Monday, January 8, 2018

Overcoming Limitations of Sequence Stratigraphy

Sequence stratigraphy has proven useful for petroleum exploration, but it is commonly misapplied. There is controversy over whether the technique can be applied to carbonate systems since it was designed to explain sand-shale systems. Some experts maintain that sequence stratigraphy is easier in carbonates because carbonates are extremely sensitive to sea level change. There is unanimous agreement, however, that low sedimentation rates often pose special problems. When sedimentation rate is moderate to high, layers within a sequence are tens to hundreds of meters thick, comfortably within the resolving power of a typical seismic wave. But when sedimentation rate is low, several sequences might fit within a seismic wavelength. Sequence stratigraphy cannot be confidently applied here, but it has been done countless times. A useful interpretation in thinly-bedded regions requires abandoning small-scale features and concentrating on larger scale, longer term processes that control the generation of sequences. 

 With this in mind, Vail and coworkers proposed a hierarchy of stratigraphic cycles based on duration and amount of sea level change. Duval and Cramez at TOTAL Exploration worked with Vail to provide subsurface examples and to expand the application to hydrocarbon exploration. The hierarchy assigns frequencies to the mechanisms of eustasy  enumerated by Fairbridge, viewed in light of plate tectonics. The first-order cycle, which is the longest, track creation of new shorelines resulting from the breakup of the continents. The second-order cycle is landward and basinward oscillation of the shoreline that lasts 3 to 50 million years. This oscillation is produced by changes in the rate of tectonic subsidence and uplift, caused by changes in rates of plate motion. 
Both first - and second order cycles may cause changes in the volume of the ocean basins resulting in long-term variations in global sea level. The third-order cycle is the sequence cycle, lasting 0.5 to 3 million years. Fourth- and higher order cycles may be correlated with periodic climatic changes.

The following example, with its low deposition rate, approaches the limit of interpretation in terms of third-order cycles. It comes from the Outer Moray Firth basin in the UK sector of the North Sea, where the initial basin shape, tectonic activity and variation in the rate of deposition add a twist to the interpretation.

Stratigraphic interpretation of the last 65 million years of sediments in the Outer Moray Firth is more difficult than in the Gulf of Mexico because slower deposition in the Central North Sea resulted in thinner units, many of which cannot be resolved by seismic waves. During this period, the Outer Moray Firth has 17 sequences totaling 5000 feet of sediments, compared to the Gulf Coast, with 10 sequences totaling 9000 ft. In the North Sea, however, depositional processes juxtaposed a variety of lithologies, provide reliable calibration points for accurate conversion of logs from depth to time using synthetic seismograms. 

In the Gulf of Mexico, this conversion is typically done with only nearby checkshots; sands and shales commonly show periodic alternation with depth at wavelength that make comparisons between seismic sections and synthetic seismograms nonunique. 

 Stratigraphy study is always preceded by structural interpretation. In addition, a paleogeographic interpretation of the Outer Moray Firth shows that late in the Cretaceous period - when the sequences under study began to deposited - a smooth basin floor sloped gently from northwest to southeast. During a relative fall in sea level, sediments were deposited as slope fans. Their seismic expressions indicate lobes with channels and some chaotic flows- large scale slumps with jumbled seismic character. As sea level rose, a wedge of out-building deltas was deposited. Sea level maximum is associated with a depositional hiatus, shown only as a thin line. Deposits synchronous with this surface may be found on what is now land in Europe, but in the basin, sediments that correspond to periods of high relative sea level are rare. 

 Why are elements of the classic Vail model missing from this sequences in this basin? One explanation is the the competing influences of tectonic uplift and sea level change. As global sea level rose and fell, continual regional uplift kept the sea from reaching levels high enough to allow formation of units typical of high relative sea level. Only once, at the top of the third sequence, does a thin layer of high relative sea level sediments appear. Another interpretation is that thin, high relative sea level sediments were deposited, but eroded and so are not preserved in the section.

 This section can also be interpreted in terms of second-order cycles. The entire set of 17 depositional sequences can be bracketed by five second-order cycles, based on physical stratigraphy and biostratigraphy. Major biostratigraphy gaps exist at the boundaries of each second-order cycle, and the boundaries can be seen to represent major changes in the depositional style of basin fill.

If the volume of earth in a study area is small enough, workstations can add a new dimension to sequence stratigraphy. In the Green canyon area of the Gulf of Mexico, interpreters concentrated on a fan deposited in a syncline on the continental slop 1 to 2 million years ago. Regional sequence stratigraphy was established using 2D seismic data and paleontologic control from six nearby wells. Zooming in on a subset of this data, interpreters assembled a series of 2D panels for 3D interpretation.

The top and base of the slope fan were interpreted over a six-block area (138 km2). The thickest part of the slope fan coincides with the stacked channels that carried shallow-water shelf and delta sands into deep water, greater than 200 m. A series of stacked channels, possibly filled with sand, is visible within the slope fan interval.

The goal of this interpretation is to identify exploration targets. Although lithology of the channel deposits is difficult to identify in the horizon slice, geology predicts that the channel will terminate in a sand-rich fan. The channel was tracked south, and a fan was discovered in the next block of seismic data.

Sequence stratigraphy continues to evolve. One area of investigation is high resolution sequence stratigraphy, which is performed at a higher resolution than seismic wavelengths, usually with log and outcrop studies. 

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