The On'nagawa Formation in Akita Sedimentary Basin is composed of siliceous mudstone and volcaniclastics. The volocaniclastic facies have been productive reservoirs of oil and gas fields in the Neogene Akita Basin, North Japan. The mudstone facies have been recognized to be potential source rocks for hydrocarbons pools in the reservoirs. After a long period of oil and gas production in the basin, a potential exploration concept of a new play in the siliceous rocks is required to keep the productivity of the basin.This study reports a basic description of physical properties of the siliceous mudstone facies in order to provide a basis for the idea of shale play in the On'nagawa Formation. Wireline logs (GR, Δt, ρb) are basic data for our description of the physical rock properties as well as descriptions of cutting lithology.
We examined dike-like chlorite rocks that replaced isotropic gabbro and dolerite in northern Oman ophiolite in order to understand the chemical budget of hydrothermal alteration of the oceanic lithosphere. During chloritization, the concentrations of Si, Ca, Na, and K decreased, while those of Fe increased. REE (rare-earth elements), except Eu, which showed a strong depletion in the chlorite rocks, were immobile during chloritization, which was caused by the downward (recharge) flow of circulated seawater. A portion of Fe was supplied from the overlying mafic extrusives, possibly through the alteration of their plagioclases. We found Ti-rich minerals such as rutile and titanite to be the reservoirs of most REE in the chlorite rocks. If the residual fluid, after chloritization, moves upward, it can realize the positive Eu anomaly of the seafloor vent fluids. And, if the fluid is transported to deeper parts of the oceanic lithosphere, rodingites, serpentinites (antigorite rocks), and diposidites with a positive Eu anomaly are formed within gabbros and mantle peridotites.
