The Kurdistan region has seen a rush of exploration activity in recent years with over 30 companies operating in what many believe to be one of the last remaining frontier hydrocarbon provinces. It forms the north-western margin of the Zagros fold belt, which has seen prolific production from fields in Turkey, Iraq and Iran. Antiforms and synforms are readily interpreted from satellite imagery, but access to the region was restricted for geopolitical reasons.
The Shaikan prospect is a doubly-plunging antiform lying 85 km northwest of Erbil near Dohuk. Pre-drill indications for oil were strong. A limestone outcrop to the north and west of the structure bleeds oil, which occurs within fractures and vugs. Field reconnaissance mapping validates satellite image analysis, defining a structural closure extending around 35 km E-W and up to 6 km N-S. Interpretation of 2D seismic suggests the presence of a compressional, possibly flower structure, with reverse faults bisecting the north and south limbs.
A single well has been drilled on the crest of the Shaikan structure. The well, completed in November 2009 was a significant discovery. It encountered Cretaceous, Jurassic and Triassic hydrocarbon reservoirs with a cumulative net pay of circa 250 metres of oil in multiple reservoirs. The well was TD’d early at 2950m in the Triassic due to an influx of high pressure gas thus the full extent of the hydrocarbon accumulation is not yet fully determined.
Microresistivity image logs were gathered through much of the well section to supplement conventional petrophysical logs, and core was cut in two limited intervals of the Jurassic Mus and Triassic Kurre Chine Formations. Five drill stem tests were conducted and confirm oil flow of up to 7480 BOPD from the Jurassic.
Borehole image log and core description results provide some insight into the subsurface structure, some at variance with initial expectations, particularly the fact that fractures are not simply orientated with respect to flexure:
- Overall structural orientation differs little with depth.
- Whilst surface structure has a strong E-W grain, fractures picked from images and goniometry tend to strike N-S to NNE-SSW, across the antiform hinge. The majority of fractures are conductive and occur in clusters with depth, some of which coincide with mud loss events, suggesting they have aperture. Ten faults were inferred. Most strike NNE-SSW; only one E-W striking fault was interpreted.
- Structural core logging reveals that fractures are commonly steep, layer-bound and discontinuous. Residual hydrocarbon coats fracture apertures. Fractures appear to be of several generations. They individually have limited porosity, but form a connected network.
- Stylolites are seen in core and borehole images, where they appear as highly conductive ‘beds’. More than one generation is present, but bed-parallel stylolites are the more common type. They are predicted to reduce vertical fluid connectivity, although stylolite associated fractures are observed that may improve layer parallel flow.
- Drilling-induced tension fractures are seen in the borehole images, striking N-S to NNE-SSW. This is subparallel to many of the conductive fractures, suggesting that they could be stress-sensitive and may be enhanced, leading to better drainage.
In addition to detailed structural information the core yielded important sedimentological information:
- The Jurassic core comprises a cyclic carbonate succession of laminated lime mudstones (locally dolomitized), bioturbated and bioclastic lime mudstones (mainly undolomitized), intraclastic packstones, in-situ breccia zones and thin detrital shale laminae. The depositional setting envisaged is a shallow, sheltered marine environment, possibly a lagoon or peritidal setting. The rapid variations of lithology and thin units may indicate limited accommodation space and frequent cyclic relative sea-level fluctuations.
An anhydrite succession in the deeper Triassic core appears to be a replacement phase of precursor carbonate or carbonate-evaporite rocks. Relict grains and ‘ghosts’ of bioclasts and burrows are evident in replacive anhydrite after carbonate lithologies. Planar, disrupted or ptygmatically folded layers occur within laminated anhydrite intervals. This rock type may be a replacement of sabkha deposits of gypsum, anhydrite, limestone and dolomite.