Petrophysical Characterization of a Complex Volcanic Reservoir, YingCheng Group, Daqing, China1

Gas-bearing reservoirs of the YingCheng Group in the northern SongLiao Basin of north-eastern China are hosted in a complex assemblage of crystalline volcanic and volcaniclastic rocks. These reservoirs display the most problematic features of the two main classes of reservoir-siliciclastic and carbonate-and present a significant formation evaluation challenge. Wide variability in lithology ranging from acidic crystalline volcanics and volcaniclastics to mafic crystalline volcanics complicates the determination of matrix properties and hence porosity. While the complexity of the rock matrix in these reservoirs rivals or exceeds most siliciclastics, their pore network has more in common with carbonate reservoirs. Although much of the reservoir-quality rock is microporous, zones ofbrecciation and leaching display a full spectrum of pore types, ranging from microporous through macroporous. This complex pore geometry combined with excess conductivity effects caused by authigenic clays and zeolites makes saturation evaluation from resistivity measurements difficult. To address the demands of these reservoirs, a novel interpretation methodology was required. This methodology is based on the use of complementary wireline logging measurements, each of which has particular sensitivity to aspects of the rock matrix, pore network geometry, or contained fluids. Integration of these measurements using interpretation models soundly based on a comprehensive core analysis dataset allows for the robust evaluation of lithology, porosity, permeability, and fluid saturations in this challenging interpretation environment. As the development of conventional gas accumulations has matured, less conventional sources are gaining in importance. Globally, volcanic rocks have the potential to host significant gas accumulations that have been historically overlooked. While these reservoirs present significant formation evaluation challenges, this study demonstrates that through the appropriate combination of technology and interpretation methodology accurate quantification of petrophysical characteristics is possible in these types of reservoirs.