Page 42 - Geology and Geochemistry of Oil and Gas

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Chapter 2

OIL- AND GAS-BEARING ROCKS

2.1. COMPOSITION OF OIL- AND GAS-BEARING ROCKS

A leading role in the rock–water–organic matter system belongs to rocks. The
rocks containing oil and gas alternate in the Earth’s crust with the rocks that do not
contain hydrocarbons. The former rocks are called petroleum sequences or petro-
leum systems. Such sequences in the sedimentary cover comprise a relatively limited
combination of rocks, but form numerous diverse combinations. Eremenko and
Ulyanov (1960) identified 15 individual lithofacies among the sedimentary rocks:
1. Limestones, dolomites.
2. Limestones and dolomites with clay (shale) interbeds.
3. Limestones and dolomites with sand (sandstone), and clay (shale) interbeds.
4. Clays (shales) with limestone interbeds and lenses.
5. Clays (shales) and sandstones (sands) with limestone (dolomite) interbeds.
6. Clays with limestone (dolomite), sandstone (sand), and marl interbeds.
7. Clays (shales) and marl with sandstone and sand interbeds.
8. Clays (shales) with sandstone and sand interbeds and lenses.
9. Clays (shales) with sand, sandstone, and conglomerate interbeds.
10. Sandstones with conglomerate interbeds.
11. Sandstones and sands.
12. Coaliferous sediments.
13. Salt- and gypsum-bearing sediments (evaporites).
14. Variegated (showing variations of colors or tints) sediments.
15. Flysh facies.
Sometimes, highly compacted (devoid of intergranular, effective porosity, and
permeability) fractured rocks are encountered. Oil can only move through fractures
in such rocks. The production of oil and gas found in the matrix of such rocks is
practically impossible. Thus, an additional group of petroleum sequences is the tight
fractured rocks. Another group is formed by the volcanic and metamorphic rocks,
which often contain oil and/or gas in fractures and, sometimes, in vugs and caverns
formed by weathering and dissolution.
A term ‘‘geologic formation’’ gained common acceptance during the recent years.
This well-forgotten term was introduced by Abraham George Werner as far back as
in 1781. Khain (1973) defined formation as follows: ‘‘ya formation is a natural and
regular combination of rocks (sedimentary, volcanic, intrusive) related by the com-
mon environment of their origin and emerging at certain evolutionary stages of the
major structural zones in the Earth’s crust’’.
Based on this definition, the lithofacies listed above may be considered as for-
mations. Generally speaking, if we treat facies as an environment in which rocks
were formed, it is possible to view formations as the product of such environments.