Fracture Development in Salt Dome Caprock, Hardin County, Texas

Lacazette, Alfred¹, Andrew R. Thomas², Dennis Kuhfal³ (1) NaturalFractures.com LLC, Houston, TX (2) Chevron Energy Technology Company, Houston, TX (3) Chevron Mid-Continent Business Unit, Houston, TX

The first East Texas oil boom was based on production from fractured limestone salt-dome caprock. Limestone caprocks are formed during salt dome ascent by: 1) Accumulation of disseminated anhydrite due to salt dissolution, 2) Conversion of anhydrite to calcite by sulfate reducing bacteria. Fracture development in salt-dome caprock is understood poorly because caprock production was exhausted long before the advent of modern reservoir characterization technologies. This contribution describes a detailed study of the fracture system of the Sour Lake Field (discovered in 1902) using oriented core, borehole image logs, crosswell seismic tomography, dipole sonic logs, petrography, isotopes, and other modern methods. We find that the caprock fracture system developed during caprock growth, that development of the fracture system is linked to salt-dome movement and therefore that it may be continuing to develop, and that the caprock was fractured both by faulting induced by dome emplacement and by mineral volume changes occurring during conversion of anhydrite to calcite. Isotopic data from the limestones (δ¹⁸O: -4 to -8 PDB, and δ¹³C: -11 to -31 PDB) suggest that fluids evolved from deep bacterial to mixed meteoric during dome ascent. Paleostress analysis based on fault-slip kinematics and extensional fracturing shows that caprock strain was due to dome inflation and that this expansion bears an overprint of the neotectonic (present-day) stress field. We interpret that previously unrecognized pockets of fractured limestone exist deep within the Sour Lake anhydrite layer and that these pockets represent a significant positive modification of traditional salt dome caprock reservoir volume. This study has direct application to oil and gas exploration and storage facilities in salt domes.