Page 108 - Hydrocarbon

CHA P T E R 6

Reservoir Description

Introduction and Commercial Application: The success of oil and gas field development is
largely determined by the reservoir: its size, complexity, productivity and the type
and quantity of fluids it contains. To optimise a development plan, the characteristics
of the reservoir must be well defined. Often the level of information available is
significantly less than that required for an accurate description of the reservoir,
and estimates of the real situation need to be made. It is often difficult for surface
engineers to understand the origin of the uncertainty with which the subsurface
engineer must work, and the ranges of possible outcomes provided by the
subsurface engineer can be frustrating. This section will describe what controls the
uncertainties, and how data are gathered and interpreted to try to form a model of
the subsurface reservoir.
The section is divided into four parts, which discuss the common reservoir types
from a geological viewpoint, the fluids which are contained within the reservoir, the
principal methods of data gathering and the ways in which these data are interpreted.
Each section is introduced by pointing out its commercial relevance.

6.1. Reservoir Geology

Introduction and Commercial Application: The objective of reservoir geology is the
description and quantification of geologically controlled reservoir parameters and
the prediction of their lateral variation. Three parameters broadly define the reservoir
geology of a field:

depositional environment
structure
diagenesis.
To a large extent the reservoir geology controls the producibility of a formation,
that is to what degree transmissibility to fluid flow and pressure communication exists.
Knowledge of the reservoir’s geological processes has to be based on extrapolation of
the very limited data available to the geologist, yet the geological model is the base on
which the FDP will be built.
In the following section, we will examine the relevance of depositional environ-
ments, structures and diagenesis for field development purposes.

6.1.1. Depositional environment

With a few exceptions, reservoir rocks are sediments. The two main categories are
siliciclastic rocks, usually referred to as ‘clastics’ or ‘sandstones’, and carbonate rocks.

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