Geometry, kinematics and dynamic characteristics of a compound transfer zone: the Dongying anticline, Bohai Bay Basin, eastern China

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Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China, 100029; School of Geosciences, China University of Petroleum, Qingdao, Shandong, China, 266580; Tel: (86) 18500507515
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/ Jianting Yang

Shengli Oilfield Branch Co., SINOPEC, Dongying, Shandong Province, China, 257061
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/ Ming Cheng

Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China, 100029
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/ Yuhong Lei

Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China, 100029
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/ Likuan Zhang

Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China, 100029
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/ Xiaoxue Wang

Research Institute of Exploration & Development, Tarim Oilfield Company PetroChina, Korla, Xinjiang, China, 841000
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/ Xin Liu

Research Institute of Exploration & Development, Tarim Oilfield Company PetroChina, Korla, Xinjiang, China, 841000
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Published Online: 2016-12-12 | DOI: https://doi.org/10.1515/geo-2016-0053

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Abstract

The Dongying anticline is an E-W striking complex fault-bounded block unit which located in the central Dongying Depression, Bohai Bay Basin. The anticline covers an area of approximately 12 km². The overlying succession, which is mainly composed of Tertiary strata, is cut by normal faults with opposing dips. In terms of the general structure, the study area is located in a compound transfer zone with major bounding faults to the west (Ying 1 fault) and east (Ying -8 and -31 faults). Using three-dimensional seismic data, wireline log and checkshot data, the geometries and kinematics of faults in the transfer zone were studied, and fault displacements were calculated. The results show that when activity on the Ying 1 fault diminished, displacement was transferred to the Ying -8, Ying -31 and secondary faults so that total displacement increased. Dynamic analysis shows that the stress fields in the transfer zone were complex: the northern portion was a left-lateral extensional shear zone, and the southern portion was a right-lateral extensional shear zone. A model of potential hydrocarbon traps in the Dongying transfer zone was constructed based on the above data combined with the observed reservoir rock distribution and the sealing characteristics of the faults. The hydrocarbons were mainly expulsed from Minfeng Sag during deposition periods of Neogene Guantao and Minghuazhen Formations, and migrated along major faults from source kitchens to reservoirs. The secondary faults acted as barriers, resulting in the formation of fault-bound compartments. The high points of the anticline and well-sealed traps near secondary faults are potential targets. This paper provides a reservoir formation model of the low-order transfer zone and can be applied to the hydrocarbon exploration in transfer zones, especially the complex fault block oilfields in eastern China.

Keywords: transfer zone; fault kinematics; stress properties; petroleum migration; Dongying Depression; Bohai Bay Basin; China

1 Introduction

Transfer zones are important structural elements in extensional regions that are locate between normal faults [1–7]. Morley et al. classified transfer zones into synthetic, divergent and convergent types according to the configuration of the major faults (Fig. 1b, 1d, 1e) and into approaching, overlapping, co-lateral and co-linear types based on the major fault terminations in planar view [1]. Through detailed studies of their structural style (Fig. 1a, 1b, 1c), displacement transfer [9], and control on hydrocarbon distribution, transfer zones are believed to be the prior locations for the development of hydrocarbon reservoirs [8–11]. The transfer zones are located at a relatively high position, can connect disparate structural units within a basin or subbasin, develop many small faults as reservoir traps and may control palaeo-drainage patterns, resulting in the deposition of coarse-grained sediments that may form reservoir rocks for hydrocarbons [12–14].

Figure 1

Common transfer zone geometries in extensional regions. (a) (b) (c) are the plain view, three-dimensional view and profile sections, respectively, of a synthetic-overlapping transfer zone. (d) Divergent-overlappingtransfer zone, with the major faults dipping away from each other. (e) Convergent-overlapping transfer zone, with the major faults dipping towards each other. (f) Convergent-overlapping and synthetic-overlapping compound transfer zone, summarized by the Dongying transfer zone, based on [1].

Previous studies have focused on large-scale transfer zones between major faults at a basin or sub-basin scale [15–17]. However, few studies have investigated these structures at a more local scale, although relatively minor faults may have a significant influence on hydrocar bon distribution [18]. The Dongying anticline in the Bohai Bay Basin has a complex structure. Over 35 normal faults are located within a 12 sq. km area in which EW, N-S and NW-SE striking faults converge. The Ying 8, -31 and -1 faults bound the anticline and control the local structure, and numerous lower-order faults are also present. Because oil was discovered in the Dongying anticline in 1964, many relatively small-scale oil accumulations in fault blocks have been discovered, and there are approximately 430 million barrels of oil in reserves [19].

To effectively develop the oilfield, the Dongying anticline has been divided into a number of discrete fault blocks. Reservoir units in these fault blocks have a medium to high water cut [20], and enhancing oil recovery is a continuous challenge. Structural studies have shown that a convergent-overlapping transfer zone is present between the Ying 1 and Ying 8 faults [21]. However, a lack of under-standing of the complex relationships between the fault block units has caused a low success ratio in recently drilled wells.

This paper attempts to analyse the geometry and kinematics of faults in the Dongying fault block using threedimensional (3D)seismic data, wireline logging data, well tops and checkshot data. Structurally, southern portion of the area is an E-W striking synthetic-overlapping transfer zone (Fig. 1b), and the northern portion forms a convergent-overlapping transfer zone (Fig. 1e). Thus, the Dongying structure is referred to as a compound transfer zone (Fig. 1f). This paper presents a reservoir model for the Dongying structure and provides a theoretical basis for future exploration activities in this area and in other similar complex fault blocks.

2 Geological background

The Dongying anticline is located in the central portion of the Dongying Depression (Fig. 2c). The Dongying Depression is part of the Jiyang sub-basin, located in the SE Bohai Bay Basin (eastern China). The Dongying Depression is bound by the Chen-Jiazhuang Uplift to the north, the Qingtuozi Uplift to the east, the Luxi-Guangrao Uplift to the south, and the Linfanjia-Binxian Uplift to the west (Fig. 2c). It measures 90 km east-west by 65 km north-south, and covers an area of 5700 km² [22].

Figure 2

Location map of Bohai Bay basin. (b) The subunits of Bohai Bay Basin, including Liaohe, Liaodong Bay, Bozhong, Jiyang Huanghua, Jizhong, and Linqing subbasins. The Dongying Depression locationed in the east part of Jiyang subbasin. (c) Secondary structural units and oilfield distributions in the Dongying Depression.(d) A N-S structural cross section with the main fault systems, and the location is shown in (c) h-h’. Modified from [17].

2.1 Stratigraphy

The Dongying Depression is filled with a thick Cenozoic sedimentary succession that comprises the Paleogene Kongdian (Ek), Shahejie (Es) and Dongying (Ed) Formations; the Neogene Guantao (Ng) and Minghuazhen (Nm) Formations; and the Quaternary Pingyuan (Qp) Formation (Fig.3) [18,19,21].

$Generalized Paleogene-Quaternary stratigraphy of the Dongying Depression, showing main sedimentary facies and the major petroleum system elements, including the Es3 and Es4 source rocks, and multiple reservoirs mainly in Es3U${\rm{Es}}_3^U$ and Ed Formations.$

Figure 3

Generalized Paleogene-Quaternary stratigraphy of the Dongying Depression, showing main sedimentary facies and the major petroleum system elements, including the Es₃ and Es₄ source rocks, and multiple reservoirs mainly in ${Es}_{3}^{U}$ and Ed Formations.

The Kongdian Formation was deposited on top of the (or over the) Mesozoic. It consisted primarily of red mudstones with thin red sandstones, and deposited in a fluvial and shallow-lake environment [24]. Above the Kongdian Formation is the Shahejie Formation, which is widely distributed in the faulted depression, but it is nearly absent on structural highs and depression edges. The Shahejie Formation is dominantly lacustrine, containing mainly source rocks and sandstone reservoir rocks, and is divided into four members (Es₄, Es₃, Es₂ and Es₁ from base to top). The Dongying Formation, which is developed in a fan delta and shallow lacustrine environment, consists of sandstones interbedded with mudstones and is an important reservoir in the depression. Uplift and erosion following the deposition of Ed resulted in a major regional unconformity, which is the main regional unconformity in the Dongying Depression. Above the unconformity, the fluvial and deltaic Guantao and Minghuazhen Formations were deposited and dominated by mudstones and sandstones.

2.2 Fault Framework

Bordered by a dominant north-bounding graben fault, the Dongying anticline forms an asymmetrical half graben (Fig. 2d), typical of many graben structures in eastern China. Multiple Mesozoic and Cenozoic faults, which controlled the structural framework and depositional process in the depression, are the key factors for structure units and sedimentary units [23]. The faults are categorized into orders depending on their scale (extent and throw) and structural characteristics. First-order normal faults are the Chennan and Gaoqing-Pinnan faults (Fig. 2c ①, ②), which defined the boundary of the depression. The first-order faults cut down into the basement and controlled the Paleogene and Neogene deposition (Fig. 2d). Second-order growth faults, including the Shengbei, Bamianhe, and Shicun faults (Fig. 2c ③ ⑧, were active during the Neogene period and divided the Dongying Depression into the Minfeng, Niuzhuang, Lijin and Boxing sags or depocenters. Third- and lower-order faults acted as hydrocarbon migration paths or screen faults for traps, and they played an important role in the accumulation and distribution of hydrocarbons. The Dongying anticline is located in the central part of the Central secondary-order fault (Fig. 2c, 2d ⑦), and the Ying 8, Ying 31 and Ying 1 faults are the third-order faults that control the local structure (Fig. 4). Because of the complex distribution of faults, the distribution of oil reservoirs is very irregular, especially in areas where the fourth-order faults developed intensively.

Figure 4

Structural map of Es2 in the Dongying anticline. Three third-order faults (Ying 8, Ying 31 and Ying 1 faults) control the localstructure, and many lower-order faults developed among them.

2.3 Petroleum Systems

Source rocks in the Dongying Depression occur in three stratigraphic intervals, ${Es}_{3}^{L}$ , ${Es}_{3}^{M}$ and ${Es}_{4}^{U}$ , which deposited in the deep lacustrine environment. The ${Es}_{3}^{L}$ unit is the main source rock interval, and ${Es}_{3}^{M}$ and ${Es}_{4}^{U}$ consist of oil shale and organic-rich mudstones. These intervals are relatively thin but they are capable of generating large quantities of hydrocarbons [25].

Previous studies have indicated that there were at least two episodes of oil generation and accumulation [26]. The first episode occurred in the late Paleogene when ${Es}_{3}^{L}$ and ${Es}_{4}^{U}$ source rocks entered the oil window. However, tectonism at the end of the Paleogene (after Ed Formation deposited) led to uplift of the source rock, terminating hydrocarbon formation. This early episode of generation was brief, and the volume of oil generated was limited. A second episode of oil generation lasted from the late Miocene to the Quaternary. With the deposition of the Neogene Ng and Nm Formations, most of the source rocks in the Es₃ and Es₄ members entered the oil window in the late Miocene. The second episode was relatively late, allowing oil to accumulate in a variety of traps formed prior to and during source rock maturation. Many reservoirs are found in the ${Es}_{3}^{U}$ and Ed members [27].

3 Data and methods

The 3D seismic data used in this paper were acquired by the Shengli Branch Company, SINOPEC, in 2008. The trace interval of the dataset is 25 m · 25 m, the time sample interval is 2 ms, and the main frequency is 25 Hz (the seismic grid is illustrated in Fig. 5). The time-domain 3D seismic dataset was used to interpret the T1, T2, T4 and T6 by tracing the peak of corresponding seismic reflections, and then the faults were interpreted in detail by interrupts or disturbances on these seismic reflections and seismic slices guided by geological concepts. Although the vertical resolution of seismic dataset is about 20 m [28], but for the study area is very small (12 sq Km), we used the 3D velocity field which provided by Shengli Oilfield to change the time-domain information to depth-domain information. The results are credible, and can interpret the changes of throw inside and outside the Dongying transfer zone.

Figure 5

The locations of seismic profiles interpreted in Fig. 6. The stress ellipse in the transfer zone between Ying 1 and Ying 8 faults was left-lateral transtension, while the stress ellipse in the transfer zone between Ying 1 and Ying 31 faults was right-lateral transtension. The advantageous and disadvantageous areas for hydrocarbon exlopration were also marked in the map.

Figure 6

(aa′-gg′) Uninterpreted and interpreted seismic profiles through the Dongying anticline showingfault characteristics of the transfer zone (for location, see Fig. 5). The trace interval is 25 m · 25 m.

Data interpretation was carried out at Shengli Oilfield Geophysical Res. Inst. where the data are held. Previous interpretations were reviewed, and new interpretations were made with a particular focus on the genetic relationships between low- and high-order faults. Firstly, the geometry of this fault block was identified. Then, after four major reflecting layers were transformed from the time to depth domain, the kinematical characteristics of the faults were determined. The extent and nature of the transfer zone were then investigated and the stress field was analysed.

Based on the source rock location, fault framework and hydrocarbon distribution, the process of hydrocarbon accumulation was studied in detail. The sealing properties of faults were first investigated. The fault activity was used to measure movement intensity. Lithologies on either side of the fault planes were used to calculate the Shale Gouge Ratio (SGR), which characterizes the fault sealing properties. Secondly, based on the knowledge of source rocks in three stratigraphic intervals, two episodes of oil generation and accumulation, and combined with the fault framework interpreted in this paper, the spatial relationship between faults, source rocks and reservoirs was investigated, and hydrocarbon migration paths were identified. Temporal relationships between faults and phases of hydrocarbon accumulations were studied and combined with fault sealing properties, allowing a reservoir formation model of the Dongying transfer zone to be constructed. Finally, locations for future exploration and production were identified.

4 Results

4.1 Structural Geometry

3D seismic data integrated with well tops and checkshot data were used to analyse the fault profiles, focusing on the relationships between low- and high-order faults. Each line of the profile was interpreted inline and cross-lines, and seven typical profiles were chosen to describe changes inthe tectonic styles (Fig. 6). Secondly, after profile interpretation, maps of the four main reflecting layers were generated correspondingly. The changes of map-view tectonic styles were also studied in detail. Vertical throw is the vertical component of the dip separation of a normal fault, measured in a vertical cross-section perpendicular to the strike of a fault [12]. In this paper, we also calculated the throws to show the faults’ geometrical changes.

4.1.1 Profile structural style

Fig. 6 shows a series of N-S oriented interpreted seismic profiles across the Dongying structure. Line locations are shown in Fig. 5. The seismic lines are briefly described below, from west (line a-a’) to east (line g-g’).

Line 3389, Fig. 6a-a′, western margin of the transfer zone

The Ying 1 fault surrounds a half graben with minor faults in the hanging wall. The Ying 1 fault, which trends NWW-SEE and dip toward the NNE (Fig. 4), has a throw of 131 m at the level of the T2 to T1 reflections which are Es₁ and Ed Formations, as described below.

Line 3413, Fig. 6b-b′, western part of the transfer zone

The offset decreases on the Ying 1 fault (throw of 87 m) compared to section a-a’. The section shows that the Ying 31 and Ying 8 faults are active, with throws of 40 m and 10 m, respectively. Two north-dipping echelon faults are present between the Ying 1 and Ying 31 faults. A roll-over anticline developed between the Ying 1 and Ying 8 faults.

Line 3432, Fig. 6c-c′, central part of the transfer zone

The offset on the Ying 31 and Ying 8 faults (throws of 131 and 79 m) has increased compared to section b-b’, while that on the Ying 1 fault has decreased (throw of 45 m). The Ying 1 fault is now the primary fault bounding the structure. The Dongying anticline disappears.

Line 3450, Fig. 6d-d′, eastern part of the transfer zone

The Ying 31 and Ying 8 fault bound a graben in this profile. The Ying 31 fault has a throw of 186 m. Three small synthetic faults are present in the hanging wall of the Ying 31 fault and are separate from the Ying 1 fault. The Ying 1 fault has a throw of 37 m and does not extend below the Sha3 member ( ${Es}_{3}^{L}$ ).

Line 3470, Fig. 6e-e′, eastern margin of the transfer zone

The Ying 1 fault has disappeared, and the graben is bounded by the Ying 31 and Ying 8 faults, both with throws of approximately 240 m. Another new roll-over anticline has increased in amplitude. Five small antithetic faults are present in the hanging wall of the Ying 31 fault. The tectonic style is very different from that at the western margin of the structure (e.g., Line 3389 in Fig. 5a).

Line 3490, Fig. 6f-f^/, eastern side of the transfer zone

Offset on the graben-bounding Ying 31 and Ying 8 faults has increased (throws of 235 m and 265 m, respectively), but the overall tectonic style is similar to that in Fig. 6e, implying that fault framework is stable.

Cross-section of the Dongying structure

A NE-SW oblique cross-section of the Dongying structure between Wells Ying 19 and Ying16 is shown in Fig. 6g-g′. Between the graben-bounding faults, a series of low-order faults are present in the respective hangingwalls, dipping to north and south

Thus, from west to east across the Dongying structure, the Ying 1 fault diminishes and then disappears, while the Ying 31 and Ying 8 faults gradually become more important and control the margins of the graben (Fig. 7). Fourth- and fifth-order faults occur in the respective hanging walls and there is a central roll-over anticline

Figure 7

Evolution of profile tectonic style in the Dongying compound transfer zone from west to east (for location, see Fig. 5), showingthe difference in profile tectonic style on each side ofthe zone.

4.1.2 Map-view structural style

The map-view structural style shows that there is a convergent-overlapping transfer zone between the Ying 1 and Ying 8 faults in the northern portion of the Dongying anticline (c.f., Fig. 1e), and a synthetic-overlapping transfer zone in the southern portion, between the Ying 1 and Ying 31 faults (c.f., Fig. 1b). The Dongying structure can therefore be described as a compound transfer zone.

At the level of the Shahejie Formation ${Es}_{3}^{L + M}$ units, the Ying 1 and Ying 8 faults do not overlap (Fig. 8, T6), and a structural horst is present between the fault tips. The strikes of the Ying 8 and Ying 31 faults are parallel, and a graben is present between them. In the ${Es}_{3}^{U}$ -Es₂ Formations, the Ying 1 and Ying 8 faults overlap and lower-order faults are present (Fig. 8, T4), and the Dongying transfer zone begins to develop. In the Es1-Ed Formations, activity on the transfer zone becomes much stronger, and the transfer zone is divided into northern and southern portions. The Ying 8 fault curves toward the Ying 1 fault. A rollover anticline developed between the Ying 1 and 8 faults, along with two groups of minor faults striking NNE-SSW and NW-SE. Between the Ying 1 and Ying 31 faults, there are eight en echelon secondary faults whose strikes curve from NW-SE to NNW-SSE (Fig. 8, T2). The transfer zone becomes less pronounced above the Ed Formation, with only a few low-order faults (Fig. 8, T1).

Figure 8

Schematic structural map of the Dongying compound transfer zone in the four main seismic reflectors, showingthe transfer zone plane tectonic style changes. The main control fault disappears in the transfer zone. See location in Fig. 4a.

4.2 Kinematic and Dynamic Characteristics of The Faults

A transfer zone is a kinematic and dynamic transition zone between major faults, and the zone transfers strain by transmitting the displacements of major faults or secondary faults [27,29]. The fault expansion index and throw can be used to characterize the strain transition.

4.2.1 Fault expansion index

The Expansion Index (EI) is the ratio of thickness variation between the layers on the footwall and in the hanging wall of a synsedimentary normal fault (formula (1)). Thus, assuming that the sedimentation rate is constant over the footwall, variable values of EI (i.e., increase or decrease of HWt relative to FWt) can be directly related to variations of the fault movement rate [31]. $EI= \frac{H W t}{F W t}$ (1)

EI: The fault expansion index

HWt: The thickness of the hanging wall in stratum n (m) FWt: The thickness of the footwall in stratum n (m)

The EI of the three major faults was determined at locations inside and outside the transfer zone. Locations are marked in Fig. 5, and results are presented in Fig. 9. The fault movement rate outside the transfer zone was greater than that inside the zone for the same time period (M>N, P>Q, S>T in Fig. 9). Characteristics of the major faults inside the transfer zone included their activity beginning later and ending earlier (Fig. 9).

Figure 9

Expansion indexes of different faults in the Dongying compound transfer zone (for location, see Fig. 5), showing the differences offault activity inside and outside the zone.

There are many secondary faults in the transfer zone, and their scales and duration of activity were smaller than those of the major faults. Activity of the secondary faults was different in the northern and southern portions of the Dongying transfer zone as they were controlled by different major faults.

The activity of the F1 fault was greater than that of the F2 fault (points U and W correspondingly), which measured in the formations between T2 and T1 reflections. Although the F1 and F2 faults were both active during the deposition of Es1 member, with their activity ceasing in the deposition of Nm Formation, the two faults displayed intense activity at different periods. The most intense activity of the F1 fault occurred in the deposition period of the Oligocene Dongying Formation, while that of the F2 fault occurred during the deposition period of the Neogene Guantao Formation.

4.2.2 Vertical Throw of the major faults

The vertical throw of the major faults in the Es1-Ed Formations was calculated every 500 m (20 lines). From line 3370 to 3490, the total regional vertical throw increases. The throw of the Ying 8 and Ying 31 faults increases, but that of the Ying 1 fault decreases from west to east (Fig. 10). In the area between line 3400 and 3450 (Table 1), the vertical throw of all three major faults is relatively small, but the total throw gradually increases because of the presence of secondary faults that transmit displacement.

Figure 10

Throw of the various faults in the Dongying compound transfer zone (for location, see Fig. 5), showing transmission of displacement in the zone.

Tab.

Table 1

Faults thickness and throw of the Dongying compound transfer zone.

4.2.3 Strain and kinematics characteristics

Transfer zones are complex strain zones. Because major faults control the boundary conditions, the stress field within a transfer zone may be different from that of any other location in a basin. During the deposition periods of Es2~Es1~Ed Formations, when the Dongying transfer zone was most active, extension in the area was oriented north-south [32]. Due to the Ying 1 and Ying 8 faults, the northern part of the transfer zone was affected by leftlateral transtension, causing the strikes of the major faults to change near their tips (Fig. 5) and adding a shear component to the secondary faults. Due to the Ying 1 and Ying 31 faults, the southern part of the transfer zone was affected by right-lateral transtension, causing the strike of the Ying 31 fault to change from NW-SE to NNW-SSE in the transfer zone.

4.3 Influence of the Major Faults and Secondary Faults on Hydrocarbon Accumulation

The Dongying compound transfer zone is located at a relatively high structural level and may provide a trap for hydrocarbons migrating updip. The major faults may provide migration conduits between source rocks in Minfeng Sag and reservoirs, while secondary faults whose activity ceased relatively earlier, may become barriers to fluid flow.

4.3.1 Spatial relationship between the faults, source rock and reservoir

The Ying 1, Ying 31 and Ying 8 faults link the Es3 and Es4 source rocks in the Minfeng Sag and the reservoir in the transfer zone, and they become the oil source fault. There are many secondary faults that form an effective oil migration framework with the oil source fault. The secondary faults cease their activity earlier and have good sealing ability, allowing many fault block traps to form. Furthermore, the rollover anticline is a favourable area for hydrocarbon accumulation [8].

4.3.2 Temporal relationship between faults and hydrocarbon accumulation

The spatial linkage of the faults and source rock is a static factor that is conducive to hydrocarbon accumulation. As a dynamic factor that favours hydrocarbon accumulation is that the activity periods of the oil source fault and screened faults are in good correspondence with the oil accumulation period [24]. In the northern Dongying transfer zone, the Ying 1 fault was still active during the deposition period of Neogene Guantao and Minghuazhen Formations. The secondary faults (F1, etc.) could conduct hydrocarbon during the deposition period of Guantao Formation, when they were weakly active, but these faults were screened in the deposition period of Minghuazhen Formation. Similar dynamic situations occur in the southern Dongying transfer zone. From the EI map, we speculated that in the deposition period of Guantao and Minghuazhen Formations, the Ying 31 fault was still active, and its activity was more intense in the deposition period of Minghuazhen Formation period (Fig. 9). It is beneficial to accumulate hydrocarbon oozed from source rock. The activity of the secondary faults (F2, etc.) ceased during the deposition period of Minghuazhen Formation, and they became well-screened faults.

4.3.3 Analysis of fault migration and sealing properties

The sealing properties of faults are key factors that controll reservoirs’fault-bounded structures. In this paper, the Shale Gouge Ratio (SGR) and the stress normal to the fault plane (δ) were used to characterize fault sealing properties [33].

Clay or shale smeared into the fault zone has been considered an effective factor impacting the sealing ability of the fault [34]. In this paper, SGR is used to estimate the amount of muddy fault smear, which is defined as the ratio between the throw of the fault and the total thickness of mudstones within the throw [34]. $S G R = \frac{\sum_{i = 1}^{n} h_{i}}{L} \times 100 %$ (2)

where L is the throw of the fault (m), h_i is the thickness of the i^th mudstone layer, and n is the number of mudstone layers within the throw. Previous studies have found that a higher SGR indicates a poorer porosity and permeability in the fault damage zone, and the fault will have good sealing ability when the shale content of the fault zone exceeds 0.7 in the Dongying Depression [34,36].

The vertical sealing ability of the fault also relies on the stress normal to the fault plane. The higher the normal stress, the better the sealing of the fault [37]. The stress on the fault plane arises from three forces: vertical stress (δ₁) which is defined as the lithostatic pressure, maximum horizontal principal stress (δ₂) and the minimum horizontal principal stress (δ₃). $δ = δ_{2} \sin α \cdot \sin θ)^{2} + δ_{3} {(\cos α \cdot \sin θ)}^{2} + δ_{1} \cos^{2} θ$ (3)

where δ is the stress normal to the fault plane (MPa), α is the angle between the fault strike and the direction of maximum horizontal principal stress (°), and θ is the fault dip angle (°).

δ₁ can be defined as follows: $δ_{1} = ρ_{b} g z$ (4)

where ρ_b is the average density of the overlying rock (we used 2.45 g/cm³ here), g is acceleration of gravity, 9.8 m/s², and z is the buried depth of the fault plane (m).

The current stress field in the Dongying anticline was studied by Zhang et al. [36]. The maximum horizontal stress is in the direction of 70°. The value of maximum horizontal stress (δ₂) and the minimum horizontal stress (δ₃) follow the burial depth: $δ_{2} = - 15.69 + 0.032 \cdot z$ (5) $δ_{3} = - 6.93 + 0.021 \cdot z$ (6)

According to rock deformation theory, when the pressure acting on mudstone exceeds the elastic limit, plastic deformation of the mudstone will occur. Physical rock test revealed that when the fault plane pressure exceeds 5 MPa, the resulting plastic flow of the mudstone will block the left leakage space near the fault plane and seal off the fault completely [38,39]. When the normal pressure is less than 5 MPa, plastic deformation of the mudstone does not occur, the leakage space near the fault plane is preserved and the fault is not completely sealed off.

Currently, the normal pressures of the two major faults in the major reservoir (Ed) are 10.24 MPa and 10.46 MPa, and SGR of the fault are 51.17% and 52.15%, which suggest that they are effective sealing faults (Table 2). Due to the extension-shear stress in the transfer zone, the fault plane pressures of the F1 and F2 faults are 22.87 MPa and 23.95 MPa, and their SGR values are 53.34% and 56.89% (Table 2), respectively, demonstrating that they also have good sealing properties.

Tab.

Table 2

Fault dip, mud content and fault plane pressure of the Dongying compound transfer zone

Previous studies have suggested that the faults can serve as flow paths when the fault is active and as barriers when the fault is inactive. Thus, the faults can be seen as a hydrocarbon effective migration path if the active period of the fault matches the hydrocarbon accumulation period. However, in most cases, the fault expansion index is used to characterize the intensity of fault activity. In the Ng-Nm period, the major faults’activity had an optimum matching relationship with hydrocarbon accumulation, while the secondary faults could not match (Fig. 9). The hydrocarbons expulsed from Minfeng Sag can migrate along major faults to reservoirs, while secondary faults act as barriers, resulting in the formation of fault-bound compartments.

4.4 Hydrocarbon Accumulations in the Dongying Transfer Zone

Hydrocarbon in the Dongying compound transfer zone was mainly generated from Es₃ and Es₄ source rock in Minfeng Sag [18, 25]. Major faults broke into the Es₄ source rock in the Minfeng Sag and were active and conductive during the hydrocarbon accumulation period. In fact, evidence demonstrates that the episodic petroleum migration model could be more geologically relevant than a continuous-flow model [40,41]. The secondary faults constituted an effective hydrocarbon conduction framework, together with the major faults during the Ng period, and became screen faults in the Nm period when their activity ceased. Under the local extension-shear stress field, the secondary faults had good sealing ability and formed many fault block traps, which could accumulate and preserve hydrocarbons effectively.

In the northern Dongying compound transfer zone, there are many secondary faults and a roll-over anticline, which preserved a large quantity of hydrocarbons in the hanging wall of the Ying 1 fault (Fig. 11). In the southern Dongying transfer zone, there are many step-shaped secondary faults adjoined to the Ying 31 fault. These secondary faults had good sealing ability in the Ng period and can accumulate substantial hydrocarbons in the hanging wall of the Ying 31 fault (Fig. 12).

Figure 11

Reservoir formation model ofthe southern Dongying compound transfer zone. The Yingl fault is the oil source fault, and the main source rock is in Es₄, buried in Minfeng sag.

Figure 12

Reservoir formation model ofthe northern Dongying compound transfer zone. The Ying 31 fault is the oil source fault, and the reservoirs are mainly found near the Ying 31 fault.

The Dongying transfer zone is a promising area for future hydrocarbon exploration. The Well Ying 13 in the rollover anticline, the Well Ying 24 area and the Well Ying 26 area, all with high points near secondary faults are advantageous hydrocarbon accumulation areas. North of the F2 fault, the west side of the Ying 31 fault and the west side of the Ying 8 fault are disadvantageous for hydrocarbon accumulation (Fig. 5).

In recent years, the drilling success ratio in the advantageous area has been higher than 86%, confirming the importance of the transfer zone in the energy exploration in the complex fault block oilfield. In the Dongying Depression, many productive wells have been drilled in the small transfer zones between third- (or fourth-) order faults, and 12 small fault block reservoirs have been found, increasing the reserve by 33.46 million barrels of oil [18].

5 Discussion

We found a compound transfer zone developing in the Dongying anticline, which is located in the central uplift of the Dongying Depression, and the transfer zone can effectively control the local hydrocarbon distribution. This transfer zone is composed of three major faults, and while in the northern part there is a convergent-overlapping type with a roll-over anticline and some secondary faults, in the southern part there is a synthetic-overlapping type with several en echelon secondary faults.

Previous studies analysed the large transfer zones in the basin scale, and some geological models have been built to interpret their geometrical characters [3, 6-8]. Most of the transfer zones follow the classifications made by Morely. In this paper, we interpreted a compound transfer zone that was controlled by fourth-order faults in a local scale, and we found that the transfer zone can also conserve regional extensional strain. The Dongying transfer zone can positively affect major faults’ activities. Along the fault strike, the displacements of major faults clearly decreased in the transfer zone, and then the major faults disappeared. Taking the Ying 1 fault for example, the displacement of the Ying fault in T1-T2 reflections is approximately more than 80 m outside the transfer zone, while in the transfer zone, it decreased to less than 50 m and finally disappeared. The secondary faults acted weakly, and their activities ceased earlier than the major faults. For example, the Ying 31 fault (major fault) was still active in the deposition period of the Guantao and Minghuazhen Formations, and its related secondary faults ceased in the latter period (Fig. 9).

In the analyses of hydrocarbon migrations in the Dongying transfer zone, we found that we should consider not only the geometrical characteristics of faults as the static factors but also the sealing properties of these faults in different periods. We used SGR and the stress normal to the fault plane to characterize fault sealing properties, combined with the faults’ activity periods. We found that during the main hydrocarbon migration period, major faults, as the oil source fault, linked the source rock and reservoir and effectively conducted hydrocarbons. The secondary faults, as screened faults, ceased movement earlier, had good sealing ability and can form plenty of fault block traps. Thus, we believe the Dongying transfer zone is an advantageous area for hydrocarbon accumulation.

6 Conclusions

This study shows that a convergent-overlapping and synthetic-overlapping compound transfer zone has developed in the Central Dongying Depression and is bounded by major faults Ying 1, Ying 8 and Ying 31. The geometry, kinematics and dynamic characteristics of the transfer zone were studied. The tectonic style changes in profile and map view across the transfer zone. At the west side of the structure is a half graben, which becomes a graben to the east. In map view, the major faults disappeared in the transfer zone, and the Ying 8 fault curved towards the Ying 1 fault from an orientation of NWW to NE towards the centre of the transfer zone. In the transfer zone, some secondary faults developed, and the Dongying roll-over anticline developed between the Ying 1 (or Ying 31) and Ying 8 faults.

The regional displacement changed in a continuous manner, and major faults acted in the same period, showing that the Dongying anticline is a compound transfer zone. From west to east, the throw of the Ying 1 fault in Ed+Es₁ Formations decreased, while those of the Ying 8 and Ying 31 faults increased, and the total throw increased gradually. Because the major faults control the stress boundary of the transfer zone, the internal stress field is different from the basin stress field and has a shearing strain component. The northern portion is dominated by left-lateral extensional-shear stress, and the southern part is dominated by right-lateral extensional-shear stress, so the secondary faults have good screen properties.

A potential trap formation model is established for the Dongying compound transfer zone. Major faults that broke into the source rock, combined with secondary faults, constructed a good hydrocarbon migration framework. During the main hydrocarbon migration period, major faults, as the oil source fault, linked the source rock and reservoir and conducted the hydrocarbons effectively. The sec- ondary faults, as screened faults, ceased movement earlier, had good sealing ability and could form plenty of fault block traps. The Well Ying 13 in the roll-over anticline, the Well Ying 24 area and the Well Ying 26 area, all with high points near secondary faults, are advantageous hydrocarbon accumulation areas for further progressive exploration and production.

Many small transfer zones developed between third- or fourth-order faults, which control the local structure. Major faults acting as oil source faults, secondary faults acting as screened faults, low-order transfer zone can effectively control the local hydrocarbon distribution. The significance of low-order transfer zones in petroleum geology can be applied to the hydrocarbon exploration in extensional basins, especially the complex fault block oilfields in eastern China

Acknowledgement

This study was supported by the Chinese National Major Fundamental Research Developing Project (Grant No. 2016ZX05008-004 and No. 2016ZX05008-006), the Chinese National Natural Science Foundation (Grant No. 41502149, No. 41372151 and No. 41102078) and the China Postdoctoral Foundation Funded Project (Grant No. 2015M570148). Shengli Oilfield Branch Company SINOPEC is deeply thanked for supplying the first-hand data and allowing the publication of this article. We thank two anonymous reviewers for their thorough and critical reviews and suggestions to improve the manuscript. We also thank Christopher Tiratsoo and Guixin Hu for their editing help on previous versions. Special appreciations are expressed to Jixiang Wang and Qinghua Chen for interesting and insightful discussions on the characters of transfer zone

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About the article

Received: 2016-02-14

Accepted: 2016-07-18

Published Online: 2016-12-12

Published in Print: 2016-01-01

Citation Information: Open Geosciences, Volume 8, Issue 1, Pages 612–629, ISSN (Online) 2391-5447, DOI: https://doi.org/10.1515/geo-2016-0053.

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