Geologic Controls on Production of Shale Play Resources: Case of Eagle Ford, Bakken and Niobrara

Abstract

Summary Recent developments in shale technology have revolutionized oil and gas production in the United States. However, there is still a strong requirement for assessing the prospectivity of emerging shale plays, both in the United States and internationally. This paper is an attempt to generalize the results from three major US shale plays: Bakken, Eagle Ford and Niobrara, and to use these to assess the prospectivity of emerging shale plays elsewhere. Porosity, permeability, total organic carbon (TOC) content, thickness, brittleness, composition and maturity of shales are all important in the generation and retention of hydrocarbons. Factors such as depositional environment, uplift and burial, proximity to porous media, presence of natural factures, and reservoir pressure distribution over geologic time all also affect the ability of shales to retain hydrocarbons and be economically productive reservoirs. As an example, in the Eagle Ford Shale, regional overpressure has been generated through disequilibrium compaction as a result of rapid burial from the Late Cretaceous to the Palaeogene. Post-burial uplift is least in the Central Eagle Ford and the generated over pressure is, therefore, best preserved there. Overpressured shale reservoirs usually have high free gas contents. This is critical for high fluid flow rates from shales. Across the Karnes Trough in the northeast of the play, there is a good seal to the Eagle Ford Formation and, hence, good production, despite the fact that the production from the overlying Austin Chalk reservoir is poor in this area. Thus, the potential of a particular shale reservoir to produce hydrocarbons could be generalized into a “retention” factor. The geologic features that control the retention and production of hydrocarbons in these three shale plays are compared and analyzed. An attempt is made to correlate these factors and their contributions over geologic time scales in order to estimate hydrocarbon in-place in each. The results obtained from the study of these three major shale plays are generalized to provide insights into the relationships between geologic features, retention and production trends for shale plays. ‘Retention’ factor charts are prepared to provide a quick assessment of the prospectivity of emerging shale resources plays. Eagle Ford The Eagle Ford play is a successful US shale play across a large part of Texas. It covers an area approximately 50 miles wide and 400 miles long and has thickness which varies from 50 to 300 feet, at a depth which ranges from 1,500 to 14,000 feet. It overlies the Buda Limestone and is overlain by the famous conventional reservoir of the Austin Chalk Formation. The Austin Chalk consists of interbedded highly fractured chalks, volcanic ash and marls. The Austin Chalk Formation and Eagle Ford Formation reservoirs are parts of the same hydrocarbon system, the hydrocarbons in both being sourced from the south dipping Eagle Ford Formation. The oil in the Austin Chalk Formation is derived from a Type II kerogen and entered the Austin Chalk reservoir through vertical migration from the underlying Eagle Ford Formation.