Page 166 - Hydrocarbon Exploration and Production Second Edition

Reservoir Description 153

not realised. Describing a reservoir can be a simple task if it has been laid down as a
thick blanket of sand, but becomes increasingly complex where hydrocarbons are
found in, for example, ancient estuarine or reef deposits. In all cases, however, there
are two main issues which need to be resolved: firstly, how much oil does the
reservoir contain (the HCIIP), and secondly, how much can be recovered (the UR).
There are a number of ways to determine these volumes (which will be explained
in Chapter 7) but the basic physical parameters for describing the reservoir remain
the same:

net reservoir thickness
porosity
hydrocarbon saturation
permeability.

At each stage of a field life cycle raw data have to be converted into information,
but for the information to have value it must influence decision making and
profitability.

6.4.1. Well correlation

Well correlation is used to establish and visualise the lateral extent and the variations
of reservoir parameters. In carrying out a correlation we subdivide the objective
sequence into lithologic units and follow those units or their generic equivalent laterally
through the area of interest. As we have seen earlier, the reservoir parameters such as
N/G, porosity, permeability, etc. are to a large extent controlled by the reservoir
geology, in particular the depositional environment. Thus, by correlation we can
establish lateral and vertical trends of those parameters throughout the structure. This
will enable us to calculate hydrocarbon volumes in different parts of a field, predict
production rates and optimise the location for appraisal and development wells.
Usually well logs are only one type of data used to establish a correlation. Any
meaningful interpretation will need to be supported by palaeontological data (micro
fossils) and palynological data (pollen of plants). The logs most frequently for correlation
are GR, density logs, sonic log, dipmetre and formation imaging tools. On a detailed scale,
these curves should always be calibrated with core data as described below.
On a larger scale, for example in a regional context, seismic stratigraphy will
help to establish a reliable correlation. It is employed in combination with the
concept of sequence stratigraphy. This technique, initially introduced by Exxon
Research, and since then considerably refined, postulates that global (eustatic) sea
level changes create unconformities which can be used to subdivide the stratigraphic
record. These unconformities are modified and affected by more local (relative)
changes in sea level as a result of local tectonic movements, climate and the resulting
impact on sediment supply. The most significant stratigraphic discontinuities used in
a sequence stratigraphic approach are
regressive surfaces of erosion, caused by a lowering of sea level
transgressive surfaces of erosion, caused by an increase in sea level
maximum flooding surfaces at times of ‘highest’ sea level.