Page 44 - Geology and Geochemistry of Oil and Gas

RESERVOIR ROCKS 21
which can be compared to those obtained by core analysis. Seismic surveys enable one
to determine some rock parameters even without drilling the wells.
Important characteristics of reservoir rocks are their capacity to contain certain
volume of oil and/or gas and their capacity to yield oil and/or gas. The first char-
acteristic is defined by porosity and the second one by permeability.

2.2.1. Porosity

The total volume of void space in the rock, including pores, vugs, and fractures, is
called the total or absolute porosity. Total porosity is the ratio of the total void
volume to the bulk volume. It is expressed as a fraction or percentage.
Some pores are not interconnected. Such isolated pores are not involved in the
fluid flow during the development and production. Besides, isolated pores may be
filled up with water, oil, or gas (irreducible fluid saturation). So, the open porosity is
identified as the ratio between the open pore volume and the bulk volume. Open
porosity is always lower than the absolute (total) one.
Some pores and channels do not permit fluid movement and turn out to be
ineffective due to their small diameter, wettability of the channel walls, and irre-
ducible fluid saturation. Thus, the ratio of the effective pore volume to the bulk
volume is called effective porosity (fraction or percentage). Effective porosity must
be always determined for a specific fluid and for the specific reservoir conditions. It is
equal to open porosity minus the irreducible fluid saturation. It may be determined
by means of petrophysics (logs) or using special field studies.
Normalized porosity (void ratio), i.e., pore volume to rock matrix volume ratio, is
sometimes used, especially in subsidence due to fluid-withdrawal studies. Porosity
depends first of all on the grain size, packing, sorting, roundness, and the mineralogy
and amount of cement. Porosity also depends on the occurrence and preservation of
vugs and fractures formed as a result of secondary alterations, and on two inter-
related phenomena, i.e., arch effect and transverse compression (Poisson’s ratio).
Structure and texture of rocks strongly affect the geometry of the pore space.
Structure of rocks means external features of rock grains (their shape, nature of
surface, etc.). Texture includes the type of interrelations between the grains and their
orientation. In particular, lamination is the most important and commonly used
texture feature. Texture of the sedimentary rocks is born during sedimentation.
Although post-depositional alterations can significantly affect the texture, recog-
nizable features of the initial texture are usually preserved. Dmitriyevskiy (1994)
called the ability of sedimentary rocks to preserve depositional features at depth ‘‘the
sedimentary translation’’. In his publications, he developed main positions of that
theory. There is a strong dependence of sedimentary translation on mineral com-
position, type of cement, and sediment thickness.
A significant influence on the interrelation between the rocks and fluids is the
specific surface area. In clastic rocks, the specific pore surface area is inversely
proportional to the grain size as follows:
s p ¼ 6ð1 fÞ=d (2.1)