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Structural slope-break zone: Key concept for stratigraphic sequence analysis and petroleum forecasting in fault subsidence basins

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Science in China Ser. D Earth Sciences 2004 Vol.47 No.9 769782 769
Copyright by Science in China Press 2004
The control of syndepositional faulting on the Eogene
sedimentary basin fills of the Dongying and Zhanhua sags,
Bohai Bay Basin
LIN Changsong1, ZHENG Herong2, REN Jianye1, LIU Jingyan1 & Qiu Yigang1
1. Department of Energy Geology, China University of Geosciences, Beijing 100083, China;
2. Petroleum Exploration and Production Institute, Sino-Petroleum and Chemistry Cooperation, Beijing 100083, China
Correspondence should be addressed to Lin Changsong (email: lincs@cugb.edu.cn)
Received September 6, 2003
Abstract The Dongying and Zhanhua sags are the major hydrocarbon exploration and produc-
tion subbasins in the Bohai Bay Basin. Integrated analysis of the sedimentary basin fills has
shown that the syndepositional faults and their arrangement styles exerted an important influ-
ence on the development and distribution of the Eogene depositional systems. The sedimentary
filling evolution of the subbasins reflects the general control of the episodic rifting process. The
major long-term active normal faults formed a series of paleogeomorphic accident or slopebreak
zones that commonly delineated the subsidiary palaeostructural units and the depositional facies
tracts and constrained the general distribution of sedimentary facies zones. The central sag
boundary fault slopebreak zones usually determined the distribution of the depocenters of ter-
restrial clastic depositional systems, particularly the lowstand fans or deltaic depositional sys-
tems, and have proven to be the economically important targets for the exploration of subtle
sandstone reservoirs. A variety of syndepositional fault arrangement patterns, including the par-
allel, en echelon, combo-like, broom-like fault systems and the fault overlap or transfer zones,
have been recognized in the subbasins. They generated distinctive geomorphic features and
exerted a direct influence on sediment dispersal and sandbody distribution during the Eogene
synrift stage. It is the key for the prediction of depositional systems tracts and reservoir sand-
stones to investigate the activity and distribution of the fault slopebreak zones and reconstruct
the structural paleogeomorphy in different basin filling stages of the basin evolution.
Keywords: episodic rifting, fault slopebreak zone, depositional pattern, Bohai Bay Basin.
DOI: 10.1360/03yd0203
The configuration and prediction of depositional
systems in a sedimentary basin have long been one of
the major tasks of basin analysis and sedimentary geo-
logical research. In terms of sequence stratigraphy,
originally established in study of tectonically table
continental margins, the sea level change has been
used to effectively interpret and predict the distribu-
tion and evolution of depositional systems in these
basins[1,2]. But in tectonically active basins, tectonism
may control predominately the architecture and evolu-
tion of the basin fills. The episodic tectonism, varia-
tion in tectonic subsidence rate and the activation of
syndepositional structure can make a significant in-
fluence on the change in accommodation, depositional
770 Science in China Ser. D Earth Sciences
rate and sediment supply[3]. The tectonostratigraphy
analysis has recently become a major international
theme of sedimentary basin analysis, particularly the
petroleum basin analysis[411]. Although comprehen-
sive analysis of tectonic and sedimentation has long
been emphasized in basin research, it is a new view of
tectonostratigraphy, combined with the analysis of
sequence stratigraphy, to reveal how the paleogeo-
morphic features generated by the activation of syn-
depositional structures can constrain the patterns of
sediment dispersal and the development of deposi-
tional systems tracts in sedimentary basins.
Mesozoic-Cenozoic coal and oil-bearing fault ba-
sins were widely developed over eastern China. In
these basins syndepositional structures were very ac-
tive and resulted in significant change in facies types
and distribution. The major factors controlling the
development and distribution of depositional systems
in these basins have been widely investigated and at-
tracted great geologic interest in China. Most of the
major oilfields in these basins are mature in explora-
tion and development, and thus, the subtle traps have
increasingly turned to be the major target of the petro-
leum exploration. It is clear that the break through in
hydrocarbon exploration will depend on the develop-
ment of effective approach for prediction of deposi-
tional systems and sandbody distribution. This has
most recently become the important research theme of
these basins[12].
Studies in the Jiyang Depression of the Bohai
Bay Basin have shown that the activation of palaeo-
structural geomorphic features generated by synde-
positional structures played an important role in con-
trol on the development and distribution of deposi-
tional systems during Eogene synrift period. The
long-term active syndepositional normal faults and
their arrangement styles control the variation in ac-
commodation and thus determine the patterns of sedi-
ment disposal and the distribution of sandbodies. The
palaeostructural geomorphic features can be recon-
structed based on study of palaeostructural framework
and basin evolution. This may provide an effective
approach for prediction of the distribution of reservoir
sandstones in these basins. The concept has been ap-
plied to the exploration of subtle sandstone reservoirs
and has achieved great success in the Bohai Bay Ba-
sin1).
The goal of this paper is to document the control
of syndepositional faults on the depositional pattern
and the distribution of potential reservoir sandbodies
formed during the synrift stage in the Dongying and
Zhanhua sags. A large amount of geological and geo-
physical data and previous research work, especially
densely penetrated borehole data and 3D seismic pro-
files provide a valid dataset for the study of the basin
fill architecture and evolution.
1 Basin setting
The Bohai Bay Basin is one of the largest Ceno-
zoic oil and gas-bearing rift basins in eastern China. It
is bounded on the north, the east and the south by the
Yinshan, Liaodong and Luxi highlands, and abuts the
Taihangshan Range on the west. The “s”-shaped basin
covers an area of about 29000 km2 and elongates
northeastward. It is separated by a few of regional up-
lifts into seven depressions or subbasin belts (fig. 1).
The Jiyang Depression is a relatively large one and
located in the southeastern part of the Bohai Bay Basin.
The study area of this paper focus on the Dongying
and Zhanhua sags within the Jiyang Depression, which
comprise the majority of the exploration and devel-
opment districts of the Shenli oilfield, the second
largest oilfield in China (fig. 1).
The Dongying and Zhanhua sags have about
6000 km2 and 3800 km2 in size respectively. They are
fault subbasins consisting of a series of fault blocks,
organized as northeast-trending complex half grabens
with master basin-bounding faults on the north margin
and hinged up to the south. These half grabens can be
usually subdivided into three palaeostructural elements:
the steep slope with master basin bounding fault, the
faulted central sag or trough, and the gentle slope
1) Pan Yanlin, Kong Fanxian, Zheng Herong et al., Tertiary deposition, structure and petroleum accumulation of the Jiyang Depression, Bohai
Bay Basin, Sino-Petroleum and Chemistry Corp., Research Report, 2000, 154.
Syndepositional fault, basin fill, Bohai Bay Basin 771
Fig. 1. Schematic map showing the location of the Jiyang Depression in the Bohai Bay Basin and the structural framework of the Dongying and
Zhanhua Subbasins, noting the structural sections across the Dongying (A) and Zhanhua (B) Subbasins.
(hinged margin) in the N-S direction, and usually
complicated and separated by N-S trending faults as a
number of sub-depressions and lower uplifts in the
E-W direction.
The Bohai Bay Basin is a Tertiary rift basin
formed by the stretching of lithosphere in the
Circum-Pacific domains in eastern China. The basin
fills of the Tertiary are composed of about 7000
10000 m terrestrial clastic lacustrine deposits. Deep
seismic survey and Global Geosciences Transects
(GGT) have revealed that the Moho underneath the
basin was obviously uplifted, showing a mirror reflec-
tion with respect to the largest basin subsidence zone.
The earth crust was thinned into 1620 km. The
analysis of the regional interaction of the Pacific and
the Tethys domains and the fault styles of the basin
indicate that the formation and evolution of the basin
is still related to the imposition of dextral strike-slip
displacement[13] (fig. 1). There are many right lateral
strike slip structures found in the Dongying and
Zhanhua sags, such as en echelon, horsetail or
broom-like and flower fault systems. The Yidong
broom-like fault systems in the Zhanhua sag and the
Shenbei and Lijing en echelon fault systems in the
Dongying sag, for instance, all resulted from dextral
strike-slip transtension.
2 Basin filling sequence and episodic rifting
The Bohai Bay rift basin formed during the Ter-
tiary and evolved generally from Eogene synrift to
Neogene postrift stages (fig. 2). The Eogene and Neo-
772 Science in China Ser. D Earth Sciences
gene sedimentary basin fills can be regarded as two
first order sequences, which are separated by a re-
gional breakup unconformity traceable throughout the
entire basin. The synrift basin fills of the Eogene sys-
tem are about 50008000 m thick and include the
Kongdian, Shanhejie and Dongying formations. The
postrift Neogene system consists of the Guangtao and
Minghuazheng formations. Four basin fill units or
second order sequences have been recognized in the
Tertiary basin fills in terms of the sequence stratigra-
phy[1,2]. The four second order sequences comprise a
transgressive to regressive depositional cycle and are
defined by a regional unconformity or regressive dis-
continuity. They are different in the types and con-
figuration of depositional systems, the distribution of
depocenters and the palaeostructural style, reflecting
that the basin filling is characteristic of episodic and
underwent several filling stages. It has been shown
that each second order sequence represents the basin
fills of one rifting episode. This has also been verified
Fig. 2. Tertiary basin filling sequence and tectonic evolution of the Jiyang Depression, Bohai Bay Basin. 1, Alluvial fan, fan delta sandstones and
conglomerates; 2, fluvial and deltaic sandy deposits; 3, lacustrine and flooding basin fine-grained deposits; 4, limestone and micrite; 5, oil shale; 6,
deep and shallow lacustrine muddy deposits.
Syndepositional fault, basin fill, Bohai Bay Basin 773
by episodic variation in tectonic subsidence rates re-
constructed based on stratigraphic data from the larg-
est subsidence central sags of the Dongying and the
Zhanhua subbasins by the backstripping technique
with the correction for sediment compaction, sediment
loading and paleobathymetry[14] (fig. 2).
The basal stratigraphic unit (SS1), corresponding
to the Kongdian Formation and resting unconform-
ablely on the pre-Tertiary systems, is about 5001000
m thick and composed of variedness conglomerate and
sandstone, sandy mudstone and represents the basin
fill of early rifting stage. The depocenters were mainly
controlled by NWW- or EW- and NE-trending growth
normal faults. The largest tectonic subsidence rate in
the deepest parts of the Dongying and Zhanhua sags is
up to 120170 m/Ma. The volcanic rocks are domi-
nated by quartz tholeiite. The overlying Shahejie For-
mation is about 10002000 m thick and can be di-
vided into three second order sequences. The lower
fourth member of the Shahejie Formation (SS2), with
a basal unconformable contact with the underlying
strata, consists mainly of alluvial and fluvial
coarse-grained deposits intercalated with shallow
lacustrine muddy deposits, while fan-delta sandstones,
shallow to deep lacustrine mudstones, reef limestone
or micrite and dolomite dominate the upper part of the
SS2. Anhydrite, halite and gypsum found in the se-
quence reflect an arid or semiarid climate condition.
This period was characteristic of varied relief in to-
pography and widely developing coarse-grained allu-
vial fan and fan-delta systems. The largest tectonic
subsidence rate is about 100140 m/Ma.
The SS3 (the third and the lower second members
of the Shahejie Formation) shows a local gentle angu-
larly unconformable and correlative conformable con-
tact with the underlying SS2. It is mainly composed of
deep lacustrine mudstone, oil shale, fluvial delta,
fan-delta and sublacustrine fan deposits, comprising a
regional transgressive to regressive cycle and repre-
senting the basin fills deposited during a strongly rift-
ing and rapid subsidence stage commencing in 42 Ma
ago. the largest tectonic subsidence rate is up to 250
350 m/Ma. The volcanic rocks are characterized by
olivine tholeiite, which indicates that the strong
stretching of the lithosphere led to upwelling and
erupting of deep mantel materials. The subsidence
centers shifted obviously from those of the SS2 and
were predominately controlled by the increasing activ-
ity of the NE-trending syndepositional faults in the
Dongying and Zhanhua subbasins.
The SS4 constitutes the upper second and the first
members of the Shahejie Formation and the Dongying
Formation, separated from the SS3 by a regional ero-
sional unconformity. It generally displays a regional
cycle from transgression to regression and is domi-
nated by shallow, deep lacustrine and fluvial deltaic
deposits onlapping most of the lower uplifts within the
basin. The faulting weakened during this period and
the largest tectonic subsidence rate is between 60
m/Ma and 100 m/Ma. The lake obviously extended
and transgression widely occurred during the period of
the first member of the Shahejie Formation, and shal-
low lacustrine fine and deltaic deposits dominated the
basin fills. Activation of some new syndepositional
faults in this stage can be observed.
The Guangtao Formation overlying the Shahejie
Formation represents the basin fills of the early pos-
trift stage of the basin evolution. The breakup uncon-
formity separating the Guangtao and the Shahejie
formations reflects the occurrence of relatively strong
tectonic uplift, as relatively large amount of erosion
along the unconformity are measured and inversed
structures (faults) are observed underneath the uncon-
formity on seismic profiles. Thick bedded,
coarse-grained braided fluvial deposits and incised
valley fills rest on the unconformity and grade upward
into meandering river system and shallow lacustrine
facies. The subsidence rate is lower, with the largest
total subsidence rate about 5080 m/Ma and the tec-
tonic subsidence rate, 2050 m/Ma. A regional rapid
downwarping with a subsidence rate up to 100 m/Ma
occurred to the period of Minghuazhen Formation and
resulted in the formation of a capping bed of shallow
lacustrine fine deposits over the entire basin. This pe-
riod of rapid subsidence seems to be simultaneously in
most Tertiary rift basins in eastern China[14].
774 Science in China Ser. D Earth Sciences
The pronounced episodic evolution of the basin
fills reflects clearly the episodic or pulse stretching
and faulting process of the lithosphere. This process
tends to be universal in most of the Mesozoic and Ce-
nozoic extensional basins in eastern China[13,14]. In
general, relatively large scale basement or intrabasin
growth normal faults in the Dongying and Zhanhua
sags were intermittently active in different rifting epi-
sodes, whereas the development of relatively small
scale subsidiary faults was usually related to certain
rifting episodes. The basin fills of each rifting episode
comprise a second order depositional cycle or se-
quence and defined by unconformities. This indicates
the decrease of tectonic subsidence rate and even up-
lifting to the end of each episode, and the origin of the
unconformities should be related to the uplift at the
end of each episode and the tectonic movement or ad-
justment before the next episode.
3 Control of syndepositional faults and their ar-
rangements on depositional patterns
Sedimentary facies mapping and comprehensive
analysis based on densely spaced borehole data and
3D seismic profiles clearly show that the patterns of
sediment dispersal and deposition in the subbasins
were mainly attributed to variation in geomorphology,
generated by the activation of syndepositional struc-
tures, particularly the growth normal faults. Because
of widespread development and intensive activation of
syndepositional faults, the change of geomorphic fea-
ture either in local or in basin-scale was predominately
constrained by syndepositional faults through the en-
tire synrift stage. It has been argued that, wherefore,
the activation and their arrangement styles of the
growth normal faults are the most important factors
controlling the depositional pattern of the synrift basin
fills.
3.1 Fault slopebreak zones
It has been clearly shown that pronounced varia-
tion in geomorphology during the synrift stage was
mainly related to the activation of relatively large scale
syndepositional normal faults. A variety of faulting
styles, such as domino or listric faulting, synthetic and
antithetic compensatory faulting and strike-slip trans-
tension can generate a series of growth normal faults
in synrift stages. Once they are formed, the relatively
large scale faults will commonly keep acting during a
long-term period due to advantageous stress concen-
tration. These major faults usually form geomorphic
accident zones bounding the geomorphic units and
sedimentary facies tracts. We referred these geomor-
phic accident zones generated by relatively large scale
and long-term acting structure or syndepositional
faults to structural or fault slopebreak zones. The fault
slopebreak zones widely developed in the Dongying
and Zhanhua sags during the Eogene synrift stage. The
growth coefficient (the ratio of the thickness of sedi-
mentary layer in footwall to that of the coeval strata in
the hanging all) of most of the master syndepositional
faults generated these slopebreak zones is between 1.3
and 2.5 in the study area (fig. 3).
In passive continental margins, the shelf slope-
break zone is a pronounced geomorphic feature de-
lineating the distribution of depositional systems tracts.
During lowstand of sea level the area upstream from
the slope is exposed or eroded, whereas the lower
slope to basin plain develops lowstand clastic wedges
and lowstand fan systems. Study shows that the depo-
sitional slope break zones that made an important in-
fluence on the development and distribution of deposi-
tional systems tracts in the rift lacustrine basin are
mostly those generated by the activation of syndeposi-
tional faults.
In the Dongying and Zhanhua subbasins, there
were a variety of step fault slopebreak zones formed by
growth normal faults along the steep slopes, the fault
central sags and the gentle slopes, which produced very
complex geomorphic features. The major fault slope-
break zones include the uplift-steep slope, steep
slope-central sag, gentle slope-central sag and gentle
slope-uplift boundary fault slopebreak zones (figs. 1, 3
and 4). They formed the boundaries defining the sub-
sidiary palaeostructural units and thus, the geomorphic
elements. Their activation and arrangement deter-
mined the palaeostructural framework and general
geomorphic feature of the basin and then exerted a
strong influence on the architecture of basin fills.
During the period of deep lake development, the fault
Syndepositional fault, basin fill, Bohai Bay Basin 775
Fig. 3. 3D seismic profile (a) and log correlation sedimentary section (b), showing the control of fault slopebreak zones on the distribution of sedi-
mentary facies tracts. HSD, Highstand deltaic deposits; LSF, lowstand sublacustrine fan deposits; FD, fandelta deposits; LMD, lacustrine muddy
deposits; SS1-SS5, second order sequences; Sb1-Sb3, third order sequence boundaries within the third member of the Shahejie Formation.
slopebreak zones forming the transitional zones from
shallow to deep lake are analogue to the shelf slope
break in the passive continental marginal basins.
3.2 Arrangement patterns of fault slopebreak zones
and the distribution of depositional systems
The Dongying and Zhanhua subbasins were in a
rifting stage of rapid subsidence in Eogene. The major
facies associations identified in the basin fills of the
Eogene include alluvial fan, fan delta, sublacustrine
fan or subaqueous fan, braided river delta, axial fluvial
delta, shallow and deep lacustrine muddy deposits.
Integrated analysis has shown that the arrangement of
fault slopebreak zones predominately determined the
general distribution of these deposits.
Along the steep slope commonly developed fan
delta and nearshore subaqueous fan complexes during
Eogene in the subbasins. The fan delta systems,
formed by alluvial fans that prograded directly into the
lake, are composed of subaerial proximal fan con-
glomerates, fandelta front sandstones and prodelta
interbedded sandstones and mudstones. These rela-
tively coarse-grained sediments usually comprise
coarsening upward deltaic sequences and commonly
show as a series of prograding complex wedges on
seismic profiles (fig. 3). In highstand of lake level,
along the fault escarpments formed plenty of
coarse-grained subaqueous fan systems, which are
dominated by gravity flow deposits and composed of
debris flow breccia, gravity flow channel fills and tur-
bidite sandstones intercalculated with dark mudstones
of deep lacustrine facies. Study shows that the multi-
ple steps of slopebreak zones generated by the activa-
tion of growth normal faults along the steep slopes
776 Science in China Ser. D Earth Sciences
Fig. 4. Schematic map showing the reconstructed geomorphic feature and the related depositional pattern of the third member of the Shahejie For-
mation in the Shiko-Bonan sags, Zhanhua subbasins.
controlled the distribution of distinctive sedimentary
facies zones in different evolutionary stages. In the
Dongying and Zhanhua sags, the steep slopes com-
monly developed two to three steps of fault slopebreak
zones. During the period of the Kongdian Formation
and the fourth member of the Shahejie Formation, the
master boundary fault slopebreak zones flanking the
north uplifts controlled the distribution of alluvial fan
and shallow lacustrine fandelta deposits. From the
period of the upper fourth and third members of the
Shahejie Formation to the Dongying Formation, these
slopebreak zones mainly confined the development of
the proximal coarse-grained deep lacustrine fandelta
and nearshore subaqueous fan systems, whereas the
central sag bounding fault slopebreak zones (second or
third fault steps) mainly constrained the depocenters of
the sandy fan delta front and sublacustrine turbidite
fan systems (fig. 3).
Along the hinged gentle slope of the subbasins
mainly received fluvial or braided river delta, clastic
shoreline and shallow lacustrine deposits. The fault
slopebreak zones generated by antithetic compensating
faults have also made an important influence on the
development and distribution of these deposits. Some
depocenters of fluvial and braided river deltaic sys-
tems were controlled by the relatively small scale
boundary growth normal faults along the uplift mar-
gins. The central sag-gentle slope boundary fault
slopebreak zones defined the depositional margin of
early basin fills or lowstand systems tracts and formed
the boundaries between the shallow and deep lake ar-
eas. These slopebreak zones usually kept relatively
large space accommodation and lower topography due
to differential subsidence and pulse faulting, resulting
in abrupt increase in the thickness and the number of
sedimentary layers, sandstone beds and depositional
cycles, and controlled the development of the high-
stand deltaic front depocenters and the distribution of
the lowstand fan wedge. These deltaic systems were
formed mainly by braided rivers from the southern
gentle slope and axial (meandering) river systems
form the east, and characterized by coarsening upward
Syndepositional fault, basin fill, Bohai Bay Basin 777
deltaic sequences consisting of relatively fine deposits.
The central sags in the subbasins were relatively
deep and rapid subsidence areas confined by the cen-
tral sag boundary fault slopebreak zones, and filled
mainly with deep and semi-deep lacustrine muddy
deposits intercalculated with sublacustrine fan and
deltaic deposits. Subsidiary fault steps and troughs can
form in the central sags, which commonly controlled
the distribution of sublacustrine fan or turbidite and
deltaic deposits, particularly those from axial sandy
clastic systems (fig. 3).
3.3 Control of fault slopebreak zones on the deposi-
tional pattern of deep lacustrine sequences of the third
member of the Shahejie Formation
Sedimentary and syndepositional fault mapping
of the Shahejie Formation based on plenty of borehole
data and 3D seismic facies analysis has revealed the
genetic relationship between the arrangement of faults
slopebreak zones and the configuration of the deposi-
tional systems (figs. 4 and 5). The third member of the
Shahejie Formation deposited during a period of deep
lacustrine basin development. In the Zhanhua subbasin,
the Shiko, Bonan and Gubei central sags, confined by
a series of central sag boundary fault slopebreak
zones, were dominantly filled with deep and
semi-deep lacustrine facies tracts (fig. 4). The largest
depth of the lake in the central sags is around 100
150 m, estimated through the decompaction of the
thickness of the fan deltaic front deposits and meas-
ured from the fandelta prograding clinoform on 3D
seismic profiles. The central sags confined by growth
normal faults are favorable for accumulation of high
quality source rocks. The Bonan-Shiko central sags
were defined by the Luobei fault to the south, Guxi
fault to the east and the Chennan-Yidong fault steps to
the north and the west. These growth normal faults
formed the geomorphic accident zones transitional
from shallow to deep lake. The lower part and the base
of these fault slopebreak zones were lower areas in
topography and usually captured a series of lowstand
delta and sublacustrine fan complexes of the third in-
terval of the Shahejie Formation, and the sandy depos-
its have been mostly penetrated and delineated based
on 3D seismic profile interpretation. They were fed
partly from the north steep slope and dominantly from
the south gentle slope and the axial sediment dispersal
system from the east Gudao uplift. Upstream from
these fault slopebreak zones commonly found incised
channels or unconformities formed during lowstand of
lake level.
Figure 5 shows the relationship between major
syndepositional faults and the development of clastic
depositional systems of the third member of the Sha-
hejie Formation in the Dongying subbasin. It is clearly
seen that from north to south there are four to five ba-
sin scale growth normal fault belts, which determine
the configuration of the basin fills. The northern steep
slope was bounded by the basin bounding Chengnan
fault belt, which controlled the general subsidence
history. This fault belt is of complex structure style
with a variety of accompanied structures. The subsidi-
ary NE-trending faults, such as the Shengbei, Bingnan
and Lijing growth normal faults are arranged as dex-
tral en echelon fault systems, forming a complex fault
slopebreak zone formed the northern boundary of the
central sag. Along this slopebreak zone the thickness
of the deposits increased abruptly and the coarse-
grained clastics from the north basin margin were
captured and deposited as a series of lowstand fan or
sublacustrine sandy aprons. Exploration to date has
proved that the central sag bounding fault slopebreak
zones are favorable for the formation of subtle sand-
stone pools in the subbasin.
Along the southern gentle slope three major NE
or NNE trending antithetic (dip to the northwest) nor-
mal fault belts, generated due to compensating and salt
doming in the central sag, were intermittently active
and formed three complex, arc-shaped fault slopebreak
zones during the period of Shahejie Formation.
Among them the Liangjialo-Xianhe fault belt extended
northeastwards and connected with the northern
Shengbei fault belt, forming an inner ring central sag
boundary slopebreak zone during the late period of the
third member of the Shahejie Formation, while the
Chengangzhuan-Wangjiagang and the Bamianhe fault
belts formed two outer ring slopebreak zones. During
the middle period of the third member of the Shahejie
778 Science in China Ser. D Earth Sciences
Fig. 5. The distribution of coarse-grained clastic systems of the third member of the Shahejie Formation and the major fault slopebreak zones in
Dongying sag. (a) Lowstand fan sandy deposits of the third member of the Shahejie Formation developed along the fault transform zone between the
Shengbei and Lijing growth normal faults; (b) depositional section across the fault slopebreak zone in the southern gentle slope.
Formation, the depocenters of the highstand fluvial
deltaic systems fed from the southern gentle slope
were mainly distributed along the Chengangzhuan-
Wangjiagang fault slopebreak zones, meanwhile the
depocenters of the lowstand fan or deltaic systems
were mainly situated in paleogeomorphic lowlands
along the Liangjialo fault slopebreak zone. These low-
stand clastic systems entered the central sag from the
west tip of the fault slopebreak zone, thickened and
extended northeastwards along the strike of the slope-
break zone. The thickened deltaic front deposits fed
from the east were also constrained by the fault slope-
break zone. The late highstand delta system of the
Shahejie Formation from the east has prograded to the
middle part of the Liangjialo fault belt (fig. 5). These
fault slopebreak zones have also proven to be the ad-
vanced area for the formation of subtle sandstone
pools.
Syndepositional fault, basin fill, Bohai Bay Basin 779
4 Structural paleogeomorphic features of distinc-
tive syndepositional fault arrangements and the
related sandy sediment dispersal systems
The syndepositional faults varied in scale, style
and arrangement, resulting from different structural
stress fields, reactivation of varied pre-existing fault
systems and gravity adjustment within a basin. Dif-
ferent arrangements of fault systems will generate dif-
ferent geomorphic features. Studies show that some
syndepositional fault arrangements, usually occurring
during the Eogene synrift stage, formed certain inte-
rior geomorphic features that exerted a significant in-
fluence on sediment dispersal and sandbody distribu-
tion in the Dongying and Zhanhua subbasins.
4.1 Comb-like structure
The so-called “comb-like structure” is one of the
growth normal fault patterns commonly found in the
study area. It is composed of one master fault associ-
ated with a series of subsidiary adjustment or com-
pensating faults that oriented perpendicularly to the
master fault. The formation of the subsidiary faults has
been interpreted as the result of compensation due to
the variation in the throw magnitude along the master
fault strike, or the superimposition of another group of
perpendicular active faults. A comb-like structure
commonly controlled the distribution of a certain fa-
cies tracts. A typical example is found in the Gubei
central sag in the Zhanhua subbasin. It consists of the
north-south-oriented Changdi-Wuhaozhuang master
fault and a number of E-W extending perpendicular
subsidiary faults (fig. 6). It is clearly shown that the
comb-like structure constrained the distribution of a
sublacustrine fan complex of the third member of the
Shahejie Formation. The sandstones of these fan sys-
tems constitute the major oil reservoirs of the Wu-
haozhuang oil field. Relatively large scale compensat-
ing W-E trending faults controlled the location of the
main channel fills, which extended basinwards along
the fault strike. We can clearly see from depositional
section and seismic profiles that the paleogeomorphic
lowlands caused by hinging wall collapse usually
contained relatively thick fan sandstones. The sand-
bodies of the third member of the Shahejie Formation
found in these areas are more than 100 m in thickness
and much thicker than in the area around. The
comb-like structure has been verified to be one of the
important fault arrangement styles controlling the dis-
tribution of sandbodies in central sags. The discovery
of this structure type provides a significant model for
exploration of subtle sandstone pools in central sags.
4.2 Fault transfer accommodation zones and fault
tips
The throw on a syndepositional fault usually de-
creases away from the central part of the fault towards
its tips[5]. The central part of the footwall is relatively
high in geomorphology owing to the flexuring uplift
and the tips of a fault are lower in topography. Thus
the transfer accommodation zone of two synthetic
syndepositional normal faults form a topographically
lower ramp, where dominant sediment transport sys-
tems are commonly captured and, in the front of the
low ramp towards the central sag, large scale deltaic or
sublacustrine complexes can be usually found. Over-
lap zones of parallel or en echelon fault systems are
also topographically lower areas, where were usually
situated the depocenters of fluvial-deltaic or sublacus-
trine fan systems.
In the Dongying subbasin it has been found that
large complex lowstand fan systems or near-shore
subaqueous fans of the Shahejie Formation accumu-
lated along the transfer accommodation zone formed
by the Bingna and Shengbei faults. The sandstone
isopach of the fan system delineated a fan-lobe shape
with an area about 60 km2 and the thickness of the
reservoir sandstones is up to 100 m (fig. 5(a)). Similar
situation is found on the fault overlap zone of the Li-
angjialo en echelon fault system, where developed a
set of thick fan sandy deposits of the third member of
the Shahejie Formation (fig. 5).
4.3 Broom-like structure
The broom-like structure is made up of a master
fault and a group of subsidiary faults that bifurcate or
splay out from one end of the major fault, such as the
Yidong and Shaojia fault systems in the Zhanhua sub-
basin and the Linshan fault system in the Huiming
780 Science in China Ser. D Earth Sciences
Fig. 6. The distribution of lowstand sublacustrine fan sandstones of the third member of the Shahejie Formation (a) and the reconstructed structural
paleogeomorphic feature related to comb-like structure (b) in Gubei sag, Zhanhua Subbasin.
subbasin. It has been shown that the master faults of
these systems usually constrained the direction of
sediment dispersaland the splayed positions of the
fault systems formed a geomorphic lowland and
commonly captured the sandy depocenters of deltaic
systems. Similarly the conjuncture of two normal
faults may form an interior, fork-shaped depositional
sink, which usually controls the location of sandy
depocenters of clastic systems. The upstream drainage
systems commonly extend basin-wards along the
strike of the master fault. The conjuncture, for instance,
formed by the EW-trending Gunan fault and the
NE-oriented Gudong normal fault, constrolled the
deltaic sandy depocenter of the Shahejie Formation in
the eastern part of the Zhanhua subbasin. The sedi-
ment dispersal (fluvial) system sourced from the
Changdi uplift, prograded southeastwards along the
strike of the Gudong fault into the Gunan central sag
and resulted in a .fork-shaped sandy depositional zone.
5 Prediction of sandstone reservoirs along fault
slopebreak zones
Exploration in the study area has shown that the
fault slopebreak zones are favorable hydrocarbon ac-
cumulation zones. And particularly the central sag
border fault slopebreak zones are of favorable condi-
tions of reservoirs, seals and sources for the formation
of subtle sandstone pools. Firstly, the central sag con-
fined by fault slopebreak zones was commonly occu-
pied by deep or semi-deep lake and deposited high
quality and thick source mudstones. Secondly, along
the central sag boundary fault slopebreak zones can
find bulk of thickened or stacked and relatively well
sorted reservoir sandstones, as the depocenters of del-
taic front deposits, lowstand fan or sublacustrine fan
systems usually tended to develop along the slope-
break zones, particularly along those with comb-like,
fork-shaped and broom-like fault systems. At the same
time, most of the sandstones deposited along the fault
slopebreak zones are connected directly to the source
rocks, and the central sag bounding faults can served
as important pathways for hydrocarbon migration. On
the other hand, these faults may also form well later-
ally sealed condition as they usually have relatively
large growth coefficients. Activation of the syndeposi-
tional faults and the rapid accumulation of sandbodies
along the slopebreak zones tend to generate a variety
of structural traps, such as fault blocks and rollovers
along the slopebreak zones either on the steep or on
the gentle slopes. When basins are inversed, the fault
slopebreak zones are prone to the concentration of
tectonic stress, favorable the formation of anticline
Syndepositional fault, basin fill, Bohai Bay Basin 781
and the enhancing of the preexisting rollovers along
the slopebreak zones. Hydrocarbon exploration to date
in the subbasins has verified that these fault slope-
break zones are economically important targets in
search of subtle sandstone pools.
6 Discussion and conclusion
It has been widely accepted that tectonic subsi-
dence rate, global sea level change and sediment sup-
ply are the major factors controlling the occurrence of
deposition and erosion. In tectonically active rift
lacustrine basins the interplay of tectonic subsidence
and lake level or climatic change should be the most
important factors determining the depositional pattern
of the basin fills. The largest tectonic subsidence rates
of the Mesozoic and Cenozoic fault basins in eastern
China are up to 250700 m/Ma[14], and it is no doubt
that the tectonic differential subsidence should exert a
strong influence on the variation in depositional base
level and sedimentary geomorphy. Paleogeomorphic
or topographic features generated by palaeostructural
activation will constrain the distribution of the in-
tro-basin drainage systems and the accumulation of the
clastic depositional systems when the dispersal sys-
tems prograde towards the central basin in lowstand.
The development and localization of incised valleys
and lowstand deltaic or sublacustrine fans seem to be
mostly attributed to the activation of grown normal
faults. When lake level is in highstand, the geomor-
phic lowlands formed by syndepositional faults along
shorelines tend to capture the deltaic depocenters;
meanwhile the subaqueous geomorphic features
caused by palaeostructure will constrain the underflow
or gravity flow systems into the central sags. Studies
in the foreland basins in western China or abroad have
also demonstrated that the structural paleogeomorphy
is one of the important factors controlling the distribu-
tion of the depositional systems[15,16]. For instance, it
has been shown that the basement thrusting may pro-
duce a series of structural slopebreak zones exerting
an obvious influence on the depositional pattern of the
foreland basin fills. It is clear that the control of struc-
tural paleogeomorphy on the depositional patterns of
basin fills is universal in tectonically active basins.
The conclusions from the discussion above are as fol-
lows:
(1) It is universal that structural paleogeomorphic
features generated by growth normal faults can make
an important influence on the depositional pattern of
basin fills in tectonically active basins. In rift basins
the development and arrangement of the fault slope-
break zones determine the general palaeostructural
framework and depositional architecture of the basin
fills. The paleogeomorphic features generated by the
arrangements of the syndepositional faults within the
fault slopebreak zones confine the local sediment dis-
persal patterns and the distribution of the sandbodies.
(2) The arrangement and activation of the synde-
positional faults in the Early Tertiary Dongying and
Zhanhua subbasins have been inferred to be the key
factors controlling the development and configuration
of the depositional systems tracts. The central sag
bounding fault slopebreak zones have proven to be the
important potential exploration targets in search of
subtle sandstone pools as the sandy depocenters tend
to develop along the slopebreak zones.
(3) To reveal the genetic relationship between the
distribution of sedimentary facies and the fault slope-
break zones through reconstruction of the arrangement
styles of the syndepositional faults and the resulted
paleogeomorphic features is the key to demonstrate
the depositional patterns of the synrift basin fills and
the distribution of sandstone reservoirs.
Acknowledgements This work was supported by the Ministry of
Science and Technology of China (Grant No. G1999043304) and the
National Natural Science Foundation of China (Grant No. 40072039).
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The formation of the North Croatian Basin, which represents the south-western marginal part of the Pannonian Basin System and the Central Paratethys Bioprovince, began during Ottnangian time (early Miocene) by continental rifting. The syn-rift phase lasted until the middle Badenian (middle Miocene), and resulted in the formation of elongated half-grabens characterized by large sediment thicknesses strongly influenced by tectonics and gradually increasing volcanism. Towards the end of the syn-rift phase sinistral strike-slip faulting took place, transverse to oblique to the master faults, which disintegrated the longitudinal structures contemporaneously with volcanic activity. The depositional environments gradually changed from alluvial and lacustrine to marine. The syn- to post-rift boundary was characterized by significant erosion of the uplift fault block footwalls. The post-rift phase extended from the middle Badenian to the end of the Pontian (latest Miocene). Tectonic influence drastically decreased, volcanism ceased, and subsidence of the basin was controlled predominantly by cooling of the lithosphere. Marine connections gradually decreased, resulting in a transition from marine to brackish, ‘caspi-brackish’ and finally fluvial-marsh environments. By the end of the Miocene the basin was finally infilled. The basin evolution was also complicated by an alternation of phases of extension and compression.
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Each tectonic setting within rift half-graben basins generates a predictable range of lithofacies architectures. Asymmetry in lithology and strata thickness is the result of lake-level fluctuations interacting with varying rates of sediment accumulation, much of which is structurally influenced. Differences in sequence geometry have implications not only for interpreting ancient rift-lake deposits but also for deposition of economically viable reservoir facies and their juxtaposition with source rocks and caprocks. -from Author
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no. 7. v + 55 pp.+ 14 fold-out plates. Tulsa: American Association of Petroleum Geologists. ISBN 0 89181 657 7.
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Both tectonics and eustasy contribute to a basin's accommodation, and hence, to observed cyclic stratal patterns, but distinguishing between the relative contribution of these components is not always straightforward. This paper addresses this problem with an example from the Upper Cretaceous foreland basin of southwestern Wyoming. Three large-scale sedimentation cycles, 500 m to 1500 m in thickness, are present in the Upper Cretaceous section. Geohistory analysis indicates that the onset of deposition of the lower, shale-prone part of each cycle corresponds with an acceleration in subsidence rate. The increases in subsidence rate were caused by thrust movements in the Sevier Fold-Thrust Belt. Deposition of the upper, progradational part of each cycle occurred during times of decreased subsidence rate and can be tied to periods of relative quiescence in the thrust belt. Superimposed on the larger scale, subsidence-related stratal packages are third- and fourth-order depositional sequences. It is proposed that the controlling mechanism, at least for the third-order sequences, is eustasy. Regardless of the mechanism, the observed stratigraphic relationships indicate that higher order fluctuations in relative sea level occurred in the Late Cretaceous that cannot be directly attributed to thrust-belt tectonics or intra-foreland basement-involved uplifts. -from Authors