PETROLEUM SYSTEMS AND ELEMENTS OF PETROLEUM GEOLOGY

Petroleum system is a core concept of petroleum geology-the study of oil and gas formation and exploration with its associated processes such as oil origin, occurrence, migration and their accumulation-which unifies interdependent processes and key elements in the formation of hydrocarbons. The formation of hydrocarbons involves processes such as trap formation, Generation-migration-accumulation of hydrocarbons and key elements which encompass source rock, reservoir rock, and seal or cap rock. Studying petroleum system helps petroleum geophysists and geochemists knowing and understanding the nature of the trap and petroleum region by basing on paleontology, stratigraphy and sedimentology of a geographic region in order to identify petroleum system. A region is said to have a petroleum system if and only if there are a presence of oil or gas seep and oozes on area or in water, with the regardless of having big quantity. According to certainty of the potentiality of evidences, petroleum systems may be known as evident, hypothetical or speculative. But also with regards to the accumulation, immigration and other key characteristics of generated oil and gas or its associated lithological unit(s)  those three types of petroleum systems fall into other two main categories which are vertical drained and horizontal drained petroleum systems. 

Elements of petroleum geology

Source rocks, trap, seal and reservoir rock are the key elements of petroleum systems which are provided by the interpretation of data from reflection seismology and electromagnetic geophysical techniques performed in a particular geographic area. Each of these elements is evaluated in a particular way to determine the potentiality of the system.

Source rock.

The source rock is a subsurface sedimentary rock unit(s) which is made of shale or limestone. It contains the precursors of hydrocarbon formation, organic matters (from decays of ancient biological species) which were subjected to high temperature for longtime. The source rock host the processes that involve in the formation oil and gas until they start to immigrate toward the upper or nearer rock(s) named reservoir due to the fluidity of oil and gas. The source rock is evaluated using the geochemistry methods. (Beaumont & Magoon, n.a) 

Reservoir rock.

This element is a kind of porous or permeable lithological unit(s) which retains the immigrating oil and gas from source rock. Oil and gas usually accumulate on the top of water and they are always there relatively to their difference of densities. The reservoir rock are basically analyzed by means of assessing their porosity a permeability but also its analysis takes ranges into various fields such as stratigraphy, structural analysis, sedimentology , paleontology and reservoir engineering disciplines. In case the reservoir has yet been identified, key characteristic crucial to hydrocarbons explorationists are bulk rock volume and net-to-gross ratio. The bulk rock volume (gross volume of the rock above the water-hydrocarbons contact) is obtained from of sedimentary packages while the net-to-gross ratio (the proportion of sedimentary packages in a reservoir rock) estimations are gotten from analogues and wire lines logs. The net volume of reserves is equal to bulk rock volume multiplied by the net-to-gross ratio.

Rv = (BRv)(NtGr).

Cap rock or seal.

It is a lithological unit(s) with low permeability which restricts hydrocarbons to escape from the reservoir. It is made of chalks, shale or evaporites. Its analysis bases on assessing the extent and thickness to know how much cap rock is efficient to oil and gas retention. According to lithological deformation that might have been happen, the cap rock may be found in various types. The tectonic movements the crust experiences cause the anticline and syncline seals and the matter of consequences of their shapes; the convex form is more enjoyable to petroleum exploration than concave one. That is why always the seismology experiments are always carried out to assess how well they can reach the reservoir by aiming at seal with a concave shape as to ease and make efficient the petroleum exploration.

Trap.

The trap is structural or stratigraphic feature that ensures a fixed and firm position of seal and reservoir which avoids the escape of oil and gas.

Maturation.

The assessment of the reservoir quality (nature) involves maturation analysis by which they know the length of time of petroleum generation or expulsion. 

Migration.

Migration is the process of moving oil and gas from the source rock to the reservoir pores when it is trapped after its generation. The main factors of the oil and gas migration are compression, buoyancy, chemical potential; thermal expansion, topography, maturation (increase in volume with time), and gravitational separation of hydrocarbons and water from each other. 

Petroleum Systems Identification.

With regardless of quantity of oil or gas found in a geographic area, the presence of any quantity shows that petroleum system exists. The identification of petroleum is done through the following steps:

Finding some indication of the presence of petroleum.

Determining the size of petroleum system which is also done through the following steps: first of all the found petroleum occurrences are genetically grouped according to geochemistry characteristics and stratigraphic occurrences. Secondarily, the source which gave rise to the genetically petroleum occurrences is identified using petroleum-source correlation methods. Thirdly, the geographic area the pod of active source found giving rise to all the genetically related petroleum occurrences.

Naming the petroleum system.

The petroleum system nomenclature is essential thing such as the way people have names to identify or differentiate them one from each other. The name of a petroleum system is compound name which has different parts showing the name of the source rock, the part showing the name of the main reservoir rock and another one showing the degree of certainty. The degree of certainty is expressed using signs such as (!) showing that the petroleum is well known, (?) expressing that the petroleum system is speculative while (.) expresses that it is hypothetical. For example the name Phosphoria-Weber (.), phosphoria indicates the name of the source rock and weber shows the main reservoir rock of the petroleum system which is hypothetical. A known system is characterized by having positive petroleum-source rock correlation, hypothetical when that correlation fails indeed with total absence of geochemical evidence. (Magoon, L.B., n.a )

Petroleum system mapping.

The petroleum system mapping is a way of portraying all the elements of the system basing on geographic (spatial), temporal and stratigraphic extents.  The geographic extent of a system is the span of all geographic area in which the active pod source is found. The system size is determined by the geographic extent while the temporal extent traced on the events chart shows the ages of important elements and events such as different process, preservation time, and critical moment of a system in its history. The burial history is explained by the stratigraphic extent which is traced basing on the lithological units of the system. 

 Practical applications of petroleum systems.         

Practical application of petroleum system study is optimizing oil and gas exploration, further researches and evaluations of the geographical area in which it is found.  Mapping and studying a system helps explorationists to predict if the trap of interest has oil or not. It also helps them locating the most likely accumulations of the petroleum province (region). The best mapping and study of petroleum system is done by linking all elements such as source rock, reservoir rock, seal and overburden rock to the processes of petroleum geology, generation-migration-accumulation.                                                                 

The system study also allows explorationists to assess risks which might be associated with the entire system, in the petroleum province. Some of those risks are finding new oil accumulations neighboring to the system being explored, that is why in petroleum geology, the oil and gas explorationists, have some risk-associated features such as prospect, play complementary prospect for optimum exploration. A prospect is a potential trap to be drilled to check if it contains sufficient oil for commercial purposes. A play is one or more geological related prospects while a complementary prospect (CP) is a prospect which is dependent on a known system. Hereby other prospects are studied without any knowledge of existing system (being explored). Therefor a better understanding of petroleum system in a province can be explained by the context bellow to associate it with exploration risks.                                                  

 PS_tot=PS_partial+CP₁+CP₂+…CPn₁+CPn.                                                                                            

PStot means the total system in a petroleum province; CPn means the possible number of unknown oil and gas accumulation that can be discovered by further exploration studies.

A prospect is determined by tree independent assessing variable. The first is a petroleum charge (fluids), the second is the trap that encompasses reservoir, and the entire trap geometry and seal rock. The third is the Timing by which they knew if the trap was formed after or if oil came after. A prospect existence depends on all of the variables above. 

P= (v1)(v2)(v3) with assessing variables (v1, v2, v3) which have to be present (1) or absent (0) hence a prospect (P) is absent if one of if one of the variable is absent