Downdip
Downdip is a directional descriptor in petroleum geology and reservoir engineering referring to the direction that lies farther down the slope of a dipping geological formation, stratigraphic surface, or structural contour, in the direction of decreasing elevation of that surface (the opposite of updip, which refers to the direction of increasing elevation of the surface), used to describe the spatial relationships of hydrocarbon reservoirs, fluid contacts, and wells within a tilted geological framework: in a dipping reservoir containing gas, oil, and water (which segregate vertically by density under gravity), the gas accumulates in the structurally highest (updip) portion of the reservoir, oil fills an intermediate structural position, and water occupies the downdip portion below the oil-water contact, so a well drilled downdip of the oil-water contact will encounter only water-saturated reservoir rock, while a well drilled updip of the gas-oil contact will encounter only gas, and a well drilled in the structural position between the two contacts will produce oil; the downdip direction is determined from the dip angle and azimuth of the formation (measured by directional surveys, dip meters, formation imaging logs, or structural interpretation of seismic data), and the practical implications of downdip versus updip position in a reservoir are fundamental to well location planning, production forecasting, water or gas injection design, and the identification of reservoir compartment boundaries.
Key Takeaways
- Downdip well locations in structurally tilted reservoirs have distinct production characteristics relative to updip locations: a downdip producer (a well drilled into the lower, water-leg portion of a tilted reservoir or near the oil-water contact) will produce water immediately (if it is below the contact) or will water out rapidly (if it is close to the contact and gravity drainage draws the contact upward toward the well as oil is produced); an updip producer (drilled into the higher-elevation, oil-column portion of the reservoir) will produce oil for a longer period before the gas-oil contact descends to the wellbore or the water-oil contact rises, depending on the production mechanism; in anticlinal traps (the most common structural trap type), the highest-elevation wells in the crestal position produce oil from the thickest oil column and have the lowest initial water cut, while wells drilled on the flanks of the anticline (in a more downdip position) are closer to the oil-water contact and have thinner oil columns and higher early water cut; the decision of where to drill a development well on an anticlinal structure requires balancing the higher initial oil rate of the crestal well (thick oil column, low water cut) against the more efficient sweep efficiency achieved by downdip water injection wells that drive oil updip toward the crestal producers.
- Downdip water injection (injecting water into the downdip, water-leg portion of a reservoir to maintain pressure and drive oil updip toward producers) is the standard secondary recovery strategy in tilted reservoirs with good vertical permeability and strong gravity segregation: by injecting water below the oil-water contact (at the downdip edge of the reservoir), the injected water reinforces the natural aquifer and creates a waterflood that sweeps oil toward the updip producers without immediate water breakthrough at the producers; this downdip injection strategy is used in many of the world's largest fields, including the giant Middle Eastern carbonate reservoirs (Arab Zone carbonate fields in Saudi Arabia, Kuwait, and the UAE) where peripheral water injection maintains reservoir pressure by injecting into the aquifer zone surrounding the reservoir, and in the Forties Field (North Sea) and other tilted sandstone reservoirs where pattern water injection with downdip injectors and updip producers achieves efficient oil displacement; the injection pressure required at the downdip injection wells must overcome the reservoir pore pressure plus the hydrostatic pressure increment from the injection well to the producer (since the injection well is at lower elevation and must push water upward against gravity), which increases the injection pressure requirement for downdip injection relative to crestal or flank injection in the same reservoir.
- Gas-oil gravity drainage (the process by which gas cap expansion drives oil downward from the gas-oil contact as pressure declines in the gas cap) is a downdip-directed production mechanism in reservoirs with active gas caps: as gas cap pressure expands (either by pressure depletion or by gas injection into the cap), the gas-oil contact descends toward the downdip water leg, displacing oil downward through the reservoir; wells drilled in the middle of the oil column (between the gas-oil contact and the oil-water contact) benefit from gravity drainage if the reservoir has adequate vertical permeability to allow oil to drain downward from above and toward the producers; in high-relief reservoirs with large vertical dimensions (such as the Norwegian North Sea Ekofisk Field with more than 300 meters of oil column), gravity drainage is a significant production mechanism that must be accounted for in reservoir simulation and well placement; the optimal well placement for gravity drainage production is not at the crestal position (where gas cap expansion would quickly gas out the well) nor at the downdip water-leg position (where water influx would flood the well), but in the intermediate oil column where the rate of gravity drainage from above balances the rate of water encroachment from below over the planned well life.
- Downdip stratigraphic traps occur when a permeable reservoir formation (a sandstone or carbonate) pinches out in the downdip direction against an impermeable lateral seal (a shale or tight carbonate), creating a trap in which the updip closure is provided by the structural dip and the downdip closure is provided by the stratigraphic pinchout rather than by a fault or fold: in this type of trap, oil or gas migrating updip along the reservoir sandstone is blocked by the downdip pinchout and accumulates in the upper (updip) portion of the reservoir against the pinchout; wells drilled updip of the pinchout will find reservoir rock, while wells drilled downdip of the pinchout will find no reservoir (the sandstone has pinched out and the formation is entirely shale); the downdip limit of the reservoir is the pinchout line (not a fluid contact), and the oil-water contact (if present) must be above the pinchout or the trap will not hold oil (oil would drain through the pinchout into the deeper, water-wet shale); classic downdip stratigraphic traps include the East Texas Field (one of the world's largest oil fields, with the Woodbine sandstone reservoir pinching out updip against the Austin Chalk on the west flank of the Sabine Uplift), numerous Cretaceous valley-fill and shoreline sandstone stratigraphic traps in the Rocky Mountain basins, and offshore turbidite pinchouts against basin-bounding faults in the Gulf of Mexico and West Africa.
- Fluid contact tilting in hydrodynamic environments can shift the apparent downdip position of the oil-water contact relative to the structurally predicted position: in a basin with active groundwater flow (where formation water is moving in the updip or downdip direction due to a regional hydraulic head gradient), the oil-water contact is tilted by the hydrodynamic force, with the downdip side of the contact displaced downward relative to the updip side (if water flows downdip) or upward (if water flows updip); the magnitude of the contact tilt is calculated from Hubbert's (1953) force diagram: tan(alpha) = (rho_water - rho_oil)/rho_oil * dh/dl, where alpha is the tilt angle, rho_water and rho_oil are the water and oil densities, and dh/dl is the hydraulic head gradient in the aquifer; in basins with strong hydrodynamic gradients (such as parts of the Williston Basin in North Dakota and Montana, where meteoric water recharges the aquifer in the Black Hills and flows basinward to the east), the oil-water contact tilt can reach 50 to 200 meters over a field scale, with the downdip side of the contact displaced significantly from its buoyancy-only position; failing to account for contact tilt when planning downdip producers can result in wells drilled into water that the structural model predicts to be in the oil column, causing expensive dry or wet holes.
Fast Facts
The term downdip and its complement updip have been fundamental vocabulary in petroleum geology since the earliest application of structural geology to oil and gas exploration in the late 19th century: the first systematic oil field mapping in Pennsylvania, Ohio, and West Virginia in the 1880s and 1890s established the relationship between anticlinal structure and oil accumulation (the anticlinal theory of oil trapping, formally articulated by I.C. White in 1885), and the positional concepts of updip (toward the crest of the anticline) and downdip (away from the crest, toward the water leg) followed directly from the structural geology of these first oil fields; the development of subsurface mapping from well data (cross-sections, isopach maps, structure contour maps) in the first decades of the 20th century made the updip-downdip distinction quantitatively precise, with contour lines on a structure map providing the dip direction at every point in the reservoir; the introduction of reflection seismic in the 1920s and its development into the primary subsurface mapping tool by the 1950s further refined the ability to determine dip direction and magnitude at the scale of individual reservoirs, replacing the ambiguity of extrapolating between sparse well control with continuous subsurface imaging; today, 3D seismic and geosteering technology allow horizontal wells to be drilled specifically staying in the updip portion of the oil column and avoiding the downdip water leg with a precision of less than 1 meter in the vertical direction, maximizing the oil column penetrated per lateral foot and minimizing water production from downdip invasion.
What Does Downdip Mean?
Downdip describes the direction of decreasing elevation along a dipping geological formation or structural surface -- the "downhill" direction in the reservoir. In a dipping hydrocarbon reservoir, gas accumulates updip (highest elevation), oil occupies an intermediate structural position, and water fills the downdip zone below the oil-water contact. Wells drilled downdip of the oil-water contact find only water. Downdip injection wells drive oil updip toward producers. Stratigraphic traps form where reservoir units pinch out in the downdip direction against impermeable formations.
Synonyms and Related Terminology
Downdip is the opposite of updip. In directional context it may also be described as downstructure or basinward (when referring to the direction toward the depositional basin center). Related terms include updip (the direction of increasing elevation along a dipping formation; the structurally highest position in a tilted reservoir; where gas accumulates in a gas-oil-water system; the preferred position for oil producers to maximize oil column and minimize early water production), dip (the angle and direction of inclination of a geological surface from horizontal, measured in degrees from horizontal and in azimuth; the fundamental descriptor of formation orientation from which updip and downdip directions are derived; measured by dipmeter or formation imaging logs in wellbores and from seismic reflection geometry in 3D surveys), oil-water contact (OWC, the depth in a reservoir below which water saturation approaches 100% and above which oil is present; the lower boundary of the oil column in a tilted reservoir; separates the updip oil-bearing zone from the downdip water zone; may be tilted by hydrodynamic flow), structural trap (a petroleum trap formed by deformation of reservoir rock into a geometry (anticline, fault block, salt dome closure) that allows oil and gas to accumulate against the downdip migration path; the most common trap type; geometry defined by updip-downdip structural relationships), and water injection (injection of water into a reservoir formation to maintain pressure and displace oil toward producing wells; most effective when injection wells are placed downdip of the oil-water contact (peripheral injection) to push water updip and sweep oil toward crestal producers).