The controlling factors of oil and gas generation from coal in the Kuqa Depression of Tarim Basin, China

The organic matter-rich rocks in the Kuqa Depression are mainly coal measures and therefore are gas prone. However, small amounts of oil also occur and appear to be derived from these coals and associated shales. The hydrocarbons have five different modes of occurrence: pure dry gas, wet gas, condensate, oil–gas mixtures and pure oil. The distribution of coal-related reservoired petroleum in the Kuqa Depression of the Tarim Basin indicates that the existence oil versus gas accumulations are largely influenced by geologic history and timing of generation.The oils and gases occur in various geological settings. The coal measures have mainly generated gas, but in different geological settings, different types of oil and gas accumulations occur. The traps containing dry gas in the Kuqa Depression formed late in the geological history, and therefore could only capture late gas generated by high maturity coaly source rocks. The traps which reservoir wet gas or condensates have a high integrity and formed when the coals had a lower maturity. The oil pools mainly occur in areas where seals are poor and in some cases fractured. The gas previously existing in these reservoirs was largely dissipated, but a small quantity of oil remained to form minor, residual oil or oil–gas occurrences.