Exploration Methods

Visible surface features such as oil seeps, natural gas seeps, pockmarks (underwater craters caused by escaping gas) provide basic evidence of hydrocarbon generation (be it shallow or deep in the Earth). However, most exploration depends on highly sophisticated technology to detect and determine the extent of these deposits.

Areas thought to contain hydrocarbons are initially subjected to a gravity survey or magnetic survey to detect large scale features of the subsurface geology. Features of interest (known as leads) are subjected to more detailed seismic surveys which work on the principle of the time it takes for reflected sound waves to travel through matter (rock) of varying densities and using the process of depth conversion to create a profile of the substructure. Finally, when a prospect has been identified and evaluated and passes the oil company's selection criteria, an exploration well is drilled in an attempt to conclusively determine the presence or absence of oil or gas.

Oil exploration is an expensive, high-risk operation. Offshore and remote area exploration is generally only undertaken by very large corporations or national governments. Typical Shallow shelf oil wells (e.g. North sea) cost $10 - 30 Million, while deep water wells can cost up to $100 Million plus. Hundreds of smaller companies search for onshore hydrocarbon deposits worldwide, with some wells costing as little as $500,000 USD.

Elements of a petroleum prospect

A prospect is a potential trap which geologists believe may contain hydrocarbons. Five elements have to be present for a prospect to work and if any of them fail, neither oil nor gas will be present.

• A source rock. When organic-rich rock such as oil shale or coal is subjected to high pressure and temperature over an extended period of time, hydrocarbons form.

• Migration. The Hydrocarbons are expelled from source rock by three density-related mechanisms: the newly-matured hydrocarbons are less dense than their precursors, which cause overpressure; the hydrocarbons are lighter medium, and so migrate upwards due to buoyancy, and the fluids expand as further burial causes increased heating. Most hydrocarbons migrate to the surface as oil seeps, but some will get trapped.

• Trap. The hydrocarbons are buoyant and have to be trapped within a structural (e.g. anticline, fault block) or stratigraphic trap.

• Seal or cap Rock. The hydrocarbon trap has to be covered by an impermeable rock known as a seal or cap-rock in order to prevent hydrocarbons escaping to the surface.

• Reservoir. The hydrocarbons are contained in a reservoir rock. This is a porous sandstone or limestone. The oil collects in the pores within the rock. The reservoir must also be permeable so that the hydrocarbons will flow to surface during production.

Terms used in petroleum evaluation

• Lead - a structure which may contain hydrocarbons.

• Prospect- a lead which has been fully evaluated and is ready to drill.

• Chance of Success - an estimate of the chance of all the elements (see above) within a prospect working, described as a probability. High risk prospects have a less than 10% chance of working, medium risk prospects 10-20%, low risk prospects over 20%. Typically about 40% of wells recently drilled find commercial hydrocarbons.

• Hydrocarbons in Place – amount of hydrocarbons likely to be contained in the prospect. This is calculated using the volumetric equation - GRV x N/G x Porosity x Sh x FVF:

• GRV - gross rock volume - amount of rock in the trap above the hydrocarbon water contact;

• N/G - net/gross ratio - percentage of the GRV formed by the reservoir rock (range is 0 to 1);

• Porosity- percentage of the net reservoir rock occupied by pores (typically 5 - 35%);

• Sh - hydrocarbon saturation - some of the pore space is filled with water - this must be discounted;

• FVF - formation volume factor - oil shrinks and gas expands when brought to the surface. The FVF converts volumes at reservoir conditions (high pressure and high temperature) to storage and sale conditions.

• Recoverable hydrocarbons – amount of hydrocarbons likely to be recovered during production. This is typically 10-50%) in an oil field and 50-80%) in a gas field.