AbstractsEarth & Environmental Science

The Tabei Uplift, located in the northern Tarim Basin, northwest China, contains the most productive hydrocarbon reservoirs. The late Cambrian strata in the Tabei Uplift present a succession of restricted platform and platform margin carbonates, which are mostly dolomitized. Based on an integrated multidisciplinary approach which incorporated one hundred and fourteen meters of core descriptions from seven wells, one hundred and fifty-two thin-section petrography studies, cathodoliuminescense microscopy examination, trace element analysis, stable and radiogenic isotopic analysis and fluid inclusion analysis, the dolomites are interpreted as: Type 1 dolomite is microcrystal to very fine-crystalline replacement dolomite associated with mudstone, wackestone and packstone with well-preserved the precursor texture of limestone. Its ??18O value ranges from -5.7 to -10.0 (??? vPDB) and ??13C value from 0.0 to -2.0 (??? vPDB). Type 2 dolomite is very fine- to fine-crystalline replacement dolomite. It consists of non-planar to planar-s, anhedral to subhedral dolomite crystals with ??18O values from -5.7 to -10.0 (??? vPDB) and ??13C value from 1.4 to -1.1 (??? vPDB). Type 3 dolomite is fine- to medium-crystalline replacement with subhedral to euhedral dolomite crystals and planar-s to planar-e, and straight compromise intercrystalline boundaries. Under CL, the Type 3 dolomite exhibits none/dull red luminescence in the crystal center, with a bright orange-red thin rim where the crystal terminates into the pore space. Its ??18O values vary from -5.9 to -11.3 (??? vPDB) and ??13C value from 0.4 to -1.4 (??? vPDB). Type 4 dolomite is medium- to coarse-crystalline dolomite cement and consists of planar, subhedral to euhedral crystals with a cloudy center and alternating dull and bright orange-red bands in clear rim under CL. Its ??18O values range from -6.9 to -12.9 (??? vPDB) and ??13C value from -0.3 to -3.5 (??? vPDB). Type 5 saddle dolomite cement is non-planar crystals characterized by curved crystal faces and wavy extinction with a wide range of ??18O values from -6.1 to -13.0 (??? vPDB) and ??13C value from -0.7 to -2.0 (??? vPDB). Type 5 saddle dolomite cement exhibits none to dull blotchy red luminescence under CL, with clear rim showing brighter red luminescent zoning. Petrographic and geochemical evidence suggests that Cambrian dolomite formed in three stages of dolomitization: 1) syn-depositional or penecontemporaneous dolomitization, formed type 1 dolomite by penesaline seawater from slightly evaporative environment; 2) burial dolomitization, produced the crystalline dolomite with different crystal size and shape at different burial depth (Types 2, 3 and 4 dolomite) by high salinity fluids with rising temperature; and 3) hydrothermal dolomitization, produced only minor amount of Type 5 dolomite and associated with igneous and volcanic activity.