Application of Milankovitch Cycle Analysis to Achieve High Definition Erosional Map From Yubei Area of Tarim Basin, China

Abstract

In the past thirty years, Milankovitch orbital cyclicity has become a principal theory to quantify the geological time scale of ancient strata. Yubei area, located in the southwest depression of Tarim Basin, is being explored for its large petroleum potential. This area has experienced strong structural deformation, uplift and denudation of multiple periods in the past tectonic evolution history. Erosional events of Middle Ordovician, Late Ordovician and Late Silurian contribute to the unconformity top of Yingshan formation in Middle Ordovician, which is one of the most important unconformity in Yubei. The main purpose of this study is to estimate the denudation thickness top of Yingshan Formation via the combination of structural and Milankovitch cycle analysis. We calculate erosion thickness on 30 wells then extrapolate the data to the whole study area referring to the structural framework. According to the Milankovitch orbital cyclicity theory, the ratio of eccentricity period (413 ka, 125 ka and 95 ka), obliquity period (36.578 ka and 30.134 ka) and precession period (19.13 ka and 16.263 ka) in the Middle Ordovician is 1.000: 0.385: 0.317: 0.201: 0.171. The sedimentary sequence with this ratio in Yingshan Formation, that coincides with the Middle Ordovician Milankovitch Cycle, can be decided by selecting thorium-potassium ratio curves from natural gamma-ray spectral logging and analyzing them by Fast Fourier Transform. Then the deposition rate during Middle Ordovician can be given by the equation: Vd=Hc/Tc, where Hc = thickness of the selected sedimentary sequence; Tc = matched orbital period; Vd = deposition rate. Using the deposition rate data and the total duration of Yingshan Formation derived from a well with a most complete sedimentary sequence, the initial deposition thickness of Yingshan Formation at other 29 well locations can be calculated. Eventually the denudation thickness is the difference between initial and current thickness. This method avoids some inaccuracy during the erosion thickness reconstruction, especially when the eroded strata are deeply buried. A valid erosional map of the study area has been drawn using denudation thickness data of wells. It shows differential tectonic deformation strength distribution in Yubei area during Early to Middle Caledonian Movement.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015