Sedimentological and diagenetic controls on burrow fillings in the upper Jurassic Hanifa formation, central Saudi Arabia: Implications for porosity and pore system variations in bioturbated reservoirs

Evaluating the petrophysical properties of carbonate strata with bioturbation necessitates an insight into the heterogeneity in rock texture within burrow fillings. This study examines five separated bioturbated intervals in the upper Hanifa Formation, Saudi Arabia, focusing on burrowing style, depositional texture, diagenetic modifications, porosity, and pore system. The main goal is to understand how the pore system and porosity vary along the vertical profile within the burrow fillings of these bioturbated intervals and how they can be influenced by factors including the burrowing style, the depositional texture, and the diagenetic alterations. Based on the burrowing style, the five bioturbated intervals are categorized into intervals with bio-retextured fabric and intervals with pipe framework burrows, as determined by the results of field studies, petrographic analysis, and medical CT scanning. Data from petrographic investigation revealed that burrow fillings in intervals with a pipe framework burrowing style (intervals 1, 4, and 5) likely deposited in similar environments exhibit minor variations in grain size, with mean grain sizes of 766 ± 752 μm for interval 1, 739 ± 433 μm for interval 4, and 514 ± 341 μm for interval 5. Porosity values from micro-CT scans show 7.15 %, 4 %, and 7.75 %, respectively. In contrast, bio-retextured intervals (intervals 2 and 3) have smaller mean grain sizes of 355 ± 206 and 402 ± 148 μm, with lower total porosity values of 1.64 % and 2.71 %, respectively. The rock texture in the burrow fillings of the pipe framework was identified as either mud-rich packstone (intervals 1 and 5) or mud-poor packstone (interval 4) based on the mud content. Petrographic analysis indicated that dolomitization is essential in preserving porosity in mud-rich packstones by preventing compaction, whereas mud-poor packstones with lower dolomite content exhibit a notable decrease in porosity due to compaction. The findings enhanced our comprehension of how porosity in burrow fillings with packstone rock texture evolved. Packstones rich in mud (≥10 %) that were originally poorly porous and permeable may become more porous as a result of selective dolomitization and subsequent dissolution. On the other hand, compaction may cause the higher porosity and permeability values of initially good-quality packstones with low mud content (<10 %) to deteriorate. When it pertains to strata with bioturbation, where direct measurements of permeability are difficult, understanding these processes is crucial in forecasting reservoir performance and fluid flow. Finally, the pore system of the burrow fillings predominantly consists of isolated pores exhibiting vertical variations in pore volumes, pore areas, and pore equivalent diameters as shown by micro-CT scanning and digital rock physics modeling.