Potential overlooked bioturbated reservoir zones in the shallow marine strata of the Hanifa Formation in central Saudi Arabia

A comprehensive study including sedimentological, ichnological, petrographic and petrophysical investigations and geostatistical modeling of the late Kimmeridgian bioturbated strata of the upper Hanifa Formation suggests that the grain-dominated burrow fills of these strata enhance the stratum porosity and permeability by creating permeable pathways in the otherwise tight mud-dominated unit. The bioturbation patterns in these strata reveal a boxwork pattern organized as a horizontal network and vertical shafts. These shafts exhibit circular to elliptical cross-sections with the long axis ranging from 0.39 to 1.65 cm that intersects at angles of 60°, 90°, and 120°. This pattern suggests the domination of Thalassinoides ichnotaxa in the studied strata. The Thalassinoides, with a bioturbation intensity ranging from 10% to 40%, contain passive fills that are characterized by coarse-grained (the majority of the grain sizes is > 0.5 mm) carbonates with a packstone texture consisting of skeletal, coated and peloidal grains. These passive fills have a porosity (a mean (µ) of 7% ± 2%) and permeability (µ of 82 ± 98 mD) substantially higher than the matrix porosity (µ of 1.5% ± 0.5%) and permeability (µ of 0.6 ± 0.6 mD). Multipoint statistics (MPS) models suggest that the Thalassinoides in these strata generated connected porous and permeable pathways even at a low bioturbation intensity (~ 10%). The MPS models also indicate that the bulk permeability of these strata increases with the bioturbation intensity and reaching up to 204 mD for models with a 40% bioturbation intensity. The similarity of these results to those published for well-studied ichnofossil-hosted tight gas reservoirs suggests that the mud-dominated units of the upper Hanifa Formation could form potential flow zones. Interestingly, however, these strata are overlooked as subsurface reservoirs, probably because of the limitations of downhole logging tools and the lack of integrated, comprehensive investigations. This study provides an illustrative example of shallow marine strata in which mud-dominated sediments could be flow units because of the impact of burrows. Hence, understanding the bioturbation trends in these strata could be beneficial for the prediction of the reservoir quality of similar strata elsewhere.