Abstract
Miocene hydrothermal activity resulted in the formation of the gas reservoir pore systems of the Nanatani Formation rhyolite lava in the Katagai gas field, Nagaoka area, Niigata Prefecture, Japan.
Based on bulk chemical composition, and XRD and EDS analysis of drill core and cuttings, hydrothermal alteration zones of rhyolitic rocks are divided into the albite (RA), illite (RS), and chlorite (RC) zones. The RA zone is characterized by three types of pore spaces ; one is primary pore spaces, interstitial pore spaces between euhedral albite crystals, and the other leached pore spaces resulted from dissolution of albite which are associated with a small amount of chamosite and monazite. Hydrothermal solution containing dissolved cation (K, Al, Fe, Mg, etc.) leached from the RA zone had migrated to the surrounding rocks, and formed the RS zone. In the RS zone, illite, quartz and carbonate minerals are filled in pore spaces and along fractures, with lesser amount of micro-sized pore spaces. Almost all pore spaces of the chlorite zone, hosted by rhyolite (RC zone) and basalt, are filled with chlorite and other altered minerals.
δ34S data of pyrite from Miocene mudstone, rhyolite and basalt show the mixing of magmatic sulfur and reduced sulfur from mudstone or basement rocks during the formation of the chlorite-illite subzones and the chlorite-pyrite subzone of the chlorite zone.
Main contribution for gas reservoirs in the Katagai gas field is the interstitial pore spaces between euhedral albite crystals in the early stage, and the leached pore system derived from albite dissolution in the next stage. These pore spaces are filled with clay minerals and other altered minerals in hydrothermal system.
