No abstract

Chalkfieldsinthe NorthSea haveseenrecent large increasesinrecovery factors andalowered limit on the effectivepermeability ofreservoirs thatcanbe developed.For example, the HalfdanFieldinDenmarkhas aneffectivepermeability of0.5mD.Asaresult the industry expects to recoversignificantly morehydrocarbons from most Chalkfi eldsbut,i naddition,therei sanadditionals ource ofupside inexistingfi elds,which ist he dynamic trap. Dueto the low permeability ofthe Chalk, hydrocarbon migration andre-equilibration occurs over geologicalt ime-scales. Exampleso fthesetypeso ftraps arethe HalfdanandS outhA rnefi eldsint he Danish sector and, possibly,the JoanneFieldinthe UK.All thesefieldshavestrongly tilted free-waterlevels (FWLs). The implications arethatreservesdownflankofdiscoveriesmaybe overlooked,aswasthe casewiththe South ArneF ield, developed 30 years afterdiscovery followingthe drillingofb etterr eservoirdownflanka ndtilted FWLs. Anotherexampleisthe HalfdanField, which wasdiscovered in1999. The oillegsinthe HalfdanandDan fieldsareconnected, but Halfdanwasnot discovered until1998, although Danhasbeenproducingsince 1972. Dynamic trappingmechanisms also suggest enormous additionalexploration potentialinthe NorthSea Chalkif the location ofmigratinghydrocarbons canbe predicted.Anewtypeoftrap-the constriction trap-i sdiscussed, which iscaused byapermeability reduction inthe migration direction causing'banking'ofhydrocarbons dueto the greaterinflux ofh ydrocarbons thano utflow overgeologicalt ime.The thickness ofpost intra-Chalk unconformity Chalki nt he Danishsector isu sed to predictt hesetraps int he Chalk: lateralt hinningofthe relatively high permeability Tor canpotentially createatrapif all the otherelements ofthe petroleum systemare inp lace.