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An Improved Method of Laser Particle Size Analysis and Its Application in Identification of Lacustrine Tempestite and Beach Bar: An Example from the Dongying Depression

Yuanfu Zhang*1,2, Chenlin Hu1,2, Xingmou Wang3, Min Wang4, Zaixing Jiang1,2, Junjie Li5
1. School of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, China;2. Institute of Earth Sciences, China University of Geosciences (Beijing), Beijing 100083, China;3.SINOPEC Shengli Geophysical Research Institute, Dongying 257022, China;4. PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, China;5. The First Oil Production plant, SINOPEC Henan Oilfield, Henan 473132, China

Grain size analysis is a common method in the study of sedimentology. For the consolidated sedimentary rocks, the traditional methods are rock slice observation and image analysis. In recent years, laser particle size analyzer is used widely in particle size analysis of sedimentary rock. Unlike the pretreatment of loose samples, the rock samples must be crushed, added acid to wipe out cement, and washed. However, in the step of washing, most of the fines component (less than 63um) in the suspended state should be inevitably lost. It will significantly affect the accuracy of particle size analysis, especially for siltstone. This paper presents a siltstone sample pretreatment method which core step is washing acid by centrifuge. Compared with traditional decantation method, the results show that the median particle size reduced 33.2μm on average. Compared with the precipitation method which is commonly used for handling loose samples, the change of solid-liquid separation time is from 12h to 10min, while the average reduction of median particle size is about 15μm.The grain size value corresponded to the cumulative volume of 10%/90% reduced 2.5μm/20.3μm on average. The percentage of the clay component less than 2μm increased 2.88% on average. The fine particle (2-4μm) and silt component (4-63μm) increased 1.71% and 5.56% on average. Based on this method, two kinds of similar lacustrine siltstone were analyzed. They are tempestite and beach bar which are difficult to identify in the Lijin sub-depression, Dongying depression, Shengli oilfield, China. The final grain-size probability plot of tempestite is the type of “one saltation component and three suspension components”. The content of suspension components can reach to 80%-90%. The beach bar is the type of “one saltation component and two suspension components”. The content of suspension components can reach to 40%-45%. They both have the characteristics of high slope which means well sorting. But they can be distinguished based on the suspension sedimentary characteristics which were preserved by maximum degree in this kind of sample pretreatment method.



Keywords:Siltstone, Grain size analysis, Sample pretreatment, Washing acid, Centrifuge.

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