Growth Faults and Syndepostional Tectonics - Coursework Example

Here is the work I’ve done following your outline and the scope of your requirements…it’s the best I can do..hope it would be a big help…..thanks” Introduction Petroleum Industry Contributions It all started in 1947, when Kerr-McGee made the first Gulf discovery at Ship Shoal 32, about 10 miles off the Louisiana coast in 18 feet of water. The firm is credited with setting the first offshore platform in the Gulf at its new field. Amazingly, that first oil field is still producing 56 years later -- and it is not alone. Many of the earliest discoveries in the Gulf are still producing today. Over the years companies have found ways to make Gulf fields of all sizes commercially viable. Of course, that was not difficult at the region’s largest field discovery, which came just two years ago when BP uncovered the giant deepwater field, Thunder Horse, in the Mississippi Canyon and Eugene Island areas. The firm estimated reserves at more than one billion barrels. But even the MO 830 field in the Mobile Bay area was commercially viable with just 82,384 estimated barrels of oil equivalent in reserves. Certainly, the deep water has been the Gulf’s exploration focus for the last decade, and that’s where many of the largest fields have been uncovered. What most people might not know is that the first discovery in water depths over 1,000 feet came in 1979 at Shell’s Cognac field in the Green Canyon area. Since then, 146 deepwater fields have been discovered. Technology has played an important role in the deepwater evolution of the Gulf and discoveries of entirely new geologic frontiers. Exploration geoscientists working in the northern Gulf of Mexico and adjacent areas developed insights into relationships between hydrocarbon occurrences and regional/ global events such as growth-fault/ allochthonous salt evolution and sea-level/ climatic cycles. Historically in the Gulf of Mexico, exploration plays have been categorized into geographic areas such as onshore, flex trend, eastern Gulf of Mexico, and deep water. Technical modifiers such as amplitude, nonamplitude, and sub-salt have also been added. Geophysical, geologic, paleontologic and drilling technologies have all played an important role at one time or another. The term Miocene is a pivotal interval in the history of the Cenozoic. Within its nearly 19 million years, profound oceanographic and climatic changes occurred. These include the transition from globally more uniform environments of the Paleogene, to the modern world where extreme climatic and oceanographic contrasts are the norm. Important Miocene climatic changes are reflected by the increasing importance of higher frequency cycles of deposition in the Gulf of Mexico. Deeper drilling technology pushed the search for upper Miocene deltaic plays onto the continental shelf. A Major change in technology occurred in the early 1970s with the advancement of true amplitude processing of reflection seismic data. "Bright Spot" exploration strategy launched a brief era of "pure" geophysical prospecting. Amplitude-related plays were chased all over the shelf and into the uppermost deep-water areas of the northern Gulf continental slope in the 1980s. 3-D seismic data became common place on the shelf and upper-slope during this time. By the 1990s discoveries on the shelf had slowed, while deep-water discoveries began to take off. Seismic advancements during the past two decades include dip move out, advanced post-stack time migration algorithms, turning wave migration, 3-D amplitude versus offset processing, ray-trace analysis, post-stack depth migration, pre-stack time migration, and pre-stack depth migration. Also, industry has been willing to acquire 3-D data sets for production as well as exploration tools. Introduction of "seismic stratigraphy" in 1977 drove the development of basin-fill and new submarine fan facies models. Major differences have been recognized between submarine fan sections in third-and fourth-order sequences deposited on a second-order relative fall of sea level, as opposed to submarine fan sections deposited on a second-order rise. Advances in biostratigraphy in the past two decades have greatly improved zonations and have allowed sequence stratigraphy to develop as an effective exploration tool. Modern computer technology has breathed new life into old exploratory field techniques such as gravity. Computer generated second-vertical derivative (SVD) gravity maps can now contrast low density salt with higher density sediment-filled minibasins, providing a high-resolution virtual image of shelf and slope structures. Location of Oil Associated with Rollover Anticlines Diagram of parts of synclines and anticlines Anticline is a wave-like fold structures include pairs of anticlines and synclines. Axial surfaces are imaginary surfaces that bisect the limbs of folds. Why is it necessary to understand the structures of sedimentary basins? The structures formed in sedimentary basins are known to act  as traps for hydrocarbons (ie natural gas and petroleum). These deposits are highly sought after as they provide great wealth to those who exploit them. The processes that lead to the formation of these deposits are not well understood, but it is known that they are associated with rifting zones. As we have seen, rift zones correspond to regions under extension and therefore sedimentary basins. Hydrocarbon traps form in permeable layers of  rock (such as sandstone) called reservoir rocks that are 'capped' by impermeable rocks (such as shale). Since water is denser than both petroleum and gas, they will not mix. Water will then force these fossil fuels through permeable rocks until they pool together in traps formed by cap rocks. Illustrated below are just a few of the types of hydrocarbon traps that can be found in sedimentary basins. As we have seen already, the structural geology of an extensional basin can be quite complex. This is also true for the types of traps. Therefore the examples that will be given are simplified versions that are typical of those found in reality. Fault traps These traps are one of the most common to be found in extensional basins. They form when a normal fault causes the displacement of reservoir rocks such that where the bed would have originally continued on as a permeable layer, it is now cut-off by the fault and instead is capped by an impermable bed. Hydrocarbons such as natural gas and oil will pool in this trap, especially if it is dipping at some angle. Fig1 A fault trap illustrating how faulting can cause reservoir rocks to become a trap for hydrocarbons Pinch-out traps These trap styles are also common in extensional basins. Unlike fault traps, these traps are purely the result of deposition processes rather than structural ones. They form as a reservoir rock layer simply tapers off into a cap rock. Water will then force hydrocarbons into these traps from below. Fig This diagram illustrates a pinch-out trap (top-right) and a fault trap (left) Anticlinal Traps These traps are diagnostic of basins that have had some compressional forces applied on them that cause folding of the sedimentary layers. These traps are therefore not particularly common in basins that have only been formed in extensional regimes. However, as discussed in the previous section, when listric faults occur, roll-over anticlines may form. These structures if large enough may in fact become traps for hydrocarbons. Fig. This is an example of an anticlinal trap. The may result from folding, irregular deposition, or in roll-over anticlines. Other Trap Types There are many other types of traps that are mined for hydrocarbons, too numerous to mention in this report. However, illustrated below are two other trap types common to extensional basins. In the centre of the image is an unconformity trap. These form when deposition of a cap rock occurs above tilted beds of reservoir rocks following a period of erosion. To the left of this is a tilted fault-block trap. These are typically formed by the rotation of domino faults mentioned in the previous section. If a permeable layer is now capped by an impermeable layer, an oil trap may form. Fig Diagram illustrating an unconfomity and tilted fault-block trap. An anticlinal trap can also be seen at the top of the image Subsidence along Gulf Coast Wetland losses are so extensive in the Gulf of Mexico Coast region of the United States that they represent critical concerns to government environmental agencies and natural resource managers. Each year, millions of dollars are spent in coastal Louisiana alone to restore wetlands and to maintain the natural ecosystem that is vital to the Nation's economy. Figure 1. Possible effects of petroleum production. Prolonged or rapid production of oil, gas, and formation water (2) causes subsurface formation pressures to decline (3). The lowered pressures (3) increase the effective stress of the overburden (4), which causes compaction of the reservoir rocks and may cause formerly active faults (1) to be reactivated (5). Either compaction of the strata or downward displacement along faults can cause land-surface subsidence (6). Where subsidence and fault reactivation occur in wetland areas, the wetlands typically are submerged and changed to open water (7). Figure is not to scale. D, down; U, up. Wetland subsidence and fault reactivation induced by oil and gas production generally have been disregarded in the Gulf Coast region because much of the wetland loss occurs in Louisiana, where many other factors contribute to coastal change (Williams and others, 1994). Understanding the influence of hydrocarbon production on wetland changes is important for predicting future wetland conditions and for planning wetland restoration projects. Hydrocarbon production is interpreted to cause movement along faults and wetland subsidence (fig. 1) if the following situations co-occur: 1. Large areas of wetland are lost at the same time and in the same places as hydrocarbons are produced 2. The cumulative volumes of fluids produced are so large that they lead to rapid declines in subsurface pressures 3. Subsurface faults near the producing reservoirs and surface faults activated after initial production have the same orientation and direction of displacement 4. Subsidence rates measured near the oil and gas wells during production are substantially higher than geological rates of subsidence Each condition by itself does not directly link wetland loss, subsidence, and hydrocarbon production, but together they are compelling indicators of causality. Subsidence and Hydrocarbon Production In the Gulf Coast region, subsidence was first linked to hydrocarbon production in the mid-1920's at the Goose Creek Field near Galveston, Tex. (Pratt and Johnson, 1926). Subsidence at Goose Creek of about 1 meter (m) was enough to change the setting from a vegetated upland to open water. Figure 2. Air photograph of the Port Neches oil and gas field in Texas in 1978 and maps showing that wetlands above the field were healthy and continuous in 1956 but deteriorated and were converted to open water by 1978. The change was caused by induced subsidence and fault reactivation resulting from hydrocarbon production. From White and Morton (1997). Relative motion along faults: D, down; U, up. A similar pattern of production-induced subsidence, which led to wetland replacement by open water, occurred at the Port Neches Field, Tex., between 1956 and 1978 (figs. 2 and 3). The similarity in area of wetland loss in 1978 compared with present conditions suggests that the subsidence was rapid initially but then slowed or possibly stopped. Figure 3. Cumulative hydrocarbon production in the Port Neches Field, Tex., from 1930 to 1994, compared with changes in faults and wetlands observed in air photographs. From White and Morton (1997). Wetlands began rapidly disappearing when the field began rapidly producing large volumes of gas in the early 1950's. Wetland loss slowed after 1978, when hydrocarbon production rates declined rapidly. Oil production is in millions of barrels; gas production is in billions of cubic feet (to convert to cubic meters, multiply cubic feet by 0.02832). Induced subsidence cannot be sustained indefinitely. Instead, the duration of surface adjustment is related to the history of production. As shown in figure 3, there was a time gap between the onset of production and the first visible evidence of surface disturbance and wetland loss. Whatever losses are occurring today, they are occurring at a much slower rate than when the wetlands deteriorated between 1956 and 1978. This reduction in rates of subsidence corresponds to the rapid decline in hydrocarbon production. Regional Depressurization (Result of Oil, Gas & Water Pumping) When large volumes of oil, gas, and associated formation water are extracted from the subsurface, the natural pressures in the reservoirs are reduced (fig. 4) and stresses around the reservoir increase (fig. 1). The increased stresses cause reservoir compaction, which, in places, leads to surface subsidence. Nearly 20 billion barrels of oil and more than 150 trillion cubic feet (4.2 trillion cubic meters) of gas have been produced from coastal Texas and Louisiana since the 1920's. Although the fluid production is concentrated within the field areas, the effect of the pressure decline extends far beyond the individual fields. Where multiple fields are producing from the same strata, regional depressurization can cause subsidence and wetland losses in the areas between the fields (Kreitler and others, 1988). Consequently, induced subsidence can be either near the fields (fig. 2) or away from the fields. Figure 4. Subsurface pressures measured in gas wells in the Port Neches Field and North Port Neches Field, Tex. (unpub. data from Fred Wang, University of Texas at Austin, Bureau of Economic Geology, 2001). Low subsurface pressures like those graphed can lead to increased overburden stress, compaction of the strata, reactivation of faults, and land-surface subsidence. Depths are in feet and meters; pressures are in pounds per square inch and megapascals. Throughout the Gulf Coast region there are many deep faults that serve as structural traps for the hydrocarbons. Some of the primary faults that trap hydrocarbons also extend upward to shallow depths near the surface (fig. 1). If the pressure drop in the producing formation is large, faults that are near the threshold of failure may be reactivated, and rocks along them may move. When the fault is active, the land area subsides on the downthrown side of the fault near the fault plane. Depending on the depth and angle of the fault, the induced subsidence may occur several kilometers away from the producing wells (fig. 1) rather than directly above the producing reservoirs (fig. 2). Subsidence and fault displacement also can be inferred from benchmark releveling surveys. The elevations of benchmarks in the Gulf Coast region are periodically resurveyed by the National Geodetic Survey. Comparing leveling surveys provides a basis for measuring rates of subsidence for the period between the dates of the surveys. For example, releveling surveys in 1965 and 1982 along Louisiana Highway 1 between Valentine and Leeville, La., showed that subsidence was greater near hydrocarbon-producing fields than between the fields (fig. 5). Figure 5. Map along Louisiana Highway 1 between Valentine and Leeville, La., showing locations of benchmarks, oil and gas fields, and shoreline and wetland losses and graph showing changes in surface elevation (in millimeters) at the benchmarks between 1965 and 1982. Subsidence was greatest near the oil and gas fields. Wetland losses from Britsch and Dunbar (1993). Elevation changes from National Geodetic Survey data. Subsidence and fault displacement around producing fields also can be measured directly by using basic coring techniques. Correlation of shallow stratigraphic markers in sediment cores provides a basis for determining the magnitude of displacement relative to some datum such as mean higher high water. Figure 6 shows that the correlated beds have been displaced progressively downward 35-63 centimeters (cm) comparedto the adjacent marsh surface. Preservationof the marsh sediments beneath ameter of water is clear evidence that themarsh surface subsided first to form theopen water area. Subsequent erosion ofthe marsh sediments was minor comparedto subsidence. One way of detecting artificially induced subsidence around a producing field is by comparing the measured rate of subsidence there with natural subsidence rates in the same region. The wetland surface at Port Neches subsided about 63 cm in 22 years (figs. 2 and 6). These values yield a historical rate of wetland subsidence of about 3 cm/yr. Some of the highest natural rates of subsidence in the Mississippi Delta of southern Louisiana are on the order of 1 cm/yr (Roberts and others, 1994). Thus, the short-term historical subsidence rate at Port Neches (3 cm/yr) is three times higher than the highest subsidence rates in a region that is known for rapid subsidence. It is 75 times higher than the subsidence rate of 0.04 cm/yr in southwestern Louisiana, which is the closest area having both a similar geologic setting and estimated subsidence rates. Environmental Implications Subsidence of 1 m or less induced by oil and gas production is not great, especially when compared to the extreme examples of subsidence such as 9 m at Wilmington, Calif., or 4.5 m at the Ekofisk Field in the North Sea. Nevertheless, the relatively small observed reductions in elevation in the Gulf Coast region are sufficient to cause dramatic changes in the affected wetland ecosystems. This demonstrates that even minor lowering of the marsh surface can translate to large wetland losses through a combination of coastal plain subsidence and marsh sediment erosion. Models of Growth Faults For shale/salt-cored anticlines, three different geometric models aid significantly in interpreting areas of poor quality seismic data and in estimating the amount of horizontal shortening and of material that was mobile. Three different geometric models can be applied: only detachment folding, detachment folding with a withdrawal basin, and a withdrawal basin only with no detachment folding. All three models result in different dip patterns, rate of growth of the anticline and associated layer-parallel strains in the pre-growth sediments. In the case of overfill of growth sediments, limb dips in the growth sediments and layer-parallel strains (elongation) decrease upward to zero at the top of the growth sediments. The elongating growth sediments would facilitate the development of normal faults that die out upward and propagate downward into elongating pre-growth sediments and die out without becoming layer-parallel. The models predict the amount of stretching at the propagating fault tip. Forward modeling of the anticlines, using the calculated/observed ductility, can then be performed to model the development of the normal fault with time. In listric normal, growth faults, changes in the curvature of growth axial surfaces can reflect changes in sedimentation rate, fault displacement rate, and/or differential compaction of hanging wall growth strata. Antithetic faults that form at the fault bend commonly propagate to the top of growth sediments present at that time. Thus, the upper termination of each antithetic fault reflects the position of the growth axial surface when that antithetic fault formed and the resulting location and shape of the growth axial surface. When combined with high-resolution chronostratigraphy, these models represents a powerful tool for interpreting the history of listric normal faults and detachment folds and for understanding possible charge, reservoir, and trap relationships in the Gulf of Mexico Hydrocarbon Province. Salt deformation plays a large role in the spatial and temporal distribution of reservoirs as well as hydrocarbon generation, migration, and entrapment. Breakthroughs in modern evaluation of salt tectonics include two basic concepts. First, differential loading drives salt tectonics and the process of diapirism is therefore more passive than had been believed. Second, lateral flow is locally important. A major component of salt tectonics is development of salt sheets, tongues, and allochthonous nappes. Passive diapirism, differential loading, extension, contraction, strike-slip faulting, and near-diapir faulting may trigger salt deformation. Different salt styles control trap styles in supra- and subsalt environments and have varying effects on sediment transport, deposition, and on hydrocarbon generation and migration. Better predictive models for reservoirs will be based on improved knowledge of mechanisms of salt (and overburden) deformation. Rock salt forms a weak layer between other lithologies and deforms as a viscous fluid. The strength and brittle nature of overburden means that density contrasts play only a limited role in salt tectonics. Salt should be viewed as a pressurized fluid and it is differential fluid pressure that drives salt flow. Salt forms an excellent detachment surface into which faults sole. Several processes are known to thin or weaken overburden or create pathways and space for salt to move into. Processes include passive diapirism, salt movement triggered by differential loading, extension, contraction, strike-slip faulting, and the presence of allochthonous salt. Differential loading produced by emplacement of a depositional lobe induces a differential fluid pressure that drives salt withdrawal and minibasin formation. Salt moves laterally into flanking areas and a bathymetric high forms adjacent to the minibasin. A feedback process is created and the minibasin receives additional sediments. The process continues until the suprasalt minibasin touches down on the subsalt strata, forming a salt weld. At that time, minibasin subsidence ceases. Once the minibasin touches down, subsidence shifts from the center of the basin to the flanks, which are still underlain by salt. Both flanks subside, creating an anticlinal turtle structure. Subsequent depocenters form on both sides of the early depocenter. A phenomenon that occurs after the deposition of some sediments such that different parts of the sedimentary accumulation develop different degrees of porosity or settle unevenly during burial beneath successive layers of sediment. This can result from location on an uneven surface, such as near and over a reef structure, or near a growth fault, or from different susceptibility to compaction. The porosity in a formation that has experienced differential compaction can vary considerably from one area to another. The differential compaction caused by shifting depocenters throughout this interval establishes a system of changing pressure gradients that influence lateral migration pathways of both salt and hydrocarbons. Changing accumulation rates and related pressure changes also directly affect hydrocarbon maturation and generation by modifying the temperature and pressure history of buried source rocks. GROWTH FAULT A type of normal fault that develops and continues to move during sedimentation and typically has thicker strata on the downthrown, hanging wall side of the fault than in the footwall. Growth faults are common in the Gulf of Mexico and in other areas where the crust is subsiding rapidly or being pulled apart. Schematic diagram of growth fault Growth faults are a particular type of normal fault that develops during ongoing sedimentation, so the strata on the hanging wall side of the fault tend to be thicker than those on the footwall side. Growth Fault is  a fault in sedimentary rock that forms contemporaneously and continuously with deposition so that the throw increases with depth and the strata of the downthrown side are thicker than the correlative strata of the upthrown side. They are particularly common in areas of high sedimentation rate and are generally associated with thick deltaic successions. Growth faults are relatively small, about 15 km to 20 km wide and 45 km to 50 km long. As sedimentation on the delta front is strongly influenced by relative change of sea level, it generally considered that sea level change also influences the formation of growth faults. Intense cyclonic storms may change sedimentation pattern near the delta lobes and may a effect the formation of growth faults. They are important for oil and gas exploration and development. Normally huge quantity of liquid and gaseous hydrocarbon deposits is found in traps associated with growth fault. The Interactions between growth faults and sedimentations An experimental basin built at St. Anthony Falls Laboratory of the University of Minnesota, is designed to model the erosional and depositional development of successions in basins associated with variations in sediment supply, base-level change and rates and geometries of subsidence. The results of the first experiment in a prototype basin (1 x 1.6 x 0.8 m) are described here, wherein the stratigraphic development associated with slow and then rapid base-level cycles in a basin with a sag geometry has been analyzed. Videotape of the experiment as well as subsequent serial slicing, photographing, and peeling of the dried strata in the basin allow interpretation of the sequence development under conditions of precisely known changes of absolute base-level, subsidence and sedimentation. Of these the only controlling parameter varied was absolute base level. The resultant fill developed under eight phases of imposed base-level change: (1) a period of no change (10 hrs.) allowing the shoreline to reach an equilibrium position; (2) a period of slow base-level fall (12 hrs.); followed by (3) a period of prolonged still stand or very slow rise (6 hrs.); (4) a period of slow base-level rise (12 hrs.) in turn followed by; (5) a period of no changes (6 hrs.) allowing shoreline to again reach an equilibrium position; (6) a period of rapid base-level fall (1.25 hrs.) followed immediately by; (7) an identical period of rapid base-level rise; and (8) a final equilibrium period (5.5 hrs.) after which the basin was drained, dried, sliced, and photographed. Changes in development of the basinal stratigraphy occurred in response to imposed base-level changes. Absolute base level had less of an effect than the combined effect of base level and subsidence (relative base-level changes). Autocyclic changes in the sedimentary system developed over short time scales, took place in the fluvial, shoreline, and slope systems although no variation in any of the controlling parameters were imposed. Sequence boundaries of differing style developed during slow and fast base-level falls. Incised valleys that formed during these times cut preexisting fluvial and deltaic deposits. During the slow fall, subsidence in the center of the basin was always great enough to cause a relative base-level rise. Incised valley development did not begin until the shoreline prograded out of the zone of relative base-level rise into the zone of base-level fall. However, once initiated, erosion cut rapidly headwards due to strong channelized flow and weak substrate. As erosion took place at the nick point, deposition occurred farther out in the basin, so that both the erosion front and the depositional fronts migrated landwards. This resulted in a single sequence boundary that had a time-transgressive history. During the rapid base-level fall, incision began upstream and grew downstream over time. Sediment deposited at the delta front were gradually exposed and incised as the fall continued. Wholesale backfilling of the incised valley did not begin until the rapid base-level rise started. As a result, the incised valley grew longer by cutting both headward and downstream over time and the total duration of incised valley development was much longer, by two fold, than the incised valley that developed during the slow base-level fall. Well-developed growth faults formed synchronous with the rapid base-level fall and during the final equilibrium period of no base-level change. The faults have a characteristic listric geometry, accommodated moderate slip, and apparently sole into coal horizons. They most likely formed due to gravity sliding down toward the basin, as with growth faults in delta slope deposits worldwide. Reservoir development within the strata is evaluated by means of gray-scale proxy for porosity. Four distinctive zones of enhanced reservoir quality occurred in the basin including: the most proximal part of the basin; the upper part of growth-fault bounded sedimentary wedges; deep-water forced regressive systems tract composed of grainflow deposits; and transgressive systems tract formed during the rapid base-level rise. This distribution of relatively porous units, all but the first of which are sharply transitional into overlying impermeable deposits, suggests that for a variety of reasons rapid sea-level cycles may produce the best reservoir units. Operation of protoype basin. SURFACE OF EXPERIMENTAL BASIN DURING RAPID AND SLOW BASE-LEVEL CHANGE The topographic development during the base level falls. STRATIGRAPHIC CROSS SECTIONS - Map view of basin showing location of three lines highlighted below. Serial slicing of basin at 2.5 cm (1 inch) intervals. Key surfaces across the basin. This block diagram shows three-dimensional perspective of the basin fill. Source area is from the back wall. FOCUS ON GROWTH FAULTS & FLOODING SURFACE - Growth faults formed before and at the beginning of the rapid base-level fall Thickening upward cycles formed during rapid transgression Normal fault with antithetic and synthetic faults Antithetic fault. Laboratory experiments to produce normal faults can simulate nature closely. This sequence of photographs from a laboratory experiment shows progressive development of a growth fault as well as synthetic and antithetic faults. An antithetic fault is a minor, secondary fault, usually one of a set, whose sense of displacement is opposite to its associated major and synthetic faults. Antithetic-synthetic fault sets are typical in areas of normal faulting. Gravity failure along the upper slope generated syndepositional faults that displaced highly mobile muds basinward of the growing lowstand sedimentary wedge. These growth faults trend generally northeast-southwest, parallel to the coastline, setting up subbasins. Higher frequency, fourth-order, sea-level fluctuations initiated even smaller minisubbasins with sedimentary and fault patterns similar to those of the larger-scale, third-order subbasins. References Britsch, L.D., and Dunbar, J.B., 1993, Land loss rates; Louisiana coastal plain: Journal of Coastal Research, v. 9, p. 324-338. Kreitler, C.W., Akhter, M.S., Donnelly, A.C.A., and Wood, W.T., 1988, Hydrogeology of formations used for deep-well injection, Texas Gulf Coast: Austin, Tex., University of Texas at Austin Bureau of Economic Geology, 204 p. (Report prepared for the U.S. Environmental Protection Agency under Cooperative Agreement No. CR812786-01-0.)>Pratt, W.E., and Johnson, D.W., 1926, Local subsidence of the Goose Creek oil field: Journal of Geology, v. 34, p. 577-590. Roberts, H.H., Bailey, Alan, and Kuecher, G.J., 1994, Subsidence in the Mississippi River Delta—Important influences of valley filling by cyclic deposition, primary consolidation phenomena, and early diagenesis: Gulf Coast Association of Geological Societies Transactions, v. 44, p. 619-629. White, W.A., and Morton, R.A., 1997, Wetland losses related to fault movement and hydrocarbon production, southeastern Texas coast: Journal of Coastal Research, v. 13, p. 1305-1320. Williams, S.J., Penland, Shea, and Roberts, H.H., 1994, Processes affecting coastal wetland loss in the Louisiana deltaic plain, in Williams, S.J., and Cichon, H.A., eds., Processes of coastal wetlands loss in Louisiana...as presented at Coastal Zone '93, New Orleans, Louisiana: U.S. Geological Survey Open-File Report 94-0275, p. 21-29. From Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu). For more information, please contact the author. American Association of Petroleum Geologists Annual Meeting, Dallas, April 18-21, 2004 Read More

The term Miocene is a pivotal interval in the history of the Cenozoic. Within its nearly 19 million years, profound oceanographic and climatic changes occurred. These include the transition from globally more uniform environments of the Paleogene, to the modern world where extreme climatic and oceanographic contrasts are the norm. Important Miocene climatic changes are reflected by the increasing importance of higher frequency cycles of deposition in the Gulf of Mexico. Deeper drilling technology pushed the search for upper Miocene deltaic plays onto the continental shelf.

A Major change in technology occurred in the early 1970s with the advancement of true amplitude processing of reflection seismic data. "Bright Spot" exploration strategy launched a brief era of "pure" geophysical prospecting. Amplitude-related plays were chased all over the shelf and into the uppermost deep-water areas of the northern Gulf continental slope in the 1980s. 3-D seismic data became common place on the shelf and upper-slope during this time. By the 1990s discoveries on the shelf had slowed, while deep-water discoveries began to take off.

Seismic advancements during the past two decades include dip move out, advanced post-stack time migration algorithms, turning wave migration, 3-D amplitude versus offset processing, ray-trace analysis, post-stack depth migration, pre-stack time migration, and pre-stack depth migration. Also, industry has been willing to acquire 3-D data sets for production as well as exploration tools. Introduction of "seismic stratigraphy" in 1977 drove the development of basin-fill and new submarine fan facies models.

Major differences have been recognized between submarine fan sections in third-and fourth-order sequences deposited on a second-order relative fall of sea level, as opposed to submarine fan sections deposited on a second-order rise. Advances in biostratigraphy in the past two decades have greatly improved zonations and have allowed sequence stratigraphy to develop as an effective exploration tool. Modern computer technology has breathed new life into old exploratory field techniques such as gravity.

Computer generated second-vertical derivative (SVD) gravity maps can now contrast low density salt with higher density sediment-filled minibasins, providing a high-resolution virtual image of shelf and slope structures. Location of Oil Associated with Rollover Anticlines Diagram of parts of synclines and anticlines Anticline is a wave-like fold structures include pairs of anticlines and synclines. Axial surfaces are imaginary surfaces that bisect the limbs of folds. Why is it necessary to understand the structures of sedimentary basins?

The structures formed in sedimentary basins are known to act  as traps for hydrocarbons (ie natural gas and petroleum). These deposits are highly sought after as they provide great wealth to those who exploit them. The processes that lead to the formation of these deposits are not well understood, but it is known that they are associated with rifting zones. As we have seen, rift zones correspond to regions under extension and therefore sedimentary basins. Hydrocarbon traps form in permeable layers of  rock (such as sandstone) called reservoir rocks that are 'capped' by impermeable rocks (such as shale).

Since water is denser than both petroleum and gas, they will not mix. Water will then force these fossil fuels through permeable rocks until they pool together in traps formed by cap rocks. Illustrated below are just a few of the types of hydrocarbon traps that can be found in sedimentary basins. As we have seen already, the structural geology of an extensional basin can be quite complex. This is also true for the types of traps. Therefore the examples that will be given are simplified versions that are typical of those found in reality.

Fault traps These traps are one of the most common to be found in extensional basins. They form when a normal fault causes the displacement of reservoir rocks such that where the bed would have originally continued on as a permeable layer, it is now cut-off by the fault and instead is capped by an impermable bed.

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