Development Regularities of the Coal-measure Source Rock in Ya-3 Member of Yacheng Formation, Well Y1, in Yanan Depression within Qiongdongnan Basin

Key words: 
• Milankovitch cycle /
• coal-measure source rock /
• Pyrolysis /
• Yacheng Formation /
• Qiongdongnan Basin

Abstract: Based on the analysis of trace element, maceral and the results of rock pyrolysis of the coal-measure source rock samples from Well Y1, the development regularities of the coal-measure source rock in Ya-3 member of Yanan depression were systematically studied combining the Milankovitch cycle division results of wavelet analysis. This research shows that:(1)Based on the 104 and 260 wavelet scale of Milankovitch cycling, combined with the sequence of core and geological test results, two medium cycles and five short-cycles were identified in Ya-3 member, which points out that the coal-measure source rock tends to thicken visibly in vertical direction, and the proportion of the source rock increases clearly upward in each cycle and it is mainly developed on the top of the medium cycle, in which ⅠA2, ⅠA3 andⅠB2 are the dominant cycles; (2)As for organic macerals, the source rock is vitrinite-rich, inetinite-poor, exinite-poor and sapropelinite-poor. Of all the organic macerals, the average concentration of vitrinite is 83%, which accounts for 44%~100% of the total content. The vitriniteis mainly composed by desmocollinite and vitrodetrinite, and the kerogen types of the source rock are primarily Type Ⅲ (humus type) and some Type Ⅱ (sapropelic-humus type) organic matter. Besides, the source rock generates gas prevailingly, and it is in the initial stage of hydrocarbon generation with low maturity. The average value of Rois is 1.12; (3) The development of the source rock in the study area is controlled by multiple factors, and the period in which the faulted depression basin transformed into a sag basin is a critical tectonic stage of coal formation. The incomplete preservation of the source rock resulted from the process of strong progradation, and the time in which regional sea level rising was also a critical period of coal formation, and the toward-right inflection point of the short-cycle represented layer which was relatively rich in coal-measure source rock.