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Int'l Dinner Meeting, Detachment folding in the Kurdish Zagros: hidden structures and missed opportunities

Wed, Apr 19, 2017 5:30-8:30pm

Abstract:

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

6:30pm Dinner

7:15pm Presentation

8:30pm Adjourn

Speaker: Dr. Greg Schoenborn, Chevron
Speaker Dr. Greg Schoenborn, Chevron
Greg Schoenborn received his Masters and Doctorate for work on the Italian Southern Alps from the University of Basel, Switzerland. After a post-doc he continued as assistant professor at the University of Neuchatel, Switzerland. Main research topic was quantitative tectonic evolution of frontal mountain belts in 3D, while teaching classes ...

Greg Schoenborn received his Masters and Doctorate for work on the Italian Southern Alps from the University of Basel, Switzerland. After a post-doc he continued as assistant professor at the University of Neuchatel, Switzerland. Main research topic was quantitative tectonic evolution of frontal mountain belts in 3D, while teaching classes and supervising Masters and PhD students. In 1998 he switched careers and accepted a job with the structural geology team at Chevron’s lab in Southern California. While staying in similar organizational functions he moved a year later to San Ramon, CA and in 2010 to Houston.


Most of his work was done as structural geology expert, focusing initially more on research, then mostly on technical service for Chevron business units. Over the years he worked in more than 50 basins around the world in all types of structural environments – extension, contraction, reactivation/strike-slip, and of course salt. Recent work mainly focused on the Middle East and the Precaspian. Besides being involved in various intra-company initiatives, committees, conference-organizations and technical networks, he mentored interns, new hires, and mid-career professionals.


A large part of his energy has always gone to teaching with more than 1350 employees taking his often week-long classes: field schools in the Canadian Rockies, Eastern California, the San Francisco Bay Area, northern Spain, and class-room schools in offices around the world.


He greatly enjoys seeing new data and trying to solve the mysteries of seemingly conflicting datasets.

Full Description
Organizer Joe Bauman, Int'l Committee Chair

When?

Wed, April 19, 2017
5:30 p.m. - 8:30 p.m.
(GMT-0600) US/Central

Event has ended

Where?

Brookhaven Country Club
3333 Golfing Green Dr.
Farmers Branch, TX 75234
United States of America