Overpressured reservoirs in Rocky Mountain region
Overpressured oil and gas reservoirs in the Rocky Mountain region are more widespread than generally recognized. Reservoir pressure gradients in excess of 0.5 psi/ft (> 11.3 kPa/m) are considered overpressured. However, most overpressured reservoirs have pressure gradient greater than 0.6 psi/ft (> 13.6 kPa/m). Although a variety of conditions can cause ovepressuring, most high pressures in the region are interpreted to be caused by the active generation of oil and gas in sequences that still contain organic matter capable of yielding thermally generated hydrocarbons. Rocks with above normal pressure in Rocky Mountain basins range in age from Late Devonian to Tertiary and are commonly associated with low-permeability (tight) reservoirs. Most overpressured reservoirs occur in Cretaceous and Tertiary sandstone sequences. Overpressuring is not common in rocks older than Cretaceous except in very organic-rich sequences, probably because lean source beds that have been heated over a long period of time are no longer capable of yielding enough hydrocarbons to maintain abnormal pressure. Statistically, nearly all overpressured reservoirs and source rocks have temperatures of about 200/sup 0/F (93/sup 0/C) or higher. In addition, available data indicate that hydrocarbon-related overpressuring does not usually occur if vitrinite reflectance values are < R/sub 1/ = 0.5% in oil-prone sequences or < R/sub 1/ = 0.7% in gas-prone sequences. Hydrocarbons expelled into widespread, high-permeability reservoirs probably migrate owing to hydrodynamic flow and buoyancy. These reservoirs usually have normal pressures.
- Research Organization:
- Geological Survey, Denver, CO
- OSTI ID:
- 6525453
- Report Number(s):
- CONF-8309274-
- Journal Information:
- Am. Assoc. Pet. Geol., Bull.; (United States), Vol. 67:8; Conference: AAPG Rocky Mountain Section meeting, Billings, MT, USA, 18 Sep 1983
- Country of Publication:
- United States
- Language:
- English
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