Abstract

This study aims to characterize the fracture network in altered Oligocene-Early Miocene basalts of the Jizan Group, which accumulated in half grabens during the continental rift stage of Red Sea evolution.  Unlike fresh basalts, the Jizan Group was affected by low temperature hydrothermal metamorphism, which plugged the original matrix porosity in vesicles, breccias, and interflow layers with alteration minerals.  On the other hand, the basalts are pervasively shattered by closely spaced fractures in several directions, which provide fracture permeability. Characterization of these fractures is essential to reducing the fracture permeability uncertainty for mineral carbonation by the dissolved CO2 process such as Carbfix.

Conventional measurements of fracture orientations and densities were initially taken at outcrops of the Jizan Group to characterize the fracture network. Terrestrial Digital Photogrammetry (TDP) and Unmanned Aerial Vehicle Digital Photogrammetry (UAVDP) surveys were conducted to acquire images covering larger areas that were then used to create 2D orthoimages and 3D models of the outcrops using Agisoft Metashape, which were analyzed for fracture geometries using QGIS and Cloud Compare, respectively. The automated analysis of fracture orientations and densities compared well with conventional manual measurements.  This gives confidence in semi-automated drone-based fracture characterization techniques in 3D, which are faster and less labor intensive, especially for characterization of large and difficult to reach outcrops.

Similar fracture geometries were observed at seven different sites along the outcrop belt of the Jizan Group, which suggests a common origin. This study found four dominating fracture sets in the Jizan Group volcanics, with a dominant trend of fractures in the NNW direction, similar to the general trend of the Red Sea. The Northern sites presented higher fracture intensity compared to the southern sites, indicating more suitable environments for carbon mineralization. Our fracture characterization will be used to construct 3D fracture permeability models of the Jizan Group for combined physical and chemical simulations of injection of dissolved CO2 from industrial sources into basalts. This will provide essential parameters to mitigate geological risks and to determine depth, spacing, and injection rates in CO2 disposal wells.

Moreover, mineralogical composition of spatially distributed samples collected from the Jizan Group volcanics were collected to investigate spatial distributions of secondary alteration minerals in the Jizan Group basalts. Epidote was observed in samples collected from southern outcrops indicating hydrothermal alteration temperatures higher than 230 C, whereas the northern sites lacked epidote and contained calcite indicating lower hydrothermal alteration temperatures. Sufficient amount of Ca in the rocks is essential for carbon mineralization to occur. Previous studies conducted by Torres (2020) suggest Ca content in the Jizan Group volcanics similar to the average Ca contents in basalts globally. The presence of sufficient amounts of Ca along with potential 3D fracture networks in the subsurface indicate feasibility for the injection of CO2 charged fluids in the subsurface of the volcanics.