Mineralogical and morphological characterization of Older Cambay Shale from North Cambay Basin, India: Implication for shale oil/gas development

Effective development of shale oil/gas reservoirs depends on the success of hydraulic fracturing. Mineralogy and morphology play a very important role in establishing brittleness and therefore controlling its hydraulic fracturing characteristics. In this paper, mineralogical and morphological characteristics of the Older Cambay Shale and the top Olpad Formation from the northern Cambay Basin, India have been characterized using well logs, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) coupled with energy dispersive X-ray spectroscopy (EDS or EDX) and atomic force microscopy (AFM) with the objective of identifying the fracturing potential of the units. Presence of mixed argillaceous mudstone, silica-rich argillaceous mudstone to argillaceous siliceous mudstone in the studied shale samples shows that the area under investigation is compositionally similar to the high clay facies of Marcellus and Barnett shale systems having low fracability. The studied formation is rich in kaolinite and chlorite, which aggregately contribute half of mineral composition ranging from 43 to 56%. Quartz, feldspar and mica combined to add approximately 30-43%, while siderite and magnesite varied between 8 and 12%. The interpreted results of mineralogical brittleness are supported by log based geomechanical properties. FESEM images at micro to nanoscale confirm the presence of various minerals with distinct crystal structure, along with inter-and-intra particle pores in the matrix. AFM scan at the nano scale shows mesopores ranging from 12 to 19 nm, with an average pore diameter of 17.57 nm and areal coverage of 10.9%. The log derived mechanical properties correlate with the mineralogy-based brittleness index (BI) of the studied shales indicating that the formations lie in the low to medium range of fracability. High clay content and low to moderate BI are likely to complicate the fracturing process of the potential resource rich area. Outcome of the present investigation demonstrates the limited shale oil/gas potential in the study area; however an extensive exploration program may unlock some sweet spots in the basin with distinctly different characteristics.