Analysis on Hydrocarbon Charging Process in the Belts of Antiformal Negative Flower Structures in the Southwestern Sag of Dongpu Depression, Bohai Bay Basin, North China

Abstract

The antiformal negative flower structure of the Shahejie Formation in the Southwest Sag of Dongpu Depression is the main target of rolling exploration evaluation, but there is limited understanding of its hydrocarbon accumulation events. In this study, firstly, based on the high-resolution 3D seismic data, the planar and profile features of antiformal negative flower structures have been depicted, and their forming mechanism has been described by cooperating with regional structural stress field evolution analysis. Meanwhile, systematic analysis of fluid inclusions has been employed to determine hydrocarbon charging events and ages in the antiformal negative flower structures. The key findings are as follows: (1) The antiformal negative flower structure in the Southwest Sag formed through three evolutionary stages: rifting at NW 282°–SE 102° (Sha-4 to Sha-3 Members), trans tensional development at NW 350°–SE 170° (Sha-2 Member to Dongying Formation), and collapse (Guantao to Minghuazhen Formations). (2) Three hydrocarbon charging events in the antiformal negative flower structure belt were categorized into two episodes, occurring, respectively, during the structure formation stage (33.3–27.3 Ma, reversing stage of Sha-2 Member to Dongying Formation) and collapse stage (9.9–4.3 Ma, new structural movement). (3) The three sets of source rocks (from strong rifting), fault-related migration pathways and traps in belts, and post-rifting regional cover exhibit a favorable temporal–spatial matching relationship, which forms a key hydrocarbon migration-accumulation site and favorable exploration target in the sag’s uplifts.

1. Introduction

Three-dimensional seismic profiles have become a crucial tool for identifying positive and negative flower structures, as well as later superimposed inversion structures associated with strike-slip movement variations [1]. Among these structures, the “antiformal negative flower structure (ANFSs)” or “composite/hybrid flower structure” exhibits a specific fracture assemblage pattern: on seismic sections, it appears as an upward-bulging “antiform” with normal fault offsets in the upper part of the mirror plane, while the lower part of the mirror plane shows a “directional” fracture arrangement. Planarly, the folds and their associated normal faults are distributed in an en echelon pattern, with their tops experiencing a certain degree of erosion [2,3,4,5]. Most researchers argue that this structural style corresponds to a type of composite flower structure formed under a strike-slip extensional regime. Driven by variations in the extensional-shear stress field, it undergoes the following three distinct evolutionary stages: the early stage, during which extensional-shear forces initiate the formation of a negative flower structure; the middle stage, in which compressional-shear forces dominate and drive the development of an antiformal positive flower structure; and the late stage, when structural collapse occurs, converting the reverse throw within the positive flower structure into normal throw [6,7,8,9,10]. However, scholars hold inconsistent views on the genetic mechanism of normal throw within the antiformal flower structure, the first perspective argues that the compressional-shear force during the middle stage is relatively weak, resulting in a smaller stratigraphic thrust amplitude than the throw generated during the extensional stage; consequently, the fault retains its normal fault nature, and the strata on both flanks arch upward toward the center to form an antiformal morphology [11]. This implies that the faults above and below the plane were formed during the same tectonic episode. The second viewpoint suggests that the formation of the antiformal structure and its associated normal throw is related to the “collapse” induced by the thinning of mudstone detachment layers within the deformed strata [12]. The third viewpoint holds that the normal throw within the antiformal flower structure is associated with the “collapse” effect induced by the load of late-stage overlying sediments, which are typically deposits formed during the post-rift sag phase [2,10]. However, both the first and second viewpoints are based on the “vertical extensional faulted anticline” theory [13,14]. They fail; however, to distinguish it from the ANFSs associated with strike-slip extensional processes, and this is one of the reasons why some researchers have long attributed the ANFSs (characterized by en echelon distribution) within such sags to vertical fault-bend folds [15].

ANFSs often develop in the footwall of extensional depression-controlling faults or at the inflection points of strike-slip faults. They exhibit a highly favorable source-reservoir-seal matching relationship, characterized by underlying hydrocarbon kitchens, vertical migration via strike-slip-extensional faults, and trap formation by ANFSs, thus serving as favorable hydrocarbon accumulation zones [16,17]. However, the hydrocarbon accumulation process of such structures remains poorly understood to date. Based on high-density 3D seismic data and drilling results from the Southwest Sag of the Dongpu Depression, this study investigates the hydrocarbon accumulation process of ANFSs using systematic fluid inclusion analysis techniques. The aim is to provide a scientific basis for decision-making regarding exploration strategies for tapping potential in mature exploration areas.

2. Geological Background

The Dongpu Sag is situated at the Southwestern end of the Bohai Bay Basin. It extends in a NNE (North–Northeast) direction on the plane, with a narrow Northern part and a wide Southern part. Tectonically, it consists of four secondary tectonic units, namely the Eastern Deep Sag Belt, Central Low Uplift Belt, Western Slope, and Western Sag Belt, covering an exploration area of approximately 5500 km² (Figure 1).

At approximately 50 Ma, under the combined influence of the Pacific Plate’s subduction direction shifting from NNW to NWW [18] and active mantle upwelling, a dextral extensional stress field with NW–SE extension developed in Eastern China. As a Cenozoic rift basin formed on a Mesozoic-Paleozoic basement under this dynamic framework, the Dongpu Sag has experienced the following three evolutionary stages: NW–SE extension during the Sha-4 to Sha-3 Members, NNW–SSE extension from the Sha-2 Member to the Dongying Formation, and overall subsidence spanning the Guantao Formation to the Quaternary (Figure 2). Lacustrine sediments, deposited from the Shahejie Formation through the Quaternary System, are present here, with the cumulative thickness of Cenozoic strata ranging between 3000 and 6000 m [19,20]. This indicates that under the synergistic influence of the remote effect of collision-subduction between the Indian and Eurasian Plates [21] and the increasingly steep subduction angle of the Pacific Plate [22], two key geological processes characterized the Paleogene in Eastern China. One involved a clockwise rotation of the extensional direction [20]; the other entailed the inversion of early extensional negative flower structures, driven by nearly E–W compressive stress developed in the middle-late Paleogene, which yielded ANFSs. After nearly 50 years of exploration and development, major hydrocarbon-bearing structural belts in the Dongpu Sag, including the Northern region and the Central Low Uplift, have generally entered a highly mature stage of exploration and development. In contrast, the Southwest Sag, with its relatively low exploration intensity, has gradually emerged as a key area for rolling exploration and edge expansion in this mature oil province. Situated between the Central Low Uplift and the Western Slope, the Southwest Sag is a narrow graben-like structure bounded by two of the following first-order faults: the Changyuan Fault to the Southwest and the Huanghe Fault to the Northeast, covering an exploration area of nearly 1000 km². Hydrocarbon exploration in the Southwest Sag commenced in the 1980s. To date, only just over 40 wells have been drilled, and merely a handful of hydrocarbon-bearing structures, including Nanhu, Zhaozhuang, Fangliji, and Nanhejia, have been identified. According to hydrocarbon resource assessments conducted during the 13th five-year plan period, the sag holds 51.6 million tons of oil resources and 34.3 billion cubic meters of natural gas resources, with a proven rate of only 1% [23].

The Cenozoic stratigraphy of the Dongpu Depression comprises the Shahejie Formation (Es), Dongying Formation (Ed), Guantao Formation (Ng), Minghuazhen Formation (Nm), and Pingyuan Formation (Qp). The Shahejie Formation can be further subdivided into four members, namely the Sha-4 Member (Es₄), Sha-3 Member (Es₃), Sha-2 Member (Es₂), and Sha-1 Member (Es₁). The Es₃ (Sha-3 Member) is subdivided into three of the following sections: the lower section (Es₃^L), middle section (Es₃^M), and upper section (Es₃^U). In contrast, the Es₄ (Sha-4 Member) and Es₂ (Sha-2 Member) each consist of only two sections: the lower section (Es₄^L, Es₂^L) and upper section (Es₄^U, Es₂^U).

3. Materials and Methods

3.1. Seismic Data Interpretation

Using newly processed seismic data in 2023 and newly drilled well distributions from the Southwest Sag, a well-tie seismic grid framework was constructed to constrain the interpretation grid. Building on this, an enhanced interpretation of target horizons was performed via well-seismic integration. From the high-density 3D seismic dataset of the Southwest Sag, attributes such as enhanced coherence, curvature, and Fault Shape Index (FSI) were extracted for characterizing faults across various stratigraphic interfaces. These attributes were then integrated with fault interpretations from seismic sections, and structural maps for each target horizon were compiled.

3.2. Fluid Inclusion Analysis

This study also collected 63 fluid inclusion samples from the ANFSs belt. Analyses including micro-fluorescence spectroscopy of individual oil inclusions, micro thermometry, and salinometry were conducted at the Key Laboratory of Tectonics and Petroleum Resources of the Ministry of Education, China University of Geosciences (Wuhan). A Maya Pro 3000 micro-fluorescence spectrometer (Provided by Guangzhou Yuanao Instrument Co., Ltd., located in Yuexiu District, Guangzhou City, China) was utilized to observe individual oil inclusions, while a Linkam heating-freezing stage coupled with a Nikon 80I dual-channel fluorescence microscope (Provided by Guangzhou Yuanao Instrument Co., Ltd., located in Yuexiu District, Guangzhou City, China) was employed for microthermometry and salinometry of fluid inclusions, with a temperature measurement error of ±0.1 °C under room temperature conditions.

To further clarify the hydrocarbon accumulation conditions of such ANFSs belts, this study collected 8 crude oil samples and performed micro-fluorescence spectroscopy analysis [25].

The analysis and observation of thin sections, cathodoluminescence, and fluid inclusions were completed at the Key Laboratory of Tectonics and Petroleum Resources (Ministry of Education), China University of Geosciences (Wuhan).

4. Results

4.1. Fault Profile Characteristics

Based on fault interpretation of 3D seismic profiles from the Southwest Sag, a set of normal fault systems, dominated by the Nanhejia Fault (F3) as the main fault, is identified in the Nanhejia structural belt of the Northern Southwest Sag. These faults splay upward toward shallow layers and converge into a large-scale strike-slip-extensional structural belt in the deep basement, with the Nanhejia Fault (F3) and Mengzhai Fault (F7) acting as the primary strike-slip-extensional fault zones.

The area of the Southwest Sag, bounded by these two regional major faults, developed a graben-like syncline during the Sha-4 to Sha-3 Members, showing structural characteristics of a negative flower structure. In contrast, the Sha-2 Member of the Dongying Formation displays an obvious antiformal (upward-bulging) morphology, with strata dipping downward on both sides. The closer to fault-concentrated zones, the more fragmented the strata and the more pronounced the antiformal amplitudes, such as in the deflecting section of the Lower Sha-2 Member in Well H302. Ultimately, a “convex-upward and concave-downward” mirror-image feature formed from the Sha-4 Member to the Dongying Formation, with the base of the Sha-2 Member serving as the mirror plane or equilibrium plane.

Additionally, within the ANFSs area, a series of fault blocks exhibit normal throw characteristics and have undergone varying degrees of tilting or rotation along fault planes, forming a set of reversed fault blocks. Meanwhile, with the central axis of the ANFSs as the approximate boundary, fault blocks on both lateral sides show an opposing dip pattern: faults on the Northwest side dip Southeastward, while those on the Southeast side dip Northwestward (Figure 3).

Figure 3. The L1 seismic interpretation section of Nanhejia ANFSs passed H302 well, North part of Southwestern Sag (see Figure 4 for location). (a) Seismic profile identification. (b) Sketch map of the seismic interpretation section of Nanhejia ANFSs.F1: Changyuan Fault; F2: Huanghe Fault; F3: Nanhejia Fault; F6: Shangzhai Fault; F7: Mengzhai Fault.

Figure 3. The L1 seismic interpretation section of Nanhejia ANFSs passed H302 well, North part of Southwestern Sag (see Figure 4 for location). (a) Seismic profile identification. (b) Sketch map of the seismic interpretation section of Nanhejia ANFSs.F1: Changyuan Fault; F2: Huanghe Fault; F3: Nanhejia Fault; F6: Shangzhai Fault; F7: Mengzhai Fault.

Figure 4. The structural maps of the Southwest Sag in Dongpu Depression (in-depth domain). (a) The base of Es₃^U; (b) The base of Es_1. F1: Changyuan Fault; F3: Nanhejia Fault F4: Fangliji Fault; F5: Linzhai Fault; F6: Shangzhai Fault; F7: Mengzhai Fault; F8: Zhaozhuangnan Fault. F9: H301 Well Block Fault; F10: F-1 Well Block Fault; F11: H302 Well Block Fault.

Figure 4. The structural maps of the Southwest Sag in Dongpu Depression (in-depth domain). (a) The base of Es₃^U; (b) The base of Es_1. F1: Changyuan Fault; F3: Nanhejia Fault F4: Fangliji Fault; F5: Linzhai Fault; F6: Shangzhai Fault; F7: Mengzhai Fault; F8: Zhaozhuangnan Fault. F9: H301 Well Block Fault; F10: F-1 Well Block Fault; F11: H302 Well Block Fault.

As the main fault, which splays upward toward shallow layers in the deep Mesozoic strata, these faults converge into a large-scale strike-slip-extensional structural belt, with the Fangliji Fault (F4) and Linzhai Fault (F8) functioning as the primary strike-slip-extensional fault zones. Within the area bounded by these two faults, a graben-like syncline developed during the Sha-4 to Sha-3 Members, whereas the Sha-2 Member to Dongying Formation features an obvious upward-bulging antiformal structure with normal characteristics, with the base of the Sha-2 Member serving as the mirror plane. Additionally, the fault blocks on both sides exhibit an opposing dip pattern in the lateral direction (Figure 5).

4.2. Planar Features of Faults

Structural maps of the base of the middle Sha-3 Member (Es₃^M) (Figure 4a) and the Sha-1 Member (Es₁) (Figure 4b) are compiled. Based on factors such as fault strike, fault throw, and extension distance, the fault system in the study area is categorized into four types: Type 1, Type 2, Type 3, and Type 4 (

Table 1. The elements of main faults in Southwest Sag of Dongpu Depression.

Fault Name	Fault No.	Type	Strike Direction	Dip Direction	Dip Angle (°)	Fault Throw (m)	Fault Active Period	Down-Cutting Horizon
Changyuan Fault	F1	Type 1	NE-NNE	SEE	40~70	1000~3500	Es4~Ng	Paleozoic
Huanghe Fault	F2	Type 1	NE-NNE	NWW	30~50	1200~3000	Es4~Q	Paleozoic
Nanhejia Fault	F3	Type 2	NNE	SEE	35~75	100~850	Es3~Nm	Paleozoic
Fangliji Fault	F4	Type 2	NE-NNE	SEE	35~50	100~1200	Es3~Nm	Paleozoic
Linzhai Fault	F5	Type 2	NNE	NW	30~60	100~600	Es3~Nm	Paleozoic
Shangzhai Fault	F6	Type 2	NE-NNE	SEE	35~50	100~1000	Es3~Es1	Es4
Mengzhai Fault	F7	Type 2	NE-NNE	NW	20~50	100~400	Es3~Nm	Es4
Zhaozhuangnan Fault	F8	Type 3	NEE	SE	35~75	20~300	Es3~Nm	Es4
H301 Well Block Fault	F9	Type 3	NEE	NW	30~60	20~200	Es3~Ed	Es3
F-1 Well Block Fault	F10	Type 3	NEE	SE	35~50	20~300	Es3~Nm	Es4
H302 Well Block Fault	F11	Type 4	NE	SE	25~45	20~50	Es2~Es1	Es2

).

Type 1 Faults: As depression-controlling faults, typified by the Changyuan Fault (F1), this category is characterized by large throw, great cutting depth, and long extension (over 100 km), with a strike of approximately NNE–SSW. Planarly, they are dominated by a NE-SE-trending right-stepping normal fault system. These faults-initiated activity during the deposition of the Sha-4 Member (Es₄) and remained active for an extended period. The Southwest Sag is precisely a large-scale graben-type faulted depression bounded by two regional faults: the Changyuan Fault (F1) to the west and the Huanghe Fault (F2) to the East [26].

Type 2 Faults: This category includes the Nanhejia Fault (F3), Fangliji Fault (F4), Linzhai Fault (F5), Shangzhai Fault (F6), and Mengzhai Fault (F7), among others. Classified as secondary faults, they are smaller in scale than Type I Faults and strike approximately NE-SE. Their main active period extended from the Sha-3 Member (Es₃) to the end of the Dongying Formation (Ed), with intense activity during the deposition of the Shahejie Formation. Faulting virtually ceased during and above the Guantao Formation (Ng). The throw of these faults typically ranges from 200 to 1000 m, with a maximum of 1200 m.

Type 3 Faults: In this area, they constitute a newly identified NEE-trending strike-slip fault system with “quasi-uniform spacing” [27], designated, respectively, as the Zhaozhuangnan Strike-Slip Fault (F8), H301 Well Block Fault (F9), and Fang-1 Well Block Strike-Slip Fault (F10). Controlled by the basement NWW-trending faults, these faults were primarily active from the Sha-2 Member (Es₂) to the end of the Dongying Formation (Ed). They cut through or displace the NNE-trending extensional fault system, complicating structural characteristics and forming structural styles such as complex fault blocks and faulted nose traps, thereby influencing the scale and potential of hydrocarbon accumulation zones in the Southwest Sag.

Type 4 Faults: Minor faults are extremely well-developed within the ANFSs belt. As mostly derivative or late-stage faults, they exacerbate structural complexity and give rise to the fragmented internal characteristics of this belt.

It follows that the ANFSs in the Southwest Sag are bounded by Type 2 fault zones, with the Nanhejia Structure (F3) in the North and the Fangliji Structure (F4) in the South as the most typical examples, exhibiting the “rise within sag” feature. Within the area delimited by Type 2 fault zones, the ANFSs belts show fragmented structures in plain view. In particular, small-scale faults are densely distributed with a spacing generally less than 1 km. Torsion zones align with the strike of these faults, which are arranged nearly parallel in the same direction, presenting an en echelon distribution (Figure 4).

4.3. Hydrocarbon Accumulation Stages and Periods

Based on micro-fluorescence spectroscopy of individual oil inclusions, microthermometry, salinometry, and the micro-fluorescence spectroscopy analysis of crude oil (

Table 2. The analytic data of micro-beam of fluorescent spectrum of crude oils in Southwest Sag.

Sample No.	Well No.	Top Depth (m)	Bottom Depth (m)	Horizon	(λmax, nm)	Q Value (Q)	QF-535	Episode
Z4-7	Z4-7	2761.8	2947.1	Es2L	577	0.92	1.76	Episode 1
Z4-3	Z4-3	2815.0	3125.9	Es2L	574	0.82	1.67	Episode 1
Z4-10	Z4-10	2753.0	2853.1	Es3U	550	0.71	1.49	Episode 2
H302	H302	3656.3	3673.0	Es2U-Es2L	518	0.27	0.91	Episode 3
H301-1	H301	3976.8	3993.4	Es2L	504	0.22	0.81	Episode 3
H301	H301	3754.8	3836.0	Es2L	493	0.27	0.81	Episode 3
H302-2	H302	4217.8	4290.8	Es3U	484	0.11	0.54	Episode 3
H302-3	H302	4406.0	4430.0	Es3U	467	0.17	0.56	Episode 3

and Figure 6). The results show that three episodes of crude oils with different maturities are present in the study area, indicating three episodes of oil charging and accumulation.

Micro-fluorescence spectroscopy analysis of individual oil inclusions reveals three episodes of oil charging with varying maturities: low-mature, mature, and relatively high-mature. In the Southwest Sag: the first episode is low-mature oil with a λ_max of 574–577 nm; the second is mature oil with a λ_max of 550 nm; the third is relatively high-mature oil with a λ_max of 467~518 nm. In terms of detection probability, the second episode (mature oil) and the third episode (relatively high-mature oil) are dominant. Additionally, by comparing maturity parameters derived from micro-fluorescence spectroscopy between individual oil inclusions and reservoir crude oils, it is found that the Sha-1 Member mainly accumulated oil from the third episode (relatively high-mature), while the upper Sha-3 Member and lower Sha-2 Member accumulated oils from the first to the third episodes (Figure 7).

Based on cathodoluminescence and fluorescence observations, the homogenization temperatures of various secondary aqueous inclusions trapped in intragranular cracks of quartz grains, transgranular quartz cracks, quartz overgrowths, and carbonate cements were primarily measured. By projecting the homogenization temperatures of aqueous inclusions contemporaneous with oil inclusions onto the burial history plot, the charging ages can be obtained [28]. By further plotting the obtained ages on a unified timeline, the division of hydrocarbon accumulation episodes and periods can be achieved [29]. In this study, parameters related to oil inclusions and their coeval aqueous inclusions from the ANFSs belt in the Southwest Sag were systematically tested. The homogenization temperature-burial history projection method was employed to indirectly determine the charging ages of hydrocarbons for each episode (Table 3). Based on these data, the division of hydrocarbon accumulation episodes and the determination of accumulation periods were conducted, with comparisons made against fault activity intensity (Figure 8). This analysis yielded the following conclusions:

(1)

The ANFSs belt in the Southwest Sag contains 1–3 hydrocarbon charging episodes and two hydrocarbon accumulation periods.

(2)

The first accumulation period occurred during the strike-slip extensional inversion stage from the Sha-2 Member to the Dongying Formation (33.3–27.3 Ma), i.e., the formation period of the ANFSs.

(3)

The second accumulation period took place during the neotectonic movement stage (9.9–4.3 Ma).

(4)

The Sha-4 to Sha-3 Members represented the main extensional stage, during which faults were highly active with large throws and controlled the development of source rocks in the sag. The Sha-2 Member corresponded to the main strike-slip extensional stage, characterized by relatively strong fault activity; this was also the formation period of the ANFSs (“rises within sags”) and the timing of the first hydrocarbon accumulation period. The late Guantao Formation coincided with the neotectonic movement stage, during which faults were reactivated. This stage also marked the onset of ANFSs “collapse”. Despite the relatively small fault throws, the second hydrocarbon accumulation period occurred during this interval.

Table 3. Data of hydrocarbon charging ages determined by homogenization temperature projecting on burial curves in the Southwest Sag.

Well No.	Depth/m	Horizon	Fluorescence Color of Oil Inclusions	QF-535	λmax Range/λmax, nm	Homogenization Temp. of Oil Inclusions/°C	Homogenization Temp. of Coeval Aqueous Inclusions/°C	Projected Age/Ma	Hydrocarbon Accumulation Period
M4	4148.2	Es3U	blue	0.9	468	113.2	129.0	28.7	Period 1
	4152.1	Es3U	green	1.8	547	139.6	153.9	7.8	Period 2
M5	3606.3	Es2L	blue	1.0	498	108.4	111.9	28.4	Period 1
	3607.4	Es2L	yellow	1.5	553	99.6	111.3	28.5	Period 1
	3607.4	Es2L	yellow	1.4	551	105.3	113.0	28.2	Period 1
	3617.9	Es2L	green	1.3	528	100.3	116.9	28.1	Period 1
M6	3507.3	Es2L	green	1.8	548	102.6	111.0	27.8	Period 1
	3507.3	Es2L	green	1.6	501	127.9	137.5	7.6	Period 2
	3507.3	Es2L	green	1.6	501	112.8	135.7	8.0	Period 2
	3514.3	Es2L	yellow	2.6	581	131.2	139.3	7.6	Period 2
	3514.3	Es2L	blue	1.0	464	124.6	143.7	7.1	Period 2
	3561.5	Es2L	green	1.7	545	132.1	141.4	7.3	Period 2
	3579.7	Es2L	green	1.6	524	96.1	106.9	28.9	Period 1
	3579.7	Es2L	yellow	1.8	561	135.2	147.0	6.2	Period 2
M601	4415.5	Es3U	green	1.8	548	130.6	144.7	28.6	Period 1
	4415.5	Es3U	blue	0.8	470	141.6	149.8	28.1	Period 1
	4417.1	Es3U	blue	1.4	527	149.2	164.2	8.9	Period 2
F3	3740.35	Es3U	green	1.5	548	119.3	140.2	6.4	Period 2
	3740.35	Es3U	green	1.4	523	123.6	136.5	7.6	Period 2
	3745.5	Es3U	green	1.7	529	126.8	136.6	7.7	Period 2
	3745.5	Es3U	yellow	1.6	551	104.1	110.8	28.5	Period 1
	3745.5	Es3U	green	1.7	529	126.1	138.4	7.2	Period 2
	3745.9	Es3U	green	1.2	505	115.7	134.2	7.9	Period 2
	3745.9	Es3U	yellow	1.1	576	126.4	138.8	7.0	Period 2
	3745.9	Es3U	green	1.4	507	102.9	112.4	28.2	Period 1
F4	4316.2	Es3M	blue	0.9	498	142.6	155.0	7.8	Period 2
	4316.2	Es3M	blue	1.2	473	114.3	129.7	28.8	Period 1
	4316.2	Es3M	blue	0.9	498	136.7	149.9	8.8	Period 2
	4430.6	Es3M	blue	1.0	477	151.8	160.3	7.4	Period 2
	3032.8	Es2L	green	1.5	528	100.9	115.6	8.0	Period 2
	3032.8	Es2L	green	1.5	528	113.2	116.9	7.9	Period 2
	3032.8	Es2L	green	1.5	528	105.9	116.6	7.9	Period 2
	3035.5	Es2L	blue	1.4	479	112.1	116.4	7.9	Period 2
	3035.5	Es2L	blue	1.5	456	102.7	115.7	8.0	Period 2
	3042.7	Es2L	blue	1.2	495	106.4	119.8	7.3	Period 2
	3042.7	Es2L	blue	1.2	495	103.6	117.2	7.5	Period 2
	3042.7	Es2L	blue	1.2	495	111.5	118.7	7.4	Period 2
S2	4101.8	Es3L	green	1.9	526	116.8	129.9	28.6	Period 1
	4103.9	Es3L	blue	0.9	477	109.3	128.7	28.8	Period 1
	4108.0	Es3L	yellow	1.9	575	114.8	127.8	28.9	Period 1
	4108.0	Es3L	blue	1.4	463	139.4	157.0	7.2	Period 2
	4110.2	Es3L	green	1.6	546	135.2	157.7	7.1	Period 2
	4285.2	Es3L	green	1.5	531	142.3	155.1	8.9	Period 2
	4285.2	Es3L	blue	1.6	496	125.7	137.6	28.5	Period 1
	4285.2	Es3L	green	1.5	531	144.6	156.5	8.8	Period 2
	4287.6	Es3L	yellow	1.5	560	126.9	138.5	28.4	Period 1
	3817.8	Es1	green	1.1	529	92.3	100.0	27.5	Period 1
H3	3773.1	Es2L	blue	1.2	497	135.3	148.7	6.6	Period 2
	3773.1	Es2L	yellow	1.2	552	109.1	111.3	28.2	Period 1
	3773.1	Es2L	green	1.6	543	136.5	146.7	6.9	Period 2
	4025.4	Es2L	yellow	1.8	553	139.8	150.2	7.8	Period 2
	4025.4	Es2L	green	1.1	532	146.2	150.9	7.7	Period 2
	4025.95	Es2L	yellow	1.5	550	130.7	143.5	9.3	Period 2
H301	3824.3	Es2L	blue	1.2	469	92.6	103.3	27.9	Period 1
	3824.3	Es2L	blue	1.6	456	114.1	128.8	7.5	Period 2
	3827.1	Es2L	yellow	1.9	564	95.4	102.7	28.1	Period 1
	3827.1	Es2L	orange	1.2	603	94.2	106.0	27.3	Period 1
	3827.1	Es2L	yellow	1.6	585	135.7	139.0	5.9	Period 2
H302	3758.9	Es2L	blue	0.7	473	102.6	115.4	28.1	Period 1
	3758.9	Es2L	blue	1.3	468	123.8	139.5	8.0	Period 2
	3758.9	Es2L	blue	1.0	497	129.9	140.6	7.8	Period 2
PS6	3856.8	Es2L	green	1.1	548	136.7	144.9	6.3	Period 2
	3856.8	Es2L	yellow	0.9	550	106.5	122.6	9.9	Period 2
	3859.4	Es2L	blue	0.9	470	96.4	107.9	28.5	Period 1
	4098.5	Es3M	green	1.4	512	144.2	166.2	4.3	Period 2
	4098.5	Es3M	blue	1.0	502	136.7	149.9	7.4	Period 2
	4100.4	Es3M	blue	1.3	473	91.2	101.2	31.3	Period 1
	4100.4	Es3M	blue	1.3	473	135.3	147.8	7.8	Period 2
	4100.4	Es3M	blue	1.3	473	139.2	154.5	6.5	Period 2
	4391.9	Es3L	yellow	2.0	551	92.5	105.3	32.4	Period 1
	4391.9	Es3L	blue	1.3	460	131.4	147.0	9.9	Period 2
	4568.0	Es3L	green	1.8	549	92.5	106.1	33.1	Period 1
	4726.9	Es3L	blue	1.2	507	105.3	111.5	33.3	Period 1
	4726.9	Es3L	blue	1.2	507	115.6	122.2	31.6	Period 1
	4726.9	Es3L	blue	1.2	507	139.2	149.4	28.2	Period 1

Table 3. Data of hydrocarbon charging ages determined by homogenization temperature projecting on burial curves in the Southwest Sag.

Well No.	Depth/m	Horizon	Fluorescence Color of Oil Inclusions	QF-535	λmax Range/λmax, nm	Homogenization Temp. of Oil Inclusions/°C	Homogenization Temp. of Coeval Aqueous Inclusions/°C	Projected Age/Ma	Hydrocarbon Accumulation Period
M4	4148.2	Es3U	blue	0.9	468	113.2	129.0	28.7	Period 1
	4152.1	Es3U	green	1.8	547	139.6	153.9	7.8	Period 2
M5	3606.3	Es2L	blue	1.0	498	108.4	111.9	28.4	Period 1
	3607.4	Es2L	yellow	1.5	553	99.6	111.3	28.5	Period 1
	3607.4	Es2L	yellow	1.4	551	105.3	113.0	28.2	Period 1
	3617.9	Es2L	green	1.3	528	100.3	116.9	28.1	Period 1
M6	3507.3	Es2L	green	1.8	548	102.6	111.0	27.8	Period 1
	3507.3	Es2L	green	1.6	501	127.9	137.5	7.6	Period 2
	3507.3	Es2L	green	1.6	501	112.8	135.7	8.0	Period 2
	3514.3	Es2L	yellow	2.6	581	131.2	139.3	7.6	Period 2
	3514.3	Es2L	blue	1.0	464	124.6	143.7	7.1	Period 2
	3561.5	Es2L	green	1.7	545	132.1	141.4	7.3	Period 2
	3579.7	Es2L	green	1.6	524	96.1	106.9	28.9	Period 1
	3579.7	Es2L	yellow	1.8	561	135.2	147.0	6.2	Period 2
M601	4415.5	Es3U	green	1.8	548	130.6	144.7	28.6	Period 1
	4415.5	Es3U	blue	0.8	470	141.6	149.8	28.1	Period 1
	4417.1	Es3U	blue	1.4	527	149.2	164.2	8.9	Period 2
F3	3740.35	Es3U	green	1.5	548	119.3	140.2	6.4	Period 2
	3740.35	Es3U	green	1.4	523	123.6	136.5	7.6	Period 2
	3745.5	Es3U	green	1.7	529	126.8	136.6	7.7	Period 2
	3745.5	Es3U	yellow	1.6	551	104.1	110.8	28.5	Period 1
	3745.5	Es3U	green	1.7	529	126.1	138.4	7.2	Period 2
	3745.9	Es3U	green	1.2	505	115.7	134.2	7.9	Period 2
	3745.9	Es3U	yellow	1.1	576	126.4	138.8	7.0	Period 2
	3745.9	Es3U	green	1.4	507	102.9	112.4	28.2	Period 1
F4	4316.2	Es3M	blue	0.9	498	142.6	155.0	7.8	Period 2
	4316.2	Es3M	blue	1.2	473	114.3	129.7	28.8	Period 1
	4316.2	Es3M	blue	0.9	498	136.7	149.9	8.8	Period 2
	4430.6	Es3M	blue	1.0	477	151.8	160.3	7.4	Period 2
	3032.8	Es2L	green	1.5	528	100.9	115.6	8.0	Period 2
	3032.8	Es2L	green	1.5	528	113.2	116.9	7.9	Period 2
	3032.8	Es2L	green	1.5	528	105.9	116.6	7.9	Period 2
	3035.5	Es2L	blue	1.4	479	112.1	116.4	7.9	Period 2
	3035.5	Es2L	blue	1.5	456	102.7	115.7	8.0	Period 2
	3042.7	Es2L	blue	1.2	495	106.4	119.8	7.3	Period 2
	3042.7	Es2L	blue	1.2	495	103.6	117.2	7.5	Period 2
	3042.7	Es2L	blue	1.2	495	111.5	118.7	7.4	Period 2
S2	4101.8	Es3L	green	1.9	526	116.8	129.9	28.6	Period 1
	4103.9	Es3L	blue	0.9	477	109.3	128.7	28.8	Period 1
	4108.0	Es3L	yellow	1.9	575	114.8	127.8	28.9	Period 1
	4108.0	Es3L	blue	1.4	463	139.4	157.0	7.2	Period 2
	4110.2	Es3L	green	1.6	546	135.2	157.7	7.1	Period 2
	4285.2	Es3L	green	1.5	531	142.3	155.1	8.9	Period 2
	4285.2	Es3L	blue	1.6	496	125.7	137.6	28.5	Period 1
	4285.2	Es3L	green	1.5	531	144.6	156.5	8.8	Period 2
	4287.6	Es3L	yellow	1.5	560	126.9	138.5	28.4	Period 1
	3817.8	Es1	green	1.1	529	92.3	100.0	27.5	Period 1
H3	3773.1	Es2L	blue	1.2	497	135.3	148.7	6.6	Period 2
	3773.1	Es2L	yellow	1.2	552	109.1	111.3	28.2	Period 1
	3773.1	Es2L	green	1.6	543	136.5	146.7	6.9	Period 2
	4025.4	Es2L	yellow	1.8	553	139.8	150.2	7.8	Period 2
	4025.4	Es2L	green	1.1	532	146.2	150.9	7.7	Period 2
	4025.95	Es2L	yellow	1.5	550	130.7	143.5	9.3	Period 2
H301	3824.3	Es2L	blue	1.2	469	92.6	103.3	27.9	Period 1
	3824.3	Es2L	blue	1.6	456	114.1	128.8	7.5	Period 2
	3827.1	Es2L	yellow	1.9	564	95.4	102.7	28.1	Period 1
	3827.1	Es2L	orange	1.2	603	94.2	106.0	27.3	Period 1
	3827.1	Es2L	yellow	1.6	585	135.7	139.0	5.9	Period 2
H302	3758.9	Es2L	blue	0.7	473	102.6	115.4	28.1	Period 1
	3758.9	Es2L	blue	1.3	468	123.8	139.5	8.0	Period 2
	3758.9	Es2L	blue	1.0	497	129.9	140.6	7.8	Period 2
PS6	3856.8	Es2L	green	1.1	548	136.7	144.9	6.3	Period 2
	3856.8	Es2L	yellow	0.9	550	106.5	122.6	9.9	Period 2
	3859.4	Es2L	blue	0.9	470	96.4	107.9	28.5	Period 1
	4098.5	Es3M	green	1.4	512	144.2	166.2	4.3	Period 2
	4098.5	Es3M	blue	1.0	502	136.7	149.9	7.4	Period 2
	4100.4	Es3M	blue	1.3	473	91.2	101.2	31.3	Period 1
	4100.4	Es3M	blue	1.3	473	135.3	147.8	7.8	Period 2
	4100.4	Es3M	blue	1.3	473	139.2	154.5	6.5	Period 2
	4391.9	Es3L	yellow	2.0	551	92.5	105.3	32.4	Period 1
	4391.9	Es3L	blue	1.3	460	131.4	147.0	9.9	Period 2
	4568.0	Es3L	green	1.8	549	92.5	106.1	33.1	Period 1
	4726.9	Es3L	blue	1.2	507	105.3	111.5	33.3	Period 1
	4726.9	Es3L	blue	1.2	507	115.6	122.2	31.6	Period 1
	4726.9	Es3L	blue	1.2	507	139.2	149.4	28.2	Period 1

Figure 8. The comparison between oil charging events and ages and faults activity intensities in the Southwest Sag of Dongpu Depression. The red dashed circles can be regarded as oil charging events occurring in the same episode.

Figure 8. The comparison between oil charging events and ages and faults activity intensities in the Southwest Sag of Dongpu Depression. The red dashed circles can be regarded as oil charging events occurring in the same episode.

This indicates a strong spatiotemporal coupling relationship between fault activity, ANFSs development, and hydrocarbon accumulation processes in the Southwest Sag. Specifically, three sets of source rocks developed during the main fault extensional period. During the strike-slip extensional period, ANFSs formed above the sag’s source rocks, accompanied by sandstone reservoirs and faulted anticline traps. The first hydrocarbon accumulation was achieved via vertical fault transport systems [30]. Following regional subsidence during the Guantao Formation, fault reactivation triggered by neotectonic movement at the end of the Guantao Formation facilitated the second hydrocarbon accumulation event.

5. Discussion

5.1. Forming Mechanism of Antiform Negative Flowers

Some argue that the normal throw of ANFSs stems from the relatively weak compressional-shear action during the intermediate stage [11,14]. This results in the stratigraphic thrust amplitude being smaller than the throw generated during the extensional stage, thus allowing the faults to retain their normal fault nature. Under the influence of differential uplift forces, the strata on both flanks arch toward the center, eventually forming an anticline morphology. The core logic of this view is as follows: during the intermediate stage of tectonic evolution, the compressional-shear stress fails to fully offset the throw from the early extensional stage, causing the faults to remain normal faults. The strata on both flanks form an anticline due to differential uplift. This perspective emphasizes the continuity and inheritance of tectonic evolution, holding that the faults above and below the mirror plane are contemporaneous tectonic products, and the normal throw is a residual result of the throw from the extensional stage.

For instance, in the study of the Doseo Depression in the Southern Chad Basin [10], normal faults formed by early tensional-shear action were not fully reversed during the intermediate compressional-shear modification due to insufficient stress intensity, manifesting as the normal throw characteristic of “antiformal negative flower structures”. Another view attributes the normal throw to the “collapse” effect triggered by the thinning of mudstone detachment layers within the deformed strata. Low-strength mudstone detachment layers often exist in the horizons where ANFSs develop. During tectonic deformation, these layers undergo thickness reduction due to plastic flow, leading to “collapse” of the overlying strata due to gravitational imbalance and the subsequent formation of normal throws.

Both of the aforementioned views are based on the “longitudinal extensional faulted anticline” theory and fail to clearly distinguish the tectonic differences between “strike-slip extensional action” and “pure longitudinal extension”. The longitudinal extension model ignores the controlling effect of strike-slip stress on fault strike and throw, making it not applicable to this area.

Previous studies [31,32] have shown that during the Es₄ to Es₃, tectonic movement was dominated by extension in the NW 282°–SE 102° direction. Structures were segmented along fault strikes and linked via “structural transfer zones”. The Southwest Sag underwent intense extensional rifting, with most faults dipping Northwestward and half-grabens as the dominant structure. During the Es₂ to Ed interval, extension primarily occurred in the NW 350°–SE 170° direction, and the Southwest Sag experienced dextral strike-slip extensional deformation. This deformation was evident not only in the left-stepping en echelon arrangement of NEE-trending faults in the caprock, en echelon fault assemblages, and local compressional-tensional positive inversion [32], but also in the formation of several ANFSs within the sag. Following regional subsidence from Ng to the Quaternary, the area subsided under the load of overlying sediments, eventually forming ANFSs (Figure 3 and Figure 5).

5.2. Hydrocarbon Accumulation Pattern

Exploration practice has demonstrated that as “rises within sags” situated above the depression, ANFSs feature highly favorable source-reservoir-caprock assemblages and migration-conduit conditions, making them favorable zones for hydrocarbon enrichment [7,8,11].

In the study area’s Nanhejia ANFSs (Figure 3), Well He-3 underwent fracturing and oil testing in the Es₂^U. The test yielded 5.5 tons of oil, 22.2 tons of water, and 2348 m³ of gas per day, and the well is currently shut in.

In the Fangliji ANFSs (Figure 5), Well PS8, located in the lower part of the structure, was tested in the Es₃^L. It produced 10.36 tons of oil and 2425 m³ of gas per day, with a cumulative oil production of 355 tons. Well, F1, situated at the structural waist, was tested in the Es₃^L sub-member. It yielded 1.18 tons of oil and 3540 m³ of gas per day, with a cumulative oil production of 39.4 tons. Well F2, located at the structural height, achieved a maximum daily production of 99.1 tons of oil and 16,000 m³ of gas. Currently shut in, it has a cumulative production of 1305.8 tons of oil and 3,468,000 m³ of gas. Well F3, 839 m North of Well F2, targets the Es₂^U. It initially produced 15,000 m³ of gas per day with no oil. To date, it has been operated intermittently, with a cumulative gas production of 3,840,000 m³.

Three sets of source rocks, the Es₁, Es₃, and Carboniferous–Permian, are developed in the Southwest Sag of the Dongpu Depression. The Es₃ source rocks are dominated by Type 2₂–3 organic matter, while the Es₁ source rocks are primarily Type 1–2₂. The Carboniferous–Permian source rocks consist of coal-bearing Type 3 organic matter. Among these, the Es₁ source rocks are in the immature-low mature stage, the Es₃ source rocks have entered the mature-high mature stage, and the Carboniferous–Permian source rocks have reached the high mature stage. The Es₃ and Carboniferous–Permian sequences are the main hydrocarbon-generating units.

Oil and gas source correlation results indicate that the Es₂ crude oil from Well S1 and Well Zhao 4-3 in the Zhaozhuang structure is derived from the Es₃^M source rock, while the Es₂ crude oil from Well M6 originates from the deeper high-mature source rock of the Es₃, and the Es₂ crude oil from Well H3 in the Nanhejia structure is sourced from the Es₃ source rock. The crude oil group components from Well F2 and Well PS8 in the central Fangliji structure show genetic affinity with the Es₃^L source rock. The natural gas pay zones in the study area, distributed in the well interval of 3522.50–4187.40 m, include Es₂, Es₃, and C-P (Carboniferous–Permian) strata with relatively deep burial depths; these gases are characterized by methane content above 85.94% and low heavy hydrocarbon content, similar to those from Well HG-2 in the Huzhuangji structure and Well W23-40 in the Wenliu structure of the Dongpu Depression. Among them, the natural gas produced from Well F2, Well F3, Well M4, Well PS8, and Well H3 is dry gas, derived from deep high-mature source rocks of the Es₃ or Carboniferous–Permian, and may have a mixed-source characteristic [33].

Based on the previous analysis of the ANFSs and, in particular, their hydrocarbon accumulation processes in the Southwest Sag, this paper summarizes their hydrocarbon accumulation model (Figure 9). This model mainly includes the following two aspects:

(1)

There are two key hydrocarbon accumulation periods in the ANFSs belt of the Southwest Sag. The first period, corresponding to the early stage from the lower Es₂ to Ed (33.3–28.2 Ma), represents the main hydrocarbon accumulation stage. During this period, intense strike-slip extensional faulting connected the three sets of source rocks with the reservoir sandstones of the Es₂ [24], forming a hydrocarbon accumulation system characterized by multi-source supply and “lower-generation and upper-accumulation” beneath the regional caprocks of the Es₁ and Ed.

(2)

The second period occurred at the end of the Ng (9.9–4.3 Ma), corresponding to the neotectonic movement stage. By this time, all three sets of source rocks had entered the large-scale hydrocarbon generation period. With weakened tectonic activity, hydrocarbons accumulated near the source. In addition to forming “self-generation and self-accumulation” hydrocarbon reservoirs in the Es₃, hydrocarbons could also migrate along strike-slip extensional faults to accumulate in the Es₂ and Es₁.

Figure 9. The oil and gas accumulation model diagram of ANFSs in the Southwestern Sag.

Figure 9. The oil and gas accumulation model diagram of ANFSs in the Southwestern Sag.

6. Conclusions

• The Southwest Sag of the Dongpu Depression has experienced three evolutionary stages: extension in the NW 282°–SE 102° direction during the Sha-4 to Sha-3 Members, strike-slip extension in the NW 350°–SE 170° direction during the Es₂ to Ed, and subsidence during the Ng to Nm. These processes have resulted in the formation of a “rise within sag” positive structural unit known as the “antiformal negative flower structure (ANFSs)”, which thereby serves as a favorable site for hydrocarbon accumulation. The ANFSs thus becomes the priority exploration target designation for mature exploration areas in the Dongpu Depression.
• Based on microscopic fluorescence spectroscopy of crude oil and systematic fluid inclusion analysis, the hydrocarbon charging episodes and accumulation periods of the ANFSs belt in the Southwest Sag were determined as follows: there are 1–3 charging events corresponding to two hydrocarbon accumulation periods; The first accumulation period occurred during the strike-slip-extensional inversion stage (33.3–27.3 Ma) from the Es₂ to Ed, i.e., the formation stage of the ANFSs; The second accumulation period took place during the neotectonic movement stage (9.9–4.3 Ma), namely the subsidence stage of the ANFSs. This method combination offers technical reference for hydrocarbon charging period identification in similar structural belts, and a replicable way to improve reservoir evaluation accuracy in analogous settings.
• Two unfavorable factors restrict hydrocarbon enrichment in the Southwest Sag. First, lake water freshening during source rock deposition led to low total organic carbon (TOC) content. Second, reservoir densification was induced by compaction and calcite cementation. However, the ANFSs belt not only features a fault transport system connecting to underlying hydrocarbon source rocks but also benefits from improved reservoir quality due to fractured damage zones associated with flower-like faults. This facilitates hydrocarbon enrichment. As a result, it can be identified as a priority target for potential tapping in mature exploration areas.