Picture
source: Permadi,A.K.2004.Diktat Teknik Reservoir 1.Bandung

This picture
shows the

**isopach map**, a map which is used in original hydrocarbon in place calculation, namely**volumetric estimation**.**Original hydrocarbon in place**is the hydrocarbon which is stored in reservoir prior to production. By this definition, the hydrocarbon in place will not show how much volume of hydrocarbon we may obtain from the formation. Although the original hydrocarbon in place does not show how much we can gain, There is thing called**Recovery Factor**, a percentage range about how much hydrocarbon we might obtain typically, whose value depend on the drive mechanism of the reservoir and the reservoir fluid type. We will see why isopach map is important here, in volumetric reserve calculation.
The
volumetric method is a method to count the volume of hydrocarbon from

**geological, log, or core data**. This is a good estimation method**before the production**data can be achieved. This method is very sensitive to the**accuracy of geological data**, also often leads to overestimation of reserves, for it does not consider the heterogeneity of the reservoir, as the geological data are obtained from samples, which may not represent the average properties of the reservoir. From the data, then a isopach map may be created.
First, it is
very important to have

**logging data**or**seismic data**, from which we can get the**shallowest**and**deepest depth**on the well where the hydrocarbon can be found. These depths are**reservoir top and bottom**. By using**planimeter**, the area of limit can be estimated, and the area difference of them in every depth elevation implies the reservoir area.
Then, we
focus on the isopach map picture above. This method is the simplest way to
estimate the initial hydrocarbon in place. As the result from planimeter has
been obtained, we need to make the map into a form of

**trapezoid pile**. the trapezoid will be created in interval of depth, in this case 5 ft depth. the top 5 ft of the reservoir could simply be assume like a “cone” or “pyramid”. Since the height of every interval is constant, then difference will be about the area of trapezoid above and below the focused trapezoid.
If the Area difference between between the trapezoid we measure and the trapezoid above it is less than 0.5 (this mostly happened in the top intervals), then
the formula would be

**Pyramid bulk volume**;
If the Area difference between the trapezoid we measure and the trapezoid above it is more than 0.5, then the formula would be

**Trapezium bulk volume**.
Let's see from the picture as the example. In case we supposed to know the volume of the lowest trapezoid, then we could see from the picture that the area of trapezoid we measure would be the area of OWC (

**oil****water****contact**). The trapezoid above it will be the reservoir area 5 ft above the OWC, and next we need to divide this reservoir area with the area of OWC. If the result is less than 0.5, we apply pyramid bulk volume, or if it is more than 0.5, we apply trapezium bulk volume.
The basic of this explanation is for the purpose of volumetric estimation, thus Petroleum Engineer could understand as far as the usage and the basic method. Geologists are the specialists who are focused in either isopach map or the planimeter itself in detail.

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