LAUSR

Abstract Seismic imaging of the Sanriku-Oki forearc basin in the Japan Trench plate convergent margin provides insights into the geological controls on hydrocarbon systems in sediment deposited on the continental slope. We perform a series of seismic attribute analyses on 3D seismic reflection data to better define the influence of geological structures (e.g., faults, slumps and gas chimneys) on hydrocarbon sources, migration pathways and reservoirs. In order to identify coal-bearing strata from the 3D seismic volume, we calculate acoustic impedance distribution by integrating seismic and logging data via waveform inversion. Two Late Oligocene to Early Miocene major coal-bearing strata are characterized as low acoustic impedance strata. Gas-bearing strata characterized by anomalously high reflection strength are widely distributed beneath a bottom simulating reflector (BSR). A greater accumulation of gas occurs in the northeast of the study area, where successive slump deposits and gas chimneys are observed. We propose that a BSR with an underlying gas accumulation in the Sanriku-Oki forearc basin developed as a consequence of the expulsion of gas from Cretaceous to Oligocene coal beds and their subsequent upward migration through gas chimneys and faults. Furthermore, the migration and accumulation of gas and gas hydrate were strongly controlled by a series of porous slump deposits. As chimneys and gas pockets occur above the edges of the slumps, gas migration processes controlled by the slumps could be important in the forearc basin. Our study based on seismic attribute analyses demonstrates that features produced by tectonic movements at plate subduction margins (e.g., faults) and associated instability processes (e.g., slumps) represent important controls on the migration and accumulation of hydrocarbons in forearc basins.