Page 37 - Hydrocarbon Exploration and Production Second Edition

24 Exploration Methods and Techniques

crucial questions in prospect evaluation is about the timing of events. The
deformation of strata into a suitable trap has to precede the maturation and
migration of petroleum. The reservoir seal must have been intact throughout
geologic time. If a ‘leak’ occurred sometime in the past, the exploration well will
only encounter small amounts of residual hydrocarbons. Conversely, a seal such as a
fault may have developed early on in the field’s history and prevented the migration
of hydrocarbons into the structure.
In some cases bacteria may have biodegraded the oil, that is destroyed the light
fraction. Many shallow accumulations have been altered by this process. An example
would be the large heavy oil accumulations in Venezuela.
Given the costs of exploration ventures it is clear that much effort will be
expended to avoid failure. A variety of disciplines are drawn in such as geology,
geophysics, mathematics and geochemistry to analyse a prospective area. However,
on average, even in very mature areas where exploration has been ongoing for
years, only every third exploration well will encounter substantial amounts of
hydrocarbons. In real ‘wildcat’ areas, basins which have not been drilled previously,
only every tenth well is, on average, successful.

3.2. Exploration Methods and Techniques

The objective of any exploration venture is to find new volumes of
hydrocarbons at a low cost and in a short period of time. Exploration budgets are in
direct competition with acquisition opportunities. If a company spends more
money finding oil than it would do to buy the equivalent amount ‘in the market
place’ there is little incentive to continue exploration. Conversely, a company which
manages to find new reserves at low cost has a significant competitive edge since it
can afford more exploration, find and develop reservoirs more profitably and can
target and develop smaller prospects.
Once an area has been selected for exploration, the usual sequence of technical
activities starts with the definition of a basin. The mapping of gravity anomalies and
magnetic anomalies will be the first two methods applied. In many cases this data will
be available in the public domain or can be bought as a ‘non exclusive’ survey.
Next, a coarse two-dimensional (2D) seismic grid, covering a wide area, will be
acquired in order to define leads, areas which show for instance a structure which
potentially contains an accumulation (seismic methods will be discussed in more
detail in the next section). Recently electro-magnetic techniques have also been
deployed at this stage to assist in the delineation of basins and the identification of
potential hydrocarbon accumulations. A particular exploration concept, often the
idea of an individual or a team will emerge next. Since at this point very few hard
facts are available to judge the merit of these ideas they are often referred to as ‘play’.
More detailed investigations will be integrated to define a ‘prospect’, a subsurface
structure with a reasonable probability of containing all the elements of a petroleum
accumulation, namely source rock, maturation, migration, reservoir rock and
trap.