NEW INSIGHT ABOUT THE DIAGENETIC HISTORY OF MID-PERMIAN SHANXI GROUP SANDSTONE IN THE NORTH CHINA BAISN - EVIDENCE FROM THE PORE SPACE EVOLUTION

Mid-Permian sandstone of Shanxi Group in associated with coal beds is studied to explore the history of porosity and permeability change of the sandstone. A diagenetic history is established based on petrographic study. Ten sandstone samples were collected from the east side (GQ samples) and the west side (TL samples) of the Taihang Mountain. At the time of deposition, the Taihang Mountain should be a submerged basement high in the basin. This barrier caused a slightly unequal distribution of detrital components in the two groups of samples. The GQ samples contain more K-feldspar and less quartz grains than the TL samples. The sandstones are made of fine-sand size quartz, chert, feldspars, and phyllitic rock fragments. The sediments are angular but well sorted and free from detrital clay. It is noted that there is an absence of other type of rock fragments. In the matrix, siderite, calcite and kaolinite are major mineral species.

Diagenetic modifications are observed in time sequence: At shallow burial, very minor silica overgrowth was the only event took place, it was directly followed by compaction and deformation of phyllitic sand grains. The mechanical breakdown of phyllite grains immediately plugged nearly all the original pore spaces and also bounded the sands into a tight sandstone. At depth greater than 2000 meters, CO₂ released from coaly material induced kaolinization of feldspars and dissolution of feldspars. Both events significantly increased the porosity of the sandstone to 5% or higher. However, the permeability remained very low and the sandstone remained to be a tight sandstone. At the last, limited amount of carbonates precipitated in the rock, but served little as a cement of the sandstone.The omni-presence of phyllitic sand suggests a young mountain range sit very close to the basin. In turn, this suggest the North China Basin at the mid-Permian time has been broken from an interior cratonic basin and became a foreland basin as one of the earliest response to the Mesozoic Yanshan Movement.