Analysis of Fold and Fault-Related Fracture Systems Development Using Image Logs at Asmari Formation in the Rag Sefid Anticline, SW Iran

Archive of SID Geopersia 9 (2), 2019, PP. 279-292 DOI: 10.22059/GEOPE.2019.264206.648410 Analysis of fold and fault-related fracture systems development using image logs at Asmari Formation in the Rag Sefid Anticline, SW Iran Mehdi Yousefi *, Seyed Morteza Moussavi, Mohammad Mehdi Khatib Department of Geology, Faculty of science, University of Birjand, Birjand, Iran *Corresponding author, e–mail: [email protected] (received: 21/08/2018 ; accepted: 03/02/2019) Abstract Image logs of 13 wells in the Rag Sefid Anticline show two systems of fractures developed under two deformation phase in the Dezful Embayment. This deformation phases are folding resulted from Zagros NE contraction and reactivation of the basement fault rotated the fold axial trace within the Hendijan-Izeh fault zone. Folding phase comprises four sets of fractures, which include axial and cross axial sets that trend parallel and perpendicular to the fold axial trace, respectively and two oblique sets that trend at moderate angles to the axial trace in the eastern part of the fold. Reactivation of the Hendijan-Izeh Fault has caused the restraining bend and dextral shear zone in the western part of the Rag Sefid Anticline. This dextral shear has produced three fracture sets which include two sets of Riedel shear fractures and an extensional set. The mean shortening directions measured from the fold and fault related fracture systems in the eastern and western parts of Rag Sefid Anticline are N022±2° and N064±1°, respectively. The measured NNE and ENE shortening orientations in the well sites is consistent with maximum horizontal stress orientations derived from earthquake focal mechanism solutions and with the absolute plate motion direction of the Arabian Plate with respect to Eurasia in the Dezful Embayment. Keywords: Asmari Formation, Rag Sefid Anticline, Fracture Systems, Hendijan-Izeh Fault, Image logs. Introduction constitute the first system, have well defined A number of regional studies have demonstrated the relationships to the fold structural elements and compressional reactivation of preexisting structures occur within the boundaries of the fold structures. within both the cover and the basement of foreland The spatial variation of these sets of fractures is a thrust belts worldwide (e.g., Alps: Lacombe & function of the spatial variation of the fold elements Mouthereau, 2002; Taiwan: Mouthereau et al., and also position within the fold-thrust belt. Many 2002). Moreover, reactivation of the intraplate studies have tried to relate development of micro basement faults and structural inversion of and meso-structural pattern of fractures respect to sedimentary basins have been documented in the fold geometrical elements, such as fold axis, foreland far from the orogens (e.g., Tapponnier et forelimb, back limb and termination (Gholipour, al., 1986; Ziegler et al., 1995; Lacombe & 1998; Wennberg et al., 2006; Inigo et al., 2012). Mouthereau, 1999; Marshak et al., 2000). The The second fracture system includes several sets of development of various trends of fracture is basement faults, and their related, subsidiary affected by reactivation of the faults. The fractures, that cut through the folded Phanerozoic understanding of the factors controlling the fracture sedimentary sequence in the Zagros Fold and patterns such as orientation, distribution, density, Thrust Belt (ZFTB). While the spatial density and spatial variation and chronology is important to variation of the fold-related fractures correlate with improve the methods used in the fractured folds themselves, the fault-related fracture system is reservoirs. Furthermore, from a regional-scale point localized along isolated linear zones, possibly of view, this understanding is of major interest for marking the boundaries of basement blocks development plans of these reservoirs, which can (Mobasher & Babaie, 2008). Because of the constitute petroleum provinces like the Southwest essential role of fractures in the hydrocarbon Zagros Mountains (Ahmadhadi et al., 2008). The production from the Dezful Embayment (Southwest high productivity of the Zagros Mountains Zagros Mountains) oil fields, many studies have reservoirs is related to the presence of two systems been done in this area to describe the fracture of fractures that have led to secondary effective population within carbonate reservoirs. Among the porosity in the carbonate reservoirs (Ahmadhadi et studied formations, the Asmari Formation is famous al., 2008). In the Southwest Zagros Mountains with regards to fracture studies because (i) this Several sets of fold-related fractures, which Oligo-Miocene formation crops out in a 1200 km www.SID.ir Archive of SID 280 Yousefi et al. Geopersia, 9 (2), 2019 long by 200 km wide belt expanding from the currently undergoing 20–25 mm/yr shortening northeast Iraq to the southwest Iran. (ii) It also (Vernant et al., 2004) and thickening as a result of includes most of the hydrocarbon reservoirs in this collision of the Arabian and Central Iran plates part of the ZFTB (e.g., Beydoun, 1992). The (Berberian, 1995). This folded belt provides a good orientation of open fractures relative to the anticline example of convergence partitioning in its NW part, axis, or relative to the Hendijan Fault trend, is the the overall NW-SE convergence between the most important reason for the separation of the fold- Arabian and Eurasian plates being accommodated related fracture and the fault related fractures. by dextral strike-slip movement at the rear of the Therefore we have applied FMI and FMS resistivity belt along the Main Recent Fault (Talebian & image logs of 13 developmental wells in the Rag Jackson, 2002) and by belt-perpendicular Sefid Anticline to help establish the kinematic shortening leading to the formation of N140 relationships of the fold and fault structures and trending folds (Hessami et al., 2001; Agard et al., analyze the tectonic history of the South Dezful 2005). The Zagros folded belt has been divided into Embayment. The main purposes of this research several morphotectonic regions. These main include: (i) identifying and distinguishing between morphotectonic regions are limited by deep seated fold and fault related fracture sets, (ii) determining basement faults (Fig. 1). Two major tectonic trends, the kinematic importance of each fracture system respectively N-S and NW-SE, are known in the and (iii) comparing the direction of shortening Arabian Shield and there is evidence for the resulted from analysis of each system and applying continuation of these trends northward into the the knowledge of the spatial variation in the Zagros orogenic belt (Bahroudi et al., 2003). In the orientation of the kinematic axes to the Zagros belt, the approximate locations and understanding of Zagros orogenic movements. geometries of the basement faults have been defined using geodetic survey, more or less precise Geological Setting epicenter/hypocenter locations, as well as The Zagros fold and thrust belt (ZFTB) trending topographic and morphotectonic analyses. The first NW-SE to nearly E-W is resulted from several group of basement faults includes the Mountain deformation events from Late Cretaceous (Vernant Front Fault (MFF), the Dezful Embayment Fault et al., 2004) (Fig. 1 a). This fold and thrust belt is (DEF), and the Zagros Foredeep Fault (ZFF). Figure 1. a Location of the Rag sefid Anticline in the Zagros folded belt. b Geological map of the Rag sefid Anticline. c Lithostratigraphy chart in the studied area since Lower Cretaceous. www.SID.ir Archive of SID Analysis of fold and fault-related fracture systems development … 281 Fault plane solutions for earthquakes along these the Gachsaran Formation (Middle Miocene), which faults indicate that they all dip about 60ᴼ NE, is the main cap rock of the Asmari reservoirs in suggesting that they now act as reverse faults Iran. The Rag sefid Anticline is located in the south although they may have acted as normal faults of the Dezful Embayment, close to the Hendijan- during the Permo-Triassic opening of the Neo Izeh Fault, one of the major NNE– SSW trending Tethys ocean (Berberian, 1995). Another group of right-lateral strike-slip fault systems in the Zagros basement faults are N-S trending faults which Simply Folded belt. The Rag Sefid structure is developed during the latest Proterozoic and Early boomerang shape and is surrounded by other Cambrian in the Arabian basement (Beydoun, adjacent structures such as Bibi-Hakimeh, Pazanan, 1992). During the Triassic and Late Cretaceous, N- Agha-Jari, Ramshir, Tangu, Zageh and Hendijan. S trending uplifted zones and basins were related to The overall axial trend is about WNW–ESE; the intermittent reactivation of this group of however, the western part of this anticline has basement faults (Sherkati & Letouzey, 2004). These NNW–SSE trend. This fold is asymmetric and faults are steeply dipping and currently undergo characterized by a high angle southwestern forelimb right-lateral strike-slip motion (Berberian, 1995; due to faults development in this limb. This Hessami et al., 2001). As presented in Fig. 1.b, structure has 54 Km length and 4-8 Km width some of these faults located in the Zagros belt, are located between 29° 50′ N and 30° 35′ N latitudes, the Hendijan- Izeh Fault (HIF), the Kharg-Mish and between 49° 42′ E and 50° 054′ E longitudes. Fault (KMF) and the Kazerun Fault (KZ). Structural This anticline has three reservoir horizons, are the information resulted from seismicity within the Khami Group (Gas bearing), Bangestan Group and Zagros belt proves that these faults are still active as Asmari Formation (Oil/Gas bearing). Based on right lateral strike-slip faults in the basement biostratigraphical data, there is a hiatus between the underlying the folded cover (Jackson & McKenzie, Pabdeh and Sarvak formations. This hiatus 1984; Berberian, 1995; Talebian & Jackson, 2002). encompasses the Gurpi, Ilam, Lafan and even The study area is located in southern west margin of Upper Sarvak formations (Fig. 1c), suggesting a the Dezful Embayment. The Dezful Embayment paleo high lineament in the Rag Sefid area corresponds to a morphotectonic region stepped (Abdollahie Fard et al., 2006).

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