Changes

  | part    = Predicting the occurrence of oil and gas traps

  | part    = Predicting the occurrence of oil and gas traps

  | chapter = Interpreting seismic data

  | chapter = Interpreting seismic data

  | frompg  = 12-1

  | frompg  = 12-25

  | topg    = 12-29

  | topg    = 12-29

  | author  = Christopher L. Liner

  | author  = Christopher L. Liner

  | isbn    = 0-89181-602-X

  | isbn    = 0-89181-602-X

}}

}}

Below is a recipe for making a classic integrated structure map from seismic data and well control. It is based on mapping one horizon at a time and must be repeated for each horizon of interest. It may not work in areas with severe static problems (i.e., lots of topography or a rapidly changing weathered layer such as glacial till). It also fails when there are extreme lateral velocity variations in the subsurface (subsalt, subthrust, etc.). When it works, this method gives a map which, by definition, matches every well exactly. It uses seismic time structure to interpolate between wells and extrapolate beyond them.

Below is a recipe for making a classic integrated structure map from [[seismic data]] and well control. It is based on mapping one horizon at a time and must be repeated for each horizon of interest. It may not work in areas with severe static problems (i.e., lots of topography or a rapidly changing weathered layer such as glacial till). It also fails when there are extreme [[lateral]] velocity variations in the subsurface (subsalt, subthrust, etc.). When it works, this method gives a map which, by definition, matches every well exactly. It uses seismic time structure to interpolate between wells and extrapolate beyond them.

==Procedure==

==Procedure==

Follow the steps listed below for each seismic event to be mapped.

Follow the steps listed below for each seismic event to be mapped.

# Make structure [[contour]] maps for key horizons using well control only.

# Pick seismic horizons.

# Calculate depth conversion velocity at locations where both well and seismic time picks exist.

# Convert time to depth by multiplying the time structure map and the depth conversion velocity map.

# Contour the integrated structure map, keeping in mind the structure map made earlier from well data only.

| Make structure contour maps for key horizons using well control only.

| Pick seismic horizons.

| Calculate depth conversion velocity at locations where both well and seismic time picks exist.

| Convert time to depth by multiplying the time structure map and the depth conversion velocity map.

| Contour the integrated structure map, keeping in mind the structure map made earlier from well data only.

==Step 1: Map structure from well data==

==Step 1: Map structure from well data==

Post well depths to key horizons and contour structure maps for key horizons using well control only. These well maps of structure should guide you when making structure maps that integrate both well and seismic data. Comparing this map with the final time structure map gives a good feel for the additional information supplied by the 3-D seismic section.

Post well depths to key horizons and contour structure maps for key horizons using well control only. These well maps of structure should guide you when making structure maps that integrate both well and [[seismic data]]. Comparing this map with the final time structure map gives a good feel for the additional information supplied by the 3-D seismic section.

==Step 2: Pick seismic horizons==

==Step 2: Pick seismic horizons==

 

[[file:interpreting-seismic-data_fig12-12.png|left|thumb|{{figure number|1}}(a) Representative line from the Glenn Pool data volume with sonic overlay and tracked events; (b) Horizon amplitude and time structure maps for the Wilcox. From Liner.<ref name=Liner_1999>Liner, Chris, Elements of 3-D Seismology: Tulsa, PennWell.</ref> Courtesy PennWell.]]

interpreting-seismic-data_fig12-12.png|{{figure number|1}}(a) Representative line from the Glenn Pool data volume with sonic overlay and tracked events; (b) Horizon amplitude and time structure maps for the Wilcox. From Liner.<ref name=Liner_1999>Liner, Chris, Elements of 3-D Seismology: Tulsa, PennWell.</ref> Courtesy PennWell.

For 2-D data, only the traveltime to each event of interest is recorded with its coordinate along the line ''t''(''x''). For 3-D data, both traveltime and amplitude at each (''x, y'') are available from the seismic data cube, ''t''(''x, y'') and ''a''(''x, y''). The traveltimes form a time structure map, and the amplitudes are a horizon slice. [[:file:interpreting-seismic-data_fig12-12.png|Figure 1A]] shows a representative line from the Glenn Pool data volume with sonic overlay and tracked events. Horizon amplitude and time structure maps for the Wilcox are shown in [[:file:interpreting-seismic-data_fig12-12.png|Figures 1B, C]].

For 2-D data, only the traveltime to each event of interest is recorded with its coordinate along the line ''t''(''x''). For 3-D data, both traveltime and amplitude at each (''x, y'') are available from the seismic data cube, ''t''(''x, y'') and ''a''(''x, y''). The traveltimes form a time structure map, and the amplitudes are a horizon slice. [[:file:interpreting-seismic-data_fig12-12.png|Figure 1A]] shows a representative line from the Glenn Pool data volume with sonic overlay and tracked events. Horizon amplitude and time structure maps for the Wilcox are shown in [[:file:interpreting-seismic-data_fig12-12.png|Figures 1B, C]].

==Step 3: Calculate depth conversion velocity==

==Step 3: Calculate depth conversion velocity==

Calculate depth conversion velocity at locations where both well and seismic time picks exist. The wells used as control do not need velocity or density logs but must penetrate the event of interest. The event depth ''z'' (measured from seismic datum) is known from well control, and the vertical reflection time ''t'' is known from the previous item. The depth conversion velocity is given by

Calculate depth conversion velocity at locations where both well and seismic time picks exist. The wells used as control do not need velocity or density logs but must penetrate the event of interest. The event depth ''z'' (measured from seismic datum) is known from well control, and the vertical reflection time ''t'' is known from the previous item. The depth conversion velocity is given by

:<math>\upsilon = \frac{2z}{t}</math>

:<math>\upsilon = \frac{2z}{t}</math>

Depth conversion velocities are posted to a map and contoured or gridded to create ''υ''(''x, y'').

Depth conversion velocities are posted to a map and [[contour]]ed or gridded to create ''υ''(''x, y'').

[[:file:interpreting-seismic-data_fig12-13.png|Figure 2]] shows a hypothetical well with important reference points as well the average velocity map for the Wilcox Formation in the Glenn Pool survey. This map has a fairly strong lateral velocity gradient, i.e., the velocity changes from about 11,400 ft/s for velocity (NE) to 10,200 ft/s (SW) in the space of just over a mile. When this occurs, time structure and depth structure can be significantly different.

[[:file:interpreting-seismic-data_fig12-13.png|Figure 2]] shows a hypothetical well with important reference points as well the average velocity map for the Wilcox Formation in the Glenn Pool survey. This map has a fairly strong lateral velocity gradient, i.e., the velocity changes from about 11,400 ft/s for velocity (NE) to 10,200 ft/s (SW) in the space of just over a mile. When this occurs, time structure and depth structure can be significantly different.

==Step 4: Convert time to depth==

==Step 4: Convert time to depth==

Convert time to depth by multiplying the time structure map and the depth conversion velocity map, i.e.,

Convert time to depth by multiplying the time structure map and the depth conversion velocity map, i.e.,

:<math>z(x,y) = \frac{\upsilon(x,y)*t(x,y)}{2}</math>

:<math>z(x,y) = \frac{\upsilon(x,y) \times t(x,y)}{2}</math>

The factor of one-half is necessary because the times are two-way vertical times and we only want the one-way depth. [[:file:interpreting-seismic-data_fig12-14.png|Figure 3]] shows the process and result for the Glenn Pool Wilcox horizon.

The factor of one-half is necessary because the times are two-way vertical times and we only want the one-way depth. [[:file:interpreting-seismic-data_fig12-14.png|Figure 3]] shows the process and result for the Glenn Pool Wilcox horizon.

==Step 5: Contour map==

==Step 5: Contour map==

[[file:interpreting-seismic-data_fig12-16.png|thumb|{{figure number|4}}zoom of the central area in the maps in Figure 5. From Liner.<ref name=Liner_1999 /> Courtesy PennWell.]]

[[file:interpreting-seismic-data_fig12-16.png|300px|thumb|{{figure number|4}}zoom of the central area in the maps in Figure 5. From Liner.<ref name=Liner_1999 /> Courtesy PennWell.]]

Contour or grid the integrated structure map with the same technique used for the wells-only depth map. This allows head-to-head comparison ([[:file:interpreting-seismic-data_fig12-16.png|Figure 4]]).

Contour or grid the integrated structure map with the same technique used for the wells-only depth map. This allows head-to-head comparison ([[:file:interpreting-seismic-data_fig12-16.png|Figure 4]]).

==Integrated map==

==Integrated map==

[[file:interpreting-seismic-data_fig12-15.png|thumb|{{figure number|5}}Comparison of the first depth map from well control only and the seismic plus well integrated depth map. From Liner.<ref name=Liner_1999 /> Courtesy PennWell.]]

[[file:interpreting-seismic-data_fig12-15.png|thumb|300px|{{figure number|5}}Comparison of the first depth map from well control only and the seismic plus well integrated depth map. From Liner.<ref name=Liner_1999 /> Courtesy PennWell.]]

The final product ''z''(''x'', ''y'') is called an integrated structure map. It honors all well-control depth points (by definition) and uses the seismic events to interpolate between these points. [[:file:interpreting-seismic-data_fig12-15.png|Figure 5]] is a comparison of the first depth map from well control only and the seismic plus well integrated depth map.

The final product ''z''(''x'', ''y'') is called an integrated structure map. It honors all well-control depth points (by definition) and uses the seismic events to interpolate between these points. [[:file:interpreting-seismic-data_fig12-15.png|Figure 5]] is a comparison of the first depth map from well control only and the seismic plus well integrated depth map.

[[Category:Predicting the occurrence of oil and gas traps]]  

[[Category:Predicting the occurrence of oil and gas traps]]  

[[Category:Interpreting seismic data]]

[[Category:Interpreting seismic data]]