Abstract

Overpressuring is ubiquitous in the Kutei Basin and occurs over a wide range of geologic ages and depths. Sequences affected by overpressuring become younger from west to east, from Eocene through Pliocene, and this is consistent with the overall easterly progradation of the sedimentary regime through the Tertiary. Overpressuring has been penetrated from as deep as 15,000 ft to as shallow as the surface. The thickness of overpressured section penetrated ranges up to more than 10,000 ft; however, the base of the overpressure has not been encountered within a sedimentary section.

The priamry mechanism for generation of the overpressuring is considered to be Disequilibrium Compaction, as a results of inefficient expulsion of pore fluids during burial. The overpressuring is pervasive in sand-deficient, fine grained clastics deposited in distal and deeper marine environments. However, relatively sand-rich sequences, deposited in more proximal settings, also experience severe overpressuring with deep burial.

Sonic log crossplots from wells have been used to define three pressure zones - Hydrostatic, Transition and Hard Overpressure. Hydrocarbons are produced from both the Hydrostatic and Transition Zones; basinwide, a large percentage of reserves occur in the Transition Zone.

Implications of overpressuring to the Petroleum System are discussed. Commercially productive hydrocarbon reservoirs have not been encountered within the Hard Overpressure Zone in the Sanga-Sanga PSC. Sands present in this zone typically exhibit characteristics of a limited reservoir and contain gas dissolved in water - so called "Coke-Bottles". Seal capacity of shales within the Transition Zone is considerably enhanced relative to shales in the Hydrostatic Zone and results in the trapping of significantly larger hydrocarbon columns. Hydrocarbon source potential in the Hard Overpressure Zone is generally poor and, due to very low expulsion efficiency, an effective kitchen in this zone is unlikely.