Fast and slow catastrophes trapping slope and basin floor plays

How slumping, sliding and folding of passive margin stratigraphy creates the elements for successful exploration while the latest seismic imaging still enables making sense of the seemingly chaotic result

The duality of existence conjured up by Erwin Schrödinger’s cat-based experiment is perhaps best extended into hydrocarbon exploration in the confined slope channel bypass play, because they are both “fantastic” in terms of potential and “monstrous” in terms of being difficult, illustrating that objects can be two opposite things at the same time. It seems unintuitive that the traps envisaged can also be both successful and not successful at the same time until observed with a drill bit; yet how would we know?

Without an up-dip barrier to flow, oil or gas migrating buoyantly up connected sands in a confined slope channel will just migrate up onto the shelf
and be lost – like tears in the rain. One such barrier that might prevent flow on to the shelf would be stratigraphic – a part of the channel where no sands were deposited or “bypass zone”. In fact, this barrier might be present anywhere up the slope channel system from the basin floor. When the basin floor structure does not avail itself of counter regional dip, as it does, for instance, in the Venus play of Namibia and Pelotas, such a barrier is required for any basin floor apron fan too.

Yet, this represents a ubiquitous and exhausting irritant for oil and gas explorers, as, despite the use of a variety of trap risk focussed proxies such
as DHI, AVO, pressure and gradient studies, such a bypass is almost always sub-seismic resolution, and just one thin cat-burglar sand, a Macavity-like
stringer of porosity, will be away on his paws with the trap.

Fortunately, a variety of syn- and post-sedimentary structural processes can even the odds in the explorer’s favour, creating very imageable channel truncation. Formed during either instantaneous or slow and long-lived catastrophes, gravity-driven processes on a variety of scales can create the required discontinuity of the basin to shelf permeability system.

Slumping and sliding

Syn-sedimentary slumping or gravity sliding can remove a deposited sand, cutting off the thief or create an up-dip barrier to flow. In the Fold-out line and Figures 1 and 2, Post-sedimentary Gravity Driven Fold and Thrust Belts (GDFTB), or Mass Transport Complexes (MTC) in Figures 3 and 4, can also be key to providing up dip barriers. Both of these, syn and post- sedimentary processes are readily imaged on modern seismic, allowing explorers to identify much lower risk traps than are available for the purely stratigraphic play.

End Members

Clastic prisms building out over passive margins appear to display a spectrum of instability. For instance, the Zambezi, Fly River, or Bengal Deltas are truly stable with few MTC’s or GDFTB’s despite the deposition of vast amounts of sediment. These clastic deltas often prograde and aggrade with a simplistic sequence stratigraphic perfection. In such settings, the chance of finding syn- or post-depositional structural traps to slope channels is somewhat reduced, although success in the Tano Delta shows that stratigraphic, or sub-seismic structural traps (Jubilee) can work.

At the alternative end of the spectrum are the Orange, Lamu and Rovuma Deltas that exhibit Ozzy Osbourne-like periods of instability (see Fold out image). In the Orange Delta, the 4 to 5 cycles of shelf building were followed by its collapse into deep water creating MTC’s, several of which are over 500 m thick.

At the end of the Cretaceous, collapse-a- thons, the slowest shelf collapse occurred, keyed onto a decollement surface close to the Turonian source rock, creating the famous Orange GDFTB. Compared to the stable margins, this instability occurs without a huge sedimentation rate, or without a steeper slope to basin floor gradient. In these cases, instability is dominated by mantle convection-driven dynamic topography changing the basin
geometry rather than sediment input and given basin geometry.

First fold of the GDFTB play

In the Rovuma Delta it is this slow-shelf collapse that created the Windjammer “first fold of the GDFTB” play, and the Barquentine “truncated by the first thrust of the GDFTB” play. Another example of a GDFTB occurs in the Mahadani basin of SE India, where the fold belt running down slope truncates not only slope channels and basin floor fans, but also cuts through leveed meandering channel complex running in near strike direction
from the huge sediment input of the Bengal fan (Figure 2 and inset).

MTCs are often found associated with the instability of the GDFTB, as seen in Figure 2, and can not only truncate slope channels and basin fans, but also truncate preceding GDFTB as seen in Mahadani Basin in India and Orange Basin of Namibia.

From Chaotic to Coherent

MTC’s have in past times appeared as chaotic sequences on seismic, comprising rarely seen coherent blocks in a sea of chaotic, reflectorless goo. However, modern de-ghosted seismic data, when viewed at an appropriate vertical and horizontal scale, can show MTC’s to be much more coherent (Figures 3 and 4).

Again, there will be a spectrum between incoherent and coherent structures, yet in these two examples from The Gulf of Papua (PNG) and Krishna
Godavari Basins (East India), modern de-ghosted seismic identifies individual thrusts and tilted fault blocks within the apparent chaos. The layering within the MTC’s suggest they are mud rich, and they can provide a perfect top seal. They can individually be 100 km in extent, up to 500 m thick and many 10’s of km’s wide. Thus, they can transfix and truncate the heads of multiple parallel channels in a region at the same time, bringing repeatable success to exploration drilling and trapping truly significant hydrocarbon resources.

Cooler for cats

Ironically, the ability to be able to image the up-dip seal of a confined slope channel or a basin floor fan makes unstable margins more straightforward to explore than those that are not. Catastrophe-derived up-dip structural trapping to slope channels or basin floor apron fans can be readily imaged on modern de-ghosted seismic data, taking the heat from the key exploration risk-element of the slope and apron fan play, and making the trap rather cooler for cats.