Periods and Processes of Oil and Gas Accumulation in the HZ-A Structure Double Paleogene Field, Pearl River Mouth Basin

Abstract

The source of oil and gas and the stages of oil and gas accumulation in the “double-Paleo” field of the HZ-A structure in the Pearl River Mouth Basin are analyzed, and the spatiotemporal coupling relationship of the key conditions of oil and gas accumulation are discussed to reconstruct the process of oil and gas accumulation. Based on previous research results, which are based the characteristics of biomarker compounds, the oil and gas in the HZ-A structure double Paleogene field came from the Paleogene Wenchang Formation hydrocarbon source rocks in the HZ26 sub-sag. By means of the casting thin section identification and inclusion homogenization temperature measurement, this paper reveals the three major hydrocarbon accumulation periods and corresponding fluid charging types in the “double-Paleo” field of the HZ-A structure in the Pearl River Mouth Basin. The results show that 13.8–10 Ma is the charging period of low mature crude oil, 10–5.3 Ma is the charging period of mature crude oil, and from 5.3 Ma is the natural gas charging period. Based on actual geological, drilling, logging, and seismic data, the key conditions for hydrocarbon accumulation in the HZ-A structure “double-Paleo” field are sorted out; that is, the source conditions are characterized by high-quality lacustrine source rocks generating early oil and late gas and a near-source continuous hydrocarbon supply. The reservoir conditions are characterized by weathering and superposition of a fracture zone that transforms into a reservoir, and a large-scale sandstone rock mass that transforms into a reservoir. The caprock conditions are characterized by the stacking of several thin mudstones that form a seal and the combination of multiple lithologies that block hydrocarbon migration. The trap conditions are characterized by multistage uplift structure traps and fracture-lithology combination control traps. The transport conditions are characterized by multi-stage cross-bed transport of source-connected faults and lateral differential transport of shallow sand in deep fractures. Finally, oil and gas accumulation models of the HZ-A structure double Paleogene field were established, and the accumulation process was reconstructed. The overall process involved three stages, with the first stage being the localized oil-displacing-water mode, the second being the large-scale oil-displacing-water mode, and the third being the late progressive gas-displacing-oil mode.

1. Introduction

With the continuous increase in the oil and gas exploration levels in the waters offshore of China in recent years, the discovered oil and gas reserves in the medium-depth and shallow layers are gradually decreasing, and the exploration targets are gradually shifting to new areas, namely, deeper layers and unconventional areas [1,2]. The exploration experience in China and internationally has proven that pre-Paleogene strata contain special traps with rich deep-layer oil and gas reserves in offshore waters and have enormous exploration potential [3,4,5,6]. Judging from the characteristics of the pre-Paleogene oil and gas reservoirs discovered thus far in China’s offshore waters, most of them are composite oil and gas reservoirs jointly composed of pre-Paleogene and overlying Paleogene strata. In recent years, some scholars have called this kind of compound oil and gas reservoir, composed of a buried hill and its overlying strata with the same tectonic evolution background, a “pan-buried hill” reservoir [7], and put forward a series of related theories on oil and gas accumulation. For example, the concealed buried hill accumulation theory of “insider fault blocking-multi-source strong filling-overpressure mudstone sealing”, the oil and gas accumulation model of “transforming slope control circle-active fault control migration-multi-stage strong charging-weak active fault control accumulation”, and the oil and gas accumulation model of “near source strong injection-ridge fault combined control” [8,9,10]. Therefore, under the guidance of the above-mentioned oil and gas accumulation theory, the HZ-A structure in the Pearl River Delta Basin in the South China Sea has, in recent years, displayed high-yield composite oil and gas reservoirs in pre-Paleogene buried hills and Paleogene delta sand bodies. The exploration results show the oil and gas distribution characteristics of “full-structure oil-bearing, multi-layer gas storage, simultaneous development of oil and gas, deep rich and shallow poor natural gas”, which breaks with the traditional understanding of hydrocarbon source rocks in this area that “oil generation is dominant but gas generation is limited”. But, at the same time, it also reflects the uncertainty of the relationship between oil and gas sources, the complexity of oil and gas accumulation conditions, and the uncertainty of oil and gas accumulation processes in the “double-Paleo” field of this area. Finally, it hinders the further expansion of oil and gas exploration achievements in “double-Paleo” field of this area. To address these issues, based on previous studies by others, we studied the timing and process of oil and gas accumulation in the HZ-A structure double Paleogene field with reference to data on the actual geology and data from drilling, logging, and seismic surveys, using experimental methods such as the thin section examination and uniform-temperature inclusion measurement. The purpose of this study is to guide future oil and gas exploration and provide strong technical support for discovering another industrial-value oil field in the HZ area.

2. Geological Setting

The Pearl River Delta Basin is located in the northern part of the South China Sea, on the edge of a Cenozoic passive continental margin. It experienced an early rifting period and a late subsidence period. The Paleogene lacustrine hydrocarbon-generating rocks developed during the rifting period are the primary hydrocarbon-generating rocks, while the delta sedimentary system that developed in the subsidence period covers most of the area from north to south and overlies the lower hydrocarbon-generating strata. Hence, the petroleum geological characteristics include terrestrial source rocks underlying marine reservoir rocks [11,12,13].

The HZ sag is located in the middle of the Zhu-I Depression of the Pearl River Delta Basin (Figure 1A), with the LF sag to the east and the XJ sag to the west. It is one of the most hydrocarbon-rich sags confirmed in the basin. Affected by faults with different orientations, the edges of the HZ sag are characterized by highly active transitional fractures, which commonly resulted in the formation of pre-Paleogene basement buried hills.

The HZ26 sub-sag is located in the southwest of the HZ sag (Figure 1B) and is the richest hydrocarbon-generating sub-sag in the Pearl River Delta Basin in the South China Sea. Two main source rocks are present: the Enping Formation and Wenchang Formation. The Enping Formation is a secondary source rock with a small contribution to reservoir formation, while the type II₁ sapropelic source rocks in the Wenchang Formation are in the low-medium maturity stage (Ro between 0.5–1.1%), with good quality (TOC between 0.37% and 6.87%), and a greater contribution to reservoir formation.

The HZ-A structure is located in the southern part of HZ26 sub-sag in the only passageway for oil and gas to migrate from the HZ26 sub-sag to the HX low high area (Figure 1B). At present, guided by the Paleogene and Pre-Paleogene buried hill joint exploration model, the first exploration well (Well 1) in this structure discovered high-yield and high-quality oil and gas reservoirs in the “double-Paleo” field, confirming the exploration potential of the “double-Paleo” field in the HZ-A structure (Figure 1C).

3. Samples and Analytical Methods

3.1. Samples

Fifteen sandstone samples were collected for fluid inclusion analysis in the double paleo field of the HZ-A structure, and six hydrocarbon-bearing inclusion samples were selected for homogenization temperature testing on the basis of microfluorescence detection. Twenty mudstone samples of the Paleogene Wenchang Formation of the HZ-A structure and 20 crude oil samples from the double paleo field were selected for GC—MS analysis to compare the characteristics of biomarkers of oil-source correlation. Twenty natural gas samples from the double paleo field of the HZ-A structure were selected for carbon isotope analysis.

3.2. Analytical Methods

3.2.1. Biomarker Analysis

The analysis and testing of biomarker compounds of source rocks and crude oil was completed at the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation of Chengdu University of Technology.

Soxhlet extractions were conducted on the powdered mudstone samples using chloroform/methanol (87:13) for 72 h before GC–MS analysis. The different components were separated from source rock extracts and oil samples. The GC–MS analysis of the saturated hydrocarbon fractions from source rock extracts and oil samples was performed on an HP6890GC/5973MSD instrument equipped with an HP5MS fused silica column (30 m × 0.25 mm i.d., film thickness of 0.25 µm). The GC oven temperature was initially set to 50 °C for 1 min, then programmed to increase to 120 °C at a rate of 20 °C/min, to 250 °C at a rate of 4 °C/min, and to 310 °C at a rate of 3 °C/min, and finally kept at 310 °C for 30 min. The mass spectrometer was operated in full-scan and selective ion monitoring modes, and the biomarker ratios were calculated by the peak areas of the compounds.

3.2.2. Carbon Isotope Analysis

The carbon isotope analysis of natural gas was completed in the experimental center of CNOOC, Shenzhen Branch.

An Optima isotope mass spectrometer was used for the carbon isotope analyses of the natural gases. The natural gas sample was separated into single components by the chromatographic column (HP-PLOTQ column, 30 m × 0.32 mm × 20 um) in the HP5890 II gas chromatograph, and single-component hydrocarbons were converted into CO₂ by a high-temperature reformer, and then directly introduced into the isotope mass spectrometer to determine the carbon isotope composition. The initial temperature of the chromatographic instrument was 35 °C. The temperature was increased to 80 °C at a rate of 8 °C/min, and then increased to 260 °C at a rate of 5 °C/min; it was then held constant for 10 min.

3.2.3. Fluid Inclusion Analysis

Fluid inclusion synthesis experiments were conducted at the Beijing Research Institute of Uranium Geology.

Doubly polished thin sections were produced from all of the sandstone samples before petrographic observation by a ZEISS Axioskop microscope. Fluid inclusion homogenization temperatures were measured by a Linkam liquid nitrogen cooling-heating stage mounted under a microscope with an attached Linkam TMS 94 control unit. Fluid inclusions were heated at a rate of 0.5 °C/min from ambient temperature until phase boundaries disappeared and then heated for 2 min. The final temperature was recorded as the homogenization temperature. These processes were repeated again to obtain a duplicate homogenization temperature for measurement precision.

4. Result

4.1. Characteristics of Biomarker Compounds

4.1.1. Hydrocarbon Source Rock

The Paleogene Wenchang Formation source rocks were derived from terrestrial-aquatic hybrid organic matter in a weakly oxidizing environment. These source rocks can be divided into two categories. One category includes deep-lake source rocks. The n-alkane distribution showed a typical two-peak structure (Figure 2A), with C₂₃TT as the main tricyclic terpene (Figure 2B), high hopanes, low orlins (Figure 2C), and “L” type as the dominant sterane distribution (Figure 2D). The other category includes shallow-lake source rocks. The n-alkane distribution showed a noticeable two-peak structure (Figure 2A), with C₂₁TT as the main tricyclic terpene (Figure 2E), relatively high hopanes and orlins (Figure 2C), and the “V” type as the dominant sterane distribution (Figure 2D).

4.1.2. Crude Oil

The crude oil of the HZ-A structure double Paleogene field had relatively similar features. The characteristics of biomarker compounds showed that the distribution of n-alkanes was mainly of two types, the two-peak type and the forward-peak type, with the two-peak type associated with Paleogene Enping Formation crude oil and the forward-peak type associated with Paleogene Wenchang Formation crude oil (Figure 3A). For the tricyclic terpane series, the crude oil of the Paleogene Enping Formation had C₂₃TT as its main peak, and the crude oil of the Wenchang Formation in the Paleogene buried hills had C₂₁TT as its main peak (Figure 3B). For the sterane series, the “L”-shaped distribution was dominant (Figure 3C).

4.2. Carbon Isotope Characteristics of Natural Gas

The natural gas in the HZ-A structure double Paleogene field is composed of hydrocarbon gas and a small amount of nonhydrocarbon gas, such as carbon dioxide. The carbon isotope values of carbon dioxide ranged from −19‰ to −13‰, the carbon isotope values of methane ranged from −40‰ to −35‰, and the carbon isotope values of ethane ranged from −30‰ to −25‰ (Figure 4A), with an overall drying factor ranging between 0.67 and 0.88 and a maturity level of approximately 1.1–1.3% (Figure 4B). Therefore, the natural gas in the HZ-A structure double Paleogene field is of typical organic origin, representing a mixture composed of mainly oil-type gas, and belongs to the category of typical wet gas.

4.3. Hydrocarbon Fluid Inclusions

4.3.1. Petrographic Characteristics of Hydrocarbon Inclusions

The results of thin-section microscopic observation of hydrocarbon inclusions in the HZ-A structure double Paleogene field show that the petrographic characteristics of the hydrocarbon inclusions were similar between the Paleogene buried hills and the Paleogene. The gas inclusions had relatively low hydrocarbon abundance, whereas the oil and oil and gas inclusions had relatively high hydrocarbon abundances, showing colorless, light yellow, or brown colors under single polarized light and light blue under fluorescence. Light oil traces emitting light blue fluorescence were observed in the Paleogene buried hill reservoirs (Figure 5A,B), where the main capture was gas–liquid hydrocarbon inclusions, which were brown under single polarized light (Figure 5C) and light blue under fluorescence (Figure 5D).

In the Paleogene reservoirs, the intergranular pores of the Wenchang Formation were generally filled with black-fluorescent asphalt (Figure 6A,B). The Enping Formation mainly captured liquid hydrocarbon inclusions and gas–liquid hydrocarbon inclusions, which were colorless or light yellow under single polarized light (Figure 6C) and light blue under fluorescence (Figure 6D).

4.3.2. Uniform Temperature of Brine Inclusions

The uniform temperature of the Paleogene Enping Formation had a large span, ranging from 92 to 129 °C, and a typical single-peak distribution, with a peak value of 108–120 °C (Figure 7A). The uniform temperature of the Paleogene Wenchang Formation inclusions ranged from 108 to 127 °C and had a narrow two-peak distribution, with the first peak occurring at 108 to 111 °C and the second (main) peak occurring at 117–120 °C (Figure 7B). The uniform temperature of brine inclusions in the Paleogene buried hills was the highest, ranging from 80 to 140 °C. The distribution had the largest range and mostly a single peak, with the primary peak at a uniform temperature of 110–130 °C (Figure 7C).

5. Discussion

5.1. Oil–Gas Source Comparison

Oil and gas sources were determined mainly based on comparisons of the geochemical features of the oil, gas, and source rocks. The goal was to understand the relationships between oil accumulation, gas accumulation, and source rock accumulation in this petroliferous basin to further delineate reliable oil and gas source areas, determine exploration targets, and effectively guide the exploration and development of oil and gas.

The results of comparing the characteristics of the crude oil in the HZ-A structure double Paleogene field to the characteristics of the HZ26 Paleogene Wenchang Formation source rocks showed that the crude oil in the HZ-A structure double Paleogene field was the product of the same set of hydrocarbon-generating rocks, with the parental material being a land–aquatic hybrid organic matter in a weakly oxidizing environment. The crude oil characteristics of the HZ-A structure Paleogene Enping Formation were similar to those of the shallow-lake source rocks in the Paleogene Wenchang Formation in the HZ26 sub-sag, and the crude oil characteristics of the HZ-A structure Paleogene Wenchang Formation and the Paleo buried hills were similar to those of the deep-lake source rocks in the Paleogene Wenchang Formation in the HZ26 sub-sag. As a result, the crude oil in the HZ-A structure “double paleo” field mainly came from the source rocks of the Paleogene Wenchang Formation in the HZ26 sub-sag (Figure 8A).

The results of comparing the characteristics of the natural gas of the HZ-A structure double Paleogene field to those of the source rocks of the Paleogene Wenchang Formation in the HZ26 sub-sag showed that the ethane carbon isotope values of the natural gas in the HZ-A structure double Paleogene field ranged from −30‰ to −25‰ and that the kerogen carbon isotope values of the Wenchang Formation of the Paleogene Wenchang Formation in the HZ26 sub-sag ranged from −29‰ to −27‰ (Figure 8B). Therefore, the natural gas in the HZ-A structure double Paleogene field mainly came from the hydrocarbon-generating rocks in the Paleogene Wenchang Formation in the HZ26 sub-sag.

5.2. Hydrocarbon Charging Time and Period

Fluid inclusions contain rich information on reservoirs and minerals and provide the best records of the history of hydrocarbon migration and accumulation, thereby facilitating hydrocarbon accumulation chronology research [14,15,16]. The uniform temperature of the brine inclusions that coexist with hydrocarbon inclusions in the reservoir can represent the stratum temperature at the time when the oil and gas entered the reservoir [17]. With reference to the burial history and thermal history, the time when oil and gas entered the reservoir can be indirectly determined. The charging period of oil and gas can then be derived.

The oil and gas charging of the HZ-A structure pre-Paleogene buried hill lasted a relatively long time. The hydrocarbon inclusions were captured from 20 Ma to the present. Hence, the time span of oil and gas charging was long. However, since the time corresponding to the main peak of the uniform temperature of the brine inclusions was concentrated from 13.8 Ma—5.3 Ma, the oil– and gas charging scale was greatest in this period, and this period represents the main oil and gas charging period of the pre-Paleogene buried hills (Figure 9A).

The oil and gas charging time of the HZ-A structure Paleogene Wenchang Formation was later than that of the pre-Paleogene buried hills. The hydrocarbon inclusions were captured from 12 Ma to the present. Its brine inclusion uniform temperature distribution featured a narrow double-peak distribution, revealing that the main oil and gas charging events occurred in two periods, with the first occurring at 12–10 Ma and the second occurring at 5.3–3 Ma (Figure 9B).

The oil and gas charging time of the Paleogene Enping Formation in the HZ-A structure was close to that of the Paleogene Wenchang Formation. The hydrocarbon inclusions were captured from 14 Ma to the present. In addition, unlike the Paleogene Wenchang Formation, the Paleogene Enping Formation of the Paleogene had a relatively concentrated hydrocarbon inclusion capture time, with the main capture period extending from 7 Ma to the present, corresponding to a large-scale hydrocarbon charging event (Figure 9C).

In summary, the main oil and gas charging periods for the HZ-A structure strata were all after 13.8 Ma. Among them, the main charging period of the pre-Paleogene buried hills was the earliest (13.8–5.3 Ma), that of the Paleogene Enping Formation was the latest (7–0 Ma), and that of the Paleogene Wenchang Formation was between the two (12–10 Ma and 5.3–3 Ma). It is known from comparing the hydrocarbon fluid maturity levels of the brine inclusions of the HZ-A structure strata that the hydrocarbon fluid maturity distribution characteristics of the HZ-A structure double Paleogene field were similar to the distribution characteristics of the main charging period of brine inclusions. Among the hydrocarbon fluids, natural gas charging occurred the latest, and asphalt (low-maturity oil deasphalted product) had a low maturity and an early charging period. Normal crude oil had a wide range of maturity distributions, indicating that crude oil had multiple charging periods. That is, the HZ-A structure double Paleogene field experienced low-maturity crude oil charging during the strata’s early accumulation stages. Accordingly, the oil and gas charging in the HZ-A structure double Paleogene field can be divided into three periods, with period 1 being 13.8–10 Ma, which mainly involved the low-maturity crude oil, period 2 being 10–5.3 Ma, during which mature crude oil was charged in large quantities, and period 3 being 5.3–0 Ma, during which a mixture of mature crude oil and natural gas was charged, with natural gas being the main form (Figure 10).

5.3. Key Factors for Oil and Gas Accumulation

The key period of oil and gas accumulation in the HZ-A structure “double-paleo” field was mostly after 13.8 Ma. By 13.8 Ma, the HZ-A structure already had the basic conditions for oil and gas accumulation, with good matching among the accumulation factors (Figure 11).

5.3.1. Sufficient Hydrocarbon Supply

The HZ-A structure had a sufficient hydrocarbon supply with high-quality lacustrine source rocks generating early oil and late gas and a near-source continuous hydrocarbon supply. Near the northern part of the HZ-A structure, the HZ26 sub-sag developed large-scale, high-maturity (Figure 12A,B), high-quality (Figure 12C), and organic-rich lacustrine sapropelic source rocks in the Paleogene Wenchang Formation [8]. Starting at 23 Ma, large amounts of hydrocarbons were generated in the form of early oil and late gas late (Figure 12D,E). Driven by a potential energy difference, the generated oil and gas resources were strongly charged into the HZ-A structure double Paleogene field and finally formed self-generated and self-stored Paleogene oil and gas reservoirs and newly generated hydrocarbons stored in the pre-Paleogene buried hill oil and gas reservoirs.

5.3.2. High-Quality Reservoirs

The HZ-A structural reservoirs developed mainly in the pre-Paleogene buried hills and the Paleogene Wenchang and Enping Formations. Among them, the pre-Paleogene buried hill reservoirs mainly formed before 47.8 Ma and were characterized by fracture zone weathering superimposed on transformed reservoirs [18]. Affected by preexisting faults in its early stage, the pre-Paleogene rocks had fracture zones with different developmental stages in their internal magmatic rock bodies, such as diorite, diabase, and granite (Figure 13A). In the late stage, these fracture zones with different developmental stages were then subjected to weathering (Figure 13B), multiphase fluid transformation, and other factors (Figure 13C), which gradually gave rise to the present pre-Paleogene buried hill reservoirs. The Paleogene reservoirs mainly formed before 33.9 Ma and as a whole were characterized as transition-zone glutenite-body large-scale reservoirs. At 38 Ma, the Paleogene Wenchang Formation developed large flower-like fan delta sedimentary sand bodies in the transition zone, and small skirt-like fan delta sedimentary sand bodies developed in the steep slope zone (Figure 13D). The lithology was mostly glutenite with large vertical thicknesses and wide horizontal extents. At 33.9 Ma, the Paleogene Enping Formation not only developed fan delta sedimentary sand bodies in the transition zone and the steep-slope zone (Figure 13E) but also developed braided river delta sedimentary sand bodies in the transition zone. The lithology was mostly sandstone, gravel sandstone, and glutenite, with a smaller sedimentary thickness and reservoir scale than those of the Wenchang Formation.

5.3.3. Composite Superimposed Caprocks

The HZ-A structural caprock mainly formed before 23.03 Ma. Although no regional caprock developed on top of the double Paleogene field, localized caprocks were abundant and diverse in lithology. In addition to the mudstone caprocks in a conventional petroleum geology sense, several types of special lithological caprocks, such as basalt, tuff, and volcanic breccia, were also seen. These caprocks had thicknesses of several meters to tens of meters and were unevenly distributed horizontally. These caprocks involved the stacking of several thin mudstones to seal and the combination of multiple lithologies to block hydrocarbon migration. Among them, the oil and gas of the Paleogene Enping Formation were mainly sealed by mudstone caprocks, with a large amount of superimposed thin mudstones above and within the reservoir. On the other hand, the oil and gas in the Paleogene Wenchang Formation and the pre-Paleogene buried hills were sealed by mudstone caprocks as well as composite caprocks composed of mudstone, basalt, tuff, and volcanic breccia. The lower limit of the thickness for blocking oil and gas could be as low as 2 m [19].

5.3.4. Multiple Types of Compound Traps

The types of HZ-A structural traps were complex and diverse and formed mostly before 23.03 Ma. Prior to 47.8 Ma, the pre-Paleogene rocks experienced accordion-like multiperiod compression and extension. With the effects of magma emplacement and stratigraphic uplift, fault-anticline stratigraphic-overlying traps eventually formed. They were jointly controlled by the oil source fault in the north and the NWW-trending fault overlapping deposits in the south (Figure 14A). Between 47.8 Ma and 38 Ma, the Paleogene Wenchang Formation was deposited on the pre-Paleogene rocks. Inheriting the shape of the pre-Paleogene rocks, the Paleogene Wenchang Formation also formed fault-anticline stratigraphic-overlying traps, which were also controlled by oil source faults in the north and NWW-trending fault overlapping deposits in the south (Figure 14B). Overall, the traps of the pre-Paleogene and Paleogene Wenchang Formations were multistage uplift structure traps. The Paleogene Enping Formation traps mainly formed from 38–33 Ma. They were mainly the fault–lithology–stratigraphic overlap traps controlled by faults and lithology and were characterized as fracture–lithology combination control traps (Figure 14C).

5.3.5. Efficient Transport System

The HZ-A structural transport system was mainly composed of the spatial combination of oil sources, faults, fractures, sand bodies, and unconformity surfaces. The transport system involved multistage cross-layer transport via faults connected to the sources and lateral migration along fractures to shallow sand bodies (Figure 15). Among them, the sub-sag controlling oil source fault was the main channel for vertical oil and gas transport in the study area. It became mostly blocked by 47.8 Ma and was only open for short periods thereafter, and this fault cut almost the entire Cenozoic sequence from the bottom to the top. Fractures, sand bodies, and unconformity surfaces were the main channels for the lateral migration of oil and gas in the study area. After the oil and gas migrated vertically via the oil source fault, they were shunted laterally through the fractures and sand bodies into the pre-Paleogene buried hills and the Paleogene, respectively. The migration difference was mainly due to the storage properties of the fractures and the sand bodies.

5.4. Oil and Gas Accumulation Process

The HZ-A structure’s sufficient oil and gas sources, high-quality reservoirs, composite superimposed caprocks, large-scale traps, and efficient transport systems, plus the effective configuration and coupling of these factors during each major oil and gas charging period, laid a solid foundation for the formation of oil and gas reservoirs. During the major oil and gas charging periods, the oil– and gas resources could first migrate vertically through the oil source faults to areas of low potential energy in the target strata in the HZ-A structure pre-Paleogene buried hills and the Paleogene Wenchang and Enping Formations and then migrate laterally through fractures and sedimentary sand bodies to the higher parts of the traps to form the reservoirs. Overall, the hydrocarbon accumulation process experienced three stages, as described below [20,21].

From approximately 13.8–10 Ma, the first stage was mainly the charging of low-maturity crude oil, forming small-scale ancient oil reservoirs via the localized oil-displacing-water mode. During this period, the crude oil generated in the low-temperature evolution stage of the source rocks gradually migrated to the double Paleogene field through three-dimensional high-efficiency mesh-like transport systems composed of faults, fractures, sand bodies, and unconformity surfaces, forming small-scale ancient oil reservoirs (Figure 16A).

From approximately 10–5.3 Ma, the second stage was mainly the charging of mature crude oil, forming large-scale ancient oil reservoirs via the large-scale oil-displacing-water mode. During this period, mature crude oil generated by the source rocks migrated and accumulated at a large scale in the double Paleogene field. As crude oil charging continued, the previously formed paleo-reservoir continued to expand and transformed into large-scale ancient oil reservoirs (Figure 16B).

From approximately 5.3–0 Ma, the third stage was mainly the charging of high-maturity oil and gas in a sequence of oil first and gas later, forming the current pattern of oil overlying gas via a localized gas-displacing-oil mode. During this period, the source rocks began to generate gas on a large scale. Under the influence of the phase-controlled miscibility mechanism, a large amount of natural gas entered the double Paleogene field following the mature crude oil in a progressive manner, displacing the oil in the ancient reservoirs successively from bottom to top in the pre-Paleogene buried hills and the Paleogene Wenchang Formation and Enping Formation to form natural gas reservoirs. Eventually, the charging stopped in the Paleogene Enping Formation due to insufficient gas supply, resulting in the current pattern of oil overlying gas (Figure 16C).

6. Conclusions

The HZ-A structure acquired the basic conditions for oil and gas accumulation by 13.82 Ma. Its sufficient oil and gas sources, high-quality reservoirs, composite superimposed caprocks, large-scale composite traps, and efficient transport system, as well as the effective configuration and coupling of these features during each major oil and gas charging period, laid a solid foundation for the final oil and gas reservoir formation.

The oil and gas in the HZ-A structure double Paleogene field mainly came from the lacustrine source rocks in the Paleogene Wenchang Formation in the HZ26 sub-sag. On the whole, it has experienced three stages and two types of oil and gas accumulation processes. The first period was 13.8–10 Ma, when the charging mainly involved low-maturity crude oil, mainly experiencing a process of localized oil-displacing-water. The second period was 10–5.3 Ma, when the charging mainly involved mature crude oil, mainly experiencing a process of large-scale oil-displacing-water. The third period was 5.3–0 Ma, when the charging mainly involved a mixture of mature crude oil and natural gas, with natural gas dominating the charging, mainly experiencing the process of progressive gas-displacing-oil.