The factors controlling the abundance and migration of heavy versus light oils, as constrained by data from the Gulf of Suez. Part I. The effect of expelled petroleum composition, PVT properties and petroleum system geometry

Abstract

In the Gulf of Suez (GOS) petroleum systems the main factors which control the abundance and migration of heavy vs light oils are (a) the composition and PVT properties of the expelled fluids, (b) the geometry of the petroleum systems and (c) reservoir flow processes. Compositional and PVT modeling of the expelled petroleum fluids from the Brown Limestone and the Thebes Formation carbonate source rocks show the following. The fluids expelled below a transformation ratio of 0.5 are heavy oils (stock tank API gravity = 10–20°), have a low GOR, and contain 20–25% asphaltenes. These fluids are formed from source rocks which have mainly low to intermediate sulfur content and normal thermal stability. The further expelled fluids, up to a transformation ratio of around 0.97, are bubble point behaving fluids (oils) with stock tank API from >20–50°, and GOR ≈ 300–800 scf/bbl. and display a progressive decrease in asphaltene content. The latest ≈3% expelled fluids represent dew point behaving fluids (gas-condensates). The heavy, low GOR oils expelled below a transformation ratio of 0.5, have a very low saturation pressure and are not prone to asphaltene precipitation, unless they are mixed with higher GOR fluids. The PVT properties of these early expelled oils permit their efficient migration. This is consistent with an overabundance of low maturity asphaltenes in the GOS discovered heavy oils. The tendency for asphaltene precipitation is increased for the fluids expelled above a transformation ratio of 0.5, due to both a decrease in their asphaltene content, and an increase in GOR. Geometrically imposed mixing of fluids with large maturity-induced compositional differences, and reservoir flow processes have a significant effect on the PVT behavior of the migrating fluids, on asphaltene precipitation, and on the distribution of heavy vs light oils.