Rock Type and Pore Throat Radius of Zubair Formation in the W Oil Field: Analysis Utilizing Core and Log Data

Abstract


Introduction
W oilfield is one of Iraq's largest oilfields, located near Basra City is one of the primary oil reserves in the W Oilfield is the Zubair sandstone from the Lower Cretaceous formation. Given its exceptional reservoir qualities, it is also one of the most significant reservoirs in the Mesopotamia basin in central and southern Iraq (Ibrahim, 2001;Awadh, 2018). Understanding the depositional model and Facies distribution is complicated by heterogeneity and intricacy (Awadh et al., 2019). The primary instrument for studying petrophysical characteristics is core testing, however, to understand rock characteristics and stratigraphic accumulation patterns, and it is not possible to obtain plugs from each well . The pore throat radius and rock type dominate the relationship between porosity and permeability (P&P). P&P can be measured directly, as in this work, using core analysis, and wireline log data can also be used to assist in the identification process. Petrophysical parameters, such as reservoir porosity, permeability, and pore throat classification are also identified. Pore characteristics are an important factor to consider when evaluating a reservoir. Pore structure influences the storage mechanism and reservoir fluid characteristics of permeable units, whereas it is a strong dominant factor for petrophysical properties and multiphase-flow features in reservoir rocks. (Zainab et al., 2019).
The hydraulic flow units (HFUs) approach has been recognized by the use of the flow zone indicator (FZI). Each HFU must have identical geological and petrophysical features and is recreated by specific FZI, and similar geological and petrophysical characteristics are expected to exist. The rock type, pore throat radius, and flow units for every unit of the Zubair Formation in Southern Iraq's W oilfield three wells used in the study were determined using core data from Well-1 and well logging. Another connection used to detect reservoir features by rock quality is the pore throat radius.

Study Area
W oilfield is approximately 50 kilometers northwest of Basra city, on the north part of the Arabian Plate in southern Iraq (Fig. 1). The oilfield is separated portions by the Euphrates River into two portions: part W (1) in the south and part W (2) in the north ( Ismail et al., 2021). A north-trending low relief anticline runs across the W Oil field. The eastern flank dips at roughly 2°, whereas the west flank dips at 3.2°-4° (western flank is steeper than eastern flank).

Geological Setting
The W oilfield is located on the Arabian Plate's outer platform, in the Mesopotamian zone stable shelf (foredeep basin) (Fouad, 2010). This basin was formed when the Neo-Tethys Ocean opened and was developed by Mesozoic and Cenozoic orogenic processes. The Mesopotamian Basin was asymmetric, like other foreland basins, with the thickest deposits in the foredeep region and the greatest accommodation space. Mesopotamian zone is divided into three subzones, Tigress, Euphrates, and Zubair subzone (Jassim and Goff, 2006). The Zubair was formed during the Early Cretaceous plate tectonic period, and rocks deposited during this period are classified as Mega sequence AP8, Thammama group. Many elongated folds of N-S to NW-SE orientation in huge oilfields in Basra Province identify the Zubair Subzone. According to Sharland's (2001), classification, Iraq's strata are grouped into 11 mega se Fig.2. Stratigraphic Colum of W Oilfield (Sharland, 2001). The Shuaiba Aptian Formation is overlain conformably and gradationally, whereas the Valanginian-Hauterivian Ratawi Formation is underlain conformably and gradationally (Jassim and Goff, 2006). The Zubair Formation is a prolific oil reservoir found in various oil fields in southern Iraq (Al-Jaberi and Al-Jafar, 2020). As seen formation comprises thick sandstones with interbedded shales and siltstones from the Lower Cretaceous (Hauterivian to lower Aptian). The Zubair Formation is divided into informal components in southern Iraq. Including the Lower Shale, the Lower Sand, the Middle Shale, the Upper Sand, and the Upper Shale (from oldest to youngest). At the W oilfield, the Main Pay member is an oil producer. At the W oilfield, the Main Pay reservoir is found at −3170 meters. Recent research focused on the upper sand member, which is divided into three informal units. Shale Intervals (K unit) divide the H and L reservoir units and are laterally continuous over the field area (Figs. 3 & 4).  The Zubair Formation comprises fluviodeltaic, deltaic, and marine sandstones, making it Iraq's most important sandstone reservoir . The NE boundary of the Zubair sandstone field on the Arabian Plate is characterized by a Facies change from delta front sandstones to shelf mudstones. The Zubair Formation sandstones dominate the basin's SW edge and thin off as it approaches Iran. In humid tropical climatic conditions, the Zubair Formation was deposited in a fluviodeltaic to the marine environment ( Fig.5).

Fluvial environment
Fluvial deposits can be found in the upper delta plain. The deposits' nature is determined by the kind of river and the environment. Fluvial ecosystems are intricate processes of erosion, sediment transport, and deposition that produce various landforms. Their sediments include anything from coarse conglomerates to sandstones and mudstone. Fluvial sandstones are often sharp-based and cross-bedded, with occasional flat bedding and cross lamination (Fig.5). Quartz with a mid-fine enormous grain. In high porosity and high permeability flow units like the H and L Unit, the clean sand is found with little to low amounts of bioturbation (Al-Dabbas et al., 2014). The best reservoir corresponds to fluvial sandstone.

Channels environment
This Facies starts with a layer of coarse-grained alluvial stone, which is followed by a layer of fine sandstone, which leads to weakly cohesive silt at the top. Energy transfer from high-energy to lowenergy sedimentation Sharp-based, upward-fining sets of the massive, trough, and tabular crossstratified and laminated sandstones interpreted as channel fills dominate the Main Pay Thickly bedded sandstone. Sharp contact at the base, Low shale content, high porosity, and high permeability.

Mouth bar environment
Massive, muddy medium sand that has been highly bioturbated. Medium porosity and low permeability are seen in high shale. Meanwhile, upward-coarsening mouth bars have been observed, thus they are uncommon. This is due to the sharp-based nature of the proximal parts of mouth bars, as well as high sediment input rates in relatively shallow water, which are considered to have induced fast progradation and the suppression of easily.

Pro delta environment
In the core, prodelta mudstones are frequently badly preserved (rubbled), although they appear to be essentially free of bioturbation, with only very weakly formed Chondrites. High shale concentration and low to no porosity are related to low bioturbation intensity in mouth bars, which reflects stressed environmental conditions (Wells et al., 2019).

Facies Analysis
The combination of tectonic development, sea-level variations, sediment supply rate, biological and physical activity, sediment transport and sedimentation processes, and climatic changes determine the sedimentology and reservoir characteristics of clastic sedimentary rocks. These processes interact in the basin to generate the stratigraphic architecture of the basin, which is the geometric organization and distribution of diverse stratigraphic tracts or sedimentary environments across time (Al-Dabbas et al., 2014). Four lithofacies were identified in the Zubair Formation.

Sandstone lithofacies
Lithofacies of sandstone that have been good sorted (Fig. 6). Within the sandstone unit, this facies represents a wide range of sand grain sizes, from medium to coarse, and well-rounded to sub-rounded grain. The sandstone in these lithofacies has a significant amount of quartz. With extremely low gammaray values that decline upward with gamma-ray log and high resistivity log values (Fig.10)  .

Shaly sandstone lithofacies
These lithofacies are classified as muddy sandstone that largely comprises quartz-dominant rocks. It is characterized by poorly sorted sandstone (Fig.7); this Facies is mostly composed of sand, with minor amounts of shale, and has high porosity and permeability, with moderate gamma-ray values in units H and L (Fig.10).

Sandy shale lithofacies
They were found in sandstone members as shale lenses (Al-Jafar and Al-Jaberi, 2019). A considerable proportion of shale and a small amount of sand (Fig. 8), with high V-shale values (Fig. 10). Rocks having quartz grains that are dominated by mud Low flow unit with high gamma-ray value and low porosity and permeability.

Shale lithofacies
This facies is mostly made up of shale. The shale lithofacies has been found throughout the Zubair sequence, which comprises shale-dominated rocks. Particularly in-unit K (Fig.9), which has strong gamma-ray values (Fig. (15), low resistivity, and a low flow rate. (Fig.10).

Rock Type
The Zubair reservoir rocks type was classified depending on porosity and permeability cross plot. Grain size and clay concentration are two major determinants of reservoir quality. Fine-grained sandstones with no clay matrix are of the highest grades. As grain size drops to fine-grained and siltsized and clay concentration rises, P&P decrease.

Pore Throat Radius
The radius of the pore throat-related pores as a gateway through which fluids pass from one pore to another is called a pore throat (Martin et al., 1997). The pore throat radius technique involves injecting mercury into the core or plug samples with a diameter of up to 35 to determine the relationship between air permeability and porosity, as shown in Table 2 and Figs. 11, 12 and 13 fundamental relationship between P&P is established using empirical data (Kolodzie, 1980). The Winland equation is used to calculate the radius of the pore throat.

Flow Units
Flow unit is distinct as the stratigraphically continuous separation of a similar reservoir process that respects the conserves the rock type features and geological structure and, temporarily it is defined as a portion of the reservoir where the petrophysical and geological properties that influence fluid flow are steady and proportionately predicted by the features of additional rock sizes (Ismail and Al-Najm, 2019) To make things easier, the flow unit is calculated using permeability, porosity, and bed thickness characteristics. In the flow units of cores, the sedimentary environment, capillary curve design, and petrophysical characteristics are all different (Abbaszadeh, 1996). The notion of a flow unit aids in the following: • Based on reservoir dynamics, quantitative description, and mapping of reservoir components.
• The numerical modeling of reservoir performance was aided by a realistic building block for reservoir zonation (Hearn et al. 1984). The diagenetic process, pore geometry descriptions, and depositional context of rock structure have all been used to identify flow units. One of these tactics is to look for correlations between reservoir quality and rock type. The reservoir quality index (RQI) and flow zone index (FZI) have been recognized as ideas for calculating flow units. Similar to the formulae, the RQI and FZI are justified. The (Øz) represents the pore volume to grain volume ratio, which may be written as: FZI = RQI (φz) (3) The (Φz) is the pore volume to grain volume ratio, which is theoretically defined as:

Discussion
Many studies focused on rock type, pore throat radius, and flow units. However, these characterizations were determined using electrofacies or restricted lithofacies data. The core description, petrophysics measurements from thin sections, and core plug P&P data are all combined in this section to create four RRTs. The Zubair Main Pay reservoir is mostly fine to extremely fine-grained sandstone with shale interspersed. Grain size and clay concentration are two major determinants of reservoir quality. Fine-grained sandstones with no clay matrix are considered one of the greatest qualities. Many samples are heterogeneous, with clean sands interbedded with clay laminations. P&P diminishes as grain size reduces to extremely fine-grained and silt-sized and clay content increases. Resultantly, the reservoir quality of these sandstones is lower than that of homogenous sandstones (Fig.15).

Reservoir Rock Type 1
This RRT has a porosity of 10%-25% and a permeability of 10-1000 mD. (Fig. 10). This RRT is a flow unit with high P&P (Figs. 11 and 12). Mega to macropores, as well as high-flow units with greater permeability flow units with pore throat sizes of approximately10-2 microns, are better for developing high flow zones (sandstone facies). Sandstone facies correspond to reservoir flow units. Pore throats are typically 10-2 microns wide. Within the H unit fluvial system, high flow zones exist. The upper L Low gamma-ray and higher permeability flow units have the best development of high flow zones (sandstone) inside the upper part of the L unit.

Reservoir Rock Type 2
This RRT has a porosity range of 15%-25%, but a permeability range of 10-100 mD, which is an order of magnitude lower than RRT 1. (Figs. 10, 12, and 13). RRT 2 is likewise a high porosity, high permeability H and L flow unit with a mild gamma-ray. Facies with shaly sand are dominated by micropores and correspond to Facies with shaly sand. Pore throats are typically 2-0.5 microns wide.

Reservoir Rock Type 3
This RRT has a high porosity but low permeability, which means it has substantial storage but low flow potential. RRT 3 is characterized by 6%-15% porosity, 10 mD permeability, and high gamma-ray (Fig. 10) and corresponds to Facies with sandy shale textures and is dominated by mesopores with throat widths ranging from 0.5 to 0.1 microns.

Reservoir Rock Type 4
The caprock is put at the K in wells 1, 2, and 3 unit's ( Fig.15), and this RRT is termed a low porosity, low permeability unit (Fig.10, 13, and 14). Given that many RRTs4 have a V shale value of >0.5, they are classified as RRT 4 shale with high gamma-ray Micro-Nanopores 0.1. Cementation associated with exposed surfaces causes low reservoir quality in various rock types.

Conclusions
The relation between petrophysical parameters (porosity and permeability) was investigated by core data and log data, with the results correlating to identify the rock type, the following are the radius of the pore throat and the flow units: • Facies caused a wide range of rock types in unit H, from sandstone Facies to shale facies.
• The most predominant rock type's in-unit L are RRT1 and RRT2, and the L unit largely comprises sandstone and shaly sand facies. Due to shale facies, the common unit K of the rock type in RRT4. • When the core data from a high-energy fluvial environment is correlated, megaspores and macropores are referred to as sandstone. Mesoporous throwbacks have shaly sand facies dominate and have a medium energy level. Shale prodelta lithofacies are microporous usually sandy shale mouth bars with micropores and nanopores. • Due to heterogeneity, the pore throat radius of units H and L ranged from nanopores to megapores.