Solid bitumen in calcite veins from the Natih Formation in the Oman Mountains: Multiple phases of petroleum migration in a changing stress field

Abstract

Solid bitumen in calcite veins in Natih limestone on the southern flank of the Jebel Akhdar Anticline provide evidence for at least two previously unrecognized petroleum migration events in the Oman Mountains. We present field observation of bituminous calcite veins combined with microscopy using highly polished thin sections. This allows imaging of solid bitumen in reflected light and study of its microstructural context in transmitted light.Straight and en échelon, black impregnated, bedding-normal Natih A calcite veins strike 110°. They formed as one of the earliest structures in an extensional stress regime at the end of Cretaceous (Turonian–Santonian) either by flexure of the Arabian plate (Wasia–Aruma break) or by subsidence during subduction. The veins always contain planar arrays of small (<10μm) intracrystal solid bitumen inclusions and may contain angular mosaic type solid bitumen associated with white, solid bitumen-free calcite and/or round, homogeneous type solid bitumen. The solid bitumen formed in multiple events and were most likely derived from the Natih B source rocks.During the first migration event, microfracturing during obduction of the Semail ophiolite and emplacement of the Hawasina thrust sheets in Santonian–Campanian times formed pathways for oil into the veins. Mosaic type solid bitumen formed during further vein growth. It has a higher solid bitumen reflectance (BRr=3.40–3.76%) combined with a high optical anisotropy compared to the underlaying thermally overmature Natih B source rock (BRr=3.10–3.14%). The higher reflectance of mosaic type solid bitumen is interpreted to result from deformation and does not reflect maximum burial temperatures which are inferred to have been about 225°C.The second migration event is indicated by low-reflective homogeneous type solid bitumen with palaeotemperatures (BRr=0.86–0.92%; about 145°C) lower than maximum burial. It occurs along pressure solution seams which cross-cut the veins showing that this is the latest event in the paragenetic sequence, interpreted to have formed after lateral intraformational oil migration from the active Natih oil kitchen located approximately 40km SW of the study area during doming and uplift of the Oman Mountains in Oligocene to Pliocene times.