Existence and geological significance of pyrite in the organic-rich shale of Lower Cambrian Niutitang Formation in Upper Yangtze region

Abstract

The existence of pyrite provides a basis for the restoration of sedimentary and diagenetic environment, also, it can be regarded as a reference for the prediction of organic-rich shale. Shales of the Lower Cambrian Niutitang Formation drilled at three different sedimentary locations (wells Yichang EYY1, Weiyuan W001-4 and Hanzhong SNY1) in the Yangtze region have been taken as samples in this study. The pyrites founded within these samples have been analyzed with XRD, microscope, scanning electron microscope and carbon and sulfur contents. By the comparison of analytical results, it was indicated that there were four types of pyrite in these samples including framboidal, cubic automorphic, strip-shaped and irregular non-self-formed. The framboidal pyrite was mostly developed, and its particle size was relatively smaller and varied in a narrow range, reflecting the synsedimentary origin, generally has a good and positive correlation with the content of organic matter, while other types of pyrite may be diagenetically originated. The morphological differences of pyrite in shale samples from three wells indicated that the sedimentary water of well SNY1 is quiet and has the strongest reductivity. The lower part of the sedimentary water of wells W001-4 and EYY1 experienced a transformation from oxygen-poor to anaerobic, and the high-abundance pyrite in the shales of well W001-4 might be affected by hydrothermal activities in the Mianyang-Changning tension trough. Compared with calcareous shale, siliceous shale and silicic-calcareous transitional shale are more favorable for the formation of pyrite. Pyrite has a positive effect on the development and preservation of organic pores, which is beneficial to the enrichment and storage of natural gas in shale reservoirs.

Keywords

• pyrite
• shale
• niutitang formation
• lower cambrian
• upper yangtz region