Formation mechanism of high-quality reservoirs of Lower Cretaceous volcaniclastic sedimentary rocks in Songliao Basin

High abundance gas reservoirs were discovered in the volcaniclastic sedimentary rocks of Wangfu faulted depression in Songliao Basin, but the distribution pattern and formation mechanism of this type of high-quality reservoirs were not clear. In the Shahezi Formation of Well Block CS6 in Wangfu faulted depression, the void space type, reservoir property, pore throat and pore diameters were studied by means of the casting thin section, helium intrusion porosimetry, mercury intrusion porosimetry (MIP) and nuclear magnetic resonance (NMR) methods, moreover, formation mechanism and main controlling factors of the high-quality reservoirs were discussed. Results showed that lithology of high-quality reservoirs consisted of the volcaniclastic sedimentary agglomerate and the volcaniclastic sedimentary breccia, and exhibited the coarsening-upward characteristics. Furthermore, three types (seven subtypes) of void spaces were identified, among which the intragranular vesicles with primary abundant pores were uniquely developed in the high-quality reservoirs of volcaniclastic sedimentary rocks, indicating this high-quality reservoir belonged to the large-pore and small-throat reservoir; porosity and permeability of volcaniclastic sedimentary agglomerate were slightly higher than those of volcaniclastic sedimentary breccia, and both were significantly higher than those of sedimentary rock in the Shahezi Formation, the porosity was closely related with the burial depth. Formation mechanisms of reservoirs in the volcaniclastic sedimentary rocks were release of volatile matters in parent rocks, grain support, dissolution and tectonic modification, among which the former two were the main controlling factors. The high-quality reservoirs over the study area were mainly distributed in the central part of the fan of volcaniclastic sedimentary rocks, among which the grain-supported, coarse-grained volcaniclastic sedimentary rocks with high content of intragranular vesicular detritus and burial depth less than 3000 m were the most favorable area for development of the high-quality reservoirs.