Received: 20 November 2016
Revised: 2 January 2017
Accepted: 17 February 2017
DOI: 10.1002/gj.2921
RESEARCH ARTICLE
The geophysical identification, characteristics, and petroliferous significance of sublacustrine fan deposits in the second member of Dongying Formation in Liaozhong Depression, Bohai Bay Basin Shiqiang Xia1 Nanan Gao2
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Jingyan Liu2
Hong Li4
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Zhen Liu1
Xu Liang5
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Dongqing Ye2
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Yanyan Chang3
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S Li
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College of Geosciences, China University of Petroleum (Beijing), Beijing, China
Large sublacustrine fan deposits have been identified within the lacustrine successions of the
2
second member of the Palaeogene Dongying Formation in the Liaozhong Depression. In this
School of Energy Resources, China University of Geosciences, Beijing, China
3
Library of China University of Petroleum, Beijing, China
study, by using comprehensive and detailed analyses of the borehole lithology, limited cores, wireline logs, and seismic facies, 4 types of sublacustrine fan deposits were identified and characterized. Then, an integrated depositional model was established based on the above investiga-
4
China National Offshore Oilfield Corporation Limited—Tanggu, Bohai Oil Research Institute, Tianjin, China
5
CNOOC Research Institute, Beijing, China
Correspondence Jingyan Liu, School of Energy Resources, China University of Geosciences, Beijing 102249, China. Present address: No.29 Xueyuan Road, Haidian District, Beijing 102249, China. Email:
[email protected]
tions. The result indicates that the geophysical characteristics of the different sublacustrine fan deposits differed from each other, in terms of their internal configuration and external geometry of seismic reflections, and stacking patterns of wireline logs, and stratigraphic position in the vertical successions. Subsequently, the model was established according to the comprehensive analysis of the geophysical characteristics and stratigraphic position. The discussion of developing conditions and hydrocarbon discoveries within different types of sublacustrine fan deposits provided robust insights regarding how to locate and evaluate reservoirs in such deposits. Furthermore, the results of this study may potentially assist in achieving a new understanding of how to identify sublacustrine fan deposits in similar lacustrine basins and may also assist in making fur-
Funding Information Natural Science Foundation of China, Grant/ Award Number: 40972081, 91328201 and U1262205; Fundamental Research Funds for the Central Universities, Grant/Award Number: 2‐9‐2013‐095 and 2010ZD07; National Key Basic Research Project, Grant/Award Number: 2011ZX05025‐002‐05, 2011ZX05001‐001‐04 and 201105002‐006
ther decisions regarding the terms of hydrocarbon migration and accumulation. KEY W ORDS
Bohai Bay Basin, Dongying Formation, Liaozhong Depression, petroliferous significance, sublacustrine fan deposits
Handling editor: S. Li
1
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I N T RO D U CT I O N
development and evolution of sublacustrine fan deposits, including tectonic movements, lacustrine fluctuations, and sediment supply, are
Sublacustrine fan deposits, which are cited from submarine fan
integrative and complex (Dasgupta, 2002; Egawa, Furukawa, Saeki,
deposits (Normark & Piper, 1969; Walker, 1965, 1978), refer to impor-
et al., 2013; Liu, Wang, Li, et al., 2014; Lowey, 2007; Lu, Wang,
tant clastic deposits and potential host reserves that have been mainly
Zhang, et al., 2011; Wang, Chen, Wang, et al., 2009b; Wang, Wang,
developed under the wave base level in lacustrine environments. The
Zhang, et al., 2009a). They share similarity in terms of these factors
sand‐rich sublacustrine fans in lacustrine rift basins have long been
due to the similar basin evolution process in the Liaozhong Depression.
of great academic and economic interest to researchers and oil
In addition, most sublacustrine fans found to date were in lacustrine rift
industry representatives in China and throughout the world, due to
basins in China, which means that the rift process provides a basic
the fact that they typically host important hydrocarbon accumulations
condition for sublacustrine fan deposits. First, the active tectonic
with excellent source‐reservoir‐cap combinations. In general, the
movements played an important role in controlling the development
sublacustrine fan depoists develop in relatively unstable environments
and distribution of sublacustrine fans. Second, landward sediment sup-
(Katz, 2001; Zhao & Liu, 1984), but the factors driving the
ply and associated delta deposits along the basin margin were also the
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Copyright © 2017 John Wiley & Sons, Ltd.
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Geological Journal. 2018;53:692–706.
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main factors to trigger the sliding and slumping to form the
The Liaozhong Depression, which is situated in the central section
sublacustrine fan. Third, frequent and rapid lacustrine fluctuations
of the Bohai Bay Basin, covers an area of approximately 6,100 km2
influenced the development of the sublacustrine fan. In conclusion,
(Figure 1). The Palaeogene Dongying Formation (Ed) developed during
the basins that share similar factors can be a potential location for
a depression stage of the Cenozoic tectonic evolution. It began at
sublacustrine fan development. The comprehensive interaction of the
approximately 32.8 Ma and ended 24 Ma, giving it a time span of
factors influencing the reservoir development and characteristics will
8.8 Ma (Figure 2). During the depositional period of the Dongying For-
lead to different exploration discoveries (Chen, Pang, Jiang, et al.,
mation, a strike‐slip episode of rifting followed (Figure 2). and subse-
2009; Chen, Wang, Li, et al., 2010; Yang and Jin, 2012). All the studies
quently, basin inversion related to the reactivation of normal faults in
described above are based on dense wells with numerous cores within
a transpressional strike‐slip regime occurred (Dong et al., 2011). The
high‐quality seismic surveys. However, how to identify and character-
fault style in the study area is especially apparent along the middle
ize and analyze the petroliferous significance of the deposits based on
and eastern margins of the Liaozhong Depression, including “flower”
geophysical data and limited cores remains controversial. Therefore,
structures, which are mainly caused by regional integrative stress of
the understanding and employment of their geophysical identification,
strike‐slip and rifting (Figure 3b). The predominant rifting style before
characteristics, and petroliferous significance are very important.
the deposition of Dongying Formation, and especially in the Liaozhong
During the past few decades, with increasing demands being made
Depression, was asymmetrical, resulting in half‐graben sequence archi-
regarding hydrocarbon resources of China, the Liaozhong Depression
tecture with a faulted escarpment to the east and gentle ramp to the
in the Bohai Bay Basin has been a focus of extensive petroleum explo-
west. However, this pattern varies along strike‐slip faults (Figure 3b).
ration and exploitation. The sublacustrine fan deposits within the study
The Dongying Formation is composed of three members (D3, D2, and
area, which have had wells drilled, have demonstrated that this sand
D1) (Figures 2 and 3a). It mainly consists of fluvial‐deltaic and shal-
body serves as a favorable exploration prospect, due to its potential
low‐to‐deep lacustrine deposits. The lithology is dominated by dark
economic value. To date, only a small number of studies have been
black mudstone in the central lake and sandstone along the basin mar-
carried out to investigate the geological significance (Tian, Zhou, &
gin. In general, it shows fining‐upward trends from a relatively coarse‐
Liu, 2010) and depositional responses and evolutions (Dong, Lin,
grain fan delta to fluvial delta deposits to fine‐grain shore‐shallow lake
Eriksson, et al., 2011; Wang, Wang, Zhou, et al., 2008; Wu, Wu, Zhang,
deposits (Figure 3a). As the target interval in this study, the thickness of
et al., 2012; Yao, Zheng, Du, et al., 2012) of sublacustrine fans in the
the Dongying Formation is much thicker in the Liaozhong Depression.
Liaozhong Depression. However, the basic geophysical identification,
In addition, more hydrocarbon discoveries were made in recent years
classification, characteristics, and petroliferous significance were found
in the sublacustrine fan of the Dongying Formation (Dong et al., 2011).
to be astonishingly rare within the scientific approach in the study area, in contrast with other similar basins, and thus, it was determined that they required further investigation and summarization.
3
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DATABASE AND WORKFLOW
In order to address these issues, a comprehensive analysis of the detailed sequence of the stratigraphic framework establishment,
The study area specifically corresponded to two oil fields, and conse-
geophysical identification, and characteristics analysis, and hydrocar-
quently, large volumes of data were available. The data used mainly
bon potential analysis were conducted. The aim of this study was to
consisted of a 3‐D seismic dataset, gamma ray, acoustic, and resistivity
highlight the geophysical identification and characteristics, and
logs from more than 25 wells, one conventional core sample from Well
petroliferous significance of four types of sublacustrine fan deposits
K, and small number of oil discoveries. The 3‐D poststack time‐
in the lacustrine rift basin within the Dongying Formation. Then, the
migrated seismic data set used for this study, which was newly
association principles of the source rock, seal rock, and traps were
reprocessed in 2011, covered 890 km2 and was characterized by a
discussed, which could potentially lead to a better understanding of
bin spacing of 25 by 25 m. The maximum vertical resolution of the seis-
the genetic implications of sublacustrine fan deposits, along with
mic data was approximately 15 to 20 m. Core photos provided the most
improvement to hydrocarbon exploration and development.
direct interpretations for the sedimentary facies. Unfortunately, only one well (Well K) in the Dongying Formation was cored, and the cores acquired from the development area of the sublacustrine fan were used
2
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G EO LO GI C A L OV E RV I EW
to identify the sedimentary characteristics. As drilling cores are limited, the lithological and logging data in the study area were employed to
The Bohai Bay Basin, which is one of the largest Cenozoic lacustrine
establish sequence stratigraphic framework and aid in analyzing the
rift basins in Northeast China, is located among the northern Yanshan,
sedimentary facies. At present, many of the wells are production wells.
eastern Liaodong, and Luxi uplifts. It was developed in response to
Only nine wells were drilled to target intervals (Dongying Formation).
compound and complex tectonic events. From a planning perspective,
The lithological association and electric logging curves from the nine
it trends northeastward in an “S” shape and covers an area of approx-
wells, including natural gamma, spontaneous potential, deep lateral
imately 30,000 km2. It is also separated by some regional uplifts and
resistivity, and shallow lateral resistivity, were used to identify sedi-
depressions. Five secondary structural units, with a northeast or
mentary facies and indicate the distribution range of the sublacustrine
north‐northeast strike, are developed in the northeastern Bohai Bay
fans. Wireline logs tied to seismic survey assist in verifying and calibrat-
Basin, in the center of which is where the Liaozhong Depression is
ing seismic interpretation, stratigraphic framework establishment, and
located (Figure 1).
depositional systems identification, according to the stacking patterns.
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Geographic location of Liaozhong Depression in Bohai Bay Basin, Northeast China [Color figure can be viewed at wileyonlinelibrary.com]
Due to the relatively low resolution of the seismic data, the wireline
logs, and cores (if available). In addition, the wireline data (gamma‐ray,
logs were employed to calibrate the seismic interpretations, identify
spontaneous, acoustic, and resistivity logs) were employed to calibrate
the sequence boundaries, and interpret the sedimentary facies, based
the seismic facies to the lithology and interpret them to sedimentary
on the stacking patterns of logs. The data of hydrocarbon discoveries
facies. Finally, after the depositional facies were classified, the seismic
provided the basis for the prediction of oil occurrence.
reflections and profiles and seismic attributes were used to delineate
The methodology employed in the study was performed as follows.
the facies distribution range, especially in sparse well or no‐well areas.
First, it was necessary to identify the regional unconformities based on
It should be noted that the method was determined to be more
3‐D seismic data, which provided the basis for understanding sequence‐
effective if there were some cores in the target interval. All of the
stratigraphic framework. The interpretation of the seismic profiles was
analyses in this study, which used all of the data mentioned above,
conducted using a standard and conventional seismic stratigraphic
potentially provide new insights into hydrocarbon prospects.
approach (Xia, Li, Wang, et al., 2015). Different order sequence bound-
Considering that there have been few previous studies, and that the
aries (in this study, third‐order sequences) were then identified, by trac-
cores are limited, this research was mainly performed based on the 3‐D
ing the unconformities and correlative conformities. In addition, the
seismic datasets and wireline logs, as detailed in the following steps.
stacking patterns of the wireline logs were used to calibrate the seismic interpretations. At this point, a sequence stratigraphic framework, which mainly included the third‐order sequences, was established. Second, the system tracts were defined in relation to the maximum flooding
4 | SEQUENCE STRATIGRAPHIC FRAMEWORK AND TYPES OF SEDIMENTARY FACIES
surfaces that are apparent on the seismic profiles as downlap surfaces, features were identified within the sequence stratigraphic framework,
4.1 | Sequence stratigraphy and sedimentary characteristics of the Dongying Formation
through the comprehensive analysis of internal configurations and
The development of stratigraphic sequences in lacustrine rift basins is
external geometry of seismic reflections, stacking patterns of wireline
generally controlled by the basins' tectonic movements, lacustrine
and as condensed sections in well logs. Third, the anomalous seismic
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FIGURE 2
The classification of sequence stratigraphy and depositional systems of the Eogene Dongying Formation in Liaozhong Depression, Bohai Bay Basin (modified from Bohai Oil Research Institute, SQ = Sequence. All these geological information is based on the seismic profile, well logs, lithology and cores) [Color figure can be viewed at wileyonlinelibrary.com]
fluctuations, sediment supplies, and climate (Cross, Baker, & Chapin, 1993;
a third‐order sequence stratigraphy with strong truncation below
Feng, Li, & Xie, 2000; Galloway, 1989; Lin, Zhang, Liu, et al., 2000; Vail,
(Figure 3b). The fourth‐order sequences were identified based on the
Audemard, Bowman, et al., 1991; Vail, Mitchum, Todd, et al., 1977;
formed surfaces, which corresponded to the sea‐level or lake‐level fluc-
Van Wagoner, Mitchum, Posamentier, et al., 1988; Van Wagoner,
tuations; base‐level changes characterized by transgression, such as the
Mitchum, et al., 1990; Lin, Liu, Zhang, et al., 2005). The characteristics
maximum flooding surface; transgressive ravine surfaces; maximum
of synrift stratigraphic sequences differ from those of postrift
regressive surfaces; and, in particular circumstances, subaerial uncon-
sequences. Therefore, each should be regarded as a separate high‐
formity (Lin et al., 2000; Feng et al., 2015). Then, based on the wireline
order sequence (Feng, Jiang, Hu, et al., 2016; Feng, Li, & Lu, 2013;
logs and seismic data, the fourth‐order sequences (parasequence sets)
Hubbard, 1988). The successions of the Dongying Formation consisted
were also recognized within the different third‐order sequences by
of six members, as follows: first member, upper second member, lower
tracing the flooding surfaces (Figure 3b). The transgressive surfaces
second member, upper third member, middle third member, and lower
were the flooding surfaces between the sandstone and overlying mud-
third member (Figure 2). The strata of the Dongying Formation were
stone (Figure 6). These were characterized by the coarsening‐upward
deposited at the synrift stage. These could be divided into three
regressive successions, and funnel‐shaped stacking patterns of the
third‐order sequences and seven fourth‐order sequences. The third‐
wireline logs were recognized at the top (Figure 7a). On the seismic pro-
order sequences were identified based on the unconformities on the
files, it was mainly shown at the bottom of the sublacustrine fan
basin margin or scoured surfaces, and the basins' formed correlative
deposits, which were vermicule‐shaped seismic reflections (Figure 3b).
conformities, which corresponded to the tectonic movements, sea‐ level or lake‐level fluctuations, base‐level changes, and so on. These features were mainly characterized by angular or minor angular uncon-
4.2
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Identification of the sublacustrine fan deposits
formities in the basin margin or paleouplifts and changed into discon-
The depositional systems that were developed in the Dongying Forma-
formities or correlative conformities in the central basin. The first
tion were found to be mainly fluvial delta, shallow‐deep lacustrine
third‐order sequence was composed of d1 and d2s, with T2 top and a
deposits (Figure 2). The lithology included brown oil‐bearing sand-
T3u bottom. These two sequence boundaries were characterized by
stone, gray sandstone, gray‐green mudstone (especially within d2x
minor angular truncation below along the margin and weak onlap above
and d1), and red mudstone. The sandstone and mudstone were often
(Figure 3b). There were also obvious lithology changes observed on the
interbedded with each other (Figure 3a). The granularity of the sand-
T2 seismic reflection surface (Figure 3a). The d2x constituted a third‐
stone was partially fine and mainly composed of siltstone, which was
order sequence. In contrast with the first sequence, it was character-
followed by fine‐grained sandstone, and muddy siltstone. Also, accord-
ized by a small number of lithology changes. However, there was a dis-
ing to the core observations, there was a large percentage of argilla-
tinct vermicule seismic reflection inside (Figure 3b). The D3 constituted
ceous deposits (Figure 6). The content of purple and variegated
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FIGURE 3
(a) Well tie correlations from west to east in Dongying Formation (see Figure 1 for location). (b) Interpretation of seismic profile (see Figure 1 for location AA’) [Color figure can be viewed at wileyonlinelibrary.com]
mudstone was approximately 46.7%, and the remainder was made up
the Liaozhong Depression. These were the channel‐filling deposits in the
of gray, gray‐green, and gray‐black shale.
upper fan with distinct bell‐shaped stacking patterns; channelized lobes
In this study, during the examination of the stacking patterns of the
in the middle fan with the characteristics of coarsening‐upward overlaid
wireline logs, the sublacustrine fan deposits could be recognized by
by upward‐fining stacking patterns; and the distant lobe in the lower fan,
the distinctive low gamma‐ray and high‐spontaneous signals within the
with interbedded thick mudstone and thin sandstone (Figure 5).
background of high gamma‐ray and low‐spontaneous sections
By analyzing the seismic reflection characteristics, it was found
(Figure 4). The logging curves were usually characterized by funnel,
that the deposits typically displayed continuous or discontinuous hum-
cylinder, and serrated bell‐shaped stacking patterns, due to the
mocky reflections, or “vermicular” reflections (Figures 4 and 5). The
responses to different microfacies (Figures 4, 5, and 6). The cylinder
discontinuous seismic reflections in the sublacustrine fan deposits
shape indicated large‐scale channel filling of the sublacustrine fan, which
were interpreted as sandstone facies. However, the continuous reflec-
displayed a fining‐upward grain size (Figures 4 and 5). The bell‐shaped
tions were interpreted as cohesive mud‐rich facies, with soft sediment
stacking patterns were mainly reflections of the sets of channelized lobes
deformations (Dong et al., 2011).
(Figure 5). The funnel‐shaped stacking patterns were then interpreted in the lower fan as having a coarsening‐upward grain size (Figure 6). The serrated stacking patterns indicated unstable hydrodynamic environ-
5
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RESULTS AND INTERPRETATIONS
ments (Figure 6). Generally speaking, it usually developed as siltstone and mudstone interbedded with minor cross‐beddings (Figure 6). The
The different paleogeomorphology typically controls the spatial
three parts of a typical sublacustrine fan deposit could be identified in
distribution of sand bodies and further controls the development of
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FIGURE 4 The sedimentary facies interpretation based on wireline logs [Color figure can be viewed at wileyonlinelibrary. com]
subsequent sequence architecture and depositional filling. All of the
seismic profile, they were characterized by a strong‐amplitude mound
depositional systems within the different paleogeomorphology pose
lens in the external geometry and chaotic reflections in the inner config-
strong influences on the predictions and evaluations of reservoirs.
urations (Figure 7b). In addition, the characteristics of stacked coarsen-
Based on the above analysis, four types of sublacustrine fan deposits
ing‐up funnel shapes below and fining‐up bell shapes above (Figure 7a)
were summarized and characterized in this study. In addition, the
were observed. The lithology was mainly composed of fine and silty sand-
hydrocarbon potential was analyzed.
stone, with single thicknesses of 12 to 25 m (Figure 7a). According to the wireline log interpretations, the porosity and permeability of Well D were
5.1
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Deep‐water sublacustrine fan deposits
respectively 31.7% and 551 md (Wu et al., 2012; Yao et al., 2012). This is of great potential importance to forming good reservoirs due to the
Deep‐water sublacustrine fan deposits refer to the deposits caused by
following reasons: Good physical properties are a postulation for good
gravity flow, which are formed in deep water areas or lake centers in
reservoirs and the adjacency to source rocks (deep‐water shale and
lacustrine basins. These deposits typically occur during times of strong
mudstone; Figures 7a and 7b). The results of previous studies have
hydrodynamic environments, thereby causing the lacustrine fluctuations
indicated that oil exists in this type of depositional system within this area
to rise, or the tectonic movements to become active. As viewed from the
(Dong et al., 2011; Wu et al., 2012; Yao et al., 2012).
FIGURE 5
The vertical successions of (left) sublacustrine fan deposits from well logs and lithology, and seismic responses of (right) sublacustrine fan deposits in Well C from crossline direction [Color figure can be viewed at wileyonlinelibrary.com]
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FIGURE 6
5.2
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ET AL.
The characteristics of wireline logs and cores from Well K [Color figure can be viewed at wileyonlinelibrary.com]
Near‐shore sublacustrine fan deposits
The near‐shore sublacustrine fan deposits in the study area refer to the
reservoirs for hydrocarbon migration and accumulation, and this was confirmed by the hydrocarbon exploration (Figure 8).
deposits distributed along the escarpment, which may have been controlled by syndepositional or strike‐slip faults (Figures 3b and 8c). From the crossline seismic profile, it could be seen that the bottom
5.3
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Fluxoturbidite deposits
and top vermicule reflections were different. The bottoms were rela-
The fluxoturbidite deposits refer to the deposits that were triggered by
tively strong‐amplitude seismic reflections (Figure 8c), with serrated
gravity, flooding, earthquakes, tsunamis, and so on, which caused
bell‐shaped stacking patterns (Figure 8a). At the same time, the tops
unstable or unconsolidated delta front deposits in areas of abrupt
were characterized by chaotic seismic reflections, with cylinder‐
variations to slump into the lacustrine basin. In the study area, the
shaped stacking patterns in vertical successions (Figure 5). The
fluxoturbidite deposits were those of large‐scale chaotic seismic
stacking patterns of the wireline logs and seismic reflections were
reflections, which could have been interpreted to some extent to be
interpreted to be channelized lobe deposits. However, the top was
a turbidite channel (Figure 9c). This was mainly controlled by the mar-
composed of channelized lobe deposits below and channel fill above
gin faults and sediment supply from the delta front along the basin's
(Figure 5). There were three types of developed reservoirs in these
margin (Figure 9c). Only two wells in the study area were drilled into
deposits: channelized lobe deposits, channel‐fill deposits, and distant
this deposit. Well E displayed mainly mudstone with serrated stacking
lobe deposits. All of these deposits easily formed into litho‐
patterns (Figure 9a). Furthermore, the seismic reflections drilled by
stratigraphic or structural‐litho traps within the appreciate settings.
Well E were of a strong amplitude, low frequency, and were continu-
The channelized lobe deposits and channel‐fill deposits were found
ous (Figure 9c). In contrast with Well E, Well N indicated low‐ampli-
to be better than the distant lobe deposits. The channel‐fill deposits
tude chaotic seismic reflections, with coarsening‐up stacking patterns
(in wells C and M) consisted of thick sandstone and conglomerates
(Figures 9b and 9c). Based on the wireline log interpretations, the
(Figures 8a and 8b). Based on the wireline log interpretations, the
porosity and permeability of Well E and Well N were 23% and
porosity and permeability of Well C and Well M were 17.9% and
509 md, and 16.5% and 786.9 md, respectively (Wu et al., 2012; Yao
618.5 md, and 20.2% and 386.9 md, respectively (Wu et al., 2012;
et al., 2012). In addition, all of these deposits were usually surrounded
Yao et al., 2012). Due to the coarse‐grain size, low‐content argillaceous
by source rock from deep lakes and were potential reservoirs.
deposits, and adjacency to the fault, they were determined to be ideal
Although the connectivity was not as good as in the other depositional
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FIGURE 7 (a) The characteristics of deep‐water sublacustrine fan from wireline log stacking patterns (see Figure 1 for location of Well D). (b) The characteristics of deep‐water sublacustrine fan from seismic response (see Figure 1 for location CC’) [Color figure can be viewed at wileyonlinelibrary.com]
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FIGURE 8 (a) The characteristics of near‐shore sublacustrine fan from wireline log stacking patterns (see Figure 1 for location of Well C). (b) The characteristics of near‐shore sublacustrine fan from wireline log stacking patterns (see Figure 1 for location of Well M) (c) The characteristics of near‐shore sublacustrine fan from seismic response (see Figure 1 for location DD’) [Color figure can be viewed at wileyonlinelibrary.com]
systems, it easily formed into good reservoirs, due to the widespread
Scholz, 1995; Scholz & Rosendahl, 1990; Soreghan & Cohen, 1996).
superimposed thick bodies of sand and the adjacency to the source
The majority of this research has employed seismic analyses, shallow
rock (Dong et al., 2011; Wu et al., 2012; Yao et al., 2012).
coring of sediment, wireline stacking patterns, and so on. However, the interpretations of the deep‐water seismic facies have mainly relied
5.4
|
Far‐shore sublacustrine fan deposits
on the studies of marine sequence stratigraphy (Galloway, 1989; Vail et al., 1977) and marine deposits (Mutti & Ricci, 1972). In the present
The far‐shore sublacustrine fan deposits in the study area are the
study, the sublacustrine fan deposits within the Dongying Formation
deposits, which were distributed in a radial direction in the lacustrine
have provided new insights into the geometry and distribution of
basin, as well as along the gentle slope, which were provided by a
deposits in deep‐water lacustrine settings (rather than marine settings),
long‐distance sediment supply. The far‐shore sublacustrine fan
as well as the sequence stratigraphic significance of sublacustrine fans.
deposits were characterized on the seismic profiles as small‐scale
The lacustrine sedimentation in the second member of the
weak‐amplitude chaotic reflections and abruptly changed into strong‐
Dongying Formation was largely dominated by multiple controlling
amplitude, low‐frequency reflections (Figure 10b). There had not been
factors.
any wells drilled into this type of deposit, and thus, it was difficult to
paleogeomorphology, sediment supply, and lacustrine fluctuations.
evaluate its lithology, wireline log stacking patterns, and hydrocarbons.
The activities of the tectonic movements, especially the fault move-
In recent years, a great deal of effort has been put forth toward
ments, played important roles in triggering the slide and slump of the
understanding the distribution and significance of sublacustrine fan
sediments along the margins or slopes, such as the fluxoturbidite
deposits in lacustrine rift basins. This has been largely due to their
deposits. The palaeogeomorphology, especially the slope gradient,
potential as models for hydrocarbon accumulations in lacustrine basins
directly influenced the deposits' distributions and types (Figures 11),
(Benvenuti, 2003; Cohen, 1990; Dasgupta, 2002; Johnson, Wells, &
such as the near‐shore sublacustrine and far‐shore sublacustrine fan
These
factors
mainly
included
tectonic
movement,
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FIGURE 9
(a) The characteristics of fluxoturbidite from wireline log stacking patterns (see Figure 1 for location of Well E). (b) The characteristics of fluxoturbidite from wireline log stacking patterns (see Figure 1 for location of Well N). (c) The characteristics of fluxoturbidite from seismic response (see Figure 1 for location EE’) [Color figure can be viewed at wileyonlinelibrary.com]
deposits. It also controlled the sequence architecture and depositional
(Stow & Mayall, 2000). At present, the most popular exploration areas
filling of the sublacustrine fan deposits. The lacustrine fluctuations
are mainly composed of some of the near‐shore basins in Mexico Bay,
influenced the development of the sublacustrine fan deposits, similar
Brazil, and West Africa, which are collectively known as the “Gold
to the tectonic movements to some extent (Dong et al., 2011; Wu
Triangle” of submarine and subaerial oil exploration. However, at the
et al., 2012). The sediment supply served as the primary factor, due
same time, abundant discoveries and breakthroughs have been made
to the fact that the sublacustrine fan deposits could not be deposited
in nonmarine lacustrine basins in China. These include the Bohai Bay,
if there was no supply of sediment.
Erdos, the Erlian Basins, and so on (Feng & Xu, 2006; Li, Zhang, Song, et al., 2004; Lin, Zheng, Ren, et al., 2003; Zou, Zhao, Yang, et al., 2009). All of these findings have presented new understandings and
6 | P E T R O L I F E R O U S SI G N I F I C A N C E F O R T H E H Y DR O C A R B O N I ND U S T R Y
orientation for submarine and subaerial hydrocarbon exploration. However, in the study area presented in this paper, there are several hydrocarbon discoveries, which are distributed throughout
Since the 1970s, research regarding submarine and subaerial turbidite
the area adjacent to Well C and Well M, in the southern and northern
deposits along passive continental margins and lacustrine rift basins
Liaozhong Depression. This is due to the fact that the Tanlu Fault
has been of great interest and economic value, for both scientific
(Liaozhong segment) passes straight through the study area with an
studies and the hydrocarbon industry. The proven reserves and
North North East (NNE) strike. The fault plays an important role in
hydrocarbon yields have continued to increase and gained a higher
controlling and influencing hydrocarbon generation, migration, and
percentage in the submarine or subaerial fields than in other fields. It
accumulation. First, the fault's activities affect palaeo‐geomorphology
can be said that, in the next several years, the research and exploration
and palaeo‐geographic framework and control the distribution of the
of submarine or subaerial deposits and their relative reservoirs will still
sedimentary facies, then they proceed to control the distribution of
be on the frontier of the hydrocarbon industry. The main target areas
the source rock and reservoir sand bodies. Second, the fault's activities
will be the submarine basins or other potential petroliferous systems
affect the formation and deformation of the structure, control the
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FIGURE 10
(a) The characteristics wireline log stacking patterns adjacent to infralittoral fan (see Figure 1 for location of Well G). (b) The characteristics of infralittoral fan from seismic response (see Figure 1 for location FF’) [Color figure can be viewed at wileyonlinelibrary.com]
formation of abundant traps, and cause the destruction of some traps.
have been proven in the Mesozoic and Cenozoic nonmarine lacustrine
Third, the fault's activities also affect the juxtaposition relation among
rift basins of China. The deposits often occur where there is a lack of
the fault, sand bodies, and unconformity surfaces and control the
oxygen, such as in deep lake zones, and are also filled with kerogen from
function and efficacy of the three as the main hydrocarbon
landward sediment supplies. All of these sedimentary settings and
translocation system (Wu, Xu, Zhang, et al., 2016; Wu et al., 2012;
sediment supplies provide the lacustrine basins with good source rock.
Xu, Peng, Liu, et al., 2016; Yao et al., 2012).
The turbidite sandstone, which have been found close to the source
In summary, sublacustrine fan deposits maintain close relationships
rock, can potentially provide a perfect location for hydrocarbon accumu-
with hydrocarbon generation and accumulation. These relationships
lation, with good upward and downward seals. In order to achieve good
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FIGURE 11
The depositional model of four types of sublacustrine fan deposits [Color figure can be viewed at wileyonlinelibrary.com]
hydrocarbon exploration results, it is of great importance to first
compared with turbidite deposits (Figure 8b). The decrease of mud
characterize the spatial distribution of the sand bodies in the
content indicated that the paleogeomorphology changed with regard
sublacustrine fan deposits. The primary difficulty with this is the
to strong fault activities. Correspondingly, the slope gradient tended
establishment of high‐resolution stratigraphic frameworks, which is
to be steep, which provided the basis for triggering gravity flow. Well
guided by the sequence stratigraphy and determination of sand bodies'
M was drilled toward the gentle zone near the bottom of slope break
distribution using excellent seismic data, wireline logs, and core qualities.
(Figure 8b). The gravity flow was easy to preserve due to the sensitive response to the changes of paleogeomorphology. The lithology showed that it contained less sand content than that of gravity flow.
7
DISCUSSION
|
However, it was composed of relatively pure glutenite with high mud content (Figure 8b). The differences in lithology explained the neces-
In the sedimentary environments of lacustrine rift basins, the density
sity of flow pattern transformation during the depositional process of
and flow velocity of sublacustrine fan change with regard to increasing
gravity flow. It was also conditioned by the fact that the differences
distance during the transformation between density current and
in the external conditions of flow pattern transformation were based
turbidite current. However, the evolution within this process is largely
on the differences of the transformation extent. As a matter of fact,
influenced by the slope gradient of paleogeomorphology, flow velocity,
if there was a stable sediment supply, the flow velocity increased as
water depth, and so on (Mulder & Alexander, 2001; Shanmugam,
the fault activities and slope gradients increased. This promoted the
2000). It is much simpler to trigger the gravity flow on a steeper slope,
transformation process and resulted in the different types of gravity
and the flow velocity increases within the steepening slope. In
flow deposits. However, due to the existence of slippage effect, the
addition, the episodic fault activities plays an important role in
debris flow can travel for a long distance on the premise of slope
controlling the episodic increasing accommodation (Martins‐Neto &
gradient (Amy, McCaffrey, & Tailing, 2009; Gani, 2004; Haughton,
Catuneatu, 2010). At the same time, the episodic fault activities also
Davis, McCaffrey, et al., 2009; Mutti, Bernoulli, Lucchi, et al., 2009).
control the slope gradient of paleogeomorphology, which promotes
When the slope gradient decreased, the deposits would be deposited
the distribution, evolution, and migration of turbidites.
as a whole. The sublacustrine fan deposits were distributed on the eastern slope during the early stage of the second member of the
7.1
|
Response to paleogeomorphology
Dongying Formation. This indicated that a gentle slope resulted from fault activities. In contrast, when the faults activated strongly in the
Before the deposition of the second member of the Dongying Forma-
late stage of the second member of the Dongying Formation, the
tion, the muddy deposits were widely distributed in the study area
locations where the deposition took place moved toward the central
(Figure 3a). However, there were some sublacustrine fan deposited in
basin. According to the seismic reflections from the seismic profile
the central lake (Figures 7a and 7b). During this period, it was mainly
and the stacking patterns of the well logs distributed within the basin
dominated by turbidite deposits with high mud content. By the time
and limited cores, it could be concluded that, as the slope gradient
of the early second member of the Dongying Formation, it mainly
resulted from the increase in the gradual strong fault activities, the
developed sandy debris flow with relatively lower mud content,
position where the deposition took place extended toward the basin.
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The depositional process and flow patterns changed not only due to
or inland sediment supply were reworked by the lake levels and depos-
the increasing slope gradient, but also due to the paleogeomorphology,
ited along the slopes. If the hydrodynamic was strong, then the
which caused the depositional position to migrate.
sediment, which was reworked by the lake levels, was transported to the deep lake areas and deposited. However, earthquakes may have
7.2
|
Response to base‐level changes
influenced the distributions of the sediment. Therefore, an integrated combination of favorable sedimentary facies regions containing the
It is suggested that the base‐level changes are the ratio of accommoda-
fine structural settings, which could become potential reservoirs, has
tion space and sediment supply, according to the theory of high‐resolution
provided the main basis for the zone selections and well proposals. It
sequence stratigraphy, which can be marked by accommodation (DA)/
is suggested in this study that more investigations are required prior
sediment supply (DS). In lacustrine rift basins, the syndepositional
to selecting the exploration targets of sublacustrine fan deposits.
faults play an important role in controlling the changes of the base level. Previous research has demonstrated that there was a positive linear relationship between fault activities and base‐level changes
8
|
CO NC LUSIO NS
(Martins‐Neto & Catuneatu, 2010; Shanmugam, 2000). In the study area, the fault activities influenced the paleogeomorphology and
In this study, the following conclusions were reached through the
associated accommodation. In addition, the types of sublacustrine fans
analysis of the geophysical identification and characteristics, and
and their depositional positions changed subsequently. When there
petroliferous significance of sublacustrine fan deposits in the second
are no obvious fault activities, the paleogeomorphology tends to be
member of the Dongying Formation in the Liaozhong Depression:
relatively extensive and flat. Under this circumstance, the sublacustrine fan deposits are characterized by weak‐amplitude and discontinuous
• In regard to the identification of sublacustrine fan deposits from
seismic reflections from seismic sections, which are interpreted to be
wireline log stacking patterns, they could be recognized by the
debris flow (Figure 10b), and indicate the response to the lower
distinctive low gamma‐ray and high‐spontaneous stacking
increasing rate of accommodation space. In contrast, when the faults
patterns within the background of high gamma ray and low
are active, they typically result in steep and bumpy paleogeomorphology.
spontaneous. The gamma‐ray stacking patterns for these deposits
This is mainly characterized by vermicule reflections from seismic
show both fining‐upward and coarsening‐upward trends, which
sections and bell‐ to cylinder‐shaped stacking patterns of well logs,
can respectively be interpreted as channel fills and lobe deposits.
which can be interpreted as turbidite deposits (Figures 7b, 8c, and 9c).
From the perspective of the seismic reflection characteristics, the
This indicates the response to the higher increasing rate of accommoda-
deposits
tion space and rapid rising of the base level. Here is an example to show
hummocky reflections, or “vermicular” reflections.
how this works. In the early stage of the second member of the Dongying Formation, the sediment supply was dominated by landward mud (Figure 3a). Due to inactive fault activities and subsequent flat geomorphology, the base level was characterized by slow rise. Correspondingly, the gentle geomorphology provided the places where the far‐shore sublacustrine fan deposition took place (Figure 10b). Then, the faults began to activate, which were mainly composed of debris flow deposits on relatively steep geomorphology (Figure 9c). Next, the base level began to rise accurately. By the time of the late second member of the Dongying Formation, the faults activated obviously due to the regional stress field, and the slope gradient increased rapidly. Mainly, the deposits from debris flow to gravity flow were shown (Figures 5 and 8c). In addition, the base level rose rapidly.
typically
displayed
continuous
or
discontinuous
• In the study area, four types of sublacustrine fan deposits were shown to contain various geophysical properties (mainly porosity and permeability). Geophysical properties control on the hydrocarbon accumulation in the turbidite sandstones: Sandstones with high porosity and permeability may have potentially high hydrocarbon saturation, whereas sandstones with low porosity and permeability may have low hydrocarbon saturation and are thus usually nonproductive, if there are pathways to source rocks and large sets of pure mudstone caprocks. • The changes in the slope gradient resulted in the transformation of flow patterns from debris flow to gravity flow. The depositional position moved toward the basin, due to the increasing slope gradient.
7.3 | Relationships between sublacustrine fan and fourth‐order sequences
• Through analyzing the relationship between the base‐level changes and fault activities, the processes of base‐level changes, types of sublacustrine fan deposits, and flow patterns were
The sublacustrine fan deposits, which have been located during the
combined, and the response among them was revealed. The
past few years in the study area, have demonstrated that they were
process of base‐level changes played an important role in control-
mainly developed at the bottom (on the top of the flooding surfaces)
ling the flow patterns.
of the fourth‐order sequences. This indicated that they were generally developed in response to the lacustrine fluctuations during the sequences' deposits. During the fourth‐order sequences' deposition,
ACKNOWLEDGEMENTS
the lake levels rose. In addition, the continuous fluctuations of the lake
The research was supported by the Bohai Oil Research Institute, China
levels played an important role in controlling the development and
National Offshore Oilfield Corporation Limited–Tanggu, Tianjin, China,
distribution of the sublacustrine fan deposits. The delta front deposits
and was funded by financial support from the Natural Science
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Foundation of China (No. 40972081, 91328201, U1262205), the Fundamental Research Funds for the Central Universities (No. 2‐9‐2013‐ 095, 2010ZD07) and the National Key Basic Research Project (Nos. 2011ZX05025‐002‐05,
2011ZX05001‐001‐04,
201105002‐006).
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AUTHOR BIOGRAPHY
Wang, J., Chen, H., Wang, H., et al. (2009b). Depositional characteristics of the large‐scale sublacustrine fan at member Yong 1 of the Yitong Graben and its response to tectonic events. Acta Geologica Sinica, 83(4), 550–557 (in Chinese with English abstract).
Jingyan Liu is an associate professor of petroleum geology in School of
Wang, J. G., Wang, T. Q., Zhang, S., et al. (2009a). Sedimentary characteristics and geophysical response of sublacustrine fan during transgress period in Songliao Basin. Acta Petrolei Sinica, 30(3), 361–366 (in Chinese with English abstract).
tigraphy in the Bohai Bay Basin, Northeast China, and Tarim Basin,
Wang, Y. J., Wang, J., Zhou, X. H., et al. (2008). Study of characteristics of Paleogene seismo‐turbidite in the central Liaozhong Depression, Liaodong Bay. Journal of Mineralogy and Petrology, 28(3), 84–89 (in Chinese with English abstract).
How to cite this article: Xia S, Liu J, Liu Z, et al. The geophys-
Energy Resources, China University of Geosciences, Beijing. Her research interests mainly focus on sedimentology and sequence straNorthwest China.
ical identification, characteristics, and petroliferous significance of sublacustrine fan deposits in the second member of Dongying Formation in Liaozhong Depression, Bohai Bay
Wu, K., Wu, J. G., Zhang, Z. Q., et al. (2012). Sedimentary model and seismic response characteristics of the sublacustrine fan in northern Liaozhong Depression. Journal of Northeast Petroleum University, 36(5), 33–37 (in Chinese with English abstract).
Basin. Geological Journal. 2018;53:692–706. https://doi.org/ 10.1002/gj.2921
