Volcanic lithofacies control the space in unconventional, rhyolitic hydrocarbon reservoirs: The Hailar Basin, NE China

Volcanic rocks are generally characterized by significant spatial variations of lithofacies. The characterization of volcanic lithofacies is an important tool to achieve accurate reservoir prediction during hydrocarbon exploration because of the type of lithofacies strictly controls the porosity and other physical and chemical properties of reservoir rocks. Studying the lithofacies of rhyolitic volcanic rocks is essential for uncovering the genesis and characteristics of volcanic reservoirs. The Lower Cretaceous Shangkuli Formation is located on the western boundary of the Hailar Basin in NE China. This basin hosts oil-bearing rhyolitic volcanic rocks and represents a natural laboratory for investigating the spatial distribution of different lithologies and volcanic lithofacies. Here, we present detailed field observations, TESCAN Integrated Mineral Analysis (TIMA), and fluorescence imaging to characterize seven types of rhyolitic volcanic rocks belonging to four main lithofacies associations. We examine the relationship between lithofacies associations and reservoir rocks. Field observations reveal the existence of volcanic conduit, pyroclastic, effusive, and extrusive lithofacies associations within the Hailar Basin. These lithofacies associations can be further subdivided into nine distinct subfacies. Among these, the extrusive lithofacies association is dominant and it is followed by pyroclastic, effusive, and volcanic conduit lithofacies associations. Intra-spherulite microfractures and pores, lithophysa cavities in the extrusive lithofacies associations, and inter-clast pores of the volcanic conduit lithofacies associations constitute the primary reservoir spaces of rhyolitic reservoirs. The lithofacies associations exert a control on the reservoir spaces of rhyolitic volcanic rocks by limiting the degree of supercooling (Delta T) and volatile content, which impact the formation of different textures in rhyolitic magma during syn-eruptive processes. We conclude that spherulitic and lithophysa rhyolites in the extrusive lithofacies associations, and the crypto-explosive breccias in the volcanic conduit lithofacies associations are promising targets for oil and gas exploration. The facies model we obtain from the Hailar Basin may be extended to similar rhyolitic rocks in other basins hosting unconventional reservoirs.