Reservoir quality evaluation of the Late Jurassic Manifa carbonate sequence in one of oilfield in the Persian Gulf

Abstract The late Tithonian Manifa carbonate is one of the important oil producing intervals in some oil fields of the Northwestern Persian Gulf. The combination of geological and petrophysical data in three key wells in one of oilfield, enabled us to clarify the effects of depositional facies and diagenesis on reservoir quality in a sequence stratigraphic framework. Nine microfacies were recognized that deposited in three facies belts: tidal flat, lagoon and leeward shoal in a homoclinal carbonate ramp. The carbonate interval of the Manifa has been subjected to a variety of diagenetic processes, including micritization, bioturbation, recrystallization, dissolution, cementation, dolomitization, dedolomitization and compaction in three realm of marine, meteoric and burial diagenesis. From the identification of sequence boundaries and maximum flooding surfaces, one third-order sequence was recognized and interpreted. The sequence boundaries are important local and regional disconformity surfaces. Investigation of reservoir quality of different facies demonstrates that diagenetic features have considerable effects on the reservoir characteristics and regulated pore system characteristics. After deposition of the shallow marine Manifa carbonate was drastically subjected to the fresh-water diagenesis in the meteoric realm. The main important meteoric diagenesis alterations including karstification, dissolution, cementation and recrystallization are mainly concentrated below disconformity surfaces of the Manifa carbonate. The poroperm properties are very good related to dissolution phenomena creating interconnected pore space. Vuggy and solution enlarged interparticle porosities are the two dominant interconnected pore types within the reservoir. Considering the reservoir significance of the Manifa carbonate in the northwest Persian Gulf, the conclusion drawn from this investigation can be utilized for a better reservoir characterization. Keywords: Manifa, Persian Gulf, Facies, Diagenesis, Sequence Stratigraphy, Reservoir Quality   Introduction Reservoir quality of carbonate reservoirs is controlled by many interrelated factors such as depositional facies, diagenetic processes and tectonic setting (Lucia 2007; Ahr 2008; Moore & Wade 2013). In fact, by controlling the pore system properties, these parameters affect the fluid flow and saturation within the reservoirs (Ahr 2008). Although, the original sedimentary properties substantially control the reservoir quality, but diagenesis may exert considerable influence on pore characteristics (Lucia 2007; Ahr 2008; Moore & Wade 2013). The reservoir quality may be enhanced or deteriorated depending on the type of dominant diagenetic processes (Ahr 2008). The Late Tithonian sequences of the Manifa carbonate is notable carbonate reservoir in some part of the Arabian Platform. Despite of this importance, a little is known about various geological and petrophysical characteristics in Iranian northwestern part of the Persian Gulf. This study is the first report of sedimentological and reservoir geological aspects of this carbonate sequences in subsurface sections of the northwestern Persian Gulf. In this study, by using a complete dataset of drilled cores, thin sections, petrophysical well log and porosity-permeability data, interpretation of reservoir quality evaluation has been carried out on the Manifa carbonate. The aims of this study are to investigate the facies characteristics, depositional environments, diagenetic alterations and evaluate pore system and reservoir properties in relation to the disconformity surface of the Manifa Formation in the northwestern part of the Persian Gulf   Material & Methods A continuous, 52-m-thick core through the Manifa Formation was logged in one of oilfield in the northwestern Persian Gulf. To differentiate the calcite from dolomite, all thin sections were stained with Alizarin Red-S (Dickson 1966). Also, in order to determine pores types and their properties, epoxy resin was injected to the samples (130 thin section), and all samples were scanned by a high-resolution (Up to 10000 DPI) scanner (CREO-IQSMART3) at the Petroleum Industry of Research Institute. Petrographic studies were carried out to identify different types of depositional facies, and based on the allochems type and their frequency, different sub-environments were distinguished. Also, the important diagenetic processes affecting the pores system were identified. Based on the combination of the results of facies/diagenesis studies the framework of sequence stratigraphy was introduced and one third-order sequences interpreted. Due to the interpreted distribution of depositional and diagenetic characteristics, petrophysical well logs were used for their correlation between the three studied wells in the studied field.   Discussion of Results & Conclusion The core description and petrographic studies resulted in the identification of nine microfacies. These microfacies were attributed to three facies belts including tidal flat, lagoon, and leeward shoal. The carbonate ramp model proposed for the Manifa Formation is comparable with the model and facies distribution presented for this interval in the neighboring area. The Manifa carbonate has been subjected to a variety of diagenetic processes including micritization, bioturbation, recrystallization, compaction, cementation, dissolution, dolomitization, dedolomitization and fracturing. Dissolution, cementation, dolomitization, compaction, along with fracturing have had the greatest impact on the pore system and reservoir properties of the Manifa carbonate. In order to establish a sequence stratigraphic framework and specify the sequence boundaries and maximum flooding surfaces for the studied formation, the results from depositional and diagenetic studies were integrated. Hence, sequence stratigraphic analysis of the studied interval has resulted in the recognition of one third-order depositional sequences with aggradational-progradational stacking pattern. At the Late Tithonian, a eustatic sea-level fall caused platform emersion and subaerial exposure at top of the Manifa carbonate. This sequences are bounded with two local and regional disconformities. From reservoir quality point of view, intense meteoric dissolution of grain-to mud-dominated facies exert a main improving effect and development of intervals with high porosity and permeability. Core poroperm data interpretation clearly demonstrate that most grain- and mud-dominated facies related to different sub-environments have a good reservoir quality due to the high impact of dissolution related to meteoric diagenesis. With respect to the effect of diagenesis on the reservoir properties, the defined diagenetic facies display a distinct trend on poroperm distribution. Porosity-permeability distribution indicating the high heterogeneity is strongly related to the effects of both depositional and diagenetic characteristics. Vuggy and solution interparticle pore spaces are two dominant pore types and show high pore connectivity. Thus, an integrated sedimentological and petrophysical reservoir evaluation of the Manifa carbonate in one of oilfield in the northwestern Persian Gulf indicate that diagenetic alterations had main role in pore system characteristics and reservoir evolution.