Geological Interpretation of Oil Entrapment in the Zubair Formation, Raudhatain Field

Abstract

ABSTRACT The purpose of this paper is to interpret geologically the oil entrapment in the Zubair Formation at the Raudhatain Field which is essentially barren at the neighbouring Sabriya Field although it is structurally high in the latter. The Zubair Formation is of Early Cretaceous age and consists of sands and sandstone intercalated with siltstones, shales and few thin limestone beds. The average total thickness of the formation is 420 m (1380 ft). The available data showed that the formation contains three potentially productive zones with many sand pools separated by shales. Analysis of the data showed that at the end of Zubair time, the upper reservoir was structurally higher at Raudhatain than Sabriya by about 16.5 m (54.4 ft), taking into account constant compaction. However, the present structural relief of the upper reservoir at Sabriya is about 29.4 m (98 ft). This gives a structural growth at Sabriya of about 46 m (152 ft). Petrographic study of the sandstones showed drastic differences in porosity. The reservoir sands are very clean quartzite with average porosity of 20%. The aquifer sandstones however, show a reduction in porosity to about 7%. It is interpreted that oil accumulation in the Zubair Formation at Raudhatain represent a case history of a "paleotrap". Oil migration occurred early before sands diagenesis. Oil entrapment was sealed off and with subsequent structural growth the oil frozen-in and did not migrate to Sabriya structural high and retained in situ. Secondary migration from the Zubair reservoir to higher horizons was postulated through growth faults at the Raudhatain. It is suggested that the relation between structural growth and reservoir characteristics should be investigated in the Arabian region to outline paleostructures which are open and presently unprospective, while they may contain important frozen-in accumulation.