Continuous Mineralogical Formation Evaluation Using Geochemical Spectroscopy Well Logs in Mantle-Crust Transition Zone: Case Study from the Scientific Oman Drilling Project

The understanding of formation properties in the mantle is relatively frontier subject in the geosciences research. To achieve the mantle sampling in the near future, Oman Drilling Project was carried out as a part of the International Continental scientific Drilling Program (ICDP) from 2017 to 2018, and both drilling and full coring operation were conducted at the former mantle-crust boundary (now exposed). Full suite of wireline log data was acquired with good quality in the drilling well and excellent quality core samples were continuously taken in the coring borehole nearby. To obtain the continuous geochemical composition of formation, the advanced pulsed neutron spectroscopy well logging was conducted first ever in scientific drilling project. The closure model, specific to igneous rocks, was applied for the spectroscopy log data processing and the result was quantitatively validated with X-ray fluorescence (XRF) measurements of the core samples with the complex mineralogy of ultramafic igneous rock, such as Oman Ophiolites. Then a mineralogical quantitative interpretation was carried out with the dry weight fraction of formation elements, and the results agreed with the core lithological description. This study confirmed that pulsed neutron spectroscopic measurements were key alternatives for continuous mineralogical quantification of igneous rocks, even without core samples, in the future mantle drilling project.