Tight carbonate reservoirs exhibit thick transition zones which usually contain large quantities of oil and therefore a significant amount of reserve might be left after secondary recovery. Therefore, the proper characterization of transition in terms of depositional facies, diagenesis and structural features are important for improved knowledge of transition zones. The nature of the mixer of the hydrocarbons with water in the transition zones face considerable production challenges, hence, require  proper  reservoir  characterization  coupled   with advanced completion and stimulation techniques for efficient oil production. Shiekah et al. (2009) defined the transition zone and predict the performance of the transition zone regarding the water saturation changes. It is well known that depositional facies mainly control the pore system geometry and architecture, primary porosity and permeability of the rocks, sedimentology and formation water chemistry near the surface water (Henares et al., 2014; Hartmann and Beaumont, 1999; Morad et al., 2010). Petrographic and  diagenetic  studies of reservoir rock samples are crucial in reservoir quality predictions as well as understanding the relationship between depositional facies and diagenesis (Bloch and McGowen, 1994; Henares et al., 2014). Diagenesis accounts for all the alteration processes which occur in the original rock or group of facies after deposition; it results in the modification of the pore network or pore throat geometries by preservation, loss, or enhancement of rock petrophysical properties. Moreover, diagenesis is used to understand and predict changes in the reservoir pore structure. It is also important to note that the influence of the diagenetic overprint on the determination of the pore system and their impact on the reservoir quality and its flow behavior (Farooq et al., 2014; Wazir et al., 2015; Aliakbardoust and Rahimpour-Bonab, 2013; Rushing et al., 2008; Zhang et al., 2009; Gomes et al., 2008).

Recently, Kolchugin et al. (2016) studied the diagenesis of the paleo-oil-water transition zone in a lower Pennsylvanian carbonate reservoir to document the strengths of a combined petrographic and geochemical study in order to investigate the actual timing of oil migration in carbonate reservoirs and its related cementation dynamics.

A few authors have described the microporosity effect as a diagenesis feature, by using thin sections and scanning electron microscope (SEM) methods (Fullmer et al., 2014; Vahrenkamp et al., 2014). Gale et al. (2005)studied the diageneses especially fractures and cementation in order to predict the fluid flow. Again Ehrenberg et al. (2014)and Major (2014)studied the diagenetic environment and regimes that result on enhancing the quality of the reservoir under investigation. Apart from these studies, it is necessary to work on characterization of transition zone sample in terms of petrographic analysis and diagenesis. Therefore, we hope that the present research work would be able to provide new aspects and information about characterizations of transition zone samples for further study by diagenesis analysis.

During this research, a petrographic study on thin sections was performed by SEM in order to understand the depositional textures, pore types, diagenetic features and visually estimate the porosity and, eventually, the permeability. A detailed analysis of the sample was done on the basis of petrographic and diagenesis features. Before continuing special core analysis (SCAL) of transition zone samples, it is necessary for reservoir quality through diagenesis. Therefore, the present work interacts the attention of reservoir engineers to primary diagenesis analysis of reservoir rock samples.  

Geological setting and environmental situation of the field

The transition zone under this study is part of a reservoir group from the  Early  Cretaceous,  deposited  during  the Aptian age. This zone is defined as the interval between the top of one of the reservoir zone and the base of the tight, argillaceous formation and represents the lower part of the Shuaiba Formation (Strohmenger et al., 2006). The field under study (Figure 1A), which is an onshore oil field in Abu Dhabi, is a double-plunging anticline trending NE-SW formed during the Campanian (Al Bloushi, 2013; Vahrenkamp et al., 2014). The thickness varies between the crusts of the structure with 190 to 130 ft at the flank. According to the regional geology, this zone was deposited within the mega sequence AP8 when the Arabian Plate was undergone several tectonic events such as the opening of the Indian Ocean that results in the formation of a new passive margin within the Neo-Tethys as shown in Figure 1B (Sharland et al., 2013). Strohmenger et al. (2006) interpreted the reservoir zone as a third-order sequence, corresponding to the AP3A 18 sequence of the composite transgressive systems tract of a second-order composite sequence. Strohmenger et al. (2006) and Al Bloushi (2013) studied the depositional environment and texture of this zone and they stated that this reservoir zone was deposited on an open platform, subtidal, and depositional environment in a carbonate ramp to basin system. The zone has various porosity ranging from <1 to 38% comprising of intragranular, intergranular, microporosity and moldic porosities. The decrease in porosity with depth from crest to flank represents the transition zone. The permeability ranges from 0.1 to 1000 mD depending on the facies. It was recognized that the permeability depends on the type of porosity and the average pore size. Again Strohmenger et al. (2006)discussed the texture of this zone. The reservoir zone is dominated by shallow marine planktonic foraminifera wackestone/packstone with interbedded (dense) bioturbated, burrowed wackestone to mudstone. The sequences start with the subaerial exposure to a transgressive period of mudstone/wackestone, then ends with a regressive interval of packstone/grainstone cycle (Strohmenger et al., 2006; Vahrenkamp et al., 2014; Van Buchem et al., 2013).

Table 1 shows the conventional core analysis (CCA) results of the three samples used in this study at ambient conditions.

Petrographic/Petrology study (Depositional textures)

The majority of the zone comprises limestones, while the dolomite is observed with the SEM only at the bottom of the zone (Figure 2A and B). There are four major textural classifications, namely wackestone, packstone, grainstone and floatstone available. There are five major types of bioclasts within the samples studied, which are thin-shelled bivalves, brachiopods, planktonic foraminifera, gastropods and echinoderms. Some of the bioclasts are aggregated bioclasts that form the grainstone texture.

Diagenetic process and features

Four major diagenetic processes have affected the transition zone including micritization, cementation, dissolution and compaction.

Micritization

Micritization in this transition zone is extensive in the bioclasts. These micritized bioclasts can form the matrix during deposition (depositional micrite) (Figure 3A). According to Alsharhan and Nairn (1990), micrite underwent recrystallization (to form microcrystalline calcite) as shown from the SEM imaging (Figure 3B). Micritization is the earliest diagenetic event in this transition zone. It destroys the internal features of the skeletal grains and thus it degrades the reservoir quality.

Dissolution

Dissolution is the leaching of the metastable bioclasts due to the presence or circulation of the meteoric waters (Tucker and Wright, 2009; Flügel, 2004). However, partial dissolution of the bioclasts is common in the upper zone of the well, which depends on the original bioclasts composition stability as shown in Figure 4A and B. The pore shape will have the shape of the originally dissolved fossil. Dissolution of carbonate rocks occurs thus leading to an enhancement of the reservoir porosity in the form of solution  moldic  voids.  The  moldic  porosity  along   with intergranular porosity is the most common type of porosity found in the upper zone of the well.

Cementation

Based on our earlier study, it can be stated that cementation is a very important factor for controlling porosity and permeability in rock (Bera and Hadi, 2016). Figure 5A and B shows drusy and blocky calcite in the present sample analysis. Saddle dolomite (Figure 5C and D) and fluorite cement (Figure 5E and F) are also present in the sample which indicates the diagenetic environment where it was precipitated and the timing of the cementation. Calcite, saddle dolomite and fluorite (CaF2) are the main cement phases in this zone. In general cementation reduces the porosity and permeability as it plugs the pores. Cement precipitation can fill the pores fully or partially based on the situation. Cement precipitation (drusy or blocky) and/or by matrix during deposition or post-deposition by compaction processes can clog the pores resulting in lower permeability (Tucker, 2009; Lucia, 2007). Drusy calcite cement is thought to be a sign of early diagenesis in a meteoric environment, whereas the saddle dolomite and the fluorite mineral are believed to be formed by circulation of hydrothermal fluids at a deep (Flügel, 2004; Tucker, 2009; Lambert et al., 2006; Hollis et al., 2010; Alsharhan and Nairn, 1990). It is also believed to have occurred after reaching burial depth of around 600 m at temperatures of around 65.6°C (Cox et al., 2010). This is considered to be a late diagenesis stage, where it replaces calcite.

Compaction

In this zone, mechanical compaction is very limited and it appears in some sections which are fractured. Fractures can enhance overall reservoir quality if they are not cemented (open fractures) (Figure 6A and B), however, some open fractures could be induced while coring or plugging the core. Most of the fractures are partially cemented (Figure 6C and D) or closed  fractures  due  to the precipitation of the cement or compaction which reduces the porosity and the permeability.

Paragenesis

In this zone, the timing is divided into two main events such as Eogenesis and Mesogenesis and telogenesis is not observed in this study.

Eogenesis

After the shallow burial of sediments, they are subjected to endolithic bacterial action that bore into such components and produces a dark micritic seam around grains. This was followed by precipitation of drusy calcite as it commonly occurs in the intergranular pore spaces (Figure 7). The water chemistry was  supersaturated  with respect to low Mg calcite, which enabled most of the drusy calcite crystals cementation, followed by blocky calcite cement. This occurs in meteoric and marine environments and proceeds on to burial environments.

Mesogenesis

During mesogenesis, there was an increase in pressure-temperature gradients, which allowed for compressional features such as fractures to occur. The second generation of cementation can occur in the remaining pores where saddle dolomite precipitates as a result of this higher temperature and pressure regime. These dolomitic precipitations are likely to be a result of the hydrothermal fluid circulation (Tucker, 2001). Blocky calcite cement is also an evidence of the late diagenetic regime, along with the presence of the fluorite minerals. After attaining an overall understanding of the diagenetic processes affecting the transition zone, by identifying the features that characterize each one, a summary of the overall diagenetic realm can be formulated by placing the occurrence of each event in order of occurrence. Figure 8 summarizes the list of events and their related effects over time.

In Eogenesis zone, there is extensive micritization observed in the sections, which contributes to the degradation of reservoir quality by diminishing primary porosity and permeability. Aside from micritization, cementation also occurred in the eogenesis in marine, mixed, and meteoric phreatic zones, which contributed to the degradation of the reservoir quality in terms of porosity and permeability. Certain events contributed to enhancing reservoir quality such as dissolution as shown in Table 2.

Mesogentic events mainly contributed to destroying porosity and permeability from the precipitations of the saddle and rhombic dolomite in pores and fractures. Diagenesis in the transition zone is very extensive especially the micritization and cementation due to the presence of the water that is responsible for the diagenetic overprint. The porosity is very low as cement is filling the pore spaces. We observed that in the upper zone, the diagenesis is less intense compared to the deeper intervals due to the presence of more oil that prevents the diagenetic processes. The diagenetic features are increasing with depth as the zone is having higher water saturations.

The main lithology is limestone with some dolomite at the bottom of the transition zone. The dolomite is observed by the SEM imaging only as the scale of investigation is different than the thin sections. A depositional facies ranging from wakestones to floatstones were found. It was also seen that while some diagenetic features such as fracturing and dissolution enhanced  reservoir  quality, others such as micritization, cementation and compaction led to a reduction in the porosity and permeability. Three types of fractures like opened, partially closed and closed are observed and therefore permeability is affected. The paragenetic sequences are mainly occurring during the eogenesis and the mesogenesis regime. 

The authors have not declared any conflict of interests.