Abstract
This paper presents a comprehensive analysis of multiscale geological, petrophysical and rock mechanical data acquired from 35 m of outcrop well core from the Late Jurassic (Kimmeridgian) Upper Jubaila Formation, Saudi Arabia. In this location the sequence is a typical shallow marine heterogeneous carbonate sequence and a direct analogue to a part of the prolific Arab D reservoir sequence in the subsurface towards the east. Four main lithofacies types were identified with a general shoaling upwards trend, varying from nodular bioturbated peloidal mudstone/wackestone facies to sharp-based intraclast-peloidal, skeletal rudstone/floatstone facies. The mineralogy in the upper section of the well cores is predominantly calcite, while the lower section is also comprised of some dolomite and quartz. Stratiform dedolomitization as a telogenetic overprint is only evident in the upper 18 m of the core and controlled by vertical flow of meteoric fluids. The core plug porosity distribution is lower than that of the subsurface equivalent reservoir zones, and is attributed to meteoric cementation. Thalassinoides burrow features in mudstone and wackestone facies in the upper sections of the core control horizontal permeability distribution at the centimeter to meter-scale. Micrite morphology and dedolomitization appear to be key controls on plug permeability, rock strength and sonic velocity. Based on the trends observed in compressive rock strength, p-wave velocity and plug porosity distributions, the cored section was divided into five main rock mechanical layers. Despite reduced porosity, the average compressive rock strength in the outcrop core is lower than that of the Arab-D subsurface equivalent rocks, indicating overall 'weakening' of the rock due to telogenetic processes and surface weathering.