1. Chapter 1 Reservoir rock:  Definition  Classification  Nomenclature  Physical Characteristics of a Reservoir rock Chapter 2 Seal rock Chapter 3.

Chapter 1 Reservoir rock:  Definition  Classification  Nomenclature  Physical Characteristics of a Reservoir rock Chapter 2 Seal rock Chapter 3 Petroleum Traps:  Definition  Types of traps 2

 Definition: A reservoir is a subsurface volume of porous and permeable rock that has both storage capacity and the ability to allow fluids to flow through it. Hydrocarbons migrate upward through porous and permeable rock formations until they either reach the surface as seepage or become trapped below the surface by a non-permeable cap rock which allows them to accumulate in place in the reservoir. Reservoir rocks are mostly sedimentary in origin. They are either mechanical or chemical deposition of solid– materials or simply the remains of animals or plant life. 4

 Classification: Classification of petroleum reservoir rock for practical use should be as simple and broad as possible. A simple, broad, Primary classification of reservoir rocks, based largely on the origin of the rock, divides them into 3 groups: 1) Fragmental (clastic). 2) Chemical and Biochemical (precipitated). 3) Miscellaneous The chief difficulty in applying any rock classification is that there are many gradational types that are hard to classify. 6

It is sometimes useful to classify a reservoir rock as of marine or non marine origin. This genetic classification may be combined with a lithologic classification, as “marine limestone”, “continental sandstone”. It is useful to place the rock in the standard geologic time scale and thereby classify it according to geologic age. This can be done by combining an age term with other term. e.g. Permian dolomite, non marine Oligocene sandstone, Devonian grit. 7

 Nomenclature: A reservoir rock formation from which petroleum is produced is commonly given a specific proper name. such names frequently begin at a well or pool during the early development stages. A name that some driller or operator may casually give to a producing formation is first used in conversation, then in newspapers and trade journals, then engineering and geologic reports, and finally in legal contracts and scientific journals. One reservoir formation may called by many names. The commonest names for reservoir rock are “pay”, 9

“pay sand”, “oil sand”, “gas sand”, “sand”, “lime”. Examples of names based on a more or less petrologic character are : “Simpson sand”, “McCloskey oolite”. The depth relative to other pay formations is shown in such names as “First sand”, Third stray” “D- 3 pay”. The geologic name of the formation is used where it can be identified underground, as in “Berea sand”, “Madison limestone”, “Oriskany sand”. A common method of naming a producing formation in the USA is to give it the name of the owner of the land on which the discovery well was drilled, 10

Such names are: “Hoover sand”, “Jones sand”. Sometimes the name of the producing formation is taken from the name of pool in which it was first found, as “Mirando sand” form “Mirando pool”. Another method is to name the producing formation after some characteristic fossil or fossil assemblage, as in “Nodosaria sand”, “Heterostegina sands”. 11

 Physical characteristics of a reservoir rock: I. Porosity (Φ): Definition: The percentage of the total volume (bulk volume) of the rock that is void space. This volume is often referred to as the pore volume and is an indication of the potential storage space for hydrocarbons within the rock structure itself. 13

Types of porosity:  Total porosity = Primary porosity + Secondary porosity The percentage of total void space with respect to the bulk volume regardless of the interconnection of the pore voids.  Effective porosity: The percentage of interconnected void space with respect to the bulk volume. It is an indication of conductivity to fluid. 14

Geological factors affecting porosity: 1) Degree of sorting:  Well sorted  Moderately sorted  Poorly sorted 2) Mode of packing:  Cubic packing: 48 % porosity  Rhombohedra packing: 26 %porosity 15

3) Compaction: It's a geological factor which reduces porosity due to overburden pressure of the overlying sediments. 4) Cementation: It's the agent which has the greatest effect on the original porosity and which affects the size, shape and continuity of pore channels. 16

Calculation of porosity:  Theoretically:  Experimentally: by Glass pycnometer, Ruska porosimeter. 17

II. Permeability: Definition: is the ability of the rock to transmit fluids. Fluids move along grain boundaries or along faults, fractures and joints. Permeability is the rock property that indicates the presence of flow channels. 18

Geological factors affecting porosity: 1) Grain size It was found that the rate of fluid flow is proportional to the square of the grain diameter, hence the finer sand the smaller the permeability. 1) Mode of packing: The effect of packing as a factor which influence permeability can be introduced as: K = 10.2 d 2 / c Where: d: diameter of spheres, cm c: packing constant depending on porosity 19

3. Clay content 4. Reactive liquids: Reactive liquids after the internal geometry of the porous. Medium, due to precipitation or corrosion. 5. Overburden pressure : When the core is removed from the formation, the confining forces are removed. The rock matrix is permitted to expand in all direction partially changing the shapes of the fluid– flow paths inside the core. 20

Calculation of permeability:  Theoretically:  Experimentally: by perm– plug method, Whole – core measurement 21

 Seal rock or Trap rock: Definition:  Seal rock can be defined as one that has pore throats too small and poorly connected to allow the passage of hydrocarbon. The geographic extent of seal rocks defines the effective limits of the petroleum system.  Confining seals are lacking, hydrocarbons escape to the surface. Therefore, the seal rock is an essential element of the petroleum system.  Two important classes of seals occur in a petroleum system:  Regional seals that roof migration hydrocarbons  Local seals that confine accumulation. 23

 Most quantitative seal data derive from studies of seal roofing local accumulations, but such field data are related only inferentially to the general properties of the regional seals that must roof and guide the migrating hydrocarbons in the petroleum system.  In petroleum system investigation, seal analysis should start with a determination of the time and place where hydrocarbon are generated and expelled. Many stratigraphic horizons display properties of a seal, but only those few that are above the mature source rock. 24

 Definition: A trap consists of a geometric arrangement of permeable (reservoir) and less-permeable (seal) rocks which, when combined with the physical and chemical properties of subsurface fluids, can allow hydrocarbons to accumulate. Trapping elements: Three main trapping elements comprise every subsurface hydrocarbon accumulation: 1) Trap reservoir: storage for accumulating hydrocarbons and can transmit hydrocarbons. 2) Trap seal: an impediment or barrier that interferes with hydrocarbon migration from the reservoir. 26

3) Trap fluids: physical and chemical contrasts especially differences in miscibility, solubility, and density between the common reservoir fluids (primarily water, gas, and oil) that allow hydrocarbons to migrate, segregate, and concentrate in the sealed reservoir. Some definitions concerning the Trap nomenclature:  Crest (culmination): is the highest point of the trap  Spill point: is the lowest point of the trap at which hydrocarbons may be contained in the trap. This lies on a horizontal contour, spill plane.  Closure of the trap: is the vertical distance crest to spill plane. Barren trap: it is a trap of permeable layer but doesn’t contain any oil due to formed after migration of H.C, no source rock, no sealing, deformation of trap. 27

A trap may contain oil, gas or both. The oil water contact (OWC): is the deepest level of producible oil. The gas oil contact (GOC): is the lower limit of the producible gas. Also it is important to gas overlies the oil as the gas has a lower density and the water has a higher density. 28

 Types of Traps: Traps are divided into 4 main categories: I. Structural traps:  Anticline trap  Fault trap  Diapiric trap II. Stratigraphic traps:  Pinch out trap  Unconformity trap  Reef trap III. Hydrodynamic traps IV. Combined traps 30

I.Structural traps: Definition:  Are traps whose geometry was formed by tectonic processes after deposition of beds involved.  Structural traps don’t occur at random. The types and distribution of them are closely related to the regional tectonic conditions and history of the region in which they are found.  Sometimes it is possible to find a trap where a single tectonic process took place, but frequently two or more of the named processes (folding, faulting, and mobility processes) are involved with equal importance in the creation of a trap. 31

1)Anticline traps: The anticline trap is a classical trap type in petroleum geology. The anticline, or fold, traps may be subdivided into 2 classes:  Compactional anticlines  Compressional anticlines 32

Compactional anticlines: Are formed by crustal tension, where crustal tension causes a sedimentary basin to form, the floor is commonly split into a mosaic of basement horsts and grabens. The initial phase of deposition infills this irregular topography. Throughout the history of the basin, the initial structural architecture usually persists, controlling subsequent sedimentary. Thus anticlines may occur in the sediment cover above deep-seated horsts. Compressional anticlines: Are most likely to be found in or adjacent to subductive troughs, where there is a net shortening of the earth’s crust. Consider best known oil province. 33

2)Fault traps: Fault trap are formed by movement of rock along a fault line. In some cases, the reservoir rock moved opposite a layer of impermeable rock. The impermeable rock prevents the oil from escaping, the fault itself can be a fault very effective trap. Although many fields are trapped by a combination of faulting, pure fault traps are rare. 35

There are 8 theoretical configurations of petroleum traps associated with faulting. These configurations are drawn on the assumption that oil can move across, but not up, the fault plane when permeable sands are juxtaposed. 36

3)Diapiric traps: Diapiric traps are produced by the upward movement of sediments that are less dense than those overlying them. In this situation the sediments tend to move upward diapirically, and may from diverse hydrocarbon traps. Diapiric traps are generally caused by the upward movement of salt or, less frequently, overpressure clay. Salt has density of about 2.03 g/cm 3. Recently deposited clay and sand have densities less than salt. As the clay and sand buried, however, they compact, losing porosity and gaining density. Ultimately, a burial depth is reached when sediments are denser than salt. Depending on a number of variables, this occur between about 800 and 1200 m. when this point is reached, the salt will tend to flow up through the denser overburden. 38

In some salt structures the overlying strata are only up- domed, whereas in others the salt actually intrudes its way upward, the latter are referred to as piercement structures. That’s why salt creates numerous traps of oil and gas. Such as: 39

II.Stratigraphic traps: Definition:  Stratigraphic traps have a geometry that is formed by changes in lithology. The lithological variations may be depositional (channels, reefs, and bars) or post depositional (truncations and diagenetic changes).  Levorsen defined as “one in which the chief trapmaking element is some variation in the stratigraphy, or lithology, or both, of the reservoir rock, such as a facies change, variable local porosity and permeability, or an up-structure termination of the reservoir rock, irrespective of the cause. 41

1)Pinchout traps: A pinchout traps, or wedged layer of reservoir rock, traps fluids and gases if surrounded by impermeable rocks. There are 2 types of pinchout traps:  Permeability pinchout  Depositional pinchout 42

Permeability pinchout:  Within otherwise continuous formations provide traps in both clastic and carbonate succession.  in sandstone reservoirs, the loss of permeability in the updip direction is due to increasing shaliness of the sand or passage into a band of secondary cementation.  More striking are the carbonate representatives of this trap category, about equally divided between updip transition from dolomitized to undolomitized limestone and from limestone (occasionally dolomite) into evaporate time equivalents. 43

Depositional pinchout:  Within otherwise a continuous succession are characteristic for in sandstones, which have much higher horizontal permeabilities than limestone.  There are 2 cases:  In first case, the sand body wedges out up the depositional dip, as a beach and bar sand do. Such sand are in trap position from outset.  In second case, the depositional pinchout is to acquire trap position, its original dip must somehow be preserved. This is easily achieved by uplift of the source area of the sand, and the consequent loading of the depositional basin by molasse sediment. 44

2)Unconformity traps: An unconformity is a break in the depositional sequence of rocks. If the underlying beds were tilted, eroded, and then covered with flat-lying impermeable rocks, then oil and gas may be trapped at the unconformity. The reservoir may be capped by almost any sedimentary rock type, but commonly by shale, or evaporites. 45

3)Reef traps: Reefs, or carbonate buildups, possess all the ancillary trapping mechanisms of buried hills: flanking clastic sediments of high porosity, facies changes over their crests, and drape anticlines in post-reef strata. They add 2 other vital characteristics of their own: they are themselves excellent reservoirs and they are commonly very well sealed. Organic reefs range in size from gigantic barrier forming structure, through large atolls, discreate bioherms, to tiny isolated bodies. Most reefs, large or small, are carbonate construction of colonies of algae, corals, mollusks, and some groups of 47

now extinct organisms such as stromatoporoids, symbiotic crinoids, brachiopods and foraminifera. As important to a reef trap as its inherent convexity and its porosity is its seal. During a transgressive depositional cycle reefs become buried by muds or marls which make excellent seals. In regressive cycle barriers reefs or atolls become overlapped by evaporites or marls from the bake-reef of lagoonal regime. 48

III.Hydrodynamic traps: This type of traps occur as part of other previous considered trap types. In this traps hydrodynamic movement of water is essential to prevent the upward movement of oil or gas. The water moving hydrodynamically down permeable beds. It may encounter upward-moving oil. When the hydrodynamic force of the water is greater than the force due to the buoyancy of the oil droplets, the oil will be restrained from upward movement and will be trapped within the bed without any permeability barrier. Hydrodynamic traps are often have titled oil-water contacts. 50

12 34 51

IV.Combined traps: Many oils and gas fields around the world are not due to solely to structure or stratigraphic or hydrodynamic flow, but a combination of 2 or more of these forces. Most of these traps are caused by a combination of structural and stratigraphical processes. Structural – hydrodynamic and stratigraphic – hydrodynamic traps are rare. 52

1. Chapter 1 Reservoir rock:  Definition  Classification  Nomenclature  Physical Characteristics of a Reservoir rock Chapter 2 Seal rock Chapter 3.

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1. Chapter 1 Reservoir rock:  Definition  Classification  Nomenclature  Physical Characteristics of a Reservoir rock Chapter 2 Seal rock Chapter 3.

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