Controls of palaeoclimate condition on facies characteristics and diagenetic processes in the Cenomanian–Turonian sequences (upper Sarvak Formation) in the Abadan Plain, SW Iran

Abstract

AbstractThe Sarvak Formation, a crucial reservoir rock in the Abadan Plain, is extensively studied due to its sedimentary attributes and diagenetic evolution, heavily influenced by tectonic activities and palaeoclimatic conditions. This research focuses on analyzing palaeoclimatic indicators within the Sarvak Formation in a selected oil field in the Abadan Plain. Through a comprehensive approach integrating core data, thin section analyses, and electron microscopy, the study characterizes various facies, diagenetic processes, and sequence stratigraphy of the formation. Five distinct microfacies representing different depositional environments, such as lagoon, shoal, reef, reef-talus, and open marine belts, are identified, suggesting a ramp-type depositional setting. The investigation also reveals a paragenetic sequence of diagenetic processes, including transitions from marine to meteoric diagenesis and from shallow to deep burial environments. Notably, two palaeoexposure surfaces are identified, characterized by meteoric dissolution, brecciation, iron oxide staining, and silicification. Scanning electron microscopy analysis indicates prevailing kaolinite and montmorillonite clay mineral assemblages, indicative of warm and humid palaeoclimatic conditions. These findings provide insights into the palaeoclimatic conditions and paleaogeographical positioning of the Arabian Plate during the Cenomanian–Turonian period, suggesting a close proximity to low latitudes near the Equator.Keywords: Sarvak Formation, Palaeoclimate, Meteoric diagenesis, Abadan Plain IntroductionThe Middle East, particularly the Arabian Plate, is renowned for its vast oil and gas reserves. Among these, the Sarvak limestone formation in Iran's Zagros Basin stands out as a crucial reservoir rock. Its sedimentary characteristics and diagenetic processes are intricately linked to past climatic and geographical conditions. Climatic factors, along with fluctuations in sea levels and tectonic activity, have shaped highly productive reservoir units within the Sarvak Formation. Understanding these influences is pivotal for comprehensive reservoir studies, as they determine sedimentary facies and diagenetic alterations, thus impacting reservoir distribution within the formation.This study aims to analyze facies and diagenetic processes in the Sarvak Formation, particularly during the Cenomanian–Turonian period. By examining the effects of long-term climate on diagenesis, it seeks to enhance understanding of reservoir characteristics and improve modeling accuracy. Ultimately, this research aims to optimize hydrocarbon recovery from the Sarvak Formation's reserves, contributing to economic development in the region. Materials & MethodsIn pursuit of our study objectives, we examined 726 thin sections derived from the Sarvak Formation in two wells (K-01 and K-02) within the study area. The cored thickness of this formation in wells one and two amounted to 242 meters and 64 meters, respectively. We employed thin-section staining using a combination of alizarin red solution and potassium ferrocyanide to distinguish between dolomite and calcite. Generally, plug samples were extracted from the cored sections of the wells, with an average interval of one meter, focusing on capturing diagenetic processes during sampling. Additionally, we analyzed 65 SEM images using the MIRA3 TESCAN device at the Research Center of Razi Metallurgical Research Center in Tehran, Iran.For limestone examination, we utilized Dunham's (1962) and Embry and Klovan's (1971) classification methods. Furthermore, we applied Flügel's (2010) classification for facies description, interpretation, and establishing a conceptual sedimentary model. Our sequence stratigraphy studies aimed at identifying third and fourth-order sequences through the T-R method (separation based on transgressive systems tract and regressive systems tract). We also conducted petrographic studies to identify deepening and shallowing-upward facies to determine the maximum flooding surface (Embry 2002). Discussion of results & ConclusionsMicrofaciesThe study of skeletal, non-skeletal, and textural components related to thin sections of the Sarvak Formation in Wells 1 and 2 of the targeted oil field was conducted using the classification methods of Flügel (2010) and Dunham (1962) to identify five microfacies (Table 1). Benthic and planktonic foraminifera, rudists, corals, and echinoderms, along with other bio-clastic components, constitute the main constituents of these microfacies. Additionally, peloids and intraclasts have been identified as the most important non-skeletal components present in the studied microfacies. Sequence stratigraphyThe sequence stratigraphic study of the Sarvak Formation has led to the identification of two third-order depositional sequences named the Cenomanian sequence and the Turonian sequence, along with six fourth-order sequences. In the following, we will focus on a more detailed examination of the third-order sequences: Cenomanian Sequence (DSS-1)The thickness of the Cenomanian sequence in Well 1 is approximately 245 meters. The transgressive systems tract (TST) of this sequence consists of restricted lagoon facies (MF-1), reef and reef talus facies (MF-2 and MF-3), and open marine lagoon facies (MF-5), which are successively positioned upwards toward the maximum flooding surface (MFS-1). The regressive systems tract (RST) in the Cenomanian sequence comprises rudist and shoal facies (MF-4), indicating a shallowing-upward trend towards the Cenomanian-Turonian sequence boundary.The upper boundary of DSS-1, known as the Cenomanian–Turonian discontinuity (CT-ES), is exposed to meteoric diagenetic processes, displaying features such as meteoric dissolution, karstification, brecciation, and the formation of paleosols (Figure 3). The lower boundary of DSS-1 has not been identified in Wells 1 and 2 of the studied oil field. However, previous studies suggest its confinement at the base by the middle Cenomanian unconformity (Alsharhan and Nairn 1997; Aqrawi et al. 2010; Sharland et al. 2001; Hollis 2011). Turonian Sequence (DSS-2)The Turonian sequence in the studied wells has a thickness ranging from 15 to 25 meters. This sequence is predominantly composed of open marine lagoon facies (MF-5) in the lower half (TST) and restricted lagoon facies (MF-1) in the upper half (RST). The upper boundary of DSS-2 is characterized by a diagenetic boundary, exhibiting features such as silicification, brecciation, meteoric dissolution, and iron oxide staining. This discontinuity, known as the middle Turonian discontinuity (mT-ES), extends throughout the Tethyan basin (Sharland et al. 2001; Mehrabi et al. 2022). In the upper part of this sequence boundary, the shale of the Laffan Formation, dated to the Coniacian, limits the upper boundary of the Turonian sequence. DiagenesisThe types of diagenetic processes that have influenced the Sarvak sedimentary sequence in the studied section are as follows: micritization, cementation, recrystallization (neomorphism), dissolution, mechanical and chemical compaction, dolomitization associated with stylolitization, fracturing, silicification, pyritization, and formation of paleosol horizons. DiscussionDuring the Late Cretaceous era, the prevailing warm and humid climate in the Zagros region had a notable impact on the composition and distribution of carbonate organisms within the shallow-water carbonate platforms (Keller, 2008). Under such climatic conditions, the saturation level of seawater with respect to CaCO3 was often below the threshold, resulting in infrequent occurrences of evaporitic facies, primary dolomites, and ooids in the Sarvak platform. Instead, the platform predominantly consisted of bioclasts (such as rudists, algae, benthic and planktonic foraminifera, and molluscs), intraclasts, and peloids.Microfacies associated with shoal facies belts offer approximate insights into the climatic context of the Sarvak Formation. As previously noted, bioclastic-peloidal shoals indicate deposition in warm and humid climates (typical of the prevailing climatic conditions over the Sarvak carbonate platform during the Cretaceous). Conversely, algal-dominated shoals, often accompanied by evaporites indicative of warm and arid climates, are not observed in the studied oil field (Tucker and Wright 1990).Furthermore, the prevalence of organisms like rudists, corals, and green algae as the primary constituents of Sarvak Formation reefs, representing a coral-algal or coral-zoan skeletal association (Tucker and Wright 1990), underscores the dominance of warm climatic conditions in low latitudes (Flügel 2004).In warm and moist climates such as those during the Late Cretaceous era, like the conditions influencing the Sarvak Formation's deposition, the intensity and development of meteoric diagenetic processes heighten. Meteoric dissolution, karstification, low-magnesium calcite cementation, silicification, fracturing, and the development of paleosol horizons are all exacerbated. Conversely, in warm and arid climates, the meteoric diagenesis intensity diminishes due to limited meteoric waters, potentially leading to minimal sediment alterations over extended periods (Mehrabi et al. 2023).For instance, prolonged meteoric diagenesis of carbonate sediments under warm and humid climates can lead to the maturation and aging of karstified intervals, the principal product of meteoric diagenesis and sedimentation under such conditions. With increased dissolution and weathering, collapse events occur, filling dissolution cavities and caves with sedimentary debris, known as dissolution collapse breccias, compromising reservoir quality (Mazzullo and Chilingarian 1992; Keller 2008).Analysis of paleosol horizons from the Sarvak Formation reveals their rich content of montmorillonite and kaolinite minerals, with elevated iron and aluminum oxides. Upon closer examination away from discontinuities, the presence of Illite and Chlorite minerals becomes apparent. The occurrence of meteoric leaching and chemical weathering during the transition from the Cenomanian–Turonian and middle Turonian boundaries led to the formation of these paleosol horizons.

Keywords

• sarvak formation
• palaeoclimate
• meteoric diagenesis
• abadan plain