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The Miyako R-1 exploratory well in Miyako-jima, Okinawa Prefecture, targeted water-dissolved gas in the Miocene Yaeyama Group was successfully drilled to a depth of 2437.14 m. Results of analyses for the petroleum geology of the well are summarized as follows. 1) The lower to middle Miocene Yaeyama Group, which contains coal-bearing sections, has a thickness of ~400 m and is unconformably overlaid by the upper Miocene to Pleistocene Shimajiri Group, which has a thickness of ~2000 m. The Shimajiri Group is subdivided into the Ookamishima, Gusukube and Henna formations in ascending order, based mainly on lithology. Dipmeter data indicate an unconformity exists within the Okamishima Formation. 2) Source rock properties (e.g., total organic carbon content and quality of organic matter) in both groups are poor except for coal-bearing sections, and all of the rocks are immature. 3) Thick sandstone sections in the Yaeyama Group and the lower part of the Shimajiri Group have good properties (e.g., porosity and permeability) and are potential reservoirs. 4) Analyses of head space gas indicate that hydrocarbons at depths of ≥1600 m are mostly thermogenic in origin and occur with microbial methane (C1), while those at depths of ≤1550 m are mainly microbial C1 with minor thermogenic C1 and C3. 5) Gases from a production test are dominantly thermogenic C1 and non-hydrocarbons (~40 % N2 and ~5 % CO2). The maturity of the thermogenic gases is estimated to be 0.8 to 0.9 % Ro equivalent based on the δC2-δ C3 plot. 6) The chemical characteristics of formation waters from a production test are similar to those of oil and gas fields in Japan, and their origin is probably altered fossil seawater, as indicated by their hydrogen and oxygen isotopic compositions.

Petroleum geology of the Miyako R-1 exploratory well, Miyako-jima, Okinawa Prefecture