Page 43 - Geology and Geochemistry of Oil and Gas

20 OIL- AND GAS-BEARING ROCKS
The most common lithofacies are: limestones and dolomites (1); and clays (shales)
with sandstone and sand interbeds (2). Less common are the petroleum sequences,
which are formed by such lithofacies as sandstones with conglomerate interbeds (10),
and clays (shales) with limestone interbeds and lenses (4). Most common facies are
marine sedimentary sections, the coaliferous facies, and the variegated deposit facies.
Petroleum sequences composed of carbonate lithofacies (1–3) prevail on the plat-
forms. Sequences comprising sandy–clayey lithofacies with carbonates (4–5), and
sandy (10–11), variegated (14), coaliferous (12) and salt-bearing (13) lithofacies are
about equally represented on platforms and in geosynclines (including foredeeps).
Petroleum sequences composed mostly of clayey (6–9) and flysh (15) lithofacies are
found predominantly or exclusively in geosynclines (including the foredeeps).
It terms of their occurrence, Bakirov (1976) subdivided petroleum sequences into:
(a) regional (i.e., developed within the entire petroleum provinces or their major
regions),
(b) subregional (i.e., developed within individual areas of petroleum provinces or
regions)
(c) zonal (occurring in an oil and gas generation zone), and
(d) local (found within individual oil or gas fields).
With respect to the fluids contained in a petroleum sequence, the rocks may be
subdivided into two classes, reservoirs and seals (caprocks), with numerous tran-
sitional forms. This is a rather tentative classification as one and the same rock can
behave differently toward different fluids. For instance, most clays permeable to
water (although sometimes with great difficulty) are impermeable to oil and gas.
Some water-saturated clays are permeable to oil but impermeable to gas (capillary
pressure at the gas–water contact is higher than at the oil–water contact). Hydro-
philic rocks with small pores prefer water to oil, whereas hydrophobic rocks behave
conversely. Depending on subsurface temperature and pressure, the salts may either
act as seals or reservoirs. Fluid flow through a rock depends not only on the rock
properties but also on fluid properties, which are very sensitive to changes in the
temperature and pressure.

2.2. RESERVOIR ROCKS
Oil and/or gas reservoir rock is understood as not only a rock that can contain
but also yield these fluids during the development and production in a given
temperature–pressure and geochemical (physicochemical) environment. The latter
statement is very important in preventing a researcher from using the various reservoir-
rock properties obtained in the laboratory. Even those determinations obtained at
reservoir conditions should be taken with a grain of salt, if the core was first exposed to
surface conditions. Physical and chemical conditions may cause alterations, such as
irreversible precipitation of salts out of solution (water, oil, gas) or, conversely, dis-
solution of salts, etc. One must keep in mind that most of such processes exhibit
hysteresis. Exposure to air (oxidation) also changes the wettability of rocks (oil-wet
versus water-wet). Logs can provide continuous record of petrophysical properties,