Controls of depositional facies and diagenetic processes on reservoir quality of the Arab Formation in the one oil field, Southern Persian Gulf

Abstract

AbstractThe Late Jurassic Arab Formation is one of the main oil reservoirs in the Middle East. To investigate factors reservoir quality controlling, the integration of the results from geological and petrophysical data was utilized. Ten main facies were recognized and grouped in five facies belts of tidal flat, intertidal, lagoon, shoal, and shallow open marine, indicating deposition of the formation on a homoclinal carbonate ramp platform. The diagenetic processes occurred in marine, meteoric, and burial diagenetic realms. According to the frequency within the reservoir, five main pore types comprising intercrystalline, interparticle, vuggy, moldic, and microporosity were determined. Four third-order depositional sequences which are correlatable with four reservoir zones (A–D) of the Arab Formation were identified. The development of grainstone facies played an important role in high reservoir potential intervals. In view of their controls on reservoir quality, diagenetic features can be categorized into two classes: (1) diagenetic processes enhancing reservoir quality that include dissolution, dolomitization, and fracturing; (2) diagenetic processes reducing reservoir quality including cementation, evaporate mineralization, overdolomitization, and compaction. Considering its layer cake nature, it is possible to use petrophysical logs for the correlation of different reservoir zones, and depositional sequences between the studied wells.Keywords: Arab Formation, Persian Gulf, Facies, Diagenesis, Pore systems, Sequence stratigraphy, Reservoir quality IntroductionThe 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 and Wade 2013). The reservoir quality may be enhanced or deteriorated depending on the type of dominant diagenetic processes (e.g., compaction, cementation, dissolution, dolomitization, and fracturing). The Arab Formation has been extensively studied in the Persian Gulf from different geological and petrophysical aspects, including facies analysis and depositional environment (Hughes 1966; Al-Saad and Sadduni 2001; Daraei et al. 2014; Sfidari et al. 2018), diagenesis, and geochemistry interpretation (Swart et al. 2005; Nader et al. 2013; Morad et al. 2012, 2019), sequence stratigraphy (Al-Husseini 2009; Al-Awwad and Pomar 2015) and reservoir characterization and rock typing (Cantrell and Hagerty 2003; Grötsch et al. 2003; Eltom et al. 2012 Marchionda et al. 2017; Assadi et al. 2018). This study aims to define the depositional facies and environments, diagenetic alterations, various pore types, and reservoir quality variations in the Arab formation in an oil field in the southern Persian Gulf. The important geological features are integrated with conventional petrophysical logs and core porosity–permeability data to delineate the reservoir and non-reservoir zones in a sequence stratigraphic framework. Material & MethodsThis study is based on data from four cored wells (A, B, C, and D) from one of the hydrocarbon fields in the southern Persian Gulf. A total of 248 m of cores (whole cores and slabbed), 201 thin sections, 420 core porosity-permeability plugs and conventional petrophysical well log data were used to investigate its impact of various geological factors on reservoir properties. To differentiate the calcite from dolomite, all thin sections were stained with Alizarin Red-S (Dickson 1966). Also, in order to determine pore types and their properties, epoxy resin was injected into all samples. Depositional facies, diagenetic processes, and pore systems were studied based on core description and petrographic analysis. Finally, controls of depositional facies characteristics and diagenetic alterations on pore types and reservoir quality were discussed in a sequence stratigraphic framework. A set of conventional well logs from the four studied wells, including GR -DT-RHOB and calculated effective porosity (PHIE), were used for reservoir zonation and correlation. Discussion of Results & ConclusionDepositional facies analysis led to the identification of 10 sedimentary facies. They are mainly distinguished based on their constituents, texture, and sedimentary structures, and belong to five sub-environments of supratidal, intertidal, lagoon, shoal, and open marine located on a homoclinal carbonate ramp. Diagenetic imprints occurred in three diagenetic environments of marine, meteoric, and burial. The pore system and reservoir characteristics were influenced by dissolution, cementation, dolomitization, evaporite mineralization, and compaction. The dolomites appear in two types as replacement or cement. The replacement dolomite is classified as fabric retentive and fabric destructive. The dolomite cement occluded intergranular, intercrystalline, and vuggy pores and fractures. Associated evaporite mineralization occurs as primary and secondary. Four third-order depositional sequences were recognized through the identification of maximum flooding surfaces (MFS) and sequence boundaries (SB), which are distinguished on the basis of facies and diagenetic characteristics. Each depositional sequence is a cycle that corresponds to a reservoir zone (Arab A, B, C, and D) within the formation. Porosity–permeability cross plots are depicted for identified lithological types, texture, depositional environment, and pore types groups of the Arab Formation in the studied field. The best reservoir intervals correspond with grainstone facies deposited in shoal sub environment that contain high values of interparticle and vuggy porosity and represented by high dissolution zone. The results obtained in this study can provide a comprehensive understanding of the reservoir characteristic of the Arab Formation in the southern Persian Gulf.

Keywords

• arab formation
• persian gulf
• facies
• diagenesis
• pore systems
• sequence stratigraphy
• reservoir quality