Significance and evolution characteristics of the isobutane/n-butane ratio of natural gas

Nian Liu,Yuanjie Wang,Ting Gao,Xinjie Shan,Chuan Cai,Nansheng Qiu,Zhenming Li

doi:10.1177/0144598719880648

Abstract

In previous studies, two conflicting conclusions existed, which were: (a) the isobutane/n-butane ratio of natural gas increases with the increasing maturity (Ro) of source rocks and (b) decreases with the increasing Ro. In this paper, the correlations between the isobutane/n-butane ratios, dryness of natural gases, and the Ro values of source rocks of 77 gas samples from Cretaceous and Tertiary in Kuqa Depression, Tarim Basin, Triassic Xujiahe Formation in central Sichuan Basin, Carboniferous–Permian in Sulige and Yulin gas field, Ordos Basin, China, and 80 shale gas samples from Mississippian Barnett Shale in the Fort Worth Basin, the United States are analyzed to reveal the evolution of the isobutane/n-butane ratios, then mathematical models of the isobutane/n-butane ratios and Ro are attempted to be established. Results show that the isobutane/n-butane ratio initially increases and then decreases with increasing Ro, both coal-derived gas and oil-type gas. Diverse types of kerogens may be responsible for the different corresponding Ro values when the isobutane/n-butane ratios of gases reach their maximum values. The initial increase in the isobutane/n-butane ratios with increasing Ro is the reason that isobutane is mainly generated at a higher rate by carbonium ion reaction of α-olefins with protons during kerogen primary cracking at lower maturity, whereas free radical reactions to form n-butane relatively quickly during oil cracking at higher maturity and isobutane cracked at a higher rate during the wet gas cracking stage may result in the terminal decreases in the isobutane/n-butane ratios. Besides, mathematical models of the isobutane/n-butane ratios of different types of natural gas and maturity are established. Therefore, the maturity of gas source rock can be obtained quickly based on the models using the isobutane/n-butane ratio combined with other component information (such as dryness, wetness, etc.), which is of great significance to the characterization of natural gas maturity and gas source rock correlation.

Highlights

Natural gas is mainly composed of methane, ethane, propane, butane, light hydrocarbon, nitrogen, carbon dioxide, and rare gas
The maturity of gas source rock can be obtained quickly based on the models using the isobutane/n-butane ratio combined with other component information, which is of great significance to the characterization of natural gas maturity and gas source rock correlation
Due to the influence of isotopic fractionation caused by pyrolysis, methane and higher hydrocarbons should become increasingly enriched in the 13C isotope with increasing thermal stress, i.e. with increasing maturity of their source rocks (Berner and Faber, 1996, 1988; Dai, 1992; Faber et al, 1988; Liu and Xu, 1999; Schoell, 1983; Stahl, 1977)

Summary

Introduction

Natural gas is mainly composed of methane, ethane, propane, butane, light hydrocarbon, nitrogen, carbon dioxide, and rare gas. Methane stable carbon isotope ratios (d13C1) for the gases from the four study areas (Cretaceous and Tertiary in Kuqa Depression, Tarim Basin, Triassic Xujiahe Formation in central Sichuan Basin, C–P in Sulige and Yulin gas field, Ordos Basin, China, and Mississippian Barnett Shale in the Fort Worth Basin, the United States) is –35.8& to –28.2&, –42.7& to –37.2&, –36.5& to –29.8&, and –45.2& to –37.9&, with the average value of –32.3&, –39.9&, –32.5&, and –41.1&, respectively. Based on the above calculation formulas, the Ro of source rocks from the four study areas (Cretaceous and Tertiary in Kuqa Depression, Tarim Basin, Triassic Xujiahe Formation in central Sichuan Basin, C–P in Sulige and Yulin gas field, Ordos Basin, China, and Mississippian Barnett Shale in the Fort Worth Basin) is 0.79%–2.47%, 0.86%–1.23%, 0.71%–2.11%, and 0.52%–1.62%, with the average value of 1.53%, 1.02%, 0.52%, and 1.02%, respectively (Table 1, Figure 4), which corresponds to the predicting maturity levels for the gas samples analyzed, based on calculation model proposed by Berner and Faber (1996) (Figure 5). The data points become rather scattered as Ro increases, which may be a reflection of the fact that iC4 and nC4 at high-maturity degree have different precursors (kerogen, the NSO (nitrogen, sulfur, oxygen) compounds, C15þ saturates, C15þ aromatics, the C6$14 fraction, etc) (Hao and Zou, 2013; Hill et al, 2003)

Findings

Discussion

Conclusions