Sequence Stratigraphy of an Eolian Gas Sand: Layering in the Permian Unayzah, a Reservoir at South Ghawar, Eastern Saudi Arabia
In the Unayzah A reservoir in subsurface Saudi Arabia, conventional wireline log correlations have historically failed to provide a geologically robust lithostratigraphy. Recent core studies, augmented by image logs, have successfully evolved a sequence stratigraphic model that has considerable applicability to geocellular modeling. Thus, a basal Sequence Boundary is marked by a Significant Desiccation Surface (SDS), overlain by a thin sheet of eolian sand. It is superseded by extensive, irregularly laminated and locally desiccated very fine-grained sandstones with rare thin siltstones. These reflect shallow water deposition with periodic desiccation in an ephemeral lake environment. They terminate in a thin, widespread upward-fining unit that represents the Maximum Extent of the Lake (MEL). Above that horizon, varying facies associations are seen, including eolian erg-centre, cross-bedded sandstones; erg margin dune and interdune deposits (with paleosols); deflation plain; ephemeral lake. Depositional cycles are recognized within each of these major facies associations that are regionally correlatable within and between the various facies tracts. When hung from the MEL as a Datum, these correlatable cycles are seen to be essentially flat-lying ‘parasequences' that are demonstrably related to fluctuating water table within the regional Unayzah setting. This stratigraphy is clearly quasi-chronostratigraphic in character, i.e. it is founded on Sequence Stratigraphic principles. This clarifies the distribution of reservoir bodies within the Unayzah A, and predicts the occurrence of intra-reservoir variability and potential compartmentalization. Reservoir characterization and modeling of these deposits is thus optimised. Outcrop analogs occur in the Cedar Mesa and Entrada sandstones of Utah.
Sequence Stratigraphy of Non-Marine Deposits
2006 AAPG International Conference and Exhibition, (November 5-8, 2006) Technical Program