Abstract:
A sequence in the earth, defined by an upper sequence boundary and a lower sequence boundary, where at least one of the boundaries includes an unconformity, is modeled by creating a restored space corresponding to the sequence by reconstructing a portion of the sequence missing because of the unconformity. The sequence in restored space has a restored top boundary having a position and a restored base boundary having a position. Stratigraphic layering is performed within the restored space sequence, using as guides the positions of the restored top boundary and the restored base boundary of the sequence in restored space.

Description:
BACKGROUND 
       [0001]    Stratigraphic layering is a process used in the modeling of the earth for geoscience purposes whereby an earth model is enhanced by introducing layers between correlated sequence boundaries to increase the resolution of the model. A depositional sequence is a stratigraphic unit composed of genetically related strata that is bounded at its top and base by unconformities or their correlative surfaces. 
         [0002]    An earth model with stratigraphic layering can be used for high-resolution facies and/or property modeling. One goal of stratigraphic layering is to create a high-resolution subdivision of a sequence which properly reflects the internal layering of the sequence. Examples of existing approaches to stratigraphic layering are shown in  FIGS. 1-3 , which show an area-depth representation of a sequence bounded by an upper sequence boundary  105  and a lower sequence boundary  110 . In top down stratigraphic layering, illustrated in  FIG. 1 , the introduced layers, e.g.  115 , are parallel to the top sequence boundary  105 . In bottom up stratigraphic layering, illustrated in  FIG. 2 , the introduced layers, e.g.  205 , are parallel to the bottom sequence boundary  110 . In proportional stratigraphic layering, illustrated in  FIG. 3 , each of the introduced layers, e.g.  305 , varies in width. The width d of each introduced layer at any given point p is proportional to the distance D between the sequence boundaries at that point. 
         [0003]    Harry E. Wheeler, in an article entitled Baselevel, Lithosphere Surface, and Time-Stratigraphy, GEOLOGICAL SOCIETY OF AMERICA BULLETIN, v. 75, pp. 599-610 (July 1964) (hereinafter “Wheeler Article”), described a way to show sequences in area-time rather than the traditional area-depth view. In the area-time view, which will be referred to as Wheeler Space, “the vertical dimension is time [rather than depth] and . . . all lithosphere surface-moments expressing the succession of events are horizontal and parallel.” Wheeler Article at 606. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIGS. 1-3  illustrate prior art methods for stratigraphic layering. 
           [0005]      FIG. 4  illustrates a sequence bounded by unconformities. 
           [0006]      FIG. 5  illustrates the gaps (restored position of time lines) in the geological record of  FIG. 4 . 
           [0007]      FIG. 6  illustrates the sequence shown in  FIG. 4  in Wheeler space. 
           [0008]      FIG. 7  illustrates the sequence shown in  FIG. 5  in Pseudo-Wheeler space. 
           [0009]      FIG. 8  shows the sequence in  FIG. 5  (or  FIG. 6 ) after stratigraphic layering has been performed. 
           [0010]      FIG. 9  shows the sequence shown in  FIG. 8  transformed to area-depth space. 
           [0011]      FIG. 10  shows a flow chart for an example restored space stratigraphic layering system. 
           [0012]      FIG. 11  shows an example environment for a restored space stratigraphic layering system. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    An example of a representation of sequence  405  in an earth model is illustrated in  FIG. 4 . The sequence  405  is bounded on one side by an upper sequence boundary  410  and on the other side by a bottom sequence boundary  415 . 
         [0014]    The sequence  405  includes three sub-sequences A, B, and C, each of which is bounded by complex curved boundaries. Sub-sequence A is bounded by lower boundary AL and upper boundary AU. Sub-sequence B is bounded by lower boundary BL (which is the same as upper boundary AU) and upper boundary BU. Sub-sequence C is bounded by lower boundary CL (which is the same as upper boundary BU) and upper boundary CU. 
         [0015]    Both the upper sequence boundary  410  and the bottom sequence boundary  415  include unconformities,  420  and  425 , respectively. An unconformity is a surface of erosion or non-deposition that separates younger strata from older rocks and represents a significant hiatus. The three sub-sequences A, B, and C terminate at the unconformity  420  on the upper sequence boundary  410  and the unconformity  425  on the bottom sequence boundary  415  indicating gaps in the geological record on either side of the sequence, as illustrated in  FIG. 5 . The gaps represent gaps in the geological record within which deposition may have occurred and then eroded away or where deposition never occurred. 
         [0016]    The existing approaches to stratigraphic layering could use top down or bottom up stratigraphic layering for portions of the sequence  405  in which the top unconformity  420  and the bottom unconformity  425  are parallel to each other, and proportional layering for other portions of the sequence  405 . 
         [0017]    In one embodiment of a restored-space approach to stratigraphic layering the sequence is transformed from the area-depth space shown in  FIG. 4  to a restored space view. For example,  FIG. 6  shows the sequence  405  from  FIG. 4  transformed into Wheeler space, which is an example of restored space. The boundaries of intervals A, B, and C, which were complexly curved in  FIG. 4 , appear as horizontal lines in  FIG. 6 . The gaps  605  and  610  in the geological record in  FIG. 4  appear on either side of the sequence. In one embodiment, the transform into Wheeler space can be performed with an off-the-shelf tool, such as the OpendTect software available from dGB. 
         [0018]    Similarly,  FIG. 7  shows the sequence after it has been transformed into Pseudo-Wheeler space, which is an example of restored space. Pseudo-Wheeler space has the appearance of Wheeler space but the vertical axis is depth rather than time. In Pseudo-Wheeler space the unconformities are split to create gaps where sediments are no longer present, such as gaps  705  and  710  shown in  FIG. 7 , without translating into the area-time domain Pseudo-Wheeler space, like Wheeler space, is an example of a restored space in which the gaps (or missing section) in the geological record are restored, and as the result of this restoration (gapping), the top and base boundaries of the sequence (e.g., top boundary  715  and bottom boundary  720  of restored sub-sequence C, respectively) are approximately parallel and horizontal. In Wheeler space, the top and base boundaries of the sequence (e.g., top boundary  615  and bottom boundary  620  of restored sub-sequence C, respectively) are perfectly parallel and horizontal. In one embodiment, the transform into Pseudo-Wheeler space can be performed using the smartSection® tool available from the Landmark Graphics Division of Halliburton Corporation. 
         [0019]    In one embodiment, stratigraphic layering is performed on the restored space version of the sequence, as shown in  FIG. 8 , in which the dashed lines represent newly added boundaries, e.g.  805 , between new sub-sub-sequences, e.g.  810 . The stratigraphic layering of each restored sub-sequence is performed using as guides the location of its restored top boundary and its restored bottom boundary. Since the sub-sequence boundaries are approximately parallel in Pseudo-Wheeler space, the new stratigraphic layering boundaries can be introduced using proportional layering. Top-down layering and bottom-up layering can also be used in many Pseudo-Wheeler space models. The existing stratigraphic layering techniques can also be used in stratigraphic layering using a Wheeler space model. In one embodiment, the new boundaries extend into the gaps  605 ,  610  or  705 ,  710 , further simplifying the application of existing stratigraphic layering techniques. In one embodiment, the new boundaries do not extend into the gaps  605 ,  610  or  705 ,  710 . 
         [0020]    In one embodiment, the deposition of sediments over time is modeled in the restored space. For example, the technique of mass balancing may be applied. Mass (or area) balancing across a depositional sequence involves keeping track of sediment volumes (or areas) deposited in different positions within the basin over time. For example, mass balance analysis might attempt to track the proportion of total sediment deposited in shallow versus deep-water environments over time. By utilizing restored-space or pseudo-Wheeler space views, all sediments deposited and then eroded over time can be properly included in the mass balance calculations. 
         [0021]    In one embodiment, facies boundaries are identified within each stratigraphic layer in the restored space. For example, analysis of well logs may allow properties, such as porosity or resistivity, to be assigned to sub-sub-sequences, or portions of sub-sub-sequences. An analysis of the properties assigned to the various sequences in the model may allow facies boundaries, such as the boundary between marine and non-marine rocks, to be identified. 
         [0022]    In one embodiment, the representation of the sequence in restored space, such as that shown in  FIG. 8 , is transformed back into a traditional area-depth space, such as that shown in  FIG. 9 . Note that in  FIG. 9  the new boundaries represented by dashed lines conform to the sequence boundaries, shown by solid lines. 
         [0023]    In practice, as shown in  FIG. 10 , a restored space stratigraphic layering system operates on a sequence in traditional area-depth space by creating a restored space corresponding to the sequence (block  1005 ). The system then performs stratigraphic layering within the restored space sequence (block  1010 ). The system then models the deposition of sediments over time in the restored space (block  1015 ). The system then identifies facies within the restored space sequence (block  1020 ). The system then transforms the restored space sequence to traditional area-depth space (block  1025 ). 
         [0024]    In one embodiment, the restored space stratigraphic layering system is stored in the form of a computer program on a computer readable media  1105 , such as a CD or DVD, as shown in  FIG. 11 . In one embodiment a computer  1110  reads the computer program from the computer readable media  1105  through an input/output device  1115  and stores it in a memory  1120  where it is prepared for execution through compiling and linking, if necessary, and then executed. In one embodiment, the system accepts inputs through an input/output device  1115 , such as a keyboard, and provides outputs through an input/output device  1115 , such as a monitor or printer. In one embodiment, the system creates an earth model in memory  1120  or modifies an earth model that already exists in memory  1120 . 
         [0025]    In one embodiment, the earth model that resides in memory  1120  is made available through a network  1125  to a remote real time operating center  1130 . In one embodiment, the remote real time operating center makes the earth model, or data from the earth model, available through a network  1135  to help in the planning of oil wells  1140  or in the drilling of oil wells  1140 . 
         [0026]    The text above describes one or more specific embodiments of a broader invention. The invention also is carried out in a variety of alternate embodiments and thus is not limited to those described here. The foregoing description of the preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.