Hydrodynamic flow is a major factor in the entrapment of petroleum in the Cretaceous "J" sandstone of the Denver basin. Two distinct fluid potential minima trend northeast across the geographic center of the basin. The downdip flow of water toward these minima has enhanced holding capacities of oil traps on the eastern and southeastern flanks of the basin and made possible gas entrapment at the Wattenburg field in the basin deep.

The present regime of downdip hydrodynamic flow in the "J" sandstone resulted from subhydrostatic pressures created by regional Pliocene uplift and erosion of approximately 600 m of sediment proximal to the Front Range. Subhydrostatic pressure development is the result of cooling which contracted the fluids more than the pore volumes, and elastic expansion of the pores which exceeded that of the water.

Present hydrodynamic conditions do not adequately explain the unusually low formation water salinities throughout the "J" sandstone. Artesian flow conditions during the Eocene flushed the "J," replacing connate water with meteoric water, conditions similar to those occurring east of the Black Hills today. Growth of the Golden fault during the Oligocene terminated the artesian flow.

Hydrodynamic effects have not been commonly recognized, due partly to the fact that it requires analyzing data which are not usually obtained. The need to understand and evaluate hydrodynamic flow is essential to the explorationist searching for subtle traps, particularly in the Rocky Mountain region.

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