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Characteristics and Cutoff of Porosity and Permeability of the Effective Volcanic Gas Reservoirs of the Lower Cretaceous Yingcheng Formation in Songliao Basin, NE China


Effective volcanic gas reservoirs of the Lower Cretaceous Yingcheng Formation in Xujiaweizi depression of northern Songliao basin are primarily found in rhyolite, rhyolitic welded tuff, trachyte and basalt. There are six types of reserve spaces together with their assemblages which indicate effective reservoirs, including unfilled vesicles, dissolved pores, inter-spherulite pores, columnar jointing fractures and tectonic fissures. Porosity of 1262 pieces of volcanic rock samples mainly ranges between 1.5% and 10%, and those of intermediate and low porosity below 10% amount to a percentage of 88.4%. Permeability of 1197 pieces of samples ranges between 0.01×10-3μm2 and 1.0×10-3μm2, and those of low permeability less than 0.1×10-3μm2 amounts to 66.7%. Consequently, these indicate the volcanic reservoirs with characteristics of intermediate to low porosity and low permeability. By means of porosity, permeability and productivity testing data from eighty wells, three methods including distribution function curves, cross-plots and statistical method are used to determine the porosity and permeability cutoff values of the four types of volcanic rocks respectively. The results of porosity and permeability cutoff of basalts are 6.2%, 0.005×10-3μm2 and trachyte 4.2%, 0.011×10-3μm2, rhyolite 5.4%, 0.046×10-3μm2, rhyolitic welded tuff 5.7%, 0.036×10-3μm2. Moreover, the results are proved by data of other twenty wells from neighbor areas with an order of accuracy amounts to be 90%. The lower limits of reservoir porosity and permeability of different types of volcanic rocks are primarily related to their reservoir heterogeneities, which were mainly controlled by reservoir space constitution and lengthwise continuity. The lower limits of porosity of trachytic and rhyolitic reservoirs which show larger thickness of continuous lengthwise layers, compared to that of less continuous basalt and ignimbrite. Synchronized mineralogical studies indicate a higher content of dark-colored minerals in basalt which is easier to form secondary fractures that resulted by a higher stress sensitivity. Moreover, dark minerals and mafic feldspars are more easily to be altered into clay minerals which expand with water and thus forming micro-fractures. As a result, permeability of basalt is the best among all the volcanic rocks, and the lower limit of reservoir permeability is significantly lower than the other three types.