REDUCING RISK IN LOW-PERMEABILITY GAS FORMATIONS: UNDERSTANDING THE ROCK/FLUID CHARACTERISTICS OF ROCKY MOUNTAIN LARAMIDE BASINS
An anomalous velocity model was constructed for the Wind River Basin (WRB) based on {approx}2000 mi of 2-D seismic data and 175 sonic logs, for a total of 132,000 velocity/depth profiles. Ten cross sections were constructed through the model coincident with known gas fields. In each cross section, an intense, anomalously slow velocity domain coincided with the gas-productive rock/fluid interval. The anomalous velocity model: (1) Easily isolates gas-charged rock/fluid systems characterized by anomalously slow velocities and water-rich rock/fluid systems characterized by normal velocities; and (2) Delineates the regional velocity inversion surface, which is characterized by steepening of the Ro/depth gradient, a significant increase in capillary displacement pressure, a significant change in formation water composition, and acceleration of the reaction rate of smectite-to-illite diagenesis in mixed-layer clays. Gas chimneys are observed as topographic highs on the regional velocity inversion surface. Beneath the surface are significant fluid-flow compartments, which have a gas-charge in the fluid phase and are isolated from meteoric water recharge. Water-rich domains may occur within regional gas-charged compartments, but are not being recharged from the meteoric water system (i.e., trapped water). The WRB is divided into at least two regionally prominent fluid-flow compartments separated by the velocity inversion surface: a water-dominated upper compartment likely under strong meteoric water drive and a gas-charged, anomalously pressured lower compartment. Judging from cross sections, numerous gas-charged subcompartments occur within the regional compartment. Their geometries and boundaries are controlled by faults and low-permeability rocks. Commercial gas production results when a reservoir interval characterized by enhanced porosity/permeability intersects one of these gas-charged subcompartments. The rock/fluid characteristics of the Rocky Mountain Laramide Basins (RMLB) described in this work determine the potential for significant, relatively unconventional, so-called ''basin-center'' hydrocarbon accumulations. If such accumulations occur, they will be characterized by the following critical attributes: (1) Location beneath a regional velocity inversion surface that typically is associated with low-permeability lithologies; (2) Anomalous pressure, both over- and underpressure, and when, less commonly, they appear to be normally pressured, they are not in contact with the meteoric water system; (3) A significant gas component in the regional multiphase fluid-flow system (water-gas-oil) that occurs beneath the regional velocity inversion surface; (4) Domains of intense gas charge (i.e., high gas saturation) within the regional multiphase fluid-flow system; (5) Compartmentalization of the rock/fluid system to a far greater extent beneath the regional velocity inversion surface than above it (i.e., convection of fluids across the regional velocity inversion surface is reduced or eliminated depending on the nature of the capillary properties of the low-permeability rocks associated with the inversion surface); (6) Commercial gas accumulations occurring at the intersection of reservoir intervals characterized by enhanced porosity and permeability and gas-charged domains; (7) Productive intersections of reservoir intervals and gas-charged domains, which are controlled by the structural, stratigraphic, and diagenetic elements affecting the rock/fluid system; and (8) No apparent meteoric water connection with the gas accumulations and gas columns up to several thousand feet in height. Because some of these critical attributes are not associated with conventional hydrocarbon accumulations, a new set of diagnostic tools are required in order to explore for and exploit these types of gas prospects efficiently and effectively. Some of these new diagnostic tools have been discussed in this report; other have been described elsewhere. In order to maximize risk reduction, it is recommended when exploring for these types of gas accumulations in the RMLB--or for that matter, anomalously pressured gas accumulations anywhere in the world--that highest priority be given to: (1) Evaluating gas distribution in the fluid system, and (2) Predicting enhanced porosity and permeability in the rock system.
- Research Organization:
- Innovative Discovery Technologies, LLC (US)
- Sponsoring Organization:
- (US)
- DOE Contract Number:
- FC26-01NT41325
- OSTI ID:
- 823311
- Resource Relation:
- Other Information: PBD: 29 Dec 2003
- Country of Publication:
- United States
- Language:
- English
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