ABSTRACT

The Permian Kennedy Group, Carnarvon Basin, Western Australia, is a shallow-marine shelf deposit of sandstone and interlaminated siltstone. The original sand was apparently arkosic but was altered by dissolution of feldspar and by other diagenetic processes into a kaolinitic, sideritic, silicified quartz arenite. After lithification, hydrocarbons moved through the sandstone, leaving microscopic solid bitumen envelopes that had formed around radioactive detrital minerals when irradiation polymerized, crosslinked, and locally solidified the oil. Most radioactive cores are monazite; a few are zircon and xenotime. Some monazite grains contain minute, elongate, randomly oriented perforations that almost certainly represent fission tracks, and are filled with bitumen.

The diagenetic sequence can be divided into three phases. Phase 1, the earliest, is an episode of complex diagenesis. Phase 2 covers the time when pores were filled with hydrocarbons and the solid bitumen envelopes formed but other diagenetic reactions were inhibited. Phase 3 begins with the entry of formation waters.

Phase 1 authigenic minerals comprise anatase, anhydrite, brookite, florencite, thorite, halite, muscovite, pyrite, quartz, possible rutile, siderite, cryptocrystalline silica, sphalerite, and xenotime. Rare thorite (ThSiO₄) locally replaces monazite. Siderite crystals contain nine zones, the main variations being in Fe and Mg, and contain solution cavities that may have formed in Phase 1. Two minerals not commonly reported as authigenic are florencite (Ce, La, Al phosphate), mainly as minute pseudocubic euhedra replacing feldspar and mica, and xenotime (YPO₄) as outgrowths on detrital zircon grains. Phase 2 diagenesis includes formation of the solid bitumen envelopes, and Phase 3 includes authigenesis of quartz, kaolinite, and zoned calcite, and perhaps solution f some florencite.

The solid bitumen envelopes surrounding the radioactive accessory minerals enclose rare minute authigenic halite and anhydrite crystals whose absence elsewhere suggests that the envelopes preserved them from solution during Phase 3. Titanium dioxide, probably from ilmenite or titano-magnetite, or both, partly replaces feldspar and mica.

Some aspects of the diagenesis are particularly significant. (1) Solid bitumen envelopes not only indicate the former presence of hydrocarbons but also enable the complex diagenetic sequence to be divided into three well-defined phases. (2) The envelopes can trap and preserve minerals apparently unable to resist solution outside the envelopes, i.e., minerals that would otherwise have disappeared. (3) The zoned siderite, calcite, and florencite indicate changes in pore-water composition in the first and third diagenetic phases. (4) Ti and some REE were mobile, at least on the scale of a thin section, during early diagenesis.