Wed, Apr 19, 2017 5:30-8:30pm
In the last several years, the Zagros belt along the NE edge of the Arabian plate generated much interest due to new opportunities in the Iraqi part of the belt. The Zagros is a highly prospective technically immature basin currently exhibiting a high exploration success rate (>50%) with medium geologic risk. Exploration targets are predominately carbonates in stacked petroleum systems at moderate drill depths ideally focused on large structures. Seismic quality is usually good in the foothills (Low Fold belt) where clastics are exposed, but the carbonates of the mountainous High Fold belt significantly degrade the seismic image. Exploration has focused primarily on surface expressed anticlines so far, but other play potentials do exist and are beginning to be pursued.
Despite often great outcrop conditions and well exposed anticlines in the High Fold belt, at least 10 wildcat wells in Iraqi Kurdistan missed the crest and drilled down steep flanks instead. In the Low Fold belt, blocks were picked up before seismic data was available that turned out not to contain the expected major thrusts and associated traps, but only a large syncline and no trap at all. Many, if not most, of these companies had extensive experience in exploring fold-thrust-belts, so what happened?
The main reason is the structural style. Most anticlines are poorly imaged seismically because surface exposure is dominated by carbonates. The lack of seismic image calls for model-based interpretation. Yet the usual fault-bend fold, fault-propagation fold and trishear fold models don’t seem to apply and with poor seismic data it is difficult to plan a well guaranteed to hit the crest.
Although the Iraqi part of the Zagros belt lacks the basal salt detachment that is famous in the SE part, numerous internal detachments dominate the style of deformation, particularly in the Triassic and younger. This led to a specific style of fold belt, which is not dominated by thrusts and stacked sheets, but by detachments, complex folds, and only the occasional thrust ramp. Most anticlines are steep-limbed detachment folds, with multiple internal detachments, locally overturned limbs, wedge-thrusts, and generally minor breakthrough thrusts that lead to many surprising geometries that are difficult to predict.
One of the major issues is forelimb-thickening by secondary folding between minor detachments, particularly in the lowermost Jurassic. Crests of deeper horizons are thus located farther hinterward, sometimes significantly (> 1km). Additionally, anticlines with two steep limbs may have a very tight core area, or reservoir horizons may be completely cut out of the visible anticline.
This type of deformation has been observed at many other locations world-wide. Detachment folds are a rather common occurrence and outcrops and image logs through steep limbs of folds often show a similar style of secondary folding in many other places (e.g., the Wyoming thrust belt, Canadian Rockies, southern Tien Shan, Northern and Southern Alps, Venezuelan Andes). However, the smaller scale of these folds tends to make this folding style less relevant for the industry. In the Zagros, however, numerous detachment horizons and rheological contrasts led to a style of deformation that has surprised even experienced operators.
Pre-Drill: In the High Fold belt seismic image is usually poor due to topography and exposed carbonates; many operators expected thrust-dominated anticlines and planned their wildcat wells accordingly.
Post-Drill: The post-drill image, however, looks rather different: This illustration after a multi-detachment analog model captures the prevalent structural regime much better. The anticline is not dominated by a thrust, but by disharmonic folding across the local detachment horizons. The relief of the anticline may increase upwards, with detachment horizons thickening and thinning. The left fold limb shows interesting secondary folding, which is actually very common throughout the Zagros at different scales. Such secondary folds are particularly common in the Lower Jurassic, impacting the location and geometry of deeper targets.
5:30pm Social Hour