Sedimentological and Stratigraphic Characterization of Late Paleozoic Unayzah Reservoirs, Central Arabia, Saudi Arabia

Abstract

AbstractThe Unayzah reservoirs are Late Carboniferous-Early Permian in age and represent deposition under variable climatic conditions in different depositional environments. The deposition of the lower Unayzah reservoirs commenced in late Carboniferous to early Permian during the global Late Paleozoic Ice Age. They comprise glaciogenic deposits of the Juwayl Formation and are divided into two members: Ghazal Member and Jawb Member. The upper Unayzah reservoirs are found in the Nuayyim Formation, which consists of post-glacial sandstones and siltstones representing a number of depositional facies of variable reservoir quality which in general indicate an arid to semi-arid depositional settings. Depositional environments in the Nuayyim Formation include ephemeral (playa) lake, ephemeral fluvial, tidal flats, estuarine, nearshore marine, and eolian environments. In Central Arabia, Unayzah reservoirs are prolific oil and gas producers and their characterization is important for production and development. This study was aimed at identifying, characterizing and understanding facies and depositional environments distribution to produce a robust depositional model for these reservoirs.Detailed description and facies analysis of over 6,300 ft of Unayzah reservoirs core were carried out. The lithofacies were grouped into facies associations, representing depositional environments. Stratigraphic correlation was performed using core and logs to discern vertical stacking patterns and establish lateral continuity.The highly variable depositional environments that were identified indicate a dramatic change in climatic conditions during the deposition times, from glaciogenic deposits in the lower section into much warmer, arid environments in the upper part of the formation. The Ghazal Member is highly variable in thickness and represents glacial valley deposits. The Jawb Member is more widely spread in the study area and consists of more variable facies than the Ghazal Member. The Nuayyim Formation contains post-glacial deposits that are more complex and variable across the study area.The results of the study contribute to the understanding of Unayzah reservoirs not only in Central Arabia but also in Eastern Saudi Arabia. It also provides new insights into the facies architecture and stacking patterns of the encountered depositional environments.