A 75 stations passive seismic network was installed in the area and operated for 10 months. The region was encountering severe seismic penetration problems and conventional seismic failed to provide any useful information. In order to spot the existence of zones of interest, in terms of hydrocarbon (gas) exploration, all available information on the study area (existing geological and geophysical information, along with well data from two local unsuccessful wells) was examined thoroughly, leading to the following remarks:
- From local geology, the formations of interest are sandstones (locally met at an approximate depth of 2km and4km, in the case when the clays act as a cap rock. These formations have the potential of containing hydrocarbons, if the local structural characteristics favor the existence of hydrocarbon traps.
- Calibration of PST data with local geology (well data) has shown that the formations of interest are characterized by a seismic wave velocity ranging between 4.5 and 4.8km/s.
- The Vp/Vs value is directly correlated to the fluid content of formations, thus giving a strong indication on the existence of zones of interest. In the case of tight sands, the Vp/Vs variations follow the following pattern:
- As porosity of the formation increases, the value of Vp/Vs decreases.
- Gas saturation results in a significant decrease of the Vp/Vs value, while water saturation increases Vp/Vs.
- Although the Vp/Vs value of gas saturated sandstones is generally lower than 1.60, in the case of less consolidated formations the expected value may be increased.
- The existence of clay or shale in the sandstone causes a small increase in the Vp/Vs values. As a result, shales are characterized by higher Vp/Vs.
Delineating areas of interest in the passive seismic tomography model is performed by cross-examining Vp and Vp/Vs distribution (figure below), searching for areas that meet specific criteria.
Detection of a possible gas indication by cross-examining Vp and Vp/Vs sections.
In the following figure (figure 8), the visualization of the upper part (top surface) of the volume of interest of each one of the three areas is presented. The x and y axes correspond to longitude and latitude values, while the z axis corresponds to depth. The color scale represents depth values. The points extracted from data are marked on the images, delineating the space actually covered by data meeting the selected criteria (in order to avoid confusion with extrapolated values).
Points meeting the gas prone selection criteria [4.5<vp
When evaluating the results presented in the above figure, it can easily be concluded that this part of the exploration block is characterized by values of Vp and Vp/Vs meeting the selected criteria at depths varying between 2.9 and 4km and could be promising in terms of hydrocarbon indications. The two unsuccessfully drilled wells were out of this area. Another two sub-regions of the exploration block have also been proposed as possible targets for successful drilling and are depicted below. As for the area B , the formation’s top surface extends a little deeper (at depths ranging from 3.8 to 5.3km) and must be thoroughly examined, in order to conclude whether the local conditions justify the appearance of a possible gas bearing formation at so shallow depths.
Finally, in the case of area C , the formation’s top surface is characterized by values of Vp and Vp/Vs meeting the selected criteria at very low depths (below 5.9km). Consequently, it seems highly unlikely that this particular area corresponds to some promising region, in terms of hydrocarbon exploration.
The above-mentioned analysis led to the conclusion that careful evaluation of the passive seismic tomography results, using all a priori available information, can result in the delineation of regions of interest for the upstream oil industry. This approach could offer an interesting option in case of exploration areas characterized by difficult topography or seismic penetration problems.