RKC Newsltter-Direct Hydrocarbon Indicators. This article will discuss a few oddball seismic facies associated with fluid expulsion-be it water or hydrocarbons, and some of the less obvious effects that hydrocarbons in the system may have. Some acronyms such as HRDZ, DHI and BSR will be mentioned. These articles may be considered as a bit of a dictionary of real-world examples-things to collect and keep in the back pocket, as one day they may help reduce a crucial risk on that prospect-as they have done in the past. We are familiar with the Direct Hydrocarbon Indicator ( DHI ) of free gas in a system where rock property contrast is appropriate to show the gas as a bright spot-such as the sub-cropping gas of Figure 1 or the stratigraphically trapped gas in Figure 2. Figure 1. Figure 2.
A lot of work is done on 3D seismic with AVO analysis, fluid cubes etc to prove the presence of hydrocarbons but this article will be more qualitative than quantitative. Occasionally free gas is seen in the section-as shown by the highly discontinuous seismic facies on Figure 3. Figure 3. In such cases we often see pull-down of the seismic geometries due to the locally much slower velocities within the gas section. Such a case is seen spectacularly on the old seismic land data of Figure 4. Figure 4. This line from the Sirte Basin in Libya shows two mounds with apparent channels under them. The channels are velocity pull-down due to the high
oil-filled porosity of the karstified reefs-the mounds just above. Individual well flow rates in these features were facility-constrained at 90,000 bopd! An important concept developed by AGSO (Geoff O Brien and colleagues) using well and seismic data in the Vulcan Sub-basin area of the Timor Sea in the 1990 s was that of the Hydrocarbon Related Diagenetic Zone ( HRDZ ). Figure 5 (from O Brien et al, 1995, APPEA Journal) is a sketch showing the concept. Figure 5. Hydrocarbons escape from a deeper trap towards the surface and on the way are attacked by oxidizing bacteria which can leave behind hard carbonates. These can produce a distinctive seismic facies of a narrow, intruding, high velocity pipe-like feature-as seen in Figure 6 in the Vulcan Sub-basin.
Figure 6. The high velocity effect can be appreciated from Figure 7 where HRDZrelated pull-up is seen on an aerial map from 3D seismic data. Figure 7. A similar HRDZ-candidate is seen in the Dampier Sub-basin on Figure 8.
Figure 8. Figure 9. 3D seismic is good to trace these features-as seen on Figure 9-where Stratimagic automated seismic facies (green feature) trace the extent of
the narrow, intruding HRDZ. It does not look like a fault and does not waveform-heal (undershoot) with depth, as it should if it is only a near-surface feature. Figure 10. Figure 10 shows a different narrow, intruding anomaly which has a nearsurface effect-possibly as a sea floor depression where free gas has created a large pock mark, or crater. Figure 11. Figure 11 is a sketch of some of the features we see, namely a small intruding anomaly which often lights up shallow porosity-presumably with free gas-but may also leave carbonate hards behind as well.
Figure 12. Figures 12 and 13 show intruding anomalies which appear to branch out as they get shallower and leave various high amplitude patches behind. Figure 13.
An important observation to make is from what apparent depth do the anomalies originate. Shallow biogenic gas can produce pock marks and other anomalies. If the anomalies come from depth then they are more likely to be thermogenic hydrocarbons and we may be able to use them to de-risk the source-generation-migration aspects of our prospect. Figure 14. Figure 14 shows three possible origin source points for these anomaliesdirectly above the source rock in the deep Gulf of Mexico. Figure 15.
Figure 15 shows a bright, patchy near-surface anomaly with a possible feeder for the hydrocarbons. This potential conduit was found only after a lot of searching and was limited to 3 or 4 traces. Figure 16. Figure 16 (Lonergan et al, 2000) summarises many elements seen when fluids (usually hydrocarbons, but can be water from overpressured, shallow turbidites and debrites) escape towards the surface. Pockmarks, or craters, may be produced and injection of sands may occur into overlying sections. Injection is an unusual phenomenon but is commonly seen on field trips to turbidite localities (for example Annot in France and Ainsa in Spain). Figure 17.
Figure 17 shows one deep water sand (green) intruding the younger section (purple) due to water escape in Ainsa. Figure 18. Perhaps Figure 18 is just imaging this injection on seismic from West Africa. Figure 19.
Figure 20. Figure 19 (Roberts et al, 2001 GCSSEPM Conference, Houston) actually images a free gas plume within the water column and Figure 20 (also Roberts) shows a surface mud volcano related to hydrocarbon escape. Figure 21. Such gas escape is seen on Figure 21 (from AAPG Explorer, February, 2002) as a series of craters on the sea floor. These sometimes have associated suites of peculiar benthonic animals adapted to living off hydrocarbons. Escaping gas can leave distinctive geometries on the sea floor-as seen on
Figure 22. Figure 22 from the Timor Sea. Such features are present at depth, if you have the frequencies to image them, on palaeo-sea floors-but you need an accurately picked surface to see them on the 3D. Figure 23.
Figure 23 shows a variety of different-sized craters on the sea floor-some appearing to be fault-related, and some not. Figure 24 shows amazing fractal detail within these craters. Figure 24. Occasionally we can see the mechanism for fluid release-as seen on Figure 25. Figure 25. Note the discontinuous seismic facies at the bottom of the figure. Surface and near-surface craters (small channel-like bodies) appear to originate
from within the discontinuous debrite, a water-logged density flow deposit that expels water when loaded by the succeeding sediments. Mud volcanoes may be related to gas escape-as seen on Figure 26 (from the Makran, Petroleum Institute of Pakistan). Figure 26. Mud diapirs, as seen on Figure 27, may be fluid escape-related also. Figure 27.
Figures 28 to 30 show reservoir sands which have been injected into the overlying shales. We see them on gamma logs, in core under UV light and on seismic. Figure 28. Figure 29.
Figure 30. The seismic facies can be difficult to see-and usually require 3D seismicnote the small, crosscutting features on Figure 30 (circled). Similar cross cutting events are seen on Figure 31 (Lonergan et al, 2000) from a field in the North Sea. Figure 31. This sand injection phenomenon within a field may be more common than the literature suggests and it could be useful to consider it anywhere there is production from deep water sands (Angel, Wanaea, Puffin, Harriet etc?). One could envisage problems with injector wells, for example, that go into injected sands. Another acronym of note is that of the BSR, or Bottom Simulating Reflector. These are caused by reflection from the base of a near-surface layer of gas hydrates. These are unusual ice-like units where gas and water are incorporated together. They may be either biogenic or thermogenic in origin (?). These units only occur in deepwater and near the water bottom, that is, in highish pressure but low temperature conditions.
Hydrates contain huge reserves of gas and many companies are currently contemplating how to access this resource. Figure 32. Figure 32 shows the distribution of some of the world s hydrates. Figure 33.
! Figure 34. Figures 33 and 34 show the seismic character of hydrates. We generally see a soft reflection from the base of the hydrates (which is often a zone of lower amplitude) and the key to identifying them is the approximate coincidence with sea floor shape as well as cross cutting of the reflectors. Occasionally high amplitudes are seen of free gas in a subcrop trap at the base of the hydrates. Figure 35. Figure 35 shows a typical deep sea feature seen in shales, especially if there are some carbonate muds around. These are large (up to several kilometres) polygons likely due to dewatering of the muds.
!! Figure 36.
Figure 37. Figure 36 shows unusual mounds in a known deep water mud section on a Seisnetics data set. We do not know what these are but thinking is along the lines of some form of fluid flow feature. Figure 37 shows unknown high amplitude effects in Cretaceous deltaics in the Barrow Basin, Western Australia. To the left, within a specific fault block, we see amplitudes increasing-possibl;y some fluid moving up a fault in to the deltaic reservoirs. Note also on the right of the section (with the ellipse) a brightening which may be a fluid effect intruding the section. So, in future as you do your seismic facies work keep an eye open for some of these more obscure seismic facies as they may be of use. Some act as near surface drilling hazards, others indicate that thermogenic hydrocarbon generation and migration has taken place, but may also indicate that your deeper target may have been breached and hydrocarbons lost-then it becomes your geochemist s issue as a secondary charge will be needed. Happy hunting!