Micropore Characteristics and Geological Significance of Pyrite in Shale Rocks of Longmaxi Formation

Key words: 
• framboidal pyrite /
• shale gas reservoir /
• nano-pore /
• quantitative characterization /
• Longmaxi Formation

Abstract: The study on shale gas reservoirs has become much more meticulously and quantitatively along with the rapid improvement of shale gas geological theories. As a common material composition of shale gas reservoirs, the mineralogical and pore development characteristics as well as geological significance of pyrite has drawn a lot of attention, especially the nanoscale pores in framboidal pyrite. By using Argon Ion Polishing-Field Emission Scanning Electron Microscopy (FE-SEM), Energy Dispersive Spectroscopy (EDS), and X-ray Diffraction (XRD) combined with Image Processing technology to study on the mineralogical characteristics of different types of pyrite in reservoir rocks and to make quantitative characterization and evaluation of nanoscale pore characteristics as well as the reservoir significance of pyrite in high quality shale reservoir rocks of middle-lower Longmaxi Formation in Southeast Chongqing area, the results showed that the framboidal pyrite was the most developed type of pyrite in the shale matrix of Longmaxi Formation with aggregate diameter ranging between 3-10 μm. The pores in the framboidal pyrite aggregate were mainly nanoscale pores developed in organic matter filling among the crystallites with diameters mainly concentrated below 100 nm. The pores in framboidal pyrite provided the reservoir with 0.7%-7% proportion of pore network in shale matrix and should be classified into Organic-Matter pores. Characterization and evaluation of classified pores based on Image Processing might provide a new approach to study on reservoir pores, being a feasible method to quantitatively study on different types of pores in different compositions. In addition to the reservoir space contributed by framboidal pyrite to the reservoir, the favorable conditions of organic matter preservation under reducing environments provided a basis for the study of high-quality reservoir formation mechanism and the prediction of high-quality reservoirs. Thus, pyrite, especially framboidal pyrite developed in shale matrix, has important geological significance for shale gas in both reservoir study and the exploration of high-quality reservoirs.