Detecting subsurface oil at the surface

New technique can reduce exploration risk by finding surface traces of hydrocarbons that have leaked from a subsurface reservoir

Alan Bailey

Petroleum News

Over the years seismic surveying has become a key technique in discovering subsurface geologic structures that might contain oil or gas, thus identifying targets for exploration drilling. But, although there has been some progress in detecting the existence of subsurface natural gas using seismic data, the data provides little or no help in determining whether oil is present in an exploration prospect. Essentially, the seismic data provides insights into the locations of subsurface petroleum traps, but does not indicate whether a trap has been filled with oil, or whether the trap is effectively sealed, to keep the oil in place.

A new technique involving the detection of trace quantities of hydrocarbons at the surface can reduce exploration risk by adding some insights into whether oil actually exists in a prospect, Andre Brown of W. L. Gore and Associates told the Pacific Section, American Association of Petroleum Geologists, on May 11. Essentially, the technique takes advantage of the seepage upward of oil from an underground oil pool, a seepage that may occur in tiny amounts but that can over time cause the accumulation of detectable amounts of hydrocarbon in the fabric of rocks above the oil reservoir.

Unlike the detection of surface seeps, involving the discovery of light hydrocarbons such as methane gas that are in the process of flowing in sizable quantities from the subsurface, the new technique involves the detection of as many as 90 distinct hydrocarbons that have accumulated at the surface over long periods of time, Brown explained.

A device involving the use of a permeable membrane and special absorbents concentrates the hydrocarbons from surface samples, with a gas chromatography-mass spectrometry detector then used to determine the mix of hydrocarbons in the sample. The precision of the detection technique is such that it is possible to distinguish between compounds that tend to occur naturally in the surface and compounds that have flowed from an underground oil pool. And, by determining the presence of a wide range of compounds, it is possible to detect the compositional “signature” of an oil pool — the precise mix of hydrocarbons that distinguish one pool from another, Brown said.

The data can be mapped, and it is also possible to seek the surface signatures of oils known to exist in a region. By evaluating seismic data in conjunction with the surface hydrocarbon data it becomes possible to refine the way in which the seismic data are processed and interpreted. Sweet spots for drilling can then be identified and the likely margins of subsurface oil accumulations can be evaluated, Brown said.