Introduction
As a result of new technology, the Greater Rocky Mountain Region (GRMR) has been the home to more giant fields “discovered” in the last 10 years than any other major US onshore province.
What’s more, it is one of only two provinces that have been growing in production during that time. Rocky Mountain and San Juan basin gas production has grown continuously (Figs. 1, 2). Note the major anticipated growth in new tight gas fields with increasing price (Fig. 3).
Although recent advances in all technological aspects of exploration and exploitation have been important in this renaissance, equally important is the geologic setting in which abundant, extremely rich source rocks for both gas and oil are liberally distributed throughout under-explored provinces.
Summary
The GRMR is a large, geologically heterogeneous area that contains numerous basins and uplifts. Although it contains a wide variety of structures generated at different times, those generated in the Lower and Upper Tertiary are commercially the most significant.
Numerous oil-prone and gas-prone source rocks and prospective reservoirs are present, and these have contributed to the presence of a large number and variety of petroleum systems. Productive and prospective reservoirs include a spectrum of carbonates and sandstones that contain matrix porosity and permeability, as well as fracture-type and coalbed methane reservoirs.
Several investigating entities have estimated the potential for future producible hydrocarbon discoveries to be 10.4 to 15.4 billion bbl of petroleum liquids and 192 to 260 tcf of gas. Of particular significance to further exploration and development potential are a class of unconventional accumulations associated with pervasive regional hydrocarbon saturation, a general absence of movable ground water, and the presence of either abnormally high or low fluid pressures. These accumulations may be dynamic and transient in nature and commonly occur in low-permeability or fractured reservoirs associated with mature source rocks in the deeper parts of typical Rocky Mountain basins.
Petroleum systems present in the Cretaceous and Lower Tertiary section will be major contributors to future hydrocarbon production, and gas will be of particular importance because of the large number of coal measures present. Gas generated by thermal or bacterial processes is present in coal beds and nearby sandstone reservoirs. Exploration and development opportunity is present in regions associated with confirmed high-generation-capacity source rocks but with little established production.
Most of the eight so-called “discoveries” of the 1990s represent hydrocarbon accumulations known through prior exploration. They had little economic significance until the development of geologic understanding, drilling, evaluation, and completion technology rendered them economically viable.
These eight giants have been developed by:
- Being “rediscovered” and exploited by new technology
- The merger of isolated coalbed methane deposits into giant fields
- The development of central basin gas deposits and their eventual merging into giant fields.
New technology applied
Application of new technology has been an important element in discovering and establishing new oil and gas reserves in the GRMR. Many developments have occurred in areas where the presence of hydrocarbon saturation was known but hydrocarbons could not be economically produced with then-existing capabilities.
The use of modern seismic acquisition and processing techniques has had a profound influence on exploration and development in recent years.
Because most Rocky Mountain reservoirs have low porosity, “bright spot” phenomena have not been notably successful in directly identifying hydrocarbon accumulations. However, high-frequency 2D, 2D swath, and 3D methods, coupled with an enhanced understanding of how reflection amplitude, frequency, anisotropy, interval velocity, and other attributes represent geologic conditions of structure and stratigraphy, have resulted in numerous successful discoveries.
Unusual or complex structures such as overthrust areas, shear zones, and meteor impact features have been interpreted through the use of analog geologic models. Similarly, reefs, mounds, channels, and other stratigraphic features have been identified. The advances in seismic technique and interpretation will have a great impact on future exploration and development.
Using some old (interval velocity studies) and new (fracture identification using anisotropy) techniques has allowed exploration for sweet spots within the basin-centered gas settings. These techniques are being exploited in several Rocky Mountain basins.
The thousands of wells drilled in the GRMR have generated an extremely large amount of basic geologic and engineering data. Modern computer techniques have been and are still being developed to sort, analyze, and plot this large volume of information.
Drilling through completion
Advances in well drilling, evaluation, and completion technology have had a significant impact in exploration and development.
Horizontal wells offer great promise for exploiting reservoirs that are thin, have low permeability, are compartmentalized or fractured, or contain viscous oil. Although the chief application of horizontal drilling in the Rocky Mountain region has been in developing fractured reservoirs, opportunity exists in many other reservoir types and conditions.
The largest oil field found in the last 10 years is Cedar Hills in North Dakota and its companion, East Lookout Butte in Montana. This field has been developed with horizontal drilling of an extremely thin (less than 10 ft) reservoir.
Many Rocky Mountain reservoirs are substantially underpressured and have been penetrated with wells utilizing standard but overbalanced mud systems. Many of these reservoirs have undergone extensive reservoir damage that can be minimized by drilling with recently developed underbalanced mud and flow control drilling systems. Utilization of downhole motors and slimhole drilling has lowered drilling expenses and increased profits in the Denver basin and made some uneconomic reservoirs viable development targets.
The development and use of formation imaging logs has aided the identification of depositional and structural features. These logs have proven especially useful in the evaluation of fractured reservoirs. Better understanding of log behavior in low-resistivity, low-contrast formations has led to better evaluation of potentially productive intervals in new or existing wells.
Hydraulic fracturing of low-permeability reservoirs in the Rocky Mountain area has produced economic production rates. Considerable progress has been made in designing less expensive and more efficient techniques, and improvements continue. Hydraulic fracture stimulation has proved successful in coalbed methane development. Cavity enlargement (“cavitation”) has also proved to be a viable technique for enhancing coalbed methane production. The resource pyramid
John Masters20 first proposed the concept of a resource triangle applied to an assessment of the economic viability of existing petroleum deposits in 1979. Thomasson21 subsequently adapted the concept, and Kuuskraa and Schmoker22 modified it into a resource pyramid (Fig. 4).
The pyramid’s apex represents a relatively small amount of oil or gas in very rich, easily found and exploited fields that have highly favorable economics.
Most of the total available resource lies in the lower portion of the pyramid, in leaner and less easily found and exploitable accumulations associated with poor or unprofitable economics. At any given time the ability to move downward from the apex of the pyramid depends on the price of the product and the finding and producing c