5. the Fossil Fuels: Petroleum I - Finding Oil

5. the Fossil Fuels: Petroleum I - Finding Oil

5. The Fossil Fuels: Petroleum I - Finding Oil 5.1 Introduction Finding petroleum is a difficult and sometimes dangerous job. It involves a large number of individuals with a wide variety of skills. These include geologists, seismologists, various types of engineers and landmen. All of these individuals play vital but specific roles in the development and ultimate production of a play. This lab conveys just a portion of the complexities involved in this important search. Figure 1: Drill pipe stacked on an offshore platform ready for drilling (photo by Erin Campbell- Stone). This lab will investigate the: • different types of petroleum wells • difference between exploration and production • the costs of exploring and producing oil • the importance of understanding geology 1 of 121 The Fossil Fuels: Petroleum I - Finding Oil 5.2 Petroleum Geology 5.2.1 Petroleum Traps 5.2.1.1 Intro Petroleum is less dense than the other fluids, mainly water, that it occurs with in the subsurface. Consequently, with time, it rises upward. If it does not encounter an impermeable layer, it will rise all the way to the Earth's surface where the lighter fractions will evaporate. This produces the oil seeps and tar pits that were important sources of early petroleum products. To prevent its loss and to form an oil field, petroleum must be trapped before it reaches the surface and allowed to accumulate. This combination of natural geologic conditions is a hydrocarbon trap. Thus, oil and natural gas companies spend considerable time, effort and money looking for the right combination of geologic conditions that in the past may have produced a hydrocarbon trap. Figure 2: The Richat Structure in the Sahara desert of Mauritania is a prime example of the type of geologic structure petroleum exploration geologist search for in their quest to locate economic hydrocarbon traps. The structure, as known as the Eye of the Sahara, consists of sedimentary layers dipping outward at 10-20o. It is deeply eroded and approximately 40 km in diameter. Source: NASA Earth Observatory (http://earthobservatory.nasa.gov/IOTD/view.php?id=2561). An accumulation of oil in a trap is a transient feature. Ultimately, the oil will seep through even "impermeable" units and reach the surface. The low permeability of a cap rock 2 of 121 1-Mar-16 Petroleum Geology simply lengthens the time it takes for this to happen. If we are lucky, we find the oil still in the trap. Bacteria also devour oil. Thus, it is not uncommon to find a trap in which the oil is no longer present, but there is evidence that it may have existed in the trap in the past. 5.2.1.2 Components The formation of an oil trap requires a number of different components and their arrangement of these components in a specific vertical sequence in the proper geologic timing. The components of an oil trap include: • a source rock is a geologic unit that accumulated organic compounds that when exposed to increased temperature were converted to oil; • a reservoir rock is the unit that has high permeability and porosity which permits the petroleum to migrate into it and accumulate. This is the unit that an oil company would drill a well into; • a cap or seal rock is the impermeable unit that slows the rise of the petroleum thereby permitting trapping. Figure 3: Left: Conventional hydrocarbon trap. Right: Stratigraphic sequence without a hydrocarbon trap. To produce a trap, these units must be arranged with the cap rock at the shallowest depth and the reservoir rock just below it. They must also be arranged so that there is a restricted space in which the petroleum can accumulate. The source rock, i.e. the rock out of which the petroleum migrates, must be below both of these units. 5.2.1.3 Types The spatial arrangements of geologic units that produce traps are quite varied but fall into four broad categories. These are: • structural traps: traps created by tectonic stresses acting on the Earth's crust to produce folds and/or faults, they act after the reservoir beds have been deposited • stratigraphic traps: traps created by changes in lithology or rock type produced during deposition of the reservoir rocks or after deposition • combination: trap formed by a combination of tectonic processes and sedimentary processes. • salt dome traps: cylinder of salt that has risen upwards, penetrating, fracturing and bending pre-existing strata to form multiple traps 1-Mar-16 3 of 121 The Fossil Fuels: Petroleum I - Finding Oil • hydrodynamic: movement of formation water prevents upward migration of hydrocarbons. Each category of trap is the result of a different set of geologic conditions. Figure 4: The three main classes of conventional hydrocarbon traps. 5.2.1.4 Structural 5.2.1.4.1 Intro Structural traps are the products of folding and faulting. They are produced where the crust has been subjected to stress. Either it has been squeezed (compression) or stretched (tension). If the stress is great enough, the crust breaks forming faults. At lower amounts of stress, it permanently deforms to form folds. Some of the types of structural traps include: anticlines; domes; and faults 4 of 121 1-Mar-16 Petroleum Geology Figure 5: The principal structural hydrocarbon traps 5.2.1.4.2 Domes A dome is a roughly circular or elliptical fold in which strata dip outward and down from a central point with the oldest strata located in the structure’s core. It approximates an overturned bowl. On a geologic map, domes are recognized by concentric, circular and closed outcrop patterns. The strike and dip symbols are oriented with their dip tailings point radially outward from the center of the dome. On the map, the oldest rocks occur in the center of the dome and become younger toward the edge. Figure 6: Cross-sections and geologic map of a domal hydrocarbon trap. Domes are ideal hydrocarbon exploration targets because if they contain the proper sequence of rocks, they may form excellent oil and/or natural gas traps. To form a trap, a 1-Mar-16 5 of 121 The Fossil Fuels: Petroleum I - Finding Oil dome must contain a cap unit (impermeable), a reservoir rock and a source rock in this depth sequence. With the rocks in this sequence, the cap rock can capture any ascending hydrocarbons and let them accumulate. 5.2.1.4.3 Anticlines Intro Anticlines are folds with an arch-like structure in which limbs dip away from center of the fold and the oldest strata are found in the core of the structure. If the axis of the anticlines is horizontal, the map pattern produced consists of a series of parallel lithologic stripes. In this pattern, the oldest rocks are exposed along the axis of the fold and they become younger outwards. A key requirement for identifying an anticline is the age relations of the rocks. If the age relations cannot be established with certainty, the proper term to use of the structure is an antiform. Figure 7: Geologic map (upper right) and cross-section (left and bottom) views of a symmetrical anticline. Trap 6 of 121 1-Mar-16 Petroleum Geology Anticlines form petroleum traps when there is a cap rock lying above a suitable reservoirs strata and both are above a source for petroleum. Figure 8: A symmetrical anticline forming a petroleum trap. The axis of the anticline runs N-S on the map with the limbs dipping east and west. Form The limbs of anticlines can dip in a wide variety of different ways. The orientation and nature of the anticline is important in determining where to drill on the anticline for hydrocarbons. 1-Mar-16 7 of 121 The Fossil Fuels: Petroleum I - Finding Oil Figure 9: Left: A symmetrical anticline. Right: An asymmetrical anticline Symmetrical Symmetrical anticlines are symmetrical with respect to a vertical axial plane, i.e. the limbs of the anticline dip in opposite directions but at the same angle. As a result, any hydrocarbon fluids are evenly distributed along the center of the anticline. Thus, the prime place to drill on these types of structures that bear oil is along the crest of the anticline. Figure 10: Cross-section through a symmetrical anticline showing that the optimum place to drill for hydrocarbons is along the crest of the structure. Asymmetrical Asymmetrical anticlines have limbs that fold dip at the different angles in opposite directions. In addition a plane through the center of the anticline is tilted from the vertical. Because of the difference in dip of the two limbs, hydrocarbon fluids trapped in such structures are not likely to be concentrated along the center of the anticline. Rather, they will be displaced toward the shallower limb side of the anticline. In this case, drilling along the crest of the anticline may actually miss the hydrocarbon accumulation and at the minimum would be displaced to one edge of the reservoir. Thus, drainage would not be optimum and additional development wells might have to be drilled and completed later to effectively produce the reservoir. 8 of 121 1-Mar-16 Petroleum Geology Figure 11: Cross-section through an asymmetrical anticline showing where to drill for hydrocarbons. Plunging Anticline If the hinge line, i.e. the intersection of the axial plane and the Earth’s surface, is not horizontal, the anticline is plunging. Plunging anticlines can be identified by the V type lithologic patterns they produce on a map. The direction of the plunge of the anticline is indicated by an arrow pointing in the direction of the plunge. If known, the dip of the plunge is noted on the map next to the arrow.

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