Application of Paleobathymetric Curve in the Sequence Stratigraphy of Well-logging Data

Key words: 
• paleobathymetry /
• sequence stratigraphy /
• well-logging

Abstract: It is well known that integrated use of outcrop, well-logging and seismic data is the key to sequence stratigraphy. Therefore, how to identify the sequence boundary by well-logging in the lacustrine sediments which are lack in the unconformity is critical step to sequence stratigraphic analysis. With an example of sequencestratigraphy of Quaternary in well Jinda1, Qaidam basin, this paper presents the basic procedures of new approach for a stratigraphic subdivision of well logs into sequences by reconstruction of paleobathymetric curve. After the log facies analysis, six kinds of lithofacies have been recognized in Quaternary which are dark-grey shale, brown shale, grey silty shale, carbargillite, shaly siltstone and siltstone. For each Lithofacies association a sedimentologically coherent paleoenvironmental interpretation has to be established. Hence, a distinct paleobathymetric range is assigned to each lithofacies unit. Based on this relative paleo-waterdepth, paleobathymetric curve of Quaternary in well Jinda1 has been established from a vertical succession of lithofacies. This water-depth curve helps to identify stratigraphic sequence of different scales.This approach is based on the identification of bathymetry turnaround trends in the lithofacies succession. The maximum regressive surface is placed at the lowest waterdepth values on the smoothed bathymetry curve. The maximum regressive surface is identical to the sequence boundary. The maximum flooding surface is placed at the deepest bathymetry as the closest approximation. This horizon separates underlying transgressive half cycle from overlying regressive half-cycle. Based on the rules above, four sequences are identified in the Quaternary sediment of Jinda1 Well. Each sequence consists of two systems tracts separated by maximum flooding surface. It is clear that there are two scales of waterdepth cycle in the bathymetry curve. More frequent fluctuation in the bathymetry curve can be considered as the response to the high order stratigraphic sequence. At last, the prospective source rock, favorable reservoir and potential caprock have been predicted.