High Resolution Sedimentology and Reservoir Properties of Ulayyah Reservoir Equivalent, Upper Jurrasic Hanifa Formation, Outcrop Approach, Central Saudi Arabia

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Saudi Digital Library
The Jurassic carbonates in Saudi Arabia host vast hydrocarbon resources. Because of their economic importance, numerous petroleum-related studies have been carried out on the Jurassic carbonates in Saudi Arabia. However, most sedimentological and micropalentological framework studies have focused only on the reservoir units themselves. A more encompassing approach is needed to enhance and improve understanding of the intra-reservoir stratigraphy, sedimentology and heterogeneity within the inter-well spacing in individual oil fields. This study uses high resolution outcrop analog to characterize a strata equivalent to the Middle Jurassic Upper Ullayah reservoir units of Hanifa Formation exposed in central Saudi Arabia. The study utilized a multidisciplinary approach involving the integration of sedimentological, chemostratigraphical, biostratigraphical and gamma ray data. The Upper Jurassic (Oxfordian-Kimmeridgian) Upper Ullayah Reservoir strata of Hanifa formation are composed of medium- to thick-bedded, mostly grainy limestones with various skeletal (brachiopods, bivalves, foraminifera, Cladocoropsis, encrusting stromatoporoid, corals and echinoderms) and non-skeletal (peloids, ooids, intraclasts, and oncoids) components. Facies analysis documents low- to high-energy environments, including, deep lagoonal, lagoonal shoal flank, and barrier sand shoal. The field analysis revealed six high frequency sequences of fourth-order depositional sequences in each of the four stratigraphic measured sections. High stand systems tracts (HST) show shallowing-upward trends in which deepwater facies are overlain by shallow-water facies. Correlation of depositional sequences in the studied sections shows that relatively shallow marine (shallow lagoonal, lagoonal shoal flank and barrier shoal sand) conditions dominated in the study area. These alternated condition with deep-water open-marine wackestone and mudstones representing zones of maximum flooding. The controlling factors on the sedimentation pattern and geometries are: (a) relative sea-level variations, at different scales: third order of eustatic/tectonic origin, fourth order and fifth order of eustatic/climatic origin (Milankovitch type) (b) input of siliciclastic sands from local high shield area. Size, morphology and reproductive characters of grainstone seem to be directly related to changes in water depth (accommodation). Spectral Gamma Ray (SGR) and chemical analysis signatures show moderate correlation with lithofacies for each high frequencies sequence, their sequence hierarchy and stacking pattern. The chemical profiles defined the regressive and transgressive phases and reflect cyclic depositional patterns within Upper Ullayah reservoir member. The geochemical data shows marked differences in character and distribution of different lithofacies, which might help in reservoir layering and zonation. This study provides outcrop analog for Upper Ullayah carbonate reservoir. It is predictive and provides quantitative information at the sub-seismic scale with regard to the distribution, size and heterogeneities of the reservoir rock lithofacies. The best reservoir potential is attributed to the cross-stratified well sorted and laminated sandy grainstone of sand shoal with a maximum porosity value of 32-25 %. Best reservoirs are located in the platform margin. The outcrop analog 3D model revealed heterogeneity of the reservoir lithofacies at a higher resolution than that in the subsurface model. Capturing the small-scale lithofacies heterogeneity is important since it influences the distribution of porosity and permeability in the subsurface reservoirs.