Downward Vertical Fluid Flow in Subsurface: Implications at Kitty Field, Powder River Basin, Wyoming: ABSTRACT

Abstract

Drill-stem test pressure data and subsurface geologic studies permit interpretation of fluid-potential relations in the Kitty field area of the Powder River basin. These relations provide substantiation of significant downward fluid flow from Mowry Shale source rocks to Muddy reservoirs in areas where maximum thicknesses of permeable sandstone are developed. Potentiometric contours indicate vertical flow becomes lateral and radiates outward from areas of high potential once confined to the Muddy aquifer system. Pressure distribution within individual reservoir beds is consistent with this interpretation. Vertical flow is presumably sustained by a combination of the following: (1) expulsion of bonded water during thermal alteration of mixed-layer clays in the Mowry Shale; 2) water generation associated with catagenesis and oil generation in the Mowry Shale; and (3) aquathermal pressuring at temperatures in excess of 200°F (94°C). Local downward vertical flow of water at Kitty field may provide the means for hydrocarbon migration from Mowry source beds to Muddy reservoirs. Additionally, pressure gradients associated with this flow may be responsible for trapping approximately 75% of the total oil column in the field. Capillary pressures can account for a maximum of about 200 ft (61 m) of the total observed 835 ft (255 m) oil column. Calculations suggest that an additional 460 ft (140 m) of oil represents the minimum hydrodynamic column and that fluid-potential gradients necessary to trap the additional 175 ft (53 m) probably exist in the field. The Muddy potentiometric surface implies a dynamic aquifer system of downdip inter-formational and vertical, cross-formational fluid flow. Early fluid migration is obscured by this present flow, but can be pressured to be updip toward the basin flanks in response to initial compaction processes. Post-Laramide (Eocene?) exposure of the Muddy aquifer resulted in recharge by meteoric waters. The interaction between meteoric downdip regional flow and local vertically downward flow at Kitty field suggests late accumulation of hydrocarbons. End_of_Article - Last_Page 947------------