Page 111 - Hydrocarbon

98 Reservoir Geology

other hand, bioturbation may homogenise a layered reservoir resulting in an
unproducible sandy shale.

6.1.1.2. Carbonate rocks
Carbonate rocks are not normally transported over long distances, and we find
carbonate reservoir rocks mostly at the location of origin, ‘in situ’. They are usually
the product of marine organisms. However, carbonates are often severely affected by
diagenetic processes. A more detailed description of altered carbonates and their
reservoir properties is given below in the description of ‘diagenesis’.

6.1.1.3. Depositional environment
Weathering and transportation is followed by the sedimentation of material. The
depositional environment can be defined as an area with a typical set of physical,
chemical and biological processes which result in a specific type of rock. The
characteristics of the resulting sediment package are dependent on the intensity and
duration of these processes. The physical, chemical, biological and geomorphic
variables show considerable differences between and within particular environments.
As a result, we have to expect very different behaviour of such reservoirs during
hydrocarbon production. Depositional processes control porosity, permeability, net to
gross ratio (N/G), extent and lateral variability of reservoir properties. Hence the
production profile and ultimate recovery (UR) of individual wells and accumulations
are heavily influenced by the environment of deposition.
For example, the many deepwater fields located in the GoM are of Tertiary age
and are comprised of complex sand bodies which were deposited in a deepwater
turbidite sequence. The Prudhoe Bay sandstone reservoir in Alaska is of Triassic/
Cretaceous age and was deposited by a large shallow water fluvial–alluvial fan delta
system. The Saudi Arabian Ghawar limestone reservoir is of Jurassic age and was
deposited in a warm, shallow marine sea. Although these reservoirs were deposited
in very different depositional environments, they all contain producible accumula-
tions of hydrocarbons, though the fraction of recoverable oil varies. In fact, Prudhoe
Bay and Ghawar are amongst the largest in the world, each containing over 20 billion
barrels of oil.
There exists an important relationship between the depositional environment,
reservoir distribution and the production characteristics of a field (Table 6.1).
It is important to realise that knowledge of depositional processes and features in a
given reservoir will be vital for the correct siting of the optimum number of appraisal
and development wells, the sizing of facilities and the definition of a reservoir
management policy.
To derive a reservoir geological model, various methods and techniques are
employed: mainly the analysis of core material, wireline logs, high-resolution seismic
and outcrop studies. These data gathering techniques are discussed in Sections 6.3,
Chapter 6 and 3.2, Chapter 3.
The most valuable tools for a detailed environmental analysis are cores and
wireline logs. In particular, the gamma ray (GR) response is useful since it captures
the changes in energy during deposition. Figure 6.3 links depositional environments