Influence of Depositional Environment on Fracture Network Characteristics: Example from the Upper Cretaceous Carbonates of Kruja zone, Montenegro

We present results from a systematic study to test whether variability in fracture properties correlates with depositional environment. The Kruja Unit forms part of the fold & thrust belt of W. Dinarides and includes a thrust-imbricate carbonate sequence overlain by Oligocene synorogenic turbiditic siliciclastics. The stratigraphic succession comprises Early Cretaceous evaporites and limestone breccias, followed by Late Cretaceous neritic carbonates and Eocene nummulitic limestones. A multi-disciplinary sedimentological, petrographic and structural analysis focused on borehole logs, core, and key outcrop sections from the coastal Kruja sequence in Montenegro. Target sites were analysed in detail regarding depositional environment, diagenetic history and key aspects of the fracture network, using a combination of field observations, graphic logs, laser scanning (lidar) and systematic sampling. Structural analysis and lidar allowed us to decipher fracture set orientations, linked fracture size-intensity, spacing and clustering. Fracture intensities recorded are generally very high, with extreme values in locally faulted anticlinal structures. Cretaceous carbonates in the area are interpreted in relation to a depositional model in which tidal flats and a lagoon are separated from the open marine environment by barrier islands composed of oolitic-bioclastic grainstones. Cyclic deposition due to rise and fall of sea level led to variable textures and anhydrite content within the limestones. Porosity in the carbonates has been affected by a protracted diagenetic history involving calcite-dolomite cementation and mechanical compaction. Stable isotope analyses indicate that carbon was mostly derived from Cretaceous seawater and host carbonate sediments, with precipitation occurring at near surface to shallow burial temperatures. Dolomitization of varying extent has affected some limestones, particularly those deposited in supratidal-intertidal (and some shoal) environments and is interpreted in relation to evaporative sabkha and seepage reflux models. Interpretation of platform depositional environment, diagenetic features and fracture characteristics provides a more robust input for understanding carbonate platform evolution or reservoir characterization models.