RKC Newsletter#7-Seismic Facies Mapping 3.

Rob Kirk (robkirkconsultants@bigpond.com)

In this final (of three) facies mapping article we will briefly discuss the role that automated facies work takes. This is usually only applied to 3D data and in my opinion is not always done by geologists who should have full “ownership” of the region’s geology.

In many facies mapping jobs the manual work often shows up shapes not seen on the automated maps (meaning amplitude extractions, coherency slices etc)-and vice versa. These methods should always be employed together (preferably manual first) but most of the time the manual work is not done-usually because of lack of experience on the part of the geoscientist concerned-and that often can have a "company bias" to it! Some companies value geology more than others!

Automated maps often only show the most obvious elements and usually require very good data to see anything.

Nearly all automated maps that I have been asked to "adjudicate" on have a little real stratigraphy (often no obvious stratigraphy) and a lot of "other"-being cross cutting of time contours, noise, mispicking etc.

Figure 36.

Figure 36 is a simple amplitude extraction (3D) which shows a series of reddish lines whose strike is changing due to Palaeozoic prograding in northern Australia which has separate sources-as shown by the yellow arrows. This amplitude extraction is actually due to mispicking but is useful nevertheless. The geophysicist ignored the prograding and picked right through them and what we are seeing as the reddish lines are the mispicks but it shows the strike of the pro-deltas nicely.

Figure 37.

Figure 37 is an automated facies map (RMS amplitude over a narrow window) which shows excellent fluvial channel geometries from the Tertiary of SE Asia.

Figure 38 shows incredible detail on Stratimagic amplitude maps from SE Asia.

The left map shows straightish distributary channels (“DCH”) and the distributary mouth bars (“DMB”) that they are feeding. The right map is a text book example showing the meandering fluvial world changing to the straight distributary channel world of the deltas. Such facies changes are usually only suggested in outcrop and on cores.

Figure 38.

Figure 39 is another amplitude map from the Tertiary of the Timor Sea showing individual wave dominated deltas in magnificent detail (meaning that the data are near the sea floor-the best area to work if you publish a lot!).

Figure 39.

The best tool that I have used for automated facies mapping is "Stratimagic"- a Paradigm product (although I am playing with Seisnetics at present-written by the same person I believe-and it is interesting).

This looks at the seismic waveform shape so includes phase, along with amplitude.

The 3D sequence that you are mapping is "calibrated" into, say, the six best waveforms that can recreate the entire dataset over a tight interval. The software then shows as coloured facies the distribution of these six waveforms over the entire area and gives various statistics on how good the fit of the modelled waveform to the real waveform is.

Figure 41 shows possible lobe and fan feeder facies on a waveform map from Gabon.

Along with waveform facies maps there are many amplitude-related variables that can be studied and different mathematical routines that can be applied.

For example, an excellent methodology is to extract the wave form at your key wells (eg wells with pay, wells with brine, a sand filled channel well and a mud plugged channel) and then see where those waveforms are distributed over the entire dataset. In the example I ran "blind" test wells and they "lit up" on the facies map as being mud and sand filled channels.

Figure 41.

Figure 42.

Figure 42 is a waveform automated facies map showing prograding in the Barrow Group, Barrow Sub-basin (north is to the right). The up and down lineations show the strike of the prograding and the left to right lineations show sand filled gullies on the slope (as calibrated by Rosily-1A).

Figure 43.

The problem with many automated tools is that the picks often have to be excellent otherwise spurious facies are readily introduced.

Figure 43 (waveform facies map) shows a greeny-red seismic facies next to a dominant blue facies-big geology change? No! The circled pieces of seismic show the slight

misspicking of the basal SB of this package where it goes from a peak to an adjacent zero crossing- a quarter of a wavelength only-but enough to create the wrong blue seismic facies.

Figure 44.

Figure 44 shows another form of automated facies mapping. All six maps are over the same interval but show varying geology. These are principle component facies maps and they actually looked like they were "filtering" the different scales of geology. PCA1 showed the most obvious geological features while PCA7 showed the very subtle ones. These were new to what was seen on "good old boy" automated maps of RMS amplitudes on Kingdom.

Figure 45 is a nice option to have to help your automated mapping, namely mixed facies mapping.

Here you simply drag whatever maps from your menu and overlay them on other maps while varying opacity to see through the maps. Isochrons with amplitudes work well.

Figure 45.

Figure 46.

Figure 46 is yet another form of facies map where two forms of mathematical processing have been used to combine different datasets together. In this case an isochron was classified with standard deviation, and with third derivative via neuronal and hierarchial classification methods!

The two resulting facies maps were different and showed complementary geological features-again not obvious on standard amplitude maps.

Figure 47.

All automated facies maps need a good deal of "ground truthing".

Figure 47 shows a map (from Angola, deep water) that has been "vetted" by looking at several vertical lines through the calibrated sequence and seeing what each facies colour means on the actual data (following on from detailed manual facies mapping so that we thought we had a good idea of the geology already). The semi opaque overlay has been laid on that area that was calibrated outside of the study interval. The whole left side of the map is dominated by structure-cross cutting time events and faults while the small lobe shown was the only area of the map deemed to have useful stratigraphy on it.

We will be seeing a lot more of spectral decomposition (“SD”) facies maps-Figure 48. It seems that stratigraphic features appear to tune over rather tight frequency ranges. I have often noted, for example, that rich, marine source rocks have very low frequencies so SD might be a tool to explore for these.

These articles hopefully give you a little flavour of seismic facies mapping-what is involved and what might come out of such work.

It is not a method which is applied sometimes-that is like saying I will use seismic sometimes! It is used all the time as a standard practice. It will give varying results-sometimes great ones and sometimes not adding much.

If your area is highly faulted or structurally deformed it might not be as useful.

 

Figure 48.

If your acreage is in shallow water with thin marine shelf units where all geometries are sub-seismic then it might not be much use. Nevertheless well sequence stratigraphy is always applicable.

A lot of us are exploring in the deep water on slopes or out in the basin and we often have good data (but few wells) and this is where this methodology may be of great use to you.

You will need to be comfortable with well logs and go out to the core shed with experts-go to Rob Seggie's and Simon Lang's core workshop in Perth (a Nautilus school), for example.

You will need to consult with the seismic experts-multiples can map up as great stratigraphy! Which seismic cube should I be using? etc etc.

You will need to be on talking terms with your biostratigrapher and geochemist. The latter has a lot of great sequence cyclicity in her data. For example in the Goodwyn- Rankin area in the Triassic there are great seismic facies which are due to the coaly shales at the maximum flooding surfaces of the mostly fluvial sequences, in between the incised valley sands, on the lower SB and the point bars, under the upper SB. Downhole TOC ("total organic carbon") variability from the geochemist were useful also.

In the deep water Gulf of Mexico “sequence boundary” picking is done by directly using bio-diversity and bio-abundance peaks to determine where the mfs are, as the wells are mostly all shale!

This work is about integration and combining disparate data sets together using a sequence-based framework-with seismic facies being just one component of the job.

References:

Abbott, W.O (1991) Operational Seismic Stratigraphy. Occidental Petroleum course, Grand Canyon, February.

Kirk, R.B. (1985), A seismic stratigraphic case history in the Eastern Barrow Sub-basin, Northwest Shelf, Australia. AAPG Memoir 39, pp. 183-208.

Prather, B., Booth, J., Steffens, G., Craig, P., (1998), Classification, lithologic calibration, and stratigraphic succession of seismic facies of intra-slope basins, deep water GOM, AAPG Bulletin, Vol. 82, No. 5.

Prather, B., Keller, F., Chapin, M. (2000), Hierarchy of deep water architectural elements with reference to seismic resolution: Implications for reservoir prediction and modelling, GCSSEPM Conference, pp. 817-835.

Sikkema, W., (2001), Woodside in house seismic stratigraphy school, Perth, Western Australia.

In this next section we continue with the Ten Commandments of Stratigraphy (by Frank Brown-

tongue slightly in cheek at times!) These were supplied by Gerhard Brink from Schlumberger.

THE 10 (Actually ~ 100!) STRATIGRAPHIC COMMANDMENTS:

THOU SHALTS AND SHALT NOTS

FOR SILICICLASTIC ROCKS-Frank Brown.*

Commandment V.

Thou  shalt  realize  that  (a)  it  may  make  little  difference  whether  unconformities  or  maximum  flooding  surfaces  are  used  as  sequence  boundaries,  as  long  as  one  understands  the  genesis  and  significance  of  each  surface.  But  it  is  seriously  important  that  stratigraphers  recognize  that  the  entire  marine  and  near  shore  sedimentary  record  comprises  cyclic  sequences  that  required  high-­‐frequency  (<3  my)  relative  falls  of  sea  level.  (b)  Thou  shalt  remember  that  if  flooding  surfaces  are  selected  as  boundaries,  the  resulting  sequence  contains  a  major  erosional  and  non-­‐depositional  hiatus  and  the  surface  has  diagenetic  importance  (especially  in  carbonates)  and  to  correlate  the  highstand  and  transgressive  shallow-­‐marine  sandstone  facies  with  off-­‐shelf  deep  water  lowstand  sandstone  facies  simply  because  they  occur  between  the  same  pair  of  marine-­‐condensed  sections  is  highly  questionable.  The  condensed  sections  are  ~time  surfaces,  but  sandstones  between  two  mfs’s  are  not  necessarily  time-­‐equivalent,  and  major  miscorrelations  can  occur  if  one  does  not  recognize  that  when  off-­‐shelf  sandstone  facies  are  being  deposited,  the  adjacent  shelf  was  subaerially  exposed.  Hence  on-­‐shelf  and  off-­‐shelf  deposition  were  temporally  unique  and  mark  the  stratigraphic  position  of  density-­‐flow  sandstones.  (c)  Thou  shalt  remember  that  although  flooding  surfaces  are  easier  to  recognize  and  trace  on  seismic  and  wireline  logs  that  is  not  justification  for  ignoring  major  subaerial  and  submarine  unconformities.    

What-is-this #5?

This image is from Seisnetics and shows part of the leading waveform around the top Miocene (carbonates) in the Carnarvon Basin, Western Australia. Lumps are interpreted to be a carbonate debrite.

What-is-this #6?

 

Here  we  have  an  overall  mounded  package  of  well  bedded,  high  amplitude  sheets  with  some  large  “climbing  waveforms”.  How  would  you  interpret  them  and  would  yopu  drill  them?