The Influence of Tectonics on the Entrapment of Oil in the Dezful Embayment, Zagros Foldbelt, Iran

Abstract

The timing of the orogenic events associated with the closure of South Tethys significantly influenced the generation, migration and entrapment of petroleum in the Zagros Foldbelt of Iran. This influence was particularly important in the Dezful Embayment, which is one of the world's richest oil provinces, containing some 8% of global oil reserves in an area of only 60,000 sq. km. In the Dezful Embayment, oil and associated gas occur in two carbonate reservoirs ‐ the Sarvak Formation of Cenomanian to Turonian age, and the Oligocene ‐ Early Miocene Asmari Formation, sealed by the evaporites of the Gachsaran Formation. The oil and associated gas are trapped in large “whaleback” anticlines which formed during the Neogene Zagros orogeny. Two excellent source rocks, the Albian Kazhdumi Formation and the upper part of the Pabdeh Formation (Middle Eocene to Early Oligocene), supplied the Asmari and Sarvak reservoirs and with them form the Middle Cretaceous to Early Miocene Petroleum System. This system was found to be independent of older petroleum systems. Two particular problems are addressed in this paper. The first is the relative timing of trap formation versus oil expulsion from the source rocks. If oil expulsion occurred prior to Zagros folding, the oil would have migrated along gently dipping ramps towards the Persian (Arabian) Gulf and Southern Iraq, and would have been trapped a long way from the source kitchen. By contrast, if oil expulsion took place when the whaleback anticlines already existed or had at least begun to develop, the oil generated would have moved almost vertically towards the nearest anticline. Secondly, we assess the type of heatflow to be used for modelling. This could be either variable or constant, depending on the stability or instability of the Arabian Platform and on subsidence variations during source rock maturation. Our conclusions can be summarized as follows. First, the paroxysmal phase of Zagros folding commenced in the Dezful Embayment towards the end of the Middle Miocene around 10 Ma ago and continued throughout the Late Miocene and Pliocene. Second, bearing in mind the remarkable stability of the Arabian Platform for some 260 Ma, during which there was almost continuous gentle subsidence between the Permian transgression and the Early Miocene, a constant heatflow was used for modelling. Burial profiles and maturity indices, such as vitrinite reflectance and Rock‐Eval parameters, demonstrate that the Kazhdumi and Pabdeh source rocks reached the onset of oil expulsion during deposition of the Agha Jari Formation between 8 and 3Ma, depending upon the location. This chronology means that oil migrated from source rocks into preexisting Zagros structures. Therefore, oil migrated over short distances to nearby traps within well‐defined drainage areas, the geometry of which can be deduced from seismic data. Moreover, the Zagros folding induced prominent fracturing which can be observed both at outcrop and in wells. This fracturing, which affects lime‐stones as well as marls, enhanced...

Genetic mechanism and development of the unsteady Sarvak play of the Azadegan oil field, southwest of Iran

“…Besides this, due to the extrusion stress decreasing, the folding amplitude as well as the deformation intensity decreased progressively following the same trend (Hessami et al 2001;Alavi 2004). The borderline along the low-amplitude Zagros trend traps, such as Ab-e Teymur and Mansuri (formed during 5-4 Ma) in the east of the Azadegan oil field, is deemed as the boundary of the surface Zagros folded zone (Hessami et al 2001;Bordenave and Hegre 2005). This means that the effect of the Zagros Trend is hardly observed on the surface outside the line, where the buried structure zone exists ( Fig.…”

“…The other type, known as the Arabian Trend-uplifting caused by basement fault ''resurrection'' and salt flow, widely distributed in a number of oil fields which are located in southeast of Iraq, Kuwait, and northeast of Saudi Arabia, with N-S trending anticlinal structures. In this type of oil field, the reservoirs are mainly Cretaceous Formations, i.e., Ilam, Sarvak in Iran and Mishrif, Rumalia in Iraq (Alsharhan 1995;Alsharhan and Nairn 1997;Beydoun 1991;Sadooni and Aqrawi 2000;Sadooni 2005;Bordenave and Hegre 2005). As shown in Fig.…”

“…Two different types of traps with different forming mechanisms are confirmed here: One type is called the Zagros Trend, located in the foothill zone (Zagros folded zone) with an NW-SE strike, and with similar trends as the anticlinal structure elongated in the Zagros Mountain range. The Miocene Asmari Formation, which is dominantly limestone and partially sandstone, is the main reservoir of the Zagros Trend oil fields (Wang et al 2011;Zhang et al 2012;McQuillan 1973McQuillan , 1974Bordenave and Hegre 2005). The other type, known as the Arabian Trend-uplifting caused by basement fault ''resurrection'' and salt flow, widely distributed in a number of oil fields which are located in southeast of Iraq, Kuwait, and northeast of Saudi Arabia, with N-S trending anticlinal structures.…”

“…Regionally, these structures are abundant in the Persian Gulf, Saudi Arabia, Kuwait, southeast Iraq, and southwest Iran (Murris 1980;Koop and Kholief 1982;Glennie 2000;Sherkati and Letouzeh 2004). In the Azadegan field, the Sarvak occurs as a result of carbonate shelf deposition in the mid-Cretaceous (Murris 1980;Alsharhan and Nairn 1997;Bordenave and Hegre 2005). In the early Paleocene (65 Ma, Fig.…”

Crustal-scale cross-sections across the NW Zagros belt: implications for the Arabian margin reconstruction

“…For more detailed stratigraphic information the interested reader should refer to the pioneering works by James & Wynd (1965) and Braud (1970), and more recent contributions by different authors (e.g. Motiei, 1994Motiei, , 1995Alavi, 2004;Bordenave & Hegre, 2005;Carruba et al 2006;Heydari, 2008;Farzipour-Saein et al 2009;Saura et al 2011).…”