ret ace ous ▁carbon ates ▁that ▁had ▁yield ed ▁discover ies ▁in ▁the ▁N asser ▁( Z el ten ), ▁Def a , ▁R ag uba ▁field , ▁and ▁Bah i ▁fields . ▁M obil ▁discovered ▁C ret ace ous ▁bas al ▁sand ▁at ▁Am al ▁in ▁ 1 9 5 9 , ▁but ▁early ▁results ▁did ▁not ▁recognize ▁its ▁full ▁potential . ▁ ▁Before ▁dr illing ▁the ▁C - 1 - 6 5 ▁well , ▁B P ▁d ril led ▁six ▁tests ▁to ▁bas ement ▁in ▁C - 6 5 , ▁ 8 0 , ▁and ▁ 8 1 . ▁None ▁had ▁high ▁hydro car bon ▁shows . ▁E oc ene , ▁Pale oc ene , ▁and ▁C ret ace ous ▁carbon ates ▁were ▁generally ▁fresh water ▁bearing , ▁and ▁bas ement ▁high s ▁were ▁d rap ed ▁in ▁Late ▁C ret ace ous ▁sh ales . ▁There ▁were ▁occas ional ▁red bed ▁v estig es ▁at ▁the ▁sh ale - bas ement ▁contact ▁and ▁bit umen ▁ve ining . ▁Many ▁well s ▁proved ▁expensive ▁due ▁to ▁lost ▁circul ation ▁in ▁Pale oc ene - C ret ace ous ▁carbon ates ▁and ▁c aving ▁in ▁underlying ▁Late ▁C ret ace ous ▁sh ales . ▁This ▁was ▁so ▁severe ▁that ▁the ▁Sar ir ▁field ▁was ▁nearly ▁by pass ed ▁because ▁oil ▁was ▁not ▁anticip ated ▁in ▁the ▁N ub ian ▁Form ation . ▁However ▁B P ' s ▁chief ▁ge ologist ▁ins isted ▁a ▁bas ement ▁core ▁be ▁taken ▁in ▁every ▁well .