The genesis of the purplish-red dolostone in the Middle Devonian Guanwushan Formation, Northwest Sichuan Basin

The Devonian Guanwushan Formation in northwest Sichuan Basin is a newly discovered important stratigraphic horizon for exploration of gas. The dolostone of the Middle Devonian Guanwushan Formation in Hejialiang and Gediba areas is with purplish-red or yellow-brown colors. Previously, the research on the genesis and forming environment of the dolostone is relatively weak. Based on studies of mineralogy, geochemistry, lithology and experimental analysis data of field samples, the genesis of the dolostone with purplish-red or yellow-brown colors is detailedly analyzed in this paper. The single layer thickness of dolostone with purplish-red or flesh-red colors is between 3cm and 50cm. It has obvious grain structure under microscopes. The preliminary analysis of its genesis is as follow:1) The purplish-red color showed in the dolostone is a secondary color, which was caused by the oxidation of the ferric element;2) Exposure, karst and collapse were common in this kind of dolostone, meaning it was formed in a limited evaporation environment with relatively high paleogeomorphology;3) Only the cores of the dolomite are with different colors, indicating that they were formed earlier than their cement outsides. It is believed that during the contemporaneous period, carbonate grains were dyed by enriched irons in a relatively confined environment, then, they were wrapped by cement outsides. Dolostone with purplish-red, flesh-red or yellow-brown colors is usually with large thickness, good reservoir, developed karst phenomenon and various reservoir space types. It is one of important indications for oil-gas exploration in the region.