Analysis of the Distribution of Onshore Sedimentary Basins and Hydrocarbon Potential in China

There are a large number of basins in China’s onshore region. The type, distribution, formation, and development of sedimentary basins have been well studied by numerous scholars since the twentieth century, and numerous oil and gas fields have been detected since 2000 [1,2]. Our understanding of the geological characteristics of these basins has also increased; however, the exploration of unconventional resources (oil shale, oil sand, coalbed gas, shale gas, etc.) requires increased study of sedimentary basins.


Introduction
There are a large number of basins in China's onshore region. The type, distribution, formation, and development of sedimentary basins have been well studied by numerous scholars since the twentieth century, and numerous oil and gas fields have been detected since 2000 [1,2]. Our understanding of the geological characteristics of these basins has also increased; however, the exploration of unconventional resources (oil shale, oil sand, coalbed gas, shale gas, etc.) requires increased study of sedimentary basins.
China has a large reserve of oil and gas, and because of the rapid development of oil and gas exploration over the past 30 years, China's hydrocarbon potential has been increasingly evaluated [1][2][3][4]. The Ministry of Land and Resources proposed a new nation-wide evaluation of the hydrocarbon reserve resources and conducted a series of studies on the geological conditions of oil and gas formation, enrichment regularity, and potential areas to be explored in the future [3]. In the past ten years, the Ministry of Land and Resources strengthened the exploration of unconventional resources. Based on previous results, this paper will systematically study the characteristics of China's onshore sedimentary basins and distribution of oil and gas. The exploration of different fossil energy also has an important impact on the application of related technology.

Sedimentary basin data
The distribution of basins is based on the following: "Map of exploration results of oil and gas basins in China" [5] "Distribution of Sedimentary Basins and Oil and Gas in China" [6] "Map of Oil and Gas Basins and Fields in China " [7], and "China Atlas of Oil and Gas Basin" [8]. The data from certain basins (Qiangtang, Songpan-Abei, South Yellow Sea, etc.) were selected from results within the national oil and gas strategic area. There are 410 sedimentary basins included in the distribution, and the total area is 421.8 × 10 4 m 2 ( Figure 1). duplicate. The details are as follows: (a) All the large basins, the middle and small basins of the West, the Northwest and the Northeast of China are selected from "Distribution of Sedimentary Basins and Oil and Gas in China" [6]. (b) The middle and small basins of the Yangtze-Platform and the Southwest of China are selected from "Map of exploration results of oil and gas basins in China" [5]. (c) The middle and small basins of the Southeast of China are selected from "Map of Oil and Gas Basins and Fields in China" [6]. (d)The boundaries of the basins (Qiangtang, Songpan-Abei, South Yellow Sea) are selected from the results of MLRSCOG [3]. (e)The oil and gas fields are selected from the "China Atlas of Oil and Gas Basin" [8] and the new oil and gas fields discovered by Petro China and SINOPEC in the last 10 years. Table 1 presents data on the exploited, detected, and show basins that contain major oil and gas reserves within China' s onshore region. The data were collected from evaluations of national oil and gas resources [3], petroleum geology annals of China, and exploration results from PetroChina, SINOPEC. Table 2 shows the oil and gas reserves of seven major basins. The data are from evaluations of national oil and gas [9].

Oil and gas resource basic data
The methods to evaluate the onshore conventional oil and natural gas resources are selected mostly on the basis of the exploration degrees and types of the basins [3]. The data of Tables 3, 4 and 5 are integrated from the results of the national oil and gas resources evaluation [3].  Table 3 presents the extent of oil and gas resource in three petroliferous craton basins. The classification and statistical analysis are based on the petroleum geology conditions of the basins and the results of oil and gas exploration.

Analysis of the sedimentary basin areas
The 410 basins ( Figure 1) can be grouped into four categories according to area. There are 11 basins larger than 10 × 10 4 m 2 : Tarim, Songliao, Ordos,      The development of large-scale sedimentary basins in China's onshore region is based on the paleo-platform (plate) or micro-plate geological background [10]. Among the basins larger than 10 × 10 4 m 2 , Tarim, Ordos, Bohai Bay, Sichuan and southern North China basins developed from the Paleozoic craton. Junggar and Qaidam basins had microplate background. Songliao and Erlian basins were the Mesozoic depression basins, and developed from the relative stable Late Paleozoic Jiangmeng The rift basin structure is created by rifting and uplifting and mainly belongs to the Jurassic-Cretaceous period. The oil enrichment layers uplift from the Middle and Lower Jurassic to the Upper Cretaceous, and they form a major oil production belt from west to east. The total oil reserve is approximately 200 × 10 8 t, which represents 30.32% of the total onshore oil resources in China.
(2) The oil enrichment zone of eastern rift basin cluster in the North China and Yangtze-Platform: Belt located between the east fault of the Taihang Mountains and Tancheng-Lujiang fault and within most of the Bohai Bay basin. The northern part contains the Yilan-Yitong basin, and the southern part includes the southern North China, Nanxiang and Jianghan basins. It is composed of a tertiary oil enrichment rift basin belt [11]. The oil reserve in this area is approximately 240 × 10 8 t, which represents 36.32% of the total onshore reserve.

Three gas enrichment craton basins: The Tarim, North China and
Yangtze paleo-plates were formed in the Early Paleozoic. They were under the Tethys Ocean, where they formed thick marine carbonate sedimentation. The plate tectonic process in the Mesozoic and Cenozoic created the Tarim, Ordos and Sichuan craton basins, and the hinterland preserved the entire Paleozoic strata. These gas enrichment craton basins contain 70.19% of the total onshore natural gas in China.

Distribution of oil and gas resources and layer series (or Era)
The long-term, multi-cycle and complex characteristics of China's tectonic evolution determined the sedimentation conditions of the multistage basins, multi-cycle sediments and multi-type and overlapping characteristics of origin basins that are controlled by different tectonic settings. Therefore, source rocks are widely distributed in China, and oil and gas are buried in layers from the Middle and Upper Proterozoic to the Quaternary (Figure 3).

Analysis of the basin hydrocarbon potential
Exploited oil and gas basins: There are 40 exploited oil and gas basins ( Table 1 and Figure 1), and the Bohai Bay, Longliao, Ordos, Sichuan, Tarim, Junggar and Qaidam basins are the seven major production bases.

Detected oil and gas basins:
The detected onshore oil and gas reserves are concentrated in Bohai Bay, Songliao, Ordoc, Sichuan, Tarim, Junggar and Qaidam basins (Table 1 and Figure 1).
Commercial hydrocarbon flows have been found in 45 basins in China. The oil reserves are over 10 × 10 8 t for Bohai Bay, Songliao, Ordos, Junggar and Tarim basins. Bohai Bay basin has the largest detected oil reserve at more than 1000 × 10 8 t. There are six basins (Ordos, Sichuan, Tarim, Songliao, Bohai Bay, and Qaidam) that have detected gas resources larger than 1000 × 10 8 m 3 . Among them, the Ordos and Sichuan basin contain more than 1 × 10 12 m 3 gas resources [10].

Oil and gas basins:
There are 17 basins with detected oil and gas reserves that have not produced oil and gas. The Qiangtang and Cuoqin basins have large areas and a high density of resources, and each has an estimated reserve greater than 1 billion tons. The Changdu, Hetao, Sanjiang and Liupanshan basins each have an estimated reserve larger than 0.3 billion tons (Table 1 and Figure 1).

Other sedimentary basin:
In addition to the previously mentions basins, 348 basins have no detectable oil and gas reserves (Figure 1).

Distribution of exploited oil and gas
China's petroleum basins can be characterized as oil enrichment rift basins and gas enrichment craton basins [11]. China's middle-and large-sized oil fields are mostly distributed in rift basins, whereas the middle-and large-sized gas fields are distributed in craton basins and foreland basins [1] (Figures 2 and 3).

Page 4 of 4
below.
(3) The distribution of the petroleum resources layer series is strongly uneven, and there are various oil and gas distribution features in different regions. In the eastern region, oil is primarily concentrated in Cenozoic and Mesozoic layers and gas is concentrated in Mesozoic layers. In the central region, oil occurs in Mesozoic layers, whereas gas occurs in Upper Paleozoic and Mesozoic layers. In the western region, oil is observed in all four layer series, with the most abundant in the Mesozoic, and gas occurs in Cenozoic and Mesozoic layers. In the southern region, rich oil deposits are found in the Upper Paleozoic and Cenozoic layers, whereas natural gas is mainly distributed in Mesozoic and Upper Paleozoic layers. The oil and gas in the Qinghai-Tibet region is mostly concentrated in the Mesozoic layers (Tables 4 and 5).

Conclusion
There are more than 400 onshore sedimentary basins in China, with 11 basins larger than 10 × 10 4 m 2 and 236 basins smaller than 0.2 × 10 4 m 2 . The development of large-scale sedimentary basins is based on the paleo-platform (plate) or micro-plate geological background.
The Tianshan-Xingmeng orogen and basin cluster in the North China/eastern Yangtze Platform are the major distribution areas of China's onshore oil reserves (66.64%).
The three major craton basins, Tarim, North China and Yangtze, host 70.19% of the total onshore natural gas reserve in China.
The onshore oil resources in China are concentrate in Cenozoic and Mesozoic layers, and the natural gas reserves are mainly distributed in Cenozoic, Mesozoic, and Upper Paleozoic layers.