Effects of clay-organic association on hydrocarbon generation in shale

Widely variable and inconsistent results are reported in many previous studies while investigating the influence of mineral matrix on hydrocarbon generation using laboratory pyrolysis experiments. Some studies show greater hydrocarbon generation in mixtures of clay minerals and organic matter while others report an inhibiting effect of clay minerals. The effect of physical association of organic matter and mineral matrices on hydrocarbon generation has not been tested yet. This thesis investigates the forms of organic carbon in shale and their physical relationship to the mineral matrix as a control on hydrocarbon yield and generation kinetics. Two distinct shale textures are compared with varying degree of surface contact between minerals and organic matter. Samples of the Micoene Monterey Formation in California, USA demonstrate a complex intimate association of amorphous organic matter with clay minerals at sub-micron scale in a nanocomposite shale fabric. Organic matter in the Permian Stuart Range Formation of South Australia is present as discrete organic particles of > 5 μm size that are largely unrelated to the mineral matrix. The relationship between organic matter and mineral surfaces is also evident in the scaling between total organic carbon (TOC) and mineral surface area (MSA). Samples with intercalated clay mineral and organic matter (nanocomposite) typical of the Monterey Formation show a strong first-order relationship between TOC and MSA (R2 = 0.91) identifying a genetic relationship between these structures leading to a molecular scale coating of organic matter on mineral surfaces. Samples from the Stuart Range Formation have a poor relationship between TOC and MSA (R2 = 0.54) indicating independence of organic matter accumulation without mineral matrix influence. Pyrolysis experiments of whole rock samples compared to the extracted kerogen isolates show greater hydrocarbon generation of the kerogen isolates than the whole rocks (containing minerals and organic matter) in both sample suites. Between the two sample sets, however, there is a significant difference of hydrocarbon generation potential (HI) between extracted kerogen and whole rock with the Stuart Range Formation showing a much greater range (56 to 210 mgHC/gTOC) than in the Monterey Formation (85 to 142 mgHC/gTOC). This is interpreted to result from greater hydrocarbon retention by the mineral surfaces unoccupied by organic matter in the Stuart Range Formation.Kinetic experiments are performed on paired samples of whole rock and theextracted kerogen component from both Monterey and Stuart Range formations using open system, non-isothermal pyrolysis at three different heating rates (0.7, 2 and 5 K/min). The majority of the samples of the Stuart Range Formation show nearly identical generation pattern for whole rock and kerogen isolate pairs in kinetic models, which is consistent with the independent distribution of organic matter and minerals. The Monterey Formation, however, show a significant clay catalytic effect by reducing the onset temperature of hydrocarbon generation by 20°C when compared to kerogen isolates. This finding is consistent with the nanocomposite fabric of samples of the Monterey Formation where reactive clay minerals are intimately associated with organic matter. The results show that shale fabric,and in particular, the physical relationship between clay mineral surfaces and organic matter at sub-micron scales can influence timing and yield of hydrocarbon generation. In turn, this association is determined by conditions in the depositional environment leading to a predictable effect between different types of shale.