INTRODUCTION

The theories of the formation of oil reservoirs consider that oil traps (structural or stratigraphic) originally were filled with water of marine origin. The oil and/or gas is believed to have entered the trap, displacing the water to some original reservoir saturation (the connate water saturation). Thus, a petroleum reservoir normally contains both petroleum hydrocarbons and water occupying the same, or adjacent, pores. Quantitative evaluation of the fluids is necessary for reservoir characterization.

The retort distillation is divided into two parts: (1) as the rock is first heated (to approximately 400 F or 204 C), water and all but the heaviest fraction of the oil present in the sample are vaporized; and (2) in the second stage of heating, the temperature is raised to about 1100 F (593 C) and the hydrocarbons remaining in the sample are vaporized, or cracked by the heat and removed as a vapor. Part of this vapor is condensable and part is not. Generally the process of cracking leaves a carbon residue within the core. Therefore, the amount of oil recovered by retort distillation is less than the amount of oil in the core. Thus, the intense heat removes water of crystallization from the clays and other hydrated minerals present in the core. The amount of water obtained is slightly greater than the amount of free water in the pores because of the added water of crystallization. Empirical correction factors (obtained from retorting cores containing known amounts of oil) are used to correct for the retorting errors. The...