Hydrocarbon generation and migration under a large overthrust: The carbonate platform under the Semail Ophiolite, Jebel Akhdar, Oman

Abstract

The Natih B source rock of the Jebel Akhdar dome in the Oman Mountains hosts at least three different generations of solid bitumen. This study presents detailed analyses of thermal maturity based on solid bitumen reflectance, Rock-Eval pyrolysis, Fourier transform infrared spectroscopy and Raman spectroscopy of matrix and solid bitumen. The Natih B source rock was sampled in different valleys (Wadis) around the Jebel Akhdar dome. Detailed petrography of organic material was conducted by optical and scanning electron microscopy (SEM). The petrographic data was used to calibrate an integrated thermal basin model of the Wadi Nakhr area. Due to ophiolite obduction the Natih B source rock was buried to 7.4 to 8km and a maximum temperature of c. 240°C. Hydrocarbon generation and associated primary migration was burial induced and took place from 78 to 66Ma with solid bitumen precipitation in the source rock. A resulting high-reflective solid bitumen generation reflects temperatures of deepest burial in the order of 225°C to 240°C. Based on an elaborated paragenetic sequence two other solid bitumen generations were linked to two oil migration events taking place during uplift (55–50Ma and 48–45Ma), after deepest burial (around 65Ma).