Investigation of reservoir characteristics, depositional setting and T–R sequences of the Lockhart Limestone of Meyal Oil Field, Pakistan: a petrophysical approach
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The present study is focused on formation evaluation of the Lockhart Limestone in two wells (Meyal-05P and Meyal-10P) located in Northern Deformed Potwar Zone of the Potwar sub-basin, Pakistan. The geological formations ranging from Triassic to Pliocene have been drilled in these wells. The formation evaluation of the Lockhart Limestone mainly involves reservoir potential evaluation, interpretation of depositional environment and transgressive–regressive sequences using petrophysical logs. In either wells, the reservoir characterization is steered by various petrophysical parameters including calculation of volume of shale, porosity, permeability and hydrocarbon saturation. The thickness of the Lockhart Limestone is 50 m and 77 m in the Meyal-05P and Meyal-10P wells, respectively. In Meyal-05P and Meyal-10P wells, the average petrophysical parameters values and ranges are given as follows: volume of shale 48% and 20%; density porosity 1–5.6% and 1–31.7%; neutron porosity 1–23% and 1–42.9%; sonic porosity 1–29% and 1–39%; effective porosity < 1–> 5% and 1–21%; and hydrocarbon saturation 92.21–99.8% and 97–99.6%. The petrophysical parameters indicate that the Lockhart Limestone of Meyal-10P well is quantitatively better reservoir than that of the Meyal-05P. In Lockhart Limestone of either wells, the permeability is < 0.1 mD. The bulk volume water deciphered the presence of vuggy and intercrystalline porosity in the Lockhart Limestone. Similarly, the lithological interpretation using logs shows mainly limestone with minor shales. Different electrofacies are interpreted from the log trends of gamma ray log such as aggrading, prograding and retrograding depositional sequences deposited in tidal channel fill, shallow water, shore line and offshore buildup and regressive-to-transgressive shore face depositional setting.
KeywordsLockhart Limestone Petrophysical logs Meyal Oil Field Reservoir properties Transgressive–regressive sequences Depositional environment
The Lockhart Limestone is well-exposed in Kohat-Potwar sub-basins, Salt Range, Trans-Indus ranges, Kala-Chitta Range, Islamabad, and Hazara-Kashmir area(s) (Shah 2009). In the Kohat area, Lockhart Limestone consists of medium to thick bedded, massive, rubbly and brecciated limestone. In the Salt Range and Trans-Indus ranges, the Lockhart Limestone consists of medium bedded and nodular limestone with minor amounts of shale and marls. In the Hazara and Kala-Chitta areas, the Lockhart Limestone is comprised of dark-colored nodular and massive limestone and intercalations of shale and marl (Shah 2009; Awais et al. 2012 and 2013). The Lockhart Limestone has been studied extensively by earlier investigators (e.g., Afzal et al. 2005; Sameeni et al. 2009; Yaseen et al. 2011; Hanif et al. 2013; Sameeni et al. 2013; Ahmad et al. 2014; Malik and Ahmed 2014; Khan et al. 2016; Bilal and Khan 2017; Khattak et al. 2017) in the context of sedimentology, biostratigraphy and sequence stratigraphy. Similarly, different researchers have investigated the diagenetic fabric of the Paleocene Lockhart Limestone exposed in the Kohat-Potwar sub-basins, Islamabad, Salt Range (central and western), Margalla Hill Range and Azad Kashmir (Ali et al. 2014; Khan et al. 2016; Bilal and Khan 2017; Khattak et al. 2017; Khan et al. 2018). However, no published work is available on reservoir characterization, sequence stratigraphy (T–R sequences) and depositional environment of the Lockhart Limestone penetrated in the Meyal-05P and Meyal-10P wells. According to Hasany and Saleem (2012), Lockhart Limestone consists of massive, argillaceous limestone in the Meyal Oil Field. The Lockhart Limestone is penetrated in few wells and is a heterogeneous quality reservoir thereby produced very limited hydrocarbons (Hasany and Saleem 2012). In the Hazara area, visually estimated 11% porosity is reported in the Lockhart Limestone (Nawaz et al. 2015). The Lockhart Limestone penetrated in the Chanda deep-01 well of the Kohat sub-basin has 9.5% porosity, 5.5% volume of shale and 70.6% hydrocarbon saturation and evinces good quality reservoir (Nawaz et al. 2015). According to Saddique et al. (2016), the Lockhart Limestone is dominated by limestone with 36 m thickness in Kahi-01 well, Kohat sub-basin, Pakistan. The limestone units are vacated by vuggy and crystalline porosities and are declared as a hydrocarbon bearing formation (Saddique et al. 2016). Ahsan and Shah (2017) have studied reservoir characteristics and depositional fabric of the Lockhart Limestone outcropped in the Hazara-Kashmir area(s). In the Hazara-Kashmir area(s), the porosity of the Lockhart Limestone ranges from 0.5% (tight) to 4% (Ahsan and Shah 2017). Ahsan and Shah (2017) reported nodular limestone and minor shales within the Lockhart Limestone of the Hazara-Kashmir area(s). The Lockhart Limestone is interpreted as carbonate ramp and represents retrogradational/progradational (transgressive/highstand systems tract) stacking pattern (Ahmad et al. 2014; Ahsan and Shah 2017). Siyar et al. (2018) interpreted reservoir properties of the Paleocene Lockhart Limestone in Chanda-01 well, Kohat sub-basin, Pakistan. They have demarcated a reservoir zone within carbonates of the Lockhart Limestone having 4% volume of shale, 5% average porosity, 4% effective porosity and 85% hydrocarbon saturation.
It is obvious from the above discussion that no such work on the petrophysical properties in combination with sequence stratigraphy of the Paleocene Lockhart Limestone of Meyal Oil Field has been done before. The current research is an effort to investigate the reservoir suitability to establish the T–R (transgressive–regressive) sequences and to interpret the depositional setting of the Lockhart Limestone using the conventional petrophysical logs.
Materials and methods
Calculation of hydrocarbon saturation
Carbonates porosity types Vb·w values (Fertl and Vercellino 1978)
Carbonates porosity types
Vuggy and intercrystalline (intergranular)
Results and discussion
Petrophysical analysis of the Lockhart Limestone of Meyal-05P well
Volume of shale (Vsh)
The interval of the Lockhart Limestone that is drilled in the Meyal-05P well contains a desirable amount of hydrocarbons. The value for hydrocarbon saturation lies in the range of 92.21–99.8% (Fig. 7).
Petrophysical analysis of the Lockhart Limestone of Meyal-10P well
Volume of shale
In Meyal-10P, most of the values for the Vb·w lie in the range of 0.015 and 0.025 (26.70%) which show the presence of vuggy and intercrystalline porosity. Also many values fall in the range of 0.025 and 0.04 (25.10%) which is an indication of intercrystalline porosity. Few values are ranging between 0.005 and 0.015 (5.50%) reflecting the vuggy porosity.
This interval of Lockhart Limestone contains a desirable amount of hydrocarbons, i.e., 97–99.6% (Fig. 11).
The permeability is calculated from the Timur Permeability equation which is less than 0.1 mD (millidarcy) for the Lockhart Limestone of both Meyal-05P and Meyal-10P wells.
Qualitative assessment of porosity for a reservoir (Rider 1996)
Qualitative assessment of porosity
Average porosity (%)
Diagenetic fabric and reservoir quality of the Lockhart Limestone: Petrographic approach
Different porosity types are reported based on the Vb·w values for carbonates of the Lockhart Limestone. In this connection, it is also worth mentioning that intergranular, vuggy, moldic and fracture type of porosities are reported through petrographic studies by Ahmad et al. (2014) in the Lockhart Limestone of the Nammal Gorge, Western Salt Range, Pakistan (Fig. 15c, e). There are variations in the values of density, neutron and sonic log porosities. All the porosity logs, i.e., density, neutron and sonic logs estimate both primary and secondary porosities. In fact, the porosity is not directly measured by these logs but they consider the physical parameters of the formation and link them to porosity estimation using mathematical calculations (Rider 1996). The density and neutron logs consider pore spaces of all sizes. Sonic log measures the interparticle/intergranular/intercrystalline primary porosity and is less sensitive for demarcating fractures and vugs, i.e., secondary porosity (Rider 1996). Furthermore, sonic log in combination with total porosity logs, i.e., density or neutron-density combo can be used to determine secondary porosity (Rider 1996).
Lithology interpretation of the Lockhart Limestone using petrophysical logs
The GR log of the studied wells is used to demarcate the shale and carbonates (reservoir) intervals within the Lockhart Limestone. A vital gizmo of lithology interpretation is density log when utilized in mishmash with neutron log. The combo of density and neutron logs is commonly used to divide limestone and dolostone in a carbonate succession. For limestone, the density and neutron logs will overlay and will be apart for dolostone, provided the correct log scale (Lucia 2007).
In Meyal-05P, the lithology is interpreted to be dominantly limestone because the GR log is below the shale baseline throughout the interval (Fig. 16). The caliper log response is also very uniform, i.e., approximately lying at 7 inches throughout the interval. The SP log curve is almost straight without any prominent deflection reflecting the presence of clean limestone. The resistivity logs (SN, LLS and LLD) show higher values and away from each other. According to Rider (1996), the density log value for limestone is 2.71 g/cc, and likewise throughout the interval, the density log values are uniform, i.e., approximately at 2.71 g/cc. The sonic log values are also close to the matrix transit time for limestone (Fig. 16).
Transgressive–regressive (T–R) sequences for the Lockhart Limestone of Meyal-05P and Meyal-10P
Sequence stratigraphic analysis using petrophysical logs is an important component of analyzing a subsurface data set. Log data allow lithology and depositional environment to be integrated with the seismic section, thus linking seismic facies, rock properties and sedimentological facies. A number of studies have been made on sequence stratigraphy interpretation using petrophysical logs but the early one is that of Wagoner et al. (1990). Later on, other publications have also been presented by different authors (Vail and Wornardt 1990; Armentrout et al. 1993; Bowen et al. 1993).
Petrophysical log-based depositional facies (electrofacies) of Lockhart Limestone of Meyal-05P and Meyal-10P
The sequence stratigraphy is well-developed from electrofacies which are characterized by distinctive properties differ from the adjacent facies (Serra 1985). Such type of facies for the Lockhart Limestone in the Meyal-05P and Meyal-10P wells have been discussed in the below section.
Different electrofacies within the Lockhart Limestone of Meyal-05P interpreted from different log trends (GR, SP, LLD and RHOB) are discussed below.
According to the investigation of Malik and Ahmed (2014) on the Lockhart Limestone of Samana Range and Daud Khel sections, the facies assemblages correspond to inner platform/lagoon, platform margin and slope settings which show deposition in the inner, middle and outer parts of platform in supratidal, intertidal and subtidal environments. Hanif et al. (2013) interpreted inner ramp lagoon, shoal and fore-shoal open marine facies associations represented by wackestone and packstone foraminiferal–algal deposits within the Lockhart Limestone. These facies are present in a cyclic order and displayed a retrograding carbonate ramp indicating the Thanetian transgressive deposits associated with eustatic sea-level rise (Hanif et al. 2013).
Electrofacies 2 This is 21-m-thick (4112–4133 m) unit having cylindrical shape indicating the aggradational sequence (Figs. 19, 21). The GR log values almost remain the same, while the resistivity log values first increased and then decreased. SP and density logs values remain same. This trend indicates the depositional setting for the keep-up carbonate at the shallow water (Figs. 19, 21; Kendall 2003).
According to Ahmad et al. (2014), three facies associations are recognized in downslope, along a distally steepened carbonate ramp platform. These facies associations correspond to inner ramp, middle ramp and ramp slope settings. The biostratigraphy implies that ramp carbonates were deposited in a single third-order depositional cycles in a highstand systems tract (Ahmad et al. 2014).
Electrofacies 3 This is 5-m-thick (4133–4137 m) rough bell shape fining upward sequence (Figs. 19, 21). The GR values for this interval go through a small variation from high value toward the lower value. While the values of density, resistivity and SP logs almost remain the same. This type of trend indicates the retrogradational sequence of the give-up carbonates at the tidal channel fill and tidal flat during the sea-level rise (Figs. 19, 21; Kendall 2003).
Bilal and Khan (2017) interpreted clastic-free shallow marine shelf conditions for the Lockhart Limestone. During the Upper Paleocene, the Lockhart Limestone containing benthonic foraminifera deposited during the transgression of the sea level (Bilal and Khan 2017).
Electrofacies 4 This is 15-m-thick (4137–4153 m) cylindrical shape interval which shows the aggradational sequence (Figs. 19, 21). The GR log values remain unchanged. Similarly, the resistivity log values also remain unchanged at the start but go toward the high value at the bottom (Figs. 19, 21; Kendall 2003).
The Lockhart Limestone is deposited in shallow to deep water of restricted inner shelf, near shore to inner shelf and inner to middle shelf environment of deposition due to the abundance of benthic foraminifera and lack of planktonic foraminifera (Khattak et al. 2017).
Electrofacies 5 This interval is 5-m-thick (4153–4158 m) with well-developed funnel shape indicating the progradational sequence (Figs. 19, 21). The GR values vary from low toward high, while the resistivity, density and SP log values remain unchanged. This type of trend indicates the depositional setting for the catch-up carbonates at the shore line (Kendall 2003). At this point, the sedimentary builds up change from clastics to carbonates during the sea-level rise (Figs. 19, 21; Kendall 2003).
According to Khan et al. (2016), the Lockhart Limestone represents a carbonate cyclic sequence marked by three, transgressive, deepening up cycles representing a gradual sea-level rise compensated by vertical accumulation of microfacies. The commencement of each cycle is clearly marked by the input of land-derived siliciclastic sediments and near shore-restricted marine faunal/floral assemblage in the inner shelf microfacies gradually thinning up section where the microfacies become deeper offshore (Khan et al. 2016). Overall the Lockhart Limestone is deposited in various sub-environments of marine environment varying from the tidal flat or tidal channel fill which has saline water influx through shallow water and shoreline which show deposition of catch-up carbonates (Figs. 19, 21; Kendall 2003).
Although the Lockhart Limestone is mainly composed of limestone, it has certain facies (electrofacies) on the basis of sea-level fluctuations. Sea-level fluctuations can easily be identified using GR log trends devised by Kendall (2003). The details of the depositional facies based on GR log (i.e., electrofacies) for the Lockhart Limestone in the Meyal-10P well are described as:
In Kotal Pass Section of north Pakistan, inner to middle neritic shelf depositional system for Lockhart Limestone has been considered based on planktonic/benthonic ratio, the total foraminiferal abundance and their diversification (Afzal et al. 2005).
According to Khan et al. (2018), the Lockhart Limestone, exposed in Taxila, is interpreted to have been deposited in the fore-shoal mid-ramp, mid-ramp and outer ramp depositional environments. This limestone is characterized by siliciclastic wackestone microfacies thereby providing the evidence that the depositional settings were changed from clastics to the carbonates (Khan et al. 2018).
Electrofacies 3 This interval is 10-m-thick (4139–4149 m) almost symmetrical shape (Figs. 20, 23). The GR values for this interval first increase and then decrease; the resistivity values decrease and also SP curve bent toward low values, while the density log values remain unchanged. This trend shows a first rise in the sea level and then falls which indicates the depositional setting of carbonates at the offshore buildups, regressive-to-transgressive shore face (Figs. 20, 23; Kendall 2003). Sameeni et al. (2009) interpreted inner, middle and outer shelf environment of deposition for mudstone, wackestone and packstone microfacies of the Lockhart Limestone.
Electrofacies 4 This is 40-m-thick (4150–4190 m) interval of cylindrical shape (Figs. 20, 23). The GR log values for this interval are almost same marking a straight line showing no variation in the values. The SP log curve goes through a greater fluctuation from high value to lower value but the overall trend is from low to high values. The resistivity log shows sharp variation from high to low value. While the density log (RHOB) value remains the same, showing the carbonate lithology. This trend shows the aggrading stacking pattern of the depositional setting of heterogeneous facies in the shallow water (Figs. 20, 23; Kendall 2003). The Lockhart Limestone is entirely of shallow marine in origin with Paleocene biotic assemblages and minor shale (Yaseen et al. 2011).
Electrofacies 5 At the lower most part of the formation is ~ 3 m thick interval (4190 m to 4193 m) having the funnel shape coarsening upward sequence. The GR values shows variations from low to high. The resistivity log curve shows fluctuations, while the SP log values first increases and then decreases at the upper and lower parts of the interval-5 respectively. This trend indicates the depositional setting of carbonates at the shore line indicating the prograding stacking pattern (Figs. 20 and 23; Kendall 2003).
Petrophysical analysis, establishment of transgressive–regressive sequences and interpretation of depositional settings of the Lockhart Limestone, based on petrophysical logs, penetrated in two wells (Meyal-05P and Meyal-10P) of the Meyal Oil Field, Potwar sub-basin has been conducted. Based on the quantitative parameters, Lockhart Limestone of Meyal-10P is a good reservoir as compared to that of the Meyal-05P. In Meyal-05P and Meyal-10P wells, the average volume of shale is 48% and 20%; the range of density porosity is 1–5.6% and 1–31.7%; range of neutron porosity is 1–23% and 1–42.9%; the range of sonic porosity is 1–29% and 1–39%; the range of effective porosity is < 1–> 5% and 1–21%; and the range of hydrocarbon saturation is 92.21–99.8% and 97–99.6%. The Lockhart Limestone of Meyal-05P is a poor quality reservoir; however, the Lockhart Limestone of Meyal-10P well is a very good reservoir. In Lockhart Limestone of both wells, the permeability is < 0.1 mD. In Meyal-05P and Meyal-10P, the interpreted lithology of the Lockhart Limestone based on the petrophysical logs is predominated by limestone and minor shale (present in Meyal-10P only). The quantitative (very low permeability and very high hydrocarbon saturation) and qualitative analysis reflects that the Lockhart Limestone is a tight reservoir. The Lockhart Limestone of both wells is deposited in different depositional environments showing the aggrading, prograding and retrograding depositional settings. The interpreted carbonate facies includes keep-up carbonates, catch-up carbonates and give-up carbonates implying fluctuations in the sea level.
The authors are thankful to Directorate General of Petroleum Concession (DGPC), Islamabad, Pakistan, and LandMark Resources (LMKR), Pakistan for providing the well data. The authors are also thankful to LMKR for providing the academic license of Geographix software.
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