**1. Introduction**

This paper attempts to summarize the current knowledge on hydraulic fracturing and production issues related to shale gas in South Africa. The observation and findings made in this work is neither totally comprehensive nor exhaustive since no data is available in the public domain on hydraulic fracturing and it associated activities in South Africa. The exploration for natural gas resources in terrestrial South Africa has been conducted since the mid-1960's, however no exploitable source could be located. Limited gas was however found in the tight shale formations of the Ecca Group at an approximate depth of 2000-4000 metres below surface. The potential current shale gas reserve in the Karoo shales is estimated to be 485 trillion cubic feet, which would make it the fifth largest shale gas field in the world [1]. In geological terms the Karoo Supergroup refers to an extensive geological sequence (100-260 million years old) which consists of sedimentary and igneous rocks. Most of the Karoo Supergroup is located in South Africa and the Great Karoo has an area of more than 600 000 km2 .

**1.** Whitehill Formation (Cape region)

[2].

properties.

**2.** Prince Albert Formation (Cape region)

**3.** Volksrust Formation (Free State and KZN regions)

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**4.** Vryheid Formation (Free State and KZN regions)

**6.** Dwyka Shales (All regions, where shallow enough).

**Figure 1.** Regional map of South Africa, showing the exploration rights and companies associated with these permits

The research into the Shale Gas deposits of the USA led to a revised set of geochemical and petrophysical parameters that are based on the criteria set by Jarvie [2] to include the following:

**1.** Total Organic Carbon and its composition (dead carbon, free gas, etc.) → 1% or more.

**2.** Kerogen Type → Determines hydrocarbon types as well as adsorption/desorption

**5.** Pietermaritzburg Formation (Natal region)

Due to present energy shortfall in South Africa, the requirement for new energy sources have gained new momentum and part of this new focus is on shale gas in Karoo type formations. The most interesting aspect of this is that the area available for natural gas development is substantially larger than just the Karoo, with exploration areas covering six of the nine provinces in South Africa [2]. The development of shale gas resources was initiated in late 2009 but were halted due to a moratorium in early 2011. This has subsequently been lifted in September 2012. There are currently five pending applications related to exploration in the Karoo (Figure 1), three belong to Shell and one each to Falcon Oil and Gas and Bundu Gas and Oil Exploration [3]. To the north is located the petrochemical group Sasol gas exploration area, however plans have been put on hold by the company until further notice. An exploration area has also been awarded to Anglo American adjacent to the Sasol area (Figure 1).

Hydraulic fracturing was developed in the United States of America in the late 1940's to assist in the stimulation of oil and natural gas wells [4-6]. The number of wells that incorporates hydraulic fracturing increases by the day since oil and gas production is increased by this technique [5].

#### **1.1. Development of shale gas in South Africa**

The Shale Gas project aims to target the carbonaceous shales of the Ecca and Dwyka Groups, but the stratigraphic units in question vary in lithological makeup along strike as one proceeds from the Cape to the Free State/Natal (KZN) regions. The initial reasoning was to only target zones of the Whitehill formation of the lower Ecca, which is a carbonaceous shale unit characterized by its distinctive white weathering in outcrop. The distribution of the Whitehill Formation with its marine setting led to further investigation into the dynamics of the Main Karoo Basin and other stratigraphic units equivalent to the Whitehill to extend the potential target areas [2]. A revised set of source rocks were identified with the main target zone now being identified as carbonaceous shales of the Lower and Upper Ecca Group with subordinate interest in the Dwyka Shales (Figure 1). The source formations have been extended to include the following:

**1.** Whitehill Formation (Cape region)

**1. Introduction**

214 Effective and Sustainable Hydraulic Fracturing

technique [5].

the following:

**1.1. Development of shale gas in South Africa**

This paper attempts to summarize the current knowledge on hydraulic fracturing and production issues related to shale gas in South Africa. The observation and findings made in this work is neither totally comprehensive nor exhaustive since no data is available in the public domain on hydraulic fracturing and it associated activities in South Africa. The exploration for natural gas resources in terrestrial South Africa has been conducted since the mid-1960's, however no exploitable source could be located. Limited gas was however found in the tight shale formations of the Ecca Group at an approximate depth of 2000-4000 metres below surface. The potential current shale gas reserve in the Karoo shales is estimated to be 485 trillion cubic feet, which would make it the fifth largest shale gas field in the world [1]. In geological terms the Karoo Supergroup refers to an extensive geological sequence (100-260 million years old) which consists of sedimentary and igneous rocks. Most of the Karoo Supergroup is located in

Due to present energy shortfall in South Africa, the requirement for new energy sources have gained new momentum and part of this new focus is on shale gas in Karoo type formations. The most interesting aspect of this is that the area available for natural gas development is substantially larger than just the Karoo, with exploration areas covering six of the nine provinces in South Africa [2]. The development of shale gas resources was initiated in late 2009 but were halted due to a moratorium in early 2011. This has subsequently been lifted in September 2012. There are currently five pending applications related to exploration in the Karoo (Figure 1), three belong to Shell and one each to Falcon Oil and Gas and Bundu Gas and Oil Exploration [3]. To the north is located the petrochemical group Sasol gas exploration area, however plans have been put on hold by the company until further notice. An exploration area

Hydraulic fracturing was developed in the United States of America in the late 1940's to assist in the stimulation of oil and natural gas wells [4-6]. The number of wells that incorporates hydraulic fracturing increases by the day since oil and gas production is increased by this

The Shale Gas project aims to target the carbonaceous shales of the Ecca and Dwyka Groups, but the stratigraphic units in question vary in lithological makeup along strike as one proceeds from the Cape to the Free State/Natal (KZN) regions. The initial reasoning was to only target zones of the Whitehill formation of the lower Ecca, which is a carbonaceous shale unit characterized by its distinctive white weathering in outcrop. The distribution of the Whitehill Formation with its marine setting led to further investigation into the dynamics of the Main Karoo Basin and other stratigraphic units equivalent to the Whitehill to extend the potential target areas [2]. A revised set of source rocks were identified with the main target zone now being identified as carbonaceous shales of the Lower and Upper Ecca Group with subordinate interest in the Dwyka Shales (Figure 1). The source formations have been extended to include

.

South Africa and the Great Karoo has an area of more than 600 000 km2

has also been awarded to Anglo American adjacent to the Sasol area (Figure 1).


**Figure 1.** Regional map of South Africa, showing the exploration rights and companies associated with these permits [2].

The research into the Shale Gas deposits of the USA led to a revised set of geochemical and petrophysical parameters that are based on the criteria set by Jarvie [2] to include the following:


From a South African perspective, the Rowsell and De Swardt's study [7] of the maturation indices pertaining to the Karoo Basin can be used to identify areas prospective for gas generation:


#### **1.2. Geology and gas plays in South Africa**

In South Africa, shales containing significant organic carbon are restricted to the Ecca Group of the main Karoo Basin, smaller basins in the northern part of South Africa and to the Bokkeveld Group in the southernmost part of South Africa [7]. These muds became buried and lithified over tens to hundreds of millions of years and generated various hydrocarbons with increasing depth of burial and increasing temperature (Figure 2). Between 2-4 km burial depth, oil is produced, between 4-5 km, wet gas is produced and between 5-6 km, dry gas, including methane, is produced. Deeper burial results in low-grade metamorphism, the termination of hydrocarbon generation and the formation of graphite from the organic material. In South Africa, shales of the Bokkeveld Group have undergone low-grade meta‐ morphism and no longer have a capacity for hydrocarbon generation. However, after com‐ prehensive investigations it was confirmed that Ecca Group shales might have the potential to generate dry gas south of the 29<sup>o</sup> S [7]. Further north, the shales have been less deeply buried and have a potential for oil generation except where younger igneous dolerite intrusions have locally increased the thermal maturity leading to the generation of dry gas [7].

Total organic carbon within the shale is an important parameter, since there is a linear relationship between total organic carbon and gas content, as in the Barnett Shale in the Fort Worth Basin of Texas [9]. Thickness is also important, as most of the gas produced is from areas where the shale is between 90 and 183 metres thick [9]. However, more recently, it has become technically possible to produce gas from shale units as thin as 10 to 15 metres [10]. Within the main Karoo Basin (Figure 3), there are reports of natural gas occurrences both at surface and at intervals in the deep wells drilled by Soekor between 1965 and 1977. Furthermore, varying quantities of gas were obtained by desorbed gas analysis undertaken by Soekor on Ecca Group shale samples retrieved from the deep well cores [7].

**Figure 3.** Distribution in time and space of the Dwyka Group, Ecca Group and Adelaide Subgroup in the main Karoo Basin, South Africa, showing lithofacies, environment and stratigraphic relationships. Modified from Fig. 7 of Veevers

**Figure 2.** Hydrocarbon generation and thermal maturation indices and maturation stages plotted against depth of

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et al. [11]. The geologic timescale is from Gradstein et al. [12].

burial [8].

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**3.** Vitrinite reflectance and Tmax (maximum temperatures that rocks were subjected to during hydrocarbon production) → Thermal Maturity with reflectance values of 1.35-2.5, Tmax values vary and can be very high (between 400°C and 580°C as seen in the Barnett

From a South African perspective, the Rowsell and De Swardt's study [7] of the maturation indices pertaining to the Karoo Basin can be used to identify areas prospective for gas

**4.** Total Organic Carbon and its composition (dead carbon, free gas, etc.) → 1% or more.

In South Africa, shales containing significant organic carbon are restricted to the Ecca Group of the main Karoo Basin, smaller basins in the northern part of South Africa and to the Bokkeveld Group in the southernmost part of South Africa [7]. These muds became buried and lithified over tens to hundreds of millions of years and generated various hydrocarbons with increasing depth of burial and increasing temperature (Figure 2). Between 2-4 km burial depth, oil is produced, between 4-5 km, wet gas is produced and between 5-6 km, dry gas, including methane, is produced. Deeper burial results in low-grade metamorphism, the termination of hydrocarbon generation and the formation of graphite from the organic material. In South Africa, shales of the Bokkeveld Group have undergone low-grade meta‐ morphism and no longer have a capacity for hydrocarbon generation. However, after com‐ prehensive investigations it was confirmed that Ecca Group shales might have the potential

and have a potential for oil generation except where younger igneous dolerite intrusions have

Total organic carbon within the shale is an important parameter, since there is a linear relationship between total organic carbon and gas content, as in the Barnett Shale in the Fort Worth Basin of Texas [9]. Thickness is also important, as most of the gas produced is from areas where the shale is between 90 and 183 metres thick [9]. However, more recently, it has become technically possible to produce gas from shale units as thin as 10 to 15 metres [10]. Within the main Karoo Basin (Figure 3), there are reports of natural gas occurrences both at surface and at intervals in the deep wells drilled by Soekor between 1965 and 1977. Furthermore, varying quantities of gas were obtained by desorbed gas analysis undertaken by Soekor on Ecca Group

locally increased the thermal maturity leading to the generation of dry gas [7].

S [7]. Further north, the shales have been less deeply buried

Shale).

generation:

**4.** Rock Eval Hydrogen Index <100.

216 Effective and Sustainable Hydraulic Fracturing

**2.** Vitrinite Reflectance of 1.35-2.5. **3.** CR/CT Ratio of about 0.85 to 0.94.

to generate dry gas south of the 29<sup>o</sup>

shale samples retrieved from the deep well cores [7].

**1.** Temperature Range → ±130°C to 170°/180°C.

**1.2. Geology and gas plays in South Africa**

**5.** Porosities and other physical properties related to gas flow.

**6.** Calculations of hydrocarbon generation, expulsion and retention.

**Figure 2.** Hydrocarbon generation and thermal maturation indices and maturation stages plotted against depth of burial [8].

**Figure 3.** Distribution in time and space of the Dwyka Group, Ecca Group and Adelaide Subgroup in the main Karoo Basin, South Africa, showing lithofacies, environment and stratigraphic relationships. Modified from Fig. 7 of Veevers et al. [11]. The geologic timescale is from Gradstein et al. [12].

It was found that only the lower Ecca Group shales [7] within the dry gas window south of latitude 29°S have comparable total organic carbon contents to those of producing shales elsewhere in the USA (Table 1). The upper Ecca Group shales, namely the Tierberg Formation [13], average only 1.2 percent organic carbon [14], which is significantly lower than the 3 to 12 percent range applicable to producing shales (Table 1). The Dwyka Group also contains black shales with between 0.1 and 4.3 percent total organic carbon, averaging 1.9 percent [15; 14]. However, these shales are thin and restricted, being interbedded with diamictite and sand‐ stone, with the thickest shales (50 to 60 m) occurring in only 3 out of 45 deep wells investigated. The lower Ecca Group comprises black, organic-rich shale of the Whitehill Formation [16] overlying dark grey shale of the Prince Albert Formation (Figure 3) [17]. The Whitehill Formation pinches out northeastwards along a line stretching from Hertzogville in the Free State to Coffee Bay in Eastern Cape Province [14]. Northeast of this line, the Whitehill Forma‐ tion correlates with the middle part of the sandstone-dominated Vryheid Formation and the Prince Albert Formation grades into shale of the Pietermaritzburg Formation (Figure 3). In the area between Coffee Bay and Harding, the Whitehill and Vryheid Formations are separated by a continuous shale succession [18].

detected in these shale formations in the USA are similar to those determined for the Whitehill, Prince Albert and Pietermaritzburg formations. Additionally, the thickness of the formations are also comparable to the Marcellus and Barnett shales. However, the Tierberg Formation and Volksrust Formation can also be possible future targets for shale gas exploration since these formations are considerably thicker than the USA counterparts but at a lower organic carbon

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**Unit or Formation Percentage organic carbon (%) Thickness (metres)**

**Table 1.** Comparative results of estimated percentage organic carbon and thickness of formation [2].

last for 30-100 years depending on gas prices and availability within the shales.

In the following sections a comparative analysis of known attributes of the Karoo shales and Marcellus Shale will be further developed as well as the impact on the hydrogeology and

The major concern to date in the Karoo is the contamination of readily accessible water supply, i.e. surface water or groundwater resources. The development of an unconventional gas field does not occur in a matter of months, with a typical initiation phase of 10 years before gas production can continuously take place [20; 3]. In the instance of South Africa, a number of issues restrict the development of an effective gas extraction project. The infrastructure for gas transport (pipelines) in South Africa is very limited since no conventional terrestrial gas fields exist within the borders of the country. The Soekor wells drilled between 1965 and 1977 have yielded only tentative clues to the availability of gas in the Karoo basin. In this regard the major gas companies have to do a comprehensive exploration and verification program that could last from 3-6 years depending on the geological complexity of the development area. This would be followed by a pilot study to evaluate the basic characteristics of the reservoir which can be done on a number of sites simultaniously over a period of 2-4 years. Finally, if the gas in place is adequate then the process can be developed into full production of gas which can

Marcellus Shale 0.3-20.0 12-270 Barnett Shale 0.5-13.0 15-300 Karoo Basin-Whitehill Formation 0.5-14.7 0.4-72 Karoo Basin-Prince Albert Formation 0.3-12.4 30-500 Karoo Basin-Pietermaritzburg Formation 0.3-11.6 0.8-420 Karoo Basin-Tierberg Formation 0.3-5.2 400-1300 Karoo Basin-Volksrust Formation 0.3-5.9 250-415 Karoo Basin-Dwyka Group 0.1-4.1 0-58

content.

hydrochemical components.

**2. Problem formulation**

The gas production probability was delineated by Rowsell and De Swardt [7] using the results of desorbed gas analysis on core samples from the deep Soekor wells. The gaseous hydrocar‐ bons (methane to pentane) are absorbed on to the fine-grained constituents of shales and can be desorbed by low-temperature acid hydrolysis [7]. Samples yielding high proportions of C1 gas (methane) and C2/C1 (Ethane/Methane) relative to C3/C1 (Propane/Methane) indicate a potential for dry gas. The trend of increasing maturity due to increasing depth of burial southwards across the basin is supported by the results from other parameters, namely vitrinite reflectance, CR/CT ratios, illite crystallinity and spore colour index. For dry gas generation, vitrinite reflectance values should be between 2 and 5 percent. In the main Karoo Basin south of latitude 29°S, values for shale of the Ecca and Dwyka Groups vary between 1.8 and 4.4 percent [7]. Branch et al. [19] measured vitrinite reflectance values between 3.5 and 5.3 percent for shale of the Whitehill Formation and between 4.0 and 6.4 percent for shale of the Prince Albert Formation in well SA1/66 in the southwestern part of the basin some 60 km north of the basin margin. These correspond to the dry gas and metamorphic maturation stages, which indicates that shales in the southern extremity of the present basin are over-mature and can no longer generate dry gas. CR/CT ratios (residual, non-volatile carbon after pyrolysis to total carbon in the kerogen or organic material) gives an indication of the ability of the shale to produce additional amounts of hydrocarbons if heated to sufficiently high temperatures with lower ratios corresponding to higher potential. The results more or less correspond to the findings of the desorbed gas analysis [7]. Illite crystallinity or Kübler index is a measure of the width in millimetres of the 10 Å diffraction peak at half its height. It gives an indication of the maturity level of the shale with decreasing indices corresponding to increasing maturity [7]. The results of Soekor's investigations indicate a trend of increasing Kübler index from south to north across the Karoo Basin in shales of the Ecca and Dwyka Groups. In the southern part of the basin, south of approximately 30°S, the average indices are less than 4 and correspond to the metagenesis stage and possible preservation of dry gas. Comparative data to the Marcellus Shale and Barnett Shale is presented in Table 1. The percentage organic carbon detected in these shale formations in the USA are similar to those determined for the Whitehill, Prince Albert and Pietermaritzburg formations. Additionally, the thickness of the formations are also comparable to the Marcellus and Barnett shales. However, the Tierberg Formation and Volksrust Formation can also be possible future targets for shale gas exploration since these formations are considerably thicker than the USA counterparts but at a lower organic carbon content.


**Table 1.** Comparative results of estimated percentage organic carbon and thickness of formation [2].

In the following sections a comparative analysis of known attributes of the Karoo shales and Marcellus Shale will be further developed as well as the impact on the hydrogeology and hydrochemical components.
