3. Materials and methods
Polished petrographic thin sections of eighty samples from the Nieniexiongla Formation in Menqu section were investigated by Nikon-LV100POL polarizing microscope. The classification of carbonate rocks we followed is the expanded Dunham (1962) classification proposed by Embry and Klovan (1971). Carbon isotope of bulk carbonate (δ13C) were measured for forty samples at the Carbonate Reservoir Key Laboratory, of the PetroChina Hangzhou Institute of Geology, using a GasBench II system coupled to a Delta V mass spectrometer (Thermo Fisher Scientific, Bremen, Germany). The instrument was calibrated with Chinese national carbonate standard GBW04405 (δ13C= 0.57‰). Values are reported in the conventional delta notation, relative to VPDB. Eight oolitic grainstone thin sections were selected and studied in the CUGB lab (China University of Geosciences, Beijing) to investigate the mineral and chemical compositions. Geochemical analysis was conducted by electron probe microanalysis (EPMA-1720(1210229S)) on polished and conductively carbon coated thin sections. The oxides of Na, Ti, Si, Co, Mg, Cr, P, Ni, Al, Mn, K, Fe, Ca were determined in this examination. Experimental method relied on general principle for electron probe microanalysis method GB/T15074-2008. A ZEISS Supra 55 field emission scanning electron microscopy (SEM) equipped with an Oxford Electrical Refrigeration Energy Dispersive Spectrometer (EDS) was used to examine the microstructure of ooids in both polished petrographic thin sections and fresh rock fragments by Secondary Electron (SE) and Backscattered Electron (BSE) modes. Prior to SEM examination, samples were washed by deionized water in ultrasonic cleaner for three times and dried to avoid contaminating, then coated by Platinum. The accelerating voltage was 15kV or 20kV and the working distance was 5 to 15 mm.