Types and formation mechanisms of authigenic quartz in the Late Paleozoic alkaline lacustrine shales, NW China

Abstract

Authigenic quartz controls the storage and mechanical properties of shales, and has attracted a growing attention in marine and continental shales. The silica solubility in alkaline lakes is high due to the high lake water pH (>9) and thus alkaline lacustrine shales generally contain abundant authigenic quartz. However, the occurrences and formation of authigenic quartz in alkaline lacustrine shales and its impact on shales remain poorly studied. To solve these problems, we systematically studied the Late Paleozoic alkaline lacustrine Fengcheng Formation in the Mahu Sag of the Junggar Basin via optical microscope and scanning electron microscope observations and elemental analyses. Four types of authigenic quartz have been identified in the Fengcheng Formation (shales): chert nodules or bands, matrix-dispersed microquartz, mineral-replacive quartz, and veins-filling quartz. The Fengcheng Formation shales contain excess SiO₂ ranging from 11.3 % to 60.4 % based on element calculation and matrix-dispersed microquartz is the dominant type of authigenic quartz, followed by cherts. Cherts and matrix-dispersed microquartz were formed during syndepositional to early diagenetic stage, sourced from pH-driven dissolution of detrital quartz grains and transformation of volcanic materials in highly alkaline water. Both chert and matrix-dispersed microquartz formation can increase the rock strength of shales while matrix-dispersed microquartz can also form massive intercrystalline pores and protect the micropores from compaction. In contrast with marine and non-alkaline lacustrine siliceous shales, alkaline lacustrine shales have a higher abundance and more formation stages of authigenic quartz, and can form high-quality shale reservoirs with high contents of organic matter and authigenic quartz in shallow-water zone.