THE SIGNIFICANCE OF SURFACE INDICATIONS OF PETROLEUM

Abstract

Seeps are of interest to the petroleum geologist because:—

they indicate a section capable of producing hydrocarbons, and very often are related to a petroleum accumulation, and

many of the Important oil-producing regions were discovered by surface indications of petroleum.

There are five main types of seeps:- those emerging from homoclinal beds exposed at the surface; those associated with beds in which the oil was formed; those arising from definite large petroleum accumulations, either bared by erosion, or ruptured by faulting; those emerging at an unconformity; and those associated with intrusions. These types of seeps are associated with, and have led to the discovery of many major oil fields throughout the world.

The reports of oil and gas seeps in Australia, however, are only meagre. This may be because:—

of a lack of exploration and documentation,

the basins are a type that do not have the conditions necessary to produce seeps,

the seeps that do exist are unrecognized. For instance, even large gas seeps may pass unnoticed in dry areas,

of a lack of petroleum.

The detection of the gaseous hydrocarbons, methane, ethane, propane and the butanes, in soils by gas chromatography could aid petroleum exploration because:—

it would enable the detection of gas seeps over a potential petroleum field that would otherwise remain undetected, and

even for small quantities of hydrocarbon gases, low ratios of methane to higher hydrocarbons indicate a petroliferous origin.

This is the technique of geochemical prospecting. It is based on three assumptions:—

It must be possible for the hydrocarbons to migrate to the surface.

The concentration of migrating hydrocarbons should not be altered by chemical reaction, bacteria, or hydrocarbons derived from another source.

An anomalous hydrocarbon concentration at the surface can be correlated with a petroleum deposit.

A search of the literature shows that, on the whole, these assumptions are correct. It would therefore appear that geochemical prospecting, particularly when used in conjunction with geological and geophysical work, can be useful for locating petroleum deposits.