The Volcanic Rocks and Hydrocarbon Accumulation in the Offshore Indus Basin, Pakistan

Abstract

To analyze the impact of volcanic rocks in the Offshore Indus Basin on hydrocarbon reservoir formation, seismic data interpretation, seismic data inversion, and sea–land correlation analysis were carried out. The results show that, longitudinally, volcanic rocks are mainly distributed at the top of the Cretaceous system or at the bottom of the Paleocene, and carbonate rock platforms or reefs of the Paleocene–Eocene are usually developed on them. On the plane, volcanic rocks are mainly distributed on the Saurashtra High in the southeastern part of the basin. In terms of thickness, the volcanic rocks revealed by drilling in Karachi nearshore are about 70 m thick. We conducted sparse spike inversion for acoustic impedance in the volcanic rock area. The results show that the thickness of the Deccan volcanic rocks in the study area is between 250 and 750 m which is thinning from southeast to northwest. Based on sea–land comparison and comprehensive research, the distribution of volcanic rocks in the Indian Fan Offshore Basin played a constructive role in the Mesozoic oil and gas accumulation in the Indus offshore. Therefore, in the Indian Fan Offshore Basin, attention should be paid to finding Mesozoic self-generated and self-stored hydrocarbon reservoirs and Cenozoic lower-generated and upper-stored hydrocarbon reservoirs.

1. Introduction

The history of offshore hydrocarbon exploration in Pakistan spans nearly 60 years. Since the 1960s, nine foreign oil companies have carried out 2D and 3D seismic surveys and drilling in Pakistan’s sea area, basically completing the general survey and local detailed survey of oil and gas in Pakistan’s exclusive economic zone. The cumulative acquisition of 2D seismic survey lines in the Offshore Indus Basin is about 70,000 km, with a grid density of 4 × 4 km in most areas and 2 × 4 km in some regions. There are five 3D seismic blocks covering an area of 8336 km².

Up to now, Pakistan has drilled eighteen offshore wells, most of which were drilled on the shelf, and only three wells (Kekra-1, Pak G2-1, Anne-1) were drilled in deep water areas. Among them, seven wells showed oil and gas, and one well found a small amount of oil and gas (PakCan-1) [1,2], which has no commercial value.

Due to the large span of acquisition years and different acquisition and processing parameters, the quality of existing seismic data is uneven, especially under the shielding effect of igneous rocks, resulting in unclear deep seismic imaging, which has a significant impact on understanding the lithology, distribution, sedimentary evolution, structural evolution, and basin structure of the deep Mesozoic strata in the study area. Thus, the current exploration focus in the Indian fan offshore basin is on the Cenozoic. The main exploration targets include the Miocene deep-water channel-fan sedimentary body [3,4,5,6] and the Paleocene isolated carbonate platform [7].

In response to the above issues, in order to increase the investigation of offshore oil and gas resources in Pakistan, the China Geological Survey Bureau, in collaboration with relevant Pakistani units, organized two survey voyages in the Indian Fan offshore basin in 2019 and 2021. Based on the above data, through seismic interpretation and stratigraphic correlation, our research team first discovered a widespread distribution of Mesozoic strata under the Deccan volcanic rocks in the Offshore Indus Basin, providing new exploration ideas.

In order to investigate the distribution of Deccan volcanic rocks in the Offshore Indus Basin and their control over Mesozoic hydrocarbon reservoirs, this research will conduct a comprehensive study on the regional geological overview, distribution of Deccan volcanic rocks in the sea and land, and the relationship between Deccan volcanic rocks and hydrocarbon reservoirs, to explore the impact of Deccan volcanic rocks on hydrocarbon reservoir formation in Pakistan’s sea area and future exploration directions. Meanwhile, achieving a breakthrough in oil and gas in Pakistan’s waters will help alleviate the country’s energy supply–demand contradiction.

2. Regional Geological Overview

The sea area of Pakistan is located at the triple junction of the Indian, Arabian and Eurasian plates [8]. According to the distribution characteristics of the Paleogene strata, it can be divided into five first-order tectonic units, which are arranged from north to south as follows: Makran Accretionary Prism, Oman Abyssal Plain, Murray Ridge, Offshore Indus Basin, and Saurashtra High (Figure 1). They are distributed in a tectonic pattern of alternating ridges and depressions [9,10,11,12,13].

The Offshore Indus Basin is located on the western edge of the Indian Plate and is a passive continental margin basin. It is bordered by the Indus River Basin in Pakistan to the northeast, the Murray Ridge to the northwest, and the Saurashtra High to the south. It covers an area of nearly 90,000 km².

The development process of the Offshore Indus Basin is divided into three stages [14]:

1. The rifting stage from the end of the Late Cretaceous to the Early Paleocene: The basin experienced prolonged volcanic eruptions, forming large areas of volcanic rock, laying the foundation for the Paleogene–Neogene basins in the study area (Figure 2).

2. The thermal subsidence stage from the Paleocene to the Eocene: Oceanic ridge expansion led to thermal subsidence, forming the first set of cover layers in the basin, and forming sea basins and sea platforms on the passive continental margins. Due to the low supply of terrigenous clastic material, shallow-water carbonate reef shoals and carbonate platforms developed on the sea platforms, and deep-sea mud shales and some turbidite sands were deposited in the low-lying areas between the platforms.

3. The Oligocene to the Quaternary Neotectonic period: Large-scale channel-bank deposits developed. As the Indian Plate moved northward and collided with the Eurasian Plate, a large amount of clastic material was transported into the basin by the Indus River, and 9000 m thick sediments accumulated since the Oligocene, developing river-delta deposits. The Miocene to Pleistocene strata is dominated by large-scale channel-bank deposits.

3. Materials and Methods

3.1. Materials

In this study, 2D seismic data and drilling data were mainly used. Among them, the 2D seismic data mainly come from the results of two survey voyages deployed by the China Geological Survey in the Offshore Indus Basin in 2019 and 2021, while the drilling data mainly involve collecting drilling data from the related historical literature.

3.2. Methods

3.2.1. Tectonic Evolution Reconstruction

GPlates is an open-source, cross-platform GIS software that plays a crucial role in plate tectonics and paleogeography [15,16]. In recent years, more and more scholars around the world have used GPlates software to reconstruct the evolution process of various plates, achieving good results [17,18,19]. We mainly used GPlates to analyze the plate activity process of the Indian Plate since the Jurassic (200 Ma), to understand the process of continental fragmentation and plate drift, and to analyze the impact of various tectonic events on the Offshore Indus Basin.

3.2.2. Seismic Data Interpretation and Inversion

According to the reflection characteristics of seismic wave field (i.e., external geometry, internal reflection structure, continuity, amplitude, frequency, and interval velocity), the Deccan volcanic rocks in the seismic profile were identified via seismic inversion combined with regional geological data. Seismic inversion is a widely used method for reservoir prediction, which estimates reservoir parameters by establishing mathematical relationships between these parameters and seismic data. It is necessary to determine sensitive inversion parameters in a study area to effectively realize seismic inversion. A petrophysical analysis was carried out using the known data from the Karachi South-1A well. It reveals that the volcanic rocks show high impedance characteristics, as shown in Figure 3. Therefore, it becomes possible to ascertain the spatial distribution characteristics of the volcanic rocks and estimate their thicknesses by using sparse spike inversion for acoustic impedance.

The specific inversion process is as follows:

• Extract the wavelet matrix $G$ according to the seismic record and well data.

• Establish the forward relationship between seismic data $d$ and impedance $Z$ based on the reflection coefficient formula and seismic convolution model, which can be given by:

  (1)$d=G·r$

  (2)$r={\displaystyle \frac{\partial \ln Z}{\partial t}}$

  where $t$ is the two-way travel time.

• Under the framework of Bayesian theory, we can compute the solution ${\mathbf{Z}}_{inv}$ for inverse problem:

  (3)$Z_{inv}={\left(G^{\mathrm{T}}G+\mu Q\right)}^{-1}{G}^{\mathrm{T}}d$

  where $Q$ is the sparse constraint term, which includes the initial model of impedance. $\mu$ is weight coefficient.

According to the inversion process, we can realize the prediction of impedance.

3.2.3. Contrast between Sea and Land

Using seismic data interpretation, drilling verification, and seismic data inversion of volcanic rock thickness and the relationship between terrestrial volcanic rocks and Mesozoic hydrocarbon reservoirs, we inferred the distribution and thickness of Deccan volcanic rocks in the sea and their control effect of Mesozoic hydrocarbon. This will provide support for Mesozoic hydrocarbon exploration breakthroughs.

4. Results and Discussion

Previous studies have suggested that the Deccan volcanic rocks in the Offshore Indus Basin were formed when the Indian plate and the Seychelles microplate broke apart during the late Cretaceous and early Paleocene and drifted northward through the Réunion hotspot, with the center located in the Deccan Traps of India [20,21] (Figure 4). The Deccan basalt in the Indian mainland is widely distributed and stratified. It is mainly located in the Deccan Plateau in south-central India and is one of the largest volcanic landforms on the Earth’s surface. The Deccan lava pile, which obscures the basement from observation over 0.5 million km², is thickest (~2000 m) along the Western Ghats region (Sahyadri range) adjacent to the western coast and thins progressively eastward and southeastward, such that along the eastern fringes of the province the lava pile is only ~200 m thick.

Sprain et al. [16] constrained the location of the Cretaceous–Paleogene boundary with high-precision 40 Ar/39 Ar data and predicted that the main eruption period of the Deccan Traps was 65–66 million years ago. Using paleomagnetic recovery to reconstruct plate tectonics is an important means of studying continental breakup and plate drift. We have collated paleomagnetic data from the study area and neighboring regions [22], and used GPlates software to reconstruct the tectonic background of the Indian plate and its surrounding regions dating back 65 million years (Figure 5). Although not very precise, it can be seen from Figure 5 that the center of the Deccan volcanic rocks is far away from the Offshore Indus Basin. Therefore, it can be speculated that the thickness of the Deccan volcanic rocks in the Offshore Indus Basin is less than that of the Deccan Traps.

The results of hydrocarbon exploration in the Pakistan land area confirm that the main hydrocarbon reservoirs in the Lower Indus Basin are located under the Deccan volcanic rocks. It proves that our research on the relationship between volcanic rocks and oil and gas reservoir formation has important practical significance.

4.1. Comparison Results between Land and Sea

4.1.1. Distribution Characteristics of Deccan Volcanic Rocks in the Pakistan Land

The Deccan volcanic rocks in the Lower Indus Basin are mainly distributed in the Early Paleocene Khadro Formation, interbedded with shale, and have a relatively thin thickness [23]. Their planar distribution is limited, with a thickness ranging from 0 to 30 m, and Jurassic and Cretaceous strata are developed below them, while hydrocarbons are mainly distributed in the Lower Cretaceous (Figure 6) [24].

Chen et al. (2017) [25] showed that the Lower Indus Basin experienced magmatic intrusion and volcanic eruptions during the late Cretaceous period, forming regionally distributed volcanic layers with a thickness of typically less than 50 m. Tian (2017) [26] interpreted the logging curve of a well in the Lower Indus Basin, revealing that the maximum thickness of the Deccan volcanic rocks can reach 760 m. The volcanic rocks are distributed in the Khadro Formation at the base of the Paleocene, with a resistivity of up to 200 ohm-meters. This is the thickest volcanic layer currently available in the Lower Indus Basin.

4.1.2. Distribution Characteristics of Deccan Volcanic Rocks in the Northeastern Offshore of the Offshore Indus Basin

In the northeastern part of the Offshore Indus Basin, four wells encountered Deccan volcanic rocks, including Karachi south-1A, Patiani Creek-1, Dabbo Creek-1, and Shaik Nadin-1, two of which penetrated through the Deccan volcanic rocks [7] (Figure 7). From Figure 7, it can be seen that the Deccan volcanic rocks are distributed from the top of the Cretaceous system to the bottom of the Paleocene, with a thickness of between 61 and 75 m (Table 1), and under which there are Cretaceous shales of varying thickness.

Khan et al. (2020) [27] conducted a study on the Karachi South-1A well, which found that volcanic rocks and shale were interbedded in the Early Paleocene Khadro Formation. Seismic inversion results showed that the thickness of the volcanic rocks in the well was only 26 m. From Figure 5 and Table 1, the thickness of the Deccan volcanic rocks encountered by the Karachi South-1A well is the largest, with a thickness of 71 m. Therefore, it is speculated that the thickness of the Deccan volcanic rocks in the northeastern coastal region of the Offshore Indus Basin is around 70 m.

4.1.3. Distribution Characteristics of Deccan Volcanic Rocks in the Kutch Basin in the East

From the Late Cretaceous to Early Paleocene period, due to the proximity of the Kutch Basin in the eastern of the Offshore Indus Basin to the Réunion hotspot (Figure 5), the Deccan volcanic rocks were widely distributed in the Kutch Basin. Currently, the Deccan volcanic rocks are commonly encountered in the land of Kutch Basin (e.g., Suthri-1 well), with a thickness of 600–800 m, while there are also a few drilling wells in the sea of Kutch Basin that encounter the Deccan volcanic rocks (e.g., GK-29A-1 well), with a thickness of 200–300 m [28].

4.2. Seismic Data Interpretation and Inversion

4.2.1. Distribution Characteristics of Volcanic Rocks on Seismic Sections

There are two uplifts in the southeastern part of the Indian subcontinent: the Somnath Ridge (SR) and the Saurashtra High (SH), which are composed of Deccan volcanic rocks as the basement. Between the two uplifts is a subbasin rich in volcanic debris (VS). Previous studies have suggested that these two nearly NE-SW uplifts are composed of nine volcanic platforms [7,29] (Figure 8), with an area of approximately 42,500 km² in Pakistan’s exclusive economic zone [30], and another 300 km² extending into India.

On the seismic profile, lava overflow structures are usually visible inside the volcanic platform, with a wide, smooth, domed top. The volcanic rock thickness is between 2000 and 3000 m, and above it develops a carbonate platform with steep slopes in the west and gentle slopes in the east. The Cretaceous–Oligocene sedimentary layers gradually overlap towards the platform, and the carbonate platform is completely covered by sedimentary layers after the Miocene epoch (Figure 9).

4.2.2. Distribution Characteristics of Volcanic Rocks on the Inversion Profile

Kumar et al. (2019) [31] comprehensively analyzed 44 drilling data and seismic reflection characteristics of the Deccan Traps volcanic rocks in the Indian Ocean. It can be concluded for Deccan igneous rocks that the P-wave velocity varies between 4.1 to 5.2 km/s, and the burial depth is between 800 to 7400 m below the sea level. We conducted sparse spike inversion for acoustic impedance in the volcanic rock area, and the results showed that the volcanic rocks showed high impedance characteristics in the seismic section (Figure 10). According to the inversion results, the roof and the bottom of the volcanic rocks can be interpreted, and then we can calculate the thickness plane figure, as shown in Figure 11. It can be found that the thickness of the volcanic rocks is thinning from southeast to northwest. The time thickness of the volcanic rocks in the volcanic platform area is about 0.3 s, and the calculated depth thickness of the volcanic rocks is about 750 m (the average velocity of the volcanic rocks is about 5000 m/s). In the southeast of the volcanic platform, the time thickness of the volcanic rocks is about 0.2 s, and the calculated thickness of the volcanic rocks is about 500 m. In the northwest of the volcanic platform, the time thickness of the volcanic rocks is about 0.1s, and the thickness of the volcanic rocks is about 250 m. According to the inversion results, the thickness of Deccan igneous rocks in the study area is between 250 and 750 m.

4.3. Control of Volcanic Rocks on Hydrocarbon Reservoir Formation

4.3.1. Volcanic Rocks and Hydrocarbon Reservoir Formation in the Lower Indus Basin

The distribution of Deccan volcanic rocks in the Lower Indus Basin of Pakistan’s land area is limited, and the thickness is generally thin. From a small number of hydrocarbon reservoir profiles in the Lower Indus Basin, Deccan volcanic rocks are usually distributed in layers at the top boundary of the Cretaceous system or at the bottom boundary of the Paleocene series (Figure 4). Four wells in the shallow water area of the Offshore Indus Basin have gas show in the Cretaceous system below the Deccan volcanic rocks (Table 1), suggesting that the Deccan volcanic rocks in the sea play a sealing role for the faults of the Mesozoic and related hydrocarbon reservoirs.

The Lower Indus Basin has developed multiple sets of source–reservoir–caprock assemblages in the Mesozoic, with the main source rock being the mudstone of the Lower Cretaceous Sembar Formation, and the mudstone of the Goru Formation serving as a secondary source rock (Figure 12) [32]. The marine shale of the Sembar Formation is distributed throughout the Lower Indus Basin, with an average thickness of 600–800 m (maximum thickness of 1400 m), organic matter abundance (TOC) of 0.5–3.5%, and an average abundance of 1.4%. The organic matter types are type II and III, basically reaching the oil window. The Sui gas field, Mari gas field, and Dhodak oil and gas field in the northern part, and the Sari gas field and Badin oil field in the southern part of the central uplift zone have all confirmed that the oil and gas come from this set of source rocks (Figure 13). The maximum TOC of the marine shale of the Goru Formation reaches 2.55%, with a thickness ranging from 300–1200 m, and has entered the stage of hydrocarbon generation [33,34,35].

The reservoirs are mainly the sandstone reservoirs of the Lower Goru Formation and the limestone reservoirs of the Parh Formation. The most important regional caprocks are the mudstones of the Upper Goru Formation, and the volcanic rocks of the Upper Cretaceous–Lower Paleocene are also important caprocks. The sandstone of the Upper Cretaceous Goru Formation in the Lower Indus Basin is in a shelf-edge deltaic sedimentary environment [36,37], with a porosity of 10.5–30% [38]. Horizontal permeability concentrates from 0.04 to 0.24 mD with an average of 0.23 mD [39]. The distribution and quality of reservoirs are mainly controlled by sedimentary facies zones, followed by diagenesis. Principal diagenetic events which have resulted in changing the primary characters of the sandstones are compaction, cementation, dissolution, and mineral replacement. The diagenetic processes have altered the original rock properties and reservoir characteristics of the Lower Goru sandstone [39].

4.3.2. Deccan Volcanic Rocks and Hydrocarbon Reservoir in the Kutch Basin

In the Kutch Basin, the Deccan volcanic rocks are distributed in the western region of the Kutch Mainland Uplift and the Kutch offshore region. The Deccan volcanic rocks are in angular unconformable contact with the underlying Precambrian basement, Bhuj Formation, Naliya Formation, and Mundra Formation, and are in parallel unconformable contact with the underlying Kori Formation; they are in unconformable contact with the overlying Naredi Formation, Matanomadh Formation, and Nakhtrana Formation.

The Cretaceous Jhuran Formation and Bhuj Formation are the main source rocks in the Kutch Basin. The Jhuran Formation shale is the best source rock in the Kutch Basin, with a TOC content ranging from 0.5 to 3%, dominated by humic substances (mainly type III, with some type II). Only the Nirona-1 well in the onshore Banni Graben has some hydrocarbon generation indicators in the Jhuran Formation, but the source rock layer is relatively thin (5–24 m). Thermal simulation indicates that the overall sedimentary sequence of the Banni Graben is immature. In the Mainland Uplift area, three wells (Lakhpat-1, Suthri-1, and Sanadra-1) have shown that the thickness ranges from 370 m to 530 m, but the vitrinite reflectance value is 0.34–0.49%, indicating immature conditions [40].

The Bhuj Formation above the Jhuran Formation consists of delta, lagoon, and shallow sea shale and some coal seams. The carbonaceous shale has a tendency to generate gas/condensate oil, dominated by type III organic matter, with a TOC value ranging from 0.1 to 10.65%. It is also a good source rock. In the Mainland uplift area, three wells (Lakhpat-1, Suthri-1, and Sanadra-1) reveal that the source rock thickness of this formation is between 65 and 165 m, with the potential to generate gas and a small amount of oil. However, its vitrinite reflectance of less than 0.5% indicates that the source rock of this formation is immature.

The Mesozoic source rocks encountered during offshore drilling are immature or on the verge of maturity (such as wells GK-22C-1 and SP-1-1). However, both of these wells are relatively close to land, while the maturity of the Mesozoic in areas with volcanic rock development on land is higher. In more distant offshore areas, the Mesozoic layer is thicker, covered by thick layers of Paleogene and Neogene. Therefore, the increased burial depth of the Mesozoic layer can meet the maturity requirements. The Deccan volcanic rocks at the end of the Cretaceous period increased the geothermal gradient, which helped accelerate the process of maturity [41].

In terms of reservoirs, the Matanomadh Formation sandstone has been confirmed by the GK-29A-1 oilfield, mainly deposited on the Deccan volcanic rocks and composed of rivers. The porosity is between 20–25% on average and the permeability is between 100–1000 mD. The Lower Eocene Jakhau Formation limestone is mainly developed in the central and outer areas of the current continental shelf and is the main reservoir of the KD-1 oilfield. The total reservoir thickness is 50 m and the net reservoir thickness is 15 m.

The most important seal in the Mesozoic is the Late Cretaceous–Early Paleocene Deccan basalt, which covers the underlying Cretaceous Naliya Formation (GK-309-1) and Bhuj Formation (GK-22C-1) reservoirs. These basaltic rocks are an important regional seal, widely distributed and extending through the Kutch Mainland uplift and continental shelf area. The widely distributed shale in the Bhuj Formation is a secondary seal, providing a good cover for the GK-22C-1 reservoir.

In summary, the Deccan volcanic rocks in the Kutch basin are not only an important caprock, but also accelerate the maturation of Cretaceous source rocks.

4.3.3. Deccan Volcanic Rocks and Hydrocarbon Reservoirs in the Offshore Indus Basin

Some studies suggest that the Offshore Indus Basin has three sets of source rocks, namely Cretaceous, Paleocene–Eocene, and Miocene source rocks [24,25,38,42]. Only two wells in the Offshore Indus Basin have encountered very thin Cretaceous source rocks. The characteristics of Cretaceous source rocks are mainly compared with those in the Lower Indus Basin, and the Paleocene–Eocene source rocks are mainly compared with those in the adjacent Kutch Basin. The Miocene source rocks have been proven present by drilling of the Pakcan-1 well, with a mudstone thickness of 300 m, an average TOC up to 2%, and an average Ro of 0.8% [43].

There are two types of reservoirs in the Offshore Indus Basin, namely shelf delta-subaqueous fan clastic rock reservoirs and Paleocene–Eocene carbonate rock reservoirs. Among them, the Paleocene–Eocene carbonate rock reservoirs are mainly distributed in the reef limestone above the volcanic rock buried hills. There are two wells in the Offshore Indus Basin that encountered carbonate rock reservoirs, and the lithology of the carbonate rock reservoir in one of the wells is biotic muddy limestone, biotic grain limestone, biotic framework reef limestone, muddy-grain limestone, and biotic framework pores and grain interstitial pores. According to the physical property analysis of this well, the porosity of the Paleocene reservoir is 27%, the lower part of the Eocene reservoir is 24%, and the upper part of the Eocene reservoir is 28%, which belongs to a good reservoir (Figure 14).

Analyzing the conditions for oil and gas accumulation and their matching relationship, the discovery of oil and gas shows and a small amount of gas in the Offshore Indus Basin indicates that there have been hydrocarbon generation and migration in this region. The configuration of reservoir accumulation is good. The deep Cretaceous source rocks entered the hydrocarbon expulsion period from the Eocene, and the shallow Paleocene–Eocene and Miocene source rocks entered the hydrocarbon expulsion period from the middle Miocene.

The sandstone, faults, and unconformities in the subaqueous distributary channel in the study area are well developed, and are good migration pathways for oil and gas. Hydrocarbon migrate along the pathways and accumulate in the Lower Cretaceous sandstone, Paleocene–Eocene carbonate rocks, and Miocene–Pliocene channel sandstone, which are capped by thick layers of Upper Cretaceous shale, Cretaceous–Paleocene volcanic rocks, and Lower Miocene and Pliocene mudstone. The carbonate reef in the area began to form in the Paleocene, and local structures and folds began to form in the Early Miocene. The time of hydrocarbon migration is the same or slightly later than the time of trap formation, and the time of hydrocarbon migration and reservoir formation matches well with the time of structural formation (Figure 15).

Therefore, it is expected to discover Paleogene–Eocene reef-type hydrocarbon reservoirs in the southeastern part of the sea area of Pakistan. Compared with the southeastern part of the sea area of Pakistan, the Deccan volcanic rocks in the northwest of the Offshore Indus Basin are limited in distribution and thin in thickness. The comparison results between land and sea suggests that the thinner Deccan volcanic rocks play a constructive role in the Mesozoic hydrocarbon reservoirs, and the area is a sedimentary center of Mesozoic Cretaceous and Neogene Paleogene–Eocene source rocks [11,39]. Therefore, it is expected to discover Mesozoic self-generated and self-preserved hydrocarbon reservoirs in the northwest of the sea area of Pakistan (Figure 16).

5. Conclusions

1. The Deccan volcanic rocks are mainly distributed in the southeastern part of the Offshore Indus Basin, consisting of nine volcanic platforms with a thickness of between 500 and 750 m. In the northwest of the basin, the distribution of the volcanic rocks is limited, with a thickness of less than 250 m, and the overall distribution is characterized by a thick southeastern and thin northwest region.

2. This study analyzed the relationship between volcanic rocks and oil and gas accumulation in the Lower Indus Basin and the Kuch Basin. The results show that the Deccan volcanic rocks developed in the Late Cretaceous–Early Paleocene period played a constructive role in the Mesozoic oil and gas accumulation in the Indus offshore. This is manifested in three aspects. First, it can provide stable caprock conditions for the underlying Cretaceous reservoirs. Second, it can improve the maturity of the underlying Cretaceous source rocks, which is conducive to the maturity and accumulation of oil and gas. Third, the volcanic uplift formed in the Late Cretaceous–Early Paleocene period can provide a place for the development of Paleocene bioherms, which is conducive to the formation of bioherm reservoirs.

3. It is expected to discover lower-generation and upper-reservoir hydrocarbon reservoirs with Paleogene–Eocene reef as reservoirs in the southeastern part of the basin, while it is expected to discover self-generated and self-reservoir hydrocarbon reservoirs with Mesozoic sandstone as reservoirs in the northwest part of the basin.

4. The advantage of this study is that through seismic interpretation and sea–land comparison of the latest collected 2D seismic data from the Offshore Indus Basin, a widely distributed Mesozoic strata was first discovered under the Deccan volcanic rocks in the study area. Combined with land exploration experience, it is believed that the Mesozoic is a potential exploration target of hydrocarbon. At the same time, by conducting seismic data inversion of volcanic rocks, the distribution range and thickness of volcanic rocks were predicted. Combined with the analysis of the control effect of volcanic rocks on hydrocarbon accumulation in the Lower Indus Basin and adjacent Kutch Basin, it is believed that volcanic rocks are beneficial for the Mesozoic hydrocarbon reservoirs in the Offshore Indus Basin. The disadvantage or limitation of this study is that although it points out the exploration potential of the Mesozoic in the study area, it is constrained by traditional exploration ideas and there is no drilling to reveal the Mesozoic strata in the study area currently. Therefore, the results of this study still need to be further verified by drilling.

Author Contributions

Conceptualization, J.S., J.L. (Jie Liang) and J.G.; methodology, J.S., J.G., and J.L. (Jing Liao); validation, J.G.; formal analysis, Q.Z. and C.Z.; investigation, J.L. (Jie Liang) and J.L. (Jing Liao); resources, J.S.; data curation, C.Z.; writing—original draft preparation, J.S.; writing—review and editing, J.S., Q.Z. and C.Z.; funding acquisition, J.S. and J.L. (Jie Liang). All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All data can be provided upon contacting the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures and Table

Figure 1. Tectonic location of the Offshore Indus Basin (The blue box indicates the scope of study area. The red lines represent the boundary lines of the plates).

Figure 2. The tectonic evolution of the Indian Plate (Reprinted with permission from Ref. [14]. Copyright 2012, Elsevier).

Figure 3. Impedance histogram from Karachi South-1A.

Figure 4. The distribution of Deccan volcanic rocks. (Reprinted with permission from Ref. [21]. Copyright 2005, Geological Society of America).

Figure 5. Restoration map of the tectonics of the Indian plate and its surrounding areas 65 million years ago.

Figure 6. The geological profile shows thin thickness of the Deccan volcanic rocks in the Lower Indus Basin (Reprinted with permission from Ref. [24]. Copyright 2017, Science Press).

Figure 7. Well section of Deccan volcanic rocks encountered in the northeastern part of the Offshore Indus Basin. (Reprinted with permission from Ref. [7]. Copyright 2019, Elsevier.)

Figure 8. Time-domain structural map of the top surface of the volcanic rock platform in the southeastern part of the Indian Fan Offshore Basin (modified according to reference [23]).

Figure 9. Seismic profile across the volcanic platform in the southeastern of study area (see Figure 1 for the location of the profile).

Figure 10. Inversion result of impedance for line 4 (see Figure 1 for the location of the profile).

Figure 11. The thinness of volcanic rocks in the Offshore Indus Basin (see the blue dotted box in Figure 1 for the scope).

Figure 12. The histogram of source–reservoir–caprock in the Lower Indus Basin (modified according to reference [32]).

Figure 13. The distribution of oil and gas fields in the Lower Indus Basin in Pakistan’s land area (Data from IHS Markit).

Figure 14. Analysis of carbonate reservoir in a well in the Offshore Indus Basin.

Figure 15. Map of hydrocarbon accumulation events in the Offshore Indus Basin.

Figure 16. Hydrocarbon accumulation model of the Offshore Indus Basin.

Table 1

Drilling wells encountering volcanic rocks in the Offshore Indus Basin.

Number	Well Name	Drilling Purpose	Completion Interval	Volcanic Rock Thickness (m)	Drilling Results	Reasons
1	Dabbo Creek-1	Cretaceous reservoir	Lower Cretaceous	61	Gas show	Deviation from structure
2	Patiani Creek-1	Cretaceous reservoir	Upper Cretaceous	36 (Not drilled through)	Gas show	Deviation from structure
3	Shaik Nadin-1	Cretaceous reservoir	Lower Paleocene Deccan volcanic rocks	70 (Not drilled through)	Gas show	Poor reservoir
4	Karachi South-1A	Cretaceous top structural trap	Cretaceous	71	dry well	Poor reservoir

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