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The Upper Jurassic–Cretaceous Petroleum System of Southern Iraq: a 3-D Basin Modelling Study

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The Upper Jurassic–Cretaceous Petroleum System of Southern Iraq: a 3-D Basin Modelling Study GeoArabia, 2013, v. 18, no. 1, p. 179-200 Gulf PetroLink, Bahrain The Upper Jurassic–Cretaceous petroleum system of southern Iraq: A 3-D basin modelling study Qusay Abeed, Ralf Littke, Frank Strozyk and Anna K. Uffmann ABSTRACT A 3-D basin model of the southern Mesopotamian Basin, southern Iraq, was built in order to quantify key aspects of the petroleum system. The model is based on detailed seismic interpretation and organic geochemical data, both for source rocks and oils. Bulk kinetic analysis for three source rock samples was used to quantify petroleum generation characteristics and to estimate the temperature and timing of petroleum generation. These analyses indicate that petroleum generation from the Yamama source rock (one of the main source rocks in the study area) starts at relatively low temperatures of 70–80°C, which is typical for Type II-S kerogen at low to moderate heating rates typical of sedimentary basins. Petroleum system analysis was achieved using the results from 1-D, 2-D, and 3-D basin modelling, the latter being the major focus of this study. The 1-D model reveals that the Upper Jurassic–Lower Cretaceous sediments are now within the oil window, whereas the formations that overlie the Yamama Formation are still immature in the entire study area. Present-day temperature reflects the maximum temperature of the sedimentary sequence, which indicates that no strong regional uplift affected the sedimentary rocks in the past. The 3-D model results indicate that oil generation in the Yamama source rock already commenced in the Cretaceous. At some locations of the basin this source rock reaches a present-day maximum temperature of 140–150°C. The most common migration pathways are in the vertical direction, i.e. direct migration upward from the source rock to the reservoir. This is partly related to the fact that the Lower Cretaceous reservoir horizons in southern Iraq directly overlay the source rock. INTRODUCTION The Jurassic–Cretaceous petroleum system of southern Iraq contains excellent source, reservoir, and seal rocks leading to the presence of several giant oil fields (Figure 1). According to detailed organic geochemical characterization, the oil, which is mainly trapped in Lower Cretaceous sandstone and carbonate reservoirs, was generated from organic matter-rich Upper Jurassic to Lower Cretaceous carbonate source rocks (Abeed et al., 2012). Evaporites and compacted mudstones seal the reservoir rocks effectively (Figure 2). Whereas new investigations have documented the maturation of the source rocks and the characteristics of the oils reservoired in this area (Pitman et al., 2004; Al-Ameri et al., 2009; Abeed et al., 2011, 2012), a detailed basin-modelling study, specifically for this area, has not yet been published. This paper is based on a 3-D basin model, focusing on the structural aspects as well as the generation, migration and accumulation of hydrocarbons within the Mesozoic sequences in the Basra region, southern Iraq. Several 1-D burial-history models have been constructed for individual wells (Figure 1), of which one for well R-172 is discussed in this study. Rumaila Well R-172 is a deep well that penetrates the Jurassic Gotnia evaporites (Figure 2). A 2-D model for the studied sequence was extracted from the 3-D model. It reveals the general geometry of the hydrocarbon structural traps. The 3-D model has been built based on the interpretation of 12 seismic sections across the study area. The data for this model are derived from general geological information on sedimentology and tectonic evolution of the area, along with several well reports as well as organic geochemical data on source rock core samples and crude oils published in Abeed et al. (2011, 2012). In order to provide the constructed model with the required data on petroleum generation characteristics, three source rock samples have been analysed for kinetic parameters. 179 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/18/1/179/4568847/abeed.pdf by guest on 01 October 2021 Abeed et al. 180 180 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/18/1/179/4568847/abeed.pdf by guest on 01 October 2021 Upper Jurassic–Cretaceous 3-D basin modelling study, southern Iraq PETROLEUM SYSTEM Only a few detailed scientific studies on the Mesozoic petroleum system of the Basra region have been published so far. A preliminary attempt to investigate the geothermal gradient of southern Iraq was written by Ibrahim (1984). Pitman et al. (2004) studied petroleum generation and migration in the Mesopotamian Basin and the Zagros Fold Belt of Iraq by applying basin modelling software. The petroleum system of a single reservoir rock (Mishrif) in three different oil fields in Basra has been studied by Al-Ameri et al. (2009). Studying an entire petroleum system within a basin requires a detailed understanding of the geological history of source, reservoir and seal rocks, as well as an analysis of the varying geological processes that affected the basin through geological time, such as erosion, uplift, or the evolution of salt diapirs. Moreover, migration pathways and traps need to be interpreted correctly to achieve a model of petroleum accumulation. The most likely source rocks for the Cretaceous hydrocarbons in the Basra region, southern Iraq, are the Upper Jurassic to Lower Cretaceous Yamama and Sulaiy formations (Al-Ameri et al., 2009; Abeed et al., 2011, 2012). The Upper Jurassic to Lower Cretaceous successions are marine carbonates with a very good hydrocarbon generation potential, which were deposited under anoxic conditions. Thickness of the succession exceeds 600 m in southern Iraq (Sadooni, 1993; Jassim and Goff, 2006). The Yamama Formation in southern Iraq was examined in wells Su-8 (Subba) and R-172 (Rumaila) and shows a predominant input of marine organic matter and a good source rock potential, both in terms of quantity and quality of organic matter (Abeed et al., 2011). The underlying Sulaiy Formation represents a similar organic facies, as indicated by carbonate content and organic matter quantity, but is much more mature; therefore, most of the petroleum generation potential has already occurred. Biomarker data are scarce for the Sulaiy Depth Lithology Oil Age Formation (m) Reservoir API S% Formation due to this high maturity. Therefore, biomarker data of the Mishrif 23 5.3 Yamama source rock were compared Cenomanian Rumaila to those of crude oils from the Zubair Ahmadi reservoir, showing a good oil-source 2,500 Mauddud rock correlation (Abeed et al., 2012). The sediments in both the Sulaiy and Albian S Yamama formations were accumulated Nahr Umr 35 3.2 in a predominantly dysoxic-anoxic environment (Al-Ameri et al., 1999). ACEOU Aptian Shu'aiba 3,000 The Sulaiy Formation can be regarded CRET as an excellent source rock, which has Barremian Zubair 29 3.6 already realized most of its petroleum generation potential. Maturity Hauterivian Ratawi parameters suggest that this formation has reached and passed peak oil generation, but has not reached the Valanginian Yamama 37 2.5 dry gas generation zone (Abeed et al., 2011). Berriasian Sulaiy 4,000 Tithonian Gotnia C Kimmeridgian Sandstone Organic Limestone 4,500 Shale Limestone JURASSI Oxfordian- Source Callovian Najma Evaporite Seal Dolomite Reservoir Evaporitic- Bathonian- Sargelu Unconformity Bajocian Dolomite Figure 2: Generalized stratigraphic section through the study area with typical oil characteristics (see Abeed et al., 2012). 181 181 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/18/1/179/4568847/abeed.pdf by guest on 01 October 2021 Abeed et al. Cretaceous carbonate and clastic rocks are the main reservoirs in southern Iraq, with the Mishrif, Nahr Umr, and Zubair reservoirs being the economically most important ones (Beydoun, 1986). In addition, Abeed et al. (2012) have demonstrated the importance of the Yamama reservoir, which contains the lightest oil of the Basra oil fields. The porosity exceeds 20% in the carbonate reservoirs and reaches values greater than 30% in the clastic reservoirs. Permeability varies from low permeability in the Ratawi reservoir (e.g. Tuba oilfield) to greater than 1,000 milliDarcy in the Zubair, Shu’aiba, and Nahr Umr reservoirs (Jassim and Goff, 2006). The Upper Jurassic Najmah limestone is the primary Jurassic reservoir. This unit consists of oolitic limestones, dolomites, and anhydrites that were deposited in a shallow-marine and transitional marine setting composed of lagoons and shoals, similar to the Arabian and southern Arabian Gulf Basin (Fox and Ahlbrandt, 2002). The Gotnia Formation is a regional seal in Iraq. It was deposited on the very shallow southern platform in a sabkha environment (Lurestan Basin), as basinal salt and laminated anhydrite and shales (Murris, 1980). It is about 200 m thick and forms a tight seal for local oil and gas accumulations in the underlying Najmah Limestone (Fox and Ahlbrandt, 2002). During Cretaceous times, several shales were deposited, which act as seal rocks. Only some of the compacted shales formed efficient seals, e.g. no producible oil was encountered in the Mauddud Formation in southern Iraq, although it is an important reservoir in the Middle East. This is either due to the inefficiency of the Ahmadi shale as a cap rock in southern Iraq or due to the presence of shale beds in the upper part of the Nahr Umr, which generally act as cap rocks in southern Iraq and which may prevent vertical charging of overlying reservoirs (Ibrahim, 1983; see Figure 2). All wells in the central, eastern, and southeastern parts of southern Iraq were drilled on structural prospects (Ibrahim, 1983). The most abundant traps of southern Iraq are elongated narrow anticlines. These structures are mainly NS-trending, possibly originating from the Neoproterozoic Nabitah orogeny. They were reactivated during the Carboniferous–Permian, Mesozoic and Tertiary (Jassim and Goff, 2006).
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