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The purpose of this study is to estimate the average flow rate of subsurface formation water through an overpressured reservoir in a significant gas field. Oil （or gas） /water contact in a field can be recognized as a representation of the hydraulic potential surface. Gradient of the potential surface is equivalent to hydraulic gradient in Darcy equation for steady flow of fluidal continuum. Supposing or sampling data of hydraulic coefficient （well data, etc.） , therefore, flow rate can be calculated based on Darcy equation. Inclined oil （or gas） /water contacts have been discovered in many fields around the world to date. A hydrodynamic theory has been proposed to explain the physical mechanism of inclined contacts （Hubbert, 1953） . Peciko gas field in the Mahakam Delta Province, East Kalimantan, Indonesia gives a typical example of inclined contact. The hydraulic gradient governing the Peciko gas field is caused by the hydraulic gradient between its adjacent hydrostatic domain and a highly over-pressured domain. The present average flow rate in the gas zone is estimated to be 3.1 mm/year. This value is less than 2.5 cm/year of subsurface fluid migration in the North Sea estimated by England et al. （1987） . The size of a water molecule （2.1×10 -10 m） indicates that the slow flow means almost no flow on average. Considering that gas accumulation in the Peciko gas field plugs the paths of fluid migration over the field, the present fluid flow averaged over the field should be less than the normal water saturated environment. Under some assumptions, the flow rate would be 3.1 mm/year; this estimate is consistent when averaging those of sandstone and mudstone.Key words： subsurface flow rate, inclined oil （or gas） /water contact, piezometric surface, hydraulic gradient, Darcy's Law, distributary channel system, overpressure, lateral pressure transfer ：浸透流レート 油（またはガス） / 水接触面 ピエゾ水頭面 動水勾配 ダーシー則 分岐チャネル系 異常高圧 水平孔隙圧伝播

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