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  3. Overview of Almond Formation

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- [Germán Merletti] So another view of the Almond Formation. So the Almond Formation is comprised of transgressive and regressive sediments, which are deposited in fluvial, coastal plain, and shallow marine depositional environments. So this is a west/east cross section of the field, and you see how the shoreline went back and forth during the Almond time. So the base of the Almond over here coincides with a maximum shoreface progradation to the east, So you don't get to see the shoreline in the seat, 'cause the system began to transgress to the west giving rise to what is informally known as the Lower Almond. This transgression records a strum plain, the Tike, and also barrier island complexes that backstep to the west. So you see how the shoreline is backstepping to the west. By the time when the shoreline was approximately in the middle of the field over here, a new turnaround happened, and the shoreline started prograting to the west, giving rise to what is informally called ... midland. So similar to the Upper Almond, the eastern part of the field, which is this, this part, exhibit, I'm sorry, the eastern part of the field, this here, exhibit a strong, shallow marine overprint. Whereas the western part of the field displays a really, almost exclusive non-marine deposits. So, a new shoreline turnaround, this time outside of the field, took place, and again, and gave rise to what is, again, an informally called Upper Almond Formation. So this Upper Almond Formation includes the more prolific gas reservoirs in this field. And this Upper Almond Formation is exclusively marine, and is mostly composed of shallow marine bars, and you may have also some deltaic facies, like shown here in red colors. So these are flat added deltas, and you may also include some deltas in the western part of the field. So this is the main depositional facies that you have for the Almond Formation. So, what is the connection between petrophysics, depositional facies, and rock quality? Perhaps the most comprehensive rock study publication was done Rick Tobin and others in AAPG in 2010. So he described, he mentioned that, that generical processing affected this complex mineralogy, It led to, basically, loss of primary porosity from flip processes, which are mechanical, compaction, pressure solution, and cementation. This section in the upper right shows mostly coarse grains, which are cemented by pores over growth, which are indicated by these red arrows. You have also some chlorite replaced grain over here, and also have some ferroan dolomite cement over here. So this is a typical ... fluvial facies in the Lower Almond. You also have secondary pores, which result from feldspar and volcanic rock fabric dissolution. You have a lot of that, and in this picture, this picture is a good example. You have some quartz grains over here. You have some plashoo clays. You have some ageratious rock fragments over here, some volcanic rock fragments. And you see how kaolinite is replacing part of these volcanic rock fragments. You have also chlorite, partially filling secondary porosity over here, on what is indicated as P2. So, micropores are found within the detrital clay matrix, the chert, and also the volcanic grains. Something interesting is that the combination of amount of certain grains ... are also the amount of cement that you have and also the amount of ... the size of the grains really have an impact on the porosity and priority terrain as you will see in the next slides. So this is an example from the Lower Almond facies. Remember that in the Lower Almond we have two type of facies. You have the coastal plain, and you have also shoreface deposits. So, as you see, you can recognize two different trends in porosity and permeability space. So you have a steeper trend for the fluvial coastal plain deposits. And this is caused by different pore structures. So, the point-count petrographic analysis indicates that shoreface facies has more secondary porosity as you can see over here. And more microporosity, as a portion of the total porosity, when compared to normaling facies. And this leads to a more tortuous porosity network and lower permeability, given the same porosity value. So this is how we, I kind of explain this difference in poro-perm space. So this difference is, in terms of pore architecture, arise from differences in primary depositional fabric and also from mineralogy. And you can see over here in this ternary plot how the shoreface has a higher proportion of feldspar, as you can see here, rock fragments, and other types of components. You see that this is, I hope that you can see the cursor, these are the fluvial facies and you see that ... It's more, kind of wipey, thin section. You have much more quartz compared to these facies, the shoreface. So, this is another example, but this time is from the Upper Almond. So, you see that the two trends that I showed you before are these two. So, you have, in this case you don't have any ... In the Upper Almond you don't have any normaling facies. But if you had probably it would plot in this dashed line. So, in the Upper Almond you have only deltaic facies and also shoreface facies. And again, you see a different trend in porosity and permeability space. The amount in this particular case, you see that the amount of cement and secondary porosity is pretty much the same for these two facies. So, but there are two textural parameters that made the difference and perhaps produced these changes in poro-perm space. So one of these textural parameters is the grain size, which is shown in this plot. You see that the shoreface deposits have a largest grain size, and also has a better-- it's not shown here, but it has a better sorting compared to the deltaic facies. It's not also shown here, but also, these shoreface facies have larger primary porosity. So the larger primary porosity and the larger the grain size, both account for higher permeability in this facies, given the same porosity. So, these are the main rock quality drivers for Upper and Lower Almond facies.