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

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.

Crystal Structure Determination of the Nonclassical 2-Norbornyl Cation

After decades of vituperative debate over the classical or nonclassical structure of the 2-norbornyl cation, the long-sought x-ray crystallographic proof of the bridged, nonclassical geometry of this prototype carbonium ion in the solvated [C7H11]+[Al2Br7]–• CH2Br2salt has finally been realized. This achievement required exceptional treatment. Crystals obtained by reacting norbornyl bromide with aluminum tribromide in CH2Br2undergo a reversible order-disorder phase transition at 86 kelvin due to internal 6,1,2-hydride shifts of the 2-norbornyl cation moiety. Cooling with careful annealing gave a suitably ordered phase. Data collection at 40 kelvin and refinement revealed similar molecular structures of three independent 2-norbornyl cations in the unit cell. All three structures agree very well with quantum chemical calculations at the MP2(FC)/def2-QZVPP level of theory.