Vesicle distribution in acidic lava flow units in the rift basins of NE China: implications for petroleum reservoir exploration

Abstract

AbstractAcidic lava flows comprise a large proportion of the total volcanic succession in the rift basins of northeast China. The objective of this study was to explore the vesicle distribution in the acidic lava flows in those basins. Two representative areas with acidic lava flow outcrops in NE China were selected for study. These lava flows share similar magma sources and chemistries. The types and inner structures of the acidic lava flows were determined in detail by measuring the shape of the flows (thickness and lateral extent) and the vertical sequences in the outcrops and shallow coring well Y1D1 in the two outcrop areas. Furthermore, the interpretation of deep borehole data and 3D seismic data from the Changling Graben of the Songliao Basin provided additional thickness and geometry data as a robust complement to the outcrop data. This study revealed that the shapes of the acidic lava flows transition from dome to tabular to braided with decreasing size. Furthermore, the vertical sequences can be classified into three zones: the upper vesicle zone, the massive core zone and the basal zone. The borehole thickness data indicate a trend in the maximum proportions occupied by the upper vesicle zone: thick flows have a relatively low proportion occupied by upper vesicle zones, whereas thin flows display a wide range in the proportion represented by upper vesicle zones. Therefore, braided acidic lava flows are more likely to have a high proportion of upper vesicle zones. Upper vesicle zones usually have high porosity and permeability and represent the primary effective gas reservoir system. The greatest lateral extents of gas reservoirs are usually in the tabular acidic lava flows, but the largest proportions occupied by gas reservoirs are usually in stacked braided lava flows. This study can help us to better understand the distribution of volcanic petroleum reservoirs in acidic lava successions where hydrocarbons are encountered but borehole data are limited. Furthermore, this study is useful for interpreting acidic lava flows in logs and seismic data from basins. Copyright © 2016 John Wiley & Sons, Ltd.