Introduction

Kurdistan is the semi-autonomous region of Iraq in the north, bordered between Syria, Turkey and Iran.  Here in the inset, you see the grey area that is considered the region of Kurdistan.

The main reservoir in this area is the Kirkuk group, which is the Oligocene section that can be broken down into 3 members, lower, middle and upper.  There are several facies within each member that go from backreef/lagoonal, to reef or shelfal and deeper water. There is a major unconformity at the top of the Oligocene.  The regional seal for this area is the Lower Fars, and our petroleum geochemistry suggests that we have a Tertiary aged source rock, most likely from basinal facies that don’t really outcrop and have yet to be drilled.

During the Tertiary, the Arabian plate is slowly moving into Eurasia and, the area in between becomes a shallow water centre for the deposition of carbonates.  There are two “margins’ of high energy carbonates on the northeast and southwest.  The prolific sediments slowly prograde into the basin from both sides, although likely more steeply dipping on the northern margin than the south, producing a narrower zone of potential reservoir rock.

As the Arabian plate continues to collide with Eurasia, the Zagros mountains are built and of course there are associated structures with this event, mainly thrusting.  The diagram above is a summary put together by my colleague Normand Begin that provides a quick structural understanding near our blocks.  Essentially, there are two phases of deformation in the thrust belt.  The first phase is a thin-skinned event occuring around 15 Ma, where there is thrusting and folding of the Oligocene and Eocene carbonates.  The detachment zone is in the Paleocene Aaliji formation.  The second phase is a thick-skinned event with thrusting at a deeper level, starting likely in the Permian and older sediments that cuts through to the Cretaceous and passively folds the over lying structures.  As a result, the phase 1 thrust sheets are folded by the phase two thrusts, leading to these anticlinal features.  The prospects are these anticlinal features.
 

Outcrop Geology Lessons

Near a section at Darzila, we see thick grainstone beds with cycles on the order of several metres.  During the Oligocene, benthic foraminifera ruled and dominate these strata.  The grainstones, and packstones were full of them, ranging from 0.1 mm to almost 1 mm in diameter in places.  

At another section in the village of Shalaii, we see similar massive beds with the grainstones.  However, above those, are recessive dolomitized beds and algal mounds.

Additionally, at the top of the Oligocene we clearly see a red zone, a discrete calcrete layer and hints of paleosol structure immediately below.

Subsurface Rocks

Switching to the subsurface, we see many of the same facies. Here are the grainstones with the entire range of mineralogies – from pure calcite to pure dolomite and almost everything in between.

In the subsurface, the boundstone facies are more prevalent, perhaps because the well locations are further downdip?  Again the mineralogy ranges from pure calcite to pure dolomite.

To round it out, here are the other facies, again with the range of mineralogies.

Interpretation

Putting this all together, here is the proposed depositional model for the Kirkuk group in this area. The rocks span the whole range from outer ramp to a bit of the inner ramp.  Most of the good reservoir quality is in the middle ramp. Reservoir quality is in the background grainstone-packstone deposition as well as in the coral rudstones which is proposed as being in and around the bioherms
Ooid shoals are shown in the diagram because they have been reported from Kirkuk field, but are not encountered in the cored intervals.

Diagenesis has a strong control on reservoir quality.  From the petrographic analyses, this is the proposed paragenesis.  Shortly after deposition, there is micritization of the grains, creating microporosity'  But in places, there is also marine cementation riming the grains ofr foraminifera.  It is likely that the marine cement has helped to preserve the depositional porosity.
Next comes dissolution of the grains, as shown in the packstone photomicrograph. Based on cross-cutting relationships, dolomitization most likely comes next.  There are many examples that show the whole range of dolomitzation, partial through to complete.  It always seems to start between the grains, supporting this chronologya.  However, in some samples with complete dolomitization and what looks like moldic porosity, it is possible that the undolomitized grains (5)  were dissolved later instead of dolomitizing a rock like this (4).
Finally, there are some late calcite and anhydrite cements.  Anhydrite, in places, completely obliterates any porosity in the rock.

In order to glean more about the timing of the diagenesis, isotopic analyses were performed.  The stable isotopic composition of the samples are dominantly in the marine realm a few outliers that are clearly modified by meteoric water.  These two points are subsurface samples at the very top of the Oligocene, just below the major unconformity.  The values are similar to the outcrop samples which have had a lot of meteoric water influence - the isotopes reflect that.
Note that the green dolomite samples also show up as marine – strongly suggesting that marine water was the source of dolomitization.  This is not surprising that in this semi-arid carbonate ramp/shelfal environment, reflux dolomitization occurs. 
But, the question is the timing – does reflux happen throughout deposition or only at the end of the Oligocene time?  The result means two different geometries for the dolomite geobody – discontinuous, or a larger body near the top.

The strontium isotopes help to figure this out.  Taking the 87Sr/86Sr ratio, assuming sea water is involved in either deposition or dolomitization (as shown by the stable isotopes), by plotting the results on the marine isotope curve, one can “predict” an age of the calcite or dolomite.  
Calcites show Oligocene age, which is makes sense, because biostratigraphy supports deposition in the Oligocene.  Dolomites show ages from Latest Oligocene to Miocene.  In other words, no dolomite has the same age as the oldest beds.  Also, anhydrite, if we believe they are precipitated from sea-water are also late Oligocene or younger in age.


What does this imply? The cartoon depicts the model.  Deposition of the Oligocene sequences (lower, middle, upper) occurs first and then dolomitization occurs near the end of Oligocene into the earliest Miocene.  This suggests there is one larger “dolomite geobody” at the top of the reservoir.  

What does this all mean for Exploration and Development?  There are two different sweet spots to target in this area.  The first, the preserved depositional reservoir quality is shown on the left. These are the limestone grainstones and rudstones that have their original depositional porosity and more importantly, their permeability preserved.  They’ve not been dolomitized at all and have had the cement rim around the grains to prevent compaction.  There are examples of grainstone with no cement rims that have been compacted and no longer have good perm.  So that point is important.


The second is a diagenetic resevoir quality.  This is the dolomitized zone enhanced with moldic porosity.  Again, the moldic porosity is important.  There are fully dolomitized beds with no moldic porosity and the permeabilities are much lower.  We also need to avoid the over dolomitized zones – there the dolomite crystal are definitely amalgamated.

But, can these sweet spots be imaged in 3D space?  The early products of a seismic inversion are shown above.  The brighter colors are presumably associated with higher porosity.  In the upper image, there is a larger porosity geobody below the green line which a surface near the top Oligocene.  The image on the lower left shows that surface.  This geobody is continuous and massive, consistent with the proposed reflux dolomitization, diagenetic reservoir quality sweet spot.

In the lower part of the cross section, there are more discrete porosity geobodies.  The lower right image is a slice through that part of the strata.  We see linear geobodies oriented roughly 280 degrees.  These are consistent with the depositional reservoir quality sweet spot, where the best grainstones are presumably in the middle ramp facies of each individual sequence.

These ideas need to be tested by additional drilling.