Chalks are a fine-grained limestone consisting of calcareous nannofossils and other microfossils. They are deposited in an open marine or pelagic environment in which the rate of accumulation is slow and the influence of terrigenous sediment input is minimal. During Late Cretaceous time, chalks were deposited in the Western Interior seaway both during times of transgression (Greenhorn Formation and lower part of Niobrara Formation) and progradation (middle part of Carlile Shale, upper part of Niobrara Formation, and parts of Pierre Shale). Deposition and distribution of the chalk tongues were controlled primarily by the variation in the supply of siliciclastics to various parts of the basin. The main source area was highlands to the west, so that chalk deposition was generally restricted to the eastern part of the seaway. Paleotectonism along basement blocks in the main part of the seaway probably had a strong influence on the dispersal of the clastics and on the geometry of individual chalk units. The chalks commonly contain features suggestive of deposition during “oceanic anoxic events”.
Because of their fine grain size, the chalks deposited have a high porosity (70-80%) and low permeability, which predictably are reduced with paleodepth of burial. In the area of current gas production in the eastern Denver Basin, Niobrara chalk reservoirs have porosities that range from 35 to 40% at depths ranging from 300 to 2,800 ft (250-850 m), and because of their low permeability require hydraulic stimulation to provide economic flow rates. Because porosity and permeability continue to decrease with
Figures & Tables
Patterns of Sedimentation, Diagenesis, and Hydrocarbon Accumulation in Cretaceous Rocks of the Rocky Mountains
In the Rocky Mountains from western Canada to Mexico, Cretaceous rocks are major sources and reservoirs for oil and natural gas, accounting for about 40% of the cumulative production to date. Resources estimates indicate that large amounts of hydrocarbons remain to be discovered in these rocks. The purpose of this volume is to examine the relationship of reservoir quality, resource evaluation, and exploration strategy to depositional environment, thermal maturity, and diagenetic history of Cretaceous rocks in the Rocky Mountain area. Chapters deal with the general characteristics of the Cretaceous Western Interior Basin and seaway, the application of organic geochemistry to hydrocarbon occurrence and exploration, principle aspects of diagenesis that affect reservoir quality and source-rock potential, and the five main depositional facies which can be recognized from west to east across the basin.