GEOMODEL CUBE HOW TO

This lateral change of facies has an impact on the geometry of the seismic horizon of HrzC ( Figure 2B). Facies is one of the key factors to control the distribution of velocity in a geological unit. The layer is a shale unit (3000m/s) truncated by a sand channel (4000m/s). In this example, the two first layers have more or less constant interval velocity of 2,500m/s and 5,000m/s respectively ( Figure 2A) while the deepest layer has a sharp lateral change of interval velocity. Interval velocities are preferred to average velocities for units with lateral changes of interval velocity ( Figure 2). The average velocity maps are then used to convert the seismic cube and/or interpretation to the depth domain. Once the average velocity for a given horizon is known at each well, a map of average velocity is defined, either as a constant everywhere, or using interpolation techniques. If only the seismic horizons need to be converted, it is done directly from the interval velocity maps.ДЄn average velocity is the mean velocity not between two horizons, as for the interval velocity, but between the ground and a given horizon (example, Figure 1B). Having done this for each geological unit, the seismic cube can be converted to the depth domain with all the seismic interpretation. For every XY location, the interval velocity value from the map is then assigned to every point of the seismic cube at this coordinate. On the contrary, if we have enough wells and if the interval velocity varies from well to well, interpolation techniques are used to generate an interval velocity map between the well interval velocities. This approach is also used when there are too few wells to interpolate the interval velocity on the map in any meaningful way. If the interval velocity is more or less constant at each well, an average constant interval velocity might be assigned to the whole unit over the lease. The interval velocity is the ratio between the delta-depth and the delta-TWT. As the well has been converted to time, the time twoвЂ‘way-time (TWT) is known too. The depth of top horizon and of the bottom horizon are known (by definition). Well tops are all we need to compute an interval velocity. The interval velocity is an integration of these local vertical variations. In a given unit, sonic shows that the velocity varies vertically. Figure 1A illustrates the concept with interval velocities computed for the shallow unit between the ground (for onshore seismic) and the first key horizon HrzA, then for the unit A between HrzA and HrzB and lastly for the unit B between HrzB and HrzC. Interval velocities are computed along each well. Instead of defining the time-to-depth conversion from the sonic log, interval velocities or average velocity are used.ДЄn interval velocity is the mean velocity between two horizons at a given XY location.

Such an approach is sometimes needed though and the topic will be discussed in more detail later in this section. It has the same level of uncertainty than extrapolating facies data or petrophysical data in 3D. It can be extremely challenging to extrapolate sonic logs data between the wells. While sonic logs are being used to convert the wells from depth to time in great details, sonic is usually not used for converting the seismic data to depth. ThatвЂ™s where time-to-depth conversion is being used. Before doing so though, the seismic interpretation must be converted to the depth domain.

GEOMODEL CUBE HOW TO

Details on how to integrate a seismic interpretation in geomodeling are covered in the two next sections. It guides the seismic interpretation of the seismic cube that is to say the picking of the horizons and faults from the seismic cube. Converting the wells to the time domain is also the time when the geophysicist must decide which seismic event can be associated with which well top. A sonic log quantifies the reverse of the instantaneous wave velocity of the rocks in the vicinity of the borehole.

Well logs, such as sonic, are used to transfer the wells to the time domain. For more details about time-to-depth conversion, the reader can refer to ( Al-Chalabi, 2014). Readers interested in depth-migration can refer for example to Jones, 2010) for more details.

There, depth-migration and not time-to-depth conversion will likely be applied.

Such approach is insufficient in complex domains such as structural plays or salt plays. There is no lateral displacement, only a vertical one. Time-to-depth conversion means that each seismic data point in the time domain will be given a depth coordinate.

Geomodel cube