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Abstract: Although oil cracking has been documented as one of the important sources of gas in many overmature marine sedimentary basins, the chemical and carbon isotopic signatures of gases of this origin are still open to question. In this study a Cambrian crude oil from the central Tarim basin, along with its main separated fractions (saturates, aromatics and asphaltenes), were pyrolyzed in sealed gold tubes to investigate how generated gases vary in chemical and carbon isotopic composition and how this variation would influence the genetic interpretation of oil cracking gas. The results indicate that the gases from cracking of aromatics and asphaltenes are much drier and more enriched in 13C than the gases from the cracking of saturates and crude oil at the same level of thermal maturity. In the experimental run of 20°C/h, the dryness index of the gases (defined as the volume percentage of C1 in C1–5) from the cracking of saturates ranges from 26.2–90.6% with the methane carbon isotope change ranging from −54.8‰ to −35.5‰, whereas the dryness index is never lower than 60.6% for the gases from the cracking of aromatics with methane carbon isotope ranging from −39.9‰ to −32.2‰. Correspondingly, experimental data for the four samples plot in different areas in diagrams designed to distinguish oil cracking gas from kerogen cracking gas, such as ln(C2/C3) vs. δ 13C2–δ 13C3 and δ 13C1 vs. δ 13C2–δ 13C3, indicating compositional variability of crude oil could assert an important influence in these diagrams. Therefore it is prudent to bring other geological constraints into consideration to avoid misinterpretation. The kinetic parameters for the bulk generation of C1–5 gas and the methane carbon isotope fractionation extrapolated to geological conditions of 2°C/Ma and an initial temperature of 50°C show that the temperatures of C1–5 gas generation from the aromatics and asphaltenes are lower than those from the saturates and crude oil due to their lower activation energies and frequency factors. Generation of C1–5 gases from the aromatics is modeled to be initiated about 122°C whereas the initiation temperature for the saturates sample is 176°C. Below 189°C (EasyRo=1.8%), the yields of C1–5 gases follow the order: aromatics>asphaltenes>crude oil>saturates. At similar thermal maturity levels, the methane carbon isotopic compositions are significantly different for the four samples, with an order of 13C enrichment: aromatics>asphaltenes>crude oil>saturates, however the difference in methane carbon isotopes becomes smaller with increasing temperature. This indicates that methane carbon isotopic values can be significantly different for gases cracked from oils that are compositionally diverse, especially in the early stage of methane generation. [Copyright &y& Elsevier]

Based on the geochemical characteristics of oil-cracking gas and kerogen-cracking gas revealed by simulation experiments and the chemical composition of natural gases in actual gas reservoirs, two kinds of natural gases with different relationships between C/C and C/C, C/C and C/C, C/C and 100×C/(C-C) were identified in the Tarim Basin, and proposed further by the authors. The relationship charts of C/C and C/C, C/C and C/C, C/C and 100×C/(C-C) can be used to effectively distinguish oil-cracking gas from kerogen-cracking gas. Petroleum geological analysis of the oil-cracking gas reservoirs showed that the distribution of oil-cracking gas is mostly related with deep-seated faults or faults with a large fault throw, and the burial depth of paleo-oil reservoir is relatively high; crude oil-cracking gas resources have been evaluated by using both forward and inversion methods. The plots of C/C vs. C/C, C/C vs. C/C, and C/C vs. 100×C/(C-C) were used to distinguish between oil-cracking gas and kerogen-cracking gas, and estimate the mixed ratios of the two kinds of natural gases in the main gas reservoirs of the platform area. [ABSTRACT FROM AUTHOR]

Abstract: Based on quantitative modeling and experimental data on n-tetracosane cracking to generate gaseous hydrocarbons in a confined system, we calculated the generation kinetics and carbon isotope kinetics of methane, ethane and propane and determined the main controlling factors influencing the variation of δ 13C2 and δ 13C3. The carbon isotope of gaseous hydrocarbons from n-C24 cracking to gas are comparable to those from crude oil reported recently, and they can be used to investigate the geochemical characteristics of crude oil cracking to gas under geological conditions. The geological model of n-C24 cracking suggests it is stable at 150–160°C and its cracking temperature ranges from 180 to 200°C, consistent with geological models of crude oil cracking currently reported. With increasing degree of thermal stress, the changes in δ 13C2 and δ 13C3 are larger than that of δ 13C1. It is shown that isotope accumulation controlled by the recharging history of a reservoir has a considerable impact on the carbon isotope distribution of natural gas. Compared to that of cumulative gas, the carbon isotope ratio of partly accumulation gas is heavier and influences the fractionation of the δ 13C2 and δ 13C3 curves to a greater extent. The geological model of n-C24 cracking to gas has been used to interpret the origin of carbon isotopic variation of natural gas in some reservoirs in the Tarim Basin. [Copyright &y& Elsevier]

Hetianhe gas field, Lungudong gas field and Tazhong gas field are marine marine-origin natural gas reservoirs in the craton area in the Tarim Basin. The natural gas is generated from Cambrian source rocks. The simulation experiment indicated that the cracking of the dispersedly dissoluble organic matter remaining in the source rocks is the main origin of marine natural gas. There are two modes to form gas reservoirs, one is the dry gas reservoir such as Hetianhe gas field, in which gas accumulated on the fault belt with violent tectonic movement, the other is condensate gas reservoir formed on the inheriting uplift such as Lunnan and Tazhong gas fields. The hybrid simulation experiment of cracking gas and crude oil indicated that crude oil accumulated on a large scale in those uplift belts at the early stage, and natural gas filled the ancient oil reservoir at the late stage, and the gas reservoirs were formed after the gas mixed with the crude oil. [ABSTRACT FROM AUTHOR]