Microfacies and Reservoir Connectivity of Shore Sandbar, Southern Indus Basin, Pakistan

Abstract

Shore sand bar reservoirs have attracted much attention as one of the target intervals with the greatest potential for petroleum exploration and development in marine sedimentary basins. Nevertheless, due to lack of effective research methods, it is difficult to analyze the rapid lateral change and heterogeneity in a sand bar reservoir, which has a major impact on the efficient petroleum development that seriously restricts the rolling evaluation and efficient development of sand-bar reservoirs. In this study, based on integrated analysis of cores, thin sections, logging, 3D seismic data, production test and dynamic data, through a combination analysis of drilling and seismic interpretation data—the petromineral composition, microfacies and reservoir connectivity of the shore sand bar in the southern Indus basin are investigated, which is used for the fine description of the sandbar reservoir. The results show that the shore sand bar is located in a relatively high-energy shore sedimentary environment, which is conducive to forming a favorable lithologic reservoir. Four sedimentary microfacies types are identified, including center bar, bar edge, inter bar and local mudstone interbeds. The sandbar microfacies are changed rapidly, and different microfacies types overlap each other, especially the inter bar and local mudstone interbeds that overlap and intersect in the center bar and bar edge, which significantly reduces the internal reservoir connectivity as well as intensifying the heterogeneity of the sandbar reservoir. The sandbar reservoir is not connected transversely, the physical properties are changing rapidly, and the sandbar reservoir is cut into several relatively independent oil reservoirs.

1. Introduction

Shore sand bars are widely distributed in the open coastal sedimentary, which suffer multiple actions of the ocean all year round. The sandbar complexes are roughly parallel or oblique to the coastline with the characteristics of narrow strips, long extension distance and large distribution scale in the plane [1,2,3,4,5]. It is generally believed that the shore sandbar is a favorable exploration target for lithologic reservoirs in the marine sedimentary basin because of its characteristics of “good transverse connectivity, high porosity and high permeability” and its close proximity to shelf mudstone [6,7,8,9]. However, with the development of oil and gas evaluation, it is found that there is a strong heterogeneity in a sandbar reservoir, which is manifested as the quick change of reservoir physical property, poor transverse connectivity and even the contradiction of oil–water relationships, which restricts the efficient evaluation and development process of oil and gas fields. In fact, the sedimentary characteristics and sand body distribution of shore sandbars are mainly affected by a series of geological factors such as coastal zone landform, water energy, sea level changes, provenance conditions and so on; the argillaceous components are widely developed in sandbar sedimentary, which leads to the complex reservoir connectivity [10,11,12].

In recent years, a series of breakthroughs have been made in detailed research of sandbar reservoirs. According to the sedimentary hydrodynamic condition and topographic variation of the coastal zone, the internal architecture and the spatial distribution of sandbar sedimentary are discussed in the aspect of geological genesis, which solves the problems of plane distribution and the development scale of effective reservoirs to a certain extent [13,14,15,16,17]. Besides, based on the numerical simulation method, the heterogeneity characteristics of a sandbar reservoir and its control on the distribution of the remaining oil are analyzed, and it is predicted that there are many stable argillaceous compartments in the interior of sandbars [15,16,17]. It is not difficult to find that a certain understanding has been obtained in sandbar reservoir architecture, internal-sand body distribution and geological genesis [18,19,20,21,22]. However, the research work on shore sandbar research is relatively insufficient, such as how to reveal a reservoir’s physical properties changes. Moreover, it is still controversial on the internal differences of shore sandbars. Therefore, it is necessary to apply high-resolution seismic data and oil field development dynamic data to sandbar sedimentary research, which can provide a useful reference for fine sandbar reservoir prediction.

2. Geological Setting

The Indus basin is located in the south of Pakistan and the total area is about 36 × 10⁴ km². It is a Mesozoic–Cenozoic sedimentary basin formed on the Paleozoic granite basement, and the sedimentary thickness ranges from 2 to 7 km. The tectonic evolution of the basin has mainly experienced five stages, including a rift stage in the Jurassic, passive continental margin stage in the early Cretaceous, diagonal extension stage in the late Cretaceous, thermal subsidence stage in the Paleogene and foreland stage since the Neogene [23,24,25,26]. From west to east, the tectonic units of the basin are a fold orogenic belt in the west part, foredeep belt in the middle part and uplift basin in the east part; furthermore, the Sulaiman and Kirthar fold belts are located in the western part. The Sulaiman and Kirthar foredeep belts are located in the middle part. The Sulaiman slope and Punjab platform are located in the northeast part. Tal slope and Sindh platform are located in the southeast part. The northeast part and southeast part are separated by the Mary–Kirthar High (Figure 1). From bottom to top, the sedimentary strata are delta sand-mudstone and carbonate platform in Jurassic; marine delta, littoral and neritic clastic rock in Cretaceous, with a small amount of volcanic rock interbeds at the top of Cretaceous [27,28]; and continental fluvial delta and lacustrine deposits since Paleogene (Figure 2).

The degree of petroleum exploration in the basin is generally low and petroleum discoveries are concentrated in the Tal slope in the southern part of the basin with a small number in the Mary–Kirthar High and offshore area [27,28,29,30]. At present, several petroleum fields have been discovered in the Tal slope structural belt; the oil reservoir types are mainly structural reservoirs with a small number of lithologic, stratigraphic or compound reservoirs [24,25,26]. The main target strata are in the lower Cretaceous, including four sets of sandstone groups, including A sand, B sand, C sand and D sand, which are the major reservoirs for petroleum exploration. The major source rocks are limestone in the Jurassic and shelf and prodelta shale in the Cretaceous. The major cap rocks are thick mud in the late Cretaceous [31,32] (Figure 2).

The Tal slope in the Indus basin is an important oil- and gas-producing base in South Asia at present. Because the various faults are well developed in the study area since the 1990s, the faulted reservoirs are the main exploration target, and a great deal of research on structural traps and oil accumulation in this area has been carried out by different scholars [23,24,25,26]. However, there are only a few studies on reservoir sand bodies. It is found that thick marine shore deposits are developed in the lower Cretaceous in the Tal slope area, which is the most favorable reservoir sand body [26,27,28]. However, in the process of evaluation and development of sandbar reservoirs, due to unclear understanding of sandbar reservoir distribution regulation, a series of problems restrict the reservoir prediction accuracy and development deployment, including the poor transverse connectivity of sandbar reservoirs, the rapid changes of sandbar microfacies and the contradiction of the oil–water relationship.

In recent years, newly 3D high-resolution seismic data and well data (i.e., wireline logs, core data, detailed test data and production data) in the Tal slope area are available to present a detailed case study of microfacies analysis and reservoir connectivity of shore sandbars. The primary objectives of this study are: (1) to analyze the petromineral composition and sedimentary facies marker of shore sandbars; (2) to identify the sedimentary microfacies types of shore sandbars and (3) to evaluate reservoir connectivity and build a sedimentary model of shore sandbars.

3. Materials and Methods

This study is based on core data, thin-section observations, well logging data, high-resolution 3D seismic data and production test data. In the study area, there are only seven wells drilled into the sandbar reservoir, and about 40 m long cores are recovered. Fifty-three samples are systematically selected to cover all the sandbar reservoir petrography and cut into thin sections for microscopic petrographic studies. The lithology, grain size, petromineral composition and sedimentary structures are interpreted through detailed core observations and thin-section descriptions, yielding precise identification and division of microfacies facies.

Wireline log data used in this study include gamma ray (GR), spontaneous potential logs (SP), deep and shallow lateral resistivity (LLD, LLS), sonic travel time (AC) and density (Den), and environmental and depth corrections are applied simultaneously for the logging data. The logging motifs include the extent–difference logging curve, shape, smoothness, top-bottom contact relation, thickness change of single sand body and curve combination mode.

The Tal slope area is entirely covered by 3D seismic data (about 150 km²). This area is characterized by an effective frequency band between 5 and 70 Hz and a peak frequency of 35 Hz. The vertical seismic resolution is 10–20 m and the two-way travel time is between 1.5 and 2.5 s in the interval containing the total A sand group in the Tal slope area. Seismic facies analysis parameters generally include internal reflection structure, geometric external shape, continuity and frequency. Thus, the different types of microfacies facies are identified from interpretation of the core calibrated logging motifs and the well calibrated seismic facies.

Besides, the production test and dynamic data are also collected for reservoir connectivity research such as the porosity and permeability data, the capillary pressure data, the crude oil daily output data, the production–pressure curve and so on. Through a combination analysis of drilling and seismic inversion data, the reservoir connectivity and sand body distribution regulation are further accurately revealed.

4. Results

4.1. Petromineral Composition

The lithology of the shore sandbar is mainly fine, siltstone and argillaceous siltstone with a small amount of medium sandstone, and the particle size is from 0.08 mm to 1.44 mm. The content of quartz particles is high (about 90~96%), while the content of feldspar and debris is low, at about 1~3% and 3~7%, respectively. The analysis of the cast thin section shows that the sand particles are moderately well sorted, the psephicity is subangular to subround, the contact mode between grains is mainly point and linear, the primary pores are developed and the content of clay impurity is low. These depositional phenomena reflects that the sediments are located in the littoral to neritic environment with strong water energy, and the sediments are screened and transformed by marine action resulting in relatively pure sand and high maturity [27,28] (Figure 3).

In addition, a certain number of marine biological fragments, mainly mollusks and bivalves, can be seen under the microscope. Bioclasts are dissolved after leaching, and mold holes are formed (Figure 3d,g). Authigenic minerals are various, such as dolomite, calcite, kaolinite and quartz authigenic margin, iron silicate minerals and some phosphate minerals such as (oolitic) glauconite have also been found (Figure 3c,h). A small amount of pyrite particles can be seen in some wells presenting a scattered distribution feature (Figure 3b,c). (Oolitic) glauconite is usually formed in a littoral to neritic environment with alkalinity, reductive situation and slow deposition and is a typical facies mineral of littoral to neritic sedimentary environments [1,2,3].

4.2. Core Sedimentary Structure

In the medium-fine sandstone, massive bedding, parallel bedding and low-angle cross-bedding are well developed, and some thin interbeddings containing biological shell are also developed (Figure 4a–c), which reflects the high-energy sedimentary environment. Storm cross-bedding is composed of a series of wide and slow wavy layers, and the top surface is hummocky, revealing the existence of storm flow below the normal wave base surface, and the lithologic composition at the bottom of the core is coarse with mud-gravel or coarse-grained sandstone interbedded with biological shell fragments. The grain size becomes fine upward and gradually changes to lenticular sandstone or mudstone with biological shell locally (Figure 4d).

The fine-siltstone sand is mainly developed with wavy laminae, wavy cross-bedding and thin argillaceous intercalation locally. In thin interbedded sections, typical sedimentary structures such as flattened bedding and horizontal bedding occur, which reflect the effect of frequent turbulence of water flow in the littoral to neritic environment [26,27] (Figure 4e–h). The fine-grained sediments such as silty mudstone and mudstone are often in the hydrodynamic environment of (weak) reduction deposition which is very conducive to the growth and activity of marine organisms. Therefore, molluscs and shells are abundant in the fine-grained sediments. In addition, the original sedimentary structures are often transformed or destroyed by biological activities, such as vertical biological caves, or the sediments are characterized by mass and partial lamellar deformation (Figure 4i–k).

4.3. Logging Motifs Characteristic

The logging motifs characteristic of sandbar is diverse. In the medium-fine sandstone, the logging motifs are medium-high extent, smooth or microtoothed box shape and the abrupt contact relationship mostly appears both at the top and bottom part which reflects the characteristics of strong hydrodynamic conditions and pure sandstone. In the fine-siltstone section, the logging motifs are mainly of medium-high extent, toothed funnel or small bell shape and the sedimentary cycles are mostly of the reverse cycle of coarse at the bottom and fine at the top. In the thin sand-mud interbedded section, finger shape or dentate funnel shape is common, reflecting the weak hydrodynamic conditions. In the fine-grained mudstone deposition, the logging motifs are low-extent and flat, reflecting the reduced hydrodynamic environment (Figure 5).

4.4. Seismic Facies Characteristic

A progradation sequence was developed in A sand from south to north [23,24]. Taking the seismic section along the provenance direction as an example, the oblique progradation seismic reflection is developed with a small development scale, and the time window of seismic wave group is less than 0.5 s, equivalent to one or two seismic events. The dip angle of the progradation is low and the different progradation reflections are roughly parallel and partially overlapped. This characteristic reflects the sedimentary environment with shallow water and a gentle slope, and the sandbar is vertically superimposed or prograded from SE to NW in the seaward direction (Figure 6).

5. Discussion

5.1. Microfacies Analysis

The paleo-geomorphology in the study area was located in the abroad and gentle slope area in the early Cretaceous. During the evolution of the basin, the southeast part continued to rise and uplift with a well-developed water system and sufficient provenance supply; while the northwest part continued to subside steadily, the coastline was broadened and a large area of shallow water sedimentary environment was formed [26,27]. The study area was located in the coastal facies belt with strong hydrodynamic action such as wave, coastal current and tide, forming a series of long strip sandbar complex deposits, which were distributed parallel to the coastal line. Drilling revealed the main litholgy was interbedded with (light) gray sandstones and dark mudstone. According to the sedimentary facies marks and drilling analysis results, four types of microfacies in the shore sandbar were identified including the center bar, the bar edge, the interbar and the local mudstone interbeds.

5.1.1. Center Bar

The center bar is in the core part of the sandbar. Drilling data reveal that the thickness of a single sandbody ranges from 3 to 6 m and the cumulative thickness is up to 15 m. The vertical rhythm is mostly positive rhythm forming the dominant reservoir (Figure 7a). The lithologic composition shows that the content of quartz particles is as high as 94%, and the lithology is mainly fine-medium sandstone and occasionally coarse-grained sandstone. The sand particles are moderately well sorted, the psephicity is subangular to subround with relatively high maturity (Figure 3a–d). The primary pores are developed in the center bar, while the secondary pores and later mold holes further improve the reservoir physical properties (Figure 3g). Parallel bedding (partially containing biological shell intercalation), low-angle cross-bedding and massive bedding are common. Besides, the shell fragments, plant stem fossils or argillaceous belts are occasionally found in core observation, which reflect the characteristics of high-speed flow and strong hydrodynamic conditions (Figure 4a–d).

The core porosity ranges from 12 to 30% with an average of 19.2%, and the permeability ranges from 50 to 800 mD, up to 1680 mD. Through the analysis of the capillary pressure curve, it is found that the primary pores are well developed in the center bar (W1 well was drilled into the center bar reservoir), the pore throat structure is good, the deflection is coarse and the discharge pressure is low. The comparison of oil well production dynamic data also reveals that the well drilled into the center bar had sustained stable production and high cumulative production confirming that reservoir performance is good in the center bar (Figure 8).

The log curves are mostly bell or box in medium and high extent, and the curve’s shape is micro-toothed or smooth. GR value is extremely low (compared with the mudstone baseline), generally less than 20 API, which reflects the characteristics of pure and clean sand (Figure 5 and Figure 7b). The seismic reflection of the center bar is the characteristics of low frequency and strong amplitude, which is easy to identify and track in seismic profile (Figure 7c).

5.1.2. Bar Edge

The bar edge is developed at the edge or flank of the center bar. The lithology is sand and mud interbeds. The thickness of single sand is less than 2 m, and the vertical rhythm is inverse rhythm (Figure 9a). Compared with the center bar, the sandstone particle is finer and mainly consisted of fine sandstone or (argillaceous) siltstone; the sandstone maturity is medium, containing a small amount of biological fragments or organic matter. The pore types include primary pores and secondary pores due to the increased argillaceous content; the pores are often partially filled with calcalous cements, argillaceous materials or a small amount of pyrite (Figure 3e,f). The sedimentary structures are well developed, such as small wavy bedding, flattened bedding and lenticular bedding indicating the turbulent sedimentary environment (Figure 4e–i). The core porosity ranges from 5% to 15%, with an average of 8.3%, and the permeability ranges from 10 to 100 mD. The pore throat structure is medium, the deflection is relatively thin and the discharge pressure is medium. The distance of the two wells is about 1500 m, but the reservoir is disconnected (Figure 1c). The reservoir properties and productivity of well W2 (drilled into the bar edge reservoir) are significantly lower than that of the adjacent well W1 (drilled into the center bar reservoir) (Figure 8).

The log curves are mostly micro-toothed funnels or finger-shaped with moderate GR values ranging from 18 to 45 API (Figure 5 and Figure 9b). The seismic reflection is characterized by moderate amplitude, medium high frequency and sub parallel combination (Figure 9c).

5.1.3. Interbar

The interbar is between the sandbars of different periods. The lithology is thick mud with thin sand interbeds. The thickness of single sand is about 1 m, and generally less than 2 m (Figure 10a). The main types of bedding are wavy bedding, lenticular bedding and horizontal bedding. Besides, strong biodisturbance phenomenon is observed, such as biological caves and a large number of argillaceous belts (Figure 4i–k). The pore type is micropore, the pore throat structure is poor, and the deflection is fine. Porosity is generally less than 6%, and permeability is less than 10 mD. The W3 well (drilled into the interbar) reveals the interbar reservoir has the characteristics of high-mud content, tight lithology and low-petroleum production. At present, the W3 well has been shut in (Figure 8).

The log curves are funnels or finger-shaped with medium and low extent, and GR value is high at greater than 45 API (Figure 5 and Figure 10b). The seismic reflection amplitude is weak, and the seismic event is disconnected (Figure 10c).

5.1.4. Local Mudstone Interbeds

Due to the frequent changes of sea level, the mudstone in the neritic facies is often associated with the shore sandbar, and the local mudstone interbeds overlap and intersect with the sandbar, forming the frequent sand and mud interbeds (Figure 10a). The lithology is mainly black and dark gray mudstone with thin layers of argillaceous siltstone locally. The sedimentary structure of the core is dominated by massive bedding and is well developed with a few small parallel beddings and lenticular laminations (Figure 4l). The logging curves are high extent, flat, linear and the GR value is greater than 70 API. This kind of mudstone interbed is not theeffective reservoir because of its low porosity and permeability.

5.2. Reservoir Connectivity Analysis

5.2.1. Profile Connectivity Analysis

Under the constraints of drilling data, with the advantage of high transverse resolution of 3D seismic data, the details of sandbar microfacies change can be better identified. The transverse variation of sandbar reservoir can be predicted on the seismic and inversion sections (Figure 11a,b). The top seismic horizon of the sandbar reservoir is T2 and the bottom is T2a; the W1 and W4 wells are drilled into the center bar reservoir, and the seismic facies is the reflection characteristic of “low frequency and strong amplitude”, and the value of the P-wave impedance is medium and high (red and yellow warm color). The W2 and W5 wells are drilled into the bar edge reservoir; the seismic facies are the reflection characteristic of moderate amplitude, medium-high frequency and subparallel combination, and the value of the P-wave impedance is medium (green color). However, the W3 well is encountered in the interbar reservoir and the seismic reflection characteristics change suddenly. The seismic event is interrupted, amplitude energy becomes weak and the value of the P-wave impedance is low (cold blue color). It is easy to see from the seismic and inversion profile that although these wells are drilled into the sandbar facies belt, the different sandbar microfacies are superimposed inside the sand bar resulting in the different seismic response.

From the corresponding inter-well sedimentary facies profile (Figure 11c), the sandbar microfacies were changed quickly resulting in rapid changes in physical properties and poor transverse connectivity of the sandbar reservoir. In addition, neritic facies mudstone is often associated with shore sandbar, which intensifies the heterogeneity of the sandbar reservoir. Due to the existence of low permeability or non-permeability mud interbeds, the complete structure-lithologic reservoirs are separated into different blocks. The actual development data also reveal that the integrated sandbar is divided into multiple reservoir systems and the oil–water interface is not uniform.

5.2.2. Plane Connectivity Analysis

The energy attenuation attribute (EAA) of the study area can better reveal the changes of sand bar reservoirs in the plane distribution. The inside area of the black dotted line in Figure 12a shows a flat sheet distribution in the plane, which is generally parallel to the direction of the coastline and is interpreted as a favorable reservoir facies zone of the sand bar. It can also be divided into three zones inside. The red and yellow warm color areas are dotted and banded. The lithology encountered by drilling is sand-rich deposition and the thickness of single sand is large (more than 3 m), this is the center bar microfacies deposition. The green area is distributed around the red and yellow warm colors and it is strip or sheet distribution. The lithology is sand and mud interbeds, the thickness of single sand is small (less than 2 m), this is the bar edge microfacies deposition. In addition, the blue color area is the interbar microfacies; the lithology is mainly mudstone interbedded with thin sand and the sedimentary environment is in the relatively low-energy area (Figure 12b).

5.3. Sedimentary Characteristics

The sandbar is located in the coastal sedimentary environment and the transgression action is strong. Due to the influence of the coastline migration, sediment provenance, wave energy transfer and other factors [33,34,35], the shore sandbars are oscillated and migrated back and forth in space, resulting in complex morphology, development scale and superposition characteristics of sand bars. The lithology is not uniform in the interior of shore sandbars [36,37] and different microfacies types of sandbodies are overlapped and cut into each other in space, leading to a rapid lateral lithology change and strong reservoir heterogeneity in the interior of sand bars.

The center bar is in the core part of the sandbar. It has a dotted and banded distribution in the interior of the sandbar, and they are not connected to the other which constitutes the dominant reservoir. The bar edge is developed at the edge or flank of the center bar. It is sheet distributed in the plane. The lithology is thin sand and mud interbeds. Interbar is developed in the relatively low energy area of the shore sandbar, the lithology is mainly argillaceous siltstone, and the scope area is small. Due to the rapid change of microfacies in the sandbar, the partial fine grain deposits in the interbar are distributed in the sandbar, which reduces the internal reservoir connectivity.

6. Conclusions

Based on the integrated investigation on the microfacies types and reservoir connectivity of the shore sandbar in the southern Indus basin, the conclusions are as follows.

(1)

The shore sandbar is located in the high-energy coastal sedimentary environment, which is conducive to form a favorable lithologic reservoir. The sedimentary microfacies types are center bar, bar edge, inter bar and local mudstone interbeds. The changes of reservoir physical properties are controlled by sedimentary microfacies. The reservoir properties of the center bar microfacies are the best and the bar edge is better than the interbar, which is poor.

(2)

It is found that the sandbar reservoir changes rapidly, and different microfacies types overlap each other, which reduces the internal reservoir connectivity and intensifies the heterogeneity of the sandbar reservoir, which significantly increases the difficulty of reservoir development. Therefore, combined with well and seismic data research, the characteristics of sedimentary microfacies and the connectivity of sand bodies are studied for detailed reservoir research. Based on the research of the characteristics and models of sedimentary microfacies, the sandbar reservoir distribution characteristics and transverse change are well predicted, which is beneficial to the evaluation and development of oil reservoirs.

(3)

As a matter of fact, with the development of oil and gas fields, sandbar reservoir distribution characteristics and transverse change are the key factors for the exploitation of the remaining reservoir potential. The effective and accurate prediction depends on the accuracy of data, such as the drilling density of the study area, the quality of the seismic data, etc. In addition, the drilling and seismic data are often inconsistent, which reduces the accuracy of prediction results. In the future, sand bar reservoir prediction research should always start from drilling data and reduce the multi-solution of seismic data, and the prediction results should conform to the geological regulation.