Karst and Early Fracture Networks in Carbonates, Turks and Caicos Islands, British West Indies

Sean A. Guidry¹, Mark Grasmueck², Andrew Gombos¹, Daniel G. Carpenter¹, Steven L. Bachtel³, and David A. Viggiano^2
1 ExxonMobil Upstream Research Company, Houston, TX
² University of Miami, RSMAS, Miami, FL
³ ConocoPhillips, Houston, TX

Full-resolution, three-dimensional ground-penetrating radar (GPR) can greatly assist in constraining the oftentimes complex spatial geometries of diagenetic heterogeneities. This emerging GPR technology provided high-fidelity images of subsurface karst dolines and early fracture networks on Providenciales, Turks and Caicos Islands, British West Indies. Whereas host rock GPR response provided surprisingly little detail about stratal architecture, reflections related to dolines revealed complex internal fill geometries (sags, antiforms) and associated fracture networks.

Used in conjunction with observations of adjacent outcrops, 3D GPR provides a method for exploring various genetic mechanisms of karstification and early fracture formation in young carbonates. Detailed fracture descriptions (e.g., aperture, fill material) and orientation measurements were collected from outcrops in the immediate vicinity of the 3D surveys, and other locations along the western margin of the Caicos Platform. Despite the fact that fracture networks on Providenciales have previously received little attention, these early fractures are relatively abundant, dominantly margin-parallel, and have great potential to impact diagenetic fluid flow regimes. Importantly, fracture systems are well established in rocks ~123,000 to 134,000 years in age. Any attempts to model early diagenesis in carbonates should not dismiss the role of these fractures as diagenetic facilitators and diagenetic “anisotropy” templates. Four karst models were evaluated to explain the observed distribution of dolines: 1) gravitational, fractured margin controlled, 2) tectonic fracture controlled, 3) antecedent topography controlled, and 4) a hybrid model. The spatial distribution of dolines on Providenciales is likely the result of a complex interplay between meteoric fluids, antecedent topography, and margin-parallel fracture systems.