**1. Introduction**

Volcanic reservoirs are widely distributed in more than 300 basins or blocks in over 20 countries and five continents and are becoming an important new area for global oil and gas resource exploration and development [1–5] (Table 1).

According to the characteristics of volcanic oil and gas reservoirs, which have already been discovered around the world, these strata have strong epochal and regional characteristics and mainly include Archean, Carboniferous, Permian, Cretaceous, and Paleogene strata. In addition, they are mainly distributed in the circum-Pacific, Mediterranean and Central Asian regions [6]. The circum-Pacific region is the main area of distribution of volcanic oil and gas reservoirs, which includes the United States, Mexico, and Cuba in North America; Venezuela, Brazil, and Argentina in South America; and China, Japan, and Indonesia in Asia. These areas are followed in importance

by the Mediterranean region and Central Asia. Some volcanic oil and gas reservoirs have also been found on the African continent, such as in Egypt, Libya, Morocco in North Africa and Angola in South Africa [7].


**Table 1.** Production statistics of global volcanic oil and gas fields.

Data Source: This table is modified from [1,7].

At present, volcanic reservoirs have been found in 14 sedimentary basins in China (Table 2) [8]. There are three main sets of volcanic strata that developed in the sedimentary basins of China: Carboniferous-Permian, Jurassic-Cretaceous, and Paleogene-Neogene [7]. In terms of spatial locations, the volcanic reservoirs in China can be divided into two parts: Eastern part and western part. The volcanic rocks in eastern China (such as the Bohai Bay Basin) mainly developed in rifted basins, which were controlled by an intracontinental rift environment that was caused by subduction of the Pacific plate under the Chinese mainland since the Mesozoic and Cenozoic [9–11]. The development of volcanic rocks in the western basins, as represented by the Junggar Basin, was closely related to the formation and closure of the Paleo-Asian and Paleo-Tethys Oceans and the orogeny that was induced by them [12–14].


**Table 2.** Distribution of volcanic rock oil and gas reservoirs in China.


**Table 2.** *Cont.*

Data Source: This table is modified from [8].

Since the first discovery of volcanic reservoirs in the San Joaquin Basin, California, USA in 1887, more than 300 volcanic-related reservoirs or oil-gas occurrences have been discovered worldwide [2,7]. The research and understanding of volcanic oil and gas exploration can be categorized into three stages: Accidental discovery stage, local exploration stage, and comprehensive exploration stage. Up to now, extensive volcanic reservoir exploration has been conducted worldwide, and many volcanic reservoirs have been discovered. Volcanic rocks have changed from being an initial "forbidden zone" for oil and gas exploration to a "target zone". Volcanic reservoirs in the sedimentary basins of China were first discovered in the northwestern margin of the Junggar Basin in 1957. At present, the exploration and development of volcanic reservoirs in the sedimentary basins of China is being comprehensively carried out, and grea<sup>t</sup> progress and breakthroughs have been made.

From a global perspective, nearly all types of igneous rocks, from basic rocks to acidic rocks and from lava to pyroclastic rocks, may have the potential to form e ffective reservoirs. Compared with conventional reservoirs, the formation of volcanic rock storage conditions is more random, which requires specific analysis based on the actual geology of specific regions [5]. At present, the proven oil and gas reserves in volcanic rocks account for only approximately 1% of the total proven global oil and gas reserves but have grea<sup>t</sup> exploration potential [4].

The Junggar Basin is a large-scale composite superimposed basin, which has undergone a complex tectonic evolution process. In the Late Carboniferous to Early Permian, the Paleo-Asian Ocean was completely closed, and the northern Xinjiang was in a post-collisional extensional environment. Large-scale mantle-derived magmatism took place in northern Xinjiang, which included the Junggar region. Volcanic rocks were widely distributed on the uplift structures of the basin [15]. Volcanic rocks in the high parts of structures have undergone long-term weathering, leaching, and dissolution, as well as transformations caused by later tectonic activities, have developed rich secondary pores and fractures, and have become high-quality reservoirs. During the process of oil and gas exploration that has extended to the deep layers of the basin, Carboniferous volcanic rocks have become an important new exploration target. The volcanic eruptions in the Junggar basin are mainly transitional facies between sea and land, with multiple eruptions, mostly small volcanoes, and the complexity of volcanic lithology increases. Most of the volcanic eruptions in foreign countries are marine facies, erupting on the seabed, and the scale of volcanoes is large.

The Hong-Che Fault Zone is located at the southern end of a fault system at the northwestern margin of the Junggar Basin. It is one of the long-term direction areas for oil and gas migration and is rich in oil and gas resources. The Carboniferous in the Hong-Che Fault Zone is an important hydrocarbon enrichment zone in the northwestern margin where various types of oil and gas reservoirs have developed, which have reserves of considerable scale and excellent exploration potential. The distribution system of faults in the Hong-Che Fault Zone is complex, and the lithology and lithofacies of the Carboniferous volcanic rocks are heterogeneous. The distribution of volcanic reservoirs is affected by lithology, and the whole region is oil-bearing but is locally enriched, and the differences are relatively large. At present, there is a lack of systematic research on the formation of volcanic reservoirs in this area, and the distribution laws of oil and gas and the main controlling factors for hydrocarbon accumulations are not clear. Therefore, it is necessary to summarize the characteristics, reservoir forming mechanisms, and main controlling factors of volcanic reservoirs, which is of grea<sup>t</sup> practical significance for predicting favorable volcanic reservoir zones and effectively developing volcanic reservoirs in this area. This knowledge can also provide a theoretical basis for the next exploration and development of Carboniferous volcanic rock-based oil and gas in the Junggar Basin.
