2 Key ingredients for petroleum accumulation

2.1 Petroleum charge

There are several ‘ingredients’ or geological conditions that are prerequisites for every subsurface accumulation of petroleum. They are petroleum charge , reservoirs, seals and traps. We will look at each of these in turn in the following sections.

Petroleum charge is an abstract concept concerning the likelihood that petroleum can form, migrate and accumulate in a body of sedimentary rocks. It depends on interactions that involve a number of factors, i.e. it concerns a dynamic system in a sedimentary basin.

An effective petroleum charge system requires:

If all these factors combine, then hydrocarbons start to fill up or charge the pore spaces in a reservoir rock. To understand petroleum charge geoscientists need to consider the nature of source rocks, maturation and its timing, and migration, and we will look at each of these in turn.

2.1.1 Source rocks

Source rocks are sediments that contain sufficient organic matter to generate petroleum when they are buried and heated. Under normal conditions very little dead plant and animal tissue is preserved in sediments. Higher concentrations tend to occur only in environments where there is unusually high productivity of organic matter, such as in coastal upwellings, shallow seas, mires and lakes. Even then, the organic matter reaching the sediment–water interface must be protected from scavengers or aerobic bacteria. If not, these microorganisms use enzymes to digest and oxidise most of the organic matter completely to produce carbon dioxide and water. Under these circumstances there is clearly no potential for the sediments to preserve sufficient hydrocarbons to constitute a source rock.

The preservation of organic matter under reducing conditions is a common factor underlying the formation of both petroleum source rocks and coal. However, most petroleum source rocks form under water, unlike coals that are almost entirely products of organic accumulations at the land surface, albeit in very wet conditions. Note that coal does generate methane and coal deposits have given rise to natural gas resources, for instance those beneath the southern North Sea. So coal can be a petroleum source rock, but usually for gas fields. Petroleum source rocks can form on the beds of freshwater lakes and brackish lagoons, but the most important ones are marine.

There have been prolonged but isolated periods in the geological past when ocean water did not circulate as it does now. Under these conditions oxygen did not reach the sea floor, leading to widespread anoxia there. Decomposition by anaerobic bacteria involves chemical processes of reduction that produce methane and hydrogen, along with hydrogen sulphide, carbon dioxide and water, but leave a residue of organic compounds that are enriched in carbon and have high molecular weight. If enough organic matter has been buried, this leads to a concentration of hydrocarbons within the source rock. Since anoxia is characterised by stagnant water – currents would bring in oxygen – source rocks are products of very low energy deposition.

What kind of sediments are likely to form under such quiet conditions?

They will contain very fine grains, mainly clay minerals, and will form mudstones (an example is shown in Figure 3a).

Anoxic conditions can also occur on a smaller scale where water circulation is restricted. For example, the present-day Oslo Fjord, Norway has an anoxic bottom layer because a shallow lip of rock prevents water from the Skagerrak from circulating around the fjord. The bottom waters of the Black Sea are also strongly reducing because it is essentially a stagnant saline lake.

Another setting that encourages preservation of organic matter is provided by shallow, often land-locked seas in tropical or subtropical latitudes. Evaporation produces a highly saline surface water layer that is denser than the underlying water column and so it sinks to form a salty layer immediately above the sea floor. Organic material derived from plants and animals that thrive in the normally saline water column sinks onto the salty sea bed. Here it remains undisturbed as only rather specialised bacteria survive in this environment. The Dead Sea is a modern example of such a system.

2.1.2 Kerogen

Organic material in buried sediments is called kerogen, a word derived from the Greek for ‘wax producer’. The concentration of kerogen in a potential source rock is usually expressed in terms of the percentage, by weight, of organic carbon in the rock. Rocks with more than 0.5% organic carbon may be effective source rocks, but prolific source rocks have more than 5% and occasionally much higher concentrations of kerogen. The world's first commercial petroleum products to be created on a large scale – in 1850 – were from black ‘oil shales’ that outcrop in the Scottish Midland Valley. These shales, or mudstones, contain more than 15% of kerogen, and when heated in sealed vessels by James ‘Paraffin’ Young they yielded the light liquid hydrocarbons from which he got his nickname.

Conventionally, kerogen is subdivided into four main types on the basis of its chemical composition, which reflects its original source material. Each type has characteristic ratios of carbon, hydrogen, and oxygen and they each generate contrasting petroleum products when they mature. Table 2 highlights the major differences between the four kerogen types in terms of their chemical properties and biological origins. Type I kerogen is comparatively rare as it is derived mainly from algal sources in lake and/or lagoonal environments: the Scottish Midland Valley ‘oil shales’ used by ‘Paraffin’ Young contain kerogen of this kind. Type II kerogen, the most abundant, is typically derived from plant debris, phytoplankton and bacteria in marine sediments; it is the common source of crude oil but also yields some natural gas. Type III kerogen comes mainly from remains of land plants found in coals and it principally generates natural gas. Type IV kerogen includes oxidised plant remains and fragmentary charcoal derived from forest fires; it has virtually no petroleum potential being devoid of hydrogen.

Table 2: Characteristics of the main types of kerogen.

Kerogen type H:C ratio O:C ratio Origin of organic material Petroleum products
Type I 1.7–0.3 0.1–0.02 Algae in lacustrine and/or lagoonal environments Light, high-quality oil and some natural gas
Type II 1.4–0.3 0.2–0.02 Mixture of plant debris and marine microorganisms Main source of crude oil and some natural gas
Type III 1.0–0.3 0.4–0.02 Land plants in coaly sediments Mainly natural gas with very little oil
Type IV 0.45–0.3 0.3–0.02 Oxidised and charred wood No petroleum potential

Box 1 summarises some of the characteristics of the Kimmeridge Clay (150 Ma), which is a mudstone sequence of Upper Jurassic age that is widespread in northern Europe. This world-class source rock is the primary reason why there is a North Sea oil industry.

Box 1: Kerogen in the Kimmeridge Clay formation.

The Kimmeridge Clay Formation is the most important source rock for North Sea oil deposits. It has an average organic carbon content of 5%, rising to 20–30% in the richest ‘oil shales’ that outcrop along the coasts of Yorkshire and Dorset in England (Figure 3a). It has an H:C ratio varying from 0.9 to 1.2.

Bacterially degraded marine algae and degraded humic matter and woody debris of land origin make up about 75% of the total carbon content. Other marine algae, land-plant spores and oxidised land-plant fragments form the remainder. The relative proportion of these constituents varies widely according to the depositional setting of the mudstones. The most organic-rich intervals developed in deeper basins where the highly anoxic bottom waters and high sedimentation rates favoured organic preservation.

Using the information in Table 2, can you suggest what kerogen type characterises the Kimmeridge Clay?

The abundance of marine algae and land-plant debris, coupled with the mid-range H:C ratio, suggests that most of the organic carbon in the Kimmeridge Clay is Type II kerogen.