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High Resolution Stratigraphic Framework Establishment of Lacustrine Shahejie Shale in Jiyang Depression, Eastern China

Abstract

The Eocene-Oligocene Shahejie Shale is one of the most important lacustrine shale oil plays in eastern China. Compared with the marine shales, lacustrine shales display greater heterogeneities of composition, structure and texture. Thus it is of great significance for the favorable enriched-strata determination of shale oil and regional comparison to define high resolution stratigraphic framework, however, there is very little published works. In this study, based on traditional strata sequence identification methods (well logging data), we applied high resolution elemental geochemistry data, so more precise information of the strata evolution and paleoenvironment changes were acquired to build the stratigraphic framework. In this study, high resolution elemental geochemical data were initially acquired by using a hand-held x-ray fluorescene (XRF) spectrometry. Totally 1010.26 m systematic core-sampling in lower Es3 and upper Es4 sections of the Shahejie Formation in the Dongying and Zhanhua depressions were analyzed in 4-5 cm interval (a few were in 10 cm interval). Data analysis is conducted through a combination of logging data from Shengli Oilfield, 74 inductively coupled plasma mass spectrometry (ICP-MS) data, 740 Rock-Eval/TOC samples data and aromatic hydrocarbon fractions biomarker characteristics of 372 samples. As the results, ratios between different major elements (e.g. Ca/Si, Si/Al) can exhibit lithologic variations in strata. Sensitive elements (e.g. Al/Ti, Sr%, Zr/Al, Fe/Mn, V/Cr, (Al+Fe)/(Ca/Mg), Ca/(Ca+Fe)) are of great reference for paleoenvironment evolution, such as paleo-climate, paleo-water-depth, paleo-oxidation. And it demonstrate that by means of moving average on different scales, effectively elemental geochemical data can identify the 4th order, 5th and 6th order sequence boundaries, providing an efficient method in detailed mapping of organic rich shales across different petroleum saource kitchens in Jiyang Depression. Favorable lithofacies (laminated argilliaceous lamestone) are mainly developed in late-transgression and early high standard system tracts with the occurence of gammacerane. It is particularly useful in the identification of mapping the exploration target in study area. Incorporation of organic geochemical data not only helps in making references about basin anoxia, but also provides constrains on source rock quality, thermal maturation, hydrocarbon fluid property and possible driving forces.