Microfacies analysis, sedimentary environment and sequence stratigraphy of the Ilam Formation (Coniacian? - Santonian) in the northwestern part of the Abadan Plain

Abstract The Ilam Formation (Coniacian? –Santonian), is among the most important oil reservoirs of the Abadan Plain, SW Iran. Despite its reservoir significance, there is no comprehensive knowledge about the geological characteristics and factors controlling reservoir quality. In this study, the Ilam Formation is investigated using detailed core description, thin section study, and conventional petrophysical well log data to explain facies characteristics, sedimentary environment, and presenting the sequence stratigraphic framework. Accordingly, a total of 280 m of cores in four key wells from three oil fields as well as 620 thin sections were described and studied. The results of facies analysis lead to the recognition of 12 microfacies grouped in four facies belts including lagoon, shoal, shallow and deep open marine and two siliciclastic petrofacies (shales) related to brackish water and shallow lagoonal environment, which deposited in a carbonate ramp. Frequency analyses of facies associations and petrophysical well log signature indicate that the sedimentary basin was deepening to the east. Based on the identified sequence boundaries and maximum flooding surfaces, one third-order sequence in the studied interval was recognized and correlated by using the petrophysical well log data in all studied wells. The maximum flooding surfaces are discriminated by the development of deep-marine facies and also high gamma-ray responses on the well logs. Keywords: Ilam Formation, Abadan Plain, Facies, Sedimentary Environment, Sequence Stratigraphy Introduction Sedimentary facies, along with the distribution of diagenetic processes and fractures, are the three most important factors controlling the pores system and the reservoir characteristics of carbonates (Lucia 2007; Ahr 2008; Moore and Wade 2013). Despite the high impact of diagenetic processes on carbonate sequences, facies analysis and reconstruction of the ancient sedimentary environment is one of the most important and primary steps in the comprehensive geological studies of the reservoir (Ahr 2008). The sequence stratigraphic framework can provide an interconnection between the characteristics of the facies associations as components of a sequence and reservoir characteristics and also facilitate reservoir evaluation (Morad et al. 2012). To identify the key sequence boundaries, the integration of microfacies and core study results is applied (Tucker 1993; Taghavi et al. 2006). In other words, by using this framework, the pattern of facies distribution and the diagenetic processes trend related to facies could be examined (Mazzullo 1994). The focus of sequence stratigraphy is on the stacking pattern of sediments in space and time framework, which is formed by the contrast of the parameters of accommodation space and sediment supply (Catuneanu 2006; Catuneanu et al. 2012). Reservoir study in the sequence stratigraphy framework can lead to the identification, description, and study of the distribution of reservoir zones (Taghavi et al. 2006). To separate the sequences, it is important to identify the two key surfaces of maximum flooding surfaces and the sequence boundaries. The carbonate rocks of the Bangestan Group include the Sarvak and Ilam formations, where they are important reservoir rocks in many oil fields in the Zagros Basin and the Persian Gulf. The Ilam Formation in the Lorestan Basin is characterized by pelagic facies whereas in the Dezful Embayment and Fars appeared with neritic facies (James and Wynd 1965). In the type section in the northwest of Kabir-Kuh anticline, the lower boundary of this formation with Surgah Formation is conformable, while from the Lorestan Basin to the southeast, the Surgah Formation was disappeared and the Ilam Formation is placed on the Sarvak Formation disconformably. The purpose of this research is to identify and introduce microfacies and determine the geometry of the Ilam Formation platform along with determining the position of the studied wells in the sedimentary model. Also, we seek to identify the third-order depositional sequences and correlate them through petrophysical logs in other studied wells. Material & Methods In this study, four key wells in three oil fields of the Abadan Plain were used to identify microfacies, sedimentary environments and determine the depositional sequences of the Ilam Formation, by describing 280 meters of cores and studying 620 thin sections. Thin sections were stained by Alizarin Red S to differentiate calcite from dolomite (Dickson 1966). To identify microfacies and sedimentary environment, integration of core descriptions and thin section petrography studies have been used. In petrographical study of microscopic thin sections, mineralogy, texture, grain size, sorting, skeletal and non-skeletal components, and fossil content in each sample were identified and described quantitatively. The Dunham classification (Dunham 1962) has been used to describe and name microfacies. Standard facies models were used for facies analysis and nomenclature (Flügel 2010). Besides, based on the pattern of facies distribution, and also the frequency of facies, the location of the studied wells and palaeogeographical directions in the carbonate platform was determined. In order to determine the general time framework of the studied interval, based on identifying and investigating the distribution of fossil associations, especially planktonic and benthic foraminifera, biozones were compared with Wynd biozones (Wynd 1965) and to provide sequence stratigraphic framework, sequence boundaries, and maximum flooding surfaces were determined based on the pattern of facies changes as well as the determination of discontinuity surfaces (Hunt and Tucker 1992; Catuneanu 2006). Finally, using petrophysical logs (GR, RHOB, NPHI, and PHIE), the identified depositional sequences in studied wells are correlated. Discussion of Results & Conclusions Based on the integration of core description results, petrographic studies, and petrophysical logs, the Ilam Formation in some fields located in the northwestern part of Abadan plain were examined from several points of view such as facies features, sedimentary environment, platform geometry, biostratigraphy, and sequence stratigraphy framework. Facies studies led to the identification of two siliciclastic (shales) petrofacies PF1 and PF2, belonging to the estuary and shallow lagoon environment, respectively, and the 12 carbonate microfacies (MF1 to MF12) belonging to the four facies belts lagoon, shoal, shallow and deep open marine which are deposited in a carbonate ramp. An examination of the facies frequency showed that deep open marine facies, with the major abundance of planktonic foraminifera in a distinct trend from west to east, showed an increased thickness. Based on this, it seems that the west of the studied transect is the landward margin of the carbonate ramp, and by moving towards the high Zagros, in the east of transect, we approach the deep parts of this carbonate platform. Examination of the available cores and microfacies changes indicate that the only identified sequence boundaries are the lower contact (base of the Laffan Member) and the upper contact (the boundary between Ilam and Gurpi formations) in the studied succession, which both of them show evidence of subaerial exposure and brecciation (Sequence Boundary type 1); Therefore, the whole studied interval was identified as a third-order depositional sequence. The shaly Laffan Member was considered as Lowstand System Tract (LST) that mainly consisted of charophyte algal facies in the continental and estuary environment. The Maximum Flooding Surface (MFS) has corresponded to the deepest carbonate microfacies whereas Regressive Surface (RS) was considered by the continental fine-grained siliciclastic sediment supply and the increasing of lagoonal carbonate facies frequency. Transgressive Systems Tract (TST) is characterized by deep open marine microfacies, Highstand Systems Tract (HST) with shallow open marine microfacies, and Falling Stage Systems Tract (FSST) is characterized by the predominance of lagoonal microfacies with continental fine-grained siliciclastic sediments supply, and finally, correlation of sequence boundaries was performed in the studied wells by petrophysical logs. The thickness of HST and FSST facies show a reverse ratio in comparison with the whole thickness of the Ilam Formation. The maximum thickness of HST facies was observed in the east of the study area (Well D), whereas the maximum thickness of FSST was observed in the west of the studied transect (Well A). This confirms that the source of the continental siliciclastic sediments was closed to the west of the study area during the relative sea-level fall. The thickness changes observed for the Ilam Formation, along with the lateral facies change, could be attributed to tectonic activities and the beginning of collision and the closure of the Neo-Tethys