Horizon Maps

Horizon mapping has the largest influence in reservoir volume.

There are generally three approaches to map horizon from well markers. One is to map the depth directly. This is the simplest. But this is probably not the most accurate.

The second approach is to model in 3D the residue of horizon depth from the vertical trend. The vertical trend is the average of the depth data for each horizon. If there is only one horizon, there is no need to remove the vertical trend. With multiple horizons, removing the vertical trend makes the various horizons share the same average and the vertical variogram becomes more consistent with the horizontal variogram. This approach is more suitable if the horizons are similar.

The third approach is to map the thickness of the zones after mapping or given a horizon. Converting depths to thickness is another way of modeling the residue. The only problems with mapping thickness are

1. When there are missing markers, the thickness is not defined.

2. For a deviated well, it is difficult to determine the correct thickness, especially before the horizons are mapped.

Mapping the thickness is usually more accurate than mapping the horizon depths under the following conditions:

1. The horizon depths are very variable but the thickness is less variable, resulting in less error.
2. There is a starting horizon that can be better mapped alone, and
3. If the effects of missing marker and well deviations are small.

The problem with missing markers and deviated wells can be avoided if the thickness is calculated between a marker and a mapped horizon, rather than between two markers.

GridSTAT attribute gridding will automatically map the horizons if markers are used and horizon grid is not already specified. In this case the thickness mapping approach will be used (97.7).

Mistie is the depth (or time) difference between well marker and horizon grid. There are two types of mistie. One is the uncertainty in the horizon grid when the horizon grid is produced from other information than the well markers, assuming that well markers are correct. A typical example is that when the horizon grid comes from seismic picks. Another type of mistie comes from the approximation when a grid is used to represent a complex surface. When a depth value is sampled from the horizon grid, there will be a sampling error when the location is different from any grid node. This error will be larger when the surface is more complex compared to the size of the grid cells.

To check the mistie, go to Tools pull down menu and select Horizon Grid. Select the desired horizon grid. Click the Sample button and select Mistie File. The result goes to *.DAT file. The number of active markers must be the same as the number of horizons in the horizon grid in order to calculate the corresponding mistie.

To correct the mistie, import the mistie into a new project, grid the mistie, and add the mistie grid to the horizon grid to form a new horizon grid. Adding mistie grid is in the same Horizon Grid panel in the Calcul button.

Zone thickness should not be negative. Usually zero thickness is allowed. If the horizon grid is mapped in GridSTAT, there is automatic check for negative thickness. To see the thickness of horizon grids, graph Top View, from the Calcul button on the left of GridOutput panel, select Func. Transform, then select Difference. The first horizon is left unchanged, but the remaining horizons become the depth differences. Go to the second layer, then go to Options pull down menu GridGraph and Set Color to reset the color for the depth difference.

See Importing Grids section for how to import grids.

To bring in an imported horizon grid (or any horizon grid prepared separate from the current project) for attribute gridding, open the attribute gridding project, from Tools pull down menu go to Horizon Grid. Select GridFile or DepthGrid then check the imported horizon grid. Click the Sample button and select Depth Grid. The depth grid for the current project will be sampled from the imported horizon grid according to the specified dimensions in the GridSize panel. If more markers are to be used, for example when there are three markers to be used but there is only one horizon grid available, use the Map button on the left of the GridSize panel and select Thick to Depth to map the other horizons as thickness to the available horizon grid. To use the resulted depth in attribute gridding (otherwise the depth grid will be automatically generated from the markers), it is necessary to go from Options pull down menu to Gridding Data panel, select Depth Grid and check the desired depth grid, then click As Horizon and Yes. Selecting No will turn off the horizon grid assignment.

When the Start button (GridSize panel) is pressed, GridSTAT checks if markers are to be used. If so, GridSTAT checks if horizon grid has been assigned (Options/Gridding Data/Depth Grid/As Horizon/Yes). If horizon grid is assigned, the depth grid will be sampled from that horizon grid (or simple duplication if the dimensions are the same), otherwise the depth grid will be generated from the markers. The process of generating the depth grid from the markers includes kriging a depth horizon from the first marker or the horizon with largest number of available markers, then mapping the rest as thickness.

The horizons are saved in HorizonDB. To bring a subset into the DepthGrid, from Tools pull down menu go to Markers, select HorizonDB, then CopyToDGrid. The horizons corresponding to the current active markers will be copied to the current depth grid.

To mapped the horizon(s) without going through the attribute gridding process, use the Map button on the left of the GridSize panel. To map one horizon at a time, turn off the other markers from Options pull down menu go to Markers.