Evaluation of Petrophysical Data for Geopressure Analysis in HPHT Settings: A Case Study of the UK Sector of the Central North Sea.

Abstract:

Drilling into high pressure-high temperature (HPHT) reservoirs requires accurate understanding of the formation fluid pressures. Many wells have been drilled in the Central North Sea but it still remains a challenge to understand its high pressure, high temperature geopressure regimes. This lack of understanding complicates the development of the deep Jurassic and Triassic aged prospects. Most of these concerns arise because the nature of the pressure transition from relatively low pressures at the top of the Chalk Group, to extremely high pressures in the deep Jurassic / Triassic reservoirs is not well known. Consequently, several models of the pressure transition zone have been proposed and tried by industry operators but with mixed success. In this study, well logs and measured pressure data have been analysed by several methods including a new tool: the Late Geopressure Indicator (LGI). It is shown that overpressure is generated by both disequilibrium compaction and late geopressure mechanisms. Disequilibrium compaction is dominant in the Cenozoic mudstones where its magnitude is related to recent burial while late geopressure dominates in deeper and older successions that lie beneath the Chalk. In the sub-Chalk settings, both the total overpressure and the prevalence of late geopressure are higher in the deeper Central Graben area (up to 8000 psi overpressure, 80% from late geopressure mechanisms) than in the basin margins (approximately 2000 psi overpressure, 15% due to late mechanisms). Contrary to some schools of thought that prefer the Chalk as the main reservoir seal, it is demonstrated that the top unit of a pressure cell can be situated anywhere between the Chalk and Heather Formation. This new approach to geopressure study offers better understanding of the cause of overpressure, shape of pressure transition zones and the location of the top of reservoir pressure cells which will help open a new window of opportunities for HPHT prospects.