geology · formation tops
Formation Top Projection (Ahead of Bit)
Projected Depth = Reference Top Depth + Horizontal Distance × tan(True Dip)
click formula to derive ↑
Inputs
ft
ft
°
Description
Projects a target formation top's expected depth ahead of the bit (or ahead of any other reference point) using a known reference depth, the horizontal distance to the projection point, and the target's true dip — the standard 'dead-reckoning' constant-dip projection technique used in real-time geosteering to anticipate where a formation top should be encountered before it is actually seen. Distinct from formation_top_subsea_depth, which converts an already-encountered pick's depth to a common datum rather than predicting a depth ahead of one.
Variables
| Symbol | Unit | Description |
|---|---|---|
| zproj | ft | Projected Formation Top Depth |
| zref | ft | The depth of the target formation top at a known reference point — typically an offset well, a pilot hole, or the last confirmed real-time log correlation while drilling. |
| D | ft | The horizontal distance from the reference point to the projection point, measured along the dip direction. Positive values project down-dip (deeper); negative values project up-dip (shallower). |
| δ | ° | The true dip magnitude of the target formation, established from offset well control, seismic interpretation, or a prior three-point/structure-gradient solution. |
Assumptions
- The target formation maintains a constant true dip and strike between the reference point and the projection point — no fold, fault, or dip reversal in between.
- Horizontal distance is measured specifically along the dip direction; if the actual path of interest runs obliquely to dip direction, the apparent (not true) dip along that specific direction should be used instead (see apparent_dip_true_dip).
- The sign convention (positive = down-dip/deeper, negative = up-dip/shallower) has been applied consistently with how the reference depth and dip were established.
Limitations
- This is a straight-line, constant-dip extrapolation — it provides no warning of an upcoming fold, fault, or dip change, and its reliability degrades the further the projection point is from the reference control point, exactly the reason real-time geosteering continuously re-anchors this projection against fresh log/drilling data rather than relying on a single static projection over a full lateral.
- The underlying formula is corroborated by general geosteering industry literature and practice (constant-dip 'dead reckoning' projection is a standard, widely described technique), but the specific source directly page-accessed this session for the formula's exact algebraic form was Danny T. Williams' U.S. Patent 8,960,326 B2 ('Formation Dip Geo-Steering Method') — patent literature, not a peer-reviewed textbook or journal source. No primary/textbook geoscience publication stating this exact projection formula was directly page-accessed this session despite searching; treat this citation as patent-corroborated only, a real citation-integrity gap consistent with (not softened relative to) the disclosed-gap treatment already applied to tvt_tst_correction, fault_throw_heave_separation, and the geochemistry/maturity batch in this discipline.
- Reports a single projected depth at a single projection distance — does not itself track a continuously updating projection along an entire lateral the way real-time geosteering software does.
Use Cases
- → Pre-drill landing point planning: Estimate where a target formation top should be encountered at a planned landing point, using dip and depth control from an offset well or pilot hole.
- → Real-time geosteering sanity check: Cross-check a geosteering interpretation's implied dip against a simple dead-reckoning projection from the last confirmed correlation point, as a quick manual sanity check alongside dedicated geosteering software.
Related Calculations
Region Notes
Global
This constant-dip projection is most reliable in structurally simple, gently-dipping settings (typical of many US onshore unconventional plays); in structurally complex areas (fold-and-thrust belts, salt-influenced basins) treat any single-point projection as a rough guide only and re-anchor frequently against fresh control.
References
Primary source
Need geoscience support? BauerCalc is just one of the tools we build. If you're looking for independent expertise in well planning, geosteering, reservoir characterization, or opportunity evaluation, visit BauerSubsurface.com.