The Origin of the Qiulitage Group Dolomite (cambrian to Ordovician) in Northern Tarim Basin

The Qiulitage Group dolomite (Cambrian to Ordovician) is one of the main hydrocarbon producing horizons in Northern Tarim Basin. Detailed petrographic and geochemical studies indicate that the origin of various dolomites of the Qiulitage Group is different. Algal laminated dolomite, micritic dolomite and grains in grained dolomite are products of penecontemporaneous dolomitization. The evidence of penecontemporaneous dolomitization is that they maintain characteristics of primary fabrics (such as algal lamination and concentric lamination of oolite); oxygen isotope composition (δ¹⁸O-5.35 to -6.50‰ PDB) is near the primary oxygen isotope composition of cambrian to Ordovician marine carbonate (δ¹⁸O -4 to -6‰PDB) the content of Na (242.8 to 334.0 ppm) and Sr (197.5 to 206.0ppm) is high and the content of Fe (558 to 563ppm) and Mn (483 to 557ppm) is low. Medium to coarse crystalline dolomites and medium to coarse crystalline dolomite cements in grained dolomite are products of deep-burial diagenetic environment. The evidence of deep-burial origin for medium to coarse crystalline dolomites is that a) medium to coarse crystal of the dolomites; b) abundant xenotopic texture; c) remnans of depositional textures in medium to coarse crystalline dolomites; d) dolomite crystal cuts fructure; e) stylolite destroyed by dolomite crystal or remnants of stylolite in dolomite; f) similarity of trace element values for the medium to coarse dolomites to the dolomites distributed along stylolite; g) oxygen isotope composition (δ¹⁸O -8.50 to - 10.75‰ PDB) is light and carbon isotope composition (δ¹⁸O + 0. 25 to + 2. 27‰PDB) is relative constant; h) the content of Na (55-87ppm) and Sr (0-32ppm) is low and the content of Fe (1663 to 1792ppm) and Mn (1396 to 1632ppm) is high; i) high fluid inclusion homogenized temperature indicats that the crystalline temperature of medium to coarse dolomites is at least 75-80℃.