Measurements of UCS in ten Austin Chalk cores located from south Texas to eastern Louisiana are heterogeneous and vary by lithofacies. Chalk-rich facies, which are higher in %Ca and lower in %Al and %Si, have higher UCS than the argillaceous clay-rich strata. This observation is consistent with outcrop measured UCS located in the same region. The importance of layered, variable-strength rocks to natural fracture development is observed in an outcrop exposure of the Austin Chalk in central Texas. Chalk-rich facies (high UCS) were more fractured and had a broader deformation area compared to clay-rich lithofacies (low UCS). Furthermore, vertical fracture heights were stunted at the boundary between high and low UCS lithofacies. This boundary can be delineated in the subsurface using derivative curves to highlight the rapid changes in UCS. To improve subsurface applicability, we constructed a DFN model from the outcrop exposure to illustrate the heterogeneous distribution of fractures in fault zones that are below typical seismic-survey resolution. This has important implications for SRV distributions and likely explains heterogeneous reservoir responses and variable flow-back percentages in hydraulically stimulated wells of the Austin Chalk.
The dominant heterogeneity in most reservoirs is the stratigraphic layering of variable lithofacies and rock properties especially in reservoirs with mixed rock strength or rigidity. The mechanical layering has a profound impact on natural fracture development and similar results should be expected in stimulated rock volumes (SRV) (Bradley and Mostafavi, 2021). The Austin Chalk (AC) is arguably the best examples of bimodal rock strength exerting control on fracture development and distributions in subsurface reservoirs. Despite the AC being a tight carbonate reservoir with low porosity and permeability, prolific production has occurred from unstimulated wells due the presences of open natural fractures encountered with horizontal drilling (Martin et al., 2011). More recent activity has demonstrated that the AC is an excellent target for hydraulic stimulation and the results have again proven that AC is a commercial producer (Boerner et al., 2013). Like many tight, unconventional reservoirs, wells developed within the AC have mixed production results and stimulation effectiveness. One reason for the unpredicted results may be the presence of low offset faults and natural fractures which are pervasive throughout the Gulf Coast basin. This study aims to illuminate the control of mechanical stratigraphy within on natural fracture development and propose a similar response within the SRV.
