| Scope and method of study. The purpose of this study was to examine the lithologic controls of tiered pressure distributions in selected basins worldwide. The study included more than 300 individual pressure-depth profiles for more than 75 sedimentary basins. From the total number of basins, nine were selected to study a more in depth view of the relationship pressures had with lithologies.;Findings and conclusions. Tiered systems contain distinct pressure domains or compartments whose distribution patterns are categorized as stepped, recessed, and ledged. Pressures in stepped basins increase with depth and form a staircase pattern of distinct pressure compartments. Recessed patterns are formed by a subnormally pressured interval, which is intercalated both above and below by normal pressures. Ledged patterns consist of an overpressured section with subjacent and superjacent normally pressured intervals.;Stepped pressures are representative of basins containing thick sections of seals with intervening hydrocarbon-bearing sandstone reservoirs with irregular subbasin relief. Recessed patterns result when underpressured hydrocarbonbearing carbonate or sandstone reservoirs are sealed from normally pressured reservoirs above and below and have encountered some tectonic activity. Ledged patterns have three distinct pressure domains: (1) a shallow, normal pressured sandstone-rich interval, (2) a shale-dominated interval that contains overpressured and gas-filled sandstones, and (3) a deeper, normally pressured interval. This deeper normally pressured domain is dominated by carbonates or sandstones that are hydraulically connected to the surface, have active water drives, and contains gas that is buoyancy trapped above the water leg. Gas accumulations in abnormally overpressured and shallow, normal pressured intervals within tiered basins are dominantly stratigraphically trapped. In contrast, deep gas in sub-ledge, normally pressured intervals typically requires anticlinal folding or faulting to trap. |
