Introduction to Petroleum Geology

Petroleum geology comprises those geological disciplines which are of greatest significance for the finding and recovery of oil and gas. Since most of the obvious and “easy to find” petroleum already has been discovered it is necessary to use sophisticated methods in the exploration of sedimentary basins. These include advanced geophysical techniques and basin modelling. There is also much more emphasis now on enhanced recovery from the producing fields. Petroleum technology has made great progress and many new tools and modelling programs have been developed, both in exploration and production.

It is however important to understand the geological processes which determine the distribution of different sedimentary rocks and their physical properties. This knowledge is fundamental to being able to successfully apply the methods now available. It is difficult to know where to start when teaching petroleum geology because nearly all the different disciplines build on each other.

This introductory chapter will provide a short and rather simple overview of some aspects of petroleum geology to introduce the subject and the problems. Most of the other chapters will then expand on what is presented here to provide a better background in relevant subjects. Since practically all petroleum occurs in sedimentary rocks, sedimentary geology forms one of the main foundations of petroleum geology. Sedimentological models are used to predict the location of different facies in the sedimentary basins, and from that the likely presence of source rocks with a high content of organic matter, reservoir rocks and cap rocks.

The distribution and geometry of potential sandstones or carbonate reservoirs requires detailed sedimentological models, and sequence stratigraphy has been a useful tool in such reconstructions. The biostratigraphic correlation of strata encountered in exploration wells is achieved by micropalaeontology (including palynology), a field developed very largely by the oil industry.

Due to the small size of the samples obtained during drilling operations one cannot rely on macrofossils; even in core samples the chance of finding good macrofossils is poor. By contrast a few grams of rock from the drill cuttings may contain several hundred microfossils or palynomorphs. These also usually provide better stratigraphic resolution than macrofossils. Reservoir rocks are mostly sandstones and carbonates which are sufficiently porous to hold significant amounts of petroleum.

The composition and properties of other rock types such as shales and salt are also important. The sedimentary environments (sedimentary facies) determine the distribution of reservoir rocks and their primary composition. Sediments do, however, alter their properties with increasing overburden due to diagenesis during burial. Diagenetic processes determine the porosity, permeability and other physical properties such as velocity, in both sandstone and limestone reservoirs. Chemical processes controlling mineral reactions are important.

Tectonics and structural geology provide an understanding of the subsidence, folding and uplift responsible for the creation and dynamic history of a basin. The timing of the folding and faulting that forms structural traps is very important in relation to the migration of hydrocarbons. Seismic methods have become the main tool for mapping sedimentary facies, stratigraphy, sequence stratigraphy and tectonic development. Marine seismics recorded from ships have become very efficient and seismic lines are shot at only a few 100 m spacing or less. Because of the rapid improvement in the quality of seismic data processing techniques, geological interpretation of seismic data has become an entirely new and expanding field.

Seismic and other geophysical data are often the only information we have, particularly for offshore exploration where drilling is very costly. Shooting seismic lines with a close spacing allows high resolution 3D seismic imagery to be produced for critical parts of sedimentary basins. By repeating a 3D reservoir seismic survey during production, one can observe how the gas/oil and oil/water contacts move as the reservoir is depleted. This is called 4D seismic because time provides the fourth dimension.