Investigation of Hydrocarbon Generation from Jurassic Coal Measures in the Southern Junggar Basin, Northwestern China, Using Closed-System Pyrolysis

Abstract

There are several sets of coal-bearing source rocks within the Lower and Middle Jurassic (J _1–2) in the southern Junggar sub-basin. To obtain a comprehensive understanding of hydrocarbon generation and the amount of potentially generated natural gas, Rock-Eval analysis was conducted on source rock samples to assess their kerogen types and quality. At the same time, maceral observation and gold-tube pyrolysis were carried out to quantify the different maceral groups and to predict the gas potential, respectively. The Rock-Eval data indicate that the investigated samples are dominated by type III kerogen and occasionally supplemented with type II kerogen. Among the six immature source rocks selected for gold-tube pyrolysis, they are vitrinite-dominated except for a dark mudstone from the Badaowan Formation that is liptinite-dominated. A positive relationship between the maximum gas yield and the liptinite content was observed. However, the accuracy of liptinite content estimation largely depends on the experience level of the organic petrographer. The optimized C _1–5 generation activation energies for the six source rocks cover a broad range between 50 and 74 kcal/mol. A relatively larger proportion of higher activation energies (74 kcal/mol) for the carbonaceous mudstone from the Badaowan Formation and also for the dark mudstone from the Sangonghe Formation implies late gas generation. Under geological conditions, the modeled gas conversion from J _1–2 source rocks in the central part of the southern Junggar sub-basin ranged from 52 to 74%, indicating good exploration potential. Notably, the computed gas generation from the dark mudstone from the Sangonghe Formation resembles that from the Xishanyao Formation. Gas generation from the Sangonghe Formation should be considered in the future.