Younes, Amgad I.1
(1) Shell International Exploration & Production, Rijswijk, Netherlands
ABSTRACT: Regional Present-Day Minimum Horizontal Stress Profile of the North Sea and Top Seal Integrity
Analysis of over 12000 RFT and 2000 LOT and LMT data points from 800 wells in the north, central and southern North Sea indicates that minimum horizontal stress in the North Sea increases linearly with depth, but jumps 500, 1250 and 750 psi at depths 5500, 11500, and 14000 feet respectively. Regional seismic interpretation shows that the first jump corresponds with the base of the Paleocene; the second with the Tor chalk, whereas the third is generally below the base Cretaceous unconformity (BCU). These jumps also correspond with known diagenetic zone boundaries: mechanical compaction, smectite to illite transformation, and chemical diagenesis below 15000 ft, which cause an increase in rock strength with depth. All rock strength data plot below a gradient of 1psi/ft, however, below the BCU, the minimum horizontal stress nearly equals the vertical stress, computed from representative density logs, with an Shmin/Sv ratio of 0.95-1.0. Common borehole breakouts and induced fractures indicate presence of differential stress, and along with the high Shmin/Sv ratio, strongly suggest that the maximum horizontal stress is greater than the vertical stress. Therefore, the present-day tectonic environment of the North Sea is dominated by strike slip faulting.
In contrast to smaller Shmin/Sv ratios above 12000 ft, examples from the Central North Sea illustrate that an increase in rock strength (high Shmin/Sv) correlate with a stronger top seal, and despite a small drilling margin, an increased prospectively of deeper targets. The dominant strike-slip environment will influence borehole trajectory and hydro-fracture design, especially in chalk reservoirs.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.