Pyrobitumen occurrence and formation in a Cambro-Ordovician sandstone reservoir, Fahud Salt Basin, North Oman

The Cambro-Ordovician Barik Sandstone reservoirs in the Fahud Salt Basin in Oman contain bitumen which may fill up to 40% of the porosity. In well Jal eel-1, this bitumen was isolated (according to kerogen procedure) and typed by NMR, elemental analysis and density measurements. The isolated bitumen is characterized by: (1) a highly aromatic character (NMR 75% C-Aro, H/C at omic ratio: 0.65), (2) a very high sulphur content (4.2%) and (3) a relativ ely high density (1.3-1.4 g/cm(3)). The insolubility and the reflectivity o f the bitumen (1.2% Vr) qualify it as a low mature pyrobitumen. The combina tion of Rock-Eval and density data was used to calculate the actual volume of the pyrobitumen in the rock, as a percentage of porosity. It was found t hat the pyrobitumen volume shows a negative correlation with total porosity , indicating that small pores are more invaded by bitumen than larger ones. Finally, closed system pyrolysis experiments, performed on oils with diffe rent NSO contents, indicate that an in situ oil with a very high content of NSO compounds is required to generate such large amounts of pyrobitumen in the pore system. These observations suggest that the precursor oil of the current pyrobitumen was a very heavy oil tentatively assumed to be the resu lt of a severe biodegradation. Basin modeling shows that the reservoir was charged already in Devonian times. A major uplift brought the oil accumulat ion near the surface during the Carboniferous and a rather regular burial t o the present day position (4500 m, 140 degrees C) (Loosveld et al., 1996). This scenario, involving a residence time at shallow depth, strengthens th e biodegradation hypothesis. The numerical modeling, which involves the IFP kinetic model for secondary oil cracking, suggests that pyrobitumen format ion is a very recent event. Inclusion of pyrobitumen particles within quart z overgrowth, containing fluid inclusions, provides an upper temperature li mit for the beginning of pyrobitumen formation which comforts the result of kinetic modelling. (C) 2000 Elsevier Science B.V. All rights reserved.