Influence of diagenetic features on petrophysical properties of fine-grained rocks of Oligocene strata in the Lower Indus Basin, Pakistan

1.1 Geological context of the study area

The Indus Basin is located between the boundary of the Indian, Asian and Arabian plates [29]. The Indus Basin is divided into two parts, Upper Indus Basin and the Lower Indus Basin [30,31]. The general stratigraphy of Lower Indus Basin including various different formations is presented in Table 1. The study area lies in the Karachi trough, part of Lower Indus Basin. The rocks are ranging from Eocene to subrecent. The lower contact of mostly exposed Nari Formation in Sindh is mostly conformable and gradational with Kirthar Formation while in our study area it is unconformable with Laki Formation and upper contact is unconformable with recent deposits. The formation consists of sandstone, shale, and subordinate limestone. The study area lies in the upper part of Nari Formation where limestone is absent; generally fossiliferous limestone is present around Karachi City [31,32]. It is mainly composed of sandstone with minor amount of shale [32] (Figure 1).

Table 1

Generalized sequence stratigraphy along with main lithology encountered in the Indus Basin Pakistan [52]

Figure 1

Geological map of Pakistan showing the study area with basic geological information and location of sampling sites.

2.1 Petrography

For petrographic thin-section observations, Leica optical microscope was used to study the rock lithology and microstructure of the Nari sandstone samples. The experiments were carried out at the Geological Survey of Pakistan (GSP), Karachi laboratory. In addition, cathodoluminescence can identify minerals and fossils, along with their microscopic features. The Leica optical microscope was used to analyse the essential microscopic features of thin section under polarized microscopes.

2.2 XRD analysis

X-ray powder diffraction (XRD) is a fast analytical method basically adapted to identify the different phases of crystalline materials. Prior to analysis, the material was powdered, finely crushed, and made very homogeneous so the average bulk composition of material is obtained. These experiments were conducted at the University of Sindh Geology Department. QXRD analysis was performed on 1 cm³ size, and the sample covering the overall sandstone mineralogy was also taken.

2.3 Scanning electron microscopy (SEM) analysis

The smooth and fresh rock chips of samples were prepared and then mounted on the sample stub of SEM. The samples were mounted with the help of the conductive double-sided carbon solution tape. The sample stub was then mounted in the sample chamber JEOL JSM-6590LV of SEM, which is also equipped with an extra and important accessory component, Bruker of energy dispersive X-ray spectrometer (EDS). The IMAGINING INTERFACE software controlled the system in JEOL JSM-6590LV in SEM whereas in the EDS , QUANTAX. The desired images were taken and controlled from computer to study the various topographic features and compositions of the samples.

2.4 Measurement of density, porosity, and permeability

Cylindrical samples were prepared for permeability and porosity measurement. The samples prepared in varying lengths and diameters. All samples were cleaned to remove the unnecessary particles which may affect results. All the samples were then dried into a humidity-controlled oven at 60°C prior to measurements.

3.1 Petrographic analysis

Thin-section observation shows that Nari Formation samples are moderate to poorly sorted. The mineralogical compositions are shown in Table 3, and the diagenetic features are shown in Figure 3. The occurrence of quartz considerably varies in proportion from 62 to 91.20%. The observed accompanying mineral assemblage includes the calcareous matrix, ferric oxides, calcite cement, clays, and lithic grains along with feldspars and mica. The proportions of all the associated minerals except quartz are shown in Figure 4. Ferric oxide, calcareous mud, and calcite observed from thin-section analysis dominated the cementing materials. These cements very densely packed these rocks, thereby resulting in poor pore spaces and subsequently leading to tight sandstones which exhibit poor porosity and permeability. From Figure 4, it was observed that the clay is the dominating mineral in samples 18-JT-8 and 18-NR-2. Many observations showed that the monocrystalline quartz was in principal quantity than the polycrystalline. At most places, the quartz grains are fractured and it may be due to the external stress (Figure 5b). The tectonic stress could be the major cause of grain breakage and fracturing (Figure 5d). The dark coloured patches within these samples display those iron oxides within these Nari sandstone samples along with calcareous mud. Both alkali feldspar (Figure 5c) and plagioclase (Figure 5c) were observed in the samples.

Figure 3

Thin-section photomicrograph of Nari Formation sandstones showing diagenetic features: (a) Bioclasts. (b) Mud interclasts along with iron oxides among quartz grains. (c) Calcareous mud and bioclasts are shown. (d) Calcite cements and calcareous muds are visible from the image. (d) Iron oxides coating quartz grains are visible from the image. (f) Calcerous muds and iron oxides are shown.

Figure 4

Mineral concentration of samples of Nari Formation other than quartz, as quartz is a highly dominated mineral within this formation.

Figure 5

Photomicrographs of sandstone samples analysed from Nari Formation showing: (a) moderately sorted, subangular to subrounded grains of quartz. (b) Monocrystalline and polycrystalline characteristics governing within these samples as quartz crystal. (c) Lithic framework grains along with observed fracture. (d) Some observed alkali feldspar grains.

3.2 Microstructural observations

Samples selected for the SEM-EDS analysis were named after their location from where they were collected, such as 18-NR-1 and 18-NR-2 were collected from station 1, and those which were collected from station 2 were labelled as JT-1, JT-2, 3, 4, 5, 8, 9, 10, 12, and 13. From SEM analysis, the various minerals identified are represented as Qtz = quartz, C = calcite, Fe = iron, and CM = clay minerals.

All samples from stations 1 and 2 were analysed in order to identify their general elemental structure. In JT-1 sample, the general concentration of elements is analysed under EDS in the form of peaks. The highest peaks of this sample are of silicon, oxygen, calcium, and iron, as shown in Figures 6 and 7. The presence of dominant minerals such as quartz, calcite, and iron sand was confirmed under SEM analysis of sample 18-JT-1. The point analysis of this sample is based on the white colour. Figure 7 indicates the highest peaks of the silicon, iron, and calcium in which the tone of the iron was confirmed under SEM analysis of sample 18-JT-1. Sample 18-JT-2 general elemental concentration indicates the highest peak of silicon, and all other peaks are averaged as shown in Figure 7. The presence of quartz was confirmed under SEM analysis of sample 18-JT-2. Further elemental concentration of sample 18-JT-3 indicates the highest peaks of silicon, oxygen, and calcium in Figure 7. The presence of quartz and calcite minerals were confirmed under SEM analysis of sample 18-JT-3. The point analysis (a) of this sample is based on the dark grey shown in Figure 7, which indicates the highest peak of silicon7. The tone of the quartz was confirmed under SEM analysis of sample 18-JT-3. The point analysis of this sample is based on the light grey indicating the highest peaks of the oxygen, silicon, and calcium (Figure 7). The tone of the calcite was confirmed under SEM analysis of sample JT-3. Further, it was found that in the samples 18-JT-4 to J18-T-8, their elemental concentration ranged from their highest peaks of silicon, oxygen, and calcium, and the presence of quartz and calcite minerals were confirmed under SEM analysis.

Figure 6

Mineral composition under SEM observation is quartz and calcite (in Figure a, b, and c sample JT-9). Quartz mineral indicates its tone as dark grey while the calcite mineral indicates its tone as light grey. The sample JT-10 (a–c) displays quartz as its dominant mineral along with iron oxides, calcite, and clay minerals. Clay minerals as kaolinite are present in JT-10 (a). The symbols represent Q = quartz, C = calcite, Fe = iron and CM = clay minerals.

Figure 7

The scanning electron microscopy images of Nari sandstone samples along with energy dispersive X-ray (EDX) spectrum revealing the different elemental composition.

The general elemental concentration of 18-JT-9 sample indicates the highest peak is of silicon and then oxygen as the lowest peak as shown in Figure 7. The presence of the dominant mineral quartz was confirmed under SEM analysis of sample JT-9, which consists of beautifully developed crystals of quartz (Figure 6). The general elemental concentration of 18-JT indicated the highest peak of silicon and oxygen. The presence of dominant mineral quartz was confirmed under SEM analysis of sample JT-10. The point analysis (Figure 7) of this sample is based on the dark black seen in the image which indicates the highest peaks of silicon, oxygen, potassium, and calcium. The tone of the clay minerals was confirmed under SEM analysis of sample JT-10.

3.3 Mineral composition

Results from XRD analysis are presented in Table 3; this shows the compositional analysis for all analysed sandstone samples. It was observed that the main components are quartz and feldspar, and the iron oxide, cement, and clay of variable mineralogy. Other minerals observed include the kaolinite, dolomite, calcite, gypsum, mica, pyrite, chlorite, and lithic fragments. The cementing materials observed are mainly comprised of carbonate (9.4%), ironic (10.6%), and micrite muds (12.8%); in the samples collected from Nari station 1, the major cementing materials are clay minerals (14.2%), ironic (4.7%), and plagioclase (2.2%). From the XRD analysis (Figure 8), it was found that the quartz content in all samples is very high, i.e., it was 91.2% and above 91.6%, which is typically quartz sandstone. Most of the samples are lithic quartz sandstone, possessing clay minerals in variable quantities as shown in Table 3. The feldspar content is very small and can be neglected. The average total clay content (24.35%) of Station 1 Formation is larger than the average total clay content (15%) in Station 2 Formation in the region (Table 4).

Figure 8

The illustration shows the X-ray diffraction patterns of two selected sandstone samples (a and b) from Nari Formation.

3.4 Petrophysical property results

Permeability and porosity data of Nari sandstones along with their slippage factor values are presented in Table 5. The results obtained were cross plotted and are shown in Figure 9, where a reasonably virtuous correlation between permeability and porosity can be perceived on the semilogarithmic plot. It is interesting to note that, the permeability value >0.1 mD, shows the linear correlation between permeability and porosity. However, the permeability and porosity values <0.01 mD are quite scattered on the semilog plot, which means that the effect of other parameters such as pore-filling clays and grain-coating clays along with cementing materials significantly affect the permeability. The cross-plotted permeability and porosity demonstrate that the trend line of samples with no clay minerals shows linear trend line; however, the sandstones possessing clay minerals, calcareous muds, and cements resulted in low and scattered permeability and porosity, which in turn makes this reservoir very poor quality. The permeability varies by several factors due to the varying mineral proportions. The main factors that control the permeability of these reservoir rocks are clay mineralogy, cementing and mud influx materials. The permeability reductions experienced imply that the Nari Formation could have a significant impact on the single-phase flow, especially when production from these reservoirs begins. Further the samples which were cemented due to diagenetic modifications have shown large reduction in permeability values. It was observed that some low-permeability sandstones exhibited overlapping permeability–porosity characteristics (Figure 10). Almost identical behaviour of permeability data could be noticed by the action of grain-coating clays and pore-filling clays (Figure 10). The overlapping was usually noticed in the data where permeability of these samples was very low, that is, less than <0.1 mD particularly for rocks with pore-filling clays and grain-coating clays. The distribution characteristic of clay mineralogy within sandstone formation is described in Figure 10. The lower permeability of Nari Formation sands is the result of the existence of clays, which is obvious from SEM and XRD data. Pore-fillings clays demonstrate that the permeability in these sands is less compared to the grain-coating clays; however, this needs further refinement for accurate prediction of permeability and porosity of Nari Formation reservoirs. Laboratory measurements of porosity and permeability of numerous sandstone and mudstone samples have demonstrated that it is mainly controlled by the clay mineralogy. From the permeability and clay content cross plot, a scatter of data is observed (Figure 11). The scatter in permeability and porosity could be the result of different grain sizes and occurrence of diverse mineral contents and is well elaborated in ref. [33].

Figure 9

The cross plot of porosity and permeability of samples from Nari sandstone formation.

Figure 10

The clay mineralogy distribution and its impact on reservoir rock properties. From left to right this illustration displays the grain- coating clays, pore-filling clays, and no clay as observed from the scanning electron microscopy images.

Figure 11

The relationship between permeability and clay content.

4.1 Diagenetic controls on quality of reservoir

The main diagenetic processes that affected the Nari Formation sandstones are the mechanical compaction, deformation, cementation, and dissolution, which play a significant role in the reservoir quality of the sandstone. It has been observed that the porosity and permeability reduction as a result of calcite cements are larger than those due to the coating cement (i.e. clay minerals). It has been reported that the calcite cement observed in the Nari sandstone formation usually exists in other sandstone formations around the globe e.g. ref. [34]. Subsequently, it is perceived that the fluid flow may exhibit difficulty in moving through pores with a higher percentage (>20%) of calcite cements [35,36,37]. On the other hand, if calcite cements are found to be in smaller amounts, that is, less than 20%, it may result in slight increase in secondary porosity [36,38]. The likely sources of calcite cements may consist of the carbonate rock fragments dissolution, dissolution of K-feldspar, the diagenesis of interclasts and calcareous muds, alteration of clay minerals such as kaolinite to illite, and the transformation of smectite to illite [4,34,38,39]. The Nari Formation sandstones mostly contain interclast muds, which leads to the likely chances of alteration of authingenic minerals. The petrographic analysis displays that calcite cements are dispersed in most of the places in the Nari Formation sandstone samples. Consequently, the key cause of calcite cements in these samples is believed to be the results of exterior mudstones and interclasts. The diagenesis that affected Nari Formation sandstones is mainly classified as early and late diagenesis (Table 6).

Table 6

Illustrates the diagenetic events experienced by Nari Formation sandstone reservoirs

4.2 Paragenetic sequence

The textural connections among the diagenetic minerals combined with the petrographic studies were conducted to establish an overall paragenetic sequence of the Nari Formation. The main diagenetic minerals identified within these sample paragenesis are predominantly the cementation by carbonate mud influx, clay minerals, and quartz cements. Diagenetic modification showed discrepancy between different samples. The diagenetic events experienced by sandstone samples of Nari Formation is summarized in Table 6.

4.3 Burial diagenetic modifications on reservoir quality

For the successful exploitation and development of any reservoir, it is essential to have a sound knowledge of their mineral compositions, microstructural characteristics, associated depositional environment and burial diagenetic history, overburden stress, and reservoir rock heterogeneity [13,14,40,41,42,43,44,45,46,47,48,49,50,51]. The main diagenetic processes that affected the Nari Formation sandstones are the mechanical compaction, deformation, cementation, and dissolution, which play a significant role in the reservoir quality of the sandstone. The following events played a significant role in controlling the porosity and permeability of the Nari sandstone.