The Strike-Slip Fault System and Its Influence on Hydrocarbon Accumulation in the Gudong Area of the Zhanhua Depression, Bohai Bay Basin

Abstract

The Gudong area contains abundant petroleum resources. Previous studies have mainly focused on the extension structure in this area, with its strike-slip characteristics remaining poorly understood. In this study, the geometry of the strike-slip faults in the Gudong area was investigated using high-resolution 3D seismic reflection and drilling data, as were their associated releasing and restraining structures. Based on the profile’s flower structure and the plane’s horsetail splay pattern, the Gudong fault in the study area can be characterized as a dextral strike-slip. Three types of strike-slip fault-associated structures can be identified in the study area: (a) a restraining bend occurring in the right-stepping area of the S-shaped Gudong strike-slip fault, (b) a restraining bend identified in the left-stepping, overlapping zone of the Gudong and Kendong faults, and (c) a releasing bend seen in the extensional horsetail splay structure at the southern end of the Gudong fault. The restraining stress induced the formation of a fault-related open anticline, which led to a significant increase in fault sealing efficiency, thereby preserving an estimated 75.479231 million tons of oil and 15.28317145 billion cubic meters of gas. Conversely, releasing transtensional stress has compromised the effectiveness of the traps, preventing hydrocarbon retention. Consequently, oil and gas have migrated upward along the horsetail faults to the top of Cenozoic formations and have then dispersed.

1. Introduction

With long, nearly straight alignments and prominent geomorphic characteristics, strike-slip faults are noteworthy features within the Earth’s crust. They form due to the large-scale movement of tectonic plates across the Earth’s surface [1,2,3]. Although theoretically straight strike-slip faults typically do not create basins or uplifts, in reality, strike-slip faults tend to exhibit bends or segments. As a result, associated releasing and restraining structures always form along strike-slip faults. In the early stages of their development, an en echelon array of faults may initially form, with these segments subsequently connecting as displacement increases [4,5,6,7]. Transpressional or transtensional deformation can occur in releasing and restraining bends or oversteps, depending on the direction of the principal stresses and the initial arrangement of the fault segments [7,8,9]. The releasing bends along strike-slip faults show negative flower structures with subsidence, whereas restraining bends along strike-slip faults often manifest as positive flower structures with discrete zones of uplift [6,7,8]. A “flower structure” is a fault pattern that is associated with strike-slip faults, resembling the shape of a flower. There are two types: positive and negative flower structures. The upward-diverging faults under compressional stress show a positive flower structure, and the downward-converging faults under extensional stress show a negative flower structure [1,3]. Hydrocarbon exploration has revealed that major commercial discoveries have often occurred within the strike-slip fault zone of the Bohai Bay basin [10,11].

The renowned Tan–Lu fault zone traverses the eastern part of the Bohai Bay Basin. In the eastern part of the basin, the Gudong area of the Zhanhua Sag is intersected by the major branches of the Tan–Lu fault zone. Situated to the east of the major branch of the Tan–Lu fault zone, the Gudong area is an excellent location for studying strike-slip faulting and its associated releasing and restraining structures. In the early stages, the lack of high-resolution seismic data prevented the identification of strike-slip faults in this area. Recently, our research identified strike-slip faults in the Gudong area (Figure 1). The Gudong area is a hotbed for oil and gas exploration. The productive zone for oil and gas is located in the middle of the Gudong fault, whereas the southern end of the Gudong fault is not suited to oil and gas extraction (Figure 1). This study proposes that whether hydrocarbons accumulate in this area is related to the releasing and restraining effects caused by strike-slip faults. Previous studies have highlighted the key influence strike-slip faults exert on oil and gas accumulation [12]. However, a more detailed understanding of these faults and their associated structures has been constrained by a lack of high-resolution seismic data. Consequently, several questions remain regarding the features of the strike-slip fault system in the Gudong area and its influence on hydrocarbon accumulation: (1) the discrimination of strike-slip faults, (2) the identification of releasing and restraining bends along the strike-slip faults, and (3) the relationship between productive and failed oil and gas zones and the releasing and restraining zones associated with the strike-slip faults. This study aimed to address this research gap by identifying the strike-slip fault system in the Gudong area of the Zhanhua depression, in the Bohai Bay basin, with an integrated analysis of high-resolution 3D seismic reflection and drilling data facilitating new interpretations. The geometry of the strike-slip faults in the Gudong area was investigated, as were their associated releasing and restraining structures. Furthermore, this study aimed to elucidate the relationship between the effects of strike-slip shearing and hydrocarbon accumulation.

2. Geological Setting

The Bohai Bay basin is a rift basin that formed during the Mesozoic and Cenozoic eras, characterized by a rhombus-shaped configuration spanning approximately 200,000 km² and situated along the eastern coastline of China [11,13] (Figure 1a). Over more than 50 years of petroleum exploration, it has become the second largest oil production basin in China after the Songliao basin [11,13]. Surrounding this basin are uplifted Precambrian basement blocks: the Yanshan massif to the north, the Luxi massif to the south, the Taihang massif to the west, and the Jiaoliao terrane to the east [11].

The renowned Tan–Lu fault zone traverses the eastern part of the Bohai Bay basin (Figure 1a). During the Mesozoic era, the Tan–Lu fault underwent left–lateral strike-slip movement, shifting to right–lateral strike-slip movement during the Cenozoic era [14,15,16,17]. While its left–lateral strike-slip activity was primarily driven by oblique subduction of the Izanagi–Pacific plate [18,19,20], the mechanism behind its right–lateral strike-slip motion remains contentious. One perspective suggests that a shift in the direction of the subduction of the Pacific plate may have contributed to the right–lateral strike-slip motion [20,21,22]. Alternatively, others contend that the right–lateral strike-slip motion mainly arose from the block escape caused by the collision of India and Asia [23,24]. The formation and evolution of the Bohai Bay basin have been influenced by several factors, including the subduction of the Pacific and Izanagi plates, mantle upwelling, and the India–Asia collision [11,13,25]. Rifting occurred in the Palaeogene period, followed by post-rifting processes in the Neogene and the Quaternary. Situated to the east of a major branch of the Tan–Lu fault zone, the Gudong area experiences its impact (Figure 1b). The productive oil and gas zone is situated in the middle of the Gudong fault, while the southern end of the fault is not conducive to oil and gas extraction (Figure 1). This study proposes that whether hydrocarbons accumulate in this area is related to the releasing and restraining effects caused by strike-slip faults. Therefore, the Gudong area is an excellent site for researching the strike-slip fault system and its impact on hydrocarbon accumulation.

3. The Data and the Method

3.1. Data and Method

This study primarily uses 3D seismic reflection data, collected in the time domain from Sinopec Shengli Oilfield Company. The coverage of the 3D seismic data is demarcated by the gray dotted line in Figure 1b. This 3D seismic reflection survey, which can image to depths of 4 s of two-way travel time (TWT), has an inline spacing of 25 m and a crossline spacing of 12.5 m (see Figure 2). The dominant frequency of the Cenozoic strata is about 40 Hz. The 3D seismic data cover an area of 1000 km², encompassing the main tectonic units in the Gudong area. The stratigraphic framework in this study was set up by correlating 3D seismic and well-logging data. The fault features were illustrated by interpretation of seismic profiles and seismic volume slices. The characteristics of the releasing and restraining bends were analyzed by fault features and the local stress was filed.

3.2. Seismic Stratigraphy

A stratigraphic framework of the study area was developed by combining the data from this study with information from previous research [12] (Figure 2). In the Zhanhua depression, the strata are categorized into two mega-sequences, labeled as E and N+Q. These mega-sequences are generally separated by angular unconformities, identified according to the seismic reflections of the lateral terminations of the strata, termed onlap, downlap, toplap, and truncation [12,26] (Figure 2). Within the Paleogene mega-sequence, two secondary unconformities have been identified, dividing it into the Shahejie (Es) and Dongying (Ed) sequences. The Shahejie sequence has been further subdivided into the first member (Es1), second member (Es2), and third member (Es3) sub-sequences [12,26] (Figure 2).

4. The Strike-Slip Fault System and Its Associated Releasing and Restraining Structures

4.1. The Strike-Slip Faults

The Tan–Lu fault zone traverses the eastern part of the study area, leading to the development of branch faults within the region. Our research has identified the Gudong fault and the Kendong fault as prominent strike-slip faults in the study area.

The Gudong fault exhibits a steep dip angle, which is nearly vertical in the seismic profiles (Figure 3a,b). Its upper part intersects with the Guantao and Minghuazhen formations (Ng, Nm) and the Quaternary formation (Q), while its lower part penetrates the base of the Cenozoic strata and engages with the basin basement (Figure 3a,b). Notably, the Gudong fault displays a typical flower structure. On the 2000 ms variance attribute section (Figure 4), a horsetail splay is clearly visible at the southern end of the Gudong fault, which is a distinctive feature of strike-slip faults. A horsetail splay refers to a pattern of multiple smaller faults that branch out from the main strike-slip fault, which are typically near the fault’s termination [7,8]. Additionally, this horsetail splay is located on the eastern side of the Gudong fault and consists of a series of normal faults (Figure 4). This suggests that the eastern block of the Gudong fault has shifted southward relative to the western block.

Within the study area, the Kendong fault also exhibits the characteristics of a typical strike-slip fault, featuring a steep dip angle (Figure 3c,d). In the seismic profile, it is apparent that the lower portion of the Kendong fault intersects with the bottom interface of the Cenozoic strata, extending into the basin basement (Figure 3c,d). Additionally, the shallow faults associated with the Kendong fault consist of flower structures, as illustrated in Figure 3d.

4.2. The Releasing and Restraining Structures along the Strike-Slip Faults

Strike-slip displacements along major faults that encounter a fault bend or a step-over are typically accommodated by deformation within the bend or step-over, either partially or entirely [6,7]. Based on the type of deformation they accommodate, these fault bends can be divided into two types: releasing bends and restraining bends [7,27]. Restraining bends, which accommodate local contractions, are characterized by topographic uplift, shortening, and the exhumation of the crystalline basement [8,27]. Conversely, releasing bends facilitate extension and are typically characterized by subsidence, extension, and significant basin sedimentation [7,8,27].

Based on our interpretation of the seismic data, three fault bends were identified, and their locations are indicated in Figure 5. Since all of the NNE-trending faults in this study area are right–lateral, the left-stepping bends among them are therefore restraining bends, and the right–stepping bends are releasing bends [7]. Based on the above criteria, three types of fault bends can be identified in the study area: (a) the restraining bend A along the S-shaped strike-slip fault (Figure 4 and Figure 5a), (b) the restraining bend B in the left-stepping, overlapping zone of the strike-slip faults (Figure 4 and Figure 5a), and (c) the releasing bend C in the extensional horsetail splay structure (Figure 4 and Figure 5a). The tectonic characteristics of the restraining and releasing bends observed in the seismic profiles provide clues about when these fault bends formed.

Restraining bend A is located in the left-stepping area of the S-shaped Gudong strike-slip fault (Figure 4 and Figure 5a). A fault-related open anticline formed at this restraining bend, characterized by the upward deformation of the Dongying, Guantao, and Minghuazhen formations (Figure 6). The associated restraining segment of the Gudong fault is nearly vertical in the seismic profile, with negative flower structures (Figure 6).

Restraining bend B is located in the left-stepping, overlapping zone between the Gudong fault and the Kendong fault (Figure 4 and Figure 5a). This restraining bend is marked by uplift and associated angular unconformities (Figure 6). In Figure 6, a prominent angular unconformity is clearly visible, where the horizontally parallel sedimentary strata overlie the tilted and truncated layers.

Releasing bend C is an extensional horsetail splay structure located at the southern end of the Gudong strike-slip fault (Figure 4 and Figure 5a). This releasing bend is composed of the NNE-trending Gudong fault and a series of NE-trending extensional faults (Figure 4). In the seismic profile, a series of normal faults have developed in a staggered arrangement (Figure 7). Notably, the strata within the releasing bend concave downward and cause synkinematic deposition above the bottom of the Dongying formation (Figure 7). This indicates that significant subsidence at the releasing bends occurred during the deposition of the Dongying formation.

5. Discussion

Based on petroleum exploration in the study area, most of the productive oil and gas wells are located along the restraining bend A of the Gudong fault and in the overstep restraining zone of the Gudong and Kendong faults. In contrast, all the exploration wells at the southern end of the Gudong fault are dry. Interestingly, these dry wells are close to depocenters and hydrocarbon source rock centers. Geochemistry data from the wells indicate that oil and gas migrated through this area but have since dissipated [28,29]. We can hypothesize that the differences in hydrocarbon accumulation in the study area are related to the releasing and restraining bends of the Gudong and Kendong faults.

5.1. The Relationship between the Restraining Bends and the Productive Oil and Gas Zone

In the study area, rock sources of hydrocarbon constitute dark gray mudstone and shale in the Shahejie formation and the lower part of the Dongying formation [12]. Previous studies have indicated that faults, sandstone layers, and unconformities are the primary pathways for oil and gas migration [30,31]. Here, oil and gas reservoirs are primarily found in the western fault-related trap of the Gudong fault, mainly in the Shahejie and Dongying formations (Figure 8). According to our study, the restraining bend A of the Gudong fault generates transpressional stress, which seals the fault plane. Therefore, oil and gas in the Gunan sag can migrate and have accumulated in the western fault-related traps along the Gudong fault. An estimated 75.479231 million tons of oil and 15.28317145 billion cubic meters of gas are preserved in this area. Meanwhile, no oil or gas can be found in the Gudong buried hill located west of the Gudong fault. Due to the transpressional stress sealing the fault plane, oil and gas have been prevented from migrating across the Kending fault and thus have not accumulated in the Gudong buried hill. A similar type of restraining bend was reported in the PL19-3 and PL25-6 structures along the Tan–Lu fault zone in the Bohai Bay basin [32]. The PL19-3 field, which has a proven oil-bearing area of 33.1 km² and oil-in-place reserves exceeding 600 million tons, is the largest offshore oil field that has been discovered in China [32]. Its associated structures are restraining bends, formed by dextral strike-slip movements, along the faults within the Tan–Lu fault zone [32,33].

In addition to the oil and gas reservoirs in the Shahejie and Donying formations within the western fault-related traps of the Gudong fault, reservoirs can also be found between the Gudong and Kendong faults (Figure 8). Our study indicates that the restraining bend B between these faults generates transpressional stress, causing an uplift of the formations between the two faults (Figure 8). This uplift forms an open anticline trap, as shown in Figure 8. Based on our analysis, oil and gas from the Gunan sag cannot cross the Gudong fault and accumulate in these traps. Data comparing the oil and the source rocks indicate that these hydrocarbons instead migrated from the Huanghekou sag along the basement unconformity to the restraining fault-related open anticline traps between the Gudong and Kendong faults (Figure 8).

5.2. The Relationship between the Horsetail Releasing Structure and the Oil and Gas Failure Zone

Most of the wells in the horsetail releasing structure, such as the Fu 23, Fu 15, Fu 291, Fu 7, and Fu 29 drilling wells, are dry (Figure 1b). Based on indicators of the maturity of C29 sterane and the abundance of 4-methyl sterane and C27-rearranged sterane in the Fu 15 well [28,29], previous research has indicated that oil and gas from the Gunan sag migrated through this area, but hydrocarbons were not retained. Our research suggests that the releasing bend at the southern end of the Gudong fault generates transtensional stress, causing the fault planes within the horsetail structure to open, and resulting in poor fault sealing in the releasing bend (Figure 9). Although oil and gas once migrated from the Gunan sag to the traps of the horsetail structure, the transtensional stress compromised the effectiveness of the traps and thus hydrocarbon retention (Figure 8). Consequently, oil and gas migrated upward along the horsetail faults to the top of Cenozoic formations and then dispersed.

6. Conclusions

In light of the Gudong area being rich in petroleum resources, the extension structure in this region has previously been studied; however, its strike-slip characteristics have largely been left unexplored. In this study, high-resolution 3D seismic reflection and drilling data were utilized to investigate the geometry of the strike-slip faults in the Gudong area, as well as their associated releasing and restraining structures. The findings were as follows:

(1) The Gudong strike-slip fault in the study area features a flower structure and a broom-like plane pattern. The angle between the main fault and subsidiary fractures is reflective of right–lateral strike-slip movement.

(2) Three types of strike-slip fault-associated structures have been identified in the study area: (a) a restraining bend in the right-stepping zone of the S-shaped Gudong strike-slip fault, (b) a restraining bend in the left-stepping, overlapping zone of the Gudong and Kendong faults, and (c) a releasing bend in the extensional horsetail splay structure at the south end of Gudong fault.

(3) Restraining stress has given rise to the formation of a fault-related open anticline in the study area, effectively sealing the fault. Consequently, oil and gas are well preserved in the restraining area, which explains the ubiquity of productive oil and gas wells in this region.

(4) Releasing transtensional stress reduced the effectiveness of the traps, preventing hydrocarbon retention. As a result, oil and gas moved upward along the horsetail faults to the top of the Cenozoic formations and then dispersed.