Page 51 - Geology and Geochemistry of Oil and Gas

28 OIL- AND GAS-BEARING ROCKS

Fig. 2.5. Mesozoic Entrada sandstone, UT, USA: (a) schematic cross-section of the fault zone; (b) his-
togram of the number of deformed layers (deformed bonds) per meter; and (c) calculated average per-
meability over a distance of 1 m (after Antonellini and Aydin, 1994). Deformed layers (deformation bands)
are 1.5 mm thick. Permeability of the wall rock is 0.005 mD; k n is the permeability normal to the fault; k p is
the permeability parallel to the fault; fault location: Delicate Arch Trail; total offset across the fault is
40 m. (Courtesy of the American Association of Petroleum Geologists.)

filling of pores and fractures as a result of re-crystallization and secondary cemen-
tation.
Beside clastic and carbonate rocks, reservoirs may be composed of volcanic rocks,
volcaniclastic rocks, and shales. Oil accumulations in such reservoirs are found in
many countries (Azerbaijan, Turkey, Cuba, USA, Indonesia, etc.)
Reservoir properties of rocks are most commonly encountered in the ultrabasic,
basic, and medium extrusive rocks, and rarely, in rhyolites. Volcaniclastic and mixed
volcanic-sedimentary reservoirs are quite common. Effective porosity in such res-
ervoirs is sometimes associated with intergranular pores and vugs, but, as a rule, with
fracturing. An important role in the formation of reservoirs included in this group