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.
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. Facies 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 between”shaley sand” and”sandy 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 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 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.
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.
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.
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.
On of the present-day Earth’s 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 :
- 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 world’s source rocks actually turn into petroleum that can be trapped and thus generate our petroleum resources
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.