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Spill Point

The spill point of a petroleum trap is the lowest structural elevation of the trap's closure — the point at which hydrocarbons would begin to migrate out of the trap laterally if the trap were filled to that depth — and defines the maximum column height of oil or gas that the trap can retain before the buoyancy of the hydrocarbons causes them to spill from the trap and migrate updip to a shallower trap or to the surface; in a structural trap (an anticline, a dome, or a fault closure), the spill point corresponds to the lowest closed contour on a depth-structure map — the saddle point or the lowest elevation on the trap's rim at which the overburden seal is breached and migration can occur; the hydrocarbon column height (the vertical distance from the free water level to the gas-water contact or oil-water contact, up to the gas-oil contact and the top of the trap) cannot exceed the spill depth minus the depth of the top of the trap (the relief or closure height) because any additional hydrocarbon accumulation above the spill depth would overflow the trap rim and migrate upward to the next accumulation point; the spill point is a critical parameter in reserve estimation because it defines the maximum possible column height that constrains the maximum possible areal extent and volume of the hydrocarbon accumulation in a fill-to-spill scenario, and because the actual free water level (determined from pressure data from the well) relative to the spill point depth reveals whether the trap is completely filled (actual free water level at or above spill point) or only partially filled (actual free water level above the top of the trap but below the spill point).

Key Takeaways

  • Spill point identification from seismic depth maps requires accurate depth conversion from the seismic two-way travel time to depth, because the spill point identification depends on finding the lowest closed depth contour on the trap structure — an identification that is sensitive to velocity model errors that can shift the depth of the trap closure up or down by tens to hundreds of meters: on a time-structure map (displaying the seismic reflection times in milliseconds rather than depths), the spill point appears as the lowest closed contour in time, but the conversion from time to depth requires a velocity model that may vary spatially across the trap due to lateral velocity variations in the overburden, making the depth-converted spill point uncertain; the uncertainty in spill point depth directly translates to uncertainty in the maximum hydrocarbon column height and therefore to uncertainty in the maximum possible reserve volume; in exploration prospect evaluation, the fill-to-spill reserve case (assuming the trap is filled to the spill point) represents the optimistic end of the reserve distribution, and the uncertainty in spill point depth is one of the sources of the P10 to P90 uncertainty range in the volumetric reserve estimate; AVO analysis and direct hydrocarbon indicators (DHI) from seismic amplitude data can sometimes constrain whether the trap is fill-to-spill (DHI anomaly extending to the spill point) or partially filled (DHI anomaly terminating above the spill point), reducing the structural uncertainty in the column height estimate.
  • Differential entrapment in a trapping system with multiple stacked closures occurs when a regional petroleum charge migrates updip and fills the deepest trap first (to its spill point), then spills into the next shallower trap, and so on up the migration pathway — creating a series of petroleum accumulations at successive structural highs along the migration route, each filling to its spill point before the excess charge moves to the next trap: this fill-to-spill migration sequence is the conceptual basis for prospect risking in updip migration plays where multiple traps exist along a common carrier bed; if the total petroleum charge (the volume of oil and gas generated and expelled by the source rock and migrated into the carrier bed) exceeds the combined spill volume capacity of the traps along the migration route, excess hydrocarbons will reach the surface and be lost to seepage; if the charge is less than the combined capacity, only the traps nearest to the source will be filled, and the distal traps along the migration pathway will be dry holes; the distribution of charge between traps depends on the relative capacities and spill points of each trap, making the analysis of the entire migration fairway (from source to all potential traps) necessary to assess the individual fill risk of any specific trap in the system; in the North Sea, the major Jurassic oil fields of the Norwegian sector (Statfjord, Gullfaks, Troll) were filled by oil migrating from the Kimmeridge Clay source rock through the Viking Graben carrier beds, with some traps filled to their spill points and excess oil spilling to shallower traps or reaching the seabed.
  • Gas-water contact (GWC) and oil-water contact (OWC) depths relative to the spill point define whether a discovered trap is fully charged, partially charged, or unfilled: a GWC at the spill point depth (confirmed by log analysis and pressure data showing the free water level at spill point depth) indicates a fill-to-spill scenario where the gas accumulated until it spilled from the trap into the next shallower trap or into the migration fairway; a GWC above the spill point (free water level shallower than the spill point) indicates that the trap was only partially charged — the available gas supply was insufficient to fill the trap to its spill capacity, or dynamic seal failure allowed gas to leak before the trap filled; a GWC at the same depth as the OWC (or OWC at the spill point with a separate GWC above it in a gas cap-oil rim system) represents the maximum fill scenario where both oil and gas are present with the water contact at the structural spill point; the actual versus maximum charge scenario has significant implications for reserves: a fill-to-spill discovery typically has higher confidence reserves because the structural controls are fully engaged, while a partially charged discovery raises the question of whether the trap leaked, whether the source was insufficient to fill the trap, or whether the well encountered an atypically low-charged segment of a trap that is better charged elsewhere.
  • Fault-controlled spill points are more complex to identify and characterize than spill points on un-faulted anticlines, because the fault that forms one side of the trap may also be the migration pathway through which hydrocarbons spill when the trap is filled: in a fault trap where the seal is provided by the juxtaposition of reservoir against shale across the fault, the spill point is the lowest structural elevation of the reservoir on the upthrown (reservoir) side at which the fault no longer provides a seal against the reservoir on the downthrown side; if the reservoir is juxtaposed against a shale seal below that depth but against another reservoir rock above, the fault ceases to provide a seal above the spill depth and hydrocarbons will migrate across the fault; the assessment of fault seal capacity (using shale gouge ratio, clay smear potential, or empirical calibrations from drilled analog traps) determines whether the fault will maintain a seal to the computed spill point or whether seal failure will limit the column height to a shallower closure than the structural spill point; in the Brent Province of the North Sea, many of the Jurassic tilted fault block reservoirs have spill points controlled by the intersection of the reservoir with the fault surface, and the column height in each fault block reflects the complex interaction of the structural dip, the fault geometry, and the fault seal capacity.
  • Spill point confirmation from well data requires the measurement of formation pressure in the reservoir and the free water level from pressure gradient analysis: in a well drilled into a petroleum trap, the pressure gradients of the oil or gas phase and the water phase (measured by repeat formation tester (RFT) or modular dynamic formation tester (MDT) pressure measurements at multiple depths) define the depth at which the oil or gas gradient intersects the water gradient — the free water level (FWL); the FWL is distinct from but related to the OWC or GWC (which is the actual depth of the observed fluid contact, which may differ from the FWL by the capillary entry pressure of the reservoir rock); the FWL depth compared to the seismically mapped spill point depth confirms whether the trap is fill-to-spill (FWL at or near the spill point) or partially charged (FWL above the spill point); if the FWL is deeper than the mapped spill point, this indicates that the structural model is incorrect (the spill point is actually deeper than mapped, possibly due to a velocity error in depth conversion), that the trap extends below the mapped closure (a separate deeper-tilted structure), or that there is a permeability barrier preventing pressure communication between the measured interval and the spill point depth; reconciling the well-measured FWL with the seismically mapped spill point is a fundamental step in the appraisal of a discovery that determines whether to revise the structural interpretation or accept the partial fill scenario.

Fast Facts

The concept of the spill point and its role in defining the maximum petroleum column height that a structural trap can retain was formalized in petroleum geology through the development of structural trap evaluation methods in the early 20th century, as geologists recognized that the structural closure — not the source rock charge or the seal capacity — was often the limiting control on accumulation size. The quantification of spill point depth from seismic data, and the distinction between fill-to-spill and partially charged traps based on the comparison of the free water level with the spill point elevation, became standard practice in exploration prospect evaluation as the resolution and accuracy of seismic depth conversion improved through the 1970s and 1980s with the adoption of three-dimensional seismic surveys and computer-aided interpretation systems.

What Is a Spill Point?

The spill point is the lowest point of structural closure in a petroleum trap — the depth on the trap's rim where hydrocarbons would begin to escape if the trap were filled beyond that level. Think of a structural trap as a geological bowl turned upside down: the arch of the anticline or the closure against a fault forms the bowl, and the spill point is the lowest point of the rim. If you pour oil into that bowl and it fills past the rim's lowest point, the oil spills out and migrates to the next structural high. The spill point therefore defines the maximum possible hydrocarbon column height in the trap — the difference in depth between the top of the trap and the spill point elevation — and hence the maximum possible trapped volume of oil and gas. In a fill-to-spill scenario, where the charge of migrating hydrocarbons was large enough to fill the trap completely, the actual fluid contacts in the reservoir will be at or near the spill depth. In a charge-limited scenario, where the trap received less hydrocarbon than it could hold, the contacts will be above the spill depth and the trap is partially filled. Distinguishing these scenarios from well data and seismic interpretation is one of the most important interpretive challenges in appraisal and development geology, because it determines whether there are additional reserves below the drilled well that could be recovered by downspill or step-out drilling.

The spill point is also called the trap closure depth or the trap low point. The concept of fill to the spill point is expressed as fill-to-spill. Related terms include structural closure (the areal extent of a petroleum trap defined by the lowest closed depth contour on a depth-structure map, with the trap containing all reservoir rock above this closure contour and the spill point being the lowest elevation on the rim of this closure), free water level (the depth at which the formation water pressure and the hydrocarbon (oil or gas) pressure are equal in the absence of capillary pressure effects, determined from the intersection of the hydrocarbon and water pressure gradients on a formation pressure versus depth plot, used to determine whether a trap is filled to its spill point), hydrocarbon column (the vertical thickness of oil or gas in a petroleum trap, measured from the free water level to the shallowest hydrocarbon-bearing point, constrained by the trap's structural closure height from the top of the trap to the spill point), fault seal (the ability of a fault zone to prevent hydrocarbon migration across the fault, determined by the clay content and shale gouge ratio of the fault rock and the juxtaposition of reservoir against seal on both sides of the fault, which controls the spill point in fault-bounded traps), and depth-structure map (a contoured map showing the depth in subsurface of a formation top or key horizon derived from seismic depth conversion or well control, used to identify structural closures, spill points, and the areal extent of petroleum traps).

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