Hydrocarbon Generation as a Mechanism for Overpressuring in Rocky Mountain Region

Abstract

Overpressured reservoirs are present in the deeper parts of most basins in the Rocky Mountain region. A variety of conditions can cause overpressuring. However, most high pressures in the region are caused by present-day or recently active oil and gas generation or generation in the last few million years in low-permeability rock successions that still contain organic matter capable of yielding thermally generated hydrocarbons. Rocks with above-normal pressure range from Devonian to Tertiary, and commonly are associated with low-permeability (tight) reservoirs. Most of the overpressuring occurs in Cretaceous and Tertiary rocks. Overpressuring in rocks older than Cretaceous is rare and where present only occurs in strata with very rich hydrocarbon source beds such as the D vonian-Mississippian Bakken Formation. Most organically lean pre-Cretaceous source beds are no longer capable of yielding enough hydrocarbons to maintain abnormal pressure. Nearly all overpressured reservoirs and source rocks have temperatures of approximately 200°F (93°C) or higher. In addition, available data indicate that hydrocarbon-related overpressuring does not generally occur if vitrinite reflectance (Ro) values are less than 0.6% in oil-prone strata or less than 0.7% in gas-prone strata. Overpressured gas-bearing strata generally have Ro values of 0.8% or higher. Maximum reservoir pressures are about equal to the fracture gradient pressure in the reservoir rocks. Some vertical fractures in reservoir rocks are probably caused by rapid expulsion of hydrocarbons from the source beds. High pressures in the Rocky Mountain region means that the rock succession is gas or oil saturated, and discrete hydrocarbon-water contacts are generally not present. Classical concepts of hydrodynamics do not appear to be applicable to these overpressured successions. The optimum depths for exploration are generally the upper 1,000-2,000 ft (305-610 m) of the overpressured succession.