A Regional Comparison of Khuff and Arab Reservoir Potential Throughout the Middle East
S. N. Ehrenberg, P. H. Nadeau, and A. A. M. Aqrawi STATOIL, Stavanger, Norway
Copyright 2005, International Petroleum Technology Conference
This paper was prepared for presentation at the International Petroleum Technology Conference held in Doha, Qatar, 21–23 November 2005.
Carbonate reservoirs from producing oil and gas fields have extreme ranges of porosity and permeability, both locally, within a single reservoir zone, and in terms of average values for entire reservoir zones.1 This study describes the latter type of variation for two major resevoir formations in the Middle East and lists the factors that seem likely to account for the striking overall differences between these units.
Compilation of average porosity and permeability data for petroleum reservoirs in the Permian/Triassic Khuff Formation and the Jurassic Arab Formation confirms and quantifies the major differences known to exist between these units. These data were provided by IHS Energy (Energy Data Information Navigator Database), as reported previously by 1. The average reservoir data are supported by a compilation of published porosity-permeability data from specific fields, where core and wireline log data provide insight into geologic controls. The geographic distribution of the combined data are shown in Figure 1.
Most Khuff reservoirs have average porosity
The above differences reflect a combination of depositional, diagenetic, and economic factors (Fig. 5). Khuff strata were deposited on an extensive, poorly-circulated, very low-relief shelf and consist in large part of interbedded mudstone and grainstones having relatively fine grain size, with major amounts of depositional calcium sulphate present. Arab reservoirs were deposited under better-circulated conditions nearer to margins facing deep intracratonic basins and thus have coarser, more grain-dominated fabrics and lesser content of chemically precipitated grains.
Overall deeper burial of Khuff reservoirs has also resulted in greater porosity loss by chemical compaction and cementation. Furthermore, Khuff deposits were likely composed of less stable mineralogy than Arab sediments because the Permian/Triassic was a time of “aragonite seas”, whereas the Jurassic was a time of “calcite seas.” The combined result of these factors is that Arab reservoirs are characterized by greater preservation of primary depositional pore types, more coarsely crystalline dolomite fabrics, and lesser plugging by anhydrite.
In addition to the above factors, however, the main reason for lower average porosity and permeability in Khuff reservoirs may be that the Khuff is almost exclusively a gas play, whereas the fluid of interest in Arab reservoirs is oil. Khuff is part of a Paleozoic petroleum system, sourced mainly from Lower Silurian hot shales in the gas window, whereas Arab is sourced mainly from Upper Jurassic shales and basinal carbonates in the oil window. As a result, much lower permeabilities are required for production of Khuff reservoirs compared to Arab reservoirs, and the characteristics of the fields in production simply reflect the respective cut-offs for economically viable flow of gas as opposed to oil.
1. Ehrenberg, S.N. and Nadeau, P.H.: "Sandstone versus carbonate petroleum reservoirs: a global perspective on porosity-depth and porosity-permeability relationships," AAPG Bulletin (2005) 89: 435-445.
Figure 1. Locations of petroleum fields for which average values are available for Khuff and Arab reservoir parameters.
Figure 2. Frequency distributions of average porosity values for Khuff and Arab reservoirs from the EDIN Database.
Figure 3. Top-reservoir depth versus average porosity for Khuff and Arab reservoirs from the EDIN Database.
Figure 4. Arithmetic average permeability versus average porosity for Khuff and Arab reservoirs from the EDIN Database (BLACK symbols) and from published data compilations (RED symbols).
Figure 5. Factors accounting for overall differences in average porosity and permeability between Khuff and Arab reservoirs.