Clay minerals with their water-filled micro porosity and their ability to exchange cations contained within pore fluids are the most common causes of LRLC reservoirs [6] . Clays, in order of their highest to lowest exchange capacity and therefore their effect on suppression of electrical logs, are Smectite (Kaolinite), mixed-Layer Smectite (Kaolinite), Illite, Montmorillonite. Distribution clay minerals in cross plot Thorium potassium and XRD results (Figure 2, Figure 3 and Table 1). The cause of the low resistivity is the presence of high cation exchange capacity clay minerals, which also have high irreducible water saturation Intervals. It can also be caused by features like Glauconitic pellets and more than 5 percent conductive materials such as pyrite. Digenetic Process like Micritization, emphasis that quality of the reservoir is influenced by various digenetic process such as Micrtization and Pyritization which have noticeable impact on declining resistivity. Porosity varies between interparticle, moldic and mudstone microporosity.

Petrographic evidence indicated that Formation was subjected to different diagenetic processes with variable intensities. This formation has been buried to a

depth of more than 2.4 km (intermediate burial realm) and has experienced micritization, bioturbation, cementation, dissolution and compaction as the main diagenetic alterations.

As an earlier diagenetic process, micritization has affected the carbonate grains in the studied formation (Figure 4(b), Figure 4(c) and Figure 4(e)). Micritized skeletal fragments are very common in the lagoonal and shoal facies (as reworked grains). Some of the uncertain grains (peloids) have formed during the complete micritization. It seems that after deposition, bioclasts were partially or completely micritized by endolithic and other microbes (micro-borer organisms) on the sea floor. This process commonly occurs in relatively low-energy, shallow-marine environments [7] [8] . Pyrite mineralization: Pyrite (FeS2) is present as barrow filling and authigenic diagenetic mineral (scattered opaque minerals both on the bioclasts and in the rock matrix). It occurs as cubic crystals with fine sizes (less than 0.3 mm). Pyritization is also seen in the bioclast grains (Figure 4(a), Figure 4(d) and Figure 4(g)). Occurrence of pyrite in organic-rich sediments indicate that this mineral may be formed by sulfate reducing bacteria (SRB), under anaerobic condition, although most of the pyrites in sedimentary rocks are of diagenetic origin [8] . Compaction: On the basis of its burial depth (>2 km) and evident compaction features, it seems that the Buwaib Formation has affected by compaction with various degrees in shallow to deep burial realms. Compaction features, such as solution seams, stylolites and compaction

gash fractures, are recorded in the both core samples and thin sections of the formation (Figure 4(f) and Figure 4(h)). Our observations have revealed that lithology (particularly clay content) exert the main control on the distribution of stylolite and seam solutions in the studied intervals. Stylolites are very common in the limestone and porous facies, particularly lithocodium-bearing facies, whereas solution seams are commonly seen in the argillaceous limestone facies these features are mostly parallel to the bedding planes. Horse-tail structures are also present. The evidence indicated that main parts of the porosity were destroyed during burial and compaction (and also through cementation). Development of pressure solution features requires a depth of more than 500 m. This process reflects the compaction due to the heavy sedimentary cover (>1 km thickness), which indicating burial diagenesis.

The most widely used pore-type classification systems for carbonate reservoirs are limited by the fact that the relation between porosity and permeability is poorly defined. Existing classification schemes for porosity-permeability data do not, in many cases, optimally integrate sedimentology, diagenesis, and flow-re- lated properties. In many carbonate reservoirs, it is therefore difficult to generate predictive models for reservoir-quality distribution, resulting insignificant uncertainty in hydrocarbon reserve calculations. Porosity distribution is a major new element in the classification. Winland and Lucia’s subdivision of inter particle porosity has been partly incorporated into the new classification system [9] [10] (Figure 5), but is now based on pore size instead of grain size and sorting. Lønøy method [11] applied to address pore throat sizes which contain Inter crystalline porosity, Chalky Limestone, Mudstone micro porosity. Pore systems are classified at class 2 and 3 Lucia and pore size varies from 0.5 to 4 micron. Pore types, are introduced: mudstone microporosity Interparticle, micromoldic and micromoldic. Mudstone micropores have extremely small pore sizes, commonly a few micrometers in diameter. Individual pores cannot be seen with a standard petrographic microscope.

However, because of the extremely small pore sizes and variable pore structure (interparticle), these pores were classified as a separate pore-type class (Figure 6 and Figure 7). Mudstone microporosity includes both true chalks and chalky microporosity. Chalk micropores are primary in origin and occur between grains of planktonic calcareous algae (coccospheres) or their component crystal plates (coccoliths). Chalky micropores are not related to chalk, but the pore structure is similar [10] . These pores occur between recrystallized mud particles and may be formed either during early meteoric leaching or deeper burial diagenesis. The pores typically form in low-energy, muddy, platform-interior facies.

Examples of petrophysical evaluation for Khatiyah, Shuiaba and Buawib formations, Mineral responses defines based on petrophysical tools (Table 2). Base on core data and NMR data these formations should be controlled to define precisely water saturation. 2 samples of MICP results for Khatiyah formation show water saturation is 20% to 45% (Figure 8).

It should be noted resistivity response these formations are very low (Figure 9 and Figure 10).

Nuclear magnetic resonance (NMR) refers to the response of atomic nuclei to magnetic fields. NMR measurements show pore-size distribution; the presence of clay, vugs; hydrocarbon properties such as viscosity; and grain-size. Knowing the value of T2 cutoff enables the amount of mobile and bound fluids to be calculated from log data. The T2 cutoff is the size boundary between small pores containing bound fluid and larger pores where the fluid is free. While various rock types have standard T2 cutoff values, the geometry of the pores and the rock’s mineralogy may shift the T2 spectrum, so laboratory core analysis experiments are used to determine the value of T2 cutoff.

MRIL (Magnetic Resolution Imager Log) spectrums indicate high volume of irreducible bound water; there is also some free fluid porosity which is mainly water with a little oil in some interval oil and water are identified [12] . Porosity spectrum also show that the total porosity is about 20% - 25% in the reservoir intervals as well as permeability varies between 5 to 200 md. In order to define water saturation precisely for the reservoir, T2 Cut off define 115 microseconds related to micro porosity carbonate to define. Irreducible water saturation defined 30% to 50% in reservoir interval. Combination between full set logs and NMR logs with NMR core results (Figure 10). Water saturation varies between 35% to 50% results extract based on 15 core samples selected for NMR analysis in oil brine and air brine condition to get information about pore size Clay bound water, Moved oil and water and residual water saturation.