Abstract: Evaluation of Austin and Buda Formations from Core and Fracture Analysis

Richard H. Snyder, J. Milton Craft

The Austin and Buda formations have been the target for active exploration owing to increased oil prices. These reservoirs are essentially totally dependent on natural fracturing for productivity. Matrix permeability is normally less than 0.5 md. The natural fracturing is present throughout the vertical interval, with fractured density ranging from one fracture per foot to more than 25 fractures per foot. These microfractures have widths ranging from 0.1 to 0.4 mm. Oil saturations measured from routine core analysis range from zero to 60% in the Austin and Buda formations, and are erratically distributed throughout the formation. Procedures for measuring fracture density, dip angle, dip direction, and other criteria needed to distinguish between natural and induced fractu es have been formulated for over 7,000 ft (2,100 m) of recovered and analyzed core from these formations. Results of these core and fracture analyses indicate the following criteria must be present for successful completion of an oil well: (1) the fracture density must be in excess of one fracture per foot, (2) residual oil saturation in the matrix must be in excess of 10%, (3) there must be some indication of matrix permeability, normally 0.01 md. It is apparent that to have sustained production from the Austin and Buda, oil saturation must be present in the matrix, as the fracture volume is extremely small and is rapidly depleted. Relationship between fracture width, fracture block height, porosity, and permeability have been developed from theoretical calculations and appear to be con irmed by well performance. These calculations indicate a fracture porosity of 0.1 to 0.25% is common throughout the Austin Chalk trend. These data also have been used to calibrate fracture-finding logs, and to assist in development of completion and simulation programs.

AAPG Search and Discovery Article #90967©1977 GCAGS and GC Section SEPM 27th Annual Meeting, Austin, Texas