ABSTRACT

In the fluvio-deltaic, quartz-arenitic sandstones of the Jurassic Khatatba Formation in the Salam field (Egypt's Western Desert), diagenesis and sedimentary facies control reservoir quality. Fluvial channel sandstones have the highest porosities (10-15%) and permeabilities (100-600 md), in part because of siderite cementation, which (1) inhibited compaction and quartz cement and (2) was later dissolved, creating intergranular secondary porosity (1/4 of total porosity). Fluvial crevasse-splay and marine sandstones have the lowest reservoir quality because of an abundance of depositional kaolin matrix and pervasive, shallow-burial Fe-dolomite cement, respectively.

Siderite precipitation was multiphase and separated by distinct dissolution events. The earliest siderite precipitated near surface, within suboxic tropical coastal swamps containing predominantly meteoric waters. Some influence of marine waters is indicated by local enrichments in Mg and Ca. The next major siderite generation shows a trend to decreasing Mn and Ca contents, and is of shallow-burial origin. The last major siderite phase is Mg rich and interpreted as deeper-burial in origin. Some dissolution occurred during shallow burial related to climatically controlled meteoric water fluxing under unconformities. The most important dissolution, however, occurred during deep burial, resulting in (1) a major corrosion surface predating the last Mg-rich zone, (2) selective dissolution of some earlier zones, and (3) secondary porosity. This burial dissolution is interpreted to have been caused by cooling of compactional waters expelled from the basin along major faults.

Other diagenetic phases observed include early-diagenetic pyrite, kaolin, quartz, bitumen, and late-diagenetic barite and illite. Kaolinite precipitated at shallow depths (< 300 m) related to climatically controlled meteoric flushing, which later recrystallized to blocky kaolinite and/or dickite with increasing burial temperatures. Quartz is the predominant cement (6.5-16 vol %) in matrix-free sandstones. Fluid-inclusion data indicate that the earliest quartz precipitated at elevated temperatures (> 130°C) during and/or after rapid Late Cretaceous burial. Quartz developed in two phases, separated by oil migration.

This paper illustrates that, in fluvio-deltaic quartz arenites deposited under the influence of humid tropical climate, reservoir quality can be largely controlled by the contrasting pathways of carbonate diagenesis followed by the different sedimentary facies. This paper also documents a case in which siderite dissolution generated significant secondary porosity in reservoir sandstones, and where both siderite cementation and dissolution took place in multiple phases during different diagenetic stages, including early, shallow-burial, and deep-burial diagenesis. The present study also shows that, in multiphase siderite cements, the earlier growth zones can be selectively dissolved and replaced by later siderite zones (i.e., recrystallized) during burial diagenesis. These findings contrast with the general thought that siderite cements are not susceptible to generation of significant secondary porosity by dissolution and that earlier-formed siderites are essentially stable during diagenesis.