Physical and Chemical Properties of Crude Oils and Their Geologic Significances

Crude oils are naturally occurring liquid phase of petroleum, composed principally of hydrocarbon compounds and are extracted from the earth in the liquid state. The hydrocarbon compounds in crude oils include paraffins ranging from pentane to pentadecane (C5 C15), alkylparaffins, naphthenes, alkylbenzenes and nuclear aromatics. Other associated matters are natural gases, hydrocarbon waxes, and salt water. Crude oil can be described as paraffin base, naphthene base or mixed base, depending on the most abundant group of hydrocarbons contained. Crude oils also contain a variety of other chemical constituents comprising of sulphur, oxygen, carbondixode, nitrogen and trace metals. Crude oils are characterized by some physical and chemical properties, which to a measurable extent, play important role in the understanding of the oils geologic nature and environment of origin, such a physical property as optical activity is dependent on hydrocarbon derivatives from organic cholesterine substances, and are destroyed at high temperatures of about 2000C. They serve as important tools for environmental analysis, reconstruction of temperature history of the oil, and correlation of crude oils of similar geologic ages. Cloud and pour points of crudes reveal the influence of low temperatures on crude oils and simultaneously, provide information about the paraffin wax content of the crude. Most of the physical properties of crude oils such as A. P. I. gravity, viscosity and coefficient of expansion, depend on reservoir pressures and temperatures, chemical composition of the oil, and sometimes, on the amount of dissolved natural gases. The chemical properties of crude oils vary in relation to changes in geotemperatures and pressure, coupled with some other elements of katagenesis. The paraffin wax and porphyrins are complex forms of hydrocarbons which have genetic relationship with living organic matters. Occurrence of isoprenoid hydrocarbons such as pristanes and phytanes, in crude oils help in the reconstruction of the genetic environments of deposition of the source rocks. Trace metal substances concentrated in oils are thought to be derivatives from sea water as secretions by marine organisms. The economic value of any crude is influenced by such properties as A. P. I. gravity, paraffin wax content and sulphur content. The aforementioned properties of crude oils apply to the Nigerian crudes. Their A. P. I. gravities change abruptly after a certain depth, where there is a marked change in temperature. Nigerian crude oils are generally light, low sulphur-bearing, and are in great demand by advanced countries.