Hydrocarbons are chemical compounds composed only of hydrogen and carbon. Petroleum is composed principally but not completely of hydrocarbons. By conventional usage petroleum is any hydrocarbon mixture that can be produced from rock through a drill pipe. The main forms of petroleum are natural gas, which does not condense at standard temperature and pressure; condensate, which is gaseous in the ground but condenses at the surface; and crude oil, which is the liquid part of petroleum [Hunt, 1979, p.28]
Chemically, petroleum is not a simple mixture of hydrocarbons but contains many other chemicals. Over 600 different kinds of hydrocarbons have been identified in petroleum and it is recognized there are thousands more, most of which will never be identified. Similarly, there are numerous chemicals that are not hydrocarbons found in crude oil. These principally contain nitrogen, sulpher and oxygen and are called NSO's. Because of this complexity geochemical reports are an important part of the information used to assess the petroleum potential of an area. Universities, are part of their training of geologists, provide a general background in geochemistry and, at the same time train specialist geochemists. It is very much due to the activity of such geological specialists that we have come to understand the origin and accumulation of petroleum. The modern petroleum geologist must have a broad background in not only basic geology, but also geochemistry, palaeobiology, geomathematics, and geophysics.
The source, reservoir trap concept for the origin and occurrence of petroleum is simple at the general level because we believe we understand the processes involved. However, in detail, there are a lot of alternative mechanism that can operate. Any remaining controversy associated with the story of the origin of petroleum is concerned with the mechanism, not the general process, involved. The process not only accounts for the presence of petroleum but also explains why petroleum accumulations are so rare; and, why today specific technical knowledge is necessary in order to assess the petroleum prospect of an area.
It is interesting to note that when I wrote the paper on South Africa's Oil Prospects I quickly recieved a job offer from AMOCO because their corporate philosophy was beginning to recognize the central importance of the source, reservoir, trap concept and they were looking for explorationists who could instill this view into their own exploration strategy. In the last 40 years the concept has become deeply entrenched as THE explanation for how petroleum forms, from organic matter entombed in sediments, as the sediments are buried and pass through the oil window.
The origin part of the sequence pertains to the fact that as organic matter is pushed deeper into the earth they are heated because of the geothermal gradient. The effect is to produce hydrocarbons by a sweating process. A rock that can produce hydrocarbons is called a potential source rock. A rock that has produced such hydrocarbons is simply called a source rock [Dow, 1978: p1586].
The movement of hydrocarbons out of the source rock is called primary migration and these hydrocarbons need an associated porous reservoir rock into which they may pass. The porous rock acts at the carrier bed for the secondary migration of the hydrocarbons i.e. a reservoir.
Secondary migration moves the hydrocarbons through the reservoir rock until their movement is blocked in some way. When further migration is stopped the hydrocarbons are trapped. If the situation is such that a fairly large volume of hydrocarbons are trapped then the hydrocarbons become concentrated. An oil reservoir that is of commercial value is termed an oil pool. Only when an oil pool is developed by drilling wells does it have a real commercial value.
An understanding of source rocks is most important in Frontier Areas of exploration. In Fairways the presence of suitable source rocks is assumed and an understanding of reservoir characteristic and structural geology is important. At the Prospect level a knowledge of traps are important. Prospect evaluation and development is the ultimate aim of petroleum geology and to me, at least, certainly the most exciting.
From the viewpoint of organic matter [OM] living organisms have three components.
Cell walls: which are formed of chemicals which often resist decay.
Cell contents: which are formed of chemicals which sometimes decay and sometimes resist decay.
Cell extrusions: which are materials passed into the external environment by life processes and which are readily reprocessed by other organism.
The cell walls and cell contents usually remain unchanged and intact until death when breakdown of these components is chiefly effected by the enzyme systems of invading organisms. The invading organisms include bacteria, fungi, and protozoans. In addition, chemical decomposition and physical breakage may remove significant amounts of dead organic debris.
In some cases decay is complete. This is particularly so in environments that have abundant oxygen [oxygenic or aerobic environments] because the bacteria living in such environments [aerobic bacteria] are very efficient degraders of organic matter. Metazoans [lower animals] also are important in oxygenic environments as scavengers of organic material,, and in turning over the sediments re-exposing the organic matter to further decay [Degens and Mopper, 1976]. In other cases the organic matter may resist decay, because it exists under aerobic conditions for only a short time or because it is deposited in an oxygen poor environment [anoxygenic or anaerobic environment]. In the oxygen poor environments the anaerobic bacteria live. These are not as efficient in degrading organic matter as are the aerobic bacteria [Claypool and Kaplan, 1974]. In addition many bacteria themselves may be preserved in an anaerobic environment and this becomes important when it is realized that bacteria are very rich in proteins and lipids, which are chemicals believed to be important in forming petroleum. With respect to all organic matter the rapidity with which it can pass from the oxygen rich to the oxygen poor environment is the most important initial factor in forming a source rock. Clearly if the environment of deposition is oxygen poor to start with then the organic matter has a greater chance of being preserved. Bacterial and fungal populations and metabolic activity increase greatly in the surficial bottom sediments if compared with the overlying water mass or with the sediments at greater depth. However, generally, some material resists decay, even under temporary aerobic condition, survives the intense geochemical activity in the surficial sediments and accumulates in the depositional environment as sedimentary matter [Forsee and McCarty, 1970; Jewell and McCarty 1971; and Kahn and Siddiqui, 1971].
We can recognize the reducing environment [anaerobic or anoxygenic] in modern environments because it is marked by a redox - potential discontinuity point [Ferschel and Riedl, 1970]. We also know something about the bacterial activity in this zone [Oppenheimer, 1960; Jorgenson and Ferschel, 1974; Sieburth, 1979; Demaison and Moore, 1980:1183]; and how different levels of degradation of macerals may be reached within similarly anoxygenic environments [Hart, 1979]. It is now clear that not only the amount of organic matter that is preserved is important but also the kind of organic matter and it's level of degradation. There are various method of determining the amount and the kind of organic matter in a sedimentary rock. These techniques will be discussed later: after considering the general classification of organic matter found in sediments.
Dead organic matter can be divided into types according to its biological origin and the form it is in. However, in practice an operational classification is used for describing organic matter found in sediments. Such a classification divides organic matter into three groups.
ADSORBED ORGANIC MATTER: which is organic matter, measured as organic carbon, that have been adsorbed onto organic and inorganic particles.
DISSOLVED ORGANIC MATTER: which is material in solution and very fine grained particles of organic matter less than 0.4 micro-millimeters in size.
PARTICULATE ORGANIC MATTER: which is particular organic matter also called macerals, 0.4 micro-millimeters in size or greater. This POM [or POC, for particulate organic carbon] can be divided into two subgroups.
Clay sized material which has a diameter less than 4.0 micro-millimeters.
Silt and sand sized material which has a diameter of 4.0 micro-millimeters or more.
Although some of the organic material in aqueous systems may become particulate by aggregation and co-precipitation with organic and/or inorganic substances most of the particulate organic matter is simply fragments of once living organisms. The particles are either derived locally [autochthonous] or are washed or blown into the depositional environment from outside the area [allochthonous]. Using the microscope the POM can be classified intro distinct clasts. These are named according to their origin: zooclasts [animal], phytoclasts [plant], scleratoclasts [fungal], and, into aggregates and co-precipitated materials [receptoclasts].
The organic chemist has other classifications of organic matter in sediments: one of which divides the material into humic and non-humic substances.
Stopes [1935] first used the term 'maceral' to describe the organic particles isolated from coal by the process of maceration [L: macerate is the verb 'to soften']. Although the general term maceral implies isolation of the organic particle by maceration it is only recently that isolated particulate organic matter has been subject to fairly intensive study and the maceral spectra [a graph of the kinds of macerals present] of rocks other than coals has become generally known.
Macerals are divided into five genetic groups according to their biological origin. These genetic groups are sub-divided according to their degree of degradation. The most important degradational sequence is that of the phytoclasts. In the phytoclasts we can detect a continuous sequence of well preserved, poorly preserved infested, degraded and gradually amorphorized material. The end product in an amorphous non-structured phytoclast. This material is important because it may not be good source rock material but can be confused with good source rock material. Associated with the degradation of phytoclasts one can recognize resins [usually as cell content precipitate], iron sulfide [both as a coating and as a total impregnation]; inertite [carbonized material]; and, peletal macerals [flakes of phytoclastic debris formed into balls and tubular masses.
In the protistoclasts amorphorization proceeds in a similar way but the cell content seems to play a more important role and take part in the formation of 'fluffy amorphous' material. This fluffy amorphous material is amorphous non-structured protistoclast. It is generally regarded as good source rock material.
Amorphous infected indeterminate macerals are another group that may resemble good source rock macerals but which may be poor source rock materials. These macerals are recognized by the inclusion of other clasts and occasionally of clay particles. Although some of this kind of material is faecal in origin much of it seems to be formed by precipitation of humin.
The relative abundance of different kinds of organic matter can be estimated by simply counting under the microscope how many of each kind of maceral are present in a rock. However, this is simply a relative measure. The estimate of the abundance of organic matter in a sample is made by simply burning off the organic matter and calculating the weight before and after combustion to provide a wt % measure. The technique is not particularly sophisticated but provides fairly accurate estimates and gives a value called the total organic carbon [TOC]. The TOC value must be interpreted with consideration of the kinds of macerals that are present: using preferable a maceral spectra of the sample.
Most major source rocks have TOC values of 2.5 wt % as the lower limit. Major exceptions are the Tertiary delta system, such as the Mississippi and the Niger, where source rocks have values of about 0.3 to 1.0 wt %, and they occur as thick successions [10,000 feet] below the reservoir. A convention in the petroleum industry is to take 0.5 wt % TOC as the lower limit for any detrital source rock. Mompers [1978] discussed how this value was selected and Jones [1980:54] pointed out the reasonableness of the figure. Jones notes that the amount or organic matter needed for an effective source rock is probably related to the value that is needed to cause primary migration. It is a value dependant upon other variables such as the geological history and the age of the rock.
The various chemical techniques used by geologist tend to isolate and study only specific fractions of the organic matter in sediments. Broadly, there are four basic procedures.
Extraction of gas contained within the pore space of the sediment; or, adsorbed onto the source of minerals.
Extraction of fulvic or humic acids using cold alkaline and acidic solutions. The material remaining is referred to as humin.
Extraction of the bitumoid fraction [Alpern, 1980:340] using warm [> 80 degrees C] organic solvents. The material remaining is referred to a kerogen. [definition of Forsman and Hunt, 1958] or kerabitumen.
Extraction of specific kinds of chemicals [e.g. amino acids] by very specific reagents. The material remaining is referred to as the stable residue.
In all four cases the extracted material is subjected to instrumentation analysis for further chemical resolution. The remaining fractions [humin, kerogen, or stable residue] form the bulk of the organic matter in a source rock. Unfortunately, the chemical characteristics of this material is difficult to asses because isolation of the material prior to further analysis often considerably modifies its physical-chemical properties [Durant and Nicaise, 1980:35].
The amount of extractable bitumenoids is a chemical procedure that has been used to estimate the amount of organic matter in a sediment. Although minimum values are available in the literature this is another figure that seems to depend on other variables. For Tertiary deltaic systems Mompers [1978] states "the amount of extractable bitumen [bitumenoids] needed in a source rock bed, before expulsion begins, is about 825-850 ppm ...or more". Smith and Bayliss [1980:D-9B] give a diagram that relates TOC and total bitumenoid content to source rock quality.
Jones and Edison [1978:1] discussed the advantages and disadvantages of studying POM using both microscope analysis and chemical procedures. They conclude that they strongly complement on another and should be used together in order to minimize mistakes.
The relationship between organic matter and the generation of hydrocarbons is primary effected by the sequential increase in temperature due to sediment burial. The length of time the rock is buried plays a smaller roll than does burial history.
Some of the organic matter is preserved almost intact during diagenesis and catagenesis. These stable substances include resins, some waxes, sporo-pollenin, chitin, some saturated hydrocarbons, pigments and their derivatives, and, some products of metabolic activity such as terpenoids and steroids [Durant, 1980:14]. Such inherited molecules are called biological markers. There are of use in tracing the origins of some source rock materials. The rest of the organic matter gradually forms new organic chemicals [liquid and gaseous molecules] from the initial organic compounds, as the temperature increases. The processes tend to be only partial as left behind are physically and chemically altered compounds. The newly formed organic materials migrate away from their point of origin, initiating primary migration.
A source rock at any stage of the temperature sequence is a very complex heterogeneous and changing chemical assemblage. One characteristic that seems to unite similar source rocks is the original type of organic matter that was preserved in the sediment at the time it was deposited. It is significant that five of the most abundant elements in the Universe play an important role in the make-up of living matter and also of petroleum: H, O, C, N, S. These elements form the basic groups of molecules of life: water, lipids [fats, oils and waxes], carbohydrates [cellulose, chitin, tannin, and lignin], adenosine phosphates, proteins, and nucleic acids. Together with the humic substances formed during degradation they are the foundation of the molecules present in a source rock. This is why the amount and type of hydrocarbons present in a source rock is related to the kinds of macerals found in the initial depositional environment: and particularly to the amount of hydrogen present in the macerals.
A useful method of classifying organic matter from the viewpoint of its source rock potential is a graphical plot of atomic hydrogen to carbon versus oxygen to carbon ratios in what is called a Van Krevelen diagram. It should be noted that it is possible to "predict the chemically H/C ratio within 0.1% approximately 85% of the time", using maceral percentages [Jones and Edison, 1978:11]. The Van Krevelen diagram is useful because on such a diagram it is possible to study the changes or potential changes in a source rock as it is heated. One can plot, for example, the changes in individual macerals, or assemblages of macerals found in source rocks. Geologists describe what are called four maturation pathways, which have been selected to illustrate common changes observed in different maceral materials. Maturation pathways I and II are closely associated and are typical of organic material that is rich in lipids [having a high H/C ratio]. Maturation pathways III and IV are also a distinct group and are typical of organic material that is rich in lignin [having a lower H/C ratio]; or, thermally altered, recycled and / or biodegraded organic matter.
The initial phase of low temperature maturation is called diagenesis. As temperature increases the amount of recoverable bitumenoids increases and within the bituminous material more and more is converted to hydrocarbons. This is the stage of oil genesis and is called catagenesis. It is during catagenesis that a source rock is active in producing hydrocarbons. If heating is continued all of the petroleum is generated and eventually the organic matter becomes carbon [graphite]. This is the stage of metagenesis.
As the temperature increases not only the chemical attributes of the organic matter change but also the physical attributes, such as color, transparency and reflectance. Fortunately, the relationship among some of these physical changes and temperature have been worked out. Thermal alteration indices are available that relate the maceral color, transparency and reflectance to the stages of diagenesis, catagenesis and metagenesis as a relative temperature measure. Using relative temperature measurements geologists were able to define the principle zone of oil genesis or the Oil Window. The important activity is that as a source rock passes through the oil window, with burial, it generates petroleum. A rock which is a potential source rock in one area can become an actual source rock in another area and produce hydrocarbons. The difference depends on the rocks relationship to the oil window. This is why it is so important to find source rocks when one is exploring for petroleum.
Compaction changes due to burial that are pertinent to source rock analysis involve the fact that as sediments are buried they compact and lithify. The compaction process drives out water, re-orientates the mineral and organic grains, and, in some cases, causes the precipitation of cements. As a process compaction is closely linked with primary migration.