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.