Print this pageDiagenetic Evolution of Cambrian Sandstones, Hassi Bir Rekaiz Area, Algeria
FRANKS, STEPHEN G., AHMED HAMEG, and LONG LIANG
Reservoir quality of Cambrian sandstones in the Hassi Bir Rekaiz area of Algeria is strongly affected by quartz and illite cement. Diagenetic kaolinite (dickite) and, to a lesser extent, siderite and dolomite, have minor effects. Quartz overgrowths are abundant in upper Cambrian quartzose, shallow marine sandstones, whereas fibrous and matted illite cement is much more abundant in the underlying micaceous, feldspathic, fluvial sandstones. Quartz-cemented sandstones have much higher permeability for a given porosity than illite-cemented sandstones.
The diagenetic sequence is, from early to late, kaolinite-illite-quartz-dolomite. Aqueous inclusions trapped at quartz overgrowth boundaries and within quartz overgrowths contain very high salinity water (>220,000 ppm NaCl). The only source for waters of this salinity are the overlying Triassic evaporites. This dearly places quartz cementation as a post-Triassic event Homogenization temperatures of fluid inclusions in quartz (95-115 degrees C) coupled with burial history modeling indicates quartz cementation in Cretaceous/Tertiary time. Rb/Sr dating suggests diagenetic illite formed 190-256 ma (Permo-Jurassic). Early diagenetic kaolinite (delta{18}O=7.1 per mil) precipitated from isotopically light (delta{18}O less than or equal to 14) meteoric waters during the early Paleozoic when Algeria lay at high latitudes. Kaolinite isotopic composition does not appear to have been reset during transformation to dickite. Late (post quartz) dolomite precipitated from waters of +4 to +11 per mil.
Generation of oil from Silurian source rocks in the source kitchen is modeled to have occurred during Cretaceous/Tertiary time and is coeval with quartz cementation. This is compatible with the abundance of petroleum inclusions in quartz cement Illite cementation significantly pre-dates oil migration. Therefore, early entrapment of petroleum may help preserve porosity in deeply-buried, quartzose sandstones, but this scenario cannot be called upon to preserve reservoir quality in the illite-cemented sandstones.