由利原油ガス田および鮎川油ガス田における中新世玄武岩質貯留岩の二次孔隙と変質作用について

抄録

Miocene hydrothermal alteration resulted in the formation of the oil and gas reservoir pore systems of the Nishikurosawa and the Onnagawa basaltic rocks in the Yurihara (YR) and the Ayukawa (AY) oil and gas fields, Akita Prefecture, Japan. This study aims to understand the porosity systematics of the altered basaltic rock oil and gas reservoirs.

Alteration zones of the subaqueous basaltic rocks have been identified as the mixed-layer clay minerals-laumontite-thomsonite zone (MLT zons), the mixed-layer clay minerals-prehnite zone (MP zone), and the alkali feldspar zone (K zone) in the YR field, and only the saponite zone (SP zone) in dolerite in the AY field. Porosity types comprise primary pore spaces (vesicles) and secondary pore spaces (fractures, leached pores, and interstitial pores). Leached pore spaces mainly resulted from saponite dissolution. The effective pore spaces were identified with blue-resin infiltration and characterized as pore space structures interconnected by fractures, the large leached pores and/or vesicles. Prehnite, laumontite, mixed-layer clay minerals, and other minerals filled some fractures and subsequent fracturing and recrystallization occurred. This resulted in the dominant characteristic observed in the effective porosity of the MP zone. In the SP zone, almost all large leached pore spaces remained unmineralized.

Sulfur isotope data from disseminated and vein-type pyrite and barite from mudstone and basaltic rocks indicate mixing of reduced sulfur from mudstone, seawater, and magmatic sulfur.

The pore characteristics, alteration mineral assemblage, bulk chemical composition of major elements and REE, and δ ³⁴S data suggest that the formation of porosity systematics during the dome-up tectonics and the circulation of hydrothermal water under the high geothermal gradient in the YR field and the remnant leached pores in the AY field mainly contributed to the formation of the hydrocarbon reservoirs.