Golden Zone Implications for Global Exploration

Paul H. Nadeau, Per Arne Bjørkum, Gillian Darke, and Øvind Steen
Statoil ASA, Stavanger, Norway

The impact of diagenetic processes on petroleum entrapment efficiency has focused the majority of the worlds oil and gas reserves into relatively narrow thermal intervals (Bjørkum and Nadeau, 1998), which we call the 'Golden Zone'. The zone's thickness is determined by the geothermal gradient as well as lithology, and can vary from circa 1 km to 3 km (Nadeau et al., 2005; Darke et al., 2004). Reservoirs within geologic traps which occur in this zone contain an exceptional number of the discovered giant and super giant accumulations. The thermally driven migration mechanisms are remarkably efficient, such that exploration risks increase exponentially in the underlying expulsion zone. Examination of drilling data indicates that the industry typically over explores the high risk expulsion zone. The geological conditions required to maintain accumulations in the expulsion zone can be specified, but thus far are realized in relatively few basin settings. These include open lateral drainage and continuous super seals. Exploration challenges remain, however, to identify remaining 'Golden Zone' accumulations in large subtle traps, particularly along the basin margins and within stratigraphic traps. This realization can open up new plays and play fairways in relatively rich/mature exploration provinces.