| Abstract |
Mudrocks have varied composition and texture, and heterogeneous nano to micro-scale pore assemblages, and therefore show distinctly different well log responses. A better understanding of the petrophysical behaviors of mudrocks will help extend the application of mudrocks in the academic and industrial fields. Mudrocks are selected from the typical sedimentary basins in China in this study, and the mudrocks are described and divided according to their composition and texture. Then mudrcoks are calibrated with geophysical well logs to understand the differently petrophysical behaviors of various mudrocks. There contain clay-rich mudrocks, mudrock pebbles, organic-matter-rich mudrock, tuff layer bearing mudrocks, pyrite-bearing mudrock, calcareous mudrocks, gypsiferous mudrock, ultra-deeply buried mudrock and fractured mudrocks, etc. There are overlaps in well log responses of various mudrocks. Clay-rich mudrocks display high gamma ray (GR), high neutron porosity (CNL), low bulk density (DEN), high sonic transit time (AC) but low resistivity (RT). Image logs can recognize the mudrock pebbles or mudstone clasts. Calcareous mudrocks show low GR, high resistivity and high DEN, and Elemental Capture Spectroscopy (ECS) log can help detect carbonate minerals. Gypsiferous mudrocks are recognized as low and fluctuated GR reading (<60 API), high DEN (>2.7 g/cm3) and high resistivity, and image logs display bright spots. The ultra-deeply buried mudrocks display high GR, high DEN (>2.6 g/cm3) and high resistivity (>50 Ω m). Organic-matter-rich mudrocks have the highest GR reaching 600 API, low DEN (<2.2 g/cm3), high CNL, high AC and resistivity reading (>100 Ω m). Tuff layers also have high GR readings, while pyrites are displayed as dark spots on image logs. Open fractures in mudrocks will result in reduction of resistivity and DEN but increasing of AC. A comprehensive understanding of geological properties and petrophysical behaviors of mudrocks will better improve the utility of mudrocks in depositional facies analysis, stratum dip determination, pore pressure estimation, source and cap rock characterization, in situ stress evaluation and pore system evaluation of unconventional hydrocarbon resources. The results have implication for better understanding of geological characteristics and petrophysical behaviors of mudrocks, and will extend the application of mudrocks in geological fields. |
, Tianyu Bai 
