Characteristics and Origin of Tuffaceous Tight Oil: Based on a reference of tight oil in Permain Pingdiquan Formation in Huoshaoshan oil field, Junggar Basin

Key words: 
• tuffs /
• black oil-bearing series /
• magmatic-hydrothermal exhalation /
• Permian /
• Junggar Basin

Abstract: The Pingdiquan Formation (also known as Lucaogou Formation) of middle Permian in Huoshaoshan oilfield, Junggar Basin, Xinjiang, is a well-known black oil-bearing series that has been considered as a set of deep, hydrocarbon-rich lacustrine mudstone in a rift basin. Comprehensive mineralogy and petrology analysis indicate that abundant rock fragments are from the explosion of alkaline-peralkaline ultrabasic magmatic rocks (mainly of carbonatite) and intermediate-acid magmatic rocks (mainly rhyolite and andesite). This is characterized by the fragments of bimodal volcanic rocks such as alkaline-peralkaline ultrabasic magmatic rocks (including alkaline feldspar, aluminum-rich quart, magma calcite, aegirine, lithium mica titanium, batisite, serpentine, etc.) and fragments of intermediate-acidic magmatic rocks (including alkaline feldspar, quart). The system of lake-swamp-hot spring is rich in mantle fluids contributing to hydrothermal alternation of above explosive matters and matrix, which was eventually deposited with normal sedimentary rock layers, and formed a set of tuff material-hydrothermal explosive rocks which are rich in mantle-derived fluids. The alternation also leads to hydrocarbon generated in deposited period. Tuff material provided heat and nutrients contributed the algae, microorganisms to promote hydrocarbons generation in lakeshore, marsh, and hydrothermal field. Abundant primary intergranular pores of tuff and analcite dissolution pores are the main reservoir space of hydrocarbons. The thickness of tuff layers varies greatly and they are intebeted in lacustrine sedimentary layers.