Lithofacies modeling of Late Jurassic in upper Ulayyah reservoir unit at central Saudi Arabia with inference of reservoir characterization

Saudi Arabia is reported to have an extensive hydrocarbon potential in the carbonates of Jurassic age. This has been proved by several studies considering their economic significance. These studies focused mainly on the petroleum related research. However, there were few studies about the reservoir characteristics focusing on the sedimentology and the micro paleontology. But still a detailed study is essential to develop a clear insight on the stratigraphic relationship between the reservoir, their sedimentological characters and their non-homogeneity between the wells of the individual oil fields. This study aims to establish 3-D geo-cellular stochastic models of the lithofacies to characterize the reservoir geometries, architecture and heterogeneity of a formation similar to the late Jurassic (Oxfordian-Kimmeridgian age). The central Saudi Arabia is represented by the late uppermost Ulayyah reservoir a representative of Hanifa Formation. The study attempts an integrated approach considering high resolution sediment analysis, spatial continuity of semi-variogram and 3-D geostatistical facies modeling. The outcrop strata consist mostly of grainstone, medium to thick limestone with different skeletal remains of foraminifera, bivalve, brachiopods stramatoporoid, corals and echinoderms. They also consist of non-skeletal components like ooids, peloids and intraclasts. The outcrop facies analysis revealed that the depositional environment ranges from relatively Outer ramp, Middle ramp and Inner ramp. The major axis of variograms (N-S) represent the lithofacies with good continuity (high degree of continuity) at the outcrop of the upper Ulayyah member. Minor axis (E-W) depicts poor continuity. The variogram of the lithofacies reflects a minor tendency of geometrical anisotropy. The geometry and architecture of the facies stacking pattern presented a genetics correlation with the variogram parameters at the outcrop scale. The 3-D model at low resolution reveals the existence of layer cake pattern. Subsequently at higher resolution a greater extent of non-homogeneity exists. Outcrop analogue of the uppermost Ulayyah reservoir equivalent is perfect for developing and understanding their basic geometry. This study will also help to get an insight into the distribution of petrophysical properties (e.g., porosity and permeability) along with the horizontal details of well spacing that might help to improve field appraisal and towards planning and development.