X-ray computed microtomography revealing the effects of volcanic, alteration, and burial processes on the pore structure of rocks from unconventional reservoirs (Songliao Basin, NE China)

Abstract

The pore structure and storage volume of rocks are crucial parameters to evaluate the favorability of hydrocarbon reservoirs. The unconventional volcanic reservoirs in the Songliao Basin (NE China) include lava and pyroclastics of different composition. Here we apply the X-ray computed microtomography (micro-CT) to reconstruct the 3D pore space of selected samples (basaltic lava, rhyolitic lava and rhyolitic pyroclastics) at depths between 2283 and 3757 m. We reconstruct the morphology and size distribution of the pores. The pore structure records the effects of superimposed volcanic, alteration, and compaction-burial processes. This latter process produces fractures in the rhyolitic pyroclastic rocks. The basaltic lavas have larger, connected pores filled by alteration minerals with a prolate shape, and unconnected, smaller, blade-like, empty pores. The hydrothermal alteration reduces the porosity and the rock is impermeable being the smaller voids unconnected. The basaltic lavas are good cap rocks but very poor reservoir rocks. The rhyolitic lava flows and pyroclastics, some of which placed in the gas zone of the Songliao reservoirs, show prevailing interconnected fractures due to burial processes and minor pores around crystals formed by syn-eruptive heterogeneous gas nucleation. The pore connectivity is ensured by the fractures and the permeability may be high, also depending on the volume, more than on the number, of the pores. Rhyolites may be good reservoir rocks. We search for possible relationships between porosity and permeability and do not find a clear relation between these two parameters as well as between depth and permeability. Although our analysis is valid within a length scale between about 20 μm and 1 cm, the methodological approach used here can be extended to other reservoirs to study the evolution of the pore systems as a function of the different processes affecting the volcanic rocks.