Organic matter occurrence and pore-forming mechanisms in lacustrine shales in China

Abstract

The evolution of pore structure in shales is affected by both the thermal evolution of organic matter (OM) and by inorganic diagenesis, resulting in a wide variety of pore structures. This paper examines the OM distribution in lacustrine shales and its influence on pore structure, and describes the process of porosity development. The principal findings are: (i) Three distribution patterns of OM in lacustrine shales are distinguished; laminated continuous distribution, clumped distribution, and stellate scattered distribution. The differences in total organic carbon (TOC) content, free hydrocarbon content (S1), and OM porosity among these distribution patterns are discussed. (ii) Porosity is negatively correlated with TOC and plagioclase content and positively correlated with quartz, dolomite, and clay mineral content. (iii) Pore evolution in lacustrine shales is characterized by a sequence of decreasing-increasing-decreasing porosity, followed by continuously increasing porosity until a relatively stable condition is reached. (iv) A new model for evaluating porosity in lacustrine shales is proposed. Using this model, the organic and inorganic porosity of shales in the Permian Lucaogou Formation are calculated to be 2.5%–5% and 1%–6.3%, respectively, which correlate closely with measured data. These findings may provide a scientific basis and technical support for the sweet spotting in lacustrine shales in China.