Forties Field: Uncertainty in Subsurface Hydrocarbon Volumes

This report focuses on the uncertainty involved in calculating subsurface hydrocarbon volumes, particularly within the context of the Forties Field. It explores various factors influencing these calculations, including porosity, hydrocarbon saturation, and net-to-gross ratio, alongside pressure, temperature, and volume parameters. The study employs both deterministic and probabilistic methods to evaluate hydrocarbon volumes, comparing the results to understand the range of possible outcomes. The methodologies used include numerical integration processes such as the Trapezoidal and Simpson's rules, as well as cell-based model approaches. The report aims to build two deterministic models based on different interpretations of the same dataset to assess uncertainties in volumetric parameters, providing insights into the viability of oil and gas exploration and production projects. The analysis highlights the importance of understanding and quantifying uncertainties in the subsurface to make informed decisions about field development and investment.

Uncertainty in Calculating Subsurface Hydrocarbon
Volumes
Abstract
The volume calculation of geological structures is one of the primary goals of interest when
dealing with exploration or production of oil and gas in general. In construction of a Forties Field
reservoir, various models are based on relationships of various factors. Integration of this factors
and the information is used in understanding of the uncertain ties involved in determining the
amount of hydrocarbons and if the project is viable financially.
This report tries to determine the analogy and correlations associated with getting and testing
data. Some of the factors that need to be determined in understanding and determination of
subsurface hydrocarbon volumes include porosity, hydrocarbon saturation, net gross ratio in
comparison with other concepts in geological models, temperature, pressure and volume
parameters. In addition, this report attempts to get results and then comparing them in terms of
probabilistic and deterministic volume evaluation of the reservoir of Forties Field.
Introduction
In the Forties Field various things were identified and quantified including effect of the
subsurface uncertainties. This uncertainty is volumes of the available hydrocarbons and their
distribution.
In this study methods, results and discussion over the same were done for the aim of field
development of Forties Field. This study intends to build two deterministic models on different
possible interpretations of the same data set. The two models can be used to evaluate some of the
uncertainties in the volumetric parameters.
The figure below shows the Forties Field and its structural setting and position.

Volume Calculation Methods
These included numerical integration processes (Trapezoidal and Simpson’s rule) and cell based
model approach.
The first two approaches are by approximation methods of definite integrals which may be
determined using numerical integration. Even when dealing with advanced methods of
integration there are many mathematical functions which cannot be integrated analytically, thus
approximation has to be used. More precisely, determining the value of a definite integral is in
fact finding the area between the horizontal axis and specified ordinates, i.e. between a curve.
These ordinates in hydrocarbon volume calculation refer to the area bound by isopachs contours
in a reservoir thickness maps.
Trapezoidal rule: trapezoidal rule is a fairly simple mathematical approach. It relates to a definite
integral denoted by ∫baf(x)dx. If f is positive, then the integral represents the area bounded by the
curve y = f (x) and the lines x = a; x = b and y = 0. If the interval of integration was divided
into n equal intervals each of width d, such that d = b−anywhere a = x0 < x1 <· · ·

< xn−1 < xn = b then the trapezoidal approximation was applied by joining the tops of the
ordinates by straight lines, and the outcome is
Simpson rule: the approximation is done by joining the tops of three successive ordinates by a
parabola, i.e. by a polynomial of degree 2 (y = a + bx + cx2). Therefore, a more precise
approximation was obtained with Simpson’s rule than with the trapezoidal rule. If parabola
passes through the points (a, f (a)), (b, f (b)), (c, f (c)), where c = 12(a + b),