The Dongying Anticline, Bohai Bay Basin, Eastern China

Open Geosci. 2016; 8:612–629

Research Article Open Access

Fei Tian*, Jianting Yang, Ming Cheng, Yuhong Lei, Likuan Zhang, Xiaoxue Wang, and Xin Liu Geometry, kinematics and dynamic characteristics of a compound transfer zone: the Dongying anticline, Bohai Bay Basin, eastern China

DOI 10.1515/geo-2016-0053 riods of Neogene Guantao and Minghuazhen Formations, Received Feb 14, 2016; accepted Jul 18, 2016 and migrated along major faults from source kitchens to reservoirs. The secondary faults acted as barriers, result- Abstract: The Dongying anticline is an E-W striking com- ing in the formation of fault-bound compartments. The plex fault-bounded block unit which located in the central high points of the anticline and well-sealed traps near sec- Dongying Depression, Bohai Bay Basin. The anticline cov- ondary faults are potential targets. This paper provides a ers an area of approximately 12 km2. The overlying succes- reservoir formation model of the low-order transfer zone sion, which is mainly composed of Tertiary strata, is cut by and can be applied to the hydrocarbon exploration in normal faults with opposing dips. In terms of the general transfer zones, especially the complex fault block oilfields structure, the study area is located in a compound transfer in eastern China. zone with major bounding faults to the west (Ying 1 fault) and east (Ying -8 and -31 faults). Using three-dimensional Keywords: transfer zone; fault kinematics; stress proper- seismic data, wireline log and checkshot data, the geome- ties; petroleum migration; Dongying Depression; Bohai tries and kinematics of faults in the transfer zone were Bay Basin; China studied, and fault displacements were calculated. The results show that when activity on the Ying 1 fault dimin- ished, displacement was transferred to the Ying -8, Ying -31 and secondary faults so that total displacement in- 1 Introduction creased. Dynamic analysis shows that the stress fields in Transfer zones are important structural elements in exten- the transfer zone were complex: the northern portion was sional regions that are locate between normal faults [1–7]. a left-lateral extensional shear zone, and the southern por- Morley et al. classified transfer zones into synthetic, diver- tion was a right-lateral extensional shear zone. A model gent and convergent types according to the configuration of potential hydrocarbon traps in the Dongying transfer of the major faults (Fig. 1b, 1d, 1e) and into approaching, zone was constructed based on the above data combined overlapping, co-lateral and co-linear types based on the with the observed reservoir rock distribution and the seal- major fault terminations in planar view [1]. Through de- ing characteristics of the faults. The hydrocarbons were tailed studies of their structural style (Fig. 1a, 1b, 1c), dis- mainly expulsed from Minfeng Sag during deposition pe- placement 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 *Corresponding Author: Fei Tian: Key Laboratory of Petroleum transfer zones are located at a relatively high position, can Resources Research, Institute of Geology and Geophysics, Chi- connect disparate structural units within a basin or sub- nese Academy of Sciences, Beijing, China, 100029; School of basin, develop many small faults as reservoir traps and Geosciences, China University of Petroleum, Qingdao, Shan- dong, China, 266580; Email: [email protected]; Tel: may control palaeo-drainage patterns, resulting in the de- (86) 18500507515 position of coarse-grained sediments that may form reser- Jianting Yang: Shengli Oilfield Branch Co., SINOPEC, Dongying, voir rocks for hydrocarbons [12–14]. Shandong Province, China, 257061 Previous studies have focused on large-scale trans- Ming Cheng, Yuhong Lei, Likuan Zhang: Key Laboratory of fer zones between major faults at a basin or sub-basin Petroleum Resources Research, Institute of Geology and Geo- scale [15–17]. However, few studies have investigated these physics, Chinese Academy of Sciences, Beijing, China, 100029 Xiaoxue Wang, Xin Liu: Research Institute of Exploration & De- structures at a more local scale, although relatively mi- velopment, Tarim Oilfield Company PetroChina, Korla, Xinjiang, nor faults may have a significant influence on hydrocar- China, 841000

Figure 1: Common transfer zone geometries in extensional regions. (a) (b) (c) are the plain view, three-dimensional view and profile sec- tions, respectively, of a synthetic–overlapping transfer zone. (d) Divergent-overlapping transfer 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]. bon distribution [18]. The Dongying anticline in the Bo- convergent-overlapping transfer zone (Fig. 1e). Thus, the hai Bay Basin has a complex structure. Over 35 normal Dongying structure is referred to as a compound transfer faults are located within a 12 sq. km area in which E- zone (Fig. 1f). This paper presents a reservoir model for the W, N-S and NW-SE striking faults converge. The Ying 8, Dongying structure and provides a theoretical basis for fu- -31 and -1 faults bound the anticline and control the lo- ture exploration activities in this area and in other similar cal structure, and numerous lower-order faults are also complex fault blocks. 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 2 Geological background approximately 430 million barrels of oil in reserves [19]. To effectively develop the oilfield, the Dongying an- The Dongying anticline is located in the central portion ticline has been divided into a number of discrete fault of the Dongying Depression (Fig. 2c). The Dongying De- blocks. Reservoir units in these fault blocks have a medium pression is part of the Jiyang sub-basin, located in the SE to high water cut [20], and enhancing oil recovery is a con- Bohai Bay Basin (eastern China). The Dongying Depres- tinuous challenge. Structural studies have shown that a sion is bound by the Chen-Jiazhuang Uplift to the north, convergent-overlapping transfer zone is present between the Qingtuozi Uplift to the east, the Luxi-Guangrao Up- the Ying 1 and Ying 8 faults [21]. However, a lack of under- lift to the south, and the Linfanjia-Binxian Uplift to the standing of the complex relationships between the fault west (Fig. 2c). It measures 90 km east-west by 65 km north- block units has caused a low success ratio in recently south, and covers an area of 5700 km2 [22]. drilled wells. This paper attempts to analyse the geometry and kinematics of faults in the Dongying fault block using three- 2.1 Stratigraphy dimensional (3D)seismic data, wireline logging data, well tops and checkshot data. Structurally, southern por- The Dongying Depression is filled with a thick Ceno- tion of the area is an E-W striking synthetic-overlapping zoic sedimentary succession that comprises the Paleogene transfer zone (Fig. 1b),and the northern portion forms a 614 Ë F. Tian et al.

Figure 2: (a) 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 theloca- tion is shown in (c) h-h’. Modified from [17].

Kongdian (Ek), Shahejie (Es) and Dongying (Ed) Forma- conformity, which is the main regional unconformity in tions; the Neogene Guantao (Ng) and Minghuazhen (Nm) the Dongying Depression. Above the unconformity, the flu- Formations; and the Quaternary Pingyuan (Qp) Formation vial and deltaic Guantao and Minghuazhen Formations (Fig. 3) [18, 19, 21]. were deposited and dominated by mudstones and sand- The Kongdian Formation was deposited on top of the stones. (or over the) Mesozoic. It consisted primarily of red mudstones with thin red sandstones, and deposited in a flu- vial and shallow-lake environment [24]. Above the Kong- 2.2 Fault Framework dian Formation is the Shahejie Formation, which is widely distributed in the faulted depression, but it is nearly ab- Bordered by a dominant north-bounding graben fault, the sent on structural highs and depression edges. The Shahe- Dongying anticline forms an asymmetrical half graben jie Formation is dominantly lacustrine, containing mainly (Fig. 2d), typical of many graben structures in eastern source rocks and sandstone reservoir rocks, and is divided China. Multiple Mesozoic and Cenozoic faults, which con- into four members (Es4, Es3, Es2 and Es1 from base to trolled the structural framework and depositional process top). The Dongying Formation, which is developed in a in the depression, are the key factors for structure units fan delta and shallow lacustrine environment, consists of and sedimentary units [23]. The faults are categorized into sandstones interbedded with mudstones and is an impor- orders depending on their scale (extent and throw) and tant reservoir in the depression. Uplift and erosion follow- structural characteristics. First-order normal faults are the ing the deposition of Ed resulted in a major regional un- Chennan and Gaoqing-Pinnan faults (Fig. 2c 1 , 2 ), which Compound transfer zone in Dongying anticline Ë 615

Figure 3: Generalized Paleogene-Quaternary stratigraphy of the Dongying Depression, showing main sedimentary facies and the major U petroleum system elements, including the Es3 and Es4 source rocks, and multiple reservoirs mainly in Es3 and Ed Formations. defined the boundary of the depression. The first-order tion paths or screen faults for traps, and they played an im- faults cut down into the basement and controlled the Pa- portant role in the accumulation and distribution of hydro- leogene and Neogene deposition (Fig. 2d). Second-order carbons. The Dongying anticline is located in the central growth faults, including the Shengbei, Bamianhe, and Shi- part of the Central secondary-order fault (Fig. 2c, 2d 7 ), cun faults (Fig. 2c 3 – 8 ), were active during the Neogene and the Ying 8, Ying 31 and Ying 1 faults are the third-order period and divided the Dongying Depression into the Min- faults that control the local structure (Fig. 4). Because of feng, Niuzhuang, Lijin and Boxing sags or depocenters. the complex distribution of faults, the distribution of oil Third- and lower-order faults acted as hydrocarbon migra- 616 Ë F. Tian et al.

Figure 4: Structural map of Es2 in the Dongying anticline. Three third-order faults (Ying 8, Ying 31 and Ying 1 faults) control the local structure, and many lower-order faults developed among them. Compound transfer zone in Dongying anticline Ë 617 reservoirs is very irregular, especially in areas where the Data interpretation was carried out at Shengli Oilfield fourth-order faults developed intensively. 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 2.3 Petroleum Systems between low- and high-order faults. Firstly, the geometry of this fault block was identified. Then, after four major re- Source rocks in the Dongying Depression occur in three flecting layers were transformed from the time to depth do- L M U stratigraphic intervals, Es3, Es3 and Es4 , which deposited main, the kinematical characteristics of the faults were de- L in the deep lacustrine environment. The Es3 unit is the termined. The extent and nature of the transfer zone were M U main source rock interval, and Es3 and Es4 consist of oil then investigated and the stress field was analysed. shale and organic-rich mudstones. These intervals are rel- Based on the source rock location, fault framework atively thin but they are capable of generating large quan- and hydrocarbon distribution, the process of hydrocarbon tities of hydrocarbons [25]. accumulation was studied in detail. The sealing properties Previous studies have indicated that there were at of faults were first investigated. The fault activity was used least two episodes of oil generation and accumulation [26]. to measure movement intensity. Lithologies on either side L The first episode occurred in the late Paleogene when Es3 of the fault planes were used to calculate the Shale Gouge U and Es4 source rocks entered the oil window. However, Ratio (SGR), which characterizes the fault sealing prop- tectonism at the end of the Paleogene (after Ed Forma- erties. Secondly, based on the knowledge of source rocks tion deposited) led to uplift of the source rock, terminat- in three stratigraphic intervals, two episodes of oil gen- ing hydrocarbon formation. This early episode of genera- eration and accumulation, and combined with the fault tion was brief, and the volume of oil generated was lim- framework interpreted in this paper, the spatial relation- ited. A second episode of oil generation lasted from the late ship between faults, source rocks and reservoirs was in- Miocene to the Quaternary. With the deposition of the Neo- vestigated, and hydrocarbon migration paths were iden- gene Ng and Nm Formations, most of the source rocks in tified. Temporal relationships between faults and phases the Es3 and Es4 members entered the oil window in the late of hydrocarbon accumulations were studied and com- Miocene. The second episode was relatively late, allowing bined with fault sealing properties, allowing a reservoir oil to accumulate in a variety of traps formed prior to and formation model of the Dongying transfer zone to be con- during source rock maturation. Many reservoirs are found structed. Finally, locations for future exploration and pro- U in the Es3 and Ed members [27]. duction were identified.

3 Data and methods 4 Results

The 3D seismic data used in this paper were acquired by 4.1 Structural Geometry the Shengli Branch Company, SINOPEC, in 2008. The trace interval of the dataset is 25 m · 25 m, the time sample in- 3D seismic data integrated with well tops and checkshot terval is 2 ms, and the main frequency is 25 Hz (the seis- data were used to analyse the fault profiles, focusing on mic grid is illustrated in Fig. 5). The time-domain 3D seis- the relationships between low- and high-order faults. Each mic dataset was used to interpret the T1, T2, T4 and T6 line of the profile was interpreted inline and cross-lines, by tracing the peak of corresponding seismic reflections, and seven typical profiles were chosen to describe changes and then the faults were interpreted in detail by interrupts inthe tectonic styles (Fig. 6). Secondly, after profile inter- or disturbances on these seismic reflections and seismic pretation, maps of the four main reflecting layers were gen- slices guided by geological concepts. Although the vertical erated correspondingly . The changes of map-view tectonic resolution of seismic dataset is about 20 m [28], but for the styles were also studied in detail. Vertical throw is the ver- study area is very small (12 sq Km), we used the 3D veloc- tical component of the dip separation of a normal fault, ity field which provided by Shengli Oilfield to change the measured in a vertical cross-section perpendicular to the time-domain information to depth-domain information. strike of a fault [12]. In this paper, we also calculated the The results are credible, and can interpret the changes of throws to show the faults’ geometrical changes. throw inside and outside the Dongying transfer zone. 618 Ë F. Tian et al.

Figure 5: The locations of seismic profiles interpreted in Fig. 6. The stress ellipse in the transfer zone between Ying 1 and Ying 8faultswas 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.

4.1.1 Profile structural style at the level of the T2 to T1 reflections which are1 Es and Ed Formations, as described below. 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 be- Line 3413, Fig. 6b-b′, western part of the transfer zone: low, from west (line a-a’) to east (line g-g’). The offset decreases on the Ying 1 fault (throw of 87m) compared to section a-a’. The section shows that the Ying Line 3389, Fig. 6a-a′, western margin of the transfer 31 and Ying 8 faults are active, with throws of 40 m and zone: 10 m, respectively. Two north-dipping echelon faults are present between the Ying 1 and Ying 31 faults. A roll-over The Ying 1 fault surrounds a half graben with minor faults anticline developed between the Ying 1 and Ying 8 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 Compound transfer zone in Dongying anticline Ë 619

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Figure 6: (aa′-gg′) Uninterpreted and interpreted seismic profiles through the Dongying anticline showing fault characteristics of thetrans- fer zone (for location, see Fig. 5). The trace interval is 25 m · 25 m. 620 Ë F. Tian et al.

Line 3432, Fig. 6c-c′, central part of the transfer zone: Line 3450, Fig. 6d-d′, eastern part of the transfer zone:

The offset on the Ying 31 and Ying 8 faults (throws of131 The Ying 31 and Ying 8 fault bound a graben in this pro- and 79 m) has increased compared to section b-b’, while file. The Ying 31 fault has a throw of 186 m. Three small that on the Ying 1 fault has decreased (throw of 45 m). The synthetic faults are present in the hanging wall of the Ying Ying 1 fault is now the primary fault bounding the struc- 31 fault and are separate from the Ying 1 fault. The Ying 1 ture. The Dongying anticline disappears. fault has a throw of 37 m and does not extend below the L Sha3 member (Es3). Compound transfer zone in Dongying anticline Ë 621

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 of thezone.

Line 3470, Fig. 6e-e′, eastern margin of the transfer zone: faults are present in the respective hangingwalls, dipping to north and south The Ying 1 fault has disappeared, and the graben is Thus, from west to east across the Dongying structure, bounded by the Ying 31 and Ying 8 faults, both with throws the Ying 1 fault diminishes and then disappears, while the of approximately 240 m. Another new roll-over anticline Ying 31 and Ying 8 faults gradually become more impor- has increased in amplitude. Five small antithetic faults are tant and control the margins of the graben (Fig. 7). Fourth- present in the hanging wall of the Ying 31 fault. The tec- and fifth-order faults occur in the respective hanging walls tonic style is very different from that at the western margin and there is a central roll-over anticline of the structure (e.g., Line 3389 in Fig. 5a).

4.1.2 Map-view structural style Line 3490, Fig. 6f-f′, eastern side of the transfer zone: The map-view structural style shows that there is a Offset on the graben-bounding Ying 31 and Ying 8 faults convergent-overlapping transfer zone between the Ying 1 has increased (throws of 235 m and 265 m, respectively), and Ying 8 faults in the northern portion of the Dongying but the overall tectonic style is similar to that in Fig. 6e, anticline (c.f., Fig. 1e), and a synthetic-overlapping trans- implying that fault framework is stable. fer 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. L+M Cross-section of the Dongying structure At the level of the Shahejie Formation Es3 units, the Ying 1 and Ying 8 faults do not overlap (Fig. 8, T6), and A NE-SW oblique cross-section of the Dongying structure a structural horst is present between the fault tips. The between Wells Ying 19 and Ying16 is shown in Fig. 6g-g′. strikes of the Ying 8 and Ying 31 faults are parallel, and U Between the graben-bounding faults, a series of low-order a graben is present between them. In the Es3 -Es2 Forma- 622 Ë F. Tian et al.

over 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).

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 varia- tion 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]. HWt EI = (1) FWt 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 Figure 8: Schematic structural map of the Dongying compound later and ending earlier (Fig. 9). transfer zone in the four main seismic reflectors, showing the trans- There are many secondary faults in the transfer zone, fer zone plane tectonic style changes. The main control fault disappears in the transfer zone. See location in Fig. 4a. 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 ofthe tions, the Ying 1 and Ying 8 faults overlap and lower-order Dongying transfer zone as they were controlled by differ- faults are present (Fig. 8, T4), and the Dongying transfer ent major faults. zone begins to develop. In the Es1-Ed Formations, activ- The activity of the F1 fault was greater than that of ity on the transfer zone becomes much stronger, and the the F2 fault (points U and W correspondingly), which mea- transfer zone is divided into northern and southern por- sured in the formations between T2 and T1 reflections. Al- tions. The Ying 8 fault curves toward the Ying 1 fault. A roll- though the F1 and F2 faults were both active during the Compound transfer zone in Dongying anticline Ë 623

total throw gradually increases because of the presence of secondary faults that transmit displacement.

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 Figure 9: Expansion indexes of different faults in the Dongying com- northern part of the transfer zone was affected by left- pound transfer zone (for location, see Fig. 5), showing the differ- lateral transtension, causing the strikes of the major faults ences of fault activity inside and outside the zone. 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

Figure 10: Throw of the various faults in the Dongying compound The Dongying compound transfer zone is located at a rel- transfer zone (for location, see Fig. 5), showing transmission of atively high structural level and may provide a trap for displacement in the zone. hydrocarbons migrating updip. The major faults may provide migration conduits between source rocks in Minfeng deposition of Es1 member, with their activity ceasing in Sag and reservoirs, while secondary faults whose activity the deposition of Nm Formation, the two faults displayed ceased relatively earlier, may become barriers to fluid flow. 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 4.3.1 Spatial relationship between the faults, source fault occurred during the deposition period of the Neogene rock and reservoir Guantao Formation.

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 4.2.2 Vertical Throw of the major faults transfer zone, and they become the oil source fault. There are many secondary faults that form an effective oil migra- The vertical throw of the major faults in the Es1-Ed For- tion framework with the oil source fault. The secondary mations was calculated every 500 m (20 lines). From line faults cease their activity earlier and have good sealing 3370 to 3490, the total regional vertical throw increases. ability, allowing many fault block traps to form. Further- The throw of the Ying 8 and Ying 31 faults increases, but more, the rollover anticline is a favourable area for hydro- that of the Ying 1 fault decreases from west to east (Fig. 10). carbon accumulation [8]. In the area between line 3400 and 3450 (Table 1), the vertical throw of all three major faults is relatively small, but the 624 Ë F. Tian et al. Area of Dongying transfer zone Ying 1 fault Ying 31 fault Ying 8 fault Regional total Area of Thickness /m Throw /m Thickness /m Throw /m Thickness /m Throw /m throw /m Dongying Faults thickness and throw of the Dongying compound transfer zone. Line hangingwall footwall hangingwall footwall hangingwall footwall transfer zone 3430 398 374 24 393 237 156 384 358 26 261 337033803390 4023400 4003410 3983420 304 400 312 3963440 312 3923450 318 983460 328 88 3903470 350 863480 / 823490 / / 382 683500 / / 42 / / / / / 8 / / / / 328 / / / / / / / 423 232 / / / / / / / / 224 447 96 / / 474 / / / 524 / 199 224 570 / 372 236 565 / 239 377 584 / 369 223 313 / 354 238 332 / 285 367 384 / 334 383 257 / 18 403 233 / 400 200 10 / 341 35 420 / 344 456 129 244 324 460 145 326 42 220 180 300 328 59 205 333 76 94 318 128 349 127 389 398 409 369 Table 1: Compound transfer zone in Dongying anticline Ë 625

4.3.2 Temporal relationship between faults and The vertical sealing ability of the fault also relies on hydrocarbon accumulation the stress normal to the fault plane. The higher the normal stress, the better the sealing of the fault [37]. The stress on

The spatial linkage of the faults and source rock is a static the fault plane arises from three forces: vertical stress (δ1) factor that is conducive to hydrocarbon accumulation. As a which is defined as the lithostatic pressure, maximum hor- dynamic factor that favours hydrocarbon accumulation is izontal principal stress (δ2) and the minimum horizontal that the activity periods of the oil source fault and screened principal stress (δ3). faults are in good correspondence with the oil accumula- δ = δ sinα · sinθ)2 + δ (cos α · sin θ)2 + δ cos2θ (3) tion period [24]. In the northern Dongying transfer zone, 2 3 1 the Ying 1 fault was still active during the deposition pe- where δ is the stress normal to the fault plane (MPa), α is riod of Neogene Guantao and Minghuazhen Formations. the angle between the fault strike and the direction of max- The secondary faults (F1, etc.) could conduct hydrocarbon imum horizontal principal stress (∘), and θ is the fault dip during the deposition period of Guantao Formation, when angle (∘). they were weakly active, but these faults were screened in δ1 can be defined as follows: the deposition period of Minghuazhen Formation. Similar dynamic situations occur in the southern Dongying trans- δ1 = ρb gz (4) fer zone. From the EI map, we speculated that in the de- where ρb is the average density of the overlying rock (we position period of Guantao and Minghuazhen Formations, used 2.45 g/cm3 here), g is acceleration of gravity, 9.8 m/s2, the Ying 31 fault was still active, and its activity was more and z is the buried depth of the fault plane (m). intense in the deposition period of Minghuazhen Forma- The current stress field in the Dongying anticline was tion period (Fig. 9). It is beneficial to accumulate hydro- studied by Zhang et al. [36]. The maximum horizontal carbon oozed from source rock. The activity of the sec- stress is in the direction of 70∘. The value of maximum hor- ondary faults (F2, etc.) ceased during the deposition pe- izontal stress (δ2) and the minimum horizontal stress (δ3) riod of Minghuazhen Formation, and they became well- follow the burial depth: screened faults.

δ2 = −15.69 + 0.032 · z (5)

4.3.3 Analysis of fault migration and sealing properties δ3 = −6.93 + 0.021 · z (6) The sealing properties of faults are key factors that con- According to rock deformation theory, when the pres- troll reservoirs’ fault-bounded structures. In this paper, the sure acting on mudstone exceeds the elastic limit, plastic Shale Gouge Ratio (SGR) and the stress normal to the fault deformation of the mudstone will occur. Physical rock test plane (δ) were used to characterize fault sealing proper- revealed that when the fault plane pressure exceeds 5 MPa, ties [33]. the resulting plastic flow of the mudstone will block the Clay or shale smeared into the fault zone has been con- left leakage space near the fault plane and seal off the fault sidered an effective factor impacting the sealing ability of completely [38, 39]. When the normal pressure is less than the fault [34]. In this paper, SGR is used to estimate the 5 MPa, plastic deformation of the mudstone does not oc- amount of muddy fault smear, which is defined as the ra- cur, the leakage space near the fault plane is preserved and tio between the throw of the fault and the total thickness the fault is not completely sealed off. of mudstones within the throw [34]. Currently, the normal pressures of the two major faults

n in the major reservoir (Ed) are 10.24 MPa and 10.46 MPa, ∑︀ hi and SGR of the fault are 51.17% and 52.15%, which sug- SGR = i=1 × 100% (2) gest that they are effective sealing faults (Table 2). Due to L the extension-shear stress in the transfer zone, the fault where L is the throw of the fault (m), hi is the thickness of plane pressures of the F1 and F2 faults are 22.87 MPa and th the i mudstone layer, and n is the number of mudstone 23.95 MPa, and their SGR values are 53.34% and 56.89% layers within the throw. Previous studies have found that a (Table 2), respectively, demonstrating that they also have higher SGR indicates a poorer porosity and permeability in good sealing properties. the fault damage zone, and the fault will have good sealing Previous studies have suggested that the faults can ability when the shale content of the fault zone exceeds 0.7 serve as flow paths when the fault is active and as barri- in the Dongying Depression [34, 36]. ers when the fault is inactive. Thus, the faults can be seen 626 Ë F. Tian et al.

Table 2: Fault dip, mud content and fault plane pressure of the Dongying compound transfer zone

Formation Fault Fault Fault plane pressure SGR/% Fault Fault Fault plane pressure SGR/% dip/∘ /MPa dip/∘ /MPa Nm Ying 31 59.03 4.85 69.71 Ying 1 65.12 4.1 72.92 Ng Ying 31 58.91 5.73 69.29 Ying1 64.38 5.8 73.86 Ed Ying 31 56.24 10.24 51.17 Ying 1 61.28 10.46 52.15

Es1 Ying 31 55.75 11.98 52.04 Ying 1 54.39 11.58 55.08 Es2 Ying 31 54.84 13.49 47.39 Ying 1 45.79 14.69 46.47 U Es3 Ying 31 53.72 15.12 58.34 Ying 1 36.22 18.78 61.06 L+M Es3 Ying 31 45.59 19.39 82.61 Ying 1 26.58 24.78 84.73 Ng F1 66.24 19.70 72.53 F2 65.49 18.71 69.34 Ed F1 64.37 22.87 53.34 F2 62.85 23.95 56.89

Es1 F1 63.15 27.60 58.19 F2 61.48 29.34 58.61

Figure 12: Reservoir formation model of the northern Dongying Figure 11: Reservoir formation model of the southern Dongying compound transfer zone. The Ying 31 fault is the oil source fault, compound transfer zone. The Ying 1 fault is the oil source fault, and and the reservoirs are mainly found near the Ying 31 fault. the main source rock is in Es4, buried in Minfeng sag.

idence demonstrates that the episodic petroleum migra- as a hydrocarbon effective migration path if the active pe- tion model could be more geologically relevant than a riod of the fault matches the hydrocarbon accumulation continuous-flow model [40, 41]. The secondary faults con- period. However, in most cases, the fault expansion in- stituted an effective hydrocarbon conduction framework, dex is used to characterize the intensity of fault activity. together with the major faults during the Ng period, and In the Ng-Nm period, the major faults’ activity had an opti- became screen faults in the Nm period when their activ- mum matching relationship with hydrocarbon accumula- ity ceased. Under the local extension-shear stress field, tion, while the secondary faults could not match (Fig. 9). the secondary faults had good sealing ability and formed The hydrocarbons expulsed from Minfeng Sag can migrate many fault block traps, which could accumulate and pre- along major faults to reservoirs, while secondary faults act serve hydrocarbons effectively. as barriers, resulting in the formation of fault-bound com- In the northern Dongying compound transfer zone, partments. 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 south- 4.4 Hydrocarbon Accumulations in the ern Dongying transfer zone, there are many step-shaped Dongying Transfer Zone secondary faults adjoined to the Ying 31 fault. These secondary faults had good sealing ability in the Ng period and Hydrocarbon in the Dongying compound transfer zone can accumulate substantial hydrocarbons in the hanging was mainly generated from Es3 and Es4 source rock in Min- wall of the Ying 31 fault (Fig. 12). feng Sag [18, 25]. Major faults broke into the Es4 source The Dongying transfer zone is a promising area for fu- rock in the Minfeng Sag and were active and conductive ture hydrocarbon exploration. The Well Ying 13 in the roll- during the hydrocarbon accumulation period. In fact, ev- over anticline, the Well Ying 24 area and the Well Ying 26 Compound transfer zone in Dongying anticline Ë 627 area, all with high points near secondary faults are advan- the static factors but also the sealing properties of these tageous hydrocarbon accumulation areas. North of the F2 faults in different periods. We used SGR and the stress nor- fault, the west side of the Ying 31 fault and the west side of mal to the fault plane to characterize fault sealing proper- the Ying 8 fault are disadvantageous for hydrocarbon ac- ties, combined with the faults’ activity periods. We found cumulation (Fig. 5). that during the main hydrocarbon migration period, ma- In recent years, the drilling success ratio in the advan- jor faults, as the oil source fault, linked the source rock and tageous area has been higher than 86%, confirming the reservoir and effectively conducted hydrocarbons. The sec- importance of the transfer zone in the energy exploration ondary faults, as screened faults, ceased movement ear- in the complex fault block oilfield. In the Dongying Depres- lier, had good sealing ability and can form plenty of fault sion, many productive wells have been drilled in the small block traps. Thus, we believe the Dongying transfer zone transfer zones between third- (or fourth-) order faults, and is an advantageous area for hydrocarbon accumulation. 12 small fault block reservoirs have been found, increasing the reserve by 33.46 million barrels of oil [18]. 6 Conclusions

5 Discussion This study shows that a convergent-overlapping and synthetic-overlapping compound transfer zone has devel- We found a compound transfer zone developing in the oped in the Central Dongying Depression and is bounded Dongying anticline, which is located in the central uplift by major faults Ying 1, Ying 8 and Ying 31. The geome- of the Dongying Depression, and the transfer zone can ef- try, kinematics and dynamic characteristics of the trans- fectively control the local hydrocarbon distribution. This fer zone were studied. The tectonic style changes in profile transfer zone is composed of three major faults, and while and map view across the transfer zone. At the west side in the northern part there is a convergent-overlapping type of the structure is a half graben, which becomes a graben with a roll-over anticline and some secondary faults, in the to the east. In map view, the major faults disappeared in southern part there is a synthetic-overlapping type with the transfer zone, and the Ying 8 fault curved towards the several en echelon secondary faults. Ying 1 fault from an orientation of NWW to NE towards the Previous studies analysed the large transfer zones in centre of the transfer zone. In the transfer zone, some sec- the basin scale, and some geological models have been ondary faults developed, and the Dongying roll-over anti- built to interpret their geometrical characters [3, 6–8]. cline developed between the Ying 1 (or Ying 31) and Ying 8 Most of the transfer zones follow the classifications made faults. by Morely. In this paper, we interpreted a compound trans- The regional displacement changed in a continuous fer zone that was controlled by fourth-order faults in a lo- manner, and major faults acted in the same period, show- cal scale, and we found that the transfer zone can also ing that the Dongying anticline is a compound transfer conserve regional extensional strain. The Dongying trans- zone. From west to east, the throw of the Ying 1 fault fer zone can positively affect major faults’ activities. Along in Ed+Es1 Formations decreased, while those of the Ying the fault strike, the displacements of major faults clearly 8 and Ying 31 faults increased, and the total throw in- decreased in the transfer zone, and then the major faults creased gradually. Because the major faults control the disappeared. Taking the Ying 1 fault for example, the dis- stress boundary of the transfer zone, the internal stress placement of the Ying fault in T1-T2 reflections is approxi- field is different from the basin stress field and has ashear- mately more than 80 m outside the transfer zone, while in ing strain component. The northern portion is dominated the transfer zone, it decreased to less than 50 m and finally by left-lateral extensional-shear stress, and the southern disappeared. The secondary faults acted weakly, and their part is dominated by right-lateral extensional-shear stress, activities ceased earlier than the major faults. For example, so the secondary faults have good screen properties. the Ying 31 fault (major fault) was still active in the deposi- A potential trap formation model is established for the tion period of the Guantao and Minghuazhen Formations, Dongying compound transfer zone. Major faults that broke and its related secondary faults ceased in the latter period into the source rock, combined with secondary faults, con- (Fig. 9). structed a good hydrocarbon migration framework. Dur- In the analyses of hydrocarbon migrations in the ing the main hydrocarbon migration period, major faults, Dongying transfer zone, we found that we should con- as the oil source fault, linked the source rock and reser- sider not only the geometrical characteristics of faults as voir and conducted the hydrocarbons effectively. The sec- 628 Ë F. Tian et al. ondary faults, as screened faults, ceased movement ear- [5] Morley, C. K., Gabdi, S. and Seusutthiya, K., Fault superimpo- lier, had good sealing ability and could form plenty of fault sition and linkage resulting from stress changes during rifting: block traps. The Well Ying 13 in the roll-over anticline, Examples from 3D seismic data, Phitsanulok Basin, Thailand. Journal of structural geology, 2007,29, 646–663. the Well Ying 24 area and the Well Ying 26 area, all with [6] Faulds, J. E., and Varga, R. J., The role of accommodation zones high points near secondary faults, are advantageous hy- and transfer zones in the regional segmentation of extended ter- drocarbon accumulation areas for further progressive ex- ranes. Geological Society of America Special Papers, 1998,323, ploration and production. 1–45. Many small transfer zones developed between third- [7] Su, J.B., Zhu, W.B., Wei, J., Xu, L.M., Yang, Y.F., Wang, Z.Q. or fourth-order faults, which control the local structure. and Zhang, Z.Y., Fault growth and linkage: Implications for tectonosedimentary evolution in the Chezhen Basin of Bohai Major faults acting as oil source faults, secondary faults Bay, eastern China. 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