Fracture intensity vs. mechanical stratigraphy in platform top carbonates: the Aquitanian of the Asmari Formation, Khaviz Anticline, Zagros, SW Iran

Abstract

Outcrop analogue studies can significantly improve the understanding of fracture distribution and their impact on fluid flow in hydrocarbon reservoirs. In particular, the outcrops may reveal details on the relationships between mechanical stratigraphy and fracture characteristics. This has been investigated in an integrated sedimentological-structural geological study in the Aquitanian sequence of the Asmari Formation on the NE limb of the Khaviz Anticline in the Zagros foothills in SW Iran. The Aquitanian sequence was deposited in a platform top setting and is characterized by well-defined bedding planes and relatively thin layers (<4 m) with rapid changes in textures from laminated peritidal mudstones to bioclast and ooid grainstones. Fractures in the studied area dominantly strike parallel to the fold axis, have a high angle to bedding and are stratabound. In the literature it is often reported that fracture spacing or the inverse fracture intensity (FI) is controlled by the mechanical layer thickness (MLT). However, in the present study area a rather poor correlation between FI and MLT was observed. Instead, the Dunham texture appears to be more important for the FI. Mud-supported textures (mudstone and wackestone) have higher FI than grain-supported (packstone and grainstone) ones. The degree of dolomitization does not appear to have any significant effect on FI within each texture class. A strong relationship between FI and MLT is observed generally in cases where there has been one single phase of extension and when interbed contacts are weak, e.g. interbedded competent limestones and incompetent shales. However, in the present study area a rather complex deformation history exists and well-developed shales between fractured carbonate layeres are lacking. It is suggested that in such cases the MLT is of minor importance for the FI, which is controlled by the texture.

Influence of structural position on fracture networks in the Torridon Group, Achnashellach fold and thrust belt, NW Scotland

“…Understanding the mode of fold formation and strain history is critical for prediction of fracture attribute variations, which can be used in a range of applications, including fractured reservoir exploration, carbon capture and storage, aquifer characterisation and civil and mining engineering. Many studies have been conducted to investigate how fracture attributes vary in carbonate thrust belts for hydrocarbon exploration, such as in the Zagros foldand-thrust belt of Iran (McQuillan, 1973(McQuillan, , 1974Wennberg et al, 2006;Wennberg et al, 2007;Awdal et al, 2013), the Italian Apennines , the Rocky Mountains of the USA and Canada (Ghosh and Mitra, 2009;Barbier et al, 2012) and the Northeastern Brooks Range, Alaska (Hanks et al, 1997). Studies on fracturing in sandstone thrust belts are much less well documented; examples include Florez-Niño et al (2005) and Iñigo et al (2012) who use fractured outcrops as analogues to low porosity, low permeability (tight) sandstone hydrocarbon reservoirs in the Sub-Andean thrust belt.…”

Major, minor element chemistry and oxygen and hydrogen isotopic compositions of Marun oil‐field brines, SW Iran: Source history and economic potential

“…The Zagros Fold Belt consists of a package of folded and faulted rocks composed of 4-7 km of mainly Palaeozoic and Mesozoic successions overlain by 3-5 km of Cenozoic siliciclastic and carbonate rocks and resting on metamorphosed Proterozoic Pan-Africa basement (Brown and Jackson, 1960;Agar, 1987;Husseini, 1989). The resulting folding and deformation during successive Alpine orogenic pulses in the Zagros Fold Belt has also created large oil traps (Bordenave and Hegre, 2005;Wennberg et al, 2006), including that of the Marun oil traps.…”

Geomechanical Modeling of Stress and Fracture Distribution during Contractional Fault-Related Folding

“…Many studies have found that fractures often develop around fault zones and anticlinal core, and that fracture spacing is positively correlated with regional differential stress [12] [13] [14]. Some researchers have suggested that lithology and combination of fractures may be more significant than regional stress [15] [16] [17] [18] [19].…”