Geneses, Sources and Accumulation Process of Natural Gases in the Hinterland of the Junggar Basin

Xiaojun Wang,Zhijun Qin,Dongming Zhi

doi:10.3389/feart.2022.843245

Xiaojun Wang, Zhijun Qin + Show 1 more

Open Access

https://doi.org/10.3389/feart.2022.843245

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Abstract

The Junggar Basin is rich in natural gas resources, but it has hardly been explored, with the proven rate being less than 9.0%. Although the hinterland of the Junggar Basin has a favorable condition for natural gas accumulation, the complex gas sources cause great trouble in the selection of zones and belts for exploration. Based on the molecular composition and stable carbon and hydrogen isotope data of 95 natural gas samples from 72 wells in this area, combined with the characteristics of structural evolution history, burial history, hydrocarbon generation and expulsion history, and fluid inclusions, this paper clarified the geneses and sources of natural gas, identified the secondary alteration of natural gas, and restored the process of natural gas accumulation. Natural gas in the hinterland was divided into four types: Type I was oil-type gas from the Lower Permian Fengcheng Formation; Type II was coal-type gas derived from the Carboniferous source rock; Type III was the mixture of Type I and II gases, which constituted the major fraction of natural gases in the hinterland; and Type IV that referred to secondary microbial gas formed by the biodegradation of crude oil. During the Late Cretaceous, the Carboniferous and Fengcheng source rocks entered the gas generation peak and a series of natural gas reservoirs were formed. However, affected by the later tectonic movements, some gas reservoirs were damaged or adjusted, and natural gas leakage and biodegradation occurred. This study has shifted the focus of natural gas exploration in the study area from the petroleum system associated with the Middle Permian source rocks to that with the Lower Permian and Carboniferous source rocks, which is of great significance for natural gas exploration in the Junggar Basin. Further, it provides an example to identify the geneses and sources of natural gas under complex conditions based on integrated geological and geochemical assessments.

Highlights

The Junggar Basin, a late Paleozoic–Cenozoic superimposed basin, is located in the north of Xinjiang Uyghur Autonomous Region in northwest China (Figure 1A), covering an area of about 130 × 103 km2 (He et al, 2018; Cao et al, 2020; Xia et al, 2021)
The alkane content of the natural gas was relatively concentrated, ranging from 72.87% to 99.01% (95.30% on average) (Table 1), the majority of which was within 90%–100% (Table 1)
Four types of natural gas have been identified in the hinterland of the Junggar Basin

Summary

Introduction

The Junggar Basin, a late Paleozoic–Cenozoic superimposed basin, is located in the north of Xinjiang Uyghur Autonomous Region in northwest China (Figure 1A), covering an area of about 130 × 103 km (He et al, 2018; Cao et al, 2020; Xia et al, 2021). In 2017, the proven oil reserves and the annual oil production in the basin reached 33.6 × 108 and 12.88 × 106 t The proven natural gas reserves in the basin are only 209.25 × 109 m3, which comes to less than 9% The gas-producing reservoirs in the eastern region are mainly volcanic rocks characterized by strong heterogeneity and poor physical properties (Dai et al, 2016; Gong et al, 2019a, Gong et al, 2019b; Gong et al, 2021). Since 2007, the annual natural gas production in the Junggar Basin has been hovering at 2 × 109 m3–3 × 109 m3

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Discussion

Conclusion