Classification of hydrocarbon-bearing fine-grained sedimentary rocks

Abstract

Fine-grained sedimentary rocks are defined as rocks which mainly compose of fine grains (<62.5 μm). The detailed studies on these rocks have revealed the need of a more unified, comprehensive and inclusive classification. The study focuses on fine-grained rocks has turned from the differences of inorganic mineral components to the significance of organic matter and microorganisms. The proposed classification is based on mineral composition, and it is noted that organic matters have been taken as a very important parameter in this classification scheme. Thus, four parameters, the TOC content, silica (quartz plus feldspars), clay minerals and carbonate minerals, are considered to divide the fine-grained sedimentary rocks into eight categories, and the further classification within every category is refined depending on subordinate mineral composition. The nomenclature consists of a root name preceded by a primary adjective. The root names reflect mineral constituent of the rock, including low organic (TOC<2%), middle organic (2%<TOC<4%), high organic (TOC>4%) claystone, siliceous mudstone, limestone, and mixed mudstone. Primary adjectives convey structure and organic content information, including massive or limanited. The lithofacies are closely related to the reservoir storage space, porosity, permeability, hydrocarbon potential and shale oil/gas sweet spot, and are the key factor for the shale oil and gas exploration. The classification helps to systematically and practicably describe variability within fine-grained sedimentary rocks, what’s more, it helps to guide the hydrocarbon exploration.

Keywords

fine-grained sedimentary rocks / classification / mineral composition / TOC content / shale oil and gas

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