Effect of volcanic activity on hydrocarbon generation: Examples in Songliao, Qinshui, and Bohai Bay Basins in China

Abstract

Numerous gas reservoirs in volcanic rocks have been found worldwide. Three gas fields with reserves of one billion cubic meters of gas related to volcanic rocks were found in China. Hence, volcanic rocks as reservoir rocks are attracting increasing attention from geologists. Volcanic activity cannot only form excellent reservoir rocks but also affect hydrocarbon generation in source rocks. The influence of volcanic rocks on the maturation of source rocks and gas generation rate in the Songliao, Qinshui and Bohai Bay Basins in China is discussed in this paper. The main effects of volcanic rocks on gas generation are as follows: Firstly, volcanic activity always leads to a high regional paleo-geothermal gradient that accelerates the thermal evolution of the source rocks. For example, although the present-day geothermal gradient is only 3.7 °C/100 m, the geothermal gradient reached up to 6.5 °C/100 m in the Songliao Basin when volcanoes were erupting on a large scale. Hence, the Cretaceous Shahezi source rocks, interbedded with two sets of volcanic rocks, had high gas generation rate and rapidly reached high-to over-mature stages (the vitrinite reflectance (Ro) is approximately 3.5% or more). The gas generation rate was 10 times higher than that of other gas-abundant basins in China, like Ordos Basin, and the gas generation intensity was up to 2.4 × 1010 m3/km2. Secondly, igneous intrusions make local source rocks reach high-to over-mature stages very quickly and generate gas promptly, even during source rock uplifting. Many areas were intruded by igneous rocks during the Jurassic to Cretaceous, for example, the paleo-geothermal gradient was as high as 5.5 °C/100 m in the Qinshui Basin. The source rocks were still generating hydrocarbons while the whole Qinshui Basin was being uplifted. The gas generated due to volcanic activity accounted for more than 70% of the total gases generated from source rocks and formed a large coal seam gas field in the Qinshui Basin. Thirdly, when source rocks developed with concomitant volcanic eruptions or igneous intrusions, these types of volcanic rocks were normally on a small scale, and therefore their areas of influence were small. Hot fluids and transition metals from volcanic activity catalyzed and accelerated the thermal evolution of the source rocks so that large amounts of low-mature hydrocarbon were generated.