Abstract
The Miocene gas reservoirs in the volcanics of the Nishiyama Central (Chuo) oil field (NC field) and the Katagai gas field (KG field) in the Niigata district, Japan, primarily consist of altered rhyolitic rocks. This study aims to understand the porosity systematics of the altered rhyolitic rock gas reservoirs. The alteration minerals in the rhyolitic tuffs and lavas are quartz, albite, adularia, illite, and chlorite with small amounts of mixed-layer clay minerals, smectite, zeolite minerals, calcite, and pyrite. Altered rhyolitic rocks contain primary pore spaces (bubbles) and secondary pore spaces (vugs, fractures, perlitic cracks, interstitial pores, and leached pores). Interstitial and leached pores formed by hydrothermal alteration comprise the pore space of the gas reservoirs. Secondary euhedral albite crystals, which replace volcanic glass, are typically present in the interstitial pores. The crystallization of secondary euhedral albite is critical to the formation of gas reservoirs. The dissolution of secondary albite resulted in the formation of the leached pores. The presence of alteration minerals, such as adularia, illite, chlorite, mixed-layer clay minerals, and quartz in the leached pores resulted in microporosity. However, adularia and mixed-layer clay minerals are absent from the main reservoir of the KG field. The diameter of the interstitial and leached pores ranges from 0.02 to 3 μm and 2μm to 2 mm or more, respectively. Micropores less than 1 μm are predominantly present in the spaces filled with clay minerals.
δ34S data for disseminated and vein-type pyrite from Miocene mudstone, rhyolitic rocks, and basalt indicate mixing of reduced sulfur from mudstone or basement rocks with magmatic sulfur.
The pore characteristics, the alteration mineral assemblage, and δ34S data in the NC and KG fields suggest that the interstitial pores formed early and the leached pores owing to hydrothermal alteration by seawater followed. Subsequently, alteration minerals filled the micropores during diagenesis.
