No Reflection Signal Can Be Good
As seismic interpreters, most of us – including the author – have developed the mindset that robust reflection events are what we first try to associate with drilling targets.
For thin-bed units, reflection amplitude increases as net pay increases within the target interval. Thin-bed interpreters conclude that “strong reflection events are good.”
In a sand-shale sequence, gas reservoirs produce P-wave bright spots. In this type of geology, interpreters focus on the boldest reflection signals to define drilling targets.
Depending on the nature of the seismic impedance contrasts in the type of geology that is being interpreted, there are exceptions to these two examples that drilling targets are associated with reasonably prominent reflection events. However, the association between robust reflection responses and drilling targets is successfully applied across many prospects and in several depositional environments.
In this article, we look at the opposite principle and describe a drilling target for which the correct mindset is: “Drill where there is no reflection signal.”
The target in this example is a thin Caddo sandstone positioned at the top of the Bend Conglomerate interval in the Fort Worth Basin.
Distribution of the sandstone is shown in map view in figure 1.
This map, based on the interpretation of well logs acquired in the labeled wells, indicates that the sandstone is distributed along a southwest-to-northeast channel-like trend. Local operators consider this particular sandstone to be an attractive drilling target, even though the average thickness is only five meters (16 feet).
At this location, the Caddo is interpreted to have been deposited in a deltaic environment that had a low accommodation space. Incised channels similar in size and shape to the trend shown in this map are therefore not unexpected features.
Two profiles (AA’ and BB’) are shown that traverse the sandstone trend and connect key calibration wells. Seismic responses along these profiles are exhibited in figure 2. The interpreted Caddo horizon is shown on the seismic sections; the circled area on each profile identifies the intersection with the sandstone trend.
Outside the circled areas, the Caddo reflection is robust because a thin carbonate layer extends across this local area and creates a significant P-wave impedance contrast with the overlying shale. Inside each circled area, the Caddo reflection is absent, or minimal, because there is no significant P-wave impedance contrast between the sandstone that infilled the erosional channel and its sealing shale.
To position a well that will penetrate this particular sand, an interpreter has to adopt the attitude that “no reflection signal is good.”
To illustrate this principle, the average reflection trough amplitude calculated in a narrow 30-ms window immediately below the top of the Caddo horizon is displayed as figure 3. The low-amplitude, southwest-to-northeast response labeled channel-fill response range on the color bar identifies the targeted incised channel that downcut through the thin carbonate layer and then infilled with sand.
This no-reflection trend is the drilling target.
The wells shown in these figures were drilled before the 3-D seismic data were acquired.