Hydrocarbon Migration

 

 

There are two types of migration when discussing the movement of petroleum, primary and secondary. Primary migration refers to the movement of hydrocarbons from source rock into reservoir rock and it is this type that the following discussion refers to. Secondary migration refers to the subsequent movement of hydrocarbons within reservoir rock; the oil and gas has left the source rock and has entered the reservoir rock. This occurs when petroleum is clearly identifiable as crude oil and gas although the gas may be dissolved in the oil. Buoyancy of the hydrocarbons occurs because of differences in densities of respective fluids and in response to differential pressures in reservoir rock.

 

 

 

The general process is as follows.
With increasing temperature the organic matter produces hydrocarbons.  This increase in temperature is simply a set result of burial of the rock.
As hydrocarbons are generated the gasses and liquids plus the remaining organic matter actually try to occupy a greater volume of the rock than the original organic matter occupied.  Because this happens to all of the organic particles in the rock and there are literally millions of such particles in a source rock, there is a definite increase in pressure within the rock.
This internal pressure increase due to the generation of hydrocarbons imposes an increased pressure on the fluids that occur in the pore spaces, that exist between the mineral grains.  This is called the pore fluid pressure.  A number of calculations have been made that show the amount of volume increase.  For a source rock with a TOC of 1% wt and assuming a 10% porosity, a volume increase in the organic matter due to hydrocarbon generation will be about 5% of the pore space.  Looked at in another way a 1% TOC is a source rock will occupy 2.5% of the volume of the rock and will generate hydrocarbons that occupy an additional  0.25% volume of the rock [Barker, 1980:25].
Clearly if one can cause an increase in the internal pressure within the source rock then a mechanism exists for the expulsion of any gaseous and liquid hydrocarbons that are generated.  One reason this idea is acceptable is it is the simplest [Occam's razor]..  It states the origin and expulsion of hydrocarbons from a source rock is a natural process that operates as the source rock is buried under more and more layers of sediments. Hunt [1977] pointed out that about 10% or less of the bitumenoids in a source rock need to migrate to produce a commercial accumulation in an associated reservoir.  As Barker [1980:25} noted this means the pressuring mechanism only need be about 10% efficient.

 

Movement as a distinct hydrocarbon liquid or gas phase, either with or without water.

Baker [1980:30] noted that at shallow depths [>6,000 feet] the only hydrocarbons present in the source rock are the biological markers. Associated with these are bacterially generated gas [biogenic gas].  Liquid hydrocarbons accumulating in shallow areas will tend to have a high content of aromatic hydrocarbons, presumably because aromatics are most soluble in water and can thus be transported further.  As the thermal generation of hydrocarbons begins [>6,000 feet] heavy oil with high NSO content tends to form.  With increasing depth [10,000 feet] thermal cracking produces a lot of low molecular weight hydrocarbons. In the deepest part of the basin liquid crude i replaced by condensate and ultimately petrogenic gas. Jones [1980:56, fig.5] presents a diagram showing the sharp drop in H/C ratio during thermal maturation of un-extracted organic matter, indicating the "ejection of the kerogen - by the kerogen", of the more hydrocarbon rich generation products. Jones [p:47] concluded that "most of the major commercial oil accumulations of the world left their source rock in a continuous oil phase". We know quite well that liquid hydrocarbons can be generated from organic matter by increasing temperature and the real question to ask is if liquid hydrocarbons did form in a specific rock body, how to they get out of the source rock into the reservoir.
Movement in solution, either with or without water.

 

Very little real-world data on the solubility of hydrocarbons under differing subsurface salinity, pressure and temperature conditions is available. Hydrocarbons do have very low solubility in water and most petroleum engineers reject solution as an important mechanism, except in special circumstances such as shallow gas fields, or elevated temperature at great depth [Tissot and Welte, 1978:296; Hunt, 1979:208-213].  It is definitely true that as peak generation approaches a source rock will contain several times the hydrocarbons required for saturation of the pore water.  As a further argument against the idea McAuliffe [1979] notes that  the molecular composition of oil bears no relationship to variations in hydrocarbon molecular solubility in water.

The solution of hydrocarbon in gas has been suggested but it is unlikely that the heavier hydrocarbons could dissolve in this way. 

 

Mechanisms connected with the origin in the source rock.

Three modes for migration of the organic component are normally suggested.
Movement as individual molecules or colloids dispersed in water.

It is possible that much of the dissolved organic matter in a source rock is not in the form of hydrocarbon but is a pre-cursor such as alcohol or an organic acid or colloidal aggregate.  These substances are more soluble than hydrocarbons [Barker, 1980:21, Tab. II].  A mechanism involving solution of a pre-cursor chemical may be operative in some circumstances for it is undoubtedly true that such materials go into solution.  Both Dickey [1975] and Magara [1980:35] rejected the colloidal aggregate ideas, partly because this would not greatly increase the solubility of the heavier hydrocarbon molecules.
Mechanisms connected with  movement in the source rock.

There are three important considerations when one examines the movement of hydrocarbons in the source rock.
Porosity of the source rock.  Both mineral grains and particulate organic matter can play a role.
Micro-pressuring in the source rock. Both micro-fracturing and aqua-thermal pressuring can be important.
Pore space chemistry of the source rock. Both desorption of water and dynamic structuring of water cab play a role.
Porosity.

 

Both grain size and pore size are related to porosity. In source rocks pore diameter is extremely small 100 angstroms at 3,000 feet and 25 angstroms at 12,000 feet].  This lower size is only slightly larger than bitumoid molecules [Hinch, 1980:7] and the old argument which precludes the possibility of continuous oil flow in fine grained source rocks due to the small pore space and capillary size is clearly not valid: as the deeper fine grain reservoirs of the Gulf of Mexico testify. It is possible that the fine pore space does stop oil leaving a source rock but in oil source rocks, which have a greater amount of organic matter, the combined effects of hydrocarbon generation and compaction cause high differential pressure effects, which more the hydrocarbons our the the source rock. 

 

Mompers [1978] discussed the distribution of particulate organic mater in source rocks.  It is important to note that POM is often located in laminations within a source rock, and the particles are larger in size than the associated clay minerals. The organic layers thus provide localized stingers or larger connecting pores exactly in the part of the source rock where hydrocarbons or their pre-cursors originate.   The heterogeneous distribution of hydrocarbons in source rocks led McAuliffe [1980:99] and Erdman [1965] to suggest hydrocarbons are generated in this organic matter and flow through it to the reservoir.  Under this idea the lower limit of 0.5% TOC in source rock would roughly represent the value below which there would be insufficient organic matter to form a 3-D network.

Micro-pressuring.

Mompers [1978] clearly outlines the characteristics of a source rock which are important in the development of micro-pathways with the rock.  The mechanism also was discussed by Snarski [1970], Dickey [1975], Magara [1978] and Tissot and Welte [1978].  Mompers' basic idea  is that as hydrocarbons are generated the increase in volume causes an increase in pressure.  At some point the pressure increase causes micro-fracturing in the rock, and the hydrocarbons migrate into the micro-fractures which lead out of the source rock.  This concept allows the hydrocarbons to migrate in a liquid phase, but does not preclude the additional or alternative migration in a pre-cursor form.  This is regarded as the main mechanism for primary migration out of the source rock.

 

Droplets, either of hydrocarbons or water, need pore space within which to migrate and if the pore space is less than the diameter of the molecule then the molecule must either deform to pass through the pore space or will block the pore space. As temperature raises with burial, the pore space becomes aqua-thermally pressured.  An example would be a source rock with 10% porosity, heated to 115oC from 85oC [which could correspond to an increased depth of burial from 10,000 feet to 14,000 feet in the Gulf Coast Basin]. Under such a change the increase in volume of the pore water will be 0.15% of the volume of the rock [Barker, 1980:25].  This increase in volume is added to the increase caused by the generation of hydrocarbons.  The result is an increase in pore fluid pressure resulting in micro-pressuring [or deformation and pushing of the oil droplets through the pore spaces].  If the source rocks are at a critical temperature then micro-pressuring could result from a very small increase in depth of burial. Hinch [1980:1] pointed out that it is difficult to explain primary expulsion of hydrocarbons via pore water expulsion during simple compaction and flushing because by the time hydrocarbons are generated in sufficient amounts most of the pore water already has been expelled by compaction and the amount of water remaining is insufficient to flush hydrocarbons from the source rock.

Pore space chemistry.

Certain minerals contain water and with increased burial the temperature and pressure increase can cause the water to be released from the minerals.  This gives an additional source of water for flushing. The clay mineral Smectite [Montmorillonite] undergoes this process. Inter-layered water is released from smectite, the resulting material reacts with potassium in the rock to form the new clay illite. This reaction may be of significance in some cases and is probably continuous from the surface to depth.

 

MECHANICS OF SECONDARY HYDROCARBON MIGRATION AND ENTRAPMENT

If an oil droplet were expelled from a source rock whose boundary was the seafloor, oil would rise through seawater as a continuous-phase droplet because oil is less dense than water and the two fluids are immiscible. The rate of rise would depend on the density difference (buoyancy) between the oil and the water phase. The main driving force then for the upward movement of oil through sea water is buoyancy. Buoyancy is also the main driving force for oil or gas migrating through water-saturated rocks in the subsurface. In the subsurface, where oil must migrate through the pores of rock, there exists a resistant force to the migration of hydrocarbons that was not present in the simple example. The factors that determine the magnitude of this resistant force are (1) the radius of the pore throats of the rock, and (2) the hydrocarbon-water interfacial tension, and (3) wettability. These factors, in combination, are generally called "capillary pressure." Capillary pressure has been defined as the pressure difference between the oil phase and the water phase across a curved oil-water interface (Leverett, 1941). Berg (1975) pointed out that capillary pressure between oil and water in rock pores is responsible for trapping oil and gas in the subsurface. A more thorough discussion of capillary pressure than is presented here is contained in Berg's paper.

To begin our discussion of the mechanics of secondary migration and entrapment and the variables involved, we look at an oil accumulation in a reservoir under static conditions.

 

A second possible process involving the pore space chemistry is a consequence of molecular reactions between the mineral grains and the pore fluids, resulting in the preferential adsorption of water onto the mineral grains, and the dynamic structuring of water close to the mineral grain [Hinch, 1980:11, figs 8-10].  Dynamic structuring takes place because the inter-molecular forces are electro-static.  Water readily reacts to these electro-static forces but the bulk of the hydrocarbons do not.   The water is adsorbed to the grains surface i.e. becomes part of the grain structure].  The water closest to the grain is most highly structured, the water nearer to the center of the space is lease structured.  Because the hydrocarbons tend to be excluded from the structured water area they will collect in the center of the pore spaces. This dynamic structuring can also present a minor expulsion mechanism because the smaller pores are more structured than the larger pores and the hydrocarbons are driven [diffused] to the larger pore spaces and micro-fractures.  Thus in a source rock there may be a built-in directional diffusion pathway prior to hydrocarbon generation.  Witherspoon and Saraf [1964] note the diffusion rate is directly related to the molecular size: gases being more effectively mobilized than liquids.