Differential hydrocarbon generation and evolution of typical terrestrial gas-prone source rocks: An example from the Kuqa foreland basin, NW China

Terrestrial gas-prone source rocks are significant for hydrocarbon gas accumulations especially in central and western basins in China. The Kuqa foreland basin in the northern margin of the Tarim Basin, NW China is rich in natural gas with recent discoveries of several large to giant gas-producing fields (e.g., the Kela-2, Dibei, Dabei-1, and Zhongqiu-1 gas fields, etc.) and a proven 3 P gas reserve of over one trillion cubic meters (TCM). This proliferous petroleum province is powered by an effective and diverse gas-prone source rock series in the basin comprising mudstone, carbonaceous mudstone, and coal seams. To date, little systematic research work has been done on the petroleum generation potential of the source rocks. This has seriously restricted the assessment of petroleum resources and further exploration in the frontier areas of the basin. A sealed gold tube pyrolysis approach was employed to simulate the oil and gas generation and cracking yields for the aforementioned three types of source rock. Compared with typical Type I and II1 kerogens, the three types of source rocks in the Kuqa foreland basin are characterized by (1) far less hydrocarbon yield (150 mg/g TOC) with comparatively larger amounts of CO2 (310 mg/g TOC), (2) a limited capacity and narrower window (0.5%-1.08% Ro) for oil gen-eration, and (3) a much earlier secondary cracking for bulk oil to light oil and gases (>0.86% Ro for mudstone and >1.08% Ro for both carbonaceous mudstone and coal). All the source rocks are gas-prone, and significant amount of gas can still be generated during an over-mature stage. The kinetic parameters obtained allow us to model hydrocarbon generation in the Kuqa foreland basin more realistically. It is concluded that (1) two episodes of hydrocarbon generation occurred with hydrocarbon gases being mainly generated during the later stage (12-3 Ma); and (2) the depocenter of the basin is capable of supplying massive high-mature gases as late as in the Quaternary, which is crucial for the formation of giant gas fields. The findings provide new insight on the reconstruction of oil and gas accumulation processes and further hydrocarbon exploration in the basin.