It is reported that the tremendous oil and gas reserves accumulation have been discovered and reported within the carbonate bearing rock formations around the globe e.g. Russia, Kazakhstan, and North America [12]. It is anticipated that the carbonate reservoirs contain 60% oil and 40% gas reserves all over the world [12]. Among them, the Middle East (e.g., Saudi Arabia, Oman, Iraq, Bahrain, Iran, United Arab Emirates, Qatar) carbonate rocks exhibits a large amount of oil around 70% and the gas reserves are almost 90% [13]. Likewise, the substantial hydrocarbon potential have also been reported that exists in Pakistan within the carbonate reservoirs rocks [14, 15]. In Pakistan, the lower Indus Basin carbonates and Kohat-Potwar sub-basin are reported to be the main carbonate reservoirs that have been attracted by many exploration and production (E&P) firms for exploitation of hydrocarbon potential [14]. These carbonate reservoir rocks are highly fractured and pretending copious challenges in reservoir characterization, production and management and poses massive technical hindrance in exploitation and drilling [14]. The fractures have significant impact on controlling the production behavior of reservoir and field development, planning and facilities installation [7, 16]. Due to such complexities of carbonate reservoirs many petroleum E&P companies have been facing copious challenges in exploiting and developing highly heterogeneous carbonate reservoirs [17]. Some of the major technical and economic challenges faced by E&P industry are that these reservoirs contains very low hydrocarbon volumes, exhibits extremely tight permeability which requires expensive stimulation treatments, longer well cleanup periods, relatively higher costs to import technologies and obtain expensive services, less service provider competitors and material suppliers, due to very tight nature of reservoir early water production issues, lower gas rates, marginal profits, complex reservoir geology and complex petrophysics [15]. In this regard, prior to development of any carbonate reservoir, it is highly recommended to address all aforementioned challenges involved in exploitation and development of reservoirs by provide accurate rock properties [17]. Many E&P companies working in Pakistan to date have conducted numerous types of surveys and have conducted research that is more specifically concentrated on geological aspects of carbonate reservoirs [14, 18, 19]. However, the petrophysical properties, mineralogy and microstructural analysis of carbonate reservoirs have not been assessed appropriately with some exceptions of [15]. For efficient exploitation and development of hydrocarbon reservoirs, it is essential to assess the carbonate rock petrophysical and microstructural properties and the factors that affect the reservoir quality. The reservoir quality is directly related to the porosity and permeability of the reservoir and is necessary to explore those aspects that control these reservoir rock properties. Since the diagenetic changes significantly affect that reservoir quality, it must be also evaluated.
Interactive Petrophysics 4.3 Crack 40
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Analysis of the carbonate samples from fossil assemblage and texture under petrographic microscope showed that these samples were deposited near fore-slope depositional environment according to [35] classification of carbonate rocks. The dolostone may be due to the increase of manganese content in sea water that would probably due to regression reducing the water depth. Thin section photomicrograph in Figure 6 displays the various petrographical descriptions and features of Jakkher Group carbonate rocks. Petrographic and SEM examination of the lower Indus Basin Jakkher Group rocks showed both primary and secondary porosity that was evident from micro-cracks and fractures. SEM observation showed that most of samples were exhibiting intergranular porosity along with associated micro pores exhibiting dense rock pore structure tending to very tight carbonate rocks. Further, it was noticed that some diagenetic processes have altered the rock porosity by precipitation and secondary filling of micrite muds within pore spaces resulting into the pore throats size reduction. Most of samples were very low in porosity and permeability due to the diagenetic alterations. These Jakkher Group carbonate rock features observed from thin section analysis and petrographical point of view are shown in Table 2 and Figure 6.
Thin section photomicrograph displays the petrographical description of carbonate rocks showing: [Plate A] point 1 displays the Nummulite and point 2 is Dolomite Rhomb. [Plate B] displays Discocyclina at point 1 and point 2 is the Peloidal Micrite. [Plate C] shows at point 1 are the Mg-Rich Dolomite crystal and at 2 is the Fe-Rich Dolomite crystal. [Plate D] the image shows the micro crack, fracture filled with carbonaceous material; and the point 2 displays the Calcisphere. [Plate E] observations are as 1, 2, 3, Quartz Crystals; 4, Micrite and point 5 is the Fe-Rich Dolomite Crystal. [Plate F] inside the image points 1, 2, 3, are the dolomite crystals surrounded by washed Nummulite walls. [Plate G] image displays the fractured Nummulite at the center surrounded by fine micrite mud. [Plate H] shows the Quartz crystal with calcite cement surrounded by coarse micrite mud. The scale bar = 20μm.
The basic properties of the samples of Jakkher Group are presented in Table 4. Few samples from Jakkher Group were sandy to argillaceous carbonates and most of them were exhibiting high volume of pure calcite and were characterized by a relatively low grain density of 2.70 g/cm3. The grain density for the samples exhibiting calcite minerals is of 2.81 g/cm3 on average. Furthermore, the grain boundary fractures could be observed within these carbonates from images of thin sections resulting into different porosity types. These samples were noticed that they developed two types of porosities (i) the porosities developed under the existence of microfractures and were ranged from 2.55 to 12.82%, with an average value of 3.86%, (ii) the porosities which developed in absence of micro-cracks and were ranging from 2.12 to 8.5% with an average value of 4.5%. Similarly, we observed a big variation in permeability values, the permeability measured were ranging from 0.013 to 5.8 mD for those samples without microcracks. The samples with obvious microfractures were exhibiting high permeability values and were discarded
From few thin sections analysis of Figure 6 Plate D displays the micro fractures resulting into high permeability values. The permeability and porosity data cross plotted showed few outliers in the Figure 9 that was due to micro cracks as observed rom thin section analysis. Furthermore, it is highly recommended that the impact of grain boundary fractures could be analyzed within these carbonates by preparing the polished surface thin sections. The permeability measured exceeding 20 mD is not included in the plot of permeability-porosity because this was resulting in big scatter on chart. The reason of such high permeability in the samples was the existence of obvious microfractures and micro-cracks as observed from one of the thin sections. These naturally fractured reservoirs would exhibit high permeability values and are expected to produce at commercial rates without application of fracturing the reservoirs for increasing permeability.
The permeability reduction due to overburden stress could significantly affect the overall output of the ultra-tight permeability reservoirs [29, 47, 48]. There have been few papers that have addressed stress sensitivity in impact on oil and gas reservoirs in Pakistan; this requires more researches attention in order to develop stress sensitivity low permeability reservoirs. Studies [29, 47, 49] have shown that the tight, low permeability reservoirs are more sensitive to the overburden stress than conventional reservoirs. The elements responsible for decreasing the permeability due to applied stress are the pore size and pore throat. However, the samples recovered from subsurface may results in formation of the micro-cracks. If confining stress is applied in laboratory, the micro-fractured samples will results in more reduction at higher overburden stress [47, 49]. From experimental observations, the absolute gas permeability has shown reduction for both samples of carbonate with increase in net stress (Figure 12). The net stress is characterized as following equation
It was observed that the permeability increases when the pore pressure was decreased even at in- situ stress, such increase in permeability was the results of gas slippage [29, 50, 51]. However, we realized that the absolute permeability of carbonate samples decreased with increase in net stress. Hence, it is clear that net effect is the reduced permeability with increase in net stress at lower ranges of stress. It was because the gas slippage effects were low due to the high pore pressure. At higher net stresses and low confining stresses, the increased flow due to slippage balances the reduction of slip corrected permeability leading to increased values of permeabilities. Stress sensitivity of permeability to low permeability tight sandstone reservoirs is well documented [48, 49], however; the permeability measurement at in-situ stress is very challenging. In present study on carbonate samples permeability (Figure 12), we found that there was greater extent of permeability reduction at lower overburden stress which is due to the microfractures existence as microcrack was observed in one of the sample. Micro-cracks could have formed along the grain boundaries of the samples that resulted in increased in permeability at lower confining stress. Other authors [47] also reported that the effect of confining stress on samples permeability is controlled by presence of micro cracks. The main explanation of changes in gas permeability will be the result of changes in effective stresses, pore pressures and are essential for assessing their impact on reservoir quality by considering their relationship between gas permeability, stresses and gas slippage. 2ff7e9595c
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