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  Chrono- vs Litho-Sequence Stratigraphy

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- So sequence stratigraphy is the study of rock relationswithin chronostratigraphy. In other words, we'relooking at certain times, so the specific time, youhad a certain sea level, you had erosional deposits coming in, and as you change sea level, you will change that system. This is different from thecommon stratigraphic concept of correlations due solely to lithology which would look for thick sands and correlate thick sands, look for thick shales andcorrelate thick shales. In this case, in thesequence stratigraphic case, you correlate along sequences that are based on time. Here we'll go through a schematic that goes through thesea level rise and fall to show you how thedistribution patterns can change as sea level goes up and the sea level goes down. In this cartoon, we have sea level rising, falling, rising, falling significantly,rising significantly, and what we'll be looking at is the distribution of the sediments with marine shales, slope silts and sands, shale silts and sands. We'll see this progress through time, and you'll see that the sediment packages either move outward, you have transgressive sand sequences, regressive sand sequences. We'll also see a bypass, which will move a proximal fan offshore. As we have sea level rise and drop, we have sands transgress. The sea level drops, they come out across the shelf, and so the sand deposit comes further toward the ocean. The marine shales go further out. Once we've reached the low stand, and start heading back to that, so this is the furthest out that the sands and silts come as we start to raise sea level again, the system, the sedimentary system, regresses back onto the shore. What happens in this case is we reach the high stand, and then we start comingback out over the shelf, and we start cutting. You can start cutting canyons, which allows sands tobypass the shelf entirely, and go significantly offshore, and this is one way toidentify possible reservoirs in more deep water areas, which used to be considerednon-conventional, but has been a majorfind in the major gulfs off of North and South America and Africa. You can see this as itprogresses with time going through the system. Once we come back, the sea level starts going up again, moving the marine sediments further in, so you can see the marinesediments coming further in, and the sands are depositedmuch closer to shore. By understanding the sequencestratigraphic concept, you can determine through time where your optimal zones were for reservoir deposition and possibly where marineshales were deposited. The reason you'd want to know where the marine shales were is those will become your ultimate seals. In this case, you've started to put seal shales over the reservoirsystem, just beneath it. By understanding sequence stratigraphy, you can identify majorsequences and packets that can be seen on seismic lines and start identifying whereyou might expect to find reservoir rocks, good reservoir rocks, maybe less permeable reservoir rocks, and, again, better reservoir rocks, and then also shale systems that can act as sealsover your reservoirs. Here's the impact of how interpretation can change how your reservoir may be connected. In this, we use thechronostratigraphic concepts from sequence stratigraphyand Walther's Law, where we're not justconnecting sand to sand, but we're using timelines. The way we find these different sequences is we look for these marine shales, maximum flooding surfaces, and those become our time boundaries because that's a given time. The sea level was in its highest point and flooded on shore, and you can even trackthese through fossils, and so you correlate looking for maximum flooding surfaces, which can just be asmall reflection on your, a small variation on your logs. You connect your reservoir systems, so you have a first sand transgression, then a flooding second sand transgression, flooding third sand transgression, flooding fourth, and you can see that if you were producing this reservoir, you may have fairlycontinuous reservoir systems. Different levels, but there is some connection between them. If you did the conventionallithostratigraphy and just connected thethickest sands together and then find the others, you see here you'd only have three, one major reservoir, and three smaller ones, but in effect, you're assuming that thisreservoir is connected, but if you look, these number four sands truly are not connectedto the number two sands.