Exploration in Kurdistan: The Importance of Regional Geology
It seems that almost every month there is news of successful exploration in Kurdistan. Whilst the geological reasons for much of this success are fairly simple to understand, predicting the best areas to target is somewhat more complex and requires a more regional geological perspective.
To begin with the simple explanations first, in Kurdistan, as in many other parts of the Middle East, there is fortuitous juxtaposition of source rock, reservoir and seal in very large thrust-fault related anticlinal traps (Figure1). Common seepage in the region testifies to a working petroleum system, principally driven by the thick, bituminous limestones of the Jurassic Sargelu and Naokelekan Formations. Reservoirs are mainly fractured carbonates at a variety of stratigraphic levels, with fracture intensity greatest along the limbs of the thrust-related anticlines.
This leads to the first complexity: fractured reservoirs. The permeability and connectivity of these is difficult to predict as is the intensity of fracturing. This will impact the flow rates from discoveries, which whilst initially high (often 3-5,000 barrels per day) can drop off dramatically. Fracturing along corridors tens of metres wide, often related to faults splaying from the primary thrust. These corridors are seen in analogous outcrop geology, but more difficult to predict in the subsurface, though an understanding of the geodynamic history of the region and therefore controls on regional stress patterns should help. The introduction of 3D seismic surveys should increase fracture detection. Moreover, not all sedimentary rocks fracture in the same way. Mechanical stratigraphy relates closely to primary sedimentary facies. Thicker, more competent beds are more susceptible to generating brittle longer open fractures than heterogeneous thinly bedded units. A study by Norsk Hydro/Statoil geologists on analogous outcrops of Cretaceous carbonates in the Iranian Zagros noted that thicker bedded highstand grainstones showed much greater fracture intensity than more muddy transgressive wackestones and packstones. The highstand grainstones are also likely to have better primary reservoir quality, especially if enhanced by diagenetic processes during relative-sea level falls. Late hydrothermal dolomitisation, commonly associated with faulting, is a further complication. It may form reservoir units with good matrix porosity as in the Qamchuqa Formation of Taqtaq but its lateral continuity is uncertain.
How then to predict the occurrence of the rock types likely to have the greater fracture intensity? Are they likely to be ubiquitously distributed throughout Kurdistan, or to be more localised? Sequence stratigraphic analysis of the regional geology can help provide an answer. This was well understood by the pioneering IPC geologists during the middle of the last century – they recognised narrow forestepping and retrograding facies belts controlled by eustatic and/or tectonic drivers of sea-level change. This leads to local facies changes (hence localised development of potential high-quality reservoir facies) and complex lithostratigraphy. One of the problems in the subsurface of Kurdistan is the use of “drillbit stratigraphy” to name rock units - i.e. the first limestone encountered after a thick section of shale must be formation x – this leads to miscorrelations and a lack of appreciation of the lateral variability in the extent and thickness of rock units.
Kurdistan remains a region where Common Risk Segment (“traffic light”) Mapping at a regional scale allows assessment of the risk on reservoir presence and effectiveness, source rock presence and maturity and seal presence and effectiveness. The starting point for such mapping is to join up all the available data not by using lithostratigraphy but by using biostratigraphically constrained sequence stratigraphy which enables gross depositional environment maps to be drawn for key stratigraphic surfaces related to the deposition of reservoirs, source rocks and seals. For example, what is the correct stratigraphic and areal distribution of source rocks? Where is the Sargelu thickest, richest and within the oil window? Are other source rocks present? Can deeper reservoirs (e.g. Triassic) be charged from the mid-Jurassic Sargelu or do we need to invoke a deeper Triassic source rock? Are some of the most organic-rich rocks acting as reservoirs comparable to resource plays in other parts of the world? For example, the Sargelu Formation is cited as one of the most important reservoirs within the Shaikan discovery. Are fractured limestone beds within the source rock the reason for the high flow rates or is there some matrix porosity?
There is still much to understand on the subsurface geology of Kurdistan but our knowledge of exploration risk is being greatly enhanced by the publication of Iraqi/Kurdish geoscience in online journals, whilst at the same time independents active in Kurdistan release geological details to encourage investment. These data can be used to improve facies mapping and subsurface prediction across the region. This will lead to better targeting of exploration drilling and more realistic estimates of reserves.
Figure 1: Typical cross-section through thrusted anticlines in the Kurdistan Zagros. The cross-section is a Neftex interpretation using in part seismic data as illustrated by Heritage Oil in their press releases and website.