Spatial patterns variations of a burrowed carbonate stratum (ramp carbonates), Upper Jurassic Hanifa Formation, central Saudi Arabia

Abstract

Strata deposited in middle carbonate ramps are often highly burrowed. Such strata are easy to recognize in the stratigraphic record and have been recorded from numerous stratigraphic sections worldwide. However, the spatial patterns of these strata are poorly known, despite being intervals of flow zones in many hydrocarbon reservoirs and water aquifers. In this study, we trace one stratum (in the upper Hanifa Formation, central Saudi Arabia) that was deposited in the middle carbonate ramp of the Late Jurassic for ~21 km, cored it in three localities, and investigated its spatial stratigraphic, sedimentological and ichnological patterns by field investigation and laboratory analysis (petrographic analysis and computer tomography (CT) scanning). The results of the field investigation show that the stratum has a similar spatial stratigraphic pattern over the study area (sharp contacts with the underlying and overlying strata), suggesting one depositional cycle product. Nevertheless, the laboratory analysis revealed that the stratum varies spatially and distinctly in rock composition, burrow percentage (BP), and burrowing style, suggesting various controls on its rock texture. The bottom interval of the stratum comprises wackestone and packstone rock texture which are interpreted as a product of bio-retexturing that took place at the time of the initial flooding stage of the deposition cycle. In the middle (5% to 10% BP) and upper intervals (20% to 40% BP), the stratum has mudstone and wackestone host rock matrix penetrated by a Thalassinoides burrow network (pipe framework) with passive infills (grain-dominated carbonates). The stratum exhibits both vertical and lateral variations in burrow percentage and burrowing styles, which are interpreted to be controlled by variations in bathymetry due to sea-level change and seafloor topography. Such variations influenced the rock texture of the studied stratum and would eventually control the spatial patterns of its petrophysical properties. The results provide insights into the nature and controls on the spatial patterns of burrowed strata of a middle ramp carbonate, patterns that can be similar to many hydrocarbon reservoirs and can be used to predict the distribution of petrophysical properties.