Page 45 - Geology and Geochemistry of Oil and Gas

22 OIL- AND GAS-BEARING ROCKS

Fig. 2.1. Paleozoic sandstones, AR, USA. Relationship between porosity and density (In: Eremenko and
Chilingar, 1996, p. 17.)

2
1
3
where s p is the specific pore surface area (cm /cm or cm ), f the porosity (frac-
tional), and d the average grain size (cm).
3
The density of sedimentary rocks ranges from 1.7 to 2.6 g/cm . In clastic rocks,
density is inversely proportional to porosity (Fig. 2.1).
Carbonate rocks often form oil and gas reservoirs (Bagrintseva, 1977; Chiling-
arian et al., 1992, 1996). Reef buildups, calcarenites, and oolitic limestones all have
primary porosity. This primary porosity substantially changes during diagenesis
upon recrystallization, dolomitization, and leaching. The latter process is very im-
portant in the karst formation. The formation of karst can begin at the stage of early
diagenesis and then continue during epigenesis (catagenesis), especially within highly
fractured rocks. Unfortunately, vugs in vuggy limestones are often filled by the
second-generation calcite and other newly formed minerals. Dolomitization proc-
esses can increase porosity up to 13.1%, whereas sulfatization and silicification can
significantly decrease porosity. Porosity due to fractures alone is around 1%,
whereas in the presence of vugs and cavities, porosity can reach 3.5%.

2.2.2. Permeability
Permeability is the ease of flow of fluids through rocks. The volumetric rate of
flow is proportional to the pressure gradient:
A Dp
Q ¼ k (2.2)
m L