CA1208535A - Method for hydrocarbon reservoir identification - Google Patents

Abstract

METHOD FOR HYDROCARBON
RESERVOIR IDENTIFICATION

ABSTRACT OF THE DISCLOSURE
A method for processing three-dimensional seismic data to define areal boundaries of hydrocarbon reservoirs. 3-D
Seismic data for an area having a producing well is examined to establish an attribute critical parameter that specifi- .
cally relates to the reservoir structure. Having identified a parameter, the 3-D data can then be examined in a selected time window for presence of the critical parameter and X, Y
gridding of its relative value. The relative values are then output to visual plot to provide a plan view of reser-voir structure.

Description

^` ~ZO~S35 METHOD FOR HYDROCARBON
RESERVOIR IDENTIFICATION

BACKGROUND OF THE INVENTION
5 1. Field of the Invention The invention relates generally to processing of three-dimensional seismic data and, more particularly, but not by way of limitation, it relates to an improved method for color analysis of 3-D seismic data to identify extent and location of oil and gas reservoir structure.

2. Description of the Prior Art The prior art has seen various types of 3-D seismic surveying schemes with varying forms of data treatment and it i8 further known to utilize color differentiation to enhance certdin types of three-dimensional or two-dimensional seismic data displays. Applicants presently know of no process for identification of particular reser-voir seismic attributes which then enable an enlarged reser-voir display through treatment and color di~play of the selected seismic attribute data.
SUMMARY OF THE INVENTION
The present invention relates to an improved method for evaluating seismic data that includes known producing strata for identification of specific attributes which may then be used to examine for similar attributes in adjoining three-dimensional seismic data. Thus, a three-dimensional survey, i.e. plural, parallel survey lines having known spacing relationship, and which include within their 3-D section volume a known producing well and reservoir, is further exa-mined to derive specific seismic attribute data for use as a - 1- ~

12(~3S35 standard in testing adjacent subterrain data to establish attribute similarities. The present invention is carried out utilizing commercially available computer equipment in conjunction with known forms of video plotter, such equip-ment being programmed to carry out the method of the presentinvention~
Therefore, it is an object of the present invention to provide a method for examining three-dimensional seismic data to establish existence and boundaries of reservoirs containing the hydrocarbons.
It is also an object of the present invention to pro-vide a method of producing a color display of a three-dimensional earth sector with color differentiated indication of oil and gas reservoir sub-strata.
F~nally, it is an object of the present invention to provide a digital computer implemented digital process for examining exi~ting three-dimensional seismic data in rela-tion to known and established producing hydrocarbon wells within the sector thereby to identify other reservoir volu-mes or continuation and extent of reservoir volumes.
Other objecta and advantages of the invention will be evident from the following detailed description when read in conjunction with the accompanying drawings which illustrate the invention~
BRIEF UESCRIPTION OF THE DRAWINGS
FIG. 1 is an idealized view of a sector of earth sur-face beneath a water covered area having well bores indi-cated therein;
FIG. 2 is a flow diagram of the program implementation for carrying out the method of the present invention;

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FIG. 3 is an output display of three-dimensional seismic data for a selected time window of the earth volume of FIG. l;
FIG. 4 is an output display of three-dimensional seismic data for a shallower time window through the earth volume of FIG. l; and FIG. 5 is a black and white representation of a multi-color display, including gradient color bar, as constructed in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
In general, the present invention provides a data pro-cessing technique for examining seismic data in the area of an established well or known hydrocarbon reservoir in order to establish areal boundaries of such sub-surface reservoir lS as well as to a~certain spacing and location for additional wells in the area. After establishment of a first, success-ful well drilling at a particular area, the present inven-tion utilizes re-examination of related seismic data, especially three-dimensional seismic data, thereby to further enlarge information about the size and shape of the hydrocarbon reservoir. Information derived is then valuable in placing offset wells with maximum success a~d reliabi-lity, proving to be a great aid in the development of oil and gas fields.
The technique may be divided generally into three steps. First, previous seismic data, e.g. 3-D data, is exa-mined to identify seismic attributes from the reservoir reflection package that can be used thereafter to define a critical parameter for that reservoir. For example, pre-vious data may show that any of pay thickness, stratum poro-sity, etc. give rise to a critical parameter that is identifiable to distinguish the oil-bearing stratum or reservoir sector. In a given case, which is exemplified throughout this presentation, seismic modeling studies of data in the area of a newly established well showed that the amplitude of the trough of data reflected from the top of the reservoir stratum was related to the amount of pay in the reservoirs.
In the second ste~ then, the seismic data was examined to establish a time window or series of time windows across

3-D seismic data in which the critical parameter would be present relative to the reservoir data. A search of the seismic data within the defined time windows is then carried out for the seismic attribute of interest, in the example here it was examined for critical trough amplitude, and the trou~h a~plitude values of the seismic attribute are saved for each trace. Then, the seismic data is searched to pick hi~hest trough amplitudes within a single defined time win-dow and this amplitude is saved for each trace o~ the three-dimensional array.
Finally, step three carries out the display of theseismic attribute values relative to one another on a plan view display, i.e. generally in a horizontal plan disposi-tion of X-Y coordinate and the relati~e X, ~ position within the 3-D survey of each attribute is used to generate such a plan view display. The relative values of the attributes are shown by mapping the values to a defined color bar. In a case to be discussed hereinafter, the trough amplitudes were displayed in color differentiation, particularly in scaled Applicon plot6 with color indicating the value of each trough amplitude relative to the rest of the trough amplitudes.
FIG. 1 illustrates a water-covered earth area 10 haviny sea bottom 12 and subterranean earth structure 14 therebe-low. For illustration purposes, a particular porous sandstratum 16 having upper interface 18 and lower interface 20 is shown wi~hin earth structure 14.
A wild cat or first well 22 has been drilled as from sea bottom bore entry 24 to a successful pay zone or reser-voir area 42 within porous stratum 160 Selection ofdrilling location for weil bore 38 would have been made by conventlonal practices utilizing prior seismic data interpretations or the like. In any event, the well 38 has proven to be a valuable well with probable good reserves of hydrocarbon. Additional offset dry holes 30 and 32 were each drilled after successive data evaluations and well site selectlons at sea bottom locations 36 and 38, respectively.
Utilizing the method of the present invention, a fourth well 22 at sea bottom location 24 was drilled to an up dip for-matlon of porous stratum 16, as at area 26, and this wellal~o proved to be good pay as previously indicated by color display of attribute data constructed in accordance with the present method. Since the first well development, and not illustrated in FIG. 1, additional successful oil and gas wells have been drllled in the same field depicted in FIG.
1, and such drill sites have been selected in accordance with reservoir attribute evaluation in accordance with the present invention.
In an earth sector of interest, such as earth sector 10 of FIG. 1, 3-D seismic energy data may be available or it lZ08S35 , .

may be specifically obtained for the purpose of evaluating the sector. Thus, and referring to sector 10, a plurality of spatially related seismic sections would be run along a plurality of parallel survey lines 50, 52, 54 . c . 56 which extend across the ~urface of earth sector 10 to a selected termination point or boundary such as line 58. In the case of earth sector 10, the seismic surveys would be run by linear marine sounding traverses as a line of successive source generations and signal receptions along each survey line is effected. After pre-processing, dynamic correction, normalization, stacking or whatever the selected procedures, the seismic data along each survey line would be established as a seismic section, probably of common depth point aligned data. Further proces~ing with observance to the spacing 15 between survey lines 50, 52, 54, etc., then enables cross relationship of the data into a three-dimensional data set.
In performing the present method, the 3-D data set reuresenting in ~ubstance the matter as shown yenerally in FIG. 1, would be considered to determine a critical para-meter exhibiting a hybrid seismic response for that dataaround reservoir area 26, the known hydrocarbon producer and the original well drilled in the sector 10. There are various seismic data processing methods and treatments which may be utillzed by the geophysicist to isolate the desirable critical parameter and it will probably result that the operator will actually have a choice of critical parameters for use, and for proving result with balance of one critical parameter against the other. In the subject case, as pre-viously mentioned, a very clear cut critical parameter ~as indicated by the maximum amplitudes oi the data trough as ~08535 reflected from the top of the reservoir stratum as this was directly related to the amount of pay in the reservoir structure.
FIG. 2 illustrates a basic data processing flow diagram which is readily implemented by the skilled artisan to per-form the necessary processing and outputing steps to imple-ment the present invention. Present practice utilizes a proyrammed digital computer, a Model 760 CYBER Digital Computer as is commercially available from Control Data Corporation. And visual output of the display is effected on such as an Applicon Plotter, available from Applicon, Incorporated of Burlington, Massachusetts. Other digital computers and image processing equipment may readily he programmed and utilized to carry out the invention ~o the similar result. In particular, the I2S Model 70 Image Procas~ Computer as produced by International Imaging Syetems, is particularly effective as it enables a very large range of color hue, value and chroma assignments.
Referring to FIG. 2, the seismic data is input at stage 70 as pre-processed and assembled three-dimensional seismic data for the earth volume of interest. The seismic data input will have been previously examined to select the desired critical parameter; for example, in the example ca~e, previous seismic modeling studies reveal that the amplitude of the trough reflected from the top of the reser-voir was related to the amount of pay within the reservoir.
A time window is then selected from the known downward tra-vel ti~es in the seismic data, such window being selected to envelope the travel time to the top of the reservoir struc-ture of interest.

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Each individual trace of the input 3-D seismic traces is then examined through the time window portion to select a critical parameter value, e.g. the largest, within the win-dow and the value for each trace is output to stage 76 whereupon the time window trace value is stored at a selected grid position that i5 related to the X-Y coor-dinates of the earth sector, e.g. a horizontal slice through earth sector 10 at approximately the depth of reservoir area 26. Once the grid is filled for the total cros~-section, the grid of values is scaled in some selected manner, i.e.
the range of values for the critical parameter as assigned to res~ective ones of a plurality of color quality values, e.g. inten~ity, hue or the like is carried out in stage 78.
The scaled grid values are then output to display in stage lS 80 to reveal a plan view indication of the critical para-meter of interest. the operation on the aeismic data may be carried out a plurality of times at a plurality of different selected time windows, each selection of which is intended to reveal still more information as to the dip and/or con-volution of the oil bearing earth structure. With each dif-ferent time window, new and different information relative to critical parameter of reservoir top structures will be revealed, and a directionally related pattern soon develops to reveal the essentlal structure acros3 the cntire earth sector that i9 being examined.
FIGs. 3, 4 and 5 represent actual output displays for the earth sector 10 of FIG. 1. In FIGs. 3 and 4, black and white are reversed for purposes of the depiction. Keep in mind that the original well 22 gave rise to the subse~went examinat~on in accordance with the present invention with a 12V85~5 view towards developing the field and drilling additional wells. Previous data inspection showed that the amplitude of the troughs was the critical parameter and 3-D seismic data for the sector was examined in the manner of the inven-tion. An examination of peaks of troughs at a time windowextending from 2.175 - 2.260 seconds depth resulted in a display pattern such as that shown in FIG. 4. It can be readily noted that the original oil well 22 is exactly on indication and that the previously selected dry holes 30 and 32 have indeed missed the reservoir sands. In FIG. 3, a subsequent examination of the 3-D data at a selected time window of 2.268 - 2.372 seconds depth, i.e. a position deeper than the previous time cut dictated by reservoir area 26, revealed a down-dip of the reservoir as well as a very large and consistent reservoir lower indication. A sub-sequent drilling of well 38 based on the indication of FIG.

4 proved out in that a good producing oil well was obtained.
FIG. 5 provides a black and white illustration of a multi-color output display of the data ~imilar to that out-put in black and white in FIGs. 3 and 4. The same 3-D
seismic data is utilized and examined over a number o~
selected time windows bracketing the approximate depth of the top of the reservoir of interest, interface 18 of FIG.
1, and the detected maximum trouyh values for each tirne win dow are scaled on the grid. Output of the grid is then carried out on such as the Applicon Plotter with grid output values ranging from dark magenta at color bar 82 through the red, yellow and cyan hues to a bright color bar white 84.
As can be seen from the tonal gradations, there are thirteen different hue/intensity combinations across the color bar -` lZ08535 and these are effected on the output display in accordance with the scaled value for each individual picture element or pixel. The background of FIG. 5 is dark magenta indicative of low scale values, and the spot indications relating to reservoir top structures shade through reds and greens to white at the highest scale indications.
The foregoing discloses a novel seismic data processing technique wherein three-dimensional data in surround of a producing well can be further examined to isolate attributes and outline hydrocarbon reservoir structure. The method has proven particularly effective in aiding well-site selection in developing oil fields. Proper picking of attributes and subsequent time window examinations of 3-D data can vir-tually eliminate drilling of dry development wells.
Changes may be made in combination and arrangement of elements as heretofore set forth in the specification and shown in the drawings; it being understood that changes may be made in the embodiments disclosed without departing from the spirit and scope of the invention.

Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for hydrocarbon reservoir indication comprising:
obtaining three-dimensional seismic data for an earth volume containing a known hydrocarbon producing earth structure;
defining a critical seismic data parameter that identifies distinctively with the producing earth structure;
examining each seismic trace within a selected time window for indication of said critical seismic data parameter and storage of a para-meter data value in a coordinate grid;
scaling the grid to represent indication intensity values; and outputing the grid to a display indicator as a plan view outline of the reservoir earth structure.

2. A method as set forth in Claim 1 which further includes:
examining each seismic trace within additional selected time windows for indication of said critical seismic data parameter and storage of a parameter data value in the coordinate grid.

3. A method as set forth in Claim 1 wherein:
said critical seismic data parameter is selected as one that gives distinctive response to the tops of reservoir earth structure.

4. A method as set forth in Claim 2 wherein:
said critical seismic data parameter is selected as one that gives distinctive response to the tops of reservoir earth structure.

5. A method as set forth in Claim 1 wherein:
the relative intensity values are output and mapped to a selected multi-color represen-tation as plural intensity ranges are assigned plural distinctive colors.