LAUSR

Gas shale was treated as a quintuple porosity system. When the source rock matures, a portion of kerogen or bitumen will be transformed into hydrocarbon, and move to displace the water out from inorganic matrix. When the kerogen is recrystallized to graphite, there will be less dissolved gas left in solid kerogen, which was assumed to be substantial in immature source rock. A series of resistivity tests showed the resistivity of shale decreases with higher vitrinite reflectance value, and proved the graphite do exist in overmature shale. With the role of natural fracture in shale analyzed, the water and hydrocarbon stored in overmature shale was summarized, and the overmature organic-rich gas shale should be treated as two porosity system. To find out which kind of pore may accommodate most of the free gas, a generalized material balance equation was presented, and the volume change with pressure drop in organic matrix and inorganic matrix can be calculated simultaneously. The field case herein proved that new material balance equation can be applied to calculate original free gas and adsorbed gas in shale or sandstone, and it was suggested that while in overmature gas shale there is free gas in inorganic pore, free gas in organic pore and adsorbed gas in kerogen, most of the free gas is in the organic pores.