Characteristics Of Pyrites In Lower Cambrian Niutitang Shales In Upper Yangtze Area And Their Geological Significance

The study of pyrite not only provides a basis for the restoration of sedimentary and diagenetic environment,but also provides a reference for the prediction of organic-rich shales.In this paper,shale samples of Niutitang Formation drilled in three different sedimentary positions(Yichang EYY1 well,Weiyuan W001-4 well and Hanzhong SNY1 well)in the east,north and west of the Upper Yangtze Plate were selected to observe the microscopic morphology of pyrite under microscope and scanning electron microscope.The organic matter,mineral composition and content distribution of pyrite in the samples were analyzed by carbon sulfur analysis,measuring the sulfur isotopic composition of pyrite(?³⁴S_py)and X-ray diffraction analysis.The study found that(1)the morphology of pyrite in the shale of Niutitang Formation in the Upper Yangtze Plate is diverse,and there are mainly four types of pyrite,including framboidal,cubic automorphic,strip-shaped and irregular non-self-formed pyrite kinds of types.Most of the pyrite is distributed in the vicinity of organic matter,accompanied by organic matter,a small number of pyrite is distributed in the vicinity of inorganic minerals or grow in inorganic minerals.Framboidal pyrite is the most developed,and its particle size is generally distributed in the range of 2?6?m,and the particle size distribution is concentrated.The content of framboidal pyrite is positively correlated with the content of organic matter,indicating synsedimentary genesis,while other types of pyrite may be diagenetic.(2)Sulfur isotopic characteristics of pyrite show that Weiyuan W001-4 and Hanzhong SNY1 are both located at the boundary between shallow and deep water shelf.Weiyuan W001-4 is located in relatively shallow water and tends to the shallow water shelf position,Hanzhong SNY1 is located in the deeper water shelf position,and Yichang EYY1 is located in the deepest water.Located in deep water shelf.(3)The morphology and particle size distribution characteristics of pyrite,as well as the relationship between average particle size and standard deviation indicate that the overall water depositional environment of the Niutitang Formation shale in the Upper Yangtze region is dominated by anoxic sulfide environment.The average content of pyrite in well SNY1 is 2.37%,mainly composed of framboidal pyrite,most of which have particle size less than 5?m.Well W001-4 has an abnormally high pyrite content(the average content is 6.6%except that the Ni-Mo deposit has a very high pyrite content),which is dominated by irregular anisotropic pyrite and framboidal pyrite.The average content of pyrite in well EYY1 is 2%,and mainly consists of framboidal pyrite.Well SNY1 showed quiet water and the strongest reducibility.The particle sizes of framboidal pyrite in Wells W001-4 and EYY1 are about 60%less than 5?m,and 5%more than 10?m,which reflect the oxygen-poor and anaerobic environment in the lower part of the sedimentary water.The high abundance of pyrite in Well W001-4 may be affected by hydrothermal action in Mianyang-Changning tensifying trough and fluvial sulfate.(4)The formation of pyrite in shale is not only dependent on water oxidation-reduction conditions,but also controlled by shale lithofacies.The study area is divided into three lithofacies,which are siliceous shale,calcareous shale and transitional shale.Siliceous shale and silicic-calcareous transitional shale are more favorable to the formation of pyrite than calcareous shale.(5)Pyrite has a positive effect on the development and preservation of organic pores.The intercrystalline pore of pyrite can act as a hydrocarbon expulsion channel,which is beneficial to the storage of shale free gas,and can improve the enrichment and storage of natural gas in shale reservoirs.Pyrite is a mineral with high brittleness.High content of pyrite is beneficial to improve the brittleness of Marine shale to some extent.