Abstract

Fracture distribution in the Giddings Austin Chalk is controlled by both structure and stratigraphy of the Austin Group. Parameters which most affect reservoir performance include fracture width, height, and spacing, as well as the number of fracture sets and their orientations. Lateral variation of these parameters is a function of structural position, while vertical change is related to stratigraphy.

The Austin Chalk productive trend is thought to coincide with the Gulf Coast Basin hingeline where extension has been concentrated during subsidence. Fracturing is attributed to normal faulting, bending over buried structures, gravity creep, differential compaction, and aquathermal pressuring. A change in structural style from faulting to flexing takes place from west to east across the Giddings Field, accompanied by a change in fracture distribution. On the west, fractures develop only in close proximity to faults whereas on the east they are more widely distributed over broad warps.

Stratigraphic factors influencing fractures include lithology, porosity, bed thickness, and ductility contrast between adjacent beds. The Austin Chalk consists of sparse biomicrite interbedded with marls, shales and clay seams. In general, thin beds are more highly fractured than thick beds, and clean limestone is more highly fractured than marl or shale. Where the more ductile marls and clays exceed a critical thickness, fractures tend to terminate within individual chalk beds, resulting in barriers to vertical flow within the reservoir.