Fault zone deformation and fracture intensity in chalk-dominated carbonates

Abstract

Cretaceous Anacacho Limestone within the Balcones fault system (central Texas) is investigated to understand fracturing of chalk-dominated carbonates in a normal faulting deformation regime. Development of opening-mode fractures is highly sensitive to mineralogy and associated mechanical behavior. Low mechanical rebound beds that have >7 % clay and <90 % carbonate generally lack well-developed opening-mode fracture sets. High-rebound beds with <7 % clay and >90 % carbonate contain opening-mode fracture networks. Away from mapped faults (with ∼10 m or greater throw), deformation is represented by opening-mode fractures in two orthogonal sets with intensities of <1 fracture/meter. The NE-SW-striking dominant set parallels regional normal-fault strike. Deformation adjacent to a ∼10-m-throw normal fault includes small-displacement (antithetic and synthetic) normal faults and opening-mode fractures. Near-fault fracture intensity reaches >5 × background levels, and decays to background intensity ∼30 m from the fault. Fluid inclusion analyses of calcite from dilational crack-seal zones along normal faults and opening-mode fractures within the fault damage zone reveal: (i) liquid hydrocarbon inclusions with ∼28–36 API gravity oil; (ii) homogenization temperatures from two-phase inclusions used to estimate burial depths of 0.9–1.5 km (oil inclusions) up to 2.4–2.9 km (aqueous inclusions); and (iii) aqueous inclusion ice-melting temperatures indicating basinal brine rather than near-surface meteoric water during vein cementation. U-Pb ages from these veins indicate Paleocene-Eocene faulting and fracturing (57.9 ± 3.2 to 38.9 ± 4.1 Ma), and limited Miocene (16.6 ± 2.2 Ma) activity. This work documents the essential role of small-displacement faults and opening-mode fractures in the migration of hydrocarbons through low-permeability chalk dominated carbonates.