Online Journal for E&P Geoscientists

Vandeginste, Veerle ^*1; John, Cedric M.¹; Cosgrove, John W.¹
(1) Imperial College London, London, United Kingdom.

Major fractures in the subsurface of the Middle East complicate hydrocarbon recovery. The location of these fractures that are undetectable on seismic sections, is unpredictable thus representing significant geohazards for exploration. To tackle this challenge and improve predictability, we undertook a study on outcrop analogues of Permian reservoirs in the Jebel Akhdar tectonic window in Oman. Since fractures in the subsurface offshore Qatar are associated with dolomite, we investigate fracture-controlled dolomitization and the extent and dimensions of the dolomite geobodies at the outcrop. Our goal was to assess a potential new workflow to predict the occurrence of fractures in analogue systems in the subsurface.

Our structural-diagenetic study in Wadi Mistal combines a fracture analysis, put into the broader regional structural framework, with a petrographic and geochemical analysis of about 130 hand samples. In addition, the dolomite dimensions were mapped in detail at the outcrop and we assessed the link between lithology and dolomite extent. We observed both stratabound and fracture-related dolomite. The majority of the small-scale fractures has a similar orientation as the main, large-scale fault. While the stratabound dolomite is characterized by small euhedral or subhedral crystals, the fracture-related dolomite is coarser and mainly anhedral. In addition, the fracture-related dolomite has much higher iron and manganese contents and more depleted δ¹⁸O values than the stratabound dolomite. Also, ⁸⁷Sr/⁸⁶Sr ratios are higher in pristine fracture-related dolomite than in stratabound dolomite and limestone and provide evidence for a ⁸⁷Sr enriched dolomitizing fluid for the fracture dolomite. The elevated ⁸⁷Sr/⁸⁶Sr ratio along with the very high iron and manganese values are interpreted as resulting from interaction of the fluids with crystalline basement. This result thus argues against interaction between the fluids and the Semail ophiolite, supporting the relevance of the Wadi Mistal outcrop as an analogue for subsurface Qatar. The structural and geochemical data may indicate an Early Santonian age for the formation of the fracture-related dolomite. Fluid inclusion work is currently carried out to gain more information in order to assess this hypothesis.

This study is part of the Qatar Carbonate Carbon Storage Centre funded jointly by Qatar Petroleum, Shell and the Qatar Science & Technology Park.