Discussion

The mineralogy results of the different producing and non-producing oil shales highlight that the non-producing oil shale samples with poor generation potential (SR1, SR2, SR3, MKK1and argillite) contain illite as the dominant clay mineral. The mineralogical composition of samples of the producing Green River Formation contains less illite, and the Orepuki oil shale shows no illite present. Illite is present in all the Waipawa Formation samples but generally in low abundance (common) (Table 4.1). The conclusion proposed from the mineralogical properties of the reference samples is that the production potential of an oil shale is inversely proportional to the amount of illite present. This is a qualitative relationship that is apparent for the rocks of the current study only, and analysis of additional shales is part of future work that can be conducted to verify the more widespread validity of this conclusion. However, in the context of the current study, the conclusion is valid. This relationship between the shale oil potential of oil shales and the content of illite is consistent with the work of Burnham (2008). This author, working on shale samples from the Green River Formation, found that illitic-rich oil shales produce less oil per unit of organic carbon. However, the magnitude of this effect was not large and the significance of the result is not certain. Weaver (1960), however, working on more than 20,000 samples from different basins in the U.S. found that the amount of montmorillonite is directly proportional to the amount of shale oil.

There is clearly a precedent for a relationship between mineralogy and oil content, but the parameters of this relationship appear to vary between studies. In the current study, where different oil shales from different countries with different origins and environments of deposition were analysed, there is no relationship between the detrital mineral (montmorillonite; which forms during weathering of silicates) and shale oil production potential. No montmorillonite was identified in the international producing or non-producing oil shales analysed except in the Mir Kalam Kala sample (MKK2). Montmorillonite was also identified present (p) or common (c) in the Waipawa Formation.

A trend in illite content, however, is observed. Therefore, the mineralogical index of oil- production potential defined in the current study is a low abundance or absence of illite, rather than a greater abundance or presence of another clay mineral, as has been defined in previous studies. This relationship is interesting because it means a shale with hydrocarbon potential can be screened quickly and inexpensively as yes it has potential or no it doesn’t, based on a simple XRD analysis of the clay mineral fraction without undertaking involved analyses and calculations to determine the quantity of illite present. This relationship warrants further verification, through analysis of a larger number of samples, so definite conclusions can be reached.

The progressive transformation of montmorillonite to illite through an intermediate mixed-layered illite/montmorillonite is the most important diagenetic clay reaction in shales (Milliken, 2005; Pevear, 1999; Pollastro, 1993). According to Meunier and Velde (2004) illite/montmorillonite is an important geothermometer in diagenetic studies and approaches to utilize this geothermometer are:

1) Vertical profiles from wells and outcrops (where illite/montmorillonite is studied through several hundred or thousand feet of sedimentary rock)

2) Paleotemperature or thermal maturity mapping on local or regional scale (where illite/montmorillonite of a particular unit or bed is studied using both outcrop and well samples to produce geothermal history map of the unit in the study area) (Pollastro, 1993).

These approaches have not been utilized in the scope of this study.

There is no montmorillonite present in the argillite and Salt Range samples but illite is dominant (d). In Mir Kalam Kala sample, MKK2, both montmorillonite and illite are common (c). In the Green River Formation samples montmorillonite is absent and illite is common (c). In the all Waipawa Formation samples from Waipawa type locality, Upper Angora Road and Old Hill Road (Porangahau) both illite and montmorillonite are common (c) except the Old Hill Road outcrop sample WFOH33 in which illite is present in minor amounts (p). In the Waipawa Formation samples from Lower Angora Road quarry both illite and montmorillonite are present in small amounts (p). This may be due to the fact that during diagenesis montmorillonite is transformed into authigenic illite with geologic time, depth of burial and temperature (Milliken, 2005). Primary factors controlling the montmorillonite to illite transformation are temperature and the

availability of potassium and aluminium ions (Hower et al., 1976; Pearson & Small, 1988; Pytte & Reynolds, 1989).

According to Hower et al. (1976), the chemical changes involved in the conversion of montmorillonite to illite are represented by the reaction:

݉݋݊ݐ݉݋ݎ݈݈݅݋݊݅ݐ݁+ܣ݈ଷା_+ܭା _{=݈݈݅݅ݐ݁+ܵ݅}ସା

Montmorillonite holds more pore water than the non-expandable clays: kaolinite, illite and chlorite and a greater pressure is required to squeeze that pore water from between the clay particles (Bruce, 1984). Water is necessary to move hydrocarbons from shales to more permeable rocks (Bruce, 1984; Weaver, 1960). For the formation of hydrocarbons an appreciable depth of burial and pressure is required (Tanaet al., 2013) and this pressure which is due to burial may cause the water-bearing clays to loose most of their pore water with only shallow burial before the majority of the hydrocarbons form (Weaver, 1960). This may explain that why the well-known Green River Formation and the historically producing Orepuki oil shales, both of which contain no expanded clay, are oil shales. By the time oil was formed not enough water was left to remove the oil from the shale (Weaver, 1960).