Advances in Arabian stratigraphy: Allostratigraphic layering related to paleo-water table fluctuations in eolian sandstones of the Permian Unayzah A reservoir, South Haradh, Saudi Arabia

In the early stages of development of the Permian Unayzah A gas reservoir at South Haradh, difficulties in intra-reservoir wireline log correlation, poor seismic character, and the recognition of multiple gas-water contacts, necessitated a detailed (core- and image log-based) geological study of the reservoir. This study revealed that the Unayzah A can be divided into two stratal units. The lower unit has poor reservoir quality. It consists of a thin, discontinuous basal eolian dune sandstone that is abruptly overlain by up to 70 ft (21 m) of very fine-grained and silty, irregularly laminated sandstones. These sediments were deposited in a very shallow ephemeral (playa) lake setting. They terminate upwards in a thin but widespread upward-fining interval that represents the Maximum Extent of the Lake (MEL horizon). In the upper unit several facies of varying reservoir quality are recognized, representing deposits that were laid down in a “mixed” eolian depositional system. Although these facies are generally common to all of the studied wells, they vary significantly in their proportions and associations from well to well. Thus a number of depositional settings are identified, which pass southwards across the study area from erg-center, eolian dune cross-bedded sandstones in the north (“dry” eolian system) through erg margin, interbedded dune and interdune deposits to very finegrained sandstones representing ephemeral lake sediments in the south (“wet” eolian system). Depositional cycles in the upper Unayzah A interval are recognized among the facies in each well, and in every case these can be related to fluctuations in the paleo-water table. When the wells are displayed in a stratigraphic section using the MEL horizon as a datum, several of these cycles are seen to be correlatable across the study area. Significantly, these sub-regional correlations are maintained both within and between the various facies tracts. They are allostratigraphic in nature with facies distribution controlled by a number of extrinsic factors including position of the paleo-water table relative to the depositional surface, rates of fluctuation of the water table, sediment availability for eolian transportation, transporting capability of the wind and rates of subsidence. Proximity to local ephemeral stream sedimentation was probably also a factor. It may be that in the long term the pulsed nature of the rising water table in South Haradh reflects the onset of a distant, pulsed rise in sea level, possibly related to the creation of the Neo-Tethys Ocean in the southern and eastern part of the Arabian Plate. Recognition of this allostratigraphic reservoir layering clearly identifies significant stratigraphic compartmentalization within the reservoir. This permits a clearer understanding of the distribution of reservoir bodies within the Unayzah A, and explains the variable gas-water contacts in the area. This conceptual stratigraphic model has been incorporated in object-based geocellular modeling for reservoir simulation at South Haradh and also at other Unayzah A gas fields in the area. Integration with geophysical (impedance) data leads to significantly improved success rates in relation to the strategic location of wells, yielding enhanced results in reservoir development.