Good Seismic Report Report Example
The seismic lines on the graph are broken into four separate sequences. Each one is separated by a boundary. The boundaries are marked in green. Each sequence also contains three system tracts, which have been highlights in light purple, orange, and yellow. These colours represent highstand system tract, transgressive system tract, and lowstand system tract. Each of the sequences relates to one another in terms the mechanism of their deposition, as well as their relation to depositional environment. However, only sequence four will be analysed extensively.
The fourth sequence is positioned from CMP 678 and 1.01 seconds TWT to the seismic line’s end at 1.5 seconds TWT. The sequence boundary for the fourth sequence is divided into three system tracts. The primary, or first system tract is the Lowstand System Tract, hereafter referred to as the LST. It is signified by the yellow colouring on the chart, beginning at the base of the sequence boundary three of the seismic line, and traveling up toward the downlap relationship. Here the LST terminates, allowing an onlap relationship to continue. The onlap relationship terminates at CMP 680, 1.03 seconds TWT. LST is used to represent the progradational parasequence set of shorelines. This movement is where rates of accommodation must be less than sedimentation rate, causing a drop in sea level.
The second assessed tract, known as the Transgressive System Tract but hereafter referred to as the TST, is evident by the orange colour. It is expressed by the onlap/downlap relationship shown in the LST until onlap’s termination, occurring at CMP 680, 1.01 seconds TWT. The maximum regressive surface is the line separating the LST and TST. The TST defines the retrogradioational arasequence set, exemplifying net landwards movement of shorelines wherein rates of accommodation occur at quicker paces concerning sedimentation. This results in the sea level rising, as well as a finer grain size regarding sand.
Highstand System Tract, otherwise known as HST, is the last system tract. It can be verified by the light purple shade. The HST occurs right after the onlap terminates, and is defined by the TST up to sequence boundary four. The maximum flooding surface is the boundary line separating the TST and HST, and shows a coarsening sand trend. Some may interpret the area above the sequence boundary four to state that the HST is parallel, meaning the sea level has not changed in any major or noticeable way. However, the line’s top right area is unclear, making it difficult to state clearly whether HST continues depositing SB4 or not. Based on the evidence, however, I would guess that it does. The parallel beds are located toward the left area of the seismic line, and show similar patterns regarding the entire series. Typically, carbondale and shale would be the marine sediment left as a deposit. As a result, marine shale has been made organism rich. Marine shale makes competent reservoir seal, consequently, as do marine carbonate sediments. Marine carbonate sediments make decent reservoir seals because as mud material decreases sandstone rises upward. Accordingly, the bottom right of the shale or carbonate may form a seal rock, leaving the top to become a source rock.
Comparable explanations were found for others; the general explanation suggested the regression trend in areas where the sea level drops. It is thought to be because the rates of sedimentation happens more quickly, or on a larger scale, than the base monitored level rate. Concerning gran size distribution, horizontally and vertically, the TST must be disturbed. Vertically, grain size would be found finer until meeting with MFS, wherein grains would begin to become courser. As suggested by Walther’s Law, horizontally the same trend would occur; first the grains would fine latterly, then coarsen laterally. Walther’s Law technically states any sediment deposited beside one another will overlap over time due to changing sea levels. This will cause them to form many vertical beds. The law then assumes neither erosion or break sedimentation between either sequence will incur, nor will any rapid environment change happen. The law is illustrated below:
Figure 1: the illustration of Walther’s law (Crisp, n.d.)
Figure 1 shows if one moves downward toward on intersecting point, meeting at the intersecting vertical and horizontal line, the individual will experience the same sequence as if they moved horizontally across the plane.
Uncertainties and Conversation:
The local geology revolves around vertical velocity variation. Skipping cycles, calibration errors, caving, etc. would all impact sonic values that have been logged. Other factors include layer dipping, and the amount of space between receivers.
Lateral variation in velocity
Lateral variation in velocity may result in a change in lithology. If the lateral velocity is ignored when plotting data or when surface water (i.e. depth of water later) during sediment measurement changes it could change the results. For example it could cause a dip or rise in the results, as exemplified in the figure below.
Figure 2 shows how the thickness of water layer could generate false dip when the seismic line is converted from TWT to depth (Geldart & Sheriff, 1995).
Distance variation of sonic velocity measurements taken from a nearby field next to the location of the desired well (CMP 740), because the party may want to move from one area to another or one sequence boundary to another.
Apparent tilt for transgressive surfaces are generated by uncertainties. In other words, lateral variations in velocity, migration, and layer dipping cause tilts.
Vo + k Z method is used in order to reduce error risk during in depth conversions. It considers different layers arranged vertically and laterally, eliminating errors occurring because of lateral variation in velocity. The method, however, falls short because it uses interval velocity to consider depth. Bartel, et al. (2006) tell us that interval velocity is not advantageous. Rock velocity, for example, changes near the borehole due to sonic log measurements and other variables. However, this method does not account for borehole measurements, as demonstrated in the chart below.
Figure 3 shows VSP data for interval and average velocities, error bars are 1ft error in depth and 0.5ms error on time pick (Bartel et al., 2006).
Figure 3 VSP/checkshot errors are average; the interval velocity average’s error magnitude is 3,000ft/s.
Figure 4 shows the checkshot data with the edited data points for interval and average velocities (Bartel et al., 2006).
Figure four shows when data belonging to interval velocity are edited, the average velocity curve swings downward while on average velocity is negative.
In sum, I suggest using methods that use average velocity while combining Vo+kZ. It should also consider eliminating effects of migration and dipping by using dip-movement correction, as well as normal dip-movement correction for the most accurate answer. Geldart & Sherrif (1995) express this process as: NMO first used in order to correct a time delaying something that offsets traces, or assumes zerp dip then DMO is used, as well as a reflection point. Essentially, the primary error is in stacking velocities, but it can be minimized by average velocities. Lateral variation in velocity, dipping layers, and migration can be minimized by Vo + kZ method, as well as NMO and DMO corrections.
Bartel, D., M. Busby, J. Nealon, & J. Zaske. (2006). Time to Depth Conversion And Uncertainty Assessment Using Average Velocity Modeling [Accessed 19th March, 2013]. Available from: http://www.onepetro.org/mslib/servlet/onepetropreview?id=SEG-2006-2166#
Crisp, E. (n.d). ROCKS, FOSSILS, AND TIME or PRINCIPLES OF STRATIGRAPHY. [Accessed 18th March, 2013]. Available from: http://www.wvup.edu/ecrisp/g103lecstratpinciples.html
Geldart, L., R. Sheriff. (1995). Exploration Seismology. Second edition. Cambridge: Cambridge university press.
SEPMSTARTA. (n.d). Walther and his law. [Accessed 18th March, 2013]. Available from: http://www.sepmstrata.org/Terminology.aspx?id=Walther