A Section Sandbox Modeling on the Evolution of Conjugate Normal
Faults
Tianwei Zhou
*
Hailong Xu and Shuang Liang
Research Institute of Petroleum Exploration and Development, CNPC, Beijing, China
Keywords: Conjugate normal faults, salt structure, Sandbox modeling
Abstract: Fault systems can impose considerable effects on structural closure concerned to oil accumulation. It is
widely known that conjugate normal faults were developed over extensional basins, but the evolution of the
special fault arrays was seldom reported. The Nanpu Sag, located in the north part of Huanghua Depression
of Bohai Bay Basin, is a Cenozoic petroliferous extensional fault-block sag. Based on the interpretation of
the 3D seismic data in the Nanpu Sag, the geometry of conjugate normal faults were described both in plane
and profile, and the evolution was analyzed using balanced cross-section restoration technique, the results
indicated that the special faults were formed in a very short geological time and also in weak extensional
setting. Further study of the fault system using a section sandbox modeling technique demonstrated that the
conjugate normal faults showing X-style were formed contemporaneously in a homogeneous geologic body
under extension. With the development of extension, new X-style faults were formed sequentially at the
sides of the pre-conjugate normal faults, all the normal faults constitute a complex conjugate style..The
evolution of conjugate normal faults in rift basins has seldom been reported systematically, this study gives
important evidence and planar sandbox model provides reliable kinematic models to further the
understanding of the description of the conjugate normal faults.
1 INTRODUCTION
The fault is a fundermental topic in fault-block basin
research. For advanced analysis of the relationship
between faults and reservoirs, the study for
evolution and deformation mechanism of faults is
essential. In the Napu sag, most former work was
focused on the description and development of fault
system(Liu, 2011; Tian, 2012), but the detailed
analysis aiming at special fault style was seldom
reported in recent years. In this paper, conjugate
normal faults found in this sag were studied.
2 GEOLOGICAL SETTING
The Nanpu sag with 1932 Km
2
area including
onshore and offshore, is a classical small but rich
petroliferous Cenozoic sag. developed on the
basement of North China block. From the fault
systems map, it is a half-graben basin faulted in the
north and onlapped in the south. The Northwest and
Northeast are bounded by Xinanzhuang fault and
Baigezhuang fault respectively. The two border
faults cut through the basement and controlled the
formation and evolution of the Nanpu Sag(Figure 1).
Within the Sag, the most newly formed normal
faults since Neogene had obvious NNE orientation,
which seemed to be controlled under later single
extension.
The Nanpu sag was in terrigenous sedimentary
environment in Cenozoic time, the strata include:
the second and third members of the Shahejie
formation in Eocene(Es
2+3
); the first member of the
Shahejie formation in Eocene and
Oligocene(Es
1
);the third member of the Dongying
formation in Oligocene(Ed
3
) ;the second member of
the Dongying formation in Oligocene(Ed
2
) ;the first
member of the Dongying formation in
Oligocene(Ed
1
) ;the Guantao formation in
Miocene(Ng) ;the Minghuazhen formation in
Miocene(Nm) and Quaternary.
Zhou, T., Xu, H. and Liang, S.
A Section Sandbox Modeling on the Evolution of Conjugate Normal Faults.
In Proceedings of the International Workshop on Environment and Geoscience (IWEG 2018), pages 219-222
ISBN: 978-989-758-342-1
Copyright © 2018 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved
219

Figure 1: The fault systems of top nm in the Nanpu Sag.
3 FAULTS EVOLUTION
ANALYSIS
Based on results of structural interpretations of 3D
pre-stack time migration data, structural styles were
analyzed in the Nanpu Sag. By the application of
balanced geological sections methods, restorations
of 29 geological sections with decompaction were
finished and Cenozoic deformation phases were
accordingly separated.Only extensional structural
styles were found in the Nanpu Sag. There are
complex structural styles in local structural belts
mainly including X-pattern, Y-pattern. The
restoration also suggested that the X-pattern faults
were formed since the deposition phase of
Nm(Figure 2). Although the X-style is complex,
almost all the faults in the style were developed in a
relative short geological time. Confined to the
resolution of strata classification, the detailed
development of X-style faults is still not clear, but it
was formed from late Cenozoic movement. It was
concluded that the lower crust in the Nanpu sag
undergone pure shear deformation since Neogene,
this mechanism can give a reasonable explaination
for the formation of the symmetrical pattern of new
faults in sedimentary cover(Figure 3).
Figure 2: Restoration of a main geological section in the
Nanpu sag.
Figure 3: Fault formation mechanism of different models
in the Nanpu sag. (a.Paleogene; b.after Neogene).
IWEG 2018 - International Workshop on Environment and Geoscience
220

4 MODEL DESIGN
Sandbox modeling, as a form of physical modeling
has proved to be an effective method for the study of
deformation mechanisms of extensional
basins(McClay and White, 1995)
Figure 4: Sketch of side view experimental model.
It was concluded that X-style faults as conjugate
normal faults, may happen easier in a very
homogeneous deformation under extension. For the
sake of homogeneity and clarity, many sandbox
modelings were conducted to determine the length
between the walls and the thickness of sand layers.
The eventual length was designed 14 cm and the
total thickness of sand layers was placed
20cm(Figure 4).
Loose quartz sands have been proven to be the
ideal material for the physical simulation of brittle
deformation in the shallow crust . The sands used in
the experiments are 0.3-0.4 mm in diameter, with
inner friction angles of 31°. In the models, one layer
of rubber sheet is used for deformation transfer
media at the basal of the sag area and connected to
the ends of the movable walls respectively. The
rubber sheet will be gradually stretched as the
motor-driven walls moves outwards, and normal
faulting will take place in the overlying sand layers.
The displacement velocity is 2.58×10
-3
s/cm and the
total displacement is 3.5 cm. The model has been
tested more than twice and similar results have been
obtained. The experiments were undertaken on the
tectonic deformation physical modeling
experimental apparatus of China University of
Petroleum, Beijing.
Although the Nanpu Sag had been controlled by
the main syndepositional faults during Cenozoic
period, for the sake of the observation of the
framework, all experiments omitted the
syndeposition process but the results could not be
affected (Diraison, et al., 2000; Keep, 2003).
5 MODEL RESULT AND
DISCUSSION
After 1 cm of extension displacement, two crossing
faults with opposite inclination appeared, and
constitute the X configuration, but still remain
obscure, the lower part of the X-style faults showed
larger displacements than the upper part, it meant
the deformation was conducted graually from
basement to cover. After 2 cm of extension
displacement, a new set of faults with opposite
inclination cut the upper part of pre-X-style faults,
all the faults constitute complex X configuration;
After 3.5 cm extension displacement, the newly
formed faults were developed at the sides of pre-
faults and cut the upper part of X-style faults
continuously. It can be imagined that the newly
shallow faults would appear with the further
extension, more and more complex X-style faults
would be shown in the sandbox modeling(Figure 5).
Figure 5: X-style faults sequence of the sandbox modeling.
6 SUMMARY
The Nanpu sag is a classical extensional basin
developed in Cenozoic time, X-style faults as a basic
structural style were developed since the deposition
phase of Nm.
It is known that almost all the X-style faults in
the style were developed in a relative short
geological time, confined to the resolution of strata
classification, the detailed development of X-style
cannot be restored by the application of balanced
geological sections method. The sandbox modeling
reproduced the development of X-style faults, it
A Section Sandbox Modeling on the Evolution of Conjugate Normal Faults
221

discovered that the single X-style normal faults were
formed easily in relative narrow geobody under
extension because of the relative homogeneous
geological background. Simple X-style normal
faults were developed under relatively weak
extension while the complex X-style normal faults
were formed under further extension. It was
concluded that the complex X-style faults in the
Nanpu sag were controlled by pure shear
deformation at the lower crust since Neogene.
ACKNOWLEGEMENTS
We would like to show our deepest gratitude to Dr.
Jianxun Zhou from China University of
Petroleum(Beijing), a respectable scholar, who
provided me with valuable guidance in this paper.
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