Page 57 - Geology and Geochemistry of Oil and Gas

34 OIL- AND GAS-BEARING ROCKS
degree of swelling and compressibility. The relatively low-temperature pore water is
retained in argillaceous rocks up to a temperature of 100 1C to 150 1C. The temper-
ature of water removal is higher when the concentration of dissolved components is
higher. Pore water is located within pores of argillaceous rocks, and at the surfaces
and along the edges of individual microblocks and microaggregates that comprise
clays. The interlayer water causes swelling in montmorillonites and in degraded illites.
The order in water molecules positioning, relative to the clay–mineral blocks and
aggregates, is rapidly altered with an increase in distance between these blocks and
aggregates. Thus, a very important information for the evaluation of the role water
plays in the formation of sealing properties is the knowledge of the structural status
of the layer in an immediate contact with the particles’ surface, and the role the
cations having different charge density play in the preservation of water molecules’
structure.
Exchange ions play a leading role in the formation of ‘‘water clouds’’ around
microaggregates and microblocks of montmorillonite minerals and an insignificant
role, with kaolinite minerals. The role played by the illite group minerals occupies an
intermediate position.
Carbonates caprocks include micro- and fine-grained, massive and laminated
limestones. Almost all limestones are dolomitized to some extent and are subject to
fracturing. This adversely affects their sealing properties (Bagrintseva, 1977). Car-
bonates with a substantial clay content have laminated texture. As a rule, this results
in a deterioration rather than an improvement of sealing properties due to the
emergence of weakness zones at the contact between different lithologies.
Evaporite seals, which are common, include salt, anhydrite, and sometimes shales.
It is a common (and probably erroneous) belief that such seals are the best and most
reliable. Brittleness of these rocks at the surface conditions contradicts that belief.
Besides, cores recovered in the Dnieper-Donets Basin and North Caspian Basin
display macro- and microscopic fractures, which sometimes cut monolithic salt
crystals. The fractures may be healed by secondary salt, but often contain traces of
oil and sometimes gas bubbles. Sometimes core samples are completely saturated
with oil. Permeability measured at the surface conditions can reach 100–150 mD and
even higher. It was established, however, that these rocks easily become plastic even
at a relatively low hydrostatic or, even, uniaxial pressure (o100 MPa) and the
properties change with temperature. Allen and Allen (1993, pp. 364, 368) considered
plasticity as an important sealing property. In this connection, they believe that salt
has the best sealing properties. They also believe that the reliability of caprock is not
directly related to its thickness.
Thus, properties of evaporites as seals change widely during the catagenesis (and
in time). Similar changes also affect the other types of seals albeit not so obviously.
Inclusions, such as organic matter, silt, clay or carbonate particles degrade sealing
properties of evaporites due to the formation of zones of weakness around such
inclusions (Klubova, 1984). A careful study of numerous logs from Dnieper-Donets
Basin showed the presence of clay interbeds between the top of accumulation and the
evaporite sequence in all cases. It appears that these interbeds in most cases act as
caprock.