ROCK MECHANICAL PROPERTIES VARIATION IN THE SEQUENCE STRATIGRAPHIC CONTEXT OF THE UPPER DEVONIAN NEW ALBANY SHALE, ILLINOIS BASIN

Rock mechanical properties of unconventional shale reservoirs are one key parameter in shale oil/gas exploration and development. Understanding the stratigraphic distribution of rock mechanical properties in black shale successions is critical for identifying potential intervals for horizontal drilling. High-resolution hardness tests (6 cm spacing) using an Equotip Bambino 2 hardness tester and portable x-ray fluorescence (pXRF) analysis (8 cm spacing) were conducted on a New Albany Shale core to study the stratigraphic variation of rock mechanical properties and the control of rock composition on hardness of shales. Element concentrations obtained by pXRF analyzer were calibrated with 44 New Albany Shale samples.

The results show that the average SiO₂ content in the New Albany Shale is 54.51%, out of which 5.51% is of biogenic origin. Biogenic silica occurs as recrystallized radiolarian, chemical precipitation in Tasmanites cysts, and microcrystalline quartz in the matrix. Hardness of shales is not controlled by total silica content, but by biogenic silica content because biogenic silica cement can form a continuous and stiff framework that hardens shales. Calcite cement also increases hardness in the presence of large amounts. Dispersed grains such as detrital quartz, feldspar, and dolomite do not contribute to hardness. The negative correlation between hardness and Al₂O₃ content suggests that clay minerals reduce hardness of shales. When examined under scanning electron microscope (SEM), soft shales are rich in clay minerals compared to biogenic-silica-rich hard shales. The influence of organic matter on hardness is offset by biogenic silica because organic matter and biogenic silica content have similar stratigraphic distribution patterns. Within the sequence stratigraphic context of the New Albany Shale, hardness increases in the transgressive systems tract (TST), reaches maximum at maximum flooding surface (MFS), and decreases in the highstand systems tract (HST). The stratigraphic distribution pattern of hardness is controlled by the distribution of biogenic silica. Therefore, the MFS interval has the highest potential for the development of natural fractures and is the optimal interval for horizontal drilling and hydraulic stimulation.