Understanding Reservoir Heterogeneities in a Lower Cretaceous Thamama Reservoir

Abstract

Carbonate reservoirs from the Lower Cretaceous Thamama Group are of major economic interest since they host some of the largest hydrocarbon accumulations in the United Arab Emirates. This study focuses on a Thamama reservoir from the Kharaib Formation and aims at complementing the regional geological understanding through the integration of newly cored and historical wells. The reservoir of interest consists of a thick organic and clay-rich dense zone at its base characterised by mud-supported, discoidal orbitolinid-rich deposits. This interpreted mid-ramp dense succession is overlain by a thick reservoir unit deposited under inner ramp environments, hence describing a large-scale shoaling trend. In detail, the reservoir succession records higher hydrodynamic conditions than the dense unit, as testified by the predominance of grain-supported textures (from packstone to grainstone). Floatstone to rudstone interbeds with grainy matrices are associated with Lithocodium/Bacinella- and rudist-rich accumulations mainly recorded in the lower and upper parts of the reservoir, respectively. A series of depositional cycles of regional significance have been recognised throughout the reservoir succession and are usually bounded by prominent stylolites, correlatable across the field. The reservoir succession is predominantly characterised by micropores, although macropores (especially vugs) also have an important contribution to the pore system. The extent and impact of dissolution is highly variable, but overall, it is responsible for the creation of most macropores (ie. secondary macropores are more abundant than primary macropores). Subsequent to dissolution, the pore system is typically heavily degraded by cementation from non-ferroan calcite cements and, to a lesser extent, by dolomite cements. An emphasis has also been put on recognising the residual hydrocarbon, the abundance of which varies considerably at field scale. To better constrain the vertical and lateral distribution of the reservoir heterogeneities, nineteen layers of interest depicting the main reservoir quality trends have been interpreted. The creation of comprehensive sets of maps – consisting of sedimentological, thickness, diagenetic and hydrocarbon staining maps – for each of these layers allowed a high-resolution understanding of the reservoir architecture. Of interest is the upward increase in reservoir quality reported towards the upper part of the reservoir unit, associated with the development of thick rudist-rich intervals, which favour the development of a macropore-dominated pore system facilitating fluid flow. By contrast, the common presence of stylolites plays a key role in the creation of baffles or barriers throughout the entire reservoir. This integrated approach has allowed a better prediction of flow units at field scale and provided valuable input data for future reservoir modelling.