Journal of Stratigraphy and Sedimentology Researches (Jun 2022)
A Quantitative analysis of reservoir heterogeneities of the Sarvak Formation in an oilfield from the Abadan Plain, SW Iran
Abstract
Abstract This study focuses on reservoir heterogeneities of the Sarvak Formation in the Abadan Plain. Coefficient of variation (CV) and Dykstra-Parsons (VDP) approaches are used for quantification of reservoir heterogeneities. To define the hydraulic flow units, rock types, and reservoir zonation, flow zone indicator (FZI), Winland R35, and Lorenz (SMLP) methods are adopted. Heterogeneities of porosity and permeability data are quantified in each rock type, HFU and reservoir zone. Then, results of sedimentological studies are integrated with petrophysical data to analyze their scale, origins, and predictability in sequence stratigraphic framework. Depositional and diagenetic heterogeneities are differentiated. Facies variations and changes in textural characteristics and sedimentary structures provided small scale heterogeneities. Meteoric dissolution beneath the Cenomanian–Turonian palaeoexposure surface formed the best reservoir zone of the formation. Cementation and compaction, mostly related to burial diagenesis, reduced the reservoir quality. Results of this study indicate that large scale heterogeneities of the Sarvak Formation are predictable in the framework of third-order sequences. Meteorically dissolved rudist dominated facies provided the best productive zones in the regressive systems tract (RST) of the Cenomanian and Turonian sequences. However, small scale heterogeneities are not easily predictable in third-order depositional sequences and systems tracts. Keywords: Sarvak Formation, Reservoir heterogeneity, Coefficient of variation, Dykstra-Parsons, Sequence stratigraphy, Abadan Plain Introduction Reservoir heterogeneity refers to the changes in petrophysical properties of rocks (i.e., porosity, permeability, water saturation, capillary pressure). They have variable origins and scales (Nurmi et al. 1990; Tiab and Donaldson 2015). Carbonate rocks host considerable amounts of hydrocarbon resources in the Middle East and around the World. Carbonate reservoirs are strongly heterogeneous because of their complex depositional and diagenetic processes (Ahr 2008; Wei et al. 2015; Tavakoli 2020). Major parts of these heterogeneities are predictable in sequence stratigraphic framework (Lucia, 2007; Rahimpour-Bonab et al. 2012; Enayati-Bidgoli and Rahimpour-Bonab 2016; Mehrabi et al. 2019). Quantification of geological reservoir heterogeneities was the subject of some carbonate reservoirs in Iran (Tavoosi Iraj et al. 2021) and other countries (Nurmi et al. 1990; Dutilleul 1993; Fitch et al. 2015). This study presents the results of quantitative analysis of reservoir heterogeneities in the Sarvak Formation in an oilfield from the Abadan Plain. The main targets of this study are; A review of depositional setting and diagenetic history of the Sarvak Formation in the Abadan Plain. Sequence stratigraphic analysis of the Sarvak Formation. Reservoir rock typing and zonation of the Sarvak Formation. Quantification of reservoir heterogeneities based on the CV and VDP approaches, and Delineation of origins, scale of occurrence, and predictability of reservoir heterogeneities in sequence stratigraphic framework. Material & Methods Dataset of this study includes 258 m of core samples, 550 thin sections, porosity–permeability measurements (532 plug samples), and petrophysical logs (GR, LLD, LLS, RHOB, and NPHI) of the Sarvak Formation in one well from an oilfield in the Abadan Plain. Facies analysis was adopted using the standard facies nomenclature schemes (Embry and Klovan 1971; Dunham 1962) and facies models (Flügel 2010). Transgressive–Regressive (T-R) method is used for sequence stratigraphic analysis (Embry 2002; Catuneanu et al. 2011). Reservoir rock types and zones are identified based on the following approaches; Winland method This method uses R35 values by using the following equation: Log R35 = 0.732 + 0.588 Log (K) – 0.864 Log (φ) Hydraulic flow units (HFU) definition using the flow zone indicator (FZI) This method was firstly presented by Amaefule et al. (1993) that uses porosity and permeability data to calculate the reservoir quality index (RQI), normalized porosity (PhiZ), and flow zone indicator (FZI): RQI = 0.0314√(K/∅e ) ∅_z = ∅e/(1-∅e ) FZI = RQI/∅z Lorenz reservoir zonation Gunter et al. (1997) proposed a method of reservoir zonation that incorporates porosity and permeability data to calculate the storage capacity (PhiH) and flow capacity (KH): KHcum= K1 (h1-h0)/Khtotal + K2 (h2-h1)/Khtotal +…. + Kn (hn-hn-1)/Khtotal PhiHcum= Ф1 (h1-h0)/Фhtotal + Ф2 (h2-h1)/Фhtotal +…. + Фn (hn-hn-1)/Фhtotal Quantitative analysis of reservoir heterogeneities For quantification of reservoir heterogeneities, two methods are uses; Coefficient of variation (CV) method: CV = Standard Deviation (STDEVA) / Mean Dykstra-Parsons (VDP) method: = = Discussion of Results & Conclusions Facies analysis indicates that the Sarvak Formation has composed of eight microfacies deposited in a ramp type carbonate platform. They include lagoon (MF8), rudistid reef talus (MF7), shoal (MFs 4 to 6), and open marine (middle- to outer ramp) facies (MFs 1 to 3). Diagenetic processes include intense meteoric dissolution and cementation, dolomitization, mechanical and chemical compaction, neomorphism, burial cementation, and fracturing. Two third-order depositional sequences are defined with two major palaeoexposure surfaces as the sequence boundaries. They are attributed to the Cenomanian–Turonian boundary (CT-ES) and middle Turonian (mT-ES) palaeoexposure events (Navidtalab et al. 2016; Mehrabi et al. 2020,2022; Bagherpour et al. 2021). Nine hydraulic flow units (HFUs), six Winland’s classes, and ten Lorenz zones have been differentiated within the Sarvak Formation. Detailed sedimentological and petrophysical properties of HFUs, WRTs, and Lorenz zones are discussed. Statistical analysis of reservoir heterogeneities is adopted by using the CV and VDP approaches for all rock types and zones. Results of these studies and measurements have revealed that reservoir heterogeneities of the Sarvak Formation are originated from the both depositional (facies) characteristics and diagenetic alterations (especially meteoric diagenesis below the palaeoexposure surfaces). The best example for the control of facies on reservoir heterogeneities is the development of rudist-dominated facies with high reservoir potential within the RSTs of third-order depositional sequences (especially Cenomanian sequence). Consequently, a major part of reservoir heterogeneity in the Sarvak Fmormation depends on the distribution of such rudist-dominated intervals. On the other hand, palaeoexposure-related diagenetic processes had major control on reservoir properties of the Sarvak Fmormation, below the CT-ES and mT-ES. In this regard, meteoric dissolution, dolomitization, and fracturing have improved reservoir properties, and, in contrast, compaction and cementation largely decreased the reservoir quality in this formation. Results of this study have revealed that large-scale reservoir heterogeneities of the Sarvak Formation are traceable in the framework of third-order depositional sequences. In this regard, high-quality (reservoir) units of this formation are concentrated withing the RSTs of third-order depositional sequences. They have composed of dissolved, grain-supported facies of reef-talus and shoal complexes. In contrast, compacted and cemented mud-dominated facies provided non-reservoir units of the Sarvak Formation in the studied well.
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