Facies Distribution and Dolomitization Along the Southeastern Shelf Margin of the Jurassic Gotnia Basin

Christoph Lehmann¹, Jeremy Goff¹, Bob W. Jones², and Alexis Anastas^3
1 BP, Houston, TX
² BP, Sunbury on Thames, United Kingdom
³ Devon Canada Corporation, Calgary, AB

Dolomite reservoirs contain a significant amount of hydrocarbons in the Middle East. The dolomites in and around the Jurassic Diyab Basin centered on present-day Saudi Arabia formed in a peritidal to shallow subtidal lagoonal environment and are associated with deposition of evaporites (e.g., Arab Formation). Those around the south-eastern Gotnia are most likely linked to migration of basinal brines to the margin during burial.

Carbonates of the Surmeh Formation formed in a shallow ramp setting on the south-eastern margin of the Gotnia Basin and contain benthic foraminifera suggesting an age range from late Early Jurassic to Late Jurassic. Palaeobathymetric analysis of the benthic foraminifera and associated calcareous algae and macrofossils indicates the existence of a number of shoaling-upward cycles. Surmeh cycles in the late Early-Middle Jurassic are typically tens of meters thick and marked by a facies at the base representing palaeobathymetries of at least several tens of meters and at the top by shoal facies representing palaeobathymetries on the order of ten meters. Mudstones and wackestones are developed at the base of the cycles and coarsen upward to oolitic and oncolitic packstones and grainstones. Diagenesis appears to have involved early extensive meteoric cementation of the grainstone shoal facies at the top and later burial dolomitization of the underlying relatively more permeable deeper subtidal facies creating 3 distinct stratigraphically distributed dolomite reservoir intervals. The Late Jurassic cycles are typically marked by facies changes from fore-shoal to back-shoal facies. The peloidal wakestones/packstones and skeletal, oolitic, peloidal grainstones are completely cemented by meteoric diagenesis and form the top seal to the underlying dolomite reservoirs. Graphic correlation data indicate that a hiatus exists at the top of the Surmeh Formation.