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  What Is Sequence Stratigraphy

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- [Paul] Sequencestratigraphy concepts is important to help us understand distribution of reservoir rocks and also source rocks and seals. It's a concept that's been around for quite a long time, and more recently, further developed by the Exxon Group under Peter Vail, and we will go through the basic concepts of this process. As I said in the introduction, this concept has beenaround for quite a long time with Walther's Law, and the important aspects are in a marine or lacustrine environment, a fairly stable geological setting, you have a progression of sediments based on hydrologicaland physical properties where you have sands near shore, finer grain material, deposits further out, clay deposits further out, and where you don't have significant impact from erosion, you have the either carbonate mud or siliceous muds that may be deposited in the marine environment. If the water level changes, say if it rises, then the system moves in shore, and you would move all these segments to the lift. That's basically how Walther's Law works and what happened more recently is the expiration boom hitin the '60s, '70s, and '80s, coupled with the conceptsof plate tectonics and ridge spreading. People were looking for universal ways to look for significant sediment packets in seismic programs that were run across major basins and ocean deposits, and the concepts that developed were changes, significantchanges in sea level, basically driven by either glaciation or ridge-spreading, wouldchange the amount of, would change the levelof global sea level, called eustasy. As you probably already know, glaciation is driven by significant forces that are mainly planetary and solar, precession, obliquity of the earth, eccentricity of orbit, solar forcing, and compositely the glaciation cycle through the earth's history as responded to this. In addition, with plate tectonics and sea mount, central ridge spreading, the spreading rate helps determine the size of the mid-ocean ridges. The faster the rate, the more volume being distributed, it raises a larger mass in the sea floor, and that raises sea level worldwide. These concepts were incorporated with Walther's Law tolook at global systems that would govern where sediments were deposited and how those changed with time, and the concept of sequence stratigraphy and sometimes called seismic stratigraphy was developed and utilized. For example, here's some curves that were developed by different authors. This one's by Hallam, and this one's by Exxon, under the Peter Vail Group. You can see that different groups, different parts of theworld have developed sea-level rise curves that show universal world-wide sea level changes, and the basic conceptsare during glaciations. You take up a lot of thewater out of the oceans so you have low sea level, and then you can have rising sea level as these glaciers melt, or you can also havevariations in sea level due to the spreading of the central ridges due to plate tectonics. Compounded with that are the depositional settings that you get in a marine terrestrialto marine environment, and so looking at this diagram, you can see playa lake systems, deltas, shelf deposits, slope deposits, and abyssal deposits, and depending on sea level, you will move what sediments are deposited in these various areas. For example, if sea level was to rise, this main plain area, this coastal plain area would flood, and you'd move the shelf deposits onshore. You might even flood this playa region and create inland seas. Different source rocks could be deposited in these areas, and the sand distribution, the silt distributions would change; if you raise sea level and move this delta far upstream, you'd get delta system up here; if you drop sea level, you'd move the delta out further, and if you dropped it far enough to go to the edge of the shelf, you'd start dumping itin the abyssal plain. You also have bypass zones, where you can have sands deposited offshore on the slope, and perhaps even on the abyssal plain. Then of course reef structures develop in different areas of the shelves.