3 Exploring for oil and gas
3.1 Detection, exploration and evaluation
It would be prohibitively expensive to explore for oil and gas on a random basis, and most of the effort would be wasted. When geological knowledge was far more limited than it is today, most of the discoveries were beneath quite obvious signs of petroleum seepage at the surface. Eventually, such easy targets ran out, although some are still being discovered. The key ingredients for petroleum accumulation which we have discussed in this unit were gleaned from the knowledge gained by drilling such targets and examining the geology around them. As more has been learned, increasingly sophisticated methods have been developed that increase the odds of making a discovery in less obvious situations. This section covers some of those methods and describes how an exploration well is drilled and evaluated.
3.1.1 Surface detection methods
Field mapping is a well established technique that has contributed to the discovery of billions of barrels of petroleum. In the pioneering era of onshore exploration the search for anticlinal structures at shallow, drillable depths usually began with the recognition of an overlying part of such a structure at the surface. These days, with ready access to various forms of subsurface data, field mapping is more commonly used to assess the structural style of a basin and to provide analogues for concealed reservoirs or source rocks. Far from being an outdated technique, modern fieldwork is becoming increasingly sophisticated as digital data collection is underpinned by Global Positioning Systems (GPS), satellite imagery and digital terrain models. In Norway's Lofoten Islands a detailed analysis of onshore fault and other structural patterns is being extrapolated offshore in order to calibrate three-dimensional (3-D) seismic data in unexplored portions of the Norwegian continental shelf.
3.1.2 Remote sensing methods
Remote sensing involves gathering information of many kinds at a distance from the object of investigation: it gives a regional picture and helps sort the likely areas to follow up from those much larger areas that are less favourable. Satellite, gravity and magnetic methods (see below) are commonly used during the early phase of exploration when a sedimentary basin, or at least a substantial part of it, is not known in sufficient detail to deploy more expensive methods. Their interpretation is simple and can be done relatively cheaply in the office. Satellite images take the form of spectral data over a wide range of wavelengths, from the visible through infrared to microwave (radar). They can sometimes detect unknown petroleum seepages. On land, the presence of a seep is often associated with a change in vegetation or soil colour, especially if the seep is of crude oil, whilst in the offshore setting rising gas bubbles may draw deep water to the surface, giving a cool thermal image. Alternatively, satellites can provide photographic imagery with an extraordinarily good resolution, sufficient to map rock exposures, analyse topography, and to locate roads, habitations and so on.
Gravity surveys are often used to analyse sedimentary basins at the regional scale. Because sedimentary rocks usually have a lower density than crystalline rocks, thick sequences of relatively low-density sediments effectively reduce the Earth's gravitational force and they are characterised by regional gravity lows. Gravity data may be collected on land, at sea or by air and they are particularly useful in areas of difficult terrain, such as jungles and deserts, where access is difficult. Regional airborne magnetic surveys can also be used to define the shape and gross structure of a basin and they are often acquired in tandem with gravity surveys. Magnetic rocks cause perturbations in the Earth's magnetic field, whereas non-magnetic rocks have little effect. Sediments are typically poorly magnetic because they do not contain large amounts of iron-rich minerals, whereas igneous rocks such as volcanic lavas often do. So sedimentary basins characteristically have a low, uniform magnetic signature that contrasts markedly with the highly variable magnetic anomalies associated with metamorphic basement rocks and near-surface volcanic intrusions. Where faults juxtapose rocks with different magnetic properties at depth, the faults show up as distinctive linear features.