A Revised Sequence Stratigraphic Interpretation of Platform Top Carbonatesof the Lower Cretaceous Shuaiba Formation,U.A.E.,Based on New Outcropand Subsurface Observations

摘要

The Lower Cretaceous carbonates of the Shuaiba Formation forms one of the most prolific carbonatereservoirs in the region.These carbonates were deposited on a shallow epeiric carbonate platform duringthe Aptian.In addition,intra-shelf basins formed on this vast platform where deeper and more restrictedcarbonates were deposited.As a result of a long term relative sea-level fall these carbonates started toprograde into the intra-shelfal Bab Basin.The platform ultimately diminished during exposure of theShuaiba platform and subsequent flooding and the deposition of deeper water carbonates and clastics ofthe Nahr Umr Formation.The observations described here concentrate on the sequence stratigraphy of theshallow carbonate platform top carbonates in outcrop and the subsurface.The internal succession of the Shuaiba Formation can be briefly described in core and outcrop asfollows:The Shuaiba Formation has been deposited on top of the deeper-water carbonates of the Hawarmember,which terminated the Kharaib platform.Based on core evidence from several Abu Dhabi oilfields,the Lower Shuaiba Fm.is composed of two depositional cycles composed of Orbitolina wackestonesshallowing upward into Lithocodium/Bacinella floatstones(Aptian 1).The succession is overlain bydeeper water Orbitolina and foraminiferal wackestones/packstones and in parts contains planktonicforaminifera.Overlying this succession are shallowing-upward sequences with Lithocodium/Bacinellafloatstones passing to Lithocodium/Bacinella rudstones or bindstones with stromatoporoids and corals(Aptian 2).The Aptian 2 section is bound by a prominent breccia that can be mapped in the subsurface around theBab basin.The breccia shows angular clasts,vugs,and in places late stage calcite and dolomite cementrimming(or filling)the pore space.In parts,breccia clasts have been incrusted by stromatoporoids andLithocodium/Bacinella after renewed flooding.This breccia has all the characteristics of a solution collapsebreccia and is the result of a major drop in relative sea-level on the platform.In outcrop(400 km awayfrom the subsurface analogue)an intraclast breccia is observed at the same stratigraphic position and can be traced kilometers along the mountain front in some wadis in the northern emirate of Ras Al Khaimah.This unit displays evidence of exposure and reworking and in areas intense pervasive later cementation.Overlying the Aptian 2 sequence boundary,the Aptian 3 succession has dominant Lithocodium/Bacinellafloatstones and locally intercalated rudist floatstones suggesting a renewed deepening.Above,the sectionpasses to massive rudist floatstones and rudstones containing mainly Caprinid,Myophorid and Caprotinidrudists and downlapping in outcrop onto the Lithocodium/Bacinella section(Aptian 3).At the base ofthe massive rudist section another breccia is exposed incorporating clasts of the underlying and overlyingmaterial.In contrast to the first breccia,this breccia displays evidence of mechanical effects involvingfluidization,pressure containment/release,and development of pipe-shaped features connected to the parentbed.Internal clasts display angularity with variably weak to strong lithification.The top of the rudist sectionis represented by a sequence boundary characterized by significant stylolitization and sharp facies change.The upper part of the section is characterized by an alternation of skeletal,peloidal,Orbitolina,algal andrudist floatstones with intercalated mudstones(Aptian 4 and Aptian 5).Close to the contact to the NahrUmr borings and pyrite nodules are prominent suggesting multiple stacked exposure surfaces leading to theultimate unconformity on the top of the Shuaiba Platform.Establishing a sequence stratigraphic framework on shallow carbonate platforms heavily relies onidentifying sequence boundaries(SBs),which are the best preserved surfaces recognized in these platformsettings.Deposits as a result of maximum flooding surfaces are often not distinct since bathymetry wa