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  Sequence Stratigraphy Terminology

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- This lesson will cover sequencestratigraphy terminology, and the purpose of thisis to help you understand the different parts ofsequence stratigraphy, how to identify them and use those in developing your geologic models for reservoirs and for seals. So, just a quick overview ofthe sequence stratigraphic images that you can have in your mind for your geologic model. What we'll be talking about are chronostratigraphic sequences. These lines here represent timelines so we're not doingstraight lithostratigraphy, but we're looking at timelines. And the beauty of this is it shows you what is going on in this particular time, what the marine sediments are doing, the marine shales are doing, the shelfal sands are doing andthe near-shore sand systems. It also can allow you to understand how you could get deep-watersand reservoirs deposited that have cut through,the sea level has fallen, you've cut canyons down through the shelf and allowed sedimentsto bypass the shelves and cause deep-water sands to deposit. So, we'll be talking aboutthese different terms. Sequences,relatively conformable. They're bounded by unconformities, and they're correlative through time. Parasequences, these are small packetswithin the sequences. And then eustasy. That's the change of sealevel on a world-wide level. So, in this concept we'retalking about world-wide levels, but remember also thatlocal perturbations, say ice rebound, isostasy caused, say around the Baltic and the North Sea, or plate tectonic faultaction from marine uplifts and shoreline uplifts along thewestern coast of California. Those can be superimposed on the world-wide sea level changes. So we'll talk first about the sequences and the terminologies. Conformable, genetically related strata. So we have a system with asea level at a certain level. You have sands, shalescoming off the continents, these erosional products,and then where those become diluted you havemarine bases offshore. And with time here wehave the transgression of sands out over themarine shales with time. And then we've had aflooding surface develop. Sea level has risen and the next phase will be depositing amaximum flooding surface. So we have unconformities. These are for our sequence boundaries. We have correlative conformities,as I said, the timelines. In the next set is the subset within sequences, or parasequences. So, within one full sequencewe'll have parasequences and these will have,again, these correlative, genetically related beds. So at a certain time wehad offshore marine shales, we had shelfal sands, and we had near-shore sands and deltas. We'll have marine flooding surfaces, and these are importantfor several reasons. And this is something goodfor you to think about because usually when you're exploring you're not looking for shales, but one thing a shale can be is it can be a major seal forsands that are beneath it. So, by understanding wheremaximum flooding surfaces are you can be looking forshales that could be seals. And you can often identifythese on seismic lines if the shales are thick enough. And, if you drill wells offshore you can often find traces of these maximum transgression of sands and the changes withinthe shales themselves. And thenthefinal event is eustasy. And this is the world-widesea level change fluctuations. So we can have high sea level. We can change this throughchanges in the volume of the ocean basin and raisingsea level or lowering it. Or, we can have thefloor rising or lowering, for example, with thecontinental formation of mid-Atlantic ridges for example, and the volume of the ridges rising and bringing up the sea level. So we can have the sealevel going up and down, or the bottom going up and down, but we're looking at world-wide effects.