Combining biodegradation in 2D petroleum system models: application to the Cretaceous petroleum system of the southern Persian Gulf basin
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Biodegradation modeling is combined with petroleum system modeling along a regional 2D transect from the southern Persian Gulf basin. An advanced basement model was considered in the model that allowed a constant temperature to enter the entire sedimentary column. Modeling results were calibrated against existing temperature and vitrinite reflectance data from nearby wells. Temperature, burial, and charge histories are carefully monitored within the studied section through geological time. Based on our modeling results, biodegradation has been effective along the migration pathway and within the eventual accumulation sites to varying extents. These findings can have practical implications for pre-drill oil quality predictions and accurate geochemical evaluations of oils and reduce offshore oil and gas exploration risks.
KeywordsSouthern Persian Gulf basin Biodegradation modeling Cretaceous petroleum system 2D petroleum system modeling
Biodegradation is a common process in many hydrocarbon reservoirs and can change the physical–chemical properties of petroleum in varying forms and extents (Connan 1984; Head et al. 2010; Head et al. 2003). Measuring the intensity of biodegradation in oil reservoirs has been the focus of many studies since the microbial alteration entails destructive effects on economic value and producibility of the oil (Larter et al. 2003, 2012; Peters and Moldowan 1993; Seifert and Moldowan 1979; Wenger and Isaksen 2002). Various factors controlling the rate of petroleum biodegradation in subsurface reservoirs have been discussed (Adams et al. 2013; Larter et al. 2003).
The essential elements and processes of the petroleum systems lodged in the Persian Gulf basin are influenced by several tectonic pulses that are discussed elsewhere (Alipour 2017; Alipour et al. 2016, 2017). This information is carefully combined with other geological and geochemical data, and a conceptual basin model is constructed in this study, so that the geohistory of petroleum systems could be recreated through time (Hantschel and Kauerauf 2009; Peters et al. 2012, 2017).
Paleogeographic reconstructions during the Upper Cretaceous of the area indicate development of intrashelf basins over the eastern edge of the Arabian Plate (Vahrenkamp et al. 2015). Deposition of organic-rich facies of the Middle Sarvak source rock took place behind the Mishrif rim, which represent favorable carrier/reservoir rock qualities in the area of our study (Al-Zaabi et al. 2010; Hennhoefer et al. 2018). The stratigraphy of the Cenomanian Sarvak–Mishrif sequence of the southern Persian Gulf basin has been discussed in more detail recently (Jodeyri-Agaii et al. 2018). In addition, recent geochemical studies on the oils from this basin have highlighted the importance of the Middle Sarvak organic-rich facies in charging some of the oilfields in the Sirri District (Alizadeh et al. 2017; Hosseiny et al. 2017).
Materials and methods
Rock-Eval 6 instrument, operated under standard procedure, was used for determining the quantity and quality of organic matter in drill cuttings of the Middle Sarvak source rock obtained from a well nearby to the modeled section (see Fig. 1). In brief, aliquots of pulverized rock samples (about 70 mg) were heated under 300 °C for 4 min to release the free hydrocarbons known as the S1 peak. The oven temperature was then ramped to 650 °C (25 °C/min rate) to measure the S2 peak, which represents hydrocarbons released via thermal breakdown of kerogen (Peters 1986). The temperature at the highest pyrolytic yield is recorded as Tmax, which gives an indication of maturation state of the organic matter (Peters and Cassa 1994).
Vitrinite reflectance measurements were made using a Leica P LED4500 microscope at a magnification of 500 × under oil immersion and a 546-nm filter. The instrument was calibrated using standard sapphire glasses with 0.494, 0.795, and 1.77 reflectance. Reflectance readings were made in random mode on vitrinite macerals following the ASTM method (D7708-11), and the reported values were arithmetic means of at least 25 measurements per sample.
Similar to the previous studies (Alipour et al. 2017), the two-dimensional model in this study included an advanced basement model underneath the sedimentary package, which comprised of the Upper Continental Crust, the Lower Continental Crust, and the Mantle (with appropriate lithofacies and thicknesses, respectively). A constant temperature of around 1333 °C was considered at the base of the Upper Mantle, and the surface temperatures were allowed to vary according to the existing geological knowledge of the area.
Temperature and vitrinite reflectance data from three wells (see Fig. 1) used for model calibration in this study
Vitrinite reflectance (VRo%)
Petroleum generation and migration
Similarly, expulsion of generated hydrocarbons begins in the trough first; whence, they begin to migrate updip within the porous Mishrif carbonates based on the Darcy’s law (Fig. 7). The directions and speed of fluid movement are controlled by lateral and vertical differences in the capillary entry pressure between the different cells, which in turn are controlled by lithofacies properties of each cell.
The updip hydrocarbon migration within the Mishrif carbonates is somewhat influenced by the Laffan Shales as also previously noted in another 2D transects in our study area (Alipour et al. 2016). Wherever these shales are thin or non-existent, migrating hydrocarbons would gain access to the overlying carbonates of the Ilam Formation (Fig. 7). Depending on the existing capillary pressure distributions, the migrating hydrocarbons would shift path from the Mishrif carbonates into the Ilam Formation. However, the net result would be an updip journey toward the SIF-1 High underneath the thick shales of the Gurpi and Pabdeh formations (Fig. 7). Therefore, sequence stratigraphic principals seem to define the drainage efficiency of the Middle Sarvak kitchen and hence play a critical role in charging of the potential traps that lie en route.
The extents to which the geochemistry of migrating hydrocarbons is affected by these successive phases of biodegradation are not clearly known at present. However, laboratory simulation studies can provide valuable estimates on compositional modifications resulting from such microbiological processes (e.g., Mishra et al. 2017). Knowledge of these processes is essential for accurate geochemical evaluation of oils from the southern Persian Gulf basin, in particular when mixing of multiple charges occurs (Alipour 2017; Alizadeh et al. 2017).
Combining biodegradation within 2D petroleum system models is an effective means for understanding the processes controlling the present-day geochemistry of petroleum accumulations. This is particularly useful for offshore hydrocarbon exploration and production, where pre-drill oil quality prediction is highly desired. Application of this technique on a regional 2D transect from the southern Persian Gulf basin indicated that biodegradation can be operative on the expelled hydrocarbons both within their migration path and in the eventual accumulation sites. The sequence stratigraphic architecture of the study area may allow shifting of migration paths between different carbonate carriers. The net result will be a cumulative influence of biodegradation on individual hydrocarbon charges that reach the accumulation sites. Understanding the occurrence and extents of these processes can help better characterize the associated petroleum systems and reduce exploration risks.
The authors are grateful to the Iranian Offshore Oil Company (IOOC) for providing the data and permission to publish. Petroleum Geology and Geochemistry Research Center (PGGRC) of Shahid Chamran University of Ahvaz is gratefully acknowledged. We are grateful for constructive comments from anonymous reviewers, which greatly enhanced the quality of this manuscript.
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