Introduction

Rock geology played an important part in the petroleum industry as it characteristic of

porosity and permeability give the ability to trap and contain the hydrocarbon within. Porosity of

the rock is referred to the interconnected space between the pore spaces in the rock which

indicate the ability of the rocks to the hydrocarbon in the subsurface while permeability is the

measure of the ability of fluid to flow through the porous rock (Nimmo, 2004). The porosity of

the porous rock can be categorized into primary and secondary porosity. Primary porosity is the

porosity that is developed by the original sedimentation process during the creation of the rock

while the secondary porosity is developed by a diagenetic process subsequent to the deposition.

Most of the common secondary porosity are the fractures and vugs. Fracture porosity is resulted

from the presence openings produced by breaking or shattering of a rock which affect all the rock

types while vuggy porosity is the process of dissolution of the more soluble portions of rock or

solution enlargement of pores or fractures (Porosity).

Geologic map are a representation of the distribution of the rocks and other geologic

materials of different lithologies and ages of the earth (Lisle, 2004). Geologist measures and

describe the rock lithologies and plots the different formations on a map that shows their

distribution. This mapping help in determining and finding the amount of hydrocarbons contains in

the reservoir beneath the earth. Isopach is an example of the map that is usually used in

determining the reserve volume of hydrocarbon in the reservoir.

Isopach is a type of contour map that connecting one point to another of equal thicknesses.

Generally the isopach maps displays the stratigraphic thickness of a rock unit with a scale ratio base

on the true vertical thickness. The isopach maps sometime used in calculating the volume of the

reserve petroleum in a formation which then help in planning the recovery methods. Thus, the

isopach maps is crucial in the petroleum industry.

Objective

The experiment was conducted with the aim to determine the capacity of the oil reservoir if maps

showing contour line (isopach) for the area of the reservoir is available.

Theory

A planimeter or platometer is a measuring device that is used to determine the area of an arbitrary

two-dimensional shape (Hanche-Olsen, 2003). The planimeter consist of three separate parts which

are tracing arms, pole arm and the pole plate. The measuring wheel that house in the roller housing

that is attached to the tracing arms is rolled along the drawing as the operator trace the contour

(The Planimeter). The movement of the roller is recorded as the measuring wheel moves

perpendicular to its axis. The perpendicular movement of the wheel is ignored as its wheel skids

without rolling. This then indicate that the planimeter only measure the distance travelled by the

roller which means that the area of a shape is proportional to the number of the turns where the

measuring wheel is rotate completing perimeter of the shape (Hanche-Olsen, 2003).

In isopach maps, the areas between the contour lines are measured using the planimeter

which then help in obtaining the gross rock volume or the capacity of the reservoir. In order to

measure the area of a reservoir contour the planimeter is moved along the contour line in a

clockwise direction and the obtained values is the area of the whole space that is within the contour

line.

Thus, the capacity in terms of volumetric value can be determined by plotting a cross

sectional graph of area that is measured by planimeter , a device that help precisely in indicating

the area of contour line in isopach map against the depth of the recorded surface. Then the area

under the graph is calculated which indicate the volume of the reservoir.

V = Ah

Where: V = Volume of reservoir (m3)

A = Area of the contours (m3)

h = Depth (m)

references

Hanche-Olsen, H. (2003). Differential forms and the polar planimeter. 1.

LISLE, R. J. (2004). Geological Structures and Maps. A PRACTICAL GUIDE, 1-115.

Nimmo, J. (2004). Porosity and Pore Size Distribution. Encyclopedia of Soils in the Environment, 1-11.

Porosity. (n.d.). Chapter 2, 1-18.

The Planimeter. (n.d.). 1-17.