FRACTURE CONNECTIVITY AND RESERVOIR CHARACTERIZATION OF THE AUSTIN CHALK

The Upper Cretaceous Austin Chalk, which extends across Texas and Louisiana, is characterized by reservoirs that produce oil, gas, and in some cases, anomalously large amounts of water. This low-permeability, low-porosity rock unit requires large connected fracture systems to store and produce hydrocarbons. Most of the large-scale fractures are parallel to regional strike with few dip-oriented fractures. This one-dimensional fracture network requires many smaller, localized fractures to maintain fluid flow in the reservoir. Reservoirs typically have low matrix permeability and contain natural fractures. Horizontal drilling has been used to enhance and connect these fracture systems to drain the reservoir more effectively. Although the formation contains continuous (unconventional) reservoirs, it behaves as a hybrid system, wherein varied geologic settings yield both continuous and conventional accumulations.

Nine oil and gas fields were grouped into low, moderate, and high reservoir communication, based on correlation values of produced water vs. hydrocarbons. Most fields in the historic trend (e.g., Giddings) display low correlation values, potentially indicating that the reservoirs are poorly-connected and thus continuous. Continuous reservoirs contain dominantly vertical, strike-parallel fractures due to local structures or movement along nearby faults, which lead to little reservoir connectivity. Masters Creek, Luling-Branyon, and Buchanan fields, however, display very high correlation values. These fields are probably conventional, because the reservoirs are well-connected and likely have oil-water contacts. In hybrid reservoirs, connected fractures are localized, and the reservoir may act as either continuous or conventional. Much of the Austin Chalk trend probably contains hybrid reservoirs, where a single fracture system type does not dominate across an entire field. In hybrid reservoirs, fracture studies would be beneficial in determining whether to drill vertical or horizontal wells. Additional research is necessary to better characterize hybrid reservoirs in the Austin Chalk and to distinguish between conventional and continuous accumulations.