**1. Introduction**

The recent global increase in fuel demand has increased hydrocarbon production at established reservoirs [1,2]. According to the United States Energy Information Administration (USEIA), Pakistan may have over 9 billion barrels of oil and 105 trillion cubic feet

**Citation:** Zhang, B.; Tong, Y.; Du, J.; Hussain, S.; Jiang, Z.; Ali, S.; Ali, I.; Khan, M.; Khan, U. Three-Dimensional Structural Modeling (3D SM) and Joint Geophysical Characterization (JGC) of Hydrocarbon Reservoir. *Minerals* **2022**, *12*, 363. https://doi.org/ 10.3390/min12030363

Academic Editor: Michael S. Zhdanov

Received: 2 February 2022 Accepted: 13 March 2022 Published: 16 March 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

of natural gas (including shale gas) reserves [3]. Pakistan's gas fields are only expected to last for about another 20 years at the most due to heavy industrial usage. Therefore, many recent works on the Lower Indus Basin (LIB) are focused on unconventional resources such as shale gas and coal seam gas [4–6]. However, focused characterization and re-evaluations of already-discovered petroleum systems are also required to evaluate reservoirs' abilities to meet the hydrocarbon requirements [7].

Reservoir characterization is a scheme that quantifies the physical and fluid properties of rock [8–12]. It also involves an understanding of reservoir structure, sedimentological heterogeneity, facies, and the quantity of hydrocarbon that exist in structural traps driven by tectonic movements, discontinuities such as faults, folds, anticlinal structures, horst and graben pop-up geometries, and duplex structures [13–18]. Advancements in 3D seismic data analysis and borehole geophysics have made it possible to characterize structural and stratigraphic features and their associated petrophysical properties with high reliability and precision, thereby reducing the risks associated with hydrocarbon exploration [19,20].

The development of three-dimensional structure models (3D SMs) is essential for reservoir description, e.g., true structural dip, fault system models (FSMs), up-dip hydrocarbon migration pathways, and quantitative geometric characterization in 3D space [21,22]. Three-dimensional structural modeling (3D SM) is divided into entity-based modeling and volume-based modeling (VBM) [23–28]. The former develops 3D SMs using a combination of four different geometric entities, i.e., point, line, surface, and body. It emphasizes the shape of geological structures and the relationship among geological bodies [29]. The latter subdivides the 3D space into discrete fields by regular or irregular voxels and emphasizes the spatial distribution of geophysical and geochemical properties. With the continuous development of reservoir geological modeling technology, the VBM, objective function, variation function, multipoint geostatistics, and static geological modeling with knowledge-driven methodology have been widely applied, significantly promoting the development and technology of 3D reservoirs geological modeling [30–36]. However, VBM is a step-change reservoir geological modeling technique that creates horizons based on depositional sequence instead of considering horizons as discrete surfaces [2,13,37].

Identifying reservoir facies and properties using seismic attributes and petrophysical analyses play an essential role in reservoir characterization. Reservoir facies classification and properties evaluation can be achieved via laboratory studies on core plugs, which are costly and time-consuming. However, seismic attributes can be used for stratigraphic-based basin depiction within a composite deposition-based structure and classify reservoir facies, thereby increasing the rate for adequate reservoir characterization. Seismic attributes, such as dip magnitude, edge enhancement, variance edge, sweetness, and root mean square (RMS) amplitude, are essential tools for delineating structural and stratigraphic characteristics, lithofacies changes, and hydrocarbon potential zones [19,38,39]. On the other hand, petrophysical analysis based on well logs offers practically continuous reservoir properties, e.g., lithology, the volume of shale (*Vshale*), average porosity (∅*avg*), effective porosity (∅*eff*), water saturation (*SW*), and hydrocarbon saturation (*Shc*). The proper analysis of these properties can significantly improve the ability to distinguish hydrocarbon-bearing zones [18,20,40,41].

The Middle Indus Basin (MIB) and Lower Indus Basin (LIB) are well known for hydrocarbon exploration in Pakistan [40]. The Kadanwari field in MIB is a significant hydrocarbon-producing field, with the early–late Cretaceous Lower Goru formation (LGF) acting as the potential reservoir. The previous studies conducted on the Kadanwari field were based on the assessment and development of the 2D fault system, porosity prediction [42], formation evaluation [40,43], and impact of diagenesis on reservoir quality prediction [44].

A systematic review of prospective observational studies found that the Kadanwari field in MIB has not yielded sufficient results for understanding the complex structural depositional environment [45]. Moreover, the distribution of key petrophysical properties and facies in the Kadanwari field are difficult to predict due to fluctuating deltaic conditions, mutable geological influences, varying hydrocarbon concentrations, regional tectonic settings, and changes in geometries. However, comprehensive research on the structural characteristics and associated tectonic extensional fault system models (FSMs) and the evaluation of reservoir characteristics and sweet spots for future drillings in the Kadanwari field is still missing.

This study, therefore, aims at evaluating the complex and heterogeneous depositional environment broadly comprised of structural and stratigraphic characteristics, distribution of associated facies, and petrophysical properties for reliable reservoir characterization, which was lacking in the previous studies. This was achieved by utilizing three-dimensional structural modeling (3D SM) and joint geophysical characterization (JGC) using seismic and well logs data. In this study, 3D SM and JGC have made novel contributions and facilitated a clearer representation of the complex and heterogeneous depositional environments, with the lateral and horizontal structural extent of reservoir horizons, FSMs (including fault geometry and orientation in 3D space), spatial facies, reservoir properties (e.g., lithology, *Vshale*, ∅*avg*, ∅*eff*, *SW*, and *Shc*), and direct hydrocarbon indicators (DHIs). In short, our presented study is crucial to characterize and evaluate reservoirs' geometrical characteristics, facies, and properties, in order to reduce uncertainties and improve the success rate of future exploration and development plans pertaining to hydrocarbons in the study area, as well as potentially in other regions around the world.

### **2. Background Geology**

Pakistan is located at the triple junction of the Indian, Eurasian and Arabian plates (Figure 1a). During the middle/late Jurassic to early Cretaceous, the Indian plate rifted away from the Gondwana landmass, forming an island continent that drifted northwards into the Tethyan Ocean [46]. This tectonic event predominantly influenced the structures and sedimentation of the MIB and LIB, which could have resulted in NE—SW to N—S rift systems. The Kadanwari field is located in the District Khairpur of Sindh Province, southeast MIB, a prolific gas-prone basin in Pakistan (Figure 1b). The latitude of the study area ranges from 27◦0408300 N to 27◦0701200 N, and its longitude varies from 69◦1209800 E to 69◦1705700 E. Tectonically, the Kadanwari field lies between two extensive regional highs, i.e., the Mari-Kandhkot High and the Jacobabad-Khairpur High (Figure 1b). In the east, it is bounded by the Indian shield; in the north by the Sargodha high; in the west by the fold and thrust belt of the Kirthar and Sulaiman Ranges, and in the south by the Jacobabad-Khairpur High [7,45–47]. Three tectonic events were responsible for the structural configuration of the study area, i.e., the late Cretaceous uplift and erosion, the late Paleocene wrench faulting, and the late Tertiary to Quaternary uplift/inversion of Jacobabad High (Figure 1b) [46]. The Jacobabad-Khairpur High was a primary contributor to the study area's structural traps and surroundings [45,47]. The final tectonic event of the late Tertiary to Quaternary was an inversion of the Jacobabad-Khairpur High, which significantly affected the Kadanwari area [45,46]. In the Kadanwari field and its surroundings, the trapping mechanism is a complex combination of structural dip, sealing faults, and loss of reservoir quality to the north. The Kadanwari field consists of several low-relief faults, forming dip closures in the subsurface and providing a stratigraphic trapping component [7,45]. The fault dip closures and the wrench faults are particularly significant as they divide the Kadanwari field into reservoir compartments [40].

The lithology stack of MIB is depicted in Figure 2a, highlighting the basin fill sedimentary deposits. The lithostratigraphic columns show the rock units encountered in the Kadanwari-10 and Kadanwari-11 wells (Figure 2b). According to [48], the shales of the Sember Formation serve as the source rock for the regional petroleum systems of the MIB. However, the reservoir sections (e.g., G and E sands) in the Kadanwari field belong to the lower Goru sand (the Cretaceous age), while the sealing is provided by the upper Goru shaly sequence [43].

shaly sequence [43].

**Figure 1.** (**a**) Location of the study area; (**b**) generalized tectonic map with the location of major oiland gas-producing fields in the study area, bounded by other gas fields, modified from [49]; (**c**) the base map shows the orientation and general information of the 3D seismic lines and wells in the Kadanwari field, MIB, Pakistan. **Figure 1.** (**a**) Location of the study area; (**b**) generalized tectonic map with the location of major oiland gas-producing fields in the study area, bounded by other gas fields, modified from [49]; (**c**) the base map shows the orientation and general information of the 3D seismic lines and wells in the Kadanwari field, MIB, Pakistan.

the Kadanwari-10 and Kadanwari-11 wells (Figure 2b). According to [48], the shales of the Sember Formation serve as the source rock for the regional petroleum systems of the MIB. However, the reservoir sections (e.g., G and E sands) in the Kadanwari field belong to the lower Goru sand (the Cretaceous age), while the sealing is provided by the upper Goru

**Figure 2.** (**a**) Generalized stratigraphic column of the study area, modified from [48]; (**b**) lithostratigraphic columns showing the rock units encountered in Kadanwari-10 and Kadanwari-11 wells. **Figure 2.** (**a**) Generalized stratigraphic column of the study area, modified from [48]; (**b**) lithostratigraphic columns showing the rock units encountered in Kadanwari-10 and Kadanwari-11 wells.

#### **3. Material and Methods 3. Material and Methods**

#### *3.1. Datasets Description and Processing 3.1. Datasets Description and Processing*

A vast volume of seismic reflection data was acquired from the MIB, Pakistan, to facilitate hydrocarbon exploration activities at different times, from the 1970s up to the modern-day [7]. The Pakistan branch of OMV (www.omv.com, Vienna, Austria) recently began a new exploration phase in the Kadanwari field by conducting a 3D seismic survey. The seismic data used in this study are 3D post-stack time migrated seismic reflection cubes stored in the SEG-Y format, wherein approximately 116 seismic inlines and approximately 181 cross-lines were used. The geometric information, e.g., total coverage area, inline interval, crossline interval, and time slice range of available 3D seismic data, were 12 km2, 24.17, 25.17, and 1800–2600 (ms), respectively. The well logs data comprise lithology, resistivity, and porosity logs (e.g., GR, SP, LLD, LLS, MSFL, RHOB, NPHI, and DT) of the Kadanwari-10 and Kadanwari-11 wells (Table 1). The available 3D seismic data and well log data were collected from the Landmark Resources (LMKR) (www.lmkr.com, Calgary, Canada) upon the request of the Directorate General of Petroleum Concessions (DGPC) (www.mpnr.gov.pk, Islamabad, Pakistan); which are available to the public A vast volume of seismic reflection data was acquired from the MIB, Pakistan, to facilitate hydrocarbon exploration activities at different times, from the 1970s up to the modern-day [7]. The Pakistan branch of OMV (www.omv.com, Vienna, Austria) recently began a new exploration phase in the Kadanwari field by conducting a 3D seismic survey. The seismic data used in this study are 3D post-stack time migrated seismic reflection cubes stored in the SEG-Y format, wherein approximately 116 seismic inlines and approximately 181 cross-lines were used. The geometric information, e.g., total coverage area, inline interval, crossline interval, and time slice range of available 3D seismic data, were 12 km<sup>2</sup> , 24.17, 25.17, and 1800–2600 (ms), respectively. The well logs data comprise lithology, resistivity, and porosity logs (e.g., GR, SP, LLD, LLS, MSFL, RHOB, NPHI, and DT) of the Kadanwari-10 and Kadanwari-11 wells (Table 1). The available 3D seismic data and well log data were collected from the Landmark Resources (LMKR) (www.lmkr.com, Calgary, Canada) upon the request of the Directorate General of Petroleum Concessions (DGPC) (www.mpnr.gov.pk, Islamabad, Pakistan); which are available to the public domain and can be utilized for scientific and research purposes. The dataset quality was first checked and harmonized in a clearly defined database. Accordingly, a base map of the 42-N TransMercator Macrocosm (UTM) zone was created using navigation and SEG-Y records to determine the orientation (dip or strike) and location of seismic lines and wells (Figure 1c).

**Table 1.** Metadata of the utilized well logs and their uses in this study.

