Abstract
The primary reservoir of the Yurihara Oil and Gas Field, Akita, Japan, is basalticrock, named “the Yurihara Basalt” that erupted onto the deep sea floor during the Middle Miocene.The “ Yurihara Basalt” is composed of an olivine-augite basalt, consisting of pillow lava, sheet flowlava, hyaloclastite, and epiclastic-hyaloclastite, and intercalates shale beds. Vesicular pillow flowsand sheet flows are the most suitable as hydrocarbon reservoir rocks in the Yurihara area. Thepore space of the reservoir is classified into three types; vesicles, dissolution pores and fractures.Lithofacies such as pillow lava and sheet flow lava recognized in the core of the YuriharaBasalt are correlatable with those of the well log data. Well-to-well correlation has revealed thatthe facies change laterally. An intensive study over the field suggests that thicker sheet flows aredistributed near the vent and change gradually and laterally to thinner sheet flows, pillow flows, hyaloclastite and epiclastic-hyaloclastite with distance. Oil producing wells are located in a pillowflow-dominant area.The direction of the lava flows was able to be determined from inner structures of sheet flows.These flow structures were identified as the characteristic arrangement of the vesicles on boreholeimages such as Schlumberger's FMS (Formation Microscanner)TM.The facies distribution was predicted by using a model of lateral facies change and informationon the flow direction. Appraisal wells drilled in the predicted oil bearing pillow flows-dominantarea resulted in a great success and confirmed oil columns. These positive results strongly suggestthat the procedure presented in this paper conrtibutes a major break-through to exploration anddevelopment of basaltic hydrocarbon reservoirs.
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