Abstract
Metamorphic rock gas reservoirs whose protolithic is sedimentary rock were found for the first time in Pingxi area, Altun foreland, Qaidam Basin. Comprehensive analysis containing core data, thin section analysis, X-ray diffraction analysis, scanning electron microscopy and confocal laser scanning shows that there are three types of shallow metamorphic rock reservoirs in Pingxi area, such as slate, “metamorphic limestone” and calc-schist which have good responses in logging and seismic data. Fractures and pores develop in epimetamorphic rock reservoir, and there are five types and eight subcategories of reservoir space. Industrial CT shows that there are differences in pores and throats of the three types of metamorphic rock reservoirs, and the fractures have a significant impact on the development of pore throats. The protolithic type is the genetic basis of epimetamorphic rock reservoirs in Pingxi area, and thin section, mineral characteristics and carbon and oxygen isotopes data show that the protolithic of epimetamorphic rock is Palaeozoic Ordovician marine sedimentary rock. Five kinds of actions, such as metamorphism, weathering, structural fragmentation, dissolution and filling, are external mechanisms of reservoir genesis. Metamorphic, weathering and structural fragmentation form fractures, and dissolution is the main cause of matrix porosity.
Copyright 2018, Shanxi Petroleum Society.
This paper was prepared for presentation at the 2018 International Field Exploration and Development Conference in Xi’an, China, 18–20 September 2018.
This paper was selected for presentation by the IFEDC Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the IFEDC Committee and are subject to correction by the author(s). The material does not necessarily reflect any position of the IFEDC Committee, its members. Papers presented at the Conference are subject to publication review by Professional Committee of Petroleum Engineering of Shaanxi Petroleum Society. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of Shaanxi Petroleum Society is prohibited. Permission to reproduce in print are restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgement of IFEDC. Contact email: paper@ifedc.org.
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Acknowledgements
(1) There are three types of shallow metamorphic gas reservoirs in Pingxi area: slate, “metamorphic limestone” and calcareous schist. The slate slabs are well developed, with palimpsest structure, high quartz and feldspar content. The metamorphic limestone has high crystalline calcite content, and calcite and sericite have layered properties. The calcareous schist is the most developed in terms of schistosity and mainly contains minerals such as quartz, calcite and biotite. The three types of reservoirs have significant response characteristics in image logging and seismic data and are easy to identify and contrast. (2) Fractures and pores develop in shallow metamorphic rock reservoir, and there are five types and eight subcategories of reservoir space. Types of fractures are high angle fracture, low angle fracture, fine mesh fracture, erosion fracture and weathering fracture. The pore type is dominated by secondary dissolving pores, which can be divided into three subgroups such as dissolved pores and caves, and a small amount of residual calcite intercrystalline pores. The fractures are closely related to the pore throats. The pores and throats of the three types of metamorphic rock reservoirs are all developed, but there are certain differences in number, volume and radius. The slate pore throats with few fractures are smaller in number, volume and radius. The “metamorphic limestone” with lots of fractures and the calcareous schist pore throats are bigger in number, volume and radius. (3) In Pingxi area, five kinds of actions, such as metamorphism, weathering, structural fragmentation, dissolution and filling, are key factors of shallow metamorphic reservoir genesis. Shallow metamorphism and weathering are important external conditions for reservoir forming. With higher weathering intensity and higher degree of metamorphism, the porosity will be better. The structural fragmentation can form both reservoir space and fluid seepage channel. The dissolution can form dissolution fractures and can also form dissolution holes and micropores. It is the fundamental cause of matrix porosity in Pingxi area. The filling effect is weak overall and has two sides.
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Xia, Z., Li, S., Liu, Z., Wang, B., Wang, Y., Xie, M. (2020). Reservoir Characteristics of Epimetamorphic Rock Gas Reservoir in Pingxi Area, Qaidam Basin. In: Lin, J. (eds) Proceedings of the International Field Exploration and Development Conference 2018. IFEDC 2018. Springer Series in Geomechanics and Geoengineering. Springer, Singapore. https://doi.org/10.1007/978-981-13-7127-1_37
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