Geologic Aspects of Tight Gas Reservoirs in the Rocky Mountain Region
Authors: Spencer Charles W. 
Published: December 2007

Summary
The Rocky Mountain region contains major gas resources in tight (low-permeability) reservoirs of Cretaceous and Ternary age. These reservoirs usually have an in-situ permeability to gas of 0.1 md or less and can be classified into four general geologic and engineering categories:marginal marine blanket,lenticular,chalk, andmarine blanket shallow.
Microscopic study of pore/permeability relationships indicates the existence of two varieties of tight reservoirs. One variety is tightly because of the fine grain size of the rock. The second variety is tight because the rock is relatively tightly cemented and the pores are poorly connected by small pore throats and capillaries. Other characteristics of tight gas reservoirs are:discrete gas/water contacts are absent in lenticular and marginal marine blanket reservoirs,most of the gas occurs in stratigraphic traps,well log analysis is difficult in tight reservoirs.many Rocky Mountain tight gas basins are either overpressured or underpressured, andformation damage may occur when wells are drilled and completed.

Introduction
Gas-bearing, tight (low-permeability) reservoirs are present in sandstone, siltstone, silty shale, and chalk in the Rocky Mountain region. Most of the gas occurs in rocks of Cretaceous and Tertiary ages. Organic-rich dark shales and coals are the source of the gas. A combined engineering and geologic effort is needed to identify, to map, and to recover gas from tight gas reservoirs. It is particularly important that geologists working on tight gas reservoir analyses be aware of problems being encountered in well log interpretation and reservoir stimulation. It is equally important for the engineer to understand the geologic differences between tight (unconventional) and conventional gas reservoirs. Tight gas reservoirs exhibit several unique characteristics compared with conventional reservoirs. One of the most significant differences is that conventional reservoirs have a reasonably consistent relationship between porosity and permeability, whereas tight reservoirs may or may not exhibit a consistent relationship between porosity and laboratory-measured permeability except that the in-situ permeability to gas is generally less than 0.1 md. The following discussion will describe some geologic characteristics of tight gas reservoirs in the Rocky Mountain region.

Types of Tight Gas Reservoirs
Fig. 1 shows basins and areas within and adjacent to the Rocky Mountains that contain tight gas reservoirs. Most of these localities also contain conventional reservoirs. Many sandstone stratigraphic intervals that are tight in the deep parts of basins have conventional reservoir characteristics at shallow burial depth. Tight gas reservoirs in the Rocky Mountains are predominantly Cretaceous and early Tertiary age. They can be grouped into four general reservoir types:marginal marine blanket,lenticular,chalk, andmarine blanket shallow.

Marginal Marine Blanket Reservoirs
Marginal marine blanket reservoirs are strata deposited on or near a shoreline that occur within a relatively predictable stratigraphic interval. These reservoirs commonly are overlain by marine shales and may be underlain by marine shale or continental deposits. They are called "blanket" for engineering reasons and normally would not be considered blanket sandstones by geologists. Tight blanket reservoirs are strata that have relatively better horizontal continuity than lenticular reservoirs. Blanket reservoirs usually respond to hydraulic fracturing in a somewhat predictable (blanket-like) manner. When the volume of fracture proppant is increased, there is a general increase in well productivity up to certain limits. Some examples of marginal marine blanket reservoirs are the Lower Cretaceous "J" sandstone in the Denver basin, the Upper Cretaceous Upper Almond formation and the Frontier formation in the Greater Green River basin, and the Upper Cretaceous Corcoran and Cozzette sandstones in the Piceance Creek basin. The locations of these basins are shown in Fig. 1.

Lenticular Reservoirs
Lenticular reservoirs are reservoirs that were deposited predominantly by rivers. These fluvial sandstones are very discontinuous and exhibit many internal permeability variations. The geometry and dimensions of these reservoirs are difficult to predict. The response of lenticular sandstones to hydraulic fracturing is very erratic, and generally the stimulation results are poorer than usually possible in marginal marine blanket sandstones. Some examples of lenticular reservoirs are fluvial sandstones in the Upper Cretaceous Mesaverde group and Tertiary in the San Juan, Uinta, Piceance Creek, and Greater Green River basins.
JPT
P. 1308^