Seismic interpretationInterpreting seismic data requires an understanding of the subsurface formations and how they may affect wave reception. This article discusses some of the key stratal interfaces and their implications for interpreting the data received. Contents 1 Application of seismic attributes 2 Seismic stratigraphy 3 Types of stratal interfaces 3.1 Flooding surface 3.2 Maximum flooding surface 3.3 Erosion surface 4 Structural interpretation 5 Imaging reservoir targets 6 References 7 Noteworthy papers in OnePetro 8 External links 9 See alsoApplication of seismic attributes All instantaneous seismic attributes (/Seismic_attributes) (amplitude, phase, frequency) can be used in interpretation. In practice, most interpreters use instantaneous amplitude (/Glossary%3AInstantaneous_amplitude), or some variation of an amplitude attribute, as their primary diagnostic tool. Amplitude is related to reflectivity, which in turn is related to subsurface impedance contrasts. Thus, amplitude attributes provide information about all the rock, fluid, and formation-pressure (/Glossary%3AFormation_pressure) conditions listed in Table 1- Geological influences on acoustic impedance. (/Geological_influences_on_acoustic_impedance)Instantaneous phase (/Glossary%3AInstantaneous_phase) is useful for tracking reflection continuity and stratal surfaces (/Glossary%3AStratal_surface) across low-amplitude (/Glossary%3AAmplitude) areas where it is difficult to see details of reflection waveform character. In general, instantaneous phase is the least used of the seismic attributes (/Seismic_attributes). Instantaneous frequency (/Glossary%3AInstantaneous_frequency) sometimes aids in recognizing changes in bed thickness and bed spacing. Anomalous values of Instantaneous frequency (/Glossary%3AInstantaneous_frequency)(negative values or unbelievably high positive values) are particularly useful for recognizing: Edges of reservoir compartments Subtle faults Stratigraphic pinchouts Hardage[1] demonstrated these applications of Instantaneous frequency (/Glossary%3AInstantaneous_frequency). Back to topSeismic stratigraphy A stratal surface (/Glossary%3AStratal_surface) is a depositional bedding plane: a depositional surface that defines a fixed geologic time. A siliciclastic (/Siliciclastic_reservoir_geology) rock deposited in a high-accommodation environment contains numerous vertically stacked stratal surface (/Glossary%3AStratal_surface). A fundamental thesis of seismic stratigraphy is that a seismic reflection event follows an impedance contrast associated with a stratal surface (/Glossary%3AStratal_surface); that is, a seismic reflection is a surface that represents a fixed point in geologic time.[2][3] The term chronostratigraphic defines this type of seismic reflection event. Because lithology varies across the area spanned by a large depositional surface, the implication of this interpretation principle is that an areally pervasive seismic reflection event does not necessarily mark an impedance contrast boundary between two fixed rock types as that reflection traverses an area of interest. The application of this fundamental concept about the genetic origin of seismic reflections to seismic interpretation is referred to as stratal-surface seismic interpretation. Tipper[4] illustrated and discussed situations in which a seismic reflection can be either chronostratigraphic or diachronous (meaning that the event moves across depositional time surfaces), depending on the vertical spacings between beds, the lateral discontinuity between diachronous beds, and bed thickness. The conclusion that a seismic reflection is chronostratigraphic or diachronous needs to be made with caution because the answer depends on the local stratigraphy, the seismic bandwidth, and the horizontal and vertical resolution of the seismic data. If two seismic reflection events, A and B, are separated by an appreciable seismic time interval (a few hundred milliseconds) yet are conformable to each other (that is, they parallel each other), then the uniform seismic time thickness between these two events represents a constant and fixed period of geologic time throughout the seismic image space spanned by reflectors A and B. An implication of seismic stratigraphy that can be invoked in such an instance is that any seismic surface intermediate to A and B, which is also conformable to A and B, is also a stratal surface (/Glossary%3AStratal_surface). Back to topTypes of stratal interfaces Page 1 of 8 Seismic interpretation -7/28/2015 http://petrowiki.org/Seismic_interpretationA key first step in seismic interpretation is to use well logs and cores to identify the three types of stratal interfaces that exist in geologic intervals of interest: Flooding surfaces Maximum flooding surfaces Erosion surfaces Back to top looding surface Flooding surfaces are widespread interfaces that contain evidence of an upward, water-deepening facies dislocation, such as contact between the following: Rooted, unfossiliferous floodplain mudstones Overlying fossiliferous marine shale A ravinement surface is a specific type of flooding surface that suggests that transgressive passage of a surf zone has eroded underlying shallower-water facies. Back to topMa imum flooding surface Maximum flooding surfaces are interfaces that contain evidence of a widespread, upward, water-deepening facies dislocation that is associated with the inferred, deepest water facies encountered in a succession of strata. A maximum flooding surface is commonly represented by a thin condensed section, typically a black, organic-rich shale with a low-diversity fossil assemblage representing deepwater, sediment-starved conditions. Maximum flooding surfaces bound and define upward-coarsening facies successions that are called genetic sequences by Galloway.[5] These genetic sequences are similar to cycles or cyclothems in other terminology.[6] [7]Back to top rosion surface An erosion surface is an interface in which there is evidence of a facies offset that indicates that an abrupt decrease in water depth occurred. If an erosion surface is widespread, truncation of older strata can be documented on well log cross sections. Some of these surfaces may be disconformities representing downcutting during periods of subaerial exposure caused by allocyclic (extrabasinal) mechanisms, such as eustatic sea-level changes. These ma or chronostratigraphic surfaces are often manifested as mappable seismic reflections. All 3seismic data volumes should be calibrated with mappable, key surfaces recognized from cores and well logs, with priority given to: flooding surfaces, maximum flooding surfaces, and erosion surfaces Back to topStructural interpretation The original use of seismic reflection data (circa 1930 through 1960) was to create maps depicting the geometry of a subsurface structure. Because many of the world s largest oil and gas fields are positioned on structural highs, structural mapping has been, in a historical sense, the most important application of exploration seismic data. hen the seismic industry converted from analog to digital data recording in the mid-1960s, digital technology increased the dynamic range of reflected seismic signals and allowed seismic data to be used for applications other than structural mapping, such as: Stratigraphic imaging Pore-fluid estimation ithofacies mapping (/Glossary%3A ithofacies_map)These expanded seismic applications have led to the discovery of huge oil and gas reserves confined in subtle stratigraphic traps (/Glossary%3AStratigraphic_trap), and seismic exploration is now no longer limited to ust mapping the structural highs.However, even with the advances in seismic technology, structural mapping is still the first and most fundamental step in interpretation.hen 3seismic data are interpreted with modern computer workstations and interpretation software, structural mapping can be done quickly and accurately. ifferent seismic interpreters use different approaches and philosophies in their structural interpretations. The technique described here is particularly robust and well documented.[8] The first step of the procedure is to convert the 3seismic data volume that has to be interpreted to a 3coherency volume. oherency is a numerical measure of the lateral uniformity of seismic reflection character in a selected data window. As the waveform character of side-by-side seismic traces becomes more similar, the coherency value for the traces approaches a value of1.0; as the traces become more dissimilar, the coherency of the traces approaches zero. All modern seismic interpretation software can perform the numerical transform that converts 3seismic wiggle-trace data into a 3coherency volume. ig1 shows an example of a horizontal time slice through a 3coherency volume from the Gulf of Mexico. The narrow bands of low coherency values that extend across this time slice are created by faults that disrupt the lateral continuity of reflection events. Fault mapping is a ma or component of structural mapping, and this type of coherency display can be used to create fast, accurate fault maps. oherency technology has evolved into the optimal methodology for detecting and mapping structural faults in 3seismic image space. Page 2 of 8 Seismic interpretation -7/28/2015 http://petrowiki.org/Seismic_interpretation(/File%3A ol5_Page_0044_Image_0001.png) ig1 ori ontal slice through acoherency olume imaging a producing area in theulf of Me ico The second step of the structural interpretation procedure is to transfer the fault pattern defined by coherency data to the associated 3seismic wiggle-trace data volume.igillustrates the pro ection of the faults in ig1 onto a vertical profile through 3seismic image space. The coherency time slice in ig1 defines the, coordinates of each intersected fault at one constant, image-time coordinate across the image space. Additional coherency time slices are made at image-time intervals of 100 or 200 milliseconds to define the , coordinates of each fault as a function of imaging depth. This procedure causes the orientations and vertical extents of faults transferred to a 3seismic wiggle-trace volume to be quite accurate. (/File%3A ol5_Page_0045_Image_0001.png) ig ertical seismic slice along crossline T ofig 1 The first-order fault labeled in igextends through the entire stratigraphic column and create large vertical displacements of strata. The second-order faults have less vertical extent and cause less vertical displacement than the first-order faults. Other structural and stratigraphic features that are common in Gulf of Mexico geology are labeled. These features are identified to indicate the imaging capabilities of seismic data. Rollover indicates fault-related flexing of bedding, which results in structural trapping of hydrocarbons. The bright spot is an example of reflection amplitude reacting as a direct hydrocarbon indicator (see changes in pore fluid in Table ). The velocity sag feature is a false structural effect caused by anomalously low seismic propagation velocity that delays reflection arrival times, leaving the misleading appearance of a structural sag. The third step of this approach to structural mapping is to interpret a series of chronostratigraphic surfaces across the seismic image space. These surfaces can be any of the chronostratigraphic surfaces (flooding surfaces, maximum flooding surfaces, and erosion surfaces) described in Sec. 2.15, depending on the amount and quality of subsurface well control available to the interpreter. If there is no well control, interpreters must use their best udgment as to how to correlate equivalent strata across a seismic image space and then ad ust their interpretation, if necessary, as wells are drilled. hen a selected stratal surface (/Glossary%3AStratal_surface) is extended across the complete seismic image space, the geometrical configuration of that chronostratigraphic surface can be displayed as a structure map. The structure map in igis one of the chronostratigraphic surfaces interpreted across this Gulf of Mexico prospect with the fault geometry information defined by coherency slices ( ig1) and vertical slices ( ig ). The producing fields shown in the map are positioned on local structural highs associated with one or more first-order faults. (/File%3A ol5_Page_0046_Image_0001.png) igTime structure map of a deep reser oir system e hibiting considerable fault induced compartmentali ation Page 3 of 8 Seismic interpretation -7/28/2015 http://petrowiki.org/Seismic_interpretationIn the lower left of the map in ig , an arbitrary profile is shown crossing the fault swarm. igdisplays a vertical section along this profile to demonstrate the degree to which faults compartmentalize producing strata. This expanded view of the seismic reflection character also reveals critical stratigraphic features, such as lowstand wedges, that are embedded in the faulted structure. (A lowstand wedge is a sedimentary wedge deposited during a period of low sea level.) This type of seismic interpretation allows stratigraphers to construct detailed models of the internal architecture of targeted reservoir systems. (/File%3A ol5_Page_0047_Image_0001.png) ig ertical seismic slice along profile ig sho ing faulted stratigraphic features igshows a second structural map constructed from a shallower chronostratigraphic surface to illustrate that less fault compartmentalization is in shallow reservoirs than in the deeper reservoirs associated with the structure shown in ig . The first-order faults still displace strata at this shallow level, but most second-order faults have terminated at deeper depths and no longer cause reservoir compartmentalization. (/File%3A ol5_Page_0048_Image_0001.png) igShallotime structure map sho ing reduced influence of second order faults compared ith the deeper structure of ig The structure maps shown in igsand are time-structure maps. These maps can be converted to depth maps once seismic propagation velocities are determined through the stratigraphic column. Back to top maging reser oir targets igshows a data window from a vertical slice of a 3seismic data volume that is centered on a targeted channel system. These data include a good-quality reflection peak labeled "reference surface." The reference surface is a reference seismic stratal surface (/Glossary%3AStratal_surface) used to construct additional stratal surfaces (/Glossary%3AStratal_surface) that pass through the targeted thin-bed interval.[9] The fluvial system is embedded in the reflection peak that occurs at 0.73 seconds at inline coordinate 120. This particular reflection peak satisfies the fundamental criteria required of a reference stratal surface (/Glossary%3AStratal_surface) used to study thin-bed sequences: Event extends over the total 3image space and has a high signal-to-noise character Event is reasonably close to the targeted thin-bed sequences that need to be studied (i.e., the strata related to the anomalous reflection waveforms labeled hannel 1approximately 90 milliseconds above the reference surface) Event is conformable to (i.e., parallel to) this targeted thin-bed sequence The third criterion is the most important requirement for any seismic stratal surface (/Glossary%3AStratal_surface)that is to be used as a reference surface. Because this reference surface follows the apex of an areally continuous reflection peak, the basic premise of seismic stratigraphy is that this reference surface follows an impedance contrast that coincides with a stratal surface (/Glossary%3AStratal_surface). Page 4 of 8 Seismic interpretation -7/28/2015 http://petrowiki.org/Seismic_interpretation (/File%3A ol5_Page_0049_Image_0001.png) ig ata indofrom aertical slice of a seismic dataolume that is centered on a targeted channel system Channel 1 igdisplays this crossline section view with four conformable surfaces (A, B,, and ) that pass through the targeted thin-bed interval added to the profile. These four surfaces are, respectively, 92, 90, 88, and 86 milliseconds above and conformable to the reference surface. isual inspection of the reflection events above and below surfaces A, B,, and shows that all these reflection peaks and/or troughs are reasonably conformable to the reference surface event. Surfaces A, B,, and can thus be assumed to be stratal surfaces (/Glossary%3AStratal_surface), or constant-depositional-time surfaces, because they are conformable to a known stratal surface (/Glossary%3AStratal_surface) (the reference surface) and are embedded in a 200-millisecond seismic window in which all reflection events are approximately conformable to the selected reference surface. The highlighted data window encircles subtle changes in reflection waveform that identify the seismic channel facies.[9] (/File%3A ol5_Page_0050_Image_0001.png) ig ata indofrom the selectedertical slice sho ing stratal surfaces lossary AStratal surfacethat tra erse the channel system The circled features in igidentify locations where stratal surfaces (/Glossary%3AStratal_surface) A, B,, and intersect obvious variations in reflection waveform. These waveshape changes are the critical seismic reflection character that distinguishes channel facies from nonchannel facies, as can be verified by comparing the inline coordinates spanned by the circled features (coordinates 60 to 70) with these same inline coordinates where this crossline (number 174) intersects the channel features labeled " hannel 1" inig . (/File%3A ol5_Page_0051_Image_0001.png) ig eflection amplitude beha ior on stratal surface lossary AStratal surface hich ismilliseconds abo eand conformable tothe selected seismic reference surface igshows reflection-amplitude behavior on stratal surface (/Glossary%3AStratal_surface) B, which is 90 milliseconds above, and conformable to, the selected seismic reference surface. This surface shows portions of the hannel 1 system in the lower right quadrant of the image. A second channel system ( hannel 2) is located in the upper right quadrant.[9] The channel-system image shown in igis a surface-based image; that is, the seismic attribute that is displayed (which is reflection amplitude in this instance) is limited to a data window that vertically spans only one data sample. hen a 1-point-thick data window is a good approximation of a stratal surface (/Glossary%3AStratal_surface) that passes through the interior of a targeted thin-bed sequence, then the seismic attributes (/Seismic_attributes) defined on that surface can be important depictions of facies distributions within the sequence, as the image in this figure demonstrates. An alternate, and usually more rigorous, way of determining facies distributions within a thin-bed sequence is to calculate seismic attributes (/Seismic_attributes) in a data window that spans several data points vertically, yet is still confined (approximately) to only the thin-bed interval that needs to be studied. The bottom stratal surface Page 5 of 8 Seismic interpretation -7/28/2015 http://petrowiki.org/Seismic_interpretation Fig. 7 Fig. 7 Figs. 68 Fig. 8 Fig. 5 Fig. 8 Location of reference surface 2 on a second vertical slice (crossline 200) through the 3D seismic data volume.[9]Fig. 9 Fig. 8 Fig. 7 Fig. 9 Three-dimensional seismic image of the targeted thin-bed fluvial channel system.[9] References Noteworthy paers in OnePetro ternal lins See also