Abstract:
In order to improve the tie between depositionally equivalent beds relative to two or more basins detected within a multi dimensional seismic volume of interest, pseudo logs based on the average of attributes derived from seismic impedance where the compaction trend is not present are created for each basin. The mean is taken over all available azimuths, following the structural variations of introduced micro layers. The correlation between the pseudo log relative to each basin enable a more reliable interpretation between the different basins from which sound exploration decision can be made. Such a process has been successfully applied to seismic data acquired in deep water environment.

Description:
FIELD OF THE INVENTION 
     The invention relates to the creation of pseudo logs based on attributes derived from seismic impedance data to improve the correlation of depositionally equivalent beds between two or more basins. 
     BACKGROUND OF THE INVENTION 
     One of the first steps in hydrocarbon exploration is to generate a consistent stratigraphic framework by interpretation of the post stack processed seismic volume to be explored. This includes the identification of depositionally equivalent beds. 
     In some structurally complex regions, such as deep water depositional environments, this first step may prove challenging because of the structural complexity which can cause differential deposition (i.e., varying thickness) and stratigraphic unconformities or discontinuities between beds with common times of deposition. By way of example, where a seismic volume includes two or more separate basins, beds and/or horizons bounding the beds within the separate basins may have been deposited at the same time, but correlation between such beds may be difficult to determine because of the unconformities, discontinuities (missing sections) or variable thicknesses between the basins. 
     Conventional techniques exist for analyzing seismic data and correlating between (i.e., tieing) separate basins. These techniques include identifying and correlating features such as surfaces (beds) and events located in the separate basins that have common times of deposition. However, the accuracy of these techniques in correlating stratigraphically equivalent events may be incorrect and/or lacking in some instances. 
     The definitive methodology for determining an accurate tie between basins is drilling a well in each basin, logging the well and sampling the data from the well bore. This is time intensive and expensive in terms of equipment and man hours. 
     SUMMARY 
     One aspect of the invention relates to a system and method for providing correlation between depositionally equivalent subsurface events between separate basins. In one embodiment, the provision of correlation between depositionally equivalent subsurface events is accomplished by operations comprising (a) obtaining a set of seismic amplitude data representing a seismic volume of interest acquired in a deep water environment, wherein the dimensions of the set of seismic data are (i) a two-dimensional position on a surface plane of the seismic volume of interest, (ii) a parameter related to seismic time and (iii) a parameter related to the amplitude derived from the signal arriving at a point in the data set defined by (i) and (ii); (b) identifying a plurality of basins in the volume of seismic amplitude data, the plurality of basins including a first basin and a second basin; (c) obtaining values for an impedance parameter related to one or both of acoustic and/or elastic impedance for locations within the volume of seismic data; (d) identifying stratigraphic layers within the first basin; (e) identifying stratigraphic layers within the second basin; (f) introducing micro-layers in between and/or within the stratigraphic layers identified in (d) for the first basin and (e) for the second basin, (g) taking the mean of the impedance parameter within each micro layer; (h) obtaining a pseudo log for each of the first basin and the second basin including the mean values obtained at (g); (i) correlating the pseudo log for the first basin with the pseudo log for the second basin; and (j) adjusting an interpretation regarding the continuity of the layers between the first basin and the second basin based on the correlation performed at (i). 
     These and other objects, features, and characteristics of the present invention, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention. As used in the specification and in the claims, the singular form of “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a time slice of a set of seismic amplitude data, according to one or more embodiments of the invention. 
         FIG. 2  illustrates a cross-sectional view of a line of seismic amplitude data crossing two basins, according to one or more embodiments of the invention. 
         FIG. 3  illustrates a view of acoustic impedance data derived from a set of seismic amplitude data, in accordance with one or more embodiments of the invention. 
         FIG. 4  illustrates a line of seismic amplitude data  4 A and acoustic impedance data  4 B related to the same portion of the seismic volume of interest, in accordance with one or more embodiments of the invention. 
         FIG. 5  illustrates a diagram showing a deep water depositional environment, according to one or more embodiments of the invention. 
         FIG. 6  illustrates the workflow used in this invention to tie a seismic volume of interest together by correlating layers within separate basins that have common times of deposition, in accordance with one or more embodiments of the invention. 
         FIG. 7  illustrates a section of a set of seismic amplitude data showing surfaces mapped along similar stratigraphic events then used to introduce microlayers, according to one or more embodiments of the invention. 
         FIGS. 8A and 8B  illustrate the flattened versions of  FIGS. 4A and 4B , according to one or more embodiments of the invention. 
         FIG. 9  illustrates a correlation of pseudo-logs related to acoustic impedance between two separate basins identified in  FIGS. 1 and 2 , according to one or more embodiments of the invention. 
         FIG. 10  illustrates a correlation of impedance information between two separate basins, according to one or more embodiments of the invention. 
         FIG. 11  illustrates an initial interpretation of seismic data from two basins, in accordance with one or more embodiments of the invention. 
         FIG. 12  illustrates an adjusted interpretation of seismic data from two basins, according to one or more embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The dimensions of seismic amplitude data representing a seismic volume of interest generally include position along a horizontal surface (or some other surface) and a parameter related to seismic time or seismic depth. Where the seismic data is three-dimensional (e.g., a seismic “data cube”), the position along a horizontal surface may be parameterized by a pair of positional parameters that describe position on a surface plane of the corresponding seismic volume of interest (e.g., an x-y position along a horizontal plane), with the amplitude of the seismic data arranged along an axis perpendicular to the surface plane of reference. 
       FIG. 1  illustrates a view of a seismic data cube  10  representing a seismic volume of interest taken along a single value for the parameter related to seismic time (e.g., a horizontal slice through seismic data cube  10 ). Generally, this type of view is known as a “time slice.” As can be seen, the seismic volume of interest includes two separate basins, labeled in  FIG. 1  as a first basin B 1  and a second basin B 2 . Although, first basin B 1  and second basin B 2  are illustrated as being directly adjacent, this is not intended to be limiting. The principles discussed below with respect to first basin B 1  and second basin B 2  could be applied to any two or more basins (or sub-basins) located within a common depositional environment. In the illustration provided in  FIG. 1 , seismic data cube  10  depicts a seismic volume of interest (including first basin B 1  and second basin B 2 ) disposed within a common deep water depositional environment. Although some aspects of the analysis discussed with respect to seismic data cube  10  are specific to this setting, it should be appreciated that the principles described below may be applied generically within other depositional settings where continuous and homogeneous layers can be identified, such as chalk for example. The scope of this disclosure includes analysis of seismic data obtained from these other depositional settings performed in accordance with the principles described. 
       FIG. 2  illustrates a sectional view of data cube  10  taken along section line C-D shown in  FIG. 1 . In the description below, various properties illustrated in the two-dimensional view of data cube  10  shown in  FIG. 2  are discussed. It should be apparent that these properties apply not only to the two-dimensional section shown in  FIG. 2 , but throughout the three-dimensional data cube  10 , and that the discussion of these properties with respect to the two dimensions shown is for illustrative purposes only. Further, it should be appreciated that the discussion below regarding the analysis performed in three dimensions could be applied to a two dimensional data set describing the subsurface structure of a single cross-section of the seismic volume of interest. 
     In the view of data cube  10  shown in  FIG. 2 , it can be seen that the seismic volume of interest includes a plurality of horizons, labeled in  FIG. 2  as H 1 -H 6 . A horizon is a surface formed at a boundary between two layers of differing composition within the strata of a seismic volume of interest. Since horizons H 1 -H 6  represent boundary changes in the composition of the strata of the seismic volume of interest, it is assumed for analysis purposes that each horizon H 1 -H 6  represents a surface within the seismic volume of interest that delineate geologic layers deposited at a common chronostratigraphic time. This is reasonable because the impetus for a composition change in the strata being deposited in one area of the seismic volume of interest would likely be an impetus for a similar change in the composition of strata being deposited in another area of the seismic volume of interest. Particularly, where the boundaries indicating such a change in composition for both areas connect and/or are of similar depth within the seismic volume of interest. 
       FIG. 3  is a depiction of acoustic impedance data that corresponds to a portion of the second basin B 2 . Acoustic impedance provides a metric related more directly to the layer properties within the seismic volume of interest, in contrast to the seismic amplitude data that primarily indicates boundaries between the beds.  FIG. 3  illustrates the manner in which beds having similar rock properties are arranged in layers, and how the boundaries of these layers form horizons H 1 -H 6  shown in  FIG. 2 . 
       FIGS. 4A and 4B  illustrate the relationship between seismic data and acoustic impedance.  FIGS. 4A and 4B  show two data sets depicting a portion of the section of data cube  10  of second basin B 2  shown in  FIGS. 2 and 3 . In the first data set,  FIG. 4A , formed from seismic amplitude data, a bed having common rock properties appears as a pair of adjacent, and separate, horizons. By contrast, in the data set formed from acoustic impedance,  FIG. 4B , the same bed is shown as a single unit having common rock properties, and being bounded on each side by the horizons shown in the first data set. Acoustic impedance is obtained through inversion of seismic data. Inversion is a known process, and can be performed, for example, on post-stack data, angle stack data, and/or other seismic data. 
     Referring back to  FIG. 2 , not only do horizons H 1 -H 6  within second basin B 2  correspond to individual chronostratigraphic times, but other basins in the same depositional environment will also include horizons that correspond to one or more of H 1 -H 6 . This is because the other basins in the same depositional environment as second basin B 2  will have been subjected to some of the same drivers of sediment deposition as second basin B 2 . By way of illustration,  FIG. 2  shows a set of horizons present in the first basin B 1 , some of which may correspond to horizons H 1 -H 6  in the second basin B 2 . Correlating such horizons, and the layers they bound, between first basin B 1  and second basin B 2  is desirable for a variety of reasons. For example, it ensures the proper identification of the same layer in each basin. Such identification is then used to make drilling decisions that encompass several millions of dollars. However, as can be seen in  FIG. 2 , a cursory inspection of data cube  10  does not enable horizons within first basin B 1  and horizons H 1 -H 6  in second basin B 2  that were deposited at common chronostratigraphic times to be correlated because corresponding horizons and layers within basins B 1  and B 2  may be found at different points along the dimension of the seismic parameter of data cube  10 . 
     In order to correlate horizons and/or layers in the first basin B 1  and the second basin B 2 , geologic markers common to both the first basin B 1  and the second basin B 2  are identified and correlated between the basins B 1  and B 2 . The geologic markers discussed below, referred to as condensed sections, are specific to deep water depositional environments. This is not intended to be limiting, and the methodology discussed below is applicable to identify other types of geologic markers found within other depositional environments. 
       FIG. 5  provides an illustration of an exemplary deep water depositional environment. Generally in deep water depositional environments (e.g., in the deposition of basins B 1  and B 2 ), material that is deposited within a basin is derived from sands, shales, and debris eroded from the neighboring shelf margin. These materials may be deposited by streams and deltas flowing into the deeper water. When sea level is relatively low, material is eroded from the continent and/or the shelf margin, and a relatively large amount of material (sands or shales) is deposited into the deep water basins. When sea level is high, less material is eroded from the shelf margins and therefore less is deposited into the basins. Typically, the materials that are deposited into the basin at high sea levels (high stand) are very fine scale materials such as fine organics and silts. The deposition is very slow, but relatively consistent; making a very thin layer of mud and organic debris. This fine organic rich material creates a relatively homogeneous thin layer within all basins affected by the sea level rise. The layer has similar rock properties and is regional in its extent. The layers formed in this manner during periods of high sea levels are referred to as “condensed sections.” 
     Condensed sections are typically regional in extent, and their homogeneous rock properties exhibit similar characteristics on seismic amplitude data. As such, condensed sections provide geologic markers for use in identifying the type of sediments (sands vs shales) and in correlating between separate basins. Identifying the condensed sections within each of basins B 1  and B 2  enables the corresponding seismic volumes to be “tied” together. When correlated accurately, those skilled in the art should understand the timing of the deposition of each basin, which will in turn enable a reconstruction of the depositional history of one or both of basins B 1  and/or B 2 . Consistent depositional events similar to those found in the deep water could also exist in other depositional environments in a regional area. Therefore, the description of the identification and correlation of geological markers consisting of condensed sections provided herein is not intended to be limiting, and the methodology discussed below could be adapted to identify other types of geologic markers found within other depositional environments. 
       FIG. 6  illustrates a method  20  of tying a seismic volume of interest together by correlating layers within separate basins that have common times of deposition. The operations of method  20  presented below are intended to be illustrative. In some embodiments, method  20  may be accomplished with one or more additional operations not described, and/or without one or more of the operations discussed. Additionally, the order in which the operations of method  20  are illustrated in  FIG. 6  and described below is not intended to be limiting. 
     In some embodiments, method  20  may be implemented in one or more processing devices (e.g., a digital processor, an analog processor, a digital circuit designed to process information, an analog circuit designed to process information, a state machine, and/or other mechanisms for electronically processing information). The one or more processing devices may include one or more modules executing some or all of the operations of method  20  in response to instructions stored electronically on an electronic storage medium. The one or more processing modules may include one or more devices configured through hardware, firmware, and/or software to be specifically designed for execution of one or more of the operations of method  20 . 
     In one embodiment, method  20  includes an operation  22 , at which a set of seismic data representing a seismic amplitude data volume of interest is obtained. The dimensions of the set of seismic data include a pair of positional parameters that describe position on a surface plane of the seismic volume of interest (e.g., an x-y position along a horizontal plane), and a parameter related to seismic time arranged along an axis that is orthogonal to the surface plane. The parameter related to seismic time may include, for example, seismic time or seismic depth. The set of seismic data also includes one or more parameters related to the propagation and/or reflection of seismic energy within the seismic volume of interest, such as, for example, amplitude, frequency, phase, and/or other parameters. As such, in one embodiment, the set of seismic amplitude data is a seismic data cube representing the volume of interest. In another embodiment, the set of seismic amplitude data is reduced to a single coordinate in the horizontal surface resulting in a two dimensional data set. 
     At operation  24 , a plurality of basins within the seismic amplitude data volume of interest is identified. In one embodiment, the plurality of basins includes a first basin and a second basin. 
     At operation  26 , values for an impedance parameter related to one or both of acoustic impedance and/or elastic impedance are determined for locations within the seismic volume of interest. These values are determined through inversion of the seismic amplitude included in the set of seismic data. The impedance parameter may include acoustic impedance or elastic impedance, and/or other parameters related to acoustic impedance and/or elastic impedance. 
     At operation  28 , a determination is made as to whether the inversion of seismic data to determine values for the impedance parameter at operation  26  was model-based or trace-based is made. If the determination was model-based, then method  20  proceeds to an operation  30 , at which the low frequency component (i.e., the compaction trend) is removed. If the determination at operation  26  was trace-based, then method  20  proceeds to an operation  32 . 
     At operation  32 , stratigraphic layers in the seismic volume of interest that are represented in the set of seismic data, are interpreted. This includes interpreting stratigraphic layers in the plurality of basins identified at operation  24 . In one embodiment, operation  32  includes interpreting stratigraphic layers in the seismic volume of interest through analysis of the impedance parameter as a function of position within the set of seismic data, as determined at operation  26 . In one embodiment, operation  32  includes interpreting horizons directly from the set of seismic data. Interpreting a given stratigraphic layer at operation  32  includes interpreting an upper layer boundary and a lower layer boundary, and associating locations represented in the set of seismic data that are between the upper layer boundary and the lower layer boundary with the given layer. It may also include a first interpretation regarding the correspondence of the stratigraphic layers between each basin. 
     At operation  34 , in the zone of interest, a plurality of non-intersecting surfaces are introduced between and eventually within each of the layers identified at operation  32 . Surfaces can also be introduced above the top of the shallowest stratigraphic layer identified at operation  32  and/or below the bottom of the deepest stratigraphic layer identified at operation  32 . Surfaces may not need to be introduced within each or any of the stratigraphic layers identified at operation  32  depending on their thickness. For purposes of illustration,  FIG. 7  shows a section of a set of seismic amplitude data where the surface  36  delineates a condensed section while the surface  40  is above another condensed section. The plurality of non-intersecting surfaces with the reference numeral  42  defines micro layers, similarly to the surfaces  36 ,  38  and  40 . As can be seen in  FIG. 7 , surfaces  42  are distributed somewhat uniformly and the shapes of the individual surfaces  42  are determined to maintain a somewhat proportional distance between adjacent surfaces  42 . As such, the shapes of surfaces  42  correspond in some regard to upper boundary  38  and lower boundary  40 . Generally, surfaces  42  are interpreted at intervals that correspond (at least roughly) to the time sampling rate of the set of seismic data (e.g., 4 milliseconds). However, surfaces  42  may be interpreted at larger intervals, such as an integer multiple of the time sampling period in some circumstances (e.g., when the seismic data are noisy or of poor quality). Such an operation is referred to by those skilled in the art as proportional slicing and the intermediate surfaces are referred to as micro layers. 
     Referring back to  FIG. 6 , at operation  44 , the mean of the values for the acoustic or elastic impedance corresponding to the micro layers is taken. In one embodiment, operation  44  includes “flattening” the set of impedance parameter data to facilitate the averaging required to determine the mean of actual values of the impedance parameters within the micro layers defined at operation  34 . “Flattening” this data means stretching the data so that layers and/or horizons depicted in the data become generally horizontal, rather than tilted and/or curved. By way of illustration,  FIGS. 8A and 8B  show flattened versions of the sections of the seismic and impedance parameter data representing B 2  shown in  FIG. 4 . 
     By way of illustration,  FIG. 9  shows schematically the process of taking the mean of the acoustic or elastic impedance when the seismic data are three dimensional. For simplicity, the micro layer  48  is shown horizontal/flattened; in practice it can be curved. In one embodiment for three dimensional data, taking the mean of the acoustic or elastic impedance is performed over all available azimuths. In one embodiment for two dimensional data, the mean is taken in the single plane of the data. The operation of taking the mean is performed successively for each basin of interest, identified at step  24 . For each basin, the mean value obtained for each micro layer is assigned to a single trace  50  referred to thereafter as a pseudo log of acoustic or elastic impedance. A vertical trace  50  indicates a trace through the set of seismic data. The x-y position of trace  50  becomes irrelevant with respect to the impedance parameter because no matter what the x-y position of trace  50  is the value of the impedance parameter along the trace is the average determined for that micro-layer in the data set of operation  44 . 
     Referring back to  FIG. 6 , at an operation  52 , pseudo logs of impedance data are obtained from each basin to be correlated. As was set forth above, the pseudo log is an accumulation of the average values determined for a given micro-layer in operation  44 . 
     At an operation  54 , geological markers (e.g., condensed sections) are identified and correlated between basins from the pseudo logs obtained at operation  52 . There will likely be a relatively large range of actual values for the impedance parameter a surface within a micro layer in the zone of interest of the seismic volume that is not a condensed section, with the actual values being centered around a baseline measurement (corresponding to zero acoustic impedance). As a result, determination at operation  44  of a mean for the values of the acoustic or elastic impedance parameter within a micro layer that is not within or simply a condensed section will result in an average value for the impedance parameter that is close to the baseline measurement (e.g., corresponding to zero acoustic impedance). Conversely, as was set forth above, condensed sections tend to have rock properties (reflected in the impedance parameter) that are relatively homogeneous. Accordingly, there will typically be relatively little deviation between the value for the impedance parameter of a micro layer within a condensed section and the actual values of the impedance parameter. Further, due to the homogeneity within condensed sections, the actual values of the impedance parameter will likely not be centered around the baseline measurement. As a result, the average values of the impedance parameter of the pseudo logs taken at operation  44  corresponding to condensed sections will be substantially greater than and/or less than the baseline measurement, while the average values of the impedance parameter taken at operation  44  within micro layers that are not part of condensed sections will generally be substantially equivalent to the baseline measurement. 
     The evaluation of pseudo logs of acoustic or elastic impedance facilitates the correlation  54  between the separate basins (or sub-basins) of interest within the seismic data volume. For example,  FIG. 10  provides a comparison of the pseudo logs of acoustic impedance parameter evaluated through each of first basin B 1  and second basin B 2 . Because of the processing provided by method  44 , horizons in first basin B 1  that correspond to horizons H 1 -H 6  in B 2  can be correlated as indicated (with the correlation coefficients provided). 
     Returning to  FIG. 6 , at an operation  56 , once the evaluation of pseudo logs has facilitated the correlation of layers, the overall interpretation of the seismic volume of interest is updated in accordance with the correlation results. For example,  FIG. 11  illustrates a view of seismic data cube  10  showing an initial interpretation of the seismic data in which areas  58   a ,  60   a ,  62   a , and  64   a  in first basin B 1  are correlated with areas  58   b ,  60   b ,  62   b , and  64   b  in second basin B 2 . However, upon performance of method  20 , the seismic data was reinterpreted, resulting in the interpretation shown in  FIG. 12  in which each of areas  60   a ,  62   a , and  58   b  is reconfigured in accordance with the correlation results of operation  54 . 
     Although the invention has been described in detail for the purpose of illustration based on what is currently considered to be the most practical and preferred embodiments, it is to be understood that such detail is solely for that purpose and that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover modifications and equivalent arrangements that are within the spirit and scope of the appended claims. For example, it is to be understood that the present invention contemplates that, to the extent possible, one or more features of any embodiment can be combined with one or more features of any other embodiment.