NanoSIMS Sulfur Isotope Studies Of Pyrite From An Early Paleozoic Marine Shale:Implications For The Sedimentary Environment

The presence of pyrite has been reported previously in the Longmaxi and Niutitang formations in southern China,and it can be an important indicator for the sedimentary environment.In order to systematically investigate the differences between the paleoenvironments of samples from the two formations,complementary experiments were conducted on six samples from the well Wei#201(three from each of the Longmaxi and Niutitang formations).The methods included Nano-scale secondary ion mass spectrometry(NanoSIMS)for in-situ sulfur isotope analysis,isotope ratio mass spectrometry(IRMS)for comparison with data from NanoSIMS,field emission scanning electron microscope(FE-SEM)for the observation of pyrite forms,inductively coupled plasma mass spectrometry(ICP-MS)for the analysis of trace elements,LECO carbon/sulfur analyzer for total organic carbon(TOC)contents,and X-ray diffraction(XRD)for the identification of the mineral compositions.Results suggest that NanoSIMS can analyze submicron pyrite grains visually,which could help to differentiate euhedra and framboids during the analyses.The sulfur isotope ratios inside the framboidal pyrite tested by NanoSIMS suggest that the formation of the framboidal pyrite is characterized by growth from the center to thesurface.The size distribution of the framboidal pyrite and the redox-sensitive trace elements of the bulk rock jointly indicate that the bottom water redox conditions of the Niutitang Formation were more reducing than those of the Longmaxi Formation.By comparison with sulfur isotope ratios of temporary seawater,the sulfur isotopic characteristics of the framboidal pyrite imply that the two formations were deposited in an open marine system,but the Niutitang environment was progressively closing while the Longmaxi environment was gradually opening.Both of Longmaxi and Niutitang formations are the favorable shale gas reservoirs in southern China.Understanding the distinction between the sedimentary environments of the two formations could contribute to an assessment of these formations as shale gas reservoirs.