Discovery of from Eolianite in the Upper Dalan Member, Permian, Offshore Fars, Iran
A laterally continuous, three metres thick bed of oolitic grainstone has been studied on cores of two offshore Fars wells (Iran), actually the largest exploited gas field. This hectokilometric, locally late burial compacted, high porosity but low permeability layer forms the top of the gas stocking interval located in the Upper Dalan Member (Permian), at the top of the informal K4 reservoir subdivision.
This layer, easily traceable between the wells, lies over high-energy marine deposits made of coarse, fauna rich, bioclastic shoals. It mainly consists of oomoldic, fine grained, azoic grainstone with coarser centimetric levels. Horizontal to oblique lamination or steep foresets were found, but no clear evidence of marine sedimentary process was observed.
The petrography reveals a pure oomoldic grainstone affected by emersion related features: pedogenetic imprints (chitonic rims and rhizolitic network), superimposed by early meteoric diagenesis (pendant, meniscus and pseudophreatic cements), prior to early total porosity inversion.
The combination of the observed sedimentary, pedogenetic and diagenetic features states for a supratidal, eolian deposition. This hypothesis is confirmed by the observation of clear pinstripe lamination, the only reliable criterion for eolian recognition.
This layer is the first eolianite discovered within the Upper Dalan Formation, and the first hydrocarbon bearing eolianite ever described in a producing interval. These sedimentary bodies, difficult to identify in subsurface, are often misinterpreted as emergent shoal deposits, misleading sequence stratigraphic interpretations. Moreover, their recognition is crucial for the correct positioning of associated sequence boundaries and for eustatic variations analysis, especially in peritidal aggrading granular intervals.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009