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Article

Basement Reservoirs in China: Distribution and Factors Controlling Hydrocarbon Accumulation

by
Qixia Lyu
1,
Weiming Wang
1,*,
Qingchun Jiang
2,
Haifeng Yang
3,
Hai Deng
4,
Jun Zhu
5,
Qingguo Liu
1 and
Tingting Li
1
1
School of Geosciences, China University of Petroleum (East China), Qingdao 266580, China
2
Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 839000, China
3
Tianjin Branch of China National Offshore Oil Corporation, Tianjin 300452, China
4
Exploration and Development Research Institute of Daqing Oilfield Company Ltd., PetroChina, Daqing 163453, China
5
Exploration and Development Research Institute of Qinghai Oilfield Company Ltd., PetroChina, Jiuquan 736202, China
*
Author to whom correspondence should be addressed.
Minerals 2023, 13(8), 1052; https://doi.org/10.3390/min13081052
Submission received: 5 July 2023 / Revised: 3 August 2023 / Accepted: 6 August 2023 / Published: 9 August 2023
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Abstract

:
The oil reserves of global basement reservoirs are 248 × 108 t and natural gas reserves are 2681 × 108 m3; they are crucial links in the future oil and natural gas exploration field and play an irreplaceable role in increasing oil and natural gas reserves and production. Based on research on the definition and classification of basement reservoirs, this study dissected three major basement reservoirs in China (i.e., the Dongping region located in the Qaidam Basin, the Bozhong 19-6 gas field located the Bohai Bay Basin, and the Central Uplift area of the Songliao Basin). The geological conditions and controlling factors of oil and natural gas accumulation in basement reservoirs in China are summarized. The results of this study are as follows: (1) Basement reservoirs can be classified into three distinct types, namely, the weathered carapace type, weathered inner type, and weathered composite type. They are characterized by a large burial depth, strong concealment, and huge reserves and are mostly distributed at the margins of continental plates and in zones with stratum intensive tectonic activity; (2) Basement reservoirs in different basins have different controlling factors. The basement reservoir in the Dongping region, located in the Qaidam Basin, has favorable geological conditions with laterally connected sources and reservoirs. In this reservoir, oil and natural gas have transferred along faults and unconformities to accumulate in uplifted areas, forming a weathered carapace-type basement reservoir controlled by structures. The Bozhong 19-6 gas field, which is situated in the Bohai Bay Basin, has favorable multiple hydrocarbon supplies of source rocks. Under the communication of faults and cracks, oil resources form a weathered inner type basement reservoir. In the Central Uplift area of the Songliao Basin, the basement reservoir exhibits a dual-sided hydrocarbon supply condition from the uplift. In this reservoir, oil and natural gas have transferred to traps through the fault and inner fracture system and have been properly preserved thanks to the extensive overlying cap rocks. It can be concluded that, after being attenuated by millions of years of weathering and leaching, basement rocks can form large-scale and medium-scale basement reservoirs with reserves of more than 100 million barrels in the presence of favorable geological conditions, such as a multi-directional hydrocarbon supply, a high brittle mineral content in the reservoirs, diverse reservoir spaces, and high-quality cap rocks.

1. Introduction

As the exploration of shallow oil and natural gas reservoirs in sedimentary basins deepens, it has become an inevitable trend to explore high-yield lithologic reservoirs in deep basements. As a lithologic reservoir type where oil and natural gas accumulate in the crystalline rock series of sedimentary basements, basement reservoirs have complex structures and strong heterogeneity, are mostly fractured reservoirs, and exhibit prominent characteristics such as large scales, high reserves, a wide distribution, multiple sets of longitudinal strata, and strong concealment. As of 2021, hundreds of industrial basement reservoirs have been discovered in more than 30 large basins around the world [1,2]. The Bach Ho oilfield located in Vietnam is one of the world’s high-yield basement reservoirs, with geological reserves of crude oil of nearly 6 × 108 t attributable to the high single-well productivity and long-term stable production at the target horizon [3]. The Mesozoic–Cenozoic Bongor Basin in Chad, West Africa—a rift basin—has experienced weathering, dissolution, and structural fracturing for 300 million years. High-quality oil and natural gas have been produced at the target horizon in this basin, with ten basement oilfields and eight high-yield oil-bearing structures having been discovered, including three 100-million-ton-class basement oilfield groups (Great Baobab, Daniela, and Raphia) and two 50-million-ton-class basement oilfields (Lanea and Birrea) [4]. Moreover, petroleum resources totaling 100 million tons have been discovered in the basement reservoirs of three locations: the La Paz oilfield located in Venezuela’s inland, the Zeit Bay gas field situated in Suez Bay, and the Augila Naafora oilfield located in the Sirte Basin of Libya [5,6,7]. International exploration practice has proved that large-scale petroleum and natural gas reservoirs can form in the basement rocks, which possess immense resource potential and represent a crucial new frontier in the realm of petroleum and natural gas exploration.
Basement reservoirs in China were discovered for the first time in the Yaerxia Oilfield in the Jiuquan Basin [8]. Through more than five decades of continuous efforts, China has made remarkable progress in the study of basement reservoirs and their exploration. It has discovered multiple basement reservoirs with considerable reserves in the Bohai Bay Basin, the Tarim Basin, and the Songliao Basin. Despite the copious petroleum and natural gas resources, the formation conditions and controlling factors of basement reservoir strata have not been thoroughly studied, mainly due to their late discovery and low exploration level, which calls for more in-depth research. This study provides a comprehensive overview of the geological characteristics of basement reservoir strata both domestically and internationally. On this basis, it classified basement reservoirs, analyzed representative basement reservoirs in basins in the western, offshore, and eastern parts of China, and summed up the geological conditions and primary controlling factors influencing basement reservoir strata. Guiding the subsequent exploration of basement reservoirs in China is the main goal of this study.

2. Definition and Classification of Basement Reservoirs

2.1. Definition

In 1960, Landes first defined basement reservoirs, proposing the hypothesis that the hydrocarbons produced by immature source rocks and accumulated within the underlying paleo-metamorphic rocks and igneous rocks of the unconformities are all referred to as basement rock petroleum deposits [9]. Many Chinese researchers also put forward some opinions on basement reservoirs. Pan considered that the basement reservoirs should also include the lower Paleozoic and middle-new Proterozoic carbonate rocks and other sedimentary rocks under the unintegrated surface of the new hydrocarbon source rocks [10]. Tian et al. consider that all petroleum and natural gas that were forged within the depths of crystalline bedrocks or in sedimentary rocks before basin formation are basement reservoirs [11]. From the perspective of crystalline basements, Wu et al. held that the oil and gas are accumulated in the metamorphic rocks and igneous rocks buried deep underground to form the basement reservoirs [12].
Overall, basement reservoirs primarily refer to petroleum and natural gas produced in young sources and accumulated in old reservoirs, which are produced by overlying or lateral source rocks prior to basin formation accumulating in basements composed of igneous rocks, metamorphic rocks other ancient rocks. Basement reservoirs are mostly distributed at plate margins and in basins formed by intensive tectonic activity.

2.2. Classification of Basement Reservoirs

Basement reservoirs, as a distinctive type of petroleum and natural gas reservoir, have been categorized based on various criteria during the exploration of petroleum and natural gas resources. Specifically, they are classified into high-, middle-, and low-elevation basement rocks according to their burial time [13]; into slope, hill, fault-block, and irregular types based on the top morphology of basement rocks [14]; and into the types of old reservoirs with a young source, old reservoirs with multiple sources, and middle-epoched reservoirs with multiple sources according to their source–reservoir configuration [15]. Moreover, they are also classified according to the main dynamic mechanism of structural evolution, the structural characteristics of basement rocks, and trap types [16]. However, these classification methods are mostly based on the geological background of a certain study area and thus are not widely applicable. By combining the research and classification results of multiple experts, this study classified the basement reservoirs into three types, namely, weathered carapace type, weathered inner type, and weathered composite type (Table 1), which differ in the oil and natural gas storage position. Specifically, for the basement reservoirs of the weathered carapace type, the weathering crusts on the surface of underlying bedrocks serve as the storage space, and unconformities serve as the shield; for the basement reservoirs of the weathered inner type, the inner fracture system formed by the development of internal highly brittle rocks serves as the storage space, and the inner interlayers serve as the shield, e.g., at the Clare Bedrock field in the western Shetland Islands, there is little evidence of a weathered crust—the oil is mostly fractured [17]; and for the basement reservoirs of the weathered composite type, as the name implies, a portion of the petroleum and natural gas is stored in weathered crusts, while the remainder is contained within the internal fracture network [18]. Comparatively, this classification method takes into account the characteristics of geological areas and oil and natural gas accumulation and is thus applicable to most basement reservoirs.

3. Distribution of Basement Reservoirs in China

The basement itself does not produce oil and gas, and the resources come from the source rock segments of several surrounding depressions. The reservoir space in the basement is divided into a weathered crust dissolution cavity (mainly controlled by weathering dissolution—the weathered crust is eroded layer by layer by weathering, and most of the rocks on the top are dissolved to form a loose porous reservoir space of weathering crust) and an inside network fracture system (mainly controlled by the lithology difference—under the influence of physical weathering and other effects, the structure of the same kind of rock inside the bedrock is different, and the mineral composition is also different; when the content of brittle minerals is high, it is easier to form cracks, and a large number of dissolution micro-cracks and pores develop along the cracks, thus forming an excellent internal network fracture reservoir space). The oil and gas are covered on the top of the basement and are directly sealed by the laterally distributed stable mudstone, clastic rock, and other sedimentary formations, forming the basement reservoirs. For example, the Paleogene Shahejie Formation and Dongying Formation of basement oilfields in Bohai Basin, east China, are lacustrine sediments with a thickness of 50~300 m, covering the entire basement belt, and they have the advantages of congenital reservoir formation.
Basement reservoirs occur primarily in areas subject to the intensive tectonic activity of plates. After a variety of geological processes such as strong collision, extrusion and subduction between plates, the strata have deep and large faults, and the basement rocks have a relative uplift or decline along the fracture direction. Over a long period, young source rocks have either directly overlain on or been in lateral contact with basement rocks, connecting to them through unconformities or a fault system. As the study of basement reservoirs in China deepens, petroleum and natural gas have been found in the basement strata of nearly ten basins, including the Bohai Bay Basin, the Pearl River Mouth Basin, and the Tarim Basin. The basement reservoirs in China have greatly varying geological reserves, including some 100-million-ton-class petroleum and natural gas fields and some only with small shows. However, basement reservoirs are widely distributed across the country (Figure 1).
Owing to different regional and structural settings, the basement reservoirs in China differ in the era of initial deposition and have developed in Archean to Mesozoic sedimentary strata (Table 2). The onshore basement reservoirs in China are mostly superimposed on Archean volcanic and metamorphic rocks and Proterozoic carbonate rocks, having undergone long-term geological evolution. The offshore basement reservoirs are Mesozoic–Cenozoic superimposed basins occurring above Mesozoic volcanic rocks, thus having younger depositional eras. Overall, Paleozoic strata are of paramount importance in the production of oil and natural gas from basement reservoirs (e.g., the Lunnan low eminence within the Tarim Basin and the Shiwu fault depression and the Central eminence within the Songliao Basin), accounting for about 35% of the total hydrocarbon-generating horizons. The Paleozoic strata are followed by Archean strata (30%), Mesozoic strata (18%), and Proterozoic strata (17%). Regardless of the geological ages, such as the Proterozoic, Archean, or Mesozoic, in more than 50% of the discovered basement oil and natural gas fields, the reservoirs are mainly composed of granites, followed by igneous rocks such as gneiss and carbonate rocks. The lithological composition is also a unique feature of basement reservoirs.

4. Dissection of Typical Basement Reservoirs in China

4.1. Basement Natural Gas Reservoir Located within the Dongping Area, Qaidam Basin, Western China

The Qaidam Basin, located within Qinghai Province in western China, is surrounded by the Altun Mountains to its northwest, the Kunlun Mountains to its south, and the Qilian Mountains to its north [19]. The Dongping region situated in the eastern piedmont of the Altun Mountains has undergone development and evolution on a foundation of basement rocks and Paleozoic granites and was later underlain by Cenozoic strata. After undergoing complex geological processes, this area formed a faulted, nose-shaped uplift. The basement natural gas reservoir in the Dongping nose-shaped uplift, discovered in 2011, is one of the onshore basement natural gas reservoirs with the most abundant reserves in China [20].
As indicated by statistics, the Dongping basement gas reservoir has proven reserves of about 52 billion m3, and its natural gas is sourced primarily from the Jurassic source rocks in the immediately adjacent Pingdong sag. With the organic matter type of Ⅱ2 and Ⅲ kerogens, the Jurassic source rocks mainly generate natural gas, followed by oil [21]. Large amounts of oil and natural gas are generated; they discharged from the Jurassic source rocks and migrated through fractures, dissolution pores, and micropores, and then the weathering crusts and Paleogene reservoirs have accumulated to form a total thickness of over 200 m. As high-quality cap rocks, the gypsum-bearing argillaceous rocks above the basement and the gypsum-bearing salt rocks in Paleogene saline lakes directly overlie the basement rocks, forming a wide range of planar contact and effectively sealing oil and natural gas [22]. In addition, various traps combined with the high degree of weathering, such as anticlinal traps and fault block traps, have further formed in the basement under the influence of structures and faulting. Oil and natural gas are preserved in the traps, forming an oil and natural gas field of the weathered carapace type ([23]; Figure 2).
During the formation of the Dongping basement reservoir, the adjacent source rocks created favorable conditions for the migration of oil and natural gas; the eminence structures and the high-quality reservoirs of paleozoic metamorphic sandstone were favorable for oil and natural gas accumulation [24]. Exposed to weathering and denudation, multiple installments tectonic movements, and the environmental effects of prolonged weathering and leaching, the foundation weathering crusts have formed a high number of cracks and dissolution pores, which have greatly aided reservoir development. Moreover, the distribution and migration of oil and natural gas were controlled by NS-trending T-shaped deep faults, while the accumulation of oil and natural gas was controlled by NW-trending multi-stage faults. These faults produce a reticular oil and natural gas diversion system that is critical in the distribution and production of oil and natural gas in the basement rocks.

4.2. Bozhong 19-6 Basement Reservoir in the Bohai Bay Basin of China

The Bohai Bay Basin is situated in the eastern part of China and is bounded by the Jiaoliao uplift, Luxi uplift, Taihang Mountains, and Yanshan ruffles region [25]. The Bozhong 19-6 basement reservoir lies in the Bozhong Basin sag, which is located at the depocenter of the Bohai Bay Basin and is encompassed by three sub-sags, thus boasting a superior geographical location. This reservoir has a developed basement composed of Archaean granites and gneisses; the Paleogene rocks including glutenites of the Kongdian Formation, the Shahejie Formation which consists of medium- to fine-grained sandstones, and the Dongying Formation that is composed of medium-grained sandstones; and the top sandstones of the Neogene Guantao and Minghuazhen formations [26,27]. Oil and natural gas accumulate in these rocks, with reserves of more than 100 billion m3, making the Bozhong 19-6 basement reservoir the largest basement gas field ever discovered to date in the Bohai Bay Basin.
The source rocks of the Shahejie and Dongying formations overlying the basement rocks of the Bozhong 19-6 basement reservoir mainly contain organic matter composed predominantly of type Ⅱ1 and type Ⅱ2 kerogens. Following extensive gas generation during the high-evolution late stage of the Bozhong sag, overpressure causes oil and gas to migrate from the upper source rock to the Archean bedrock, Kongdian Formation, Guantao Formation, and Minghuazhen Formation over a long distance through unconformities, structural fractures, dissolution pores, and intergranular pores. Subsequently, they were sealed by mudstones of the Paleogene Shahejie Formation and Mesozoic tight clastics with a total thickness of 500 m, forming a basement reservoir of the weathered inner type ([28]; Figure 3).
Before the Cenozoic, the Bozhong 19-6 area experienced multiple periods of tectonic movements; this results in the provision of organic matter as a source for rocks in three areas [29]. Specifically, oil and natural gas produced by the Shahejie Formation source rocks in the southwestern sag migrated directly with oil and natural gas source faults [30]. The dominant sag and southern sag, on the other hand, are far from the Bozhong 19-6 area, and these depressions accumulate oil and natural gas after a long-distance migration. Furthermore, due to the repeated uplift and denudation of the strata, significant dissolution fractures have developed at a depth of 300 m below the weathering crust interface. The fracture horizon contains about 90% metamorphic granites. The high brittle mineral content greatly improves the storage conditions of basement reservoirs, making the fracture horizon the most stable reservoir with the most favorable physical properties in the Bozhong 19-6 area [31]. The overlying contiguous Paleogene cap rocks are subjected to a higher fluid pressure than the reservoir, and the pressure difference that results stops the oil and natural gas from moving higher. As a result, the Paleogene cap rocks further seal the oil and natural gas and improve the degree of natural gas enrichment, establishing suitable preservation conditions for the creation and dispersion of the basement reservoir.

4.3. Basement Reservoirs Located in the Central Uplift of the Songliao Basin in Eastern China

The Songliao Basin is located in eastern China, stretching across Heilongjiang, Jilin, Liaoning, and Inner Mongolia [32]. The Central Uplift is a banded, nearly-NS-trending uplift and lies between the fault depressions of Xujiaweizi and Gulong. Its basement rocks are composed of Proterozoic and Paleozoic metamorphic and igneous rocks and are underlain by five strata as cap rocks, namely, the Huoshiling, Shahezi, Yingcheng, Denglouku, and Quantou formations [33]. Four wells with industrial gas flow and eight wells with low-yield gas have been drilled in the basement, suggesting great potential [34].
In the Central Uplift, the basement of oil and natural gas primarily derived from the Shahezi Formation—the deep, dominant source rocks on the fault depressions of both sides of Xujiaweizi and Gulong, located in the Songliao Basin, and the organic matter of this formation is predominantly composed of type III kerogen [35]. A significant quantity of oil and natural gas was produced by the dark mudstones inside and the seams of the Shahezi Formation. After passing through reservoir spaces, such as dissolution pores and fractures, the oil and natural gas are predominantly congregated within tight and strongly heterogeneous basement weathering crust, and some of them accumulated in the inner reservoirs composed of metamorphic granites. The oil and natural gas are sealed by the 100–200 m-thick mudstone and sandstone cap rocks in the Denglouku or Quantou Formation in the Central Lift [36]. They accumulate within basement rocks and various traps of overlapping and draped strata, forming a weathered composite-type basement reservoir.
In the Central Uplift, the oil and natural gas derive from hydrocarbon-generating depressions on both sides of the uplift. Moreover, the basement rocks are intimately exposed to the Shahezi Formation source rocks, and the extensive contact area is conducive to oil and natural gas aggregation in the weathering crust of both the Central Uplift and Inner Uplift. The basement reservoir consists mainly of granites and is thus prone to developing dissolution fractures and pores, which contribute to high storage performance. The large faults in the Central Uplift are all SN-trending and can effectively connect the source rocks to the weathering crust reservoirs, making it easy to form a basement reservoir of the upper weathering shell type. Some fractures that do not penetrate the entire depth can serve as a foundation for the development of an inner-type basement reservoir. As cap rocks, the Denglouku and Quantou formations are in direct contact with the basement, providing favorable conditions for oil and gas storage (Figure 4).

5. Factors Governing the Development of Subterranean Reservoirs in Basement Rocks

5.1. Long-Term Weathering and Leaching Are Essential Preconditions of High-Quality Basement Reservoirs

Subjected to tectonic movements such as the Indosinian and Yanshanian movements, the basements of the sedimentary basins in China experienced compression and collision at varying degrees and were then uplifted or lowered, forming uplifts or sags [37]. As a result, the surface is susceptible to weathering and leaching. After full contact with solar radiation, wind, atmospheric water, and biological factors, the surface undergoes physical and chemical changes, forming loose deposits, weathering crust reservoirs, and complex karst systems (Figure 5). The early fractures in the basement rocks intensified the effects of weathering and leaching, which increased the reservoir thickness. Meanwhile, prolonged weathering and leaching have sped up the development of storage spaces, such as reticular fractures and subsequent collapse holes, as well as improved storage physical features, thus providing the groundwork for the establishment of superior basement reservoirs [38].

5.2. Brittle Minerals Serve as the Fundamental Basis for the Formation of Fractured Basement Reservoirs

The high-yield basement reservoirs discovered so far in China are composed of granites and gneisses, and a few of them are carbonate reservoirs. These basement reservoirs were subjected to multi-stage tectonism, prolonged weathering, and erosion over the course of their evolution. Moreover, more weathering fractures and inner fractures can be formed in the basement rocks due to the presence of highly brittle minerals, providing a basis for forming high yielding deposits [39]. Statistics show that granites are the most favorable plutonic in basement rocks (Figure 6). They have high brittle mineral contents. Accordingly, they are prone to forming fractured reservoirs when suffering weathering and leaching, thus further improving the reservoir permeability. Granites are followed by carbonate rocks, which experience fewer fractures due to tectonic stress, weathering, and denudation. As the burial depth increases, their reservoir stimulation weakens progressively. Gneisses are poorly suited as reservoir lithologies. They are generally tight and have high ductility and can form only a few open fractures under the influence of stress, which in turn affects the reservoir’s ability to accumulate. Phyllites and slates are the least favorable reservoir lithologies, as they are generally thinly laminated. Owing to their high toughness, they tend to become compacted and form only a few fractures under the effects of weathering and denudation, consequently reducing the connectivity of the basement reservoirs.

5.3. Multiple Hydrocarbon Supplies of Source Rocks Are the Key to the Elevated Production of Basement Reservoirs

Basement rocks cannot generate hydrocarbons themselves, and the oil and natural gas come primarily from the overlying strata or adjacent rocks of origin that are connected to the foundation stones through deep faults, unconformities, and inner fractures [40]. The oil and natural gas then travel to and deposit in reservoirs, and within the BoZhong sag, they originate from Paleogene source rocks located in multiple horizons above the basement rocks [41]. They migrated over a short distance through major channels such as faults and fracture networks. The short migration is characterized by its brevity and minimal hydrocarbon loss, rendering it advantageous for the scale agglomeration of oil and natural gas reserves. The Dongping area in the Qaidam Basin, which is adjacent to the dominant Mesozoic hydrocarbon-generating sag, has been a persistent target for oil and natural gas, running the move over the long term. Thanks to the multiple hydrocarbon supplies around, the traps have accumulated significant quantities of oil and natural gas, forming a large basement reservoir (Figure 7). Overall, the source rocks of the basement reservoirs in China all occur in Paleogene–Neogene strata, and the basement reservoirs are characterized by a continuous hydrocarbon supply from multiple horizons and orientations. The distance between the source rocks and basement rocks and the multi-directional hydrocarbon supply mode directly determine the degree of aggregation of basement hydrocarbons.

5.4. Effective Configuration of Diverse Storage Spaces Enhancing the Connectivity of Basement Reservoirs

Basement rocks have long been subjected to weathering, dissolution, and multi-stage tectonic movements. Accordingly, basement reservoirs have complex and diverse storage spaces that can be broadly categorized into three groups: (1) unconformities resulting from geological evolution; (2) deep faults formed by tectonism; and (3) pores and micro-fractures formed by the fluid dissolution of weathering crust and inner fillings. The combination of the three types of storage spaces can greatly improve the reservoir’s permeability (Figure 8). Despite the random distribution in different parts of the basement rocks, micro-fractures are well interconnected. They connect dissolution pores to deep faults and unconformities, thus forming multiple oil and gas transport systems, which provide the most important connection condition for basement reservoirs [42]. Therefore, the effective arrangement of the storage spaces can significantly improve the reservoir’s ability to accumulate and improve connectivity between basement traps, thereby facilitating the distribution and transmission of oil and natural gas.

5.5. Stably Distributed High-Quality Cap Rocks Control the Accumulation Range of Basement Petroleum Fields

Owing to the effects of sedimentary environments, some voids and fractures in the weathering crusts’ upper portion of bedrocks are cemented and filled by minerals such as calcites and gypsum, forming local top-sealing cap rocks for the oil and natural gas within the underlying basement storage stratum. Although the cap rocks of this type can seal only part of the oil and gas, they are thick, on average (generally more than 10 m), and have an excellent sealing capacity. During the sedimentary process, regional gypsum-bearing salt rocks and mudstones were formed as cap rocks with the continuous evaporation of formation water. Despite their small thickness (generally more than 1 m), the gypsum-bearing salt rocks and mudstones are widely distributed and stable and have a high sealing capacity. Therefore, they can still effectively seal the underlying oil and gas [13]. Two types of high-quality overlying strata from the Paleogene–Neogene era are stably rendered in large- and medium-scale basement reservoirs discovered in China to date. Among them, the high-pressure dark mudstone cap rocks deliver the highest sealing performance, followed by the cap rocks of mudstones with an oxidation color and gypsum-bearing rocks. Although these cap rocks and reservoirs are not necessarily continuous in the formation time, they are in direct contact and match well with each other in space. Therefore, these cap rocks can effectively seal oil and gas and control the distribution range of basement reservoirs (Figure 9).

6. Conclusions

(1)
Basement reservoirs refer to reservoirs formed as oil and natural gas born in overlying or lateral source rocks migrating to as well as accumulating in metamorphic rocks or igneous rocks under a sedimentary sequence. They have complex and diverse structures and can be classified as an upper weathering shell type, inner type, or mixed type depending upon the location of oil and natural gas enrichment.
(2)
Basement storage strata are distributed at plate margins and in areas with intensive tectonic activity. After experiencing long-term structural evolution, weathering, and leaching, the basement rocks in basins have developed in longitudinal strata from the Archean to the Mesozoic periods. The Dongping nose-shaped uplift in the Qaidam Basin of western China is a long-term destination for oil and natural gas relocation and enrichment. The basement reservoir within the uplift enjoys a favorable source–reservoir configuration. Moreover, the oil and natural gas are trapped beneath the storage stratum by stably distributed ultra-thick cap rocks, forming a basement storage stratum of an upper weathering shell type. For the Central Uplift in the eastern Songliao Basin, the sags on both sides of the uplift jointly supplied hydrocarbons, and oil and natural gas relocated to weathering crust reservoirs and inner reservoirs, forming a mixed-type basement reservoir. The Bozhong 19-6 gas field located within the Bohai Bay Basin has multi-directional hydrocarbon supplies. Oil and natural gas in the field relocated to and accumulated in the inner fractures through faults and fractures, thus forming a basement reservoir of the inner type.
(3)
The effective coupling of long-term weathering and leaching, multi-directional and short-distance hydrocarbon supplies, the presence of highly brittle minerals in basement rocks, various storage spaces, and cap rocks with an excellent sealing capacity are the prerequisites for forming large-scale and medium-scale basement reservoirs.

Author Contributions

Conceptualization, W.W.; methodology, Q.L. (Qixia Lyu) and W.W.; formal analysis, Q.L. (Qixia Lyu) and W.W.; data curation, J.Z.; writing—original draft, Q.L. (Qixia Lyu); writing—review editing, W.W.; visualization, Q.L. (Qingguo Liu) and T.L.; project administration, Q.J., H.Y. and H.D. All authors have read and agreed to the published version of the manuscript.

Funding

This study was jointly funded by the general program of the National Natural Science Foundation of China (42272145, 41672125), the general program of the Shandong Natural Science Foundation (ZR2020MD027), and the forward-looking major science and technology program of the 14th five-year plan of PetroChina (2021DJ0203).

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflict of interest.

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Minerals 13 01052 g001
Figure 1. Map showing the distribution of major basement oil and natural gas reservoirs in China.
Figure 1. Map showing the distribution of major basement oil and natural gas reservoirs in China.
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Figure 2. Profile view of the basement natural gas reservoir in the Dongping area (N1-Q: Miocene to Quaternary; E3: Oligocene; E1+2: Palaeocene to Eocene; J1: Jurassic).
Figure 2. Profile view of the basement natural gas reservoir in the Dongping area (N1-Q: Miocene to Quaternary; E3: Oligocene; E1+2: Palaeocene to Eocene; J1: Jurassic).
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Figure 3. Section of the Bozhong 19-6 basement reservoir (N1g: Guantao Fm of Miocene; N1-2m: Miocene to Minghuazhen Fm of Pliocene; E3d: Dongying Fm of Oligocene).
Figure 3. Section of the Bozhong 19-6 basement reservoir (N1g: Guantao Fm of Miocene; N1-2m: Miocene to Minghuazhen Fm of Pliocene; E3d: Dongying Fm of Oligocene).
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Figure 4. Geological Reservoir Section within the Paleo-Central Uplift of the Songliao Basin [34].
Figure 4. Geological Reservoir Section within the Paleo-Central Uplift of the Songliao Basin [34].
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Figure 5. Scheme of weathering and leaching (modified after Yi et al., 2020 [32]).
Figure 5. Scheme of weathering and leaching (modified after Yi et al., 2020 [32]).
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Figure 6. Pie chart of lithologies of basement reservoirs in China.
Figure 6. Pie chart of lithologies of basement reservoirs in China.
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Figure 7. Sketch of the multi-directional natural gas source rocks supply located in the Dongping area, Qaidam Basin (Red arrows indicate the direction of oil and gas accumulation; the red dotted line indicates the accumulation trend of oil and gas).
Figure 7. Sketch of the multi-directional natural gas source rocks supply located in the Dongping area, Qaidam Basin (Red arrows indicate the direction of oil and gas accumulation; the red dotted line indicates the accumulation trend of oil and gas).
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Figure 8. Types of storage spaces. (A) Pore from well Jiantan-1 in the Qaidam Basin; depth: 4648.3 m; dissolution pores. (B) Well Dongping-17 in the Qaidam Basin; depth: 4559.35 m; dissolution pores along fractures. (C) Well Dongping-103 in the Qaidam Basin; depth: 3230.6 m; dissolution fractures. (D) Well Dongping-17 in the Qaidam Basin; depth: 4461 m; dissolution pores along fractures. (E) Core from well Longtan-3 in the Central Uplift; high-angle reticular fractures. (F) Well Fs2 in the Central Uplift; depth: 1884.10 m; mineral dissolution pores. (G) Well BZ19-G in the Bozhong area; 4537 m; fractures. (H) Well BZ19-B in the Bozhong area; depth: 4432 m; intergranular pores.
Figure 8. Types of storage spaces. (A) Pore from well Jiantan-1 in the Qaidam Basin; depth: 4648.3 m; dissolution pores. (B) Well Dongping-17 in the Qaidam Basin; depth: 4559.35 m; dissolution pores along fractures. (C) Well Dongping-103 in the Qaidam Basin; depth: 3230.6 m; dissolution fractures. (D) Well Dongping-17 in the Qaidam Basin; depth: 4461 m; dissolution pores along fractures. (E) Core from well Longtan-3 in the Central Uplift; high-angle reticular fractures. (F) Well Fs2 in the Central Uplift; depth: 1884.10 m; mineral dissolution pores. (G) Well BZ19-G in the Bozhong area; 4537 m; fractures. (H) Well BZ19-B in the Bozhong area; depth: 4432 m; intergranular pores.
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Figure 9. Diagram of oil and natural gas sealing and aggregation modes (Oil and gas (yellow arrow) migrates along fractures and faults (red dashed line) to form traps).
Figure 9. Diagram of oil and natural gas sealing and aggregation modes (Oil and gas (yellow arrow) migrates along fractures and faults (red dashed line) to form traps).
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Table 1. Basement reservoirs type diagram and characteristic description.
Table 1. Basement reservoirs type diagram and characteristic description.
TypeProfile of ReservoirGeological Characteristics
Weathered carapace typeMinerals 13 01052 i001After weathering crusts underwent denudation and leaching, oil and gas migrated to and accumulated on top of the weathering crusts in the presence of faults and unconformities acting as shields, thus forming reservoirs. They can be further classified into the erosion, structural, and rock mass eruption types according to the types of traps on the top of weathering crusts.
Weathered inner typeMinerals 13 01052 i002Under the influence of heterogeneous minerals, a complex inner fracture system was formed, in which oil and gas accumulate and form reservoirs. Depending on their inner structural characteristics, they can be further classified into fault-block, fault, formation, and permeation types.
Weathered composite typeMinerals 13 01052 i003Owing to the long-term effects of geological structures, weathering, and leaching, both weathering crusts and inner fractures are well developed. As a result, oil and gas accumulated on top of weathering crusts and inner fractures, forming reservoirs.
Table 2. Characteristics of typical basement reservoirs in China.
Table 2. Characteristics of typical basement reservoirs in China.
BasinLocationReservoir LithologyReservesBedrock Era
Bohai Bay BasinDongshengpu oilfield in the Damintun sagGranite + gneiss0.26 × 108 tAr
Bohai Bay BasinJing’anpu oilfield in the Damintun sagCarbonate rock2.1 × 108 tPt
Bohai Bay BasinXinglongtai in the Liaohe depressionGranite + gneiss0.17 × 108 tAr
Bohai Bay BasinRenqiu in the Jizhong depressionCarbonate rock0.55 × 108 tPt
Bohai Bay BasinBozhong 19-6 gas field in the Bozhong sagGranite + gneiss1700 × 108 m3Ar
Bohai Bay BasinPenglai 9-1 oilfield in the Miaoxibei upliftGranite0.25 × 108 tAr
East China Sea BasinLishui sagGranite1.57 × 108 tMz
Qiongdongnan BasinSongnan low upliftGranite5000 × 108 m3Mz
Pearl River Mouth BasinHuizhou sagGranite2.67 × 108 tMz
Hailar BasinBeier sagClastic rock1811 × 104 tAr
Qaidam BasinDongping area in the eastern section of the piedmont of the Altun MountainsGranite + gneiss520 × 108 m3Pz
Tarim BasinTabei upliftCarbonate rock10 × 108 tPt
Jiuquan BasinYaerxia oilfieldGranite + gneiss2591.3 × 104 tPz
Songliao BasinShiwu fault depressionIgneous rock1917 × 108 m3Pz
Songliao BasinCentral upliftGranite + gneiss6031 × 108 m3Pz
Songliao BasinEastern Changling fault depressionIgneous rock542 × 108 m3Pz
Songliao BasinDehui fault depressionIgneous rock210 × 108 m3Pz
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Lyu, Q.; Wang, W.; Jiang, Q.; Yang, H.; Deng, H.; Zhu, J.; Liu, Q.; Li, T. Basement Reservoirs in China: Distribution and Factors Controlling Hydrocarbon Accumulation. Minerals 2023, 13, 1052. https://doi.org/10.3390/min13081052

AMA Style

Lyu Q, Wang W, Jiang Q, Yang H, Deng H, Zhu J, Liu Q, Li T. Basement Reservoirs in China: Distribution and Factors Controlling Hydrocarbon Accumulation. Minerals. 2023; 13(8):1052. https://doi.org/10.3390/min13081052

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Lyu, Qixia, Weiming Wang, Qingchun Jiang, Haifeng Yang, Hai Deng, Jun Zhu, Qingguo Liu, and Tingting Li. 2023. "Basement Reservoirs in China: Distribution and Factors Controlling Hydrocarbon Accumulation" Minerals 13, no. 8: 1052. https://doi.org/10.3390/min13081052

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