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## Esam

*             				   ɡ    ǡ    				    ɡ       				        .     				         ʡ  				            				̡ ...      .**
**  * Pay  				zone*             				         .   				         "   				"     " "     				    .       				           .  				    *   				Spill  				Plane*  *   				Crest*  .**
**  				          ǡ  				   *   				Oil Water Contact-OWC*  *   				Gas  				Oil  				Contact-GOC*    ҡ         				          				.**
**  				           				   1985         				  *      				Tectonic*        ɡ  .  				           				     * Crustal  				Shortening*           16   				           				   ɡ       				           				 ɡ         				ɡ   ѡ ޡ ɡ .   				          				   *   				Crustal  				Tension* *   				Deep  				Seated  				Horsts*.**
**  				       ϡ     				   ʡ         				 ""            				             				 .          				    ڡ      				             				 ɡ          				    .**
*  *     				             				ɡ      ɡ    				          				 ɡ        ""   				            				  ɡ        				    .       				          .  				           				        ɡ   				           				       ʡ      				   ͡    ǡ   				  *   				Crystallization*  .**
**  				       ǡ     				            				ǡ           				 .           				            				     .**
**  				         ޡ   				   ޡ        				   ޡ       				.       * Pinch-out*   				  .          				 ɡ            				      .     				         				  ɡ     ɡ   				            				 .**

** *   				Channels*    				            				ɡ     .    *   Barrier  				Bar  				Traps*               				ơ     *   				Well*-*sorted*.               				ɡ   .**

**  				           				  ɡ         				 * Cementation*   				      .      				            				       .       				              				ɡ    "" 				*  				Dolomitization*    				      ɺ     				          .**

** *   				Erosion*             ɡ  				            				  .**

**  				          				              				 ɡ         				           				           				  *   				Buoyancy*   				.**

*  *      				              				   ɡ         				   .       				*   				Fault*               				 ǡ          .  				          				          .**

**  				           				     ǡ      				       .     				       ɡ   				     ϡ         				   .**

** * *
**  				            				            				          				           				  ɡ       				           				   ѡ        				   .       				   ɡ         				 ɡ    .**

**  				    ʡ   ǡ   				     ѡ     				ѡ           				  ڡ       .   				         				ɡ ǡ      ǡ  				            				    ǡ      				     .**
**  				         ѡ  				        ǡ   				           				          				   .        				    ֡        				           .**


**  				   ޡ         				           				ɡ            				    .         				    ѡ       				          ѡ  				              				   ͡         				        ѡ    				       .     				    ѡ       				 .**

**  				  ѡ          				 * Well  				Casing*   				    ѡ       				 ѡ    ɡ     				        ǡ  				            				 .          				      .     				          				ѡ           				  ѡ          				   .       				  ѡ  ѡ ǡ    				  .        				   *   				Tertiary  				Recovery*.**

          
**  				          				         				     ʡ     				   ʡ      				           				   .**

**  				           				  ɡ     *   				Petroleum  				Condensates* * *

**  				          				 ɡ    ɡ     				            				           				.            				    .        				            				  20 - 30%      .    				              				           				       ѡ    				            				 .**

**  				             				 .         				 ɡ        ʡ   				     *   				Bottom  				Hole  				Pressure* ɡ      .      				             				 40 - 50%           				         .**

**  				     ء       				        ɡ   				             				.           				    ֡     *   				Mule  				Head  				Pumps*. 				   				          .  				            				 ѡ     ѡ      				    .**
**  				             				           				  .        				  .         				-   -           				           				ʡ       *   				Emulsions*      .         				    ʡ       				      .      				             				          				      ǡ   				.**

** *   				Formation  				Water*         ʡ     				      ʡ        				    .        				         				         ɡ   				             				  ء           				            				            				80%     .**

**  				   ʡ        				         				.           				(  ѡ         				    -         -   				  ʡ           				       ʡ  ǡ  				 ǡ     .**

**  				        ߡ     				            				          				          				       ͡     				           ա  				    ɡ       				  .**

**  				     ӡ      				      3000 /   1974  				    /    6100    ѡ   				   .          				          ɡ  				             				      ء       				   .**



**  				            				             				         				 .            				       .    				            				         .     				       *   				Fractures*    ʡ         				          				      .**

**  				            				      ѡ    				            				    .         				        ѡ   				         .**

**  				            				            				   ǡ        				.           				      50%  70%   				    .**

**  				          				    ɡ     				ɡ         				    .     				           				     * Colloids*   				  ʡ          				    ɡ       				.**

**  				      -      				   -        .  				    ʡ *   				Sulfonates*   ߡ*   				Carboxylates*. 				       .    				   ʡ *   				Glycols*   ʡ  ʡ *   				Glucosides*  *   				Polysaccharides*.    ڡ      Ǻ   				            				       .**

**  				           				      ʡ     				ɡ        *   				Immiscible  				Displacement*   				      ʡ     				  ʡ      *   				Emulsions*      ֡   *   				Miscible  				Flooding*.**

**  				       ʡ    				   ǡ        				ɡ      90%     				        .**

**  				          				         				 .         60%  				     .**

**  				           				    ɡ     				.          ̡  				          .  				            				      ǡ       				      .**

**  				             				           ѡ    				   ȡ   .      				            				          .*See More:

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## Esam

In addition to the requirement that source rock exists for the generation of hydrocarbons, and that reservoir rock exists for the storage and production of the generated hydrocarbons, traps must also exist to trap, or seal, the hydrocarbon in place forming a hydrocarbon reservoir.
The fluids of the subsurface migrate according to density. As previously discussed, the dominant fluids present or potentially present are hydrocarbon gas, hydrocarbon liquid, and saltwater. Since the hydrocarbons are less dense than the saltwater, they will tend to migrate upward to the surface, displacing the heavier water down elevation. These fluids will continue to migrate until they encounter impermeable rock, which will serve as a reservoir seal or trap. These impermeable rocks serving as reservoir seals, of which shales are among the most common, are referred to as _confining bed_s or _cap rocks_. Traps exist because of variations in characteristics of rocks of the subsurface. If impermeable rock does not exist, the hydrocarbons will migrate to the surface and dissipate into the environment. In order for a hydrocarbon reservoir to exist, a proper sequence of events must have occurred in geologic time.
*Traps can be classified as:*
*structural trap:*
 is a shifting or alteration in the horizontal formations of the earth's crust.  The alteration is caused by the physical processes of plate tectonics, continental drift, earthquakes, rifting or the intrusion of salt, shale or serpentine.  The intrusion forms faults and folds in the original horizontal formations thus creating the traps necessary for reservoirs Other structures common to hydrocarbon reservoirs are folds and faults
*type of Structural Traps:*
*1)     Anticline Traps:*
Sedimentary beds are generally deposited in horizontal parallel planes over a geographic region, so that many of these sediments will be of essentially uniform thickness over This trap may exist as a simple fold or as an _anticlinal dome_.that region. If geologic activity should occur, resulting in the folding of these sediments, the result may be the formation of hydrocarbon reservoirs in anticlinal traps. Two major potential advantages of the anticlinal trap reservoir are the simplicity of the geology and the potential size of the trap and therefore of the hydrocarbon accumulation. The high part of the fold is the_ anticline_, and the low part of the fold is the _syncline_. Since the hydrocarbons are the less dense of the subsurface fluids, they will tend to migrate to the high part of the fold. Consider the hydrocarbon reservoir illustrated in Figure 18. Hydrocarbon reservoir rock,where shale is the cap rock formation of this hydrocarbon reservoir. Sedimentary beds are deposited in a water environment, as indicated by the presence of limestones and shales. During or after lithification, geologic activity causes folding of the sediments. After folding and lithification, the sandstone has a 100% _connate_ water saturation. Millions of years later, hydrocarbon generated in source rock down elevation from this anticlinal fold is forced from its source rock into the water-saturated, permeable sandstone. Since hydrocarbon is less dense than the water, it begins to migrate up elevation, displacing the heavier water down elevation. As it migrates upward, pressure decreases. At some point in this migration, the reservoir fluid pressure might equal the bubble point pressure of the original hydrocarbon combination. From this point upward, gas is being released from the hydrocarbon. Since the gas is so much less dense than the oil or the water, it will migrate more rapidly toward the top of the anticlinal trap. This process of migration and fluid separation according to density may continue over millions of years in geologic time, until finally, equilibrium is achieved as the hydrocarbon fluids accumulate within the trap formed by the  impermeable shale cap rock When this condition of equilibrium is finally achieved, there will be a gas zone (gas cap) on top of an oil zone and then a water zone beneath the oil zone. 




*2)    Fault Traps :* 
Fault implies fracturing of rock and relative motion across the fracture surface. Consider a possible sequence of geologic events that, in geologic time, . Sedimentary beds are deposited in a water environment, as indicated by the presence of shales and limestones. During or after lithification,geologic events result in uplift of these original horizontal sediments, and fracturing and tilting above sea level, so that the surface rocks are exposed to erosion. During uplift, the rocks are fractured and slippage occurs along the _fault plane_.This brings the shale across the fault so that it seals the tilted sandstone below the fault. Millions of years later, hydrocarbon generated in its source rock down elevation from the fault is forced into the connate water-saturated sandstone. Since the hydrocarbon is less dense than the water, it will migrate up elevation, displacing the heavier water down elevation. This upward migration will continue until it reaches the fault and is trapped by the impermeable shale. If the faulting had not occurred, the hydrocarbon would have continued to migrate upward until it was dissipated at the surface into the environment. Since faulting occurred, the shale provides the necessary seal, resulting in the existence of the hydrocarbon reservoir.Notice that, in this example, if slippage had occurred to a greater extent, there would have been flow into the permeable sandstone above the fault. The hydrocarbon would have been lost to the surface, and no reservoir would have been formed.This situation illustrates the significance of geologic probability.
What is the probability that the relative motion across the fault would have resulted in a reservoir seal being formed?Geologic events must occur in the proper sequence, resulting in the proper geologic conditions for a reservoir to exist. The North Sea hydrocarbon environment is an excellent example of the
significance of this geologic probability. Of the hydrocarbon generated in the source rock of the North Sea, it is estimated that less than 10% was trapped. Over 90% of the hydrocarbon was lost back to the surface in geologic time and dissipated into the environment because traps were not present. Fault traps leading to the presence of hydrocarbon reservoirs are often difficult to define because of the complexity of the geology.



 
*3)    Salt Dome Traps:   * 
Consider the salt dome geologic system illustrated in Figure and a possible sequence of geologic events that could lead to the formation of this salt dome environment. A major portion of a continental plate was below sea level at a point in geologic history. Due to geologic events, this region rose above sea level, trapping inland a salt water sea. As geologic time passed,the climate changed to a desert environment. This event could have resulted from movement of the continental plate near tothe equator. In this arid desert environment, water evaporated from the salt water sea, leaving the salt residue on the dry seabed. As millions of years passed in the desert environment,sand blew over the salt to cover and protect the salt sediment.Later geologic events resulted in the sinking of the region below sea level, followed by tens of millions of years of sedimentation in the resulting water environment. As time passed, lithification occurred. The desert sand became sandstone, and the salt became rock salt (sedimentary salt).
After lithification, this salt bed was impermeable. It also had two properties significantly different from typical shale, sandstone or limestone:
 It was less dense, with a measurably smaller specific weight.
 At subsurface overburden pressures and subsurface
temperatures, the rock salt was a plastic solid (it was highly deformable).
The combination of this lesser density and plasticity resulted in a buoyant effect if flow possibilities existed. Geologic events caused fracturing of overlying confining rocks. The salt, forced upward by the overburden pressures, began to flow plastically back to the surface, intruding into the overlying rock structures
to lift, deform, and fracture them. The intruding salt was solid,yet geologically deformable. It might intrude at an average rate of only 1 inch per 100 years, yet on a geologic time basis, such deformation is highly significant. This rate would result in 10inches in 1,000 years, or 10,000 inches (833 ft) in 1 million years. In a geologic time period of only 10 million years, this salt dome could intrude to a height of over 1.5 mileoverlying structures. Obviously, a vertical subsurface structure1.5 miles high is geologically significant. Since the salt isimpermeable, the region around the perimeter of the salt domeis an ideal geologic environment for hydrocarbon traps. The tendency of the intruding salt to uplift the rocks as it intrudesenhances the separation of the less dense oil from the more dense salt water by reducing the area of the oil-water contact.The fracturing of surrounding rocks due to the intruding salt and the lifting of the rocks above the salt dome also provide an environment for the existence of fault traps and anticlinal traps in addition to the salt dome traps around the perimeter of the
dome. A salt dome region, therefore, is an excellent geologic environment for all three types of traps discussed so far.An excellent example of a salt dome trap is Spindle top near Beaumont, Texas. The first major discovery and resultant initial oil boom at Spindle top occurred in 1901. Through the 1890s Patillo Higgins had promoted drilling for oil outside Beaumont.He concluded that it was an excellent geologic environment for hydrocarbon reservoirs, because he noted a location near Beaumont where the surface elevation was 15 ft higher than the surrounding land. This rise was a circle approximately 1 mile in diameter. He concluded that this indicated high points in the underlying geology. In 1901, Captain Anthony Lucas drilled a wildcat well at this location, resulting in the Spindle top discovery. Future drilling confirmed that this reservoir existed as an anticlinal dome trap, with the dome created by the uplift of overlying rocks by an intruding salt dome in the subsurface,creating the surface indication of what the subsurface geology might be.The second oil boom at Spindle top began in the mid-1920s.
When further wells were drilled, it was discovered that fault trap and salt dome trap reservoirs existed around the circumference of the salt dome. The drilling pattern for the wells drilled during this later activity was almost a perfect circle as these circumferential reservoirs were developed.


* stratigraphic trap:*
The stratigraphic trap is a change in the lithology of the rock sequence.  This change is caused by erosional forces or changes in rock type within a limited areal extent.  An unconformity is an erosional feature where a portion of the geological sequence is eroded and an impermeable rock is deposited on top of a porous formation.  The process of erosion will enhance or create the porosity and permeability necessary for the existence of a petroleum reservoir.  Other stratigraphic trapping include channel sand deposits surrounded by shale, growth of limestone reefs and the formation of barrier islands or sand bars along the ancient shoreline.



 
*Classification of stratigraphic trap:*
*-Primary Stratigraphic Traps:*
 These traps result from deposition of elastic or chemical materials. Shoestring sands, lenses, sand -----es, bars, channel fillings, facies changes, strand-line (shoreline) deposits, coquinas, and weathered or reworked igneous materials are classified as elastic sedimentary deposits and can result in stratigraphic traps. An ancient sand-filled stream channel meander has cut into older south-dipping shales and created a perfect stratigraphic trap.
The shale plug served as the seal for reservoirs within a west-plunging structural nose. Hydrocarbons are trapped in the truncated up dip portions of the reservoirs. Organic reefs or biohenns and biostromes are the primary chemical stratigmphic traps; they are built by organisms and are foreign bodies to the surrounding deposits .The Strawn and Cisco-Canyon series are limestone reefs that have had younger
the seal. Differential compaction of the thicker shales on the Type of stratigraphic trap : flanks of the reef as compared with the thinner shale at the crest has created structural closure in younger overlying formations. Hydrocarbon accumulations have occurred in the Cisco and Fuller formations as a result of this differential compaction. Additional traps in other reservoirs arc the result of up dip permeability and porosity barriers and are either primary or secondary stratigraphic traps.
*Secondary Stratigraphic Traps:*
 Traps of this type were formed after the deposition of the reservoir rock by erosion and/or alteration of a portion of the reservoir rock through solution or chemical replacement. Secondary tratigmphic traps actually should fall into the combination-trap classification because most are associated with or are the result of structural relief during some stage of development of porosity and permeability or limitation of the reservoir rock. However, many of the so-called typical stratigraphic traps fall into this category, and it is felt that it would be impossible tochange the historical usage of this term. Therefore, secondary stratigraphic traps are defined for this discussion as those traps created after deposition and having limitations caused by lithology changes.
Erosion creates a major part of these through truncation of the reservoir rock. On-lap deposition (when the water is encroaching landward), off-lap deposition (when the water is regressing), and the chemical alteration of limestone result in many secondary stratigraphic traps) It is a truncation of the Woodbine formation as it approaches the regional Sabine uplift.  A certain amount of leaching of the cementing material by waters over the unconformity has resulted in increased porosity and permeability in the field as compared with similar Woodbine sands in the deeper portions of the East Texas basin.

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## Esam

*Examples of   Stratigraphic Traps:*
* 1)pinchout:*

are the result of the changes in deposition of the sediment. Thick layers of mud are covered by thinner layers of sand from migrating shoreline, or by the sand deposited by large rivers. As sea level changes, or rivers migrate, the different sand and mud layers are interwoven creating lenses or pinch-outs. These sand layers allow the petroleum to accumulate and the mud rock layers trap the petroleum. can create traps by burying truncated sandstone or limestone layers with layers of mudstone. 


* 2) Carbonate Reef:*

are great places to trap oil. The open cavities between the corals create excellent reservoirs, and when the reef is buried by mud, the oil becomes trapped. Many of the large oil and gas fields in west Texas are found in buried age reef.


*  3)Sandstone Lens:*

Lenses - Layers of sand often form lens like bodies that pinch out. If the rocks surrounding these lenses of sand are impermeable and deformation has produced inclined strata, oil and natural gas can migrate into the sand bodies and will be trapped by the impermeable rocks. This kind of trap is also difficult to locate from the surface, and requires subsurface exploration techniques.


*   4) Faces Change:*

Consider the deposition near a shoreline of a continent, as distance from the shoreline increases. From the shoreline out into the body of water, the particle size decreases from gravel to pebbles, to sand, to silt, to mud. When lithification occurs, the silt-to-mud size particles, form shale. Therefore, in the same sedimentary bed, as distance from the original shoreline increases, the rock grades from sandstone, through a transition zone, to shale. Assume that, after lithification, with further sediments having been deposited on this original sediment, a geologic event results in uplift and tilting of this sediment, so that the shale is up dip from the sandstone, as illustrated in Figure 24. The dip of a bed is the angle its plain makes with the horizontal.
Later in geologic time, hydrocarbon generated in its source rock at lower elevations is forced into the connate water-saturated sandstone and begins to migrate up elevation, displacing the heavier water down elevation. This hydrocarbon will continue to migrate until it encounters the impermeable shale at the transition zone within the rock. It is trapped as a result of the change of permeability within the sedimentary bed, as the transition occurs from sandstone to shale or from permeability to no permeability. This transition of properties within the rock sediment is called a facies change.


Through the transition zone, the transition occurs from sandstone to shaley sandstone, to sandy shale, to shale. As to the distinction betweenshaley sand andsandy shale, as long as the rock has sufficient porosity and permeability to be considered an acceptable reservoir rock, it is classified as sandstone. However, when either property has reduced sufficiently within the transition zone so that the rock can no
longer be considered an acceptable reservoir rock, it is considered shale.
*Combination Traps:*
Combination traps are structural closures or deformations in which the reservoir rock covers only part of the structure. Both structural and stratigraphic changes are essential to the creation of this type of trap. Traps of this nature are dependent on stratigraphic changes to limit permeability and structure to create closure and complete the trap. Up dip shale-outs, strand-lines, and facies changes on anticlines, domes, or other structural features causing dip of the reservoir rock create many combination traps. Unconformities, overlap of porous rocks, and truncation are equally important in forming combination
traps. Faulting is also a controlling factor in many of these traps. Asphalt seals and other secondary plugging agents may assist in creating traps.
* Examples  of  Combination traps:* 
*1)Traps Associated with salt domes:*

A salt dome is a mass of NaCl (Sodium Chloride) generally of a cylindrical shape and with a diameter of about 2 km near the surface, though the size and shape of the dome can vary. This mass of salt has between pushed upward from below through the surrounding rock and sediments into its present position. The source of the salt lies as a deeply buried layer that was formed in the geologic past. Salt is an evaporate. Salt beds were formed by the natural evaporation of sea water from an enclosed basin; in Louisiana, this occurred in Permian or Jurassic time. Subsequently, the precipitated salt layer is buried by successive layers of sediments over geologic time until segments of it begin to flow upward 

toward the surface of the earth .The origin of salt domes is best explained by the plastic-flow theory. Salt has a density of 2.2 under standard conditions. But at a depth of about 12,000 feet, the mass of the overlying sediments exerts a compressive, downward force, density decreases and salt begins to flow like a plastic substance. A small fracture in the overlying, higher density sediments or a slightly elevated mass of salt above its surroundings would trigger the upward movement. Once this upward salt movement begins, salt from elsewhere in the salt bed moves into the region surrounding the salt plug to replace the salt that is flowing upward to form the salt plug. The upward movement of the salt plug, or dome, continues as long as there is sufficient source of salt "feeding" the dome OR until the upward movement is halted by a more rigid formation.   Once equilibrium is reached, upward movement of the salt dome ceases, but may begin again if sufficient sediments are added to the weight of the overburden which again increases the load pressure on the parent salt mass. In Louisiana, the age of the salt domes is dependent upon which side of the Cretaceous reef structure you are on. The domes are oldest on the north side and youngest on the south side. This also corresponds to the age of the hydrocarbon deposits discussed earlier. 


* 2) Unconformity*
Consider the sequence of geologic events summarized in Figure 23. Sedimentation occurs over millions of years in a water environment, resulting in horizontal, parallel, sedimentary beds. Lithification occurs, followed by uplift and tilting above sea level. As a result of being uplifted above sea level, erosion occurs over millions of years, removing rocks down to an erosionalsurface, or unconformity. Following erosion, the region subsides again below sea level and is followed by millions of years of sedimentation in a water environment. After lithification, the first sediment on top of the unconformity is impermeable shale. The unconformity represents a discontinuity in the geologic system, because there is a geologic time discontinuity between the rocks above the unconformity and those below it. Millions of years after this sequence of events, hydrocarbon that is generated in source rock at lower elevations is forced into the connate water-saturated sandstone. Due to its lesser density, it migrates upward through the permeable sandstone, displacing the heavier water down elevation. When the hydrocarbon reaches the unconformity, it is trapped. This trap is a stratigraphic trap, and this particular type of stratigraphic trap is referred to as an unconformity, or truncation. The specific type of unconformity illustrated here is an angular unconformity. 




Notice that the hydrocarbon trap would not have existed had thefirst sedimentary bed above the unconformity not been impermeable after lithification. Again, the proper sequence of geologic events was necessary in order for the trap to exist.
* 3)Other Traps:*

 Many other traps occur. In a combination trap, for example, more than one kind of trap forms a reservoir. A faulted anticline is an example. Several faults cut across the anticline. In some places, the faults trap oil and gas (fig- ). Another trap is a pier cement dome. In this case, a molten substance-salt is a common one-pierced surrounding rock beds. While molten, the moving salt deformed the horizontal beds. Later, the salt cooled and solidified and some of the deformed beds trapped oil and gas (fig-). Spindle top was formed by a pier cement dome.


*Lenticular Traps:*
Oil and gas may accumulate in traps formed by the bodies of porous lithofacies (rock types) embedded in impermeable lithofacies, or by the pinch-outs of porous lithofacies within impermeable ones, as seen in Fig. 2.10.
Examples of such lenticular traps include: fluvial sandstone bodies embedded in flood basin mud rocks, deltaic or mouth-bar sandstone wedges pinching out within offshore mud rocks, and turbid tic sandstone lobes embedded in deep marine mud rocks. Similar traps occur in various
limestones, where their porous lithofacies (e.g. oolithic limestone or other calcarenites) areembedded in impermeable massive lithofacies; or where porous bioclastic reefal limestones pinch out in marls or in mud rocks.
The approximate percentages of the worlds petroleum reservoirs associated with those major trap types are given in Fig. 2.11.






On of the present-day Earths surface, over half of the continental areas and adjacent marine shelves have sediment covers either absent or too thin to make prospects for petroleum accumulation. Even in an area where the buried organic matter can mature, not all of it results in petroleum accumulations. The following statistical data may serve as a fairly realistic illustration [49]:
 Only 1% by vol. of a source rock is organic matter,
 < 30% by vol. of organic matter matured to petroleum,
 > 70% by vol. of organic matter remains as residue and
 99% by vol. of petroleum is dispersed or lost at the ground surface in the process of
migration, and only 1% by vol. is trapped.
These data lead to the following estimate: only 0.003 vol.% of the worlds source rocks actually turn into petroleum that can be trapped and thus generate our petroleum resource*s*
 What is the difference between each of the three trap types in terms of how they were formed?
Answer: A Structural trap is formed by tectonic processes AFTER deposition of the reservoir beds involved while a Stratigraphic trap is created during deposition of the reservoir beds. A Combination trap is formed by a combination of processes present in the sediments DURING the time of deposition of the reservoir beds AND by tectonic activity that occurred in the reservoir beds after their deposition*.*

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