Oil Field Formation
>>Hydrocarbon Formation, Source rock, and Migration
The vast majority of petroleum geologists and chemists believe that crude oil and gas were formed through biogenesis, but a few believe in – or have at least not completely ruled out the possibility of – abiogenic formation. One abiogenic hypothesis asserts that “iron carbide and water percolating from the surface reacted to form acetylene which grew into longer or more complex hydrocarbons” deep in the earth’s mantle. Another abiogenic hypothesis “purports that gaseous hydrocarbons, mostly methane, existed in the earth’s atmosphere…” and these “hydrocarbons rained down and formed the basis for today’s hydrocarbon deposits – or at least some of them” (Raymond and Leffler 2006, p. 68).
Biogenesis is a much more widely accepted explanation. The biogenic formation hypothesis asserts that hydrocarbons are derived from hydrogen and carbon rich organic material. These organic materials – along with nitrogen – are hypothesized to have been deposited and trapped in sediments millions of years ago. Because organic material is subject to bacterial decomposition, the deposits that eventually transformed into the hydrocarbons produced today must have been deposited in an oxygen deprived environment – one in which decomposers could not survive. Today, the combination of a nutrient-rich surface water environment and an oxygen deprived nutrient fallout zone is rare. Organic-rich sediments are therefore not common. It has been estimated that “less than 1 percent of all sedimentary rocks contain more than 5 percent organic carbon” (Deffeyes 2005).
In order for hydrocarbons to form and nature, deposits of organic-rich sediments must be buried deep within the earth’s crust so that the organic material can be broken down through the application of heat and pressure. With insufficient application of temperature or pressure, the hydrocarbons are considered ‘immature’, meaning that they have not transformed into an energy-dense, and therefore economically valuable, form. In order to reach maturity, the hydrocarbons themselves must metamorphose and this requires both heat and pressure. Some forms of bitumen are immature hydrocarbons. If this is the case, they need to be processed (pressure cooked) in order to reach maturity.
Subsurface heat and pressure is strongly and positively correlated with depth, but depth is not the sole predictor of heat and pressure. Heat is largely influenced by the average surface temperature. In mild climates the average surface temperature may be somewhere around 62ºF, but at the depths of the deepest oceans, the temperature is closer to 32ºF. (Hyne 2001, p. 150) Subterranean heat is also impacted by the presence of or proximity to magma. Similarly, pressure depends on the density of the overlain rocks, and because the density of rocks varies widely, pressures at any given depth cannot be perfectly predicted by depth alone. Anomalies in pressure and temperature are important to understanding the so called ultra-deep ‘pre salt’ hydrocarbons traps that have recently been discovered in Brazilian waters. ‘Pre salt’ formation will be discussed in the section on exploration and in the section on production.
The ‘kitchen’ (or oil window) describes the temperature and pressure ranges under which the organic matter in organic-rich sedimentary rocks is transformed into the wide range of energy dense hydrocarbon molecules discussed above. There is some minor disagreement regarding the upper and lower bounds of the oil window. Deffeyes states that the top of the pressure window is roughly 7,500 feet, and the floor is 15,000 feet (Deffeyes 2005). Hyne states that the upper and lower bounds are 7,000 and 18,000 feet, respectively (Hyne 2001). The temperature in the oil kitchen must be at least 175ºF, but if the temperature exceeds 300ºF for a long enough period of time, crude oil is transformed into graphite and natural gas. Other petroleum geologists likely have slightly different ceilings and floors. Their disagreement indicates that the window should not be taken as an absolute limit, but often it is. As we shall see in a later discussion of the deep offshore ‘pre salt‘ deposits recently found off the coast of Brazil (and likely to be found off the coast of Angola as well), it is folly to mistake a rule of thumb for an absolute limit.
That said, it is commonly believed that should a hydrocarbon rich sedimentary rock be buried more deeply than the bottom of the oil window – that is if a hydrocarbon rich sedimentary rock is subjected to higher pressures and/or temperatures that characterize the floor of the oil kitchen – the hydrocarbon molecules will further metamorphose into methane and graphite. As discussed in a previous section, methane cannot exist as a liquid at normal atmospheric temperatures regardless of the amount of pressure applied. For this reason, methane must either be liquefied at the source through a combination of high pressure and cryogenic liquefaction, or be transferred in a gaseous state through pipelines. The infrastructure requirements of building pipelines to every methane producing reservoir are cost prohibitive. As a result, much wellhead methane is produced as ‘stranded gas’, and is either flared on site, used to power turbines, or reinjected into the reservoir.
The final requisite for the formation of large ‘pools’ of subterranean hydrocarbon formation is time. Hydrocarbons can be dated, and the age of reservoir hydrocarbons ranges from just over a million years (the crude found in some fields in the Gulf of Mexico are roughly 2 million years ago) to more than 600 million years (Hyne 2001, p. 156). Most crude produced today is between 10 and 270 million years old (http://prodigyoilandgas.com/oil-and-gas-science.html). The age of oil is commonly misunderstood, though. It is often asserted that because our youngest oil is over a million years old, it must take that long to form. This is a partially correct statement because it certainly takes that long for the masses of organic rich sediment to be deposited, subducted (or covered), lithified, and cooked. But, it is important to keep sight of the fact that under the right conditions, hydrocarbons of various molecular structures can be produced much quickly under laboratory conditions. We see that this is the case with bio-fuel production.
Ergo, an unfortunate and all-too-common misperception, is that time is not a limiting factor. It is incorrect to assume that we can produce hydrocarbons from biomass as fast as required to meet the demands of economic growth. The problem with this line of reasoning is that the masses of organic material required to produce the quantity of crude that the world consumes every day took millions of years to form. The concentrated solar energy in a barrel of oil was either gathered over a great expanse of landscape or was produced from a small piece of land that absorbed solar energy for thousands of years (if not more). The problem, then, is that we extract *far* more oil – orders of magnitude more oil – during any period of time than is captured and stored through photosynthesis.
Biogenesis, as described here, is bound up with physical earth processes of erosion, transportation, deposition, lithification, and diagenesis. So let us now turn our attention to the formation of organic rich sedimentary rocks.
Sedimentary rocks are formed from the sediments created through erosion. Sediments of small particle size (silt and clay) are carried in suspension in both air and more commonly water. Particles of slightly larger size – sand – can be transported by wind or water through the process of saltation. If you’ve ever sunbathed on the beach on a windy day and watched sand particles bounce along the surface, you’ve seen the process of saltation. The same process occurs in aquatic environments. Particles larger than sand, however cannot be transported by the wind. They can, however, be pushed or rolled slowly along a river bed through the process of creep. Water born sediments are deposited when the stream load surpasses the stream’s carrying capacity (or – as is often the case – when a stream deposits its effluent into a lake, sea, or ocean), or when the wind dies down.
In some cases, the sediment deposits are heterogeneous with sediment size ranging from small to large. In other cases, the particles are well sorted before deposition. Typically, because large particles are most resistant to transportation, they settle first, and the smallest particles of clay settle only under calm conditions. Smaller sediments can also be carried and deposited by the wind. Sand dunes are at once erosional and depositional features.
A final and important category of sedimentary rocks is the carbonate sedimentary rock known as limestone. Limestone – calcium carbonate (CaCO3) – is created from the deposition of calcium-rich shells of sea creatures (such as diatoms).
Once deposited, whether through wind, hydrological, or biological processes, sediments transform into sedimentary rocks through the addition of pressure and time. The twin processes of compaction and chemical cementation is known as lithification. It can be broadly stated that lithification reduces porosity and permeability as pore spaces are compressed and filled with the mineral precipitates which cement the sedimentary particles together. Any chemical, physical, or biological transformations that take place after lithification (exclusive of surface weathering and metamorphism) are known as diagenesis. Like the process of lithification, and with the exception of dolomitization and rare cases of dissolution, diagenesis further reduces both porosity and permeability.
In order of particle size, we see that sand-sized particles lithify into sandstone, silt to siltstone, clay to shale, and shells to limestones. When poorly sorted sediments compact and lithify, conglomerates are formed. Under specific conditions limestone is transformed into dolomite through the aptly named process of dolomitization – the geochemical process where magnesium (Mg) ions from the evaporation of seawater replace calcium (Ca) ions in calcite and form dolomite. The chemistry of dolomite is similar to limestone, with the exception of the addition of magnesium (CaMg(CO3)2).
Most hydrocarbons that are formed in permeable source rocks (sandstones, conglomerates, limestones, and dolomites) migrate because they are under intense pressures. These pressures come from two sources: the weight of the rocks above, and the weight of the water column above. Because hydrocarbons are lighter than the brine (groundwater with more than 100% salt saturation – only possible under intense pressure and at high temperatures) with which they compete for pore space, hydrocarbons tend to rise and supplant the brine.
Unless the migrating hydrocarbons become trapped by an impermeable caprock, they escape to the surface where they are broken down through bacterial and other physical and organic processes. It has been estimated that roughly 90 percent of hydrocarbons find their way to the surface, which means that only 10% of all oil ever created becomes trapped (Deffeyes 2005). In the cases where the migrating hydrocarbons meet an impenetrable caprock, the hydrocarbons will form reservoirs as the hydrocarbons displace groundwater and pool. Here it is important to understand that the caprock need not necessarily be absolutely impenetrable. That said, Deffeyes is quick to point out that that it would only take 100 million years for a one billion barrel oil field to completely deplete if the depletion rate is only 1 drop per second (Deffeyes 2005).
Often layers of halite (soluble salt layers created through precipitation as terrestrial waterbodies evaporate) transform into impenetrable caprock as the soluble salt dissolves in the groundwater leaving impermeable and insoluble anhydrite behind. A key feature of salt is that it is much lower density than most other rocks. Salt layers also behave like very low viscosity liquids. As a consequence of these two properties, salt layers often bubble towards the surface creating salt domes. As the salt domes climb and break through overlying strata, hydrocarbon traps often form on the flanks of the rising dome.
Occasionally, hydrocarbons are trapped in the source rock itself – i.e. they do not migrate. In fact, this is always the case when hydrocarbons are formed in siltstone or shale, but it is only very rarely the case for sandstones, limestones, and dolomites. Because siltstone and shale are impermeable, the hydrocarbons do not migrate, nor do they pool. Instead they are spread evenly throughout the rock matrix. Should hydrocarbon rich siltstones or shale be deformed, the fractures and faults that are formed release the newly exposed surface hydrocarbons. These newly released hydrocarbons tend to migrate relatively quickly because fluid flows through fractures at a much higher rate than the rate at which hydrocarbons flow through even the most porous of unfractured rock matrices.
Despite the vast quantities of solar energy that has been captured and transformed in to chemical energy through the process of photosynthesis over the last 600 million years, only a very small portion of this chemical energy has become trapped in oil reservoirs. As Deffeyes explains, seven ingredients are essential to reservoir creation: 1) a marine environment in which organisms flourish near the surface, but 2) the fallout/accumulation zone is oxygen deprived; 3) the accumulation zone must then undergo lithification, and the rock which is produced through compaction and cementation must retain some level of porosity; 4) the organic rich sedimentary rock must also meet a minimum level of permeability or be highly fractured; 5) the organic rich source rock must be buried to at least 7,000 feet, but never exceed 18,000 feet; 6) the rock must reach a temperature of 175ºF, but not exceed 300ºF; and 7) at least one rock layer between the oil reservoir and the surface must be ‘impermeable’. Deffeyes goes on to state that all seven criteria must be met for an oilfield to be formed. “Nature has a funny way of grading exams…” says Deffeyes, “you answer seven questions; your grade on the overall exam is the lowest grade that you got on any one of the seven,” and if you fail to meet any one of these criteria you will not have an oilfield (Deffeyes 2005, p. 16).
Hence we see that less than 1 percent of all sedimentary rocks contain more than 5 percent organic carbon (Deffeyes 2005), and that in sedimentary rock basins, only 30 to 70 percent of the organic matter in the source rock that has been buried deep enough but not too deep generates gas and oil (Hyne 2001). We also see that “of all the gas and oil that forms in a sedimentary rock basin, only 0.3 to 36 percent is ever trapped, and that on average only 10 percent of the gas and oil is trapped. The rest of the gas and oil either did not get out of the source rock, was lost during migration, or seeped into the earth’s surface” (Hyne 2001, p. 153). We also see that only 2 of the four erosional sedimentary rocks, sandstones and conglomerates, permit hydrocarbons to flow. The other two sedimentary rocks, shale and siltstones are too tight to permit the free flow of hydrocarbons.