Significance of Aeolian Deposits Resultant Drift Direction, Implications for Reservoir Prediction, Utilizing Subsurface and Analogue Data

Abstract

Petroleum is common and abundant in wind-laid rocks in many places in the world, and aeolian deposits typically form good reservoirs or pathways for hydrocarbon migration due to their high initial porosity and permeability, and the generally great lateral continuity of preserved elements. Subsequently, heterogeneity in such aeolian successions, e.g., arising due to the juxtaposition of dune and interdune elements, can be problematic; therefore, there is a need to develop quantitative models for predicting the arrangement of such elements in subsurface successions. The Artinskian-Kungurian Upper Unayzah Formation displays widespread evidence of deposition in a continental setting under the influence of an arid to semi-arid climatic regime, commonly in a wind dominated, aeolian setting. In the study areas, a number of distinctive aeolian depositional facies are recognized from subsurface cores and image logs, including: aeolian sand dune, interdune/damp sandflat, sand sheets/dry sandflat and ephemeral playa lake palaeo-environments. The facies occur with a high degree of vertical and areal repeatability throughout the studied successions. The study further investigated distribution of azimuthal variability of the cross-bedded sandstone's aeolian sand dune facies in the Lower Permian, utilizing microresistivity borehole image logs. A palaeo-wind data study involved the analysis of image log data from several wells. To investigate the palaeo-wind directions from the measured subsurface azimuthal data, a hierarchical approach was employed as follows: (i) azimuth data recorded for each identifiable bed, (ii) averaged over each bedset within each well, (iii) averaged over each well, (iv) averaged over each area, and (v) averaged over the four studied areas. This rigorous approach resulted in the identification of a resultant drift direction toward east-northeast (relative to the present day), with a range that varied between 40° and 100° with a mean direction of 72 azimuth degrees. The final stage of this study was to analyze wind data obtained from modern analogues to further understand and evaluate the sand drift directions and potentials. Identification of the dominant palaeo-wind direction served as the basis for developing a model for palaeo-environmental spatial and temporal variations in dune distribution and planform morphological type throughout the studied areas. This model then formed a principal tool used for field development plans by enabling prediction and mapping of porosity and permeability distributions.