Depth imaging shows salt-like shale features on Scotian slope

Mark B. Gordon, Robert H. Newding, John O'Dowd
GX Technology Inc.
Houston

Paul J. Harvey, Jack MacDonald
Nova Scotia Petroleum Directorate
Halifax

Velocity modeling and prestack depth imaging by GX Technology (GXT) show a previously unrecognized deformation mechanism in the area of our test lines, south-southwest of Sable Island. Shale, not salt, tectonics dominates the deformation of the late Cretaceous and Tertiary section on the upper to mid slope in this area.

A layer that resembles an allochthonous salt sheet results in the formation of minibasins¹ and structural highs. However, the distinctively lower velocities indicate that this layer cannot be predominantly salt, and we believe that it is mobile shale instead.

Growth faults that form the margins of these minibasins continue up in the section as far as the sea floor. Below the top Cretaceous unconformity, the prestack depth migrated data reveal details that would have been obscured in conventional time data. These images indicate brittle faulting, possibly related to the shifting of the depocenter between the slope basin and the shelf basin. Prestack depth migrations can provide the images needed for regional exploration of this complex frontier basin.

Why depth imaging?

Depth imaging consists of building a velocity model in depth and using it as input for running prestack depth migrations. The process is interpretive and iterative and commonly requires repeated applications of model building and migration for the best solution.

Depth imaging is beneficial where substantial lateral velocity variation occurs such as salt features,² gas charged sediments,³ fold/thrust belts,⁴ and fault shadows.⁵ Depth imaging yields better results than conventional time processing for these data types because the velocity model in the depth domain corrects for the errors caused by the velocity variations.

Depth imaging has been in general use as an exploration tool for approximately five years although it was developed and used in research settings earlier.⁶ Depth imaging is now used routinely to identify subsalt prospects in the Gulf of Mexico, the North Sea, and elsewhere. Depth images also have the obvious advantage of more closely resembling geologic sections because the finished product is a depth display, not time.

The preferred tools for model building are reflection tomography and migration velocity analysis.

Tomography provides a continuous global solution to velocity within a layer and thus effectively accommodates anomalous lateral variations in velocity. Tomography is particularly effective shallow in the section where velocity errors must be corrected to prevent errors from being propagated at depth.

Migration velocity analysis is generally used deeper in the section. These analyses are performed at discrete locations on data that are migrated. Signal to noise is thus improved and ray path problems associated with dip and complex structure are minimized.

Both velocity determination methods are generally limited in depth to the length of the cable. Thus for regional tectonic and basin analysis, data with relatively long offsets and extended recording lengths are essential.

Nova Scotia location

In collaboration with the Nova Scotia Petroleum Directorate, GXT tested depth imaging in the Scotian basin by working portions of two lines located south-southwest of Sable Island (Fig. 1) [98,637 bytes].

The water depths along the lines vary from 500 to 2,500 m. Wade and MacLean7 call this region the "slope diapiric province." The continental margin of Nova Scotia formed in the Late Triassic and Jurassic as a result of rifting between North America and Africa. A thick section of Triassic to Cenozoic sedimentary sequences was deposited on this passive continental margin.

Imaging problem

Significant gas reserves and some oil reserves have been discovered on the Nova Scotian shelf in the vicinity of Sable Island.

Construction of a major gas pipeline system slated to begin carrying gas in November 1999 from six of these gas fields to markets in New England has resulted in the development of oil and gas infrastructure in Nova Scotia, making future development much more economic.

The deepwater Nova Scotian slope that is the focus of this article presents a frontier exploration province with the possibility of large, untapped oil and gas reserves. In preparation for a land sale that closes on Apr. 29, 1999, seismic contractors have been acquiring state-of-the-art seismic data in the Scotian basin, especially where the lands lie in deeper water on the Scotian slope.

Use of these improved data is important because of the difficulty of seismic imaging in this geologically complex region. Some of the features that we hoped to resolve included the following:

1. Discrimination of salt versus shale based on the velocity structure, and
2. Imaging of the deep structure below the highly reflective Upper Cretaceous unconformity, especially on the upper continental slope.

Imaging methods

Geophysical Service Incorporated (GSI) provided GXT with 2D seismic reflection data collected in 1998 for test purposes. These data have a 7,500 m offset range.

The data were initially processed in the time domain with algorithms that optimally enhanced the data for depth processing. Very careful attention was given to the attenuation of the water bottom and ensuing multiples. However, it was not possible to remove all the multiple effects.

These time-processed gathers were entered into our tomography program. The highest energy, most continuous events were picked and then inverted for velocity and depth determinations. Tomography calculates interval velocities that will flatten interpreted events within the common reflection point gathers across the entire offset range in depth. For the Scotian basin, tomography gave excellent results to at least 5,000 m and deeper in parts of the section.

A velocity grid was calculated from the tomography results. The grid was then used to run a Kirchhoff prestack depth migration of the data. After migration, the interpreted horizons were adjusted, and focusing analyses were used to determine velocities at depths greater than 5,000 m. The data were then migrated again. Several iterations of focusing plus migration were run to determine the optimum velocity depth model.

Imaging results

The imaged lines are from the "slope diapiric prov- ince." ⁷ Previous workers show many salt diapirs in this area.

Salt of the Late Triassic Argo formation is well known in the Scotian basin and has been drilled in many wells.⁸ However, we do not believe that the features on our lines can be predominantly salt because the velocity of the salt-like features is too low (Fig. 2) [63,274 bytes].

We believe that these salt-like features are formed by a mobile shale layer. The deformed shale layer is highlighted with arrows in Fig. 3. [104,161 bytes] The feature on the left appears similar to allochthonous salt sheets of the Gulf of Mexico. A minibasin has formed between this feature and a diapir located farther to the northwest. We believe that shale moved out of the minibasin and formed an allochthonous shale sheet and shale diapir just as salt withdraws from minibasins in the Gulf of Mexico.

Based on seismic character, we correlate the bright reflector on the northwest side of Fig. 3 to the Upper Cretaceous unconformity shown on many seismic lines tied to wells8 such as Fig. 4. [280,520 bytes] Regionally, this reflector represents a prominent Jurassic carbonate bank complex that was deposited along the ancestral shelf edge forming the LaHave platform. This Jurassic-Late Cretaceous carbonate platform remained exposed until the Late Cretaceous and controlled the location of the paleoslope and the sediment infill that spilled across it. This regional feature also controlled the slope of the shelf-slope break.

This horizon is approximately at the same level as the deformed shale layer, but without more deep well data we cannot be certain that the shale is the same age as the unconformity. Thus we can only speculate that the shale, though present in the upper to mid slope area as demonstrated by the Tantallon M-41 and Shubenacadie H-100 wells,⁸ was prevented from becoming mobile on the northwest side of the line because of a carbonate cap. Absence of the carbonate in the deepwater may have allowed the shale to deform in a much more plastic manner.

The imaging of stratigraphic sequences above the Upper Cretaceous unconformity is excellent (Fig. 5a) [707,062 bytes] In this zone, we see stratigraphic wedges that thin either toward the deeper basin (to the southeast or left on Fig. 5a) or toward the Scotian shelf. Thus we believe that the depocenter may have been shifting from basinward to shelfward. However, we cannot exclude the possibility that the effect is caused by stratigraphic changes in the third dimension.

The Upper Cretaceous unconformity is highly reflective and does not allow much energy to penetrate deeper in the section. Initially, we believed that we were not as successful imaging below this unconformity. However, we are now convinced that the imaging is good whereas the geology is complicated. Stratigraphic thin-outs are visible just below the unconformity (Fig. 5b).

The deeper part of the seismic section exhibits much faulting and steep bed tilts (Fig. 5c). We believe that the lack of continuity in the deep section is due to the complex structure and that the imaging is good to 12 km, the maximum depth that was migrated. This depth covers the entire zone of current economic interest.

The stratigraphic wedges above the Upper Cretaceous unconformity and the faulting at depth may be related. The brittle faulting at depth may be causing the shifting in depocenters higher in the section. This idea could be tested by imaging more lines close to and crossing test line a.

Implications for exploration

The structural complexity of the slope diapiric province makes correct seismic imaging with conventional time data difficult. However, use of prestack depth migration with the same dataset reveals many previously undocumented features.

Additionally, we found that the high resolution velocities determined in the iterative prestack depth migration process could be used to reinterpret the tectonic origin of the basin. A tectonic interpretation is a major facet of any exploration model for this area. The depth-imaged data can then be utilized to evaluate the prospectivity of the basin, and to high grade areas for further exploration.

Thus depth imaging is a key component of regional analysis in this structurally complex region. Though used primarily for prospect specific applications, depth imaging is proving to be valuable for regional tectonic and basin studies. The depth imaging of data from the Scotian basin demonstrates its ability to test models, verify lithologies, and obtain superior imaging results.

Due to the relative expense of depth imaging compared to conventional time processing, its use for regional exploration may be best accomplished with industry groups in order to optimize the economics. Additionally, the geological input from the participants is essential to the final result.

Acknowledgment

We thank GSI for providing us with the seismic data and allowing us to publish the results.

References

1. Koch, A., Mathur, V., Nagy, R., and Snyder, F., Methodology for minibasin ranking in the deepwater Gulf of Mexico, Houston Geological Society Bull., Vol. 41, No. 1, 1999, pp. 23-25.
2. Ratcliff, D.W., and Weber, D.J., Geophysical imaging of subsalt geology, The Leading Edge, Vol. 16, 1997, pp. 115-142.
3. Sempere, J.E., and Hardy, P.B., Reflection tomography and velocity model building in an area characterized by shallow gas, Offshore Technology Conference, 1998, pp. 479-486.
4. Zhu, J., and Lines, L.R., Comparison of Kirchhoff and reverse-time migration methods with applications to prestack depth imaging of complex structures, Geophysics, Vol. 63, 1998, pp. 1,166-76.
5. Fagin, S., The fault shadow problem: its nature and elimination, The Leading Edge, Vol. 15, 1996, pp. 1,005-13.
6. Jeannot, J.P., Faye, J.P., and Denelle, E., Prestack migration velocities from focusing depth analysis, SEG Expanded Abstracts, 56th annual international meeting, 1986, pp. 438-440.
7. Wade, J.A., and MacLean, B.C., The geology of the southeastern margin of Canada, Part 2: Aspects of the geology of Scotian basin from recent seismic and well data, in Keen, M.J., and Williams, G.L., eds., Geology of the continental margin of eastern Canada: Geology of Canada, Geological Survey of Canada, Ottawa, 1990, pp. 190-238.
8. MacLean, B.C., and Wade, J.A., 1993, Seismic markers and stratigraphic picks in Scotian basin wells, East Coast basin atlas series, Dartmouth, Atlantic Geoscience Centre, Geological Survey of Canada, 1993, 276 p.

The Authors