Are Faults Necessary Migration Conduits? A Simple Migration Model Says No.

We often observe petroleum accumulations in association with faults, especially in deltaic systems (Gulf coast, Niger delta, Mahakam delta, Nile delta ... and rift systems (Most basins in South East Asia, North Sea ...). Often in the literature the assumption is made that the faults act as path ways for migration up such systems. Here I make a simple argument that migration via faults is not necessary, or even possible in order to explain the distribution.  

This cross section is from the Hindel field, in Mahakam delta, Indonesia. Some obviously active faults cut through the large number of stacked oil and gas reservoirs.  This is very typical of deltaic systems. Question is, did the oil and gas migrate up the faults to charge these reservoirs?

 

Figure 1. Cross section through Handil Field, Mahakam delta, Indonesia, Antony Reynolds, 2016. There are some 500 stacked reservoirs vertically.

I have made a very simple model of the geology in Trinity (our popular 3D migration modeling software). There are only two (yes only 2) variables in this model. Capillary displacement pressure, Pd, and buoyancy. The faults are assumed to have a higher Pd than the shales, meaning NO migration along or across the faults. Oil is injected from below the field where the source is at. As the column of each accumulation grows and exceeds the capillary displacement pressure of the shale above, migration continues through the shale and into the next reservoir. It is amazing that such a simple model can explain the distribution of petroleum pools so well. So the first obvious conclusion is that the faults are NOT necessary to act as conduits for the filling of the reservoirs. In fact, if we allow migration along the faults, we could not form the accumulations.    

Figure 2. Simple capillary model to explain the accumulations in stacked reservoirs in a deltaic system. The faults are sealing.

With the high net to gross in this field, it is very conceivable that juxtaposition of sand on sand may allow migration across the faults. Below I made some of the sand-on-sand locations low capillary pressure so migration across faults is allowed. The patterns are a bit more complex and the main difference is that the shallow reservoirs between the faults are charged in such a case, compared to figure 1. 

Figure 3. Same as figure 2, except migration is allowed through some of the sand-on-sand juxtapositions. This is by lowering the capillary pressure of the faults at the juxtaposition locations. 

The second one is more reasonable compared to the actual field. Also keep in mind this is only a 2D model. Vertical migration may happen in different locations, and lateral migration along structure strike is also possible.  

An important observation I have made of similar fields in many basins that the sands in between large faults in these compressional flower structure are less frequently charged. I interpret this as indicating although cross fault migration is possible, but less frequent.  The faults are acting as barriers and thus creating migration shallows for vertical migration.  

I have paid attention to these observations to get a better understanding on migration. And I have always been able to explain them with a simple capillary model like this. This applies to stacked reservoirs in 3 ways against salt as well. 

A lot of papers mention faults are migration conduits, without further elaboration. In discussions I have had with colleagues and friends, I find that more than 50 of us would invoke faults as migration conduits. Some go as far as to believe no charge of shallow reservoirs is possible without faults. But when asked how can the oil come up the faults and fill a reservoir, but not leak up the fault the same way it came (both sealing and leaking). The answer is usually more strenuous and unconvincing, and usually involves some exotic episodic behavior. 

I think the main reasons geologists like to invoke faults for migration are 1) accumulations are often associated with faults, and 2) there have been a misconception that shales are "impermeable". The association argument works both ways, and the opposite is that faults act as seals so 3 way traps can be traps. The permeability is never zero for a shale, they are often quite permeable, and low-permeability is not the reason that oil is trapped below a shale, it is the entry pressure that is holding the column. Entry pressure is a finite pressure and can only hold a finite column. Additional oil will simply leak through.   

Since the simple assumption that faults are seals (vertically and laterally) can explain the distribution of accumulations beautifully, I suggest we stick to an Occam's Razor model.

I am not ignorant of anecdotal evidence for oil migrating though faults, most obviously the seeps along faults at surface, bitumen filled faults, among others. But for forming large accumulations that we observe we should assume faults, in most cases are sealing. If we look at accumulations in basins across the global, we can hardly find any fields that don't have faults across the structure - if faults are leaking, and not leaking, we would have no predictive power. Some of these have been there for many millions of years.

There, I said it.