Types and Origin of Dolostones in Tarim Basin, Northwest China: Petrographic and Geochemical Evidence

Abstract:In the Tarim Basin of northwestern China hydrocarbon deposits have been discovered in parts of the thick strata of Cambrian dolostones. Based on petrographic study, six types of dolostone have been distinguished: Type‐1, pink mud‐bearing silty crystalline dolostone (PMSD); Type‐2, gypsum‐ and salt‐bearing fine crystalline dolostone (GSFD); Type‐3, fine crystalline dolostone with dolomite crystals with cloudy core and clear rim (CCFD); Type‐4, deep gray mud‐bearing silty crystalline dolostone (GMSD); Type‐5, euhedral coarse crystalline dolostone (ECD); and Type‐6, xenotopic coarse crystalline dolostone (XCD). Applying petrographic and geochemical methods, the genesis of the dolostones is studied in this paper. Normally, Type‐1 dolostone shows U‐ and Mo‐depleted characteristics, reflecting a more oxidized formation environment; High δ8O and the purple color are consistent with formation of Sabkha dolostones on a supratidal flat. Types 2, 3, 4 dolostones show strata formation, similar REE patterns and 87Sr/86Sr ratios with contemporaneous limestones, suggesting a penecontemporaneous origin from seawater. Types 5 and 6 dolostones commonly occur as interbedded rocks, indicating secondary genesis after diagenesis. Type‐6 dolostone has the highest order degree (OD) values (average 0.86), the lowest oxygen isotope values and positive Eu anomalies, which are consistent with previously reported hydrothermal dolostones. Differently, Type‐5 shows euhedral texture, higher δ18O value, similar REE characteristic and 87Sr/86Sr ratios in comparison with contemporaneous limestones, suggesting that this type might have been dolomitized by down‐transferring evaporated seawater during shallow burial stage. Dolostone fluid sources, formation environments and crystallizing dynamics are summarized and possible genetic models for the six types are proposed.