Hand Contouring Workshop
INSTRUCTOR: Sia Agah
COURSE LENGTH (DAYS): 3 Days
AVAILABILITY: Public & In-House
ATTEND AN UPCOMING CLASS:
WHO SHOULD ATTEND: Geologists, geophysicists, petrophysicists, reservoir engineers and managers who are exploring for and developing oil and gas fields in unconventional, basin-centered petroleum systems. Basic knowledge of well log evaluation is recommended.
COURSE DESCRIPTION: To teach certain accurate methods of subsurface interpretation and correct method of contouring and provide exposure to hand-contoured map examples with the aim that when computer-generated maps, which are deemed geologically unreasonable or invalid, the course participant would be able to manually edit that output by creating additional control points and contours to generate a geologically valid map.
Examples of manual editing requirements include occasions when data are widely scattered (e.g., porosity, net sand and net pay values from wells), mapping 2-D seismic data, dealing with poor resolution deep 3-D seismic events, fault blocks which may have been mapped incorrectly as structurally incompatible (lack of contour compatibility) by the computer, or the mapping software is incapable of correct net pay contouring.
To avoid generating poor quality or incorrect maps on workstations (assuming the seismic picks and well data are correct) is to manually insert control points and in some cases contours prior to gridding. The ideal way to become proficient in generating accurate contour maps on the workstation is by “hand contouring” various maps such as structure, fault surface, isochpach and isochore.
- Why do hand contouring in the age of 3-D seismic and computers?
- Rules of contouring and methods of contouring by hand.
- Correct understanding and mapping of vertical components of faults, throw and vertical separation.
- Understanding of contour compatibility or continuity of structural attitude across faults.
- Fault patterns and additive property of faults (a balancing principle), with contouring examples..
- Contouring dense 3-D data set, with possible “screw fault” interpretation, and widely-spaced well and 2-D data, including mapping a reef reservoir with incompatible top and base surfaces.
- Generating a stratigraphic oil play by creating channel sand and porosity contour maps.
- Generating net pay maps for edge-water reservoirs by generating top- and base reservoir (derivative) structure maps, net-to-gross ratio, and net sand (derivative) maps.
COURSE CONTENT: Short lectures and up to 13 exercises, several requiring generating multiple contour maps, including structure, isochore, net-to-gross ratio, porosity, net sand, net pay and derivative maps, made by cross contouring of other relevant maps.
Construct a uniformly-dipping fault surface map by triangulation and mechanical contouring. Participants learn: 1) How to place contouring points by mathematical interpolation; 2) the mapped contours would be equally spaced and parallel; 3) the concept that if the geological surface being mapped (fault surface, unconformity surface, formation top or base, etc.) is uniformly dipping, then once only two contouring points are identified, that surface could be projected up- and down-dip by equal-space contouring method; and 4) determine the map scale of a uniformly dipping surface if the dip value is known and the horizontal scale is omitted on that map.
Contour an area of en-echelon, plunging folds represented by a dense subsurface data set. Participants learn that: 1) Depth contours should be parallel, but not equally spaced; 2) that in the case of loose interpolation or extrapolation of data, there is the probability of inadvertent break of contouring rules and ending with a map that has “dangling contour”, rendering that map 3-dimensionally impossible.
Problem 3 (A & B)
Contour two sets of dense, 2-D seismic data sets, in highly faulted extensional tectonic settings. Participants learn that despite of using small contour intervals, honoring the control points, and the faults having large vertical offsets, there would be the possibility of inadvertently mapping: 1) Faults across which contours are not compatible; or 2) “screw or propeller faults”, i.e., faults which incorrectly appear to change their sense of displacement along their strike.
Contour a dense data set from a 3-D seismic survey in an extensional structural setting. Participants learn that despite numerous control points, interpreting a low-relief structure could result in certain problems, such as mapping faults which should have compatible up-thrown and down-thrown blocks, but do not show contour compatibility, or intersecting faults with intersection points that do not conform to the additive property of faults or the law of volume conservation.
Map a faulted, plunging anticline by using widely scattered well data. Participants learn the importance, and power, of contour compatibility when interpreting faults with compatible hanging wall and footwall blocks. In this exercise, there are 30 control points in the western fault block and only 4 in the eastern fault block. Maintaining structural continuity across the fault and using interpretive contouring by projecting the structure contours from one fault block to the other and adjusting the contour value by the vertical separation of the fault, the block with only 4 control points is mapped accurately and shows the correct form of the plunging structure.
Whereas, mapping the same data set using computer, with a software that treats faults as opaque barriers, it is not possible to contour across the fault. Hence, the resulting output would be an incorrect map of two structurally incompatible fault blocks.
Make a time-structure map of a compensating, normal fault pattern. Participants learn to contour two opposite dipping fault surfaces (synthetic and antithetic), and delineate the intersection, and the tip lines, of the two faults. The synthetic fault is listric, and is mapped by parallel contouring method, whereas the antithetic fault is uniformly dipping, and is contoured equally spaced.
Using well data, interpret an un-faulted carbonate reef oil reservoir, with incompatible top and base surfaces. Participants learn that by independently contouring top and base of the reef and making an isochore contour map with the posted well data, errors are likely to occur. Since an isochore value is the true vertical thickness between the top and the base of the unit, then away from well control points wherever top and base structure contours cross, the difference in contour values should be equal to the isochore value at that point. If not, that interpretation is three dimensionally incorrect. Problem 8 demonstrates this fact, which is a method of quality controlling the interpretation.
Map a potential unconformity trap. Participant learns: 1) How to map an unconformity trap, including its wedge zone where the top and base of the unit are truncated by the unconformity surface; and 2) the unit’s gross isochore in the wedge zone is contoured as the relief between the unconformity surface and the base of the unit, using the Wharton Method.
Net sand interpretive contouring, using dense subsurface control points and amplitudes extracted from a 3-D seismic survey. Participant learns how to contour net sand values (hard data) by visually incorporating the extracted seismic attributes (soft data) to constrain the shape of the channels in a deltaic depositional environment example.
Map a stratigraphic, channel sand oil prospect. Participant learns to construct the following maps by the parallel contouring option, and then in combination with the oil-water contact, delineate a stratigraphic trap: 1) Structure of the top of sand; 2) sand porosity; 3) net sand isochore; 4) prospect outline, which covers the area between a specific net sand cut-off contour value, located monoclinally up-dip of the oil-water contact.
Net pay isochore map of a fault dependent, low relief, edge-water oil reservoir by using the Wharton Method. Participants learn to determine the reservoir volume of a this unit by constructing the followings: 1) Top of reservoir structure contour map; 2) gross interval isochore map; 3) base reservoir derivative map (by adding gross interval values to top reservoir structure); 4) base reservoir map, utilizing posted well depths; 5) net-to-gross ratio map; 6) net sand derivative isochore map (by cross contouring gross interval and net-to-gross ratio maps); and 7) net pay isochore derivative map, using the Wharton Method, that shows the full thickness oil area and the oil wedge. Participants will further learn that in the full thickness area net sand equals net pay, but in the wedge zone, where the reservoir is not totally filled with hydrocarbon, net pay contours should represent the structural relief between hydrocarbon-water contact and the reservoir top.
Net pay isochore map of a reverse-faulted, high relief, plunging, asymmetric, edge-water, anticline reservoir. Similar to Problem 12, participants have to make preliminary maps, including 1) top and base reservoir structure contour maps; 2) gross interval and net-to-gross ratio isochore maps; 3) net sand derivative isochore map from gross interval and net-to-gross maps; and 4) net pay isochore map. However, unlike the case in the previous exercise, this reservoir is tightly folded and reverse-faulted. As a result it has a very narrow wedge zone and wide full thickness areas, in both the hanging wall and the footwall fault blocks. Participants learn that top- and base structure contours should demonstrate compatibility of shape, and the fault blocks should be structurally compatible by honoring the fault’s vertical separation. Otherwise, the interpretation would be three dimensionally impossible.