It is widely accepted that hydrocarbon gases can be generated from either primary cracking of marine and lacustrine kerogen or secondary cracking of oil. The yields and composition of the generated hydrocarbons (including chemical and isotopic ones) and temperature, maturity as well as formation time are determined both in natural and laboratory systems. Coal are believed a gas-prone source rocks, however, the generation of hydrocarbon gases from coal is still not well understood, especially for the gases from the secondary cracking of oil in coal. In many cases, the coal-derived oil-like liquid hydrocarbons generated are restrained in coal and its expelling efficiency is very low. Even in laboratory conditions, the liquid hydrocarbons can not be isolated completely from their host coal by routine extraction methods. Some or most of the liquid hydrocarbons are regarded as a part of the coal itself. The open system such as RockEval and PyGC are common-used laboratory methods to study the gas generation of coal, which are not suitable for researching secondary cracking. These factors make the secondary cracking of coal less understood than marine and lacustrine source rocks. The understanding to the coal-derived oil cracking gases has impeded the identification of the source, origin and formation process of many gas pools found in Tarim, Ordos and Sichuan basins of China.
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