Analysis of Hydrocarbon Enrichment in Tight Sandstone Reservoirs in the Eastern Baiyun Depression

Abstract

Based on the special geological background of the east and north slopes of the Baiyun Depression, the development conditions of Paleogene structure–lithology traps, the development conditions of high-quality reservoirs and the difficulty in characterizing the distribution characteristics are studied in this paper. It is concluded that the eastern Baiyun is located on the Baiyun–Liwan continental–oceanic large-scale intershell separation system, with a complex tectonic background and a tectono-sedimentary pattern of “fault and uplift interlocking and uplift and depression interphase”. The palaeo source sink system of the low bulge in the east of Yundong is restored, the favorable position of reservoir collective development and the favorable characteristics of reservoir–cap assemblage are clarified, and the paleo-geomorphology and sedimentary filling evolution law are clarified. Guided by the drive of oil and gas accumulation, three types of large and medium-sized structure–stratigraphic traps have been implemented in the eastern Baiyun system, including the convex inclined end, the restricted fault gully and the magmatic floor intrusion, and the corresponding oil and gas accumulation models have been perfected. By studying the structure, source and sink system and trap characterization of the eastern Baiyun basin, the development conditions and exploration direction of the large and medium-sized Palaeogene traps are systematically summarized.

1. Introduction

Because of the increasing difficulty of oil and gas discovery, structure–lithology and stratigraphic lithology reservoirs are very important in exploration in deep-water and deep-formation conditions [1,2]. At present, many large and medium-sized oil and gas fields composed of lithologic and structural hydrocarbon reservoirs have been discovered in China, especially in the Baiyun Depression [3]. The Baiyun Depression is a hydrocarbon-rich depression with good formation conditions for middle–deep lithology/stratigraphic traps and great resources and development potential [4,5]. However, it is still faced with relatively single reservoir locations, mainly small oil and gas fields, and the diagenesis of the middle–deep reservoir is affected by other factors [6]. Therefore, further research should be carried out. The upper member of the Enping Formation in the north slope of the Baiyun Depression is a large delta with continuous progradation, which belongs to the progradation reflection structure, and the source is mainly from the NW direction. The Baiyundong area has obvious mid-depression uplift landforms, and provenance analysis is difficult and needs to be combined with paleogeomorphology analysis, heavy minerals, debris components, seismic facies and seismic sedimentology analysis techniques. There are typical differences in the relationship between sedimentary facies types and the spatial allocation of sedimentary bodies in the two regions in different sedimentary periods. In addition, several different types of source–sink systems are developed in the study area, and there are essential differences among them.

The exploration of the Baiyun Depression can be divided into three stages. The first stage is 1979–2001, which is the initial study and exploration stage. During this stage, the major targets were the large structural traps in the Panyu uplift and Dongsha uplift, but no industrial discoveries were made. The second stage is 2001–2015. After the failure in the uplifts, we turned to the northern and eastern slopes near the hydrocarbon generation centers. In the northern slope, breakthroughs were made in the lower Miocene by drilling the structural traps controlled by antithetic normal faults, which also showed bright spot characters and several important gas fields were found. In the uplift area between the eastern center and the Yunli Uplift, middle–large gas fields were discovered in deep-water fans and shelf-edge deltas from structural and lithological traps, including the first large deepwater gas field, Liwan 3-1. In the northeastern slope, three important oilfields with light crude oil were also found in this stage. During this peak period, the most important medium–shallower traps were drilled. So, after that, we turned to new areas, such as the west and southwest, new layers, such as middle–deep layers, and new trap types, such as stratigraphic–lithology traps. Many wells were drilled, with no significant discoveries. Several wells targeted in the middle–deep layers of the Paleogene also failed due to high temperature, high compaction, uncertainties in the traps and accumulation.

The exploration of the Paleogene is still faced with several problems, such as the fact that there are relatively few reservoir locations, mainly small oil and gas fields, and the diagenesis of the middle–deep reservoir is affected by other factors [6]. In the Baiyun Depression, relatively few wells have been drilled into the target layer, the burial depth is generally large, and the reservoir diagenesis has undergone a long history of diagenesis. The diagenesis of the reservoir is affected by various geological processes. The difficulties in analyzing and characterizing the structural and stratigraphic lithologic traps are as follows: the thickness of the target sand body is often less than the vertical resolution of the seismic layer, and the boundary of the seismic reflection abnormal body is often fuzzy. However, through the coupling analysis of a high-resolution 3D seismic data source–sink system, seismic sedimentological analysis technology, combined with the existing drilling data and the analysis of structural paleogeomorphic background, source tracing, mother rock type and other macro factors, we can better study, analyze and predict the potentially favorable stratigraphic lithology traps.

Based on the theory of the “source–sink” system in continental basins, guided by sequence stratigraphy, modern sedimentology, seismic sedimentology and reservoir geology, and constrained by a high-frequency tectonic-sequence stratigraphic framework, a structure–sequence stratigraphic framework is established by means of a tectono–palaeo-landform–slope break zone analysis and the quantitative identification of sequence stratigraphy by earthquakes in wells. The sedimentary filling evolution and structure–lithology/stratigraphic trap development models of different tectonic zones in the target area are studied.

In this project, two targets on the north slope of the Baiyun and the eastern slope of the Baiyun are studied: dividing the structure–sedimentary units in the target area, characterizing complex structure–lithology trap sand bodies in the North slope of the Baiyun, analyzing the coupling of source and sink system in the east rift period, evaluating and predicting the physical properties of middle and deep quality reservoirs and evaluating the favorable Tibet-forming zones and stratigraphic lithology traps. In view of the different core problems in the two research targets, a series of comprehensive seismo-sedimentary–petrological studies based on high-resolution 3D seismic data and supplemented by limited reservoir physical property testing and analysis data are required.

2. Geological Settings

2.1. Baiyun Depression Structure

The Pearl River Estuary Basin is located on the vast continental shelf and continental slope margin in the north of the South China Sea, with Taiwan and the Hainan Islands as the borders in the east and west, respectively. It covers an area of about 17.5 × 10⁴ km² and is one of the largest oil-bearing basins in offshore China with a NE–SW distribution. Located in the southern margin of the South China continent, the Pearl River Estuary Basin is a Cenozoic oil-bearing basin formed on the base of complex folds in Paleozoic and Mesozoic under the complex continental dynamic background under the influence of the intersection of the Pacific plate, Indian plate and Eurasian plate [7,8,9].

The Pearl River Estuary Basin is dominated by the NE-trending fault system, which, together with the NWW-trending fault, controls the uplift and depression pattern of the basin. It has a north–south zoning and east–west zoning structure and is mainly divided into five tectonic units (Figure 1): the northern uplift belt, the northern depression belt, the central uplift belt, the southern depression belt and the southern uplift belt [10,11,12]. The northern depression zone is composed of the Zhu-I Depression and Zhu-III Depression, and the Zhu-II Depression is in the southern part of the Pearl River Estuary Basin.

The Baiyun Depression is in the second basin of two oil-bearing belts in offshore China, in the northeast of Zhu II Depression in the Pearl River Estuary Basin, and is a transitional gas-rich depression basin located in the continental slope area of the northern continental margin of the South China Sea, is a long-term stable sinking negative tectonic unit [11,13,14,15,16]. The Baiyun depression has an area of more than 20,000 km² and a water depth of 200–3000 m. About 70% of the depression has a water depth of more than 500 m. The maximum sedimentary thickness of the Cenozoic exceeds 11 km, making it the largest depression in the Pearl River Estuary Basin and the sedimentation and sedimentation center of the basin [17,18,19].

The Baiyun depression is generally a compound graben structure separated by low protrusions, with an NEE strike, and the Panyu low uplift and slope belt on its north side. The west side is a Yunkai low bulge and steep slope zone; the southern part is a Yunkai low bulge, the southern uplift zone and the fault step zone. The east side is the Dongsha uplift, and a low uplift is developed in the depression (Figure 1). Overall, it can be divided into the Baiyun main depression (about 4800 km²), Baiyun East depression (about 512 km²) and Baiyun West depression (about 900 km²), in which the north side of the main depression of the Baiyun depression is a gentle slope zone, the south side is a steep slope fault step zone, and the fault structure is not completely controlled by the fault center overall [14,20].

During the sedimentary period of the Enping Formation, the Baiyun Depression was in the late stage of rift evolution, and the fault features were obvious, with a wide “half graben” feature of “steep in the south and gentle in the north and faulting in the south and overrunning in the north”. Under the joint action of fault retention and deep thermal attenuation, the Baiyun Sag settled as a whole, and the sedimentary range expanded continuously. It related to the Liwan Sag to the southeast, and the sedimentary background was shoreline shallow sea and limited sea, which ended the sedimentary pattern of “depression separation” during the sedimentary period of the Wenchang Formation.

2.2. Baiyun Depression Sedimentary Strata

Controlled by regional tectonic evolution, the Baiyun Depression in the Pearl River Estuary Basin is filled with sediments from the lake before the sea and is a continental deposit from Paleocene to early Oligocene, including the Shenhu Formation (Tg-T90), Wenchang Formation (T9-T80) and Enping Formation (T80-T70) [21,22]. Since the Late Oligocene, there have been Marine and continental transition facies and Marine sediments, including Zhuhai Formation (T70-T60), Zhujiang Formation (T60-T40), Hanjiang Formation (T40-T20), Yuehai Formation (T20-T10), Pliocene Wanshan Formation and the Fourth Series (T10-T0) (Figure 2).

The Shenhu Formation was deposited in the Shenhu Movement period of the Paleocene. It is an intermontane basin deposit at the early stage of the rift, with river and alluvial fan deposits at the early stage in the basin, mid–deep lacustrine facies deposits at the late stage, and fan delta and alluvial fan at the basin margin. The stratigraphic distribution of the Shenhu Formation is limited.

The Wenchang Formation, deposited after the Zhuqiong movement, is widely distributed in the basin, mainly developing semi-deep lake-to-deep lake mudstone with a thickness ranging from 1000 m to 2000 m, and is the main source rock formation in the Pearl River Estuary Basin. Because the basin continued to settle after the second act of the Zhuqiong Movement, the Enping Formation has a wider development range than the Wenchang Formation [23,24,25]. It is mostly lacustrine and swamp facies deposits, a coal-bearing system, and one of the main source rock systems in the Pearl River Estuary Basin.

The top and bottom boundaries of the Zhuhai Formation are T70 and T60, respectively, which correspond to the South China Sea movement and Baiyun movement. After the South China Sea Movement, the Pearl River Estuary basin entered the depression period from the fault depression period, the basin had a large area of subsidence, seawater began to invade, and most of the basin changed into the coastal shallow sea and delta environment of the transition phase, which is one of the main reservoir systems in the basin.

The Zhujiang Formation was deposited after the Baiyun movement, when the basin entered the depression stage, the second-order sea level continued to rise, and the shelf slope break migration caused the Baiyun depression to enter the deep-water environment. The Zhuhai-I Depression and the central uplift zone are shallow Marine environments, the old Pearl River Delta develops, and the Dongsha uplift develops a carbonate platform, which is also one of the main reservoirs in the basin.

During the sedimentary period of the Hanjiang Formation, it was mainly composed of delta and Guanghai shelf deposits, during which many large-scale transgressive regressions occurred. The Dongsha Movement had a profound influence on the sedimentary system, and the lithology was mainly sandstone, mudstone and siltstone.

During the sedimentary period of the Yuehai Formation, Wanshan Formation and the Fourth Series, the sea level continued to rise, and the environment of the delta, shallow sea shelf and Guanghai mainly developed.

2.3. Source Rock Formation and Characteristics in Baiyun Depression

The Baiyun depression is characterized by lacustrine facies of the Eocene Wenchang Formation, transitional facies of the Lower Oligocene Enping Formation and Marine source rocks of the Upper Oligocene Zhuhai Formation. The development of lacustrine source rocks in the Wenchang Formation is controlled by fault basins, which are mainly distributed in the margin of depression-controlled faults. The source rocks of the Enping Formation include delta coal measures and shallow Marine mudstones, which are mainly distributed in depressions and troughs. The distribution of natural gas in the Baiyun sag is controlled by source rocks. The oil and gas found on the north slope of the Baiyun Sag is basically the same origin as coal-derived oil and gas, and the Ro value of natural gas maturity is in the range of 1.5%~1.8%, mainly from the coal measure source rocks of the Enping Formation. A number of gas fields, such as Panyu 30-1, have been found, and the gas reservoir is highly filled. The gas characteristics of Liwan 3-1 gas reservoirs in the south of the Baiyun Depression are different. The gas is wet, and its maturity is slightly lower than that of the natural gas on the north slope, and the maturity Ro value is about 1.5%. The natural gas mainly originates from the Marine mudstone source rock of the Enping Formation, and the condensate may also come from the Marine source rock of the Enping Formation [26].

The average total organic carbon content of deep lacustrine mudstones in the Wenchang Formation is 2.94%, the average chloroform bitumen “A” content is 0.23%, and the H/C atomic ratio of kerogen is mostly 1.5~1.0 [27,28]. The organic matter types are mainly II₁ type, and some are I type, indicating that the Wenchang Formation has a good source rock. At the same time, the hydrocarbon generation potential of the source rocks of the Wenchang Formation has been confirmed. For example, crude oil rich in C₃₀-4 methylstane from the Wenchang Formation was found in well LGH-1, which reflects that the source material of the oil is middle–deep lacustrine algae and plankton in a reducing environment [28].

A large set of Marine mudstones is developed in the Zhuhai Formation. The TOC values of the mudstones in the Baiyun Depression are mostly between 1.0% and 1.5%, and the S₁ + S₂ values are mostly between 2 and 4 mg/g. The hydrogen index values and the maximum pyrolysis peak temperature chart indicate that the mudstones in the Zhuhai Formation are type II₂ and the reflectance R_o values of the whole petrovite range from 0.43 to 0.53%. These data show that the Marine source rocks of the Zhuhai Formation in this area are a set of effective source rocks.

2.4. Reservoir–Cap Combination Characteristics and Trap Types in Baiyun Depression

Four sets of favorable reservoir–cover assemblages developed vertically in the Baiyun Depression, namely, the lacustrine delta assemblage of the Wenchang Formation, the Marine and continental transitional delta assemblage of the Enping Formation, the continental shelf delta assemblage of the Zhuhai Formation and Zhujiang deep-water fan assemblage of the Zhujiang Formation.

The sand–mudstone association in the lacustrine delta of the Wenchang Formation and the sand–mudstone association in the Marine and continental transitional delta of the Enping Formation have lower reservoir physical properties due to their deep burial depth and high compaction degree. Through comprehensive reservoir analysis of multiple exploration Wells on the north slope, it is found that the sandstone porosity is obviously affected by the compaction of buried depth. The greater the buried depth, the smaller the porosity, and if the depth is greater than 3700 m, and the porosity drops below 10%. However, the delta sand body, the middle–deep lacustrine mudstone and the shallow Marine mudstone are finger-like interactions, which can form effective reservoir-cap assemblages. Source rocks in the Niger Delta of Nigeria, the large shallow water braided river delta of North Kanafon of Australia, the Daqing Changyuan delta and the Huagang Formation delta of Xihu Depression are in direct contact with reservoir rocks, all of which have the characteristics of self-generated and self-stored reservoirs [29,30,31,32]. Judging from the breakthrough of deep natural gas exploration in China in recent years, such as Kuqa depression and Xihu Depression, deep traps have a high degree of charging [19,33,34,35], the development of sweet reservoir, large reserves and other characteristics, from the perspective of reservoir formation, the large delta of the Enping Formation–Zhuhai Formation is large in scale and has superior reservoir conditions. Therefore, the deep structure–lithology trap in the Baiyun Depression is of great value not only as a potential strategic exploration direction but also as a potential exploration target.

The Zhuhai Formation is a shallow Marine shelf environment, a large shelf delta developed in the Baiyun Depression during this period and the delta advanced southward to the south side of the Baiyun depression, where a slope turbidite fan reservoir formed by front collapse may also have developed [13]. The sand body of the shelf delta has the characteristics of large area distribution. Drilling data show that the average porosity of the delta sandstone of the Zhuhai Formation is 15%~20% [36], with high porosity and good reservoir physical properties. In the highly constructive delta, the pre-delta mudstones with rapid sand–mudstone interlayer accumulation and thick layer development during the advancing process easily form mud mounds, diapirs, etc., forming good reservoir–cap assemblages and lithologic traps. Moreover, a large-scale transgression occurred in the middle of the Zhuhai Formation, forming widely distributed mudstones, which can be used as regional cap beds.

During the Zhujiang Formation and Hanjiang Formation period, the Baiyun depression was strongly settled, forming a deep-water environment on the continental slope, and the slope break zone moved northward. The Pearl River deep-water fan system developed in the continental slope area, the deep-water fan developed in the low area inside the sequence and the vertical superposition was multi-stage. Because the Pearl River Delta is dominated by sandstone, the deep-water fan caused by gravity flow at the front of the delta is also mostly a sandy reservoir. The deep-water fan sand body revealed by well LGH-2 has a porosity of up to 27%, which is a high-quality reservoir. Moreover, the deep-water fan is surrounded by extremely thick deepwater mudstone, making it a good reservoir cap combination [5,27].

Under the influence of four sets of reservoirs and cap assemblages, the trap types of the Baiyun depression are the lithologic trap of the deep-water fan of the Zhujiang Formation, the tectonic trap of the shallow Marine shelf delta of the Zhuhai Formation, the tectonic–lithologic trap and litho-stratigraphic trap of the transitional facies delta of the Enping Formation, the tectonic trap of the lacustrine facies delta of the Wenchang Formation and the litho-stratigraphic composite trap.

3. Structure and Multiple Reconstruction Characteristics of Eastern Baiyun Depression

3.1. Structural Characteristics and Tectonic Belt Division

In the Lower Wenchang period, the Baiyun depression was generally a balanced rift period controlled by NWW and NEE faults, and there were several depositional centers in the eastern part of Baiyun. From south to north, it developed successively: high angle isotropic fault zone, isotropic and reverse regulated tectonic zone, reverse fault step tectonic zone and isotropic fault step tectonic zone.

During the Upper Wenchang–Enping Formation, the eastern Baiyun was generally in the dissociation rift period and fault depression transition period, which formed the main depositional center pattern transformation in the eastern Baiyun [37]. From south to north, syn-reverse regulatory tectonic belt, syn-reverse graben tectonic belt and syn-fault tectonic belt are developed successively. The structural characteristics of the Enping Formation depression have obvious inheritances to the upper Wenchang period and gradually transition to the fault depression transition period. With the continuous action of magmatic floor encroachment, the sub-depression is subjected to horizontal extension–dissociation and continuous extinction.

From south to north, syn-reverse regulatory tectonic belt and syn-reverse graben tectonic belt develop, and the fault-control effect is obviously weakened.

3.2. Multiple Transformation Characteristics of “Extension-Detachment-Differential Magma Penetration”

The concept of a detachment fault, first proposed by Davis in 1980, is the floor fault of an imbricated thrust, applied to extensional structures and defined as “a large low-angle normal fault or extensional fault between a crystalline metamorphic basement complex and an overlying sedimentary cover”. Lister, Davis et al. also proposed that the detachment fault is a near-horizontal plow-type large normal detachment fault, which is accompanied by a metamorphic core complex. Under the extensional background, during the vertical downward cutting of normal faults, the dip angle of the fault is slow to nearly horizontal due to the downward cutting to the lower crust or the upper mantle ductile layer, and the upper wall of the fault moves in the form of slippage. This type of fault is called the detachment fault.

3.2.1. Extension-Detachment Fault

According to the early studies of extensional structures in the basin area of the United States by Lister, Davis and other scholars, the detachment fault is a near-horizontal plow-type large normal slip fault, which is accompanied by a metamorphic core complex. The upper and lower detachment disks divided by detachment faults have different deformation characteristics. The upper detachment disk is a brittle deformation zone composed of an imbriculate normal fault series or domino normal fault series, while the lower detachment disk is a metamorphic core complex composed of mylonite, gneiss, etc. Therefore, under the extensional background, during the vertical downward cutting of normal faults, the dip angle of the faults is softened to nearly horizontal due to the downward cutting to the lower crust or the upper mantle ductile layer, and the upper wall of the faults moves in the form of slippage. This type of fault is called a detachment fault. The detachment fault may be a low-angle fault, or it may be a shovel or flat fault.

The Upper Wenchang period in the east of Baiyun was mainly in the dissociation depression stage, which was affected by horizontal extension–dissociation, and mainly developed a two-stage dissociation system. As shown in Figure 3, the primary separation plane is the low-angle white cloud main fault (MF), which presents obvious characteristics of long horizontal extension distance, large fault scale and large separation depth. The second-order detachment surface is characterized by multiple low-angle faults (YDFs) with a small scale and detachment depth, which converge on the main Baiyun fault section. The structure and structure–sedimentary strata patterns of the depression in eastern Baiyun are mainly controlled by the first-order disassembly system, while the second-order disassembly system is more likely to regulate and control the local structural features of the depression.

3.2.2. Differential Magma Penetration

The widely developed volcanic activities in and around the South China Sea have recorded important information about the evolution of the South China Sea and related deep dynamic processes, and the study of igneous rocks is of great significance for understanding the mantle evolution, crust–mantle interaction, seafloor spreading and the formation of the deep-sea basin in the South China Sea [38]. Since the late Cenozoic, the passive continental margin of the South China Sea has experienced frequent and intense magmatic activity due to the superimposed effects of various factors.

According to the detailed interpretation of drilling data and 3D seismic data, the magma floor invasion occurred in different degrees during T83-T70 in the eastern Baiyun Depression. Subsequently, the magma eruption mainly developed the overflow phase of volcanic eruption during the South China Sea movement, and the development period was relatively concentrated. The magma of volcanic eruption effusive facies mainly presents the form of a zonal distribution in the Baiyun depression, and the occurrence of effusive facies magma in different regions has certain similarities, but the development times, profile characteristics and development forms are different to some extent. The strata lifted upward, thinned down and contained more upper super cusps in the interior, forming a mantle structure, which reflected the syn-sedimentary nature of volcanic activity. If the formation caused by volcanic magma or volcanic activity rises above the surface and is exposed upon the lake’s surface, it provides a source to the surrounding area and can form a certain prograde reflection structure, indicating the direction of the source.

Because the Baiyun depression is located in the crustal thinning zone, magma intrudes into the strata from the mantle or erupts to the surface and then condenses into igneous rocks. This study shows that the volcanic rocks’ development age is related to the crust and mantle equalization, the northward migration of the shelf slope line, the strong subsidence during the depression, and the southward transition of the south ocean mid-ridge. In the early stage, it was mainly a high-angle fault depression, and, in the later stage, the strata were uplifted and separated into low-angle fault depositional centers and migrated to the main Baiyun depression.

The main period of Paleogene magmatic floor encroachment in eastern Baiyun is T83-T70 (Figure 4). Relatively speaking, the development age of extrusive magma is mainly concentrated in the period from 23.8 Ma to 17.5 Ma, while the magma that developed before and after 18.5 Ma developed in a large range in the three main areas of the Baiyun depression, showing a near east–west distribution in space. Under the influence of magmatic floor invasion, magmatic activity is frequent, and the strata of T80 and below show obvious deformation characteristics. Among them, the magma floor invasion is the most intense in the LGH-2 well area, and the accommodating space and formation thickness in the area with strong magma floor invasion are small. In contrast, other regions with weak magma floor invasion have larger space and thickness.

3.3. Main Tectonic Regions and Characteristics of Magmatic Floor Encroachment

The Baiyun depression and its adjacent areas are mainly dominated by piercing structures, most of which are developed along faults. Magmatic floor encroaching structures are mainly developed in the eastern region, and structural geomorphological transformation of bulges or uplifts in the study area is carried out along the fault weak zone or the depression center. A small number of magmatic floor encroaching structures are also developed in the Yunkailow uplift and Baiyun Nanwa area. Generally, the Mesozoic basement that has not been transformed by volcanic magma floor encroaching usually presents a chaotic reflection structure (Figure 5). The magmatic diapiric in the eastern Baiyun basin developed along the depression margin, mainly in the following structures (Figure 5): LGH (A), LZY (B), WXD (C), providing potential conditions for the implementation and evaluation of related types of traps. The upward migration of deep plastic material and its tectonic action on overlying rock is called Diapirsm, and the tectonic deformation formed by the floor invasion is called the floor invasion structure. Common floor encroachment structures in sedimentary basins are formed by the floor encroachment of salt rock, gypsum rock and mudstone, while magmatic floor encroachment structures are formed by floor encroachment of magmatic rock. They mainly develop in sedimentary strata in the form of intrusion and belong to a type of high-temperature floor encroachment. The Baiyun depression and its adjacent areas are mainly dominated by piercing structures, most of which are developed along faults. The main time of magma floor invasion is determined by the contact relation between the magma body and the surrounding strata, the thickness of overlying strata, the thickness correlation of both sides and the upper floor invasion fault [17,18]. The magma floor encroaching structure of the Baiyun depression mainly developed in the eastern region, and a small number of magma floor encroaching structures also developed in the Yunkai low bulge and Baiyun Nanwa area. The development time is the Nanhai Age and Dongsha Age, and the development scope of the Dongsha Age is wider.

The volcanic or magmatic floor transgression in the eastern Baiyun mainly developed in the periphery of the Yundong low bulge and Yunli high, and the structural geomorphological transformation of the bulge or high in the study area was carried out along the fault weak zone or depression center. For example, the small bulge in the sub-depression of the northern margin of the Yunli Low bulge was generally modified by volcanic activities. In general, the Mesozoic basement that has not been transformed by volcanic magma floor intrusion usually presents a chaotic reflection structure (Figure 5), while the bulges or uplifts that have been transformed by magma floor intrusion show a “layered” structure of magma outflow flow, showing a certain dome-like strong reflection, and the strata on both sides of the uplift show a drag phenomenon, indicating a potential metamorphic core complex development area (such as WXD uplift). In addition, the locally developed hidden volcanoes lead to strong uplift and folding of the strata, plastic swelling and shrinkage deformation can be seen, gravity slip faults developed in the late stage and a few crown faults developed on the top.

The three structures LGH (A), LZY (B) and WXD (C) strongly reformed the strata, resulting in the present basement interface unconsolidated and the reformed stratigraphic structure pattern of the lower Wenchang, providing potential conditions for the implementation and evaluation of related types of traps. The interior of the magma intrusion is generally chaotic or blank, and the upper part has associated faults. The influence area of magmatic floor encroachment development is large and widely distributed, which often has a great influence on the structure of the depression, the response characteristics of structure–sedimentary filling and the migration and evolution of sedimentary centers in the study area.

Among them, the LGH and WXD have a large scale of floor invasion, and the horizontal separation and extension distance are far (13.3–17.8 km), showing obvious low angle characteristics (15.6°~23.7°). The LZY magma floor transgression is small in scale (Figure 5), and the horizontal separation extension distance is short (5.6–8.3 km), mainly characterized by a medium–high angle (32.7°~45.8°). Therefore, the intensity of magma floor encroachment is different, and, as shown in the figure, the differential magma floor encroachment mainly leads to the overall decrease of the accommodative space, and, the more intense the magma activity, the smaller the accommodative space; on the contrary, the accommodative space is larger, but the overall trend is decreased. The occupied space due to magmatic activity reduces the thickness of the strata available for deposition, and the main source area changes, adjusting the superposition pattern of lithology combination and favorable configuration of reservoir and cap combination with good conditions for subsequent formation.

In the Lower Wenchang period, the eastern Baiyun, as a whole, belongs to the equilibrium rift period and is controlled by two sets of high-angle faults, namely, the NWW and NEE, which control the uplift structure. The overall structural–sedimentary filling model is as follows: the development of multiple sedimentary centers, the coexistence of main and secondary depressions, the overall plane presented a sedimentary pattern of uplift and depression, the development of a fan delta, braided river delta and river equal multiple sedimentary facies coexistence.

In the Upper Wenchang period, due to the strong horizontal extension, the eastern Baiyun, as a whole, belonged to the dissociation rift period, which was mainly the horizontal extension–dissociation process and suffered from the magma floor invasion with different intensity, which transformed the structure of the depression, resulting in the gradual disappearance of the small depression and the migration of the depositional center to the main Baiyun depression.

3.4. Tectonic Sedimentary Response

The overall structure–sedimentary filling pattern is as follows: multiple sedimentary centers migrated and gradually evolved to form a single sedimentary center, small uplift structures and sub-depression structures gradually transformed to a single center of the main depression of the white cloud, small uplift structures gradually disappeared, the area of the sedimentary facies in the middle and deep lake gradually decreased, the protruding edges gradually submerged underwater, and braided-river delta and shoreline bar developed.

During the Enping Formation, there was obvious inheritance with the upper Wenchang period, and the study area was transitioning to the faulting transition period. At the same time, the study area was still subjected to a certain degree of horizontal extension-dissociation and relatively weak magmatic floor invasion, and the secondary depression in eastern Baiyun continued to die out and the depositional center continued to migrate to the main depression. In the eastern Baiyun, the small-scale sub-depression is further reduced, the overall number of upper depressions continues to decrease and the depositional center continues to migrate from the secondary depression to the main Baiyun depression, gradually forming a single central deposit. The water body in the study area is relatively shallow and is mainly composed of braided river delta and shoreline shallow lake facies.

The eastern Baiyun suffered from magmatic floor encroachment with different intensities, and the degree of floor encroachment results in a great difference in the distribution pattern and distribution of the sedimentary system in the study area: the magmatic floor encroachment intensity is large, and the direct erosion and denudation are the source; the intensity of magma floor invasion is from medium to small, which affects the degree of pinching of preexisting sediments and regulates the transport channels of Upper Wenchang and Enping sedimentary systems. At the same time, based on the study of the tectono-sedimentary response under complex fault-uplift joint control in the eastern Baiyun, it can be clear that the Lower Wenchang tectono-lithologic trap is mainly to find the large sedimentary development area under the control of large sources and sinks in the background of the uplift and depression. The upper Wenchang tectonic-lithologic trap is mainly to find the large sedimentary development area controlled by a large source and large sink and the lower Wenchang sedimentary transformation area under the background of the demolition–magmatic floor invasion.

Since the Yundong low bulge was subjected to strong magma floor encroaching during the Chang–Enping Formation period above, the strata were then subjected to sedimentary uplift and exfoliation, and the area at the southern inclined end of the bulge was gradually submerged underwater. Due to the strong water energy, it was conducive to the formation of a shoreline beach bar facies storage collective.

4. Basic Characteristics of Paleogene Reservoir Formation Elements in the Eastern Baiyun Depression

4.1. Development Characteristics of Source Rock

The evaluation of source rock quality mainly includes three aspects: abundance, type and maturity of organic matter. The abundance of organic matter directly determines the hydrocarbon generation capacity and quantity of the source rock. The type of organic matter is a parameter to measure the hydrocarbon production capacity of organic matter and determines whether the product is mainly oil or gas. The analysis shows that the source rocks of the Wenchang Formation in the east of Baiyun Juwa reached the mature stage in the late deposition of the Zhuhai Formation (about 23 Ma).

4.2. Reservoir Development Characteristics

Oil and gas accumulation and accumulation in sedimentary basins are a series of physico-chemical processes of oil and gas generation, migration and accumulation in dynamic fields controlled by temperature, pressure and potential energy. The source of clastic material in the Paleogene reservoir in the Pearl River Estuary Basin is diverse, complex and variable. Based on the above analysis, the sources of the favorable reservoir sand bodies of the Paleogene in the eastern Baiyun are mainly palaeouplift and Yundong low bulge. After weathering denudation and moving along certain transport channels, the corresponding deltas are formed in the slopes and fault channels at the edge of the uplift or uplift, and a certain scale of shoreline beach bar facies storage collective is developed in local underwater low uplift.

The paleogene reservoir studies revealed by the existing wells (LGH-4, LGH-6, and WXD-5) in eastern Baiyun are mainly located at the margin of the Yunli low uplift and the western margin of the Yundong low uplift. Overall, the location distribution of the three exploration wells is relatively dispersed, among which WXD-5 is distributed in the western margin of the Yundong low uplift and reveals the filling deposit of faultditch 1, while LGH-4 and LGH-7 are distributed in two different structures in the northern margin of the Yunli low uplift.

According to the relevant sample research of drilling, the above three wells are mainly lithologically composed of lithologic quartz sandstone and lithologic sandstone, but there are great differences in rock types, grain size and diagenetic types (Figure 6).

The LGH-4 well is located on the east side of the north margin of the Yunli low uplift, and its lithology is mainly uneven-grained sandstone with gravel, low content of mud base and poor sorting. The Wenchang Formation can be found kaolinite-rich gravel sandstone. LGH-7, located in the west of the north margin of the Yunli low uplift, is mainly composed of clastic sandstones with large differences in the content of the mud matrix and poor sorting. WXD-5 is located on the western margin of the Yundong low bulge, and its lithology is mainly quartz sandstone with a high content of mud matrix and great sorting difference. Wenchang Formation can be seen in tuffaceous solution microporous sandstone. Due to the development of volcanic tuffaceous matter at the bottom of well WXD-5, volcanic tuffaceous matter easily blocks the roar channel. Therefore, although the porosity of the Wenchang Formation in well WXD-5 is large, the permeability is low, showing the characteristics of low permeability to ultra-low permeability. Relatively speaking, the reservoir property of the Enping Formation is relatively good, showing certain characteristics of “middle porosity and permeability”.

Fractures can be seen in the eastern Baiyun basement, but there is also chlorite development. Chlorite can effectively inhibit the occurrence of quartz secondary enlargement and reduce the filling pores of cement, which plays a role in improving the reservoir. The autologenic chlorite requires a relatively iron-rich sedimentary environment. It is clear from the previous comprehensive regional geological study that there were relatively common volcanic eruptions in the eastern Baiyun during the Upper Wenchang and Enping periods, and the pyroclastic rocks and tuffaceous rocks were relatively developed, which provided corresponding favorable basic conditions for the development of the chlorite rim.

Through analysis, the target of the WXD-5 region is a high-energy sedimentary facies zone, which has the basic conditions for the development of a high-quality reservoir. WXD-6 well drilling reveals that the porosity of the coarse-grained sandstone reservoir in the EP Formation can reach 14.4%, and the permeability can reach 18.1 mD.

The WXD-7 region mainly developed a braided river delta sedimentary system restricted by fault channels, which is conducive to the development of a large braided river delta system under the geological factors of abundant source supply, large space for formation deposition, high energy hydrodynamics, long-distance transport, easy formation of low mud content and a high maturity sand body. The wells near the WXD-7 region reveal that the porosity of the fault gully traps reservoir in the eastern Baiyun Depression is about 15–20%, and the permeability is about 10~20 mD.

4.3. Development Characteristics of Reservoir and Cap Combination

The Yunli uplift and Yundong low uplift have developed a few favorable nose-shaped structural belts, which are located in the east wing of the main depression of Baiyun and have developed a number of secondary depressions around them and have rich hydrocarbon source potential. As long as effective reservoir and cap combinations can be implemented and located on favorable oil and gas migration paths, related traps with exploration potential can be formed. Through the oil and gas display revealed by the existing drilling and lithology combination analysis, the advantageous oil and gas migration and accumulation range (the area circled by the dotted line) in the eastern Baiyun area can be preliminarily defined, that is, the advantageous accumulation driving range (Figure 7).

The western side of the Yunli low uplift and the western side of the Yundong low bulge in the eastern part of Baiyun are adjacent to the east side of the main depression of Baiyun and the related source rocks of the secondary depression and are on the favorable path of oil and gas transport, and the transport distance is relatively close, so they belong to the advantageous source transport and accumulation path and have high hydrocarbon migration and accumulation potential. Through the analysis of the relevant cross-well seismic profile, the seismic facies characteristics of the sand–oil-rich lithologic assemblage are mainly medium–strong amplitude-to-intermittent reflection, while those of the mud-rich lithologic assemblage in the eastern Baiyun area are mainly medium–weak amplitude-to-continuous reflection.

The LGH-8 well site showed mud-rich deposits and, occasionally, thin sand bodies developed deposits. At the LGH-7 well location, the lower Wenchang Formation develops a large set of sandstone, the upper Wenchang–Zhuhai Formation develops a thick mudstone and mud-sand combination, the sand body is relatively thin, and the upper Wenchang Formation can be seen as an oil layer. At the LGH-4 well location, there is a large set of sand conglomerates in the lower Wenchang Formation, but no oil and gas show. The well LGH-9 developed large sections of mudstone in the T70-T80 period, abnormal gas measurement in the Enping Formation period, and overpressure accumulation in the Lower Wenchang period. The well WXD-5 shows oil formation in both the Enping Formation and Wen3 Member. The Enping Formation is well developed, while the Wen 3 Member is poorly developed.

In summary, the relevant drilling results in the eastern Baiyun area reveal the following: (1) The Lower Wenchang developed thick sand conglomerate (LGH-4 and LGH-7), but there was no oil and gas display—high-quality sand bodies near hydrocarbon sources or favorable transport high locations should be sought. (2) The Upper Wenchang–Enping Formation: thin sand formation (WXD-5 and LGH-7)—thick and high-quality reservoir sand bodies should be implemented within the effective range of reservoir formation drive. Therefore, the most restricted oil and gas exploration in the eastern Baiyun area is the lack of high-quality sandstone reservoirs located in the favorable accumulation driving zone, which is also the focus of this study.

5. Optimal Selection and Main Distribution of Favorable Trap Targets in the Eastern Baiyun Depression

5.1. Target Optimization Criteria

(1) There is a background of large source and sink favorable for sand body development; (2) the seismic reflection structure and boundary conditions are clear; (3) storage cover combination conditions are superior; (4) it is located in the direction of advantageous oil and gas migration and accumulation; (5) the structure–stratigraphy–lithology traps can be formed on a large scale.

5.2. The Target of WXD-5 Region

5.2.1. Basic Elements of Trap

The main favorable target strata of the WXD-5 region are the Upper Wenchang–Enping stage, which is a medium and large lithologic trap controlled by the convex inclined end. The evaluation objectives of this trap are (1) realizing the potential of the Wenchang Formation and (2) expanding the exploration of buried hills. The main risk is the physical condition of the reservoir.

5.2.2. Typical Seismic Profiles and Seismic Attributes

Through the analysis of the relevant cross-well seismic profile, the seismic facies characteristics of the sand–oil-rich lithologic assemblage are mainly medium–strong amplitude-to-intermittent reflection, while those of the mud-rich lithologic assemblage in the eastern Baiyun area are mainly medium–weak amplitude-to-continuous reflection. The minimum amplitude attribute map of the target layer shows that the target area has developed beach bar sand bodies formed by lake wave transformation, transportation, and re-sedimentation, with high-energy facies zones and improved physical properties (Figure 8). The seismic profile exhibits strong amplitude characteristics, with sand bodies overlapping the basement in the southeast direction, thinning the sand bodies and weakening the amplitude. The amplitude attribute plan can clearly observe the changes in amplitude strength, which can predict the distribution range and boundaries of the sand bodies.

The two typical seismic profiles of the WXD-5 region target can clearly show the relevant structural characteristics of the shore-bar reservoir collective (Figure 9). The seismic profiles in both parallel and vertical shoreline directions show abnormally strong amplitude reflection, and the in-phase axis distribution can be seen inside. The target system is a type of shoreline beach bar with a convex inclined end and underwater low uplift developed during the sedimentary period of the Upper Wenchang and Enping formations.

Fractures can be seen in the eastern Baiyun basement, but there is also chlorite development. Chlorite can effectively inhibit the occurrence of quartz secondary enlargement and reduce the filling pores of cement, which plays a role in improving the reservoir. Through analysis, the target of the WXD-5 region is a high-energy sedimentary facies zone, which has the basic conditions for the development of a high-quality reservoir.

The main source rocks of the WXD-5 region target come from the east wing of the Baiyun main depression and LGH-2 Depression (WC-EP), and there are two different hydrocarbon accumulation modes: (1) the near-source slope transport hydrocarbon accumulation mode mainly supplied by the Baiyun main depression; (2) hydrocarbon supply in LGH-2 depression is dominated by near-source fault (vertical)-sand (lateral) co-controlled transport model.

5.3. The Target of WXD-7 Region

5.3.1. Basic Elements

The target of the WXD-7 region structure is a large and medium-sized lithologic trap controlled by a restricted faulted channel and a multi-stage restricted delta development with obvious features of foreproduct reflection in the inner development, and its advancing distance is relatively long.

Under the tectonic target of the WXD-7 region, three stages of delta development can be identified in Wenchang, and there are certain rules and differences in the evolution of the delta stages. During the upper Wenchang–Enping Formation, the delta scale or provenance supply is mainly represented by mud-rich deposits, and the regional transgressive mudstone of the late Zhuhai Stage can serve as the effective cover layer of the lower Wenchang delta sand body. Form a relatively high-quality storage cap combination.

The volcanic structure, as a whole, is mound-like, and the sedimentary strata overlie the magma channel, forming a certain overlying anticline in the upper part of the rock mass. A series of near-source pyroclastic rocks are mainly developed on both sides, with a medium–strong amplitude-short axial reflection. The sedimentary period of the Wen 6 period of the WXD-7 region target shows an unusually high pyroclastic value, and the volcanic channel is mainly located at the root of the fault channel, and the influence of pyroclastic material on the fault channel of the WXD-7 is mainly in the Wen 6 period, while the volcanic activity of the Wen 5 and Wen 4 periods is obviously weak, mainly showing the characteristics of delta deposition.

The structure target of the WXD-7 region mainly develops fault-filled restricted delta and the development direction is gradually progradated along the long axis, showing moderate to strong hydrodynamic characteristics, and the inferred grain size is mainly from medium to coarse-grained sandstone.

The WXD-7 region mainly developed a braided river delta sedimentary system restricted by fault channels, which is conducive to the development of a large braided river delta system under the geological factors of abundant source supply, large space for formation deposition, high energy hydrodynamics, long-distance transport, easy formation of low mud content and high maturity sand body.

5.3.2. Hydrocarbon Accumulation Model

The structural target of the WXD-7 region is located at the west margin of the eastern low bulge. The main strata studied in this study are the sedimentary strata of Member 4 and Member 5 of the Wenchang Formation, which belong to the stratigraphic trap type. The water depth is about 680–1210 m, and the peak depth is about 3950 m. LGH-8 well drilling reveals that the source rock of the Wenchang Formation has good oil generation potential. Therefore, according to the comprehensive study, the main source rocks of the WXD-7 region structural target come from the east wing of the Baiyun main depression and LGH-2 Depression (WC-EP), among which, the trap area of the Wen4 Formation and the Wen5 formation in this study is about 44 km² and 52 km², respectively.

The hydrocarbon accumulation model of the WXD-7 region structure target is mainly a source-sand near-source accumulation model, and the east wing of the main depression mainly provides source rock, that is, hydrocarbon supply in the eastern wing of the main depression (Figure 10). The target of the WXD-7 region is the lateral transport system of the sand body, and the typical self–reservoir cap combination is developed. The well-developed reservoir sandstone of the Wenchang Formation also develops a good cover layer of sand–mud interlayer, showing a self-generation and self-storage model. In the development period of the Wenchang Formation, especially in the fourth and fifth members of the Wenchang Formation, relatively high-quality reservoirs, namely, delta sandstone, were developed. At the same time, the thick mudstone developed in the Zhuhai Formation is a good sealing cap layer, which is conducive to the formation of a high-quality reservoir cap combination. In the migration path, the structural target belongs to the dominant migration type of near-source slope, the delta sand body and the source rock can be finger-like contact, the oil and gas belong to near-source migration, and the oil and gas migration efficiency is high. Therefore, based on the analysis of various factors, it is concluded that the target of the WXD-7 region structure develops a large-scale delta system, the combination conditions of the source reservoir and cap are superior, and the structure is located on the favorable path of oil and gas migration and accumulation, which is conducive to the large-scale accumulation of reservoirs, but there are certain exploration risks in terms of reservoir physical properties and trap conditions.

According to current geological research in the eastern region of the Baiyun Depression, the combination of overpressure systems, favorable reservoirs and basement structures controls the favorable areas for oil and gas. The periphery of large nose-shaped uplifts, shallow burial depths and relatively low geothermal gradients provide favorable conditions for the development of large-scale stratigraphic overlap and ancient buried hill composite enclosures, with favorable reservoir formation conditions, trap boundary conditions and high-quality large-scale reservoirs being the main controlling factors. Due to the influence of diagenesis, the physical properties of the middle and deep reservoirs are generally poor, while the thick and coarse-grained sandstone in the high-energy phase zone still has the conditions for developing good physical reservoirs under larger burial depths, which is also the target we are looking for.

6. Conclusions

Aiming at problems such as the development conditions and distribution characteristics of high-quality reservoirs in the eastern Baiyun Depression, this study carried out a series of targeted studies focusing on the special geological background of the eastern Baiyun Depression and reached the following important conclusions:

(1)

The eastern Baiyun depression is located on the Baiyun–Liwan continental–oceanic large-scale intershell separation system. During the Wenchang–Enping Formation of the Paleogene, the eastern Baiyun Depression is characterized by a structure–sedimentary pattern of “fault and uplift interlocking and uplift and depression interphase”. The dissociation collapse stage (upper Wenchang) and fault depression transition stage (Enping Formation) are mainly characterized by toughness, horizontal extension–dissociation and thermal subsidence, accompanied by magmatic floor invasion and uplift structure, subdepression extinction and sedimentary transformation. The magma floor invasion intensity is large, the source of direct erosion and denudation, and the magma floor invasion intensity is from medium to small, which affects the pinching degree of preexisting sediments and regulates the transport channels of the Upper Wenchang and Enping sedimentary systems.

(2)

The favorable locations for reservoir development in the study area were the WXD-5 region and WXD-7 region target areas.

The medium and large lithologic trap (WXD-5 region target) is located at the inclined end of the western margin of the low bulge in eastern Yunnan. The reservoir type is the shoreline beach bar of the Upper Wenchang–Enping Formation and the basement buried hill of the Mesozoic. It can supply hydrocarbons from the eastern wing of the Baiyun Dongwa and Liwan 3 Wa, and be transported by the near-source slope, fault and sand body, so it has great exploration potential. In the large and medium-sized lithologic trap (WXD-7 region target), located in the west margin of the Yundong low bulge, the reservoir type is a braided river delta sand body of the Lower Wenchangwen 4 and Wen 5 members. It is located on the raised slope of the east wing of the Baiyun main depression, the reservoir (lower Wenchang Delta) cap (Zhuhai Formation mudstone) combination conditions are superior, the hydrocarbon near-source migration and accumulation efficiency is high and the scale of accumulation is favorable.