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  2. Thin and Massive Beds

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- Here I am showing a cross-section of seismic through the reservoir, and also I am showing an RMS amplitude of the reservoir to show the interpreted depositional model. In the area under study we have two reservoirs, A1 and A2. The A1 reservoir is lying shallow to the A2 reservoir. All the reservoirs have different depositional history. The shallower reservoir was deposited in a northwest-southwest trending channel-levee system. It forms a combination stratigraphic/structural trap. You can see in the seismic cross-section how the different reservoirs on-lap on each other, and we can also see, on the right, a log of the discovery well. It was drilled in 1984. We can see here that the A1 reservoir is shown, shallower to the A2. From the logs, in detail I will show in the next slide how the resolution is so challenging. So much resistivity against the gamma ray are suppressed. We can see that as we go to the next slide. So here in the lower reservoir sands, they were deposited as sand backfilling an erosional deep water turbidite channel. Now in this slide I'm showing core photograph on the left, and the associated log signature on the right. So, on the core photograph, you can see, this is a one-foot interval. I am showing my power-pointer from the top to the bottom. This hole is one-foot interval, and this section here is core photograph in plain light. Not very clearly seen in plain light, we can see everything is very dark. But if you look in the UV light, so on the third tract we shield the photograph in UV radiation, and you see that the oil-saturated zones are highlighted in the UV light photographs. We can clearly see the pay here sitting in this cleanly-laminated sand. When I say cleanly-laminated, I have shown a rectangle box here. This interval is 1.5 inches, so if we want to measure the thickness of this small streak of that thinly-laminated pay, it's in the range of centimeters. So we can say that we have really challenging, thinly-laminated pay here. And also, the porosity and the permeability of these intervals is very good enough that they can contribute to slow. The porosity of these thinly-laminated streaks here is up to 30 to 32%. That's why evaluation of these thinly-laminated sands is very crucial because it contributes to 15 to 20% of the missing net pay, and that is why the wells have been over-performing expectations. This also shows the importance of taking core in the selected intervals. Also it highlights the importance of selecting the core plugs. It is very crucial how we select core plugs in these intervals for our routine core measurements of porosity and saturation. And I will go in detail in later slides to explain what are the issues that we face when we try to sample logs in such thinly-laminated core intervals. The arrow here shows the interval of the core in logs. So here on the log scale, this whole interval here I am pointing is ten feet. So we have the small, one feet interval here. This interval is representation of that core interval, here. In the logs, you can see the gamma ray track, the resistivity, and the RHOB, which is the density log. You can see how very suppressed these logs are. It is really challenging to get any pay out of these values of logs. As you go down, you have massive beds, good sands and massive beds, and you can see how well the resistivity is showing up and doing its job here, and giving a very good resolution. So when we calculate our rock properties, when we calculate porosity and then we go ahead and calculate saturations using Archie, this can never be missed. It is very nicely calculated to show up in our saturation, and what will be missed is zones like these, where it will be really suppressed, and if you look at the zone here, we are not sure whether this ten feet are we missing or this ten feet we are missing, so that's why advanced interpretations like Thomas-Stieber become crucial. In the next slide, I'm showing the thin and massive bed stone core. So here we can see this is the interval A2. This is the sand which lies below the A1 sand. The A1 sand is very thinly-laminated, and the A2 sand is massive sand, which lies at the bottom of the A1 sand. The A2 sand looks nice, thick, homogenous on the logs, you can see from the gamma ray, the resistivity, and the density. It is a very simple, conventional approach to be taken for A2 sand, and we should not have any issues with generating the sulfate summaries for this interval. The core photographs show that the A2 sand is mostly massive bed, but there are some thin beds between the massive beds, which is shown here in the UV light photographs. The lower reservoir sand is more massive, with a few thin beds. That is what I'm showing here in this core photograph. Again, the core photographs are very, very revealing in such places where we don't really see that on the log. We could miss this pay if we don't have core photographs to tell us that we have some hidden pay which is not seen in logs. That's why in this study the core has been very crucial.