Case studies

Geological analysis

Seismic exploration in the Canadian Foothills, as in many thrust belt terranes, is difficult and expensive because of extremely rugged topography and steeply dipping, high-velocity rocks at surface. Even high-fold data bought with an expensive source effort can give sections too saturated with noise to interpret — in an area where lines cost millions of dollars to shoot and wells tens of millions to drill, a better method of seismic imaging is required.

The area is characterised by multiple stages of thrust faulting. The centre of the image shows a series of duplex thrust features above a sub-horizontal decollement surface and capped by a roof thrust. Several individual duplexes that are excellent exploration targets can be seen on the MultiFocusing section; the roof thrust above them carries the sediments exposed at surface as a large anticlinorium.

MultiFocusing greatly improves the signal-to-noise ratio and gives a clear image of the complex sub-surface structure. The conventional processing not only fails to reveal the detail of the duplexes — it does not even show the large-amplitude anticlinorium that can be mapped at surface.

Geological analysis

MultiFocusing raised the signal-to-noise ratio enough to interpret every key horizon. Areal interlocking of those horizons then let the target groups of objects be mapped, and several 2D lines were processed over the area, allowing the interpreter to build a 3D map that clearly delineated these complex structures. The results of this case study have been confirmed by drilling.

The region belongs to the Timan-Pechora hydrocarbon basin. All Timan-Pechora swells trend northwest and have a horst-imbricate structure, broken into wedge-like blocks that include anticlinal structures — an indication of good hydrocarbon prospects throughout the entire geological section. The swells developed during the Carboniferous and Lower Permian; the local structures that further complicated the subsurface were complete by the early Triassic.

The time sections clearly display an inconsistency in the structural stages. Ordovician–Silurian deposition took place under trough conditions on the right-hand side of the sections, where the formations are considerably thicker than in the western areas on the left, where sedimentation took place in a shallower environment. The left-hand part shows the autochthonous tectonic dislocations, the right-hand part the allochthonous zones; evidence of stretching is visible on the tops of the anticline hinges in the Permian interval, which produced the fan-like configuration of the structure.

Geological analysis

The area’s geological history includes numerous complex structural forms caused by active tectonic processes — a salt core, for example, has been identified in the central part of the time section.

On the section processed by the conventional CMP method the internal structure of each formation is shown only in a general sense, and the salt lens can only be inferred. MultiFocusing not only displays the tectonic structure of the site, it also shows details of the fault zone confining the salt lens on the west. The salt body is well imaged on the east, the pinchout of the relevant stratigraphic interval is clearly defined, and the bottom of the salt shows up exceptionally well.

Geological analysis

The MultiFocusing result is a more accurate seismic image than the conventionally processed section. Not only is the continuity of the reflections improved, stratigraphy is revealed in great detail, with much better reflector continuity and bandwidth through the shallow Jurassic to Cretaceous section.

More impressive still is the deeper part of the section. This interval was a structural depression unevenly filled with Palaeozoic sediments; paleo-faulting, onlap, offlap, erosional truncation and possible reefing can all be read easily on the MultiFocusing section, and are stratigraphic detail the conventional CMP method cannot come close to achieving.

Geological analysis

The complex overburden, combined with strong lateral velocity variations and deformed, tectonised interfaces, made this land dataset a major imaging challenge. Gravity and magnetotelluric surveys had supplied only the basic shape of the basin.

MultiFocusing shows a remarkable overall improvement in structural resolution over the conventional processing in the poor-data zones — below salt domes, overthrusts, shallow basalt layers and permafrost — and a significant increase in signal-to-noise ratio. Supergathers and the MultiFocusing moveout compensation are what buy that: stacking a large number of traces across many CMP gathers increases the stacking power, which is particularly useful for low-fold coverage, and the moveout correction itself is stretch-free.

The reprocessing led to a better understanding of the structural and sedimentary development of the basin, and gave the exploration team a more reliable tool for maturing a drillable exploration prospect.