Marine shales make up the major part of the Cretaceous sequence in the Western Interior. These shales often contain at least 5% organic carbon and are source beds for oil and/or gas. Under certain conditions, these shales may also serve as important reservoir rocks.
Cretaceous shales of the Western Interior may be divided into two groups: those deposited during progradations and those deposited during transgressions. Transgressive shales form thin sequences of mainly claystone, contain abundant sapropelic (marine) organic matter, and constitute major oil-source beds. These shales can be productive fractured-reservoirs with the following prerequisites: (1) abundant hydrogen-rich organic matter, (2) thermal maturity with respect to oil generation, (3) wel1-developed natural fracture system, and (4) brittleness.
The second, volumetrically large group consists of marine shales deposited during progradations. The progradational shales are commonly thick and coarser grained, and contain smaller amounts of organic carbon, most of which is nonmarine in origin. Typical progradational deposits consist of composite bedsets of thinly interbedded claystone, siltstone, and sandstone. Although these sequences are characterized by extremely low vertical permeability, their horizontal permeability and porosity may be significant.
In thermally immature, progradational shale sequences, such as those of the northern Great Plains, major resources of biogenic gas occur at shallow depths. These reservoirs are characterized by relatively high porosity and low permeability, and require hydraulic fracture stimulation to achieve effective gas production. The reservoirs also have very high irreducible water saturation values because of large internal surface area and are dirficult to evaluate with geophysical logs.