Laser Confocal Scanning Microscope Analysis on Micro-Pore Structures and Occurrence State of Organic Matter in Tight Rocks

Abstract

The micro-pore structure and occurrence state of organic matter in tight rocks are two important factors to decipher the origin of tight oil reservoirs. With the laser confocal scanning microscopy (LSCM) technique, we use an Argon laser excitation of 488 nm wavelength and a two-photon laser excitation of 768 nm wavelength to respectively examine and establish the standard spectrograms of the minerals, crude oil and its light and heavy hydrocarbon components. Quantitative examination and three-dimensional images of the micro-pore structures and distribution of organic matter in the tight rocks (including siltstone and mudstone) are conducted in the Songliao Basin, NE China. In the siltstone, the content of light hydrocarbon components in crude oil is more than that of heavy hydrocarbon components. The light components mainly occur in intergranular pores, cracks and fractures with a pore radius size of 1.3–37.3 μm. The volume fraction of light components is 14.8% and the hydrocarbon-bearing conditions are advantageous. In the mudstone, however, the distribution of organic matters with different components of source rock is inhomogeneous. The organic matters occurring in the calcite vein in the mudstone are light hydrocarbon components of crude oil, whereas the algae enrichment layer in the mudstone mainly contains heavy hydrocarbon components of crude oil. The generated hydrocarbon from the algae enrichment layer is expelled into the calcite vein. The mudstone is therefore the major contributor to oil source and reservoir for the hydrocarbon generation in the Songliao Basin. Our results provide microscopic evidence for migration and accumulation patterns of hydrocarbon in tight rocks and will have significant applications for further petroleum exploration and exploitation.KeywordsLCSMTight rocksMicro-pore structureOrganic mattersOccurrence stateSongliao basin