied to seismic survey will assist in verifying and calibrat-
3
DATABASE AND METHODOLOGY
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All data employed in this study were provided by Shenzhen Branch of CNOOC Ltd. And the data that will be utilized include 3‐D poststack seismic datasets, wireline logs, core samples and thin sections, and other related data.
ing seismic interpretation, stratigraphic framework establishment, and depositional systems identification according to the stacking patterns and seismic reflections. That is because the resolution of seismic data is far lower than well data. Core data, no matter side coring or drill coring, provide geologists and researchers the most direct and accurate image of depositional systems and relative macro sedimentary environment, characteristics of sequence boundary, while thin sections provide micro grain size, contact type.
3.1
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Seismic data
The subset of seismic data pertinent to this study consists of about 188 km2 of a poststack, time‐migrated, recently reprocessed 3‐D seis-
3.3
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Other related data
mic merging volume (included one gathered in 2003 and the other
This mainly consists of cumulative frequency curve of grain size and
gathered in 2014), with a bin spacing of 12.5 × 25 m and a two‐way
thin sections. All these can be a great help in establishing sequence
travel time trace length of 5,000 ms. It is wide‐angle gathered in the
stratigraphic framework and identifying depositional systems.
FIGURE 3 Schematic map showing the location of wells and tectonic unit divisions of the study area (green line in Figure 1) in Pearl River Mouth Basin, South China Sea [Colour figure can be viewed at wileyonlinelibrary.com]
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3.4
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Methods
According to the database provided, the workflows designed are used to study sequence architecture and depositional evolution of Enping Formation. Based on the comprehensive analysis of previous studies, seismic data, well data, and other related data about the study area, fine structural interpretation of 3‐D seismic merging volume profiles provides the basis for establishing sequence stratigraphic framework (Xia et al., 2015). In this step, different‐order unconformities (mainly second‐order and third‐order) are recognized based on the seismic reflections and wireline log stacking patterns. The unconformities recognized characterize the structural style. Then well tie–seismic calibration is essential for the tracing of sequence boundary. All this process mentioned above are to reveal the sequence architecture of the target
stratal architectures. Each stratal architecture defines a particular genetic type of deposits with a unique geometry and sediment dispersal pattern within the basin. All these associations in time and space reveal the evolution of sequence architecture and depositional systems (Catuneanu et al., 2009). This new method has considerably improved our insights into how sedimentary basins accumulate and preserve sediments and has become a highly successful and widely used exploration technique in the search for natural resources (Catuneanu, 2002). The acquirement of high‐resolution geophysics and geochemistry data makes it possible to obtain new progress in stratigraphic architecture analysis and depositional evolution characterization (Catuneanu, 2002; Catuneanu, 2006; Catuneanu et al., 2011; Catuneanu, Martins‐ Neto, & Eriksson, 2012; Embry, 2009; Lin, 2009; Lin, Xia, Shi, 2015).
interval. The types of depositional facies need to be defined based on the sedimentary characteristics of cores, stacking patterns of logs, reflection characteristics of seismic events, and other related geological data. In addition, stratal slicing of 3‐D seismic volume may often significantly image the horizontal distribution and vertical evolution of depositional systems; it aims to take advantage of techniques from seismic sedimentology to determine the spatial depositional evolution (Xia et al., 2013; Zeng, Ambrose, & Villalta, 2001).
5.1
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Sequence classification
The traditional method in establishing sequence stratigraphic framework is to trace the unconformities and correlative conformities, facies discontinuities, and flooding surfaces based on the comprehensive analysis of core observation and description, stacking patterns of wireline logs, interpretation of seismic profiles, and outcrop measurements. The establishment of sequence stratigraphic framework is essential for reconstruction of palaeo‐geography and strategic deci-
4 | T HE R E S U L T S O F S E I S M I C M E R G I N G DA T A
sions for hydrocarbon exploration and exploitation (C. S. Lin, 2009). In this paper, we employ 3‐D seismic merging data, wireline logs, cores, and so on to establish a chronological framework. The results
Seismic merging process with different azimuths is a systemic process.
indicate that one second‐order sequence and four third‐order
It has to take azimuth, data regulation, static correlation, random noise
sequences are identified through integrated analysis of the data men-
attenuation, signal noise ratio, asymmetric prestack time migration,
tioned above during Enping deposition period. The division of differ-
and so on into consideration to make signal noise ratio and continuity
ent‐order sequences is in response to regional tectonic stages. The
of seismic data greatly improved and seismic imaging in deep buried
second‐order sequence identified based on regional unconformities
depth more clear. The results of seismic merging data indicate that
and correlative conformities are mainly influenced by episodic tectonic
the seismic events are more continuous. In addition, the fault shadows
movements. While the third‐order sequence is identified based on the
are much clear, which makes it easy to identify. The seismic merging
unconformities along the basin margin or scoured surfaces and correl-
data reveals more details in internal architecture and stacking patterns
ative conformities in the basin centre that are formed corresponding to
of channel deposits (Figure 4). This provides the basis for the descrip-
tectonic movements, sea‐level/lake‐level fluctuations, base‐level
tion and characterization of river channel deposits. Besides this, it also
changes, and so on in the basin. These features mainly are character-
helps the subsequent work contents in this study.
ized by angular or minor angular unconformities along the basin margin or palaeo‐uplifts and also change into disconformities or correlative conformities in the central basin. All these features can be traceable
5 | S E Q U EN C E C L A S S I F I C A TI O N A N D ARCHITECTURE
throughout the study area. There are also some sand sections/units defined due to the needs for hydrocarbon development. These sand sections/units are divided in the chronological framework, and
Sequence stratigraphy is widely accepted as a new and effective
therefore, they also present geological chronological meanings to some
method of stratigraphic analysis by both academic and industry practi-
extent.
tioners at home and abroad (Catuneanu, 2002). And also, it highlights facies types, facies relationships, and stratal architecture within a chronological framework. However, the identification of sequence bound-
5.2
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Sequence architecture
ary provides the basis for chronological framework. Classic sequence
The base (T80) and the top (T70) of Enping Formation are bounded by
stratigraphic framework was established by identifying the unconfor-
second‐order unconformities that are generated corresponding to
mities on basin margin and correlative conformities in the basin centre.
regional tectonic movements. However, the base (T80) is defined by a
The identification of sequence boundary contributes a lot to the inter-
regional subaerial unconformity on the palaeo‐uplift and correlative
pretation of chronological framework and the analysis of sequence
conformity in the basin centre, with truncation below and onlap above
architecture and depositional evolution within it. The analysis firstly
the surface from the seismic profile (Figure 5). The unconformity can
starts from the division of framework into several different‐order
also be identified from the logs and lithology that is characterized by
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FIGURE 4
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The comparisons among seismic data newly reprocessed [Colour figure can be viewed at wileyonlinelibrary.com]
FIGURE 5 Sequence architecture and depositional evolution interpretation of 3‐D seismic profile in Lufeng Depression (See Figure 3 for location of section line) [Colour figure can be viewed at wileyonlinelibrary.com]
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abrupt changes in lithology and stacking patterns in wireline logs. The logs below the surface appear as high‐amplitude gamma ray, while it
6 | SEDIMENTARY FACIES TYPE AND D E P O S I T I O N A L E V O LU T I O N
appears as low‐amplitude gamma ray, serrated bell‐shaped stacking patterns above the surface (Figure 5). The top (T70) is defined by minor
The identification of depositional facie types and investigation of
angular unconformity also with truncation below and onlap above the
depositional evolution have long been a hot topic in sedimentary geol-
surface. It is also featured in wireline logs especially. The stacking pat-
ogy and basin analysis. Over the last decades, significant progress has
terns above the surface are serrated funnel‐shaped, coarsening‐upward
been made based on comprehensive analysis of seismic datasets,
sandstone to siltstone while it appears as high‐amplitude gamma ray,
wireline logs, core samples, laboratory tests, and field outcrops. In this
coarse‐grained sandstone with interbedded shale (Figure 5).
study, seismic sedimentology is employed to extend single sedimen-
In accordance with the major unconformity identified above (T70
tary facies derived from observation of cores, stacking patterns of well
and T80), four third‐order sequences can be recognized within Enping
logs, and internal configuration and external geometry of seismic data
Formation (Figure 5). And the division is based on the cycle changes in
to the entire study area and provide plain view of sedimentary facies.
wireline logs to a larger extent with seismic reflection characteristics as
The study will assist in determining depositional evolution in time
references. They generally keep a record of local transgressive–
and space within Enping Formation and evaluating favourable zones
regressive depositional cycles. The sequences of sq1, sq2, sq3, and
for hydrocarbon exploration and exploitation. In addition, it will also
sq4 are the target intervals of this paper. Sq1 develops in response
help analyse the depositional response to tectonics, climate, lake level
to base‐level rising process (Figure 5). The basal boundary surface of
fluctuation, and sediment supply.
sq1 appears as unconformity with obvious truncation below and onlap above the surface from seismic profile. It is overlapped on T80 surface
6.1
Depositional facies type
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on the palaeo‐uplift margin, and there is only LF‐L drilled. It cannot be traceable throughout the basin due to its depositional characteristics.
Based on seismic data, wireline logs, cores, and thin sections, numer-
The bottom surface of sq1 appears as weak‐amplitude, discontinuous
ous types of sedimentary facies are determined in the study area:
reflections. It also pinches out upward along the margin of the
(a) braided channel deposits; (b) subaqueous distributary channel
palaeo‐uplift. The thickness of sq1 tends to be thinner from central
deposits; (c) lacustrine shale deposits; (d) beach‐bar deposits; and
basin to palaeo‐uplifts. It is composed of several low‐amplitude bell‐
(e) maximum flooding surface deposits.
shaped stacking patterns that may be interpreted as distributary channel in terms of wireline logs in vertical successions. The sq2 and sq3
6.1.1
also develops in response to base‐level rising process. The bottom sur-
The Figure 6 shows some of the detail of the braided channel channel
face appears as medium‐strong amplitude, continuous reflection with
deposits as preserved in well LF‐B within a fully offshore setting. The
obvious onlap above. The thickness of sq2 and sq3 does not change
multiple fining‐upward lithology associations in well LF‐B are com-
|
Braided channel deposits
too much in horizontal direction. They consist of thick cylinder‐shape
posed by a series of 3–4 channels, each about 10–30 m deep
coarse‐grained sandstone interbedded with thin shale that can be
(Figure 6). The core data show multistacked medium‐ to coarse‐
interpreted as braided channel. And 2,980 sand section/unit develops
grained sandstones interbedded with very thin shale (Figure 6a). This
on the top of sq3 successions. It is characterized by fining‐upward
means high energy settings during deposition. It is composed of multi-
sandstone and cylinder‐bell shape stacking patterns. The sq4 is com-
ple thick fining‐upward successions from lithology and wireline logs of
posed of entire transgressive to regressive depositional cycle. The
well LF‐B. Each succession begins with a distinct scoured surface at its
lower cycle is characterized by fining‐upward, cylinder‐bell shape
base that indicates an erosion of underlying strata (Figure 6b). The
sandstone, while the upper cycle appears as coarsening‐upward funnel
shaly barrier below the scoured surface may develop in response to
shape sandstone. In addition, there is a maximum flooding surface
river channel migration. However, below the scoured surface, small‐
identified between base‐level transitions. In this sequence, there are
scale wedge cross‐bedding, parallel bedding deposited within good‐
many sand sections/units developed, such as 2,900 sand section/unit,
sorted, subrounded sandstones are observed in cores (Figure 6a and
2,880 sand section/unit, 2,850 sand section/unit, which are the basic
6c). In contrast, heterogenetic grain‐size sandstones with no obvious
units for depositional evolution analysis (Figure 5). Among them,
sedimentary structure are also observed in cores (Figure 6b).
2,980 sand section/unit is dominated by relatively thick sandstone
From the seismic section, it can be seen that the incised braided
and thin shale showing multiple fining‐upward trend. It is characterized
channels are characterized by superimposed stacking patterns in chan-
by high‐amplitude serrated bell‐shape and cylinder‐shape stacking
nel belts. The width–depth ratio of channel belts is low. It shows weak
patterns (Figure 5). In contrast with 2,980 sand section/unit, 2,900
amplitude, uncontinuous reflections (Figure 7). It is believed that this
sand section/unit show single fining‐upward stacking patterns and
kind of braided channel is mainly developed in response to relatively
colour‐deepening lithology that may indicate an increase in accommo-
low accommodation space.
dation space (Figure 5). The 2,880 sand section/unit is characterized
In thin sections of well LF‐D at 3226.12 m measured depth
by low‐amplitude finger‐shape stacking patterns in wireline log and
(Figure 8), matrix in the braided channel deposits is high (as much as
light grey medium‐grained sandstone, while the 2,850 sand section/
20–30%). Grains are angular and poorly sorted, indicating short‐
unit shows high‐amplitude serrated cylinder‐shape stacking patterns
medium transport distances. In addition, the clastic particles adjacent
and light grey medium‐grained sandstone interbedded with thin
shoreline setting are affected by tides, waves, and winds and are there-
green‐grey shale (Figure 5).
fore relatively better sorted and more rounded. The thin section in
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Characteristics of braided channel in stacking patterns and cores (See Figure 3 for well location) [Colour figure can be viewed at wileyonlinelibrary.com]
FIGURE 6
FIGURE 7 The seismic characteristics of braided channel (See Figure 3 for location of seismic section) [Colour figure can be viewed at wileyonlinelibrary.com]
Characteristics of braided channel in thin section made from sandstone from well LF‐D at 3,226.12 m measured depth, showing/ displaying poorly rounded angular grains. The thin section was made from left core; upper thin section photomicrograph was taken in plane‐ polarized light, while lower thin section was taken in cross‐polarized light (modified from Zheng, internal report; see Figure 3 for well location) [Colour figure can be viewed at wileyonlinelibrary.com]
FIGURE 8
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plane‐polarized light shows fine‐medium grain‐size and good‐sorted
geomorphology. Hence, the wireline logs are not serrated and charac-
character (Figure 8). It is characterized by subrounded, point‐contact,
terized by small‐scale bell‐shape and finger‐shape stacking patterns. It
grain‐supported. The inter‐granular space is filled by siltstone and shale.
is predominantly composed of fining‐upward, medium‐ to coarse‐
However, in cross‐polarized light the thin section appears as high con-
grained, sandstones locally and black shale on the whole (Figure 6d).
tent and maturity of quartz. The clastic constituents are mainly com-
There is obvious scoured surface at its base, indicating an erosion of
posed of quartz and feldspar. It is characteristic of medium‐sorted,
underlying strata (Figure 6e). The lithology and colour gradually fines
subangular or subrounded with intergranular pore and intergranular
and lights in ascending order that may represents an increase in accom-
emposieu that indicates short‐medium transport distance and strong
modation space and water depth.
reconstruction (Figure 8).
It can be seen from the seismic section that it is characterized by
Based on grain size statistics from microscope observation, the
separated, strong amplitude, and uncontinuous reflections that indi-
cumulative frequency curve of sandstones has been calculated. The
cate relatively high accommodation space than that of braided channel
result shows four linear segments on the cumulative frequency curve,
(Figure 11). The width–depth ratio of subaqueous distributary channel
which can be interpreted as rolling, bouncing, and suspension popula-
is higher than that of braided channel.
tions, respectively. Among them, it is characterized by 8% rolling population, 82% bouncing population, and 10% suspension population. However, there are 62% lower saltation population and 20% upper saltation population. The rolling population exhibits low gradient linear character, while the suspension population shows the same. The bouncing population can be further divided into two segments, because many sediments are composed not of one single grain size population but rather of a combination of subpopulations. Each sub‐population may be defined by dynamic considerations or by supply characteristics. In the natural environment, individual subpopulations may be present in
6.1.3
|
Lacustrine deposits
The lacustrine deposits in the study area are mainly include deep lacustrine shaly deposits and beach bar deposits. Each of them are characterized by their own sedimentary and geophysical characteristics. Generally, the lacustrine deposits are mainly composed of sharp‐based and sharp‐topped sandstone units that range in thickness from 0.5 to 10 m, have lateral continuity of hundreds of metres, and occur interbedded within delta front mudstone units.
varying proportions from site to site. The one in well LF‐C shows distinct tractive currents that drive the sediment move forward. Sandstone samples from well LF‐C have a quartzo‐feldspathic
Deep lacustrine shaly deposits The deep lacustrine shaly deposits consists of dark grey to black lami-
composition with mean Q, F, and L values of 68%, 15%, and 17%,
nated mudstone or marl that commonly contains centimetre‐ to
respectively (Figure 9). Glauconite is a minor but characteristic compo-
millimetre‐scale planar layers of siltstone to very fine sandstone
nent of sandstone beds observed from cores and related test in the
(Figure 10d and 10e). It is characterized by high gamma ray, low resis-
study area (Figure 10). The interpreted source area for the Enping For-
tivity, and serrated dactylitic stacking patterns adjacent to mudstone
mation may consist mainly of medium‐distal plutonic and medium‐
base line. It usually occurs interbedded with pro‐delta turbidites. No
grade metamorphic rocks, with contributions from volcanic rocks
fossils are observed from cores (Figure 10d and 10e).
(Zhou, Ru, & Chen, 1995). The Enping Formation underwent rapid burial during the Oligocene and Miocene and associated cementation.
Beach bar deposits
Most of the mud‐poor sandstones are well cemented; however, as the
It includes main part and flank of beach bar deposits, and it is predom-
mud content increases, the rocks become less well cemented and
inantly composed of good‐sorted, coarsening‐upward medium to fine
therefore more friable.
sandstones containing broken shell debris interbedded with thin mudstones. In addition, there are occasional centimetre‐thick beds that is
6.1.2
Subaqueous distributary channel deposits
rich in algae debris that indicates a relatively high depositional energy
The facies associations of subaqueous distributary channel deposits in
and suggests that deposition occurred adjacent to a coarse clastic
the study area are developed in response to relatively flat
input point to the basin (Figure 10a). The serrated geometry shows
|
The cumulative frequency curve and petrological characteristics of braided channel (please see Figure 5 LF‐C for depth and relevant depositional characteristics). Q = quartz; F = feldspar; R = rock debris [Colour figure can be viewed at wileyonlinelibrary.com]
FIGURE 9
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FIGURE 10 Characteristics of lacustrine mud deposits and beach bar deposits in stacking patterns and cores (See Figure 3 for well location) [Colour figure can be viewed at wileyonlinelibrary.com]
The seismic characteristics of subaqueous distributary channel (See Figure 3 for location of seismic section) [Colour figure can be viewed at wileyonlinelibrary.com]
FIGURE 11
high‐energy sedimentation, while the bell‐shape stacking patterns of
progradational stacking patterns takes place during continued base
logs indicate fining‐upward trends locally. The main part of beach bar
level rise at the shoreline, when the sedimentation rates start to out-
deposits is dominated by grey, good‐sorted, medium‐ to fine‐grained
pace the rates of base level rise. Hence, it can be identified on seismic
sandstones with no distinct sedimentary structures from core samples
profiles as surfaces of parallel or subparallel reflections along the basin
(Figure 10b). In contrast, the flank of beach bar deposits appears as off‐
margin and downlap reflections in the basin centre and characterized
white fine‐grained sandstones interbedded with thin shaly belt on the
by high gamma‐ray signals within sequences in the Enping Formation.
whole. It shows a general coarsening‐upward and a decrease in shale
High gamma‐ray signals are a result of high concentrations of organic
content on a scale of 10–20 cm from cores. The small‐scale wavy
matter and radioactive elements (Figure 5).
cross‐bedding appears at the middle to top of cores, while soft‐sediment structure develops at the bottom (Figure 10c).
6.2
|
Depositional evolution
6.1.4 | Maximum flooding surface deposits within Enping Formation
reveal the evolution of depositional systems during the research based
Maximum flooding surface (Catuneanu, 2002; Frazier, 1974) is defined
on the data quality (especially seismic reflection characteristics) and
relative to the T–R curve, making the end of shoreline transgression.
work scope (De Bruin, Hemstra, & Pouwel, 2007; De Groot, De Bruin,
The non‐linear chronostratigraphic slicing technique is employed to
And therefore, it separates retrogressive strata below from progressive
& Hemstra, 2006; Jia et al., 2014). Non‐linear chronostratigraphic
strata above. In nonmarine successions, the maximum flooding surface
slices are generated by a data‐driven approach of dip‐steering in the
is distributed at the top of fining‐upward deposits. The position of this
Wheeler transformed domain (Figure 12a), aiming at complex seismic
surface may be indicated by an abrupt increase in fluvial energy, from
reflections such as truncations and erosional or depositional hiatuses
braided systems to delta systems. The presence of prograding strata
in the deposits of braided‐delta in Enping Formation (De Groot et al.,
above identifies the maximum flooding surface as a downlap surface
2006; De Bruin et al., 2007). Dip‐steered tracking starts from one
on seismic data. The change from retrogradational to overlying
selected inline, cross‐line position at all seismic sample positions that
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ET AL.
FIGURE 12 Non‐linear chronostratigraphic slices of Enping Formation in Lufeng Depression, Pearl River Mouth Basin. (a) Data‐driven approach of dip‐steering in the Wheeler transformed domain (De Groot et al., 2006; De Bruin et al., 2007; Jia et al., 2014). (b) Sequence stratigraphic framework of Enping Formation for area with 3‐D seismic data. Chronostratigraphic bounding surfaces on a representative seismic profile and a sequence stratigraphic framework of Enping Formation. The starting position is defined in the thickest strata of the 3‐D seismic data [Colour figure can be viewed at wileyonlinelibrary.com]
lie between the two mapped horizons that define sequence bound-
sedimentation. The establishment of regional chronological frame-
aries (Figure 12a) (De Groot et al., 2006; Jia et al., 2014; Zeng, 2010).
work (mainly including second‐order and third‐order sequences) by
From the starting position with the thickest strata in the seismic area
tracing and correlating unconformities along the basin margin or
(Figure 12b), each horizon grows in all directions by following the local
correlative conformities in the basin centre is essential for tectonic
dip and azimuth information at the evaluation point to find the corre-
palaeo‐geography reconstruction and strategic hydrocarbon study in
sponding two‐way time position at neighbouring traces (Figure 12a)
sedimentary basins. In contrast, the establishment of high‐resolution
(De Groot et al., 2006, De Groot, Huck, De Beuin, Hemstra, & Bedford,
chronological framework and systems tracts (mainly including
2010). Finally, numerous chronostratigraphic slices can be obtained
fourth‐order and fifth‐order sequences) provides the basis for deposi-
from the 3‐D seismic data (Figure 12b). Because of the lateral exten-
tional systems dissection and potential reservoirs distribution in key
sion of a particular chronostratigraphic event, slices are effective to
zones (Lin, 2009; Lin et al., 2015).
assist in revealing the spatial evolution of depositional systems (De Groot et al., 2006). Modern sequence stratigraphy theory has become an essential
In this paper, we employ cores, wireline logs, network seismic data, and arc length seismic attribute to depict plan views of sedimentary facies and reveal the time–space evolution in terms of sand sections.
paradigm and methodology in bringing depositional process into chro-
The 2,980 sand section was deposited at early period of rifting‐
nological framework of geological history combining with the polycy-
depression stage (Figure 2). During this depositional period, the rising
clic or rhythm evolution of the earth. It provides revolutionary and
base level resulted in an increase in accommodation space and lacus-
challenging insights in investigating depositional process in chronolog-
trine level. Due to the relatively flat palaeo‐geomorphology (Figure 4),
ical framework and plays an important role in evaluation and prediction
it was dominated by braided delta deposits combined with cores,
of reservoirs. The approach operates with numerous geological data (i.
wireline logs, seismic data, and cumulative frequency curve (Figures 6,
e., cores, thin sections, wireline logs, and seismic data).
8, and 9). The channel deposits usually consisted of high proportion of
The depositional fillings in sedimentary basins can be divided into
stacked coarse‐ to gravelly coarse‐grained sandstones with thin shale
sequence stratigraphic units corresponding to multiple‐order cycle of
or mudstone interbedded (Figures 6 and 9). This lithology assemblage
12
XIA
ET AL.
is easy to cause distinct differences in seismic response. Figure 12a
channel width and an increase in curvature (Figure 13a). In addition,
shows arc length seismic attribute extracted based on the 2,980 sand
there are strong seismic responses of arc length value between channel
section. And it can be seen that the seismic response varies from place
deposits that can be interpreted as shale (well LF‐B and LF‐A) in sub-
to place. It shows relatively low arc length value on north‐eastern part
aqueous channel deposits (Figure 13a). The interpretation of channel
and high arc length value on south‐western part of the study area,
deposits in southern part remains unclear due to the faults (Figure 3).
respectively. This indicates a complete subaerial braided channel
The 2,900 sand section was deposited at the middle period of
deposits on low seismic response region (confirmed by depositional
rifting‐depression stage (Figure 2). Due to gradually rising base level
characteristics of well LF‐D) and several subaqueous distributary chan-
and subsequent increasing accommodation space and lacustrine level
nel deposits (confirmed by depositional characteristics of well LF‐B).
(Figure 5), braided delta began to retrograde. The seismic response of
From the plan view (Figure 13a), abrupt facies changes are mainly
main braided channel deposits retrograded to the north‐eastern part.
caused by a complex interplay between sediment supply and lacustrine
In contrast, the seismic response of subaqueous distributary channel
fluctuation. Although 2,980 sand section was deposited at an early
deposits covered most of south‐eastern part of study area with low
period of rifting‐depression stage, the increasing accommodation space
arc length value, while there are large coverage of high arc length value
resulted in the rising of lacustrine level that made subaerial braided
that may be interpreted as interdistributary deposits. The overview of
channel flow into the lake and develop subaqueous distributary channel
seismic response during 2,900 sand section deposition rotated coun-
deposits. There are bigger differences between seismic response of
terclockwise in comparison with that during 2,980 sand section depo-
subaqueous distributary channel deposits and that of subaerial braided
sition. In addition, the width of subaqueous channel increased a lot and
channel deposits. The differences mainly are reflected in a decrease in
subaqueous channel connected gradually. The channel range in
FIGURE 13 Attribute maps of arc length extracted from stratal slicing and evolution of braided delta in Enping Formation. The delta retroprogrades landward gradually indicating as sea level stepping landward in response to base‐level rise [Colour figure can be viewed at wileyonlinelibrary.com]
XIA
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ET AL.
thickness from 8 to 30 m and extend from 300 to 500 m in width dur-
no matter subaerial part or subaqueous part have relatively good
ing this period. The subaqueous distributary channel deposits are com-
sorting and rounding, which is detrimental to reservoir quality. The
posed of fine‐ to coarse‐grained, poorly sorted sandstones based on
porosity of these sandstones ranges from 15% to 35%, and the perme-
the depositional characteristics of cores and stacking patterns of
ability is between 300 and 3,000 md, more commonly, between 400
wireline logs. Due to the rising lacustrine level, the log responses show
and 1,100 md. Dissolution of feldspar and debris produced secondary
low‐amplitude serrated bell‐shape stacking patterns and the shale con-
pores that have contributed to a higher porosity and permeability. In a
tent and thickness increased alike (Figure 13b).
temporal framework, the most favourable sandstone reservoirs in
The 2,880 sand section was deposited at the late period of rifting‐
Enping Formation are located in braided channel deposits of 2,980
depression stage. During this period, the base level began to fall; how-
and 2,900 sand section and mouth bar deposits of 2,880 and 2,850
ever, the lacustrine level still rose slowly. The subaerial braided delta
sand section. In a spatial framework, the relative lowstand braided
deposits were covered by wide lake. The width of subaqueous distrib-
channel or subaqueous distributary channel deposits capped by rela-
utary channel decreased. The north‐eastern sediment supply
tive highstand interchannel, prodelta, or deep‐water mudstones con-
decreased, while the northern sediment supply increased. The sedi-
stitute litho‐stratigraphic traps with the greatest potential.
ments from two source areas accumulated and presented lobate geometry (Figure 13c). The channel range in thickness from 1 to 8 m
7
and extend from 150 to 300 m in width during this period.
|
DISCUSSION
The 2,850 sand section was deposited when the base level and lacustrine level fell. The coverage of subaqueous distributary channel
The development and distribution of sequences and depositional sys-
deposits increased and moved southward (Figure 13d). The north‐
tems in non‐marine reflect palaeo‐geographic changes that were
eastern part developed more interdistributary channel deposits than
strongly influenced by tectonics, sediment supply, lacustrine level fluc-
that in the northern part, while the case is just the opposite in terms
tuations, and climate changes. The balance between rates of potential
of subaqueous distributary channel deposits (Figure 13d).
accommodation generated by tectonic subsidence versus sediment
In summary, the depositional characteristics derived from cores,
supply and lacustrine level fluctuation influences the type of lacustrine
wireline logs, seismic profiles, and attributes reveal the time–space evo-
basin and sequence architecture and depositional filling within it (Lin,
lution of Enping Formation in terms of sand sections. The extending
2009, Lin et al., 2015). Most lacustrine basin fillings display an evolu-
characteristics of braided delta deposits rotated counterclockwise that
tion of facies associations that represent overfilled, balanced‐fill, or
may indicate the source and amount of sediment supply from 2,980
underfilled lakes. The following discussion evaluates controls on
sand section to 2,850 sand section on the whole. In addition, the transi-
sequence architecture and depositional evolution in the Enping
tion from subaerial braided channel deposits to subaqueous distributary
Formation in terms of these controls.
channel deposits represented an increase in accommodation space and lacustrine level generally. But, to be specific, during 2,980 sand section deposition, the braided delta deposits were controlled by syn‐depositional faults and sediment supply. As water depth deepened and lacustrine level rose, it retrograded to north‐eastern part during 2,900 sand
7.1 | Influence of long‐term tectonic uplift on sequence architecture and combination with regional tectonic evolution
section deposition. The most part of study area was covered by a wide
The tectonically active nonmarine lacustrine basins commonly devel-
lake and mainly characterized by subaqueous distributary channel
oped a series of unconformities with complicated stacking patterns.
deposits. As the accommodation space began to decrease and lacustrine
The major unconformity contact may change significantly from high
level rose slowly during 2,880 sand section deposition, subaqueous dis-
angular contact on the basin margin, low angular contact along the
tributary channel deposits started to rotate seriously due to sediment
basin slope to parallel or correlative contact in the basin centre. This
supply changes. The subaqueous distributary channel deposits further
phenomenon reflects the palaeo‐geomorphology changes from the dif-
extended to the whole study area due to falling lacustrine level and
ferent structural belts, such as uplift, marginal slope, and depression. In
increasing sediment supply when 2,850 sand section was deposited.
the high uplift belt developed in response to regional stress field, the multiple uplifting may result in composition of numerous unconfor-
6.3
|
mities. The top (T70) and bottom (T80) of Enping Formation that were
Prospecting prediction
characterized by distinctive unconformities may be identified from
The time–space evolution analysis of depositional systems provides
palaeo‐uplift to depression based on the seismic reflections and log
the basis for reservoir prediction and reveals sand body distributions
responses. These composite unconformity belts showed truncation or
in terms of sand section. Within the Enping Formation in LF13‐1 area,
onlap triangular unconformity belts on the palaeo‐uplift or along the
major oil reservoirs are developed in lithology that range from fine‐ to
marginal palaeo‐slope. And then they gradually changed into minor
coarse‐grained
important
angular or parallel unconformity belts from marginal slope to basin
reservoirs of Enping Formation are found in braided delta deposits,
centre. The composite unconformity belts indicated a distinct uplift
especially in braided channel deposits, some of which have been dem-
and erosion process. It concluded a major unconformity characterized
onstrated by well drilling. These reservoirs are formed where thick
by obvious high angular truncation or onlap seismic reflections (such
delta‐front sandstones are stacked to form stratigraphic traps or com-
as T80) and several secondary unconformities characterized by minor
plex structural‐stratigraphic traps. Fine‐grained inter‐channel deposits
angular truncation seismic reflections (such as T70) or abrupt changes
sandstones.
The
economically
most
14
XIA
ET AL.
in stacking patterns of wireline logs or sourced surface in cores. The
results in progradation, whereas underfilled accommodation results in
major unconformity resulted from regional tectonic event, indicated a
retrogradation. During stages of negative accommodation, processes
relatively large amount of erosion and long‐term exposure. The regional
of subaerial exposure and erosion are typically accompanied by
tectonic event during Enping Formation was part of episodic rifting as a
progradation in the basinward parts of the depositional profile.
feature distinguishing the northern margin of the South China Sea. And
The base level during 2,980, 2,900, and 2,880 sand section depo-
this feature was proven by later offshore data. Previous studies had
sitions was rising, and therefore, it resulted in an increase in accommo-
demonstrated three rifting episodes with confidence in the study area
dation space (positive accommodation) and water depth. Continuous
based on seismic and well data. The first rifting episode started in the
rising lake level and weakening sediment supply contributed a lot for
Late Cretaceous and ended in the Early Eocene with continental beds
depositional trends of retrogradation due to weak subsidence. Espe-
and predominant clastic deposits filling in the small rifts. The second
cially, during 2,880 sand section deposition, subaqueous distributary
rifting episode (which is the main period Enping Formation deposited)
channel deposits retrograded northward obviously. In contrast, the
took place in the Middle Eocene and resulted in a new generation of
base level gradually fell during 2,850 sand section deposition. How-
rifts filled with lacustrine successions. During this period, it was charac-
ever, the decreasing accommodation space did not exert an immediate
terized by dark shale with sandstone interbedded. And also, it devel-
effect on lacustrine level. The lacustrine level still rose slowly and sub-
oped fan delta deposits along the palaeo‐uplifts and braided delta
aqueous distributary channel deposits progradated basinward.
deposits. The third rifting episode activated in the Late Eocene and lasted until the Late Oligocene (Ru & Pigott, 1986; Wang & Li, 2009; Zhou et al., 1995).The distribution of different composite unconformity
8
|
CO NC LUSIO NS
belts represented the location of palaeo‐uplift that formed in response to compound and complex tectonic events. The wedge‐shaped triangle
1. Based on an integrated analysis of 3‐D seismic merging datasets,
unconformity belts were composed of truncated and onlap belts. The
wireline logs, cores, a total of one second‐order sequence and
former referred to the unconformity generated by truncation of sec-
four third‐order sequences defined by regional and local
ondary compound unconformities or sequence boundaries by the
unconformity surfaces, respectively, are recognized in Enping For-
major unconformity. This unconformity belts separate the downward
mation, Huilu Low Uplift. The second‐order sequence was
parallel unconformity or correlative conformity with an initial truncated
bounded by regional erosional unconformities on seismic profile,
point. The onlap triangular unconformity belts were formed in response
while third‐order sequences were mainly based on wireline logs
to secondary unconformities or sequence boundaries on major uncon-
and cores. The second‐order sequence generally consists of an
formity. The inclination angle of secondary unconformity or sequence
entire cycle and shows fining‐upward trend characterized by
boundaries against major unconformity was employed to reveal the
amalgamated channel‐fill beds at the base that pass upwards into
uplift and deformation intensity in general. The hydrocarbon explora-
floodplain‐dominated and lacustrine and delta successions. The
tion in many nonmarine lacustrine basins has demonstrated that the
third‐order sequences usually are composed of rising hemicycle
wedge‐shaped triangular unconformity belts are potential zones for
and characterized by fining‐upward trends in general. The
discovering large‐scale stratigraphic traps, and also, it is of greatest
palaeo‐geomorphology, especially palaeo‐uplift during Enping
importance to characterize unconformity belts for palaeo‐geomorphol-
period, plays an important role in controlling sequence architec-
ogy reconstruction and reservoir prediction.
ture. The unconformity belts controlled by palaeo‐uplift show truncation or onlap triangular unconformity belts on the palaeo‐ uplift or along the marginal palaeoslope. The distribution of differ-
7.2 | The influence of base level on depositional filling
ent composite unconformity belts represented the location of
Base level is widely regarded as a global reference surface to which con-
tectonic events. The inclination angle of different order unconfor-
tinental denudation and marine aggradation tend to proceed
mities or sequence boundaries against major unconformity can
(Catuneanu, 2002). This surface is dynamic, moving up and down
reveal the uplift and deformation intensity in general.
palaeouplift that formed in response to compound and complex
through time relative to the centre of Earth in parallel with eustatic rises
2. The Enping Formatin records deposition within a fluvial and lacus-
and falls in sea level. During the moving process, accommodation space
trine environment accumulated in the basin during rifting‐depress
is generated. The accommodation space defines the space available for
stage. The stratigraphic unit encompasses four facies associations
sediments to fill. The origin is closed related to base‐level changes.
based on the seismic and drilling data, complemented by thin sec-
Accommodation space may be modified by the interplay between var-
tions, SEM, and other relative geology information ascribed to (a)
ious independent controlling factors. In this paper, the controlling fac-
braided channel deposits, (b) subaqueous distributary channel
tors mainly consist of sediment supply and lacustrine level
deposits, (c) lacustrine deposits, and (d) maximum flooding surface
fluctuations. Depositional trends of aggradation, erosion, progradation
deposits. The depositional evolution in Enping Formation consist
and retrogradation may be explained by changes in accommodation or
of progradation and retrogradation. The retrogradation from
by the interplay between accommodation and sediment supply. Posi-
2,980 to 2,880 sand section indicates an imbalance between weak
tive accommodation promotes sediment aggradation, whereas negative
sediment supply and strong lacustrine level fluctuations, while the
accommodation results in downcutting. During stages of positive
case is just the opposite referring to the progadation from 2,850
accommodation, sediment supply in excess of available accommodation
sand section.
XIA
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ET AL.
3. Favourable oil‐bearing potential for the study area is proven, marked by relatively high porosity and permeability. In addition, this suggests that braided delta deposits should be the primary target of petroleum exploration within it, because the presence of a large‐scale, laterally continuous reservoir is likely. The work such as this study would help reduce the risk in petroleum exploration. Relative lowstand channel deposits capped by relative highstand prodelta or deep lacustrine mudstones may
ACKNOWLEDGEMEN TS The Petroleum Development of Oil Research Institute, Shenzhen Branch Company, China National Offshore Oilfield Corporation Limited is thanked for their kind cooperation, valuable inputs, and helpful discussions; Then the authors gratefully acknowledge joint financial support from the National Key Basic Research Project (Grants 2011ZX05025‐002‐05, 2011ZX05001‐001‐04, and 201105002‐006), the Natural Science Foundation of China (Grants 91328201, 40972081, 40372056, and U1262205), the Fundamental Research Funds for the Central Universities (Grant 2010ZD07), Natural Science Project for Province
(Grants
D2015209075,
2017209236
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ZD2016077) and also would like to show their appreciation to Professor Hongwen Deng, who provided the opportunity to participate in this research project, and to Professor Zhen Liu for providing us with invaluable logistical support concerning this topic. We are grateful to anonymous reviewers for the critical and constructive comments on
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Pearl River Mouth Basin, South China Sea Part A: The braided delta deposits of Cenozoic Enping Formation. Geological Jour-
