Polyphase thermal evolution in the Infra-Cambrian Ara Group (South Oman Salt Basin) as deduced by maturity of solid reservoir bitumen

Abstract

Petrographic and organic geochemical analyses on solid reservoir bitumen from the Late Precambrian to Early Cambrian Ara Group indicate polyphase thermal evolution in the South Oman Salt Basin (SOSB). The Ara Group constitutes a complex petroleum system in the deep subsurface (3–5 km) of interior Oman. Salt diapirs encase isolated, partly highly overpressured carbonate bodies (intra-salt “stringers”) which represent a predominantly self-charging hydrocarbon system. A large number of stringer cores show that reservoir quality is decreased by the presence of solid reservoir bitumen. By using different microscopy techniques, this solid reservoir bitumen has been identified within pores, pore throats and microfractures of the carbonates. Solid bitumen reflectance measurements (BR r) were converted to vitrinite reflectance (VR r), using a new improved calibration. Vitrinite reflectance geothermometry shows that maximum paleo-temperatures have a highly heterogeneous distribution. Paleo-temperatures of 380 °C were obtained from coke-like solid reservoir bitumen, which most likely formed from thermal cracking of oil. In our model, this process is related to hydrothermal fluids, deriving from deeper pre-salt strata of the SOSB, which entered the intra-salt carbonate stringers during times of basement tectonic activity via hydrofractures. This caused the formation of high maturities (“pyrobitumens”) in close vicinity to the fractures and precipitation of “migrabitumens” with increasing distance to the HT source, which is supported by the geochemical composition of the solid reservoir bitumens. The influx of these fluids into the carbonate stringers is considered to represent a major contribution to their strong overpressures.