Exploration in mature hydrocarbon zones has both advantages and disadvantages. The biggest advantage is that generally numerous wells have already been drilled. These wells can be used for calibration purposes and new techniques can be tested on known well locations before they are applied to proposed drilling spots. The most obvious disadvantage lies in the fact that the easily discovered gas and oil reservoirs have already been drilled and depleted, which makes exploration in such areas very risky.
We are discussing a case study in a highly-developed area with numerous wells that we can utilize as control points. The largest challenge lies in mapping reservoirs that have not been drilled and are not connected to previously discovered gas fields, and therefore have not been drained. We have been applying high resolution spectral decomposition together with detailed 3D structural mapping and coherence technology to reach this goal.
The first step in our interpretation workflow is the detailed interpretation of the seismic amplitude data. A time horizon is picked over the zone of interest. We incorporate wells that are drilled within the vicinity of the picked horizon whenever possible. Reservoirs at our target depth have characteristically strong troughs at the top followed by a well defined peak. The maximum trough amplitude is the strongest seismic amplitude indicator for gas reservoirs.
The time horizons are used for structure mapping and to produce maximum trough horizon maps. Spectral decomposition transforms seismic amplitudes as a function of space and time to spectral amplitudes as a function of space, time and frequencies. Various methods are commercially available. We apply the high resolution discrete Wigner-Ville transform for finding previously undrilled gas sands but also for investigating the geological setting for possible connections to producing gas fields. The horizon of interest covers a producing gas field that is easily recognizable on seismic amplitude data as a strong negative reflectivity.
The polarity used for this study states that a decrease in acoustic impedance is represented by a negative reflectivity. The amplitude anomaly extents over a larger area and was tested by further wells, which did not intercept gas. The spectral amplitudes confirmed the areas with known gas saturation distribution. It also indicates that the dry wells were perforated outside of a channel system. Furthermore, it shows an anomalous area in the shape of a point bar and another anomaly to the south that does not seem to be connected to the gas reservoir. Both anomalies point towards commercial gas discoveries.
The study area is located onshore and has been developed in the past. The high demand for energy sources and the subsequent high gas price makes it nevertheless an attractive area to explore for undiscovered gas sand. The presented results are part of a larger study that concentrates on hydrocarbon bearing geological intervals that have not been drilled during previous exploration phases. The first step in our exploration workflow consists of picking horizons on reflections of seismic data.
