Evidence for the development of a superpermeability flow zone by bioturbation in shallow marine strata, upper Jubaila Formation, central Saudi Arabia

Superpermeability (super-k) flow zones in carbonate reservoirs are thought to be controlled by bioturbation—related burrow networks, e.g., burrow pores of Thalassinoides. To improve the current understanding, this study investigated Thalassinoides and associated passive fill in the Upper Jubaila Member of the Upper Jurassic Jubaila Formation, central Saudi Arabia. Analysis of the elemental and mineralogical compositions of powder samples from the passive fill and burrow matrix, and thin sections of selected burrow matrix samples were conducted to determine the origin of the passive fill. The goal, herein, is to infer the flow properties of the bioturbated strata of the Upper Jubaila Member based on the occurrence of this passive fill in the burrow pores of the Thalassinoides. Compared to the burrow matrix, the analyzed passive fill exhibit marked differences in elemental composition, suggesting a much younger and different origin than that of the marine carbonates of the Upper Jubaila Member. A CaO plus MgO concentration below 28% and an elevated SiO₂ plus Al₂O₃ concentration above 45% suggest a clay-dominated passive fill. The reddish and brownish colorations of the passive fill are attributed to the presence of iron oxyhydroxides, as indicated by the elevated Fe₂O₃ concentration (a mean of 5.8% ± 1.7%), and a mineralogical composition with elevated percentages of goethite and hematite. The normalized rare earth element (REE) pattern of the passive fill is consistent with that of lithogenous (siliciclastic) sources, which are likely deposited in a well-oxygenated environment, as suggested by the redox-sensitive elements and normalized REEs. These sources probably originate in hot humid environment, as indicated by the chemical index of alteration (CIA) (>80). The above geochemical data suggest that the passive fill was derived from soils similar to Mediterranean Terra Rossa soils and postdate the formation of burrow pores in the Thalassinoides-bearing strata. This could have occurred after exposure of the Upper Jubaila Member. Terra Rossa soil most likely reached the Thalassinoides-bearing strata from the surface through vertical fractures and karst features, filled burrow pores, fractures along bedding plains, and even intercrystalline pores. The occurrence of Terra Rossa soil in the burrows and intercrystalline pores of the Thalassinoides-bearing strata implies that the pore system of the Upper Jubaila Member possessed connected pathways that enabled soil infiltration followed by simultaneous precipitation in the burrows and intercrystalline pores. The presence of these connected burrow pores containing shafts with diameters ranges from 1 to 5 cm suggests a superpermeable storage capacity and presence of a superflow unit in the Upper Jubaila Member. This study provides an excellent example to better understand the controls of super-k zones in carbonate reservoirs.