The generation and migration of petroleum in coal-bearing strata and the evaluation of its potential

Abstract

 Based on research results of Turpan, Yanji and Bayanhaote basins where oils from coal are formed, the generation, primary migration and hydrocarbon generation potential for oils from coal-bearing strata are discussed in this paper. It is pointed out that hydrocarbon generation characteristics for coal-bearing strata in paludal facies differ from those of source rocks in lacustrine facies in many aspects. The types of organic matter in coal-bearing strata are inferior, resulting in low unit hydrocarbon generation potential, low organic matter transformation rate, while its organic matter abundance is high and has a wide variation range. Hence, its total hydrocarbon generation potential is high and depends mainly on organic matter abundance. Accordingly, quantitative evaluation and classification standards for hydrocarbon generation of coal and mudstone in coal measures are proposed respectively in this paper, based on organic matter abundance and the correlative studies on the corresponding parameters. The bitumen in coal measures is also thought to be the main contributor to hydrocarbon generation in accordance with the studies on structural Chemistry. As to the primary migration, soluble high-molecular-weight compounds are hard to be expelled out, due to the high absorptivity of coal, but hydrocarbons, especially for saturates, are easy to be expelled out, causing the ratio of saturates to aromatics lower than 1 and the increase of δ~(13)C in chloroform bitumen. As a result, oils from coal are mainly light oils and condensates. In mature stage, the expulsion efficiency for source rocks in coal measures (including coal) is estimated to be 25%～30% and 50% or so respectively for mature and high mature stages.