Filling characteristics, reservoir features and exploration significance of a volcanostratigraphic sequence in a half-graben basin -A case analysis of the Wangfu Rift Depression in Songliao Basin, NE China

It has long been considered that thicker volcanostratigraphic sequences should, in theory, contain more favourable reservoir conditions than in thinner sequences, particularly within large-scale stratovolcanoes found in sedimentary basins. However, drilling results from the Wangfu Rift Depression (WRD) in the Songliao Basin, China, show that the thick intermediate volcanostratigraphic sequence contains only one or two layers of favourable reservoir characteristics over approximately 1000 m of volcanic successions. The volcanostratigraphic sequence that is common in the graben basin has a particular distribution pattern as a result of reservoir and origin processes. The Eruptive Interval Unconformity Boundary (EIUB) is introduced and the burial history is investigated in order to determine the reservoir characteristics and origin. We also identify what controls the gas pool accumulation in the thick intermediate volcanostratigraphy. According to the reworked volcanic rocks and weathered crust patterns, the volcanostratigraphy of the basin has 13 wells containing one EIUB, and 2 wells with two EIUBs. The exposed and burial histories of the volcanostratigraphic sequence have produced a variety of pore space including primary pores, secondary pores and secondary fractures. The secondary pores are the most dominant, followed by primary pores and finally fractures. These pore spaces form a reservoir with moderate to low porosity and permeability and the most productive reservoirs are located 40-80 m below the EIUB. The secondary pores are primarily controlled by weathering and leaching as well as deep-burial alteration. Due to the migration of fluids from the overlying strata into the volcanic rocks of the Huoshiling Formation below, the top most layer dissolves first, which significantly enhances the secondary porosity of this layer found 40-80 m below the EIUB. Thus, the number of EIUBs should control the number of favourable reservoir layers. Based on analysis of the reservoir, source and cap rocks in the study area, we conclude that the best exploration targets in these thick volcanostratigraphic sequences, specifically in half-graben basins, are the volcanic inherited-uplift slope areas, followed by the inherited-uplift of the sag area and the uplifted zones. The findings presented in this work provide an insight of the evolution of porosity in other volcanic systems elsewhere and contribute to the likelihood of locating the favourable exploration targets for thick volcanostratigraphic sequences in half-graben basins.