The pressure architecture of seventy five sedimentary basins was examined to determine spatial distribution of normal and abnormal pressure. Basins with like depositional histories have similar distribution patterns that are generally classified as either linear or tiered. Linear systems are common in dynamic basins and represent a systematic increase in pressure with increasing depth. Tiered systems occur in both dynamic and senile basins and 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 bounded 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 shale with intervening hydrocarbon-bearing sandstone reservoirs. Recessed patterns result when underpressured hydrocarbon-bearing carbonate or sandstone reservoirs are sealed from normally pressured reservoirs above and below. Ledged patterns have three distinct pressure domains: (1) a shallow, normal pressured sandstone-rich interval, (2) a shale-dominated interval that consists of overpressured mudrocks and 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 and oil accumulations in abnormally overpressured and underpressured intervals within tiered basins are dominantly stratigraphically trapped. In contrast, trapping of deep gas in sub-ledge, normally pressured intervals is facilitated by anticlinal folding or faulting.
In the Anadarko Basin, the overpressured Woodford shale can be superjacent to normally pressured Hunton Group carbonates. This juxtaposition creates the potential for fracturing fluid diversion during completion. Similar conditions exist in other basins where "frac barriers" needed to contain hydraulic stimulations are thin or absent and resource play intervals overlie lower pressured, water-charged permeable strata.