Cenozoic Sedimentary Characteristics of the East China Sea Shelf Basin and an Evaluation of the Suitability of Geological Storage of Carbon Dioxide in the Saline Water Layer

Abstract

Carbon sequestration in the sea area, with the advantages of high storage potential, high safety, low environmental impact, and a long storage period, serves as an important way to achieve the goals of “carbon peaking and carbon neutrality”. The East China Sea Shelf Basin has a large area, and the target layer of carbon storage has a large thickness and wide distribution. Due to the high tectonic stability of the basin and the absence of earthquakes of magnitude 7 or higher, the basin is a subcooled–subthermal basin in terms of geothermal field characteristics. In addition, the basin has a high degree of oil and gas exploration and development, some of the reservoirs are in production depletion, the oil and gas transmission pipeline network is well developed, and the development engineering data are informative. Therefore, it enjoys the characteristics of shallow seawater depth; being close to the surrounding industrial areas; and having a short transmission distance, large effective space, and mature engineering conditions. Among the oil-and-gas-bearing basins in China’s waters, the suitability of carbon dioxide (CO₂) geological storage in the saline water layer of the East China Sea Shelf Basin is highly suitable. The analytic hierarchy process (AHP) is used to comprehensively evaluate the suitability of CO₂ geological storage in the saline water layer of each tectonic unit in the basin, and it is concluded that Taipei Depression and Zhedong Depression are highly suitable zones, Changjiang Depression and Haijiao Uplift are moderately suitable zones, and Yushan East Uplift, Hupijiao Uplift, and Pengjiayu Depression are generally suitable zones.

1. Introduction

According to scientific research, excessive carbon emissions will lead to global warming, the greenhouse effect, and extreme weather. The greenhouse effect, as the most direct and serious problem, will lead to a series of climate problems such as increased surface temperature, increased regional precipitation, an increased number of typhoons, and changes in the circulation field [1,2]. The IEA study concluded that there are four main types of countermeasures to achieve carbon neutrality: carbon substitution, carbon emission reduction, carbon recycling, and carbon sequestration, and it is expected that carbon sequestration will contribute about 14% to carbon neutrality by 2060 [3]. In addition, more and more scientists are turning their attention to the utilization of CO₂, such as carbon-based composites that are promising for practical use, and so on [4,5].

There are many forms of CO₂ storage, such as hydrate replacement storage [6], displacement of crude oil storage [7], and so on. If divided according to the location of sequestration, CO₂ sequestration can be classified into two categories: carbon sequestration on land and carbon sequestration in the sea. In China, suitable and low-cost storage sites for most CO₂ emission sources can be found in Ordos, Bohai Bay, Songliao, and northern Jiangsu basins [8], while the eastern coastal regions, such as Shandong, Jiangsu, Hebei, and Guangdong provinces, are the major CO₂ emission provinces, producing CO₂ emission sources for which suitable storage sites cannot be found within 250 km. Theoretically, the terrestrial Bohai Bay Basin and the northern Jiangsu Basin can provide storage sites for coastal provinces and cities. However, the high population density, low land resources, and concentration of major engineering facilities in these areas pose certain safety risks. In contrast, the Bohai Sea Basin, the South Yellow Sea Basin, the East China Sea Shelf Basin, the Pearl River Estuary Basin, the Beibu Gulf Basin, the Yingge Sea Basin, and the Southeast Qionghai Basin in China’s waters are all adjacent to land and have been studied to a higher degree. Therefore, these areas have obvious advantages in terms of site space, environmental impact, and risk controllability, and they can be used as sequestration sites for major coastal CO₂ emission sources. Meanwhile, compared with land-based areas, carbon storage in marine areas has the advantages of high storage potential, high safety, low environmental impact, and long storage period [9,10].

The geological bodies that can be used for CO₂ storage include deep saline water formations, depleted oil and gas fields, basaltic rocks, and so on. Among them, deep saline water formations are widely distributed in most sedimentary rock basins around the world. Geological storage of CO₂ in saline water layer refers to the injection of supercritical-state CO₂ [11] into the saline water layer, after which CO₂ is sequestered into geological bodies under a series of petrophysical and stratigraphic binding, dissolution, and mineralization [12]. This type of storage, due to its large storage capacity, mature technology, high safety, and low cost, is an important means of CO₂ storage [13,14,15,16]. Recent years have witnessed the increasing attention of the international community toward CO₂ storage in the saline water layer, and the Japanese demonstration project of subsea carbon storage has sequestered 300,000 tons of CO₂ in the deep saline water layer near the port of Tomakomai, Hokkaido [17]. The U.S. and U.K. have completed assessments of CO₂ drive and storage potential in the North Sea continental shelf and Gulf of Mexico oil and gas fields [18], respectively.

In this paper, we attempt to evaluate the suitability of basin-level and zone-level CO₂ geological storage by analyzing the geological conditions of CO₂ geological storage in the East China Sea Shelf Basin. To achieve this goal, we collected a large amount of data on the geological conditions of CO₂ storage in the East China Sea Shelf Basin and used the analytic hierarchy process (AHP), and we established a three-level index system, selected 12 specific indicators, and established a zone-level suitability evaluation index system for CO₂ geological storage areas in the East China Sea Shelf Basin. Finally, we comprehensively evaluated the suitability of geological storage of CO₂ in each zone, which will provide a basis for site selection for the implementation of CO₂ storage in sea areas.

2. Geological Background of the East China Sea Shelf Basin

The geotectonics of the East China Sea shelf basin is located at the interface of the Eurasian, Pacific and Indo-Australian tectonic plates, and belongs to the overlapping interval between the Huaxia tectonic domain and the Tethys and West Pacific tectonic domains [19,20,21,22,23]. The basin ranges between 25°22′ and 33°38′ N latitude and 120°50′ and 129°00′ E longitude. Its geographic orientation is north–east, with a length of about 1150 km, a width of 90 to 300 km, and an area of 23.96 × 10⁴ km², making it the largest sedimentary basin on the offshore shelf of China.

The East China Sea Shelf Basin is a double-layered structure with a lower fault and an upper depression and is a continental margin rift basin in nature. The basin is divided into the western fracture zone, the central uplift zone and the eastern fracture and depression zone [20]. There are seven primary geological tectonic units in the East China Sea Shelf Basin, namely, the Zhendong Depression, Changjiang Depression, Taipei Depression, Pengjiayu Depression, Hupijiao Uplift, Haijiao Uplift, and Yuyudong East Low Uplift [22,23] (Figure 1).

In the East China Sea Shelf Basin, oil and gas exploration started in 1974. The former Marine Petroleum Exploration Bureau of the Ministry of Petroleum Industry, the Shanghai Marine Geological Survey of the Ministry of Geology and Mining, China National Offshore Oil Corporation and Sinopec Shanghai Marine Branch have undertaken a lot of exploration work, completing a total of more than 300,000 km of 2D seismic surveys and about 4000 km² of 3D seismic surveys. The areas with a high degree of exploration are the Lishui Depression of the Taipei Depression and the Xihu Depression of the Zhendong Depression. Incomplete statistics show that a total of 25 wells have been drilled in the Taipei Depression, and 109 wells have been drilled in the Xihu Depression.

By the end of 2019, 2 oil fields and 15 gas fields (Figure 1) had been established, with complete geological information and development data and a clear understanding of the geological features. The cumulative proven geological reserves of oil are 40 million tons, and the cumulative proven geological reserves of natural gas are nearly 300 billion sq. m. In 2019, crude oil production was 390,000 tons, with cumulative oil production of over 6 million tons, and natural gas production was 1.2 billion sq. m., with cumulative gas production of over 17 billion sq. m. It should be especially noted that the Lishui 36-1 gas field, located in the Lishui Depression of the Taipei Depression, is currently in the middle and late stages of development, with the degree of condensate gas and condensate recovery at 54% and 87%, respectively. Because of its large number of depleted oil and gas reservoir distribution areas, it is a favorable area for implementing geological storage of CO₂ in the saline layer.

The East China Sea is a shelf–slope–trench–island–arc–basin system with a water depth of 20~3500 m. Its land slope is steep, and there is a complex topography, with trenches, troughs, and ridges. The East China Sea continental shelf is one of the widest shelves in the world. The shelf starts from the low tide line of the coastal zone and slopes gently to the southeast until the outer edge of the shelf turns at a water depth of 140~160 m. Its slope is gentle, with an average slope of about 58″, and is divided into the inner shelf and outer shelf by 50~60 m (Figure 2). Most of the shelf basins in the East China Sea have water depths of 20–140 m, with an average depth of 72 m. Therefore, the shallower and flatter the water bodies and the smaller the seafloor undulations, the more favorable the implementation of carbon sequestration projects.

3. Cenozoic Sedimentary Characteristics

3.1. Stratigraphic Distribution Characteristics

The thickness of the sedimentary strata is a key parameter affecting the geological storage potential of CO₂. Under the same geological conditions, the thickness of sedimentary strata is proportional to the geological storage potential of CO₂ in the saline layer. In addition, the thickness of sedimentary strata also has an important influence on the geological storage effect of CO₂. If the formation thickness is less than 800 m, the temperature and pressure conditions for supercritical CO₂ geological storage cannot be achieved; if the formation thickness exceeds 3200 m, the porosity and permeability of the formation become poor, and the injectability and cost of CO₂ injection will be greatly increased. Therefore, the 800~3200 m stratum is selected as the target layer for carbon sequestration.

The Cenozoic of the East China Sea Shelf Basin includes the Paleoproterozoic Yueguifeng Formation, Lingfeng Formation, Mingyuefeng Formation, Baoshi Formation, Pinghu Formation and Huagang Formation, and the Neoproterozoic Longjing Formation, Yuquan Formation, Liulang Formation, Santan Formation and Donghai Group. The sediment thickness of the Paleocene is 1000~12,000 m, showing the distribution characteristics of thick in the east and thin in the west, thick in the east and west and thin in the middle. The largest sedimentation center is in the central part of the Zhendong Depression, where the sedimentation thickness of the Hupijiao Uplift and Haijiao Uplift is relatively small, and the sedimentation thickness is only 1000 m. The sedimentation thickness of the Neoproterozoic is 1000~5000 m, the largest sedimentation center is in the central and southern part of Zhendong Depression, and the maximum sedimentation thickness of Neoproterozoic in the western part of the basin is 2000 m.

The 800~3200 m target layer is distributed in the whole basin, and the locations where the sedimentary thickness exceeds 3500 m are mainly located in the eastern Zhedong Depression and the western Yangtze Depression and Taipei Depression (Figure 3). The giant thick sedimentary stratum can provide sufficient storage space for CO₂ geological storage.

3.2. Sedimentary Evolutionary Characteristics

During the Cenozoic Paleocene period, the East China Sea Shelf Basin was mainly in the faulting period, with a large difference in terrain elevation. With the continuous invasion of seawater from south to north, the western slope of Lishui Depression in the southern part of Taipei Depression developed a number of large-scale deltaic–shallow–lacustrine and deep-lacustrine sedimentary systems, and the Yangtze Depression was mainly lacustrine and fluvial sedimentary; in the Eocene period, the basin turned into a fault depression transition period, with local uplift on the western side. With another large-scale intrusion of seawater, most of the tectonic zones inside the basin were submerged. The central and eastern parts of the basin gradually evolved into a whole shallow marine sediment, only on the western side of the Minzhe uplift zone and the northern side of the Yandang low relief and Yushan relief, the coastal marine sediment was developed; the Oligocene basin turned into the depressional period, until the end of the Oligocene, the large-scale uplift of the western side of the basin led to the large-scale denudation of the sedimentary strata, and only in the eastern part of the basin, the shallow marine sediment remained in the Keelung depression. The large delta developed in the south of the Lishui Depression continued to advance eastward to the west of the Keelung Depression; during the Miocene, the basin subsided again, with shallow coastal marine sediments dominating in the south of the basin and deltaic–shallow lacustrine phases in the north. The west side of the basin mainly develops fluvial deposits (Figure 4).

4. Geological Conditions of CO₂ Storage

4.1. Rupture and Seismic Activity

Fracture and seismic activities, as key factors affecting the safety and stability of CO₂ geological storage, may lead to a significant reduction in CO₂ storage life and safety. Therefore, the CO₂ geological storage site should avoid the area with strong seismic activity, high earthquake magnitude, and active fracture development as much as possible.

The East China Sea Shelf Basin is an extension of the South China massif to the land area, and three groups of fault systems, NE–NNE oriented, NEE oriented, and NW–NWW oriented, are mainly developed. Among them, NE–NNE- and NEE-oriented fractures form the regional tectonic framework of the basin, while NW–NWW-oriented fractures are generally formed later and are dominated by tensor–torsional fractures.

Fracture and seismic activities, as key factors affecting the safety and stability of CO₂ geological storage, may lead to a significant reduction in CO₂ storage life and safety. Therefore, the CO₂ geological storage site should avoid the area with strong seismic activity, high earthquake magnitude and active fracture development as much as possible.

The East China Sea Shelf Basin is an extension of the South China massif to the land area, and three groups of fault systems, NE–NNE oriented, NEE oriented, and NW-NWW oriented, are mainly developed. Among them, NE–NNE- and NEE-oriented fractures form the regional tectonic framework of the basin, while NW–NWW-oriented fractures are generally formed later and are dominated by tensor–torsional fractures.

Some scholars have pointed out that when the magnitude is less than 6, it is suitable for marine CO₂ geological storage. When the magnitude is between 6 and 8, it is less suitable for marine CO₂ geological storage. When the magnitude is greater than 8, it is not suitable for marine CO₂ geological storage [24,25].

Seismic activity is strongest in the plate suture zone or plate margin. The southeastern part of the East China Sea and the Okinawa Trough Basin near the Ryukyu Island Arc are areas of strong earthquakes, with a maximum magnitude up to 8. These strong earthquake zones are prone to strong earthquakes, which have a negative impact on the safety of carbon dioxide geological storage.

The basement of the East China Sea Shelf Basin is a natural extension of the southeastern Chinese continent into the sea, and the ancient deep major fractures basically do not cut through the Fourth System and are less dense, so the seismicity is relatively weak. Only one earthquake of magnitude 6~7 has occurred in the Taipei Depression of the southern basin, and no earthquake of magnitude greater than 7 has been recorded. Several earthquakes with magnitudes of 6 to 7 have occurred on the edge of the basin (Figure 5). It meets the safety requirements of CO₂ geological storage in the East China Sea Shelf Basin.

4.2. Geothermal Gradients of the Geothermal Field

The geothermal field is the thermal distribution field between the surface and interior of the Earth. It can be described by seawater temperature, geothermal gradient, and geothermal heat flow value. Under the same geological conditions, the seawater temperature does not increase very much, and the higher the geothermal gradient and geothermal heat flow value, the higher the temperature of the carbon reservoir and the lower the density and viscosity of CO₂ and, thus, the lower the potential and safety of CO₂ geological storage.

The whole East China Sea region shows a low geothermal gradient and geothermal heat flow value in the stable shelf area, and the basin is relatively cold, while the geothermal gradient and geothermal heat flow value are higher in the deep-water basin and trough, and the basin is relatively hot (Figure 6). Since the average geothermal gradient in the East China Sea Shelf Basin is 33.2 °C/km, and the geothermal heat flow value is 70.4 mW/m², the geological storage potential and safety of CO₂ are high.

4.3. Carbon Reservoir and Capping Layer

In the fan–delta phase sandstone of the Paleocene Yueguifeng Formation and the upper part of the Lingfeng Formation, and in the shoreline sand dam and deltaic sand body of the Mingyuefeng Formation, an effective carbon storage layer is formed (Figure 7). The porosity of the Mingyuefeng Formation averages 17.14%, and the porosity of the Yueguifeng Formation averages 9%; in the lower part of the Upper Eocene Pinghu Formation, the tidal channel sand of the riparian system, the riparian sand and the submerged divergent river sand body of the deltaic system are developed, and the carbon storage layer has good physical properties, with porosity ranging from 16.28 to 25.7% and an average porosity of 20.99%. The lower part of the Oligocene Huagang Formation, mainly distributed in the central depression area, has good physical properties relating to its carbon storage layer in the river–lake phase system of the coastal sand and the lake delta sand, with an average porosity of about 20%; regarding the Lower Miocene Longjing Formation, being self-storing and self-capping, with sandstone, siltstone and mudstone interbedded, its lower part contains coal seams, and the bottom is sand conglomerate, with a porosity of 5~29% and an average porosity of 17%.

The Eocene Baoshi Formation has light gray mudstone, siltstone, and brown-yellow mudstone, which have good capping ability. The upper part of the Pinghu Formation has dark gray mudstone interbedded with siltstone and contains a small amount of coal seam, which also has certain capping ability and can be used as a capping layer; the upper part of the Huagang Formation of Oligocene and Longjing Formation of Miocene develop dark gray and gray-green mudstone interbedded with siltstone and sandstone and siltstone interbedded with mudstone, which can be used as a general capping layer.

Through the comprehensive analysis of the spatial configuration of carbon reservoir and capping layer, it is concluded that four sets of reservoir-capping assemblages can be formed in the Cenozoic of the East China Sea Shelf Basin, namely, the Lower Miocene Longjing Formation, the Oligocene Huagang Formation, the Upper Eocene Pinghu Formation, and the Paleocene Yueguifeng Formation (Figure 7).

4.4. Summary

According to the Cenozoic sedimentary characteristics and geological conditions of CO₂ storage, the East China Sea Shelf Basin has a large area with a large thickness and wide distribution of carbon storage target layers; at the same time, the carbon storage layer is widely developed, the basin has high tectonic stability, there is no earthquake record of magnitude 7 or higher, and the geothermal field characteristics analyze that it belongs to the subcooled–subthermal basin. In addition, the basin has a high degree of oil and gas exploration and development, and some of the reservoirs represented by the Lishui 36-1 gas field are in the production depletion period, with a well-developed oil and gas transmission pipeline network and informative development engineering data. It is characterized by shallow seawater depth, close proximity to the surrounding industrial zone, short transmission distance, large effective space, and mature engineering conditions. The comprehensive evaluation concludes that the suitability of CO₂ geological storage in the East China Sea Shelf Basin is high.

5. Evaluation Methodology for the Suitability of Geological Storage of CO₂ in the Area

5.1. Evaluation Methodology

The method of evaluating the suitability of CO₂ geological storage is mainly AHP.

AHP is a decision-making method proposed by Satty, a professor at the University of Pittsburgh, in the 1970s [28]. AHP is a systematic and hierarchical analysis method combining qualitative and quantitative criteria. The characteristic of this method is to use less quantitative information to mathematize the thinking process of decision-making on the basis of in-depth research on the nature, influencing factors and internal relations of complex decision-making problems so as to provide multi-objective, multi-criteria or simple decision-making methods to tackle complex decision-making problems without structural properties. It is a model and method for making decisions about complex systems that are difficult to quantify completely [29].

The principle of AHP is that according to the nature of the problem and the overall goal to be achieved, the AHP decomposes the problem into different constituent factors and gathers and combines the factors at different levels according to the interrelated influence and affiliation relationship between the factors to form a multi-level analysis structure model, so that the problem finally boils down to the determination of the relative weight of the lowest level (plans, measures, etc. for decision-making) relative to the highest level (total goal) or the arrangement of the relative order of pros and cons.

Its decision-making mindset is to decompose a complex problem into component factors and group these factors according to the dominant relationship to form an ordered hierarchical structure. Then, the relative importance of the factors in each level is determined by a two-by-two comparison and then synthesized within the hierarchy to obtain the total order of importance of the decision factors relative to the goal. Because of the characteristics of combining qualitative and quantitative decision factors and the advantages of flexibility and simplicity, the AHP is widely used in many socio-economic fields, such as energy system analysis, urban planning, economic management, and scientific research evaluation.

5.2. Indicator System

This paper is concerned with the evaluation of the suitability of geological storage of CO₂ saline aquifers. Based on the reference of the related literature [30,31,32] and combined with the basic geological conditions of the study area, the suitability evaluation system is established, and the system framework can be divided into three levels: criterion level, indicator level, and result level (Figure 8).

The criterion layer refers to the decision factors affecting the evaluation of the suitability of the CO₂ geological storage and contains the criteria and evaluation indexes to be considered. Specifically, it covers five criteria such as the research degree of the study area, regional tectonic stability, geological storage potential of CO₂ saline layer, geothermal conditions, and conditions for the implementation of a geological storage project.

The indicator layer, also called the sub-criteria layer, consists of several specific evaluation indicators. In this paper, each indicator is assigned 9, 5 and 1, which correspond to the evaluation results of high suitability, medium suitability, and general suitability, respectively, and the evaluation score of each evaluation indicator is derived. Then, according to the importance of each indicator, the weight value is determined, and finally, the weighted average method is applied to calculate the CO₂ geological storage suitability score value. The evaluation indexes specifically include the following:

• Exploration degree: the higher exploration degree reflects the higher knowledge of the sedimentary basin zone and the higher reliability of obtaining evaluation indicators.
• Data support: This is an auxiliary indicator to measure the suitability of CO₂ geological storage in sedimentary basins in terms of quality and quantity of suitability evaluation data. The more data related to the CO₂ geological storage suitability evaluation index, the higher the data reliability, and the higher the reliability of CO₂ geological storage suitability evaluation results.
• Seismic intensity: This item is evaluated by the occurrence of earthquakes in the past 100 years. Specifically, the larger and more frequent the earthquakes occurring in the area, the lower the suitability of CO₂ geological storage.
• Fracture activity: This item considers whether there are active fractures in the basin zone. The better the closure of the fractures, the higher the suitability of CO₂ geological storage.
• Basin area: This item refers to the area of the basin zone boundary projected on the plane, which to some extent determines the basin CO₂ storage potential. Theoretically, the larger the sedimentary area of the basin, the more favorable the geological storage of CO₂.
• Sediment thickness: This item refers to the sediment thickness of the stratum as the target layer of carbon sequestration from 800 to 3200 m, which directly affects the size of the CO₂ storage potential.
• Carbon reservoir characteristics: This item refers to the lithology and average porosity of the CO₂ reservoir; the higher the porosity, the more favorable the geological storage of CO₂.
• Capping layer characteristics: This item refers to the lithology and continuity of the capping layer overlying the CO₂ reservoir.
• Geothermal heat flow value: This item is used to characterize the heat per unit area transferred and distributed from the earth’s interior to the surface, which is related to the regional tectonic and geological characteristics and directly controls the overall thermal environment of the CO₂ reservoir. The higher the heat flow value of the earth, the more unfavorable the geological storage of CO₂.
• Geothermal gradient: This item reflects the warming rate of the formation with depth and is present as an important factor affecting the geological storage potential of CO₂.
• Offshore distance: This item is the distance of the basin zone from the coast. As the distance from the shore becomes farther, the cost of transporting and injecting CO₂ becomes higher, and the technical difficulty becomes greater, which is not conducive to CO₂ geological storage.
• Depth of seawater: This item refers to the depth of seawater in the basin zone; the greater the depth, the greater the technical difficulty and the higher the cost. In the case of seawater depths greater than 200 m, more advanced and expensive storage processes are required.

The result layer is the output of the zone CO₂ geological storage suitability evaluation based on the weighted average calculation of each evaluation index weight value. It is specifically classified as high suitability, medium suitability, and general suitability.

According to the above definition, the zone level suitability evaluation index system of CO₂ storage in the East China Sea Shelf Basin is established (

Table 1. Evaluation index system for the suitability of CO₂ geological storage zones in the East China Sea Shelf Basin.

Criterion Level	Indicator Level	Weight	Value
			9	5	1
Research degree	Exploration degree	0.05	Under development	High degree of exploration	Low degree of exploration
	Data support	0.05	Sufficient and reliable data	Less sufficient and reliable data	Insufficient data
Tectonic stability	Earthquake intensity	0.05	<6	6~8	>8
	Fracture activity	0.05	Good sealing, no cracks	Generally closed, medium cracks	Poor sealing, large cracks
CO2 storage potential	Basin area (104 km2)	0.2	>10	5~10	<5
	Sediment thickness (m)	0.15	>3200	3200~1500	<1500
	CO2 reservoir characteristics	0.05	Average porosity ≥20%	Average porosity 10~20%	Average porosity ≤10%
	Capping layer characteristics	0.05	Mudstone, high continuity	Silty mudstone, horizontal continuous	Siltstone with mudstone, poor continuity
Geothermal conditions	Geothermal heat flow (mW/m2)	0.05	<50	50~70	>70
	Geothermal gradient (°C/100 m)	0.05	<3	3~5	>5
Conditions for project	Offshore distance (km)	0.15	<100	100~200	>200
	Depth of seawater (m)	0.1	<80	80~150	>150

).

6. Assessment of the Geological Conditions of CO₂ in Each Zone of the East China Sea Shelf Basin

6.1. Zhedong Depression

The Zhedong Depression, located in the eastern part of the East China Sea Shelf Basin, is the largest sedimentary depression in the region, with an area of about 11.46 × 10⁴ km². It is also the Cenozoic sedimentary center of the East China Sea Shelf Basin, and the largest subsidence center is in the central and southern parts of the Zhedong Depression. Among them, the Xihu Depression has been explored to a high degree, with more than 150 wells of various types drilled in the area, and the Pinghu oil and gas field is currently being developed. The 800~3200 m carbon sequestration target layer has large stratigraphic sediment thickness and uniform distribution (Figure 3).

In terms of the carbon storage layer, the lower part of the Pinghu Formation of the Upper Eocene Formation in the East Zhejiang Depression develops tidal channel sand, coastal sand, and submerged divergent river sand bodies of the delta system, with good physical properties of the carbon storage layer and 15–20% porosity; the lower part of the Huagang Formation of the Oligocene Formation is mainly distributed in the central depression area, with the coastal sand body and lake delta sand body of the river–lake phase system and with good physical properties in the carbon storage layer, with about 20% average porosity; the lower middle Xintiandi Longjing Formation is self-storing and self-capping, with sandstone, siltstone and mudstone interbedded; the lower part contains coal seam, the bottom is a sand conglomerate, the porosity is 5~29%, and the average porosity is 17%.

As for the capping layer, the upper part of the Pinghu Formation is dark gray mudstone with siltstone and contains a small amount of coal seam, which also has a certain capping ability and can be used as a capping layer; the upper part of the Huagang Formation of the Oligocene and that of the Longjing Formation of the Miocene develop dark gray and gray-green mudstone and siltstone interbedded with siltstone and sandstone, which can be used as a general capping layer.

Since the offshore distance of Zhendong Depression is more than 300 km, and the depth of seawater is about 100 m, the implementation conditions of CO₂ geological storage are average.

6.2. Changjiang Depression

The Changjiang Depression, located in the northwest of the East China Sea Shelf Basin, is part of the western depressional zone, with an area of about 1.65 × 10⁴ km². Two wells are drilled, among which Meifeng I is the first well in the Changjiang Depression. The largest thickness of stratigraphic deposits in the 800~3200 m carbon sequestration target layer is in the western part of the depression, while the rest of the area has a relatively uniform thickness distribution (Figure 3).

The Meirenfeng I well revealed that the Paleocene Meirenfeng Formation lacustrine deltaic–riverine sandstones and the Eocene Changjiang Formation riverine sandstones can be used as carbon reservoirs [33]. The sandstone of the Meirenfeng Formation accounts for 42%, with a total thickness of about 504 m. The sandstone porosity is 17~20%, and it is unequal-grained feldspathic clastic sandstone. The sandstone of the Yangtze Formation accounts for 59%, with a thickness of about 442 m. The composition of the sandstone is mainly quartz, followed by feldspar, which has the disadvantages of poor sorting, sub-angularity, and muddy cementation.

The lacustrine mudstone of the Paleocene Meirenfeng Formation and the river mudstone of the Eocene Changjiang Formation are capping layers, and lithologically speaking, the capping layers are of average quality.

The offshore distance of Changjiang Depression is less than 200 km, and the depth of seawater is about 50~80 m, so the conditions for implementing geological storage of CO₂ are good.

6.3. Taipei Depression

The Taipei Depression, located in the southwestern part of the East China Sea Shelf Basin, is part of the western depressional zone with an area of about 5.27 × 10⁴ km². The extent of oil and gas exploration in the area is high, second only to the Xihu Depression. The 800~3200 m carbon sequestration target layer has the largest thickness of stratigraphic deposits in the Lishui Depression in the southern part of the depression, while the rest of the area is thicker and evenly distributed (Figure 3).

According to the statistical analysis of the drilling data, the carbon storage layer in the Lishui Depression of the Taipei Depression is mainly developed in the Paleocene. The measured porosity is concentrated in 0–25% and is normally distributed, and the sandstone reservoir level gradually deteriorates from top to bottom in the longitudinal direction [34]. The main carbon reservoirs are developed in the near-source lakebed fan, fan delta, and delta of the Yueguifeng Formation; the shoreline sand dam, fan delta, delta, and lakebed fan of the Lingfeng Formation; and the gravity flow channel, delta, shoreline sand dam, near-source lakebed fan, and carbonate terrace of the Mingyuefeng.

The mudstone of the middle and lower part of the Eocene Wenzhou Formation and the upper part of the Oujiang Formation, the mudstone of the middle and lower part of the Paleocene Mingyuefeng Formation, and the middle and upper part of the Lingfeng Formation can be used as high-quality regional capping layers; the mudstone of the Paleocene Yueguifeng Formation, the mudstone of the Lower Lingfeng Formation, the upper part of the Mingyuefeng Formation, and the mudstone of the lower part of the Eocene Oujiang Formation are local capping layers.

Since the offshore distance of Taipei Depression is less than 100 km, and the depth of seawater is about 80~100 m, the implementation conditions of CO₂ geological storage are good.

6.4. Pengjiayu Depression

The Pengjiayu Depression, located in the southern part of the East China Sea Shelf Basin, is part of the western depressional zone, with an area of only about 0.8 × 10⁴ km². Taiwan oil companies have carried out more oil and gas exploration activities in the area. In total, 800~3200 m of the carbon sequestration target layer has a large stratigraphic sediment thickness and uniform distribution (Figure 3).

Due to the lack of first-hand investigation data, this paper can only speculate based on the regional tectonic and sedimentary evolution process and conclude that the average porosity of its carbon storage layer is lower than that of the Taipei Depression in the north, and the lithology and continuity of the capping layer is better than that of the Taipei Depression.

The Pengjiayu Depression is about 240 km away from Fuzhou City, Fujian Province in the west and 60 km away from Keelung City, Taiwan Province in the south, and the depth of the seawater is about 100 m; therefore, the implementation conditions of CO₂ geological storage are average.

6.5. Hupijiao Uplift

Hupijiao Uplift, located in the north of Changjiang Depression, has an area of about 1.22 × 10⁴ km² and only one drilling well in the area. The 800~3200 m carbon storage target layer has a thin stratigraphic sediment thickness compared to that of the depression area, being 2000 m on average, with a uniform thickness distribution (Figure 3).

The carbon storage layer in the area is mainly riverine and shallow lacustrine sandstone, and the capping layer is mainly shallow lacustrine mudstone with poor continuity.

The offshore distance of the Hupijiao Uplift is more than 500 km, and the conditions for implementing geological storage of CO₂ are poor.

6.6. Haijiao Uplift

The Haijiao Uplift, located between the Changjiang Depression and the Taipei Depression, is adjacent to the West Lake Depression of the Zhe Dong depression in the west, with an area of about 1.43 × 10⁴ km², with no drilling in the area. The fracture orientation is mainly SWW–NEE [35]. The 800~3200 m stratigraphic deposition thickness of the carbon sequestration target layer is thinner in the western part near the Zhendong Depression, generally less than 2000 m, while the thickness in the eastern part near the land side is larger, generally greater than 2000 m (Figure 3).

The carbon storage layer of the Haijiao Uplift is mainly the sandstone of the lower part of the Oligocene Huagang Formation. Analogous to the Xihu Depression, this area is characterized by wide distribution, large thickness, and good connectivity, and it belongs to a relatively high-quality carbon reservoir.

In terms of the capping layer, there are no drilled wells to confirm. However, from the analysis of seismic data from the Xihu Depression to the Haijiao Uplift, a set of marine fine-grained sediment with mudstone as the main layer is deposited above the bottom of the Santan Formation of Pliocene, which is about 300 m thick, and several layers of mudstone are more than 30 m thick with stable distribution; the upper part of Miocene is developed in a set of mudstone with a thickness of about 500 m and which is terminal, and several layers of mudstone are more than 30 m thick. The two sets of mudstone concentrations have a relatively stable distribution on the uplift and can be used as a good capping layer [36].

Since the offshore distance of the Haijiao Uplift is less than 150 km, and the depth of seawater is about 100 m, the implementation conditions of CO₂ geological storage are good.

6.7. Yushandong Low Uplift

The Yushandong Low Uplift, located in the north of the Changjiang Depression, covers an area of about 2.13 × 10⁴ km². There are two drilled wells in the north of the area and five drilled wells in the south. The 800~3200 m carbon storage target layer thickness varies widely, between 2000 and 2400 m (Figure 3).

The carbon storage layer in the zone is mainly shallow coastal marine sandstone, and the capping layer is mainly shallow coastal marine mudstone, so it is presumed that both the carbon storage layer and the capping layer are in poor condition.

Since the distance from the Yushandong Low Uplift is 150~200 km, and the depth of seawater is about 100 m, the conditions for the geological storage of CO₂ are average.

7. Comprehensive Evaluation of the Suitability of CO₂ Geological Storage in Each Tectonic Unit of the East China Sea Shelf Basin

The evaluation of the CO₂ geological storage suitability of tectonic units (zone level) is aimed at selecting suitable zones for target area selection. The evaluation is graded according to the overall score: those with a score of 7 or more are considered to be highly suitable zones, those with a score of 5–7 are considered to be moderately suitable zones, and those with a score of less than 5 are considered to be generally suitable zones (

Table 2. Assignment and Scoring Table of Zone Level Suitability Evaluation.

Zones	Exploration Degree	Data Support	Earthquake Intensity	Fracture Activity	Basin Area	Sediment Thickness	Reservoir Characteristics	Capping Layer Characteristics	Geothermal Heat Flow	Geothermal Gradient	Offshore Distance	Depth of Seawater	Overall Rating	Suitability Evaluation
Taipei Depression	9	9	9	9	5	9	9	9	5	5	9	5	7.4	Highly suitable
Zhedong Depression	9	9	9	9	9	9	9	9	5	5	1	5	7.0	Highly suitable
Changjiang Depression	1	5	9	9	1	9	9	5	9	9	5	9	6.0	Moderately suitable
Haijiao Uplift	1	1	9	9	1	5	9	9	9	9	5	5	5.0	Moderately suitable
Yushandong Low Uplift	1	5	9	9	1	9	5	5	1	1	1	5	4.0	Generally suitable
Hupijiao Uplift	1	1	9	9	1	5	5	5	9	9	1	5	4.0	Generally suitable
Pengjiayu Depression	1	1	9	9	1	9	5	5	1	1	1	5	3.8	Generally suitable

).

According to the final comprehensive evaluation results, the Taipei Depression and Zhedong Depression are highly suitable zones, the Changjiang Depression and Haijiao Uplift are moderately suitable zones, and the Yushandong Low Uplift, Hupijiao Uplift and Pengjiayu Depression are generally suitable zones.

8. Discussion

The activity of natural earthquakes in the East China Sea Shelf Basin and its surrounding areas have been discussed previously. When selecting the site for offshore CO₂ geological storage, a relatively quiet area of seismicity should be chosen to avoid sudden large-scale leakage of CO₂.

In order to analyze the impact of the seismicity of the East China Sea Shelf Basin on the security of CO₂ geological storage, we have specifically counted the data of natural earthquakes that occurred in the East China Sea Shelf Basin and its surrounding areas in the last decade [26]. From Figure 9, over the last decade, a total of 1166 earthquakes with a magnitude of 2.6 or above have been recorded in the East China Sea Shelf Basin and its surrounding areas, including 1028 earthquakes with a magnitude of 4.9 or below, 131 earthquakes with a magnitude of 5–6, and 7 earthquakes with a magnitude of 6–7. The largest earthquake occurred on 13 November 2015, located in the northern section of the Okinawa Trough in the eastern part of the East China Sea Shelf Basin (31.0009° N, 128.8729° E), with a magnitude of 6.7 and a depth of 12 km.

In general, seismicity frequently occurs in the eastern outer margin of the East China Sea Shelf Basin, especially in the Okinawa Trough basin, which belongs to the back arc basin formed by the lithosphere expansion of the Ryukyu trench. Although the earthquake intensity in this area is not large, the frequency is high. Due to the relatively stable basin basement in the East China Sea Shelf Basin, undeveloped deep and large faults, and weak seismicity, in the last decade, only in the southeast edge of the basin near Taiwan Island, two earthquakes of magnitude 6 or above occurred in December 2014 and May 2016, respectively, with a focal depth of more than 240 km, which has little impact on the shallow layer.

Therefore, the seismicity of the East China Sea Shelf Basin has been relatively weak in the past decade. Due to the frequent seismicity in the southeastern and eastern outer edge of the basin, the western part of the basin is more stable than the eastern part, which is more conducive to the implementation of CO₂ geological storage.

9. Conclusions

In this paper, we first collected a large amount of data on the geological conditions of CO₂ storage in the East China Sea Shelf Basin, including the degree of oil and gas exploration, the thickness of Cenozoic sediments, faults and seismic activities, and geothermal field characteristics. According to a comprehensive evaluation, the suitability for geological storage of CO₂ in the East China Sea Shelf Basin is highly suitable. Secondly, using the analytic hierarchy process, we established a three-level index system, selected 12 specific indicators, and established a zone-level suitability evaluation index system for CO₂ geological storage areas in the East China Sea Shelf Basin; finally, we comprehensively evaluated the suitability of geological storage of CO₂ in each zone, which will provide a basis for site selection for the implementation of CO₂ storage in sea areas.

• Carbon sequestration in the sea, as one of the four major countermeasures to achieve carbon neutrality, enjoys the advantages of large storage potential, high safety, low environmental impact, and a long storage period and thus serves as an important way to achieve the goal of “carbon peaking and carbon neutrality”.
• The East China Sea, with a large shelf basin area, has the characteristics of large thickness and wide distribution of carbon sequestration target layer, high stability of basin structure, and subcooling–subthermal basin in the analysis of geothermal field characteristics. At the same time, due to the high degree of oil and gas exploration and development in the basin, the engineering conditions are ripe for implementing the geological storage of CO₂ in the saline layer. By comparing the hydrocarbon-bearing basins in China, it can be found that the saline water layer in the East China Sea Shelf Basin is highly suitable for CO₂ geological storage.
• Using less quantitative information to mathematize the thinking process of decision-making, providing a simple decision-making method for complex decision-making problems with multiple objectives, criteria, or unstructured characteristics. The use of AHP to evaluate the suitability of CO₂ geological storage can ignore the attributes of the geological data itself. The practice has proven that applying AHP to the suitability evaluation of CO₂ geological storage is an efficient and practical method.
• With the help of AHP, the suitability of each tectonic unit in the East China Sea Shelf Basin for CO₂ geological storage is comprehensively evaluated, and it is concluded that Taipei Depression and Zhedong Depression are highly suitable zones, Changjiang Depression and Haijiao Uplift are moderately suitable zones, and Yudong Uplift, Hupijiao Uplift, and Pengjiayu Depression are generally suitable zones.
• We have completed the suitability evaluation of CO₂ geological storage at the basin level and zone level, and the next step is to carry out the suitability evaluation of CO₂ geological storage at the target area level. At the same time, we will carry out dynamic monitoring of CO₂ geological storage and build a comprehensive monitoring and warning system that integrates geological safety, environmental safety, and ecological safety, including CO₂ migration, escape, accompanying earthquakes, water acidification, and ecological damage.