Bioturbation-Controlled Petrophysical and Mechanical Properties of the Lower Jurassic Carbonates: Marrat Formation, Central Saudi Arabia

Bioturbation can substantially change petrophysical and mechanical properties of carbonate strata. To develop deeper understanding in this concept, this study examined bioturbated and fractured carbonate strata in the Lower Marat Formation in central Saudi Arabia using integrated methodology including field and laboratory observations. Examining 10-meter outcrop, distinct vertical trends were observed: bioturbation decreases upward with to some extent decrease in bed thickness, while fracture density increases. A remarkable mineralogical contrast was discovered in the burrow fillings, with dark grey dolomite-filled burrows overlying brown de-dolomite-filled burrows at the middle of the section. The uppermost units show lower bioturbation intensities. Porosity measurements, on 1.5-inch plugs, show no distinction between de-dolomite- and dolomite-filled burrows, with majority of the samples have porosity less than 10%. However, the de-dolomite-rich fillings exhibit higher dissolution features and occurrence of rhombohedral molds compared to dolomite-filled burrows. Consequently, the average spot permeability of de-dolomite-filled burrows is one order of magnitude higher (~2.1 mD) than to the dolomite-filled burrows (~0.4 mD). Isotopic analysis (δ13C and δ18O) have shown that the de-dolomitization might occurred during short subaerial exposure after dolomitization. These results indicate distinct paragenetic sequence wherein the bioturbation have enhanced the dolomitization process, leading to preferential dolomitization within burrows. Subsequently, as these strata were exposed shortly after deposition, they underwent de-dolomitization process that created rhombohedral molds and slightly enhanced the permeability. In general, fracture intensity shows a reverse correlation with bioturbation intensity, with R2 of about 0.5. The higher intensity fractures with low bioturbation intensity, at the uppermost part of the section, provided further pathways for diagenetic fluids to precipitate calcite cement within interparticle and intraparticle pores. This process has exerted more pronounced influence on the upper beds, resulting in reduced porosities compared to the lower beds. This study provides insights into the mechanism by which bioturbation influences the petrophysical and mechanical properties of carbonate strata, insights that are important to understand their storage and flow capacity.