New Insight into the Kinetics of Deep Liquid Hydrocarbon Cracking and Its Significance

Abstract

The deep marine natural gas accumulations in China are mainly derived from the cracking of liquid hydrocarbons with different occurrence states. Besides accumulated oil in reservoir, the dispersed liquid hydrocarbon in and outside source also is important source for cracking gas generation or relayed gas generation in deep formations. In this study, nonisothermal gold tube pyrolysis and numerical calculations as well as geochemical analysis were conducted to ascertain the expulsion efficiency of source rocks and the kinetics for oil cracking. By determination of light liquid hydrocarbons and numerical calculations, it is concluded that the residual bitumen or hydrocarbons within source rocks can occupy about 50 wt.% of total oil generated at oil generation peak. This implies that considerable amounts of natural gas can be derived from residual hydrocarbon cracking and contribute significantly to the accumulation of shale gas. Based on pyrolysis experiments and kinetic calculations, we established a model for the cracking of oil and its different components. In addition, a quantitative gas generation model was also established to address the contribution of the cracking of residual oil and expulsed oil for natural gas accumulations in deep formations. These models may provide us with guidance for gas resource evaluation and future gas exploration in deep formations.

Carbon and hydrogen isotope fractionation for methane from non-isothermal pyrolysis of oil in anhydrous and hydrothermal conditions

“…The E a for methane generation is mainly distributed in the range between 52 and 73 kcal/mol (Figure 4(a)). This broad distribution should be attributed to the cracking of different components with diverse thermal stabilities, which include C 14+ NSOs, C 14+ aromatics and saturates, C 6–14 hydrocarbons and C 2–5 hydrocarbon gases (Behar et al., 2008; Tsuzuki et al., 1999; Vandenbroucke et al., 1999; Zhao et al., 2017). Although the yield of methane in pyrolysis with water is slightly higher than that in anhydrous pyrolysis, the conversions of methane at certain temperature in two conditions are essentially identical (Figure 4(b)).…”

Gas Generation and Its Carbon Isotopic Composition during Pyrite-Catalyzed Pyrolysis of Shale with Different Maturities

“…In contrast, during open-system pyrolysis, the secondary cracking of organic matter is very limited and can be neglected. Therefore, during the primary cracking of organic matter, the high temperature of 500 • C promoted the enrichment of 12 C in the CH 4 and 13 C in the C 2+ hydrocarbons generated.…”

“…These results indicate that the addition of pyrite can lead to stronger kinetic isotope effects, as shown in Figure 7. The radicals containing 12 C had faster reaction rates when catalyzed by the active sulfur in pyrite, leading to the more negative δ 13 C values of CH 4 and C 2 H 6 . It was found that, when catalyzed by pyrite, the δ 13 C values for CH 4 and C 2 H 6 at 400 • C underwent greater declines than at 500 • C. The weaker catalytic effect of pyrite at 500 • C is due to the more complete cracking of kerogen at higher temperatures.…”

“…During shale pyrolysis, the gas generation behavior also follows the mechanisms of thermal cracking of chemical bonds and the combination of radical fragments [12]. Pyrolysis conditions, such as the temperature, residence time and catalyst used, have been found to significantly affect gas generation characteristics [13].…”

“…Gases were also found to become isotopically heavier as shale matures [21]. This can be chemically explained by the kinetic isotope effect (KIE), the preferential cracking and recombination of 12 C bonds in kerogen leading to different generation rates of hydrocarbon carbon isotopes [22,23]. However, the effect of heating temperature on the carbon isotopic composition of gas generated from heated shale has not been well analyzed, which has potential engineering applications in the monitoring of formation heat treatments during thermal recovery processes.…”