Three-dimensional seismic data have significantly expanded the envelope for exploration in challenging offshore frontiers, providing valuable insight into geological structures, stratigraphy and lithology of target areas. However, the ability to gain detailed interpretation of the data for depositional environments has been impacted by several phenomena, notably sparse spatial sampling and interference from background noise. This limitation presents obstacles to obtaining comprehensive and accurate geological analyses critical for exploration and development.
In offshore environments seismic data traditionally have been acquired using pressure sensors to reconstruct the wavefield, often producing artificial noise that distorts the image. Streamers typically enable good sampling in the inline direction but provide poor sampling in the crossline direction due to operational or economic constraints. The gap between streamers in offshore environments can be as much as 500 m (1,640 ft). Additionally, seismic data frequently are interpreted without qualifying which depositional setting they represent.
Ancient and modern analogs to help interpret 3-D seismic data from rock onshore outcrops generate information only along horizons defined by the erosion of the strata, yielding 2-D information. Satellite images of modern depositional environments provide a more complete overview of the geologic setting but lack vertical dimension.
A newer approach integrates the acquisition, processing and interpretation of 3-D seismic data by combining an omnidirectional sampling technique and color processing to deliver realistic high vertical resolution and lateral sampling of the subsurface. Especially suited to frontier regions where no well data are available, the method results in more efficient exploration and completion times while maintaining data quality and reducing operational cost and environmental exposure. The acquired data can be used for the entire E&P cycle.
Uniform dense sampling in vertical and lateral directions facilitates direct interpretation of seismic data for continuous depositional environments. Analysis of color-processed 3-D seismic images along lithologic horizons enables the richer interpretation that reveals a wealth of information such as dewatering and overpressure for both the original depositional lithologies and post-depositional diagenetic and tectonic overprint. In cases where no well data are available for calibration, interpretation of polygonal fractures can be linked directly to the lithology to further aid in stratigraphic interpretation.
Mapping a complex depositional environment
A study offshore Newfoundland, Canada, successfully tested the approach to map a complex depositional environment from shelf to basin in areas dominated by sands and shales. The IsoMetrix marine isometric seismic technology provided true 3-D measurements for full-bandwidth imaging of fine-scale structures in the subsurface in all directions—vertically along the streamer and crossline between streamers—to acquire detailed imaging from the seabed to the reservoir.
The ability to isometrically sample multimeasurement data enhances the definition of geological features in deep and shallow subsurface environments, resulting in a marked improvement in fine spatial sampling compared to conventional 3-D surveys. This enables wider streamer separation, increasing subsurface coverage by more than 100% without compromising data integrity.
The multisensor streamer data allow deghosting of the wavefield by eliminating noise bounce-back that interferes with the ability to distinguish signals. Once the signals are separated from the noise, isometric seismic technology can accurately map the wavefield between the streamers in all directions. The data are then color-processed for an analysis that can be easily interpreted.
The eXchroma chromatic geology extraction technique provides a structurally sharp visualization of the seismic data in true red-green-blue color. A regional 3-D view of the color-processed seismic data yields images similar to satellite images acquired from modern earth surfaces. However, the color-processed images can be generated for any lithostratigraphic horizon in the subsurface, meaning depositional environments can be studied in their context of geologic time.
The Newfoundland study, conducted from May to October 2016, encompassed an area of 9,038 sq km (3,490 sq miles) on the northern slope of the Flemish Bank from the Flemish Cap to the Orphan Basin. The objective was to map the depositional environment from shelf to basin in the modern shale-dominated environment near the seafloor and in the sand-dominated environment.
After taking into consideration noise attenuation and migration, 3-D seismic data were processed along stratigraphic horizons like stacks of satellite images, with about 100 m (328 ft) between streamers. Three horizons were selected for interpretation: H1 near the base Tertiary, H2 at about 300 m (984 ft) below the seafloor and the seafloor. The study focused on the Tertiary basin between the first horizon and the seafloor, which is dominated by sands and shales.
Extraction of depositional geobodies indicated that shales were likely deposited from shelf currents transporting fine sediments down the slope of the Flemish Cap, while sand contourites were deposited by seafloor currents almost perpendicular to the slope currents. (Source: Schlumberger)
Effective seismic interpretation in frontier exploration
First, isometric seismic technology was applied to acquire and process data, analyzing seismic sections for polygonal fault patterns at distinct horizons. Then, seismic data image processing generated color images that were draped over these horizons to gain a detailed interpretation of the depositional environment in map view, a process defined as horizon image texturing. Finally, the horizon analysis was conducted in 3-D to place the interpreted depositional features in their geological context. After gaining an understanding of the depositional environment, individual depositional features were isolated for more detailed investigation.
The omnidirectionally sampled seismic data combined with the color-processed images provided the basis for the 3-D geological interpretation. The horizon texture for H1 revealed the significant presence of polygonal faults in the basin with thick deposits, while only a few polygonal faults were seen at the shelf toe.
This suggests that horizon image texturing of seismic data can provide clues regarding the depositional environment of strata mapped from seismic data. In this case, the application yielded details of the deposition near the base Tertiary at H1.
The presence of polygonal faults correlates with dewatering processes and also indicates the deposition of shale sediments from the Flemish Cap. Formations without polygonal faults were associated with predominantly sandy facies, where dewatering can release pressure upward. Sandy contourites near the northwest edge of the cap suggested the presence of strong marine longshore currents at the time of deposition.
The simultaneous occurrence of polygonal faults, interpreted as indicators for shales and contourites interpreted as sand bodies, suggests that two styles of deposition occurred at the same time. Shales were likely deposited from shelf currents transporting fine sediments down the slope of the Flemish Cap, while sand contourites were deposited by seafloor currents almost perpendicular to the slope currents.
Depositional environment mapping from the IsoMetrix 3-D seismic data indicated that two styles of deposition occurred at the same time. (Source: Schlumberger)
The discovery of the longshore stable sand contourites is important for exploratory drilling prospects and would have been overlooked without the more detailed geological interpretation gained through high vertical resolution and continuous lateral sampling. The resolution of two different depositional environments at the same time also verified that color processing of omnidirectionally sampled seismic data is a highly sensitive and effective method for analyzing frontiers for exploration and development.