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Characterization of Organic Matter Hosted Pores in Cambrian Shale, South China: Implication of a Role of Lithofacies Difference to Pore Development

Abstract

The TOC-rich high mature marine Cambrian Shale in southern China has significant shale gas resource potential. However, a large variation in shale gas content is a challenging issue for resource development. Controls to pore development, in particular organic matter (OM) hosted pores, remains unclear. In order to understand a role of lithofacies variation to OM hosted pores development in the Lower Cambrian shale, the core samples were collected from the Lower Cambrian Niutitang Formation in EYY1 vertical well, which is located at the eastern margin of the Yangtze platform and has daily average shale gas production of 3000 MCF. Thin sections observation, X-ray powder diffraction analysis, scanning electron microscope (SEM) with Argon ion milled samples as well as energy-dispersive spectrometry (EDS) analysis were employed for lithofacies identification and pore classification. The results show that calcareous mudstone and siliceous mudstone are two major lithofacies in the Lower Cambrian Niutitang Formation. The calcareous mudstone contains 50-60% carbonates, 15-20% quartz and 10-15% clay minerals, and the siliceous mudstone has 50-60% quartz and 25-30% clay minerals. TOC content of the siliceous mudstone is 2-3 times higher than that of the calcareous mudstone. The size and shape of OM particles and the degree of OM hosted pores development are obviously different in two major lithofacies and highly depend on mineralogy and rock fabrics. Based on the size, shape and association with minerals, residual kerogen and pyrobitumen which is solid residue from oil cracking, can be partially differentiated. Residual kerogen is more carbon-rich (C/O ratio = 7:2) than pyrobitumen which tends to be oxygen-rich (C/O ratio = 5:3). The calcareous mudstone mainly contains massive carbon-rich residual kerogen with its surface porosity of less than 5%, and pyrobitumen with less pores developed. In contrast, the siliceous mudstone has amorphous residual kerogen with its surface porosity ranging from 5% to 15%, and also contains sponge pore-rich pyrobitumen with the surface porosity of 20-30%. The porosity of OM hosted pores in siliceous mudstone is greater than that of calcareous one, likely resulting from the different type of algae or bacteria associated with silica or calcium-rich environments in the early Cambrian. Our observation also shows that OM hosted pores are abundantly developed in the vicinity of clay minerals. The clay minerals may retain more generated oil by means of the adsorption of bitumen on their surfaces, and enhance oil cracking to gas via catalytic effect and lead to OM hosted pore development in pyrobitumen. Therefore, the clay-rich siliceous mudstone may be additional factor to be considered in the sweet-spot prediction of the Cambrian shale gas.