Cyclostratigraphy of the Tertiary Lacustrine Sediments in the Nanpu Oil Field of the Bohai Basin, China

Changlin Wu, Dag Nummedal

The Nanpu depression is a sub-basin of the Bohai rift basin in northeast China. A spectrum of lacustrine sedimentary cycles was revealed by time series analysis of both high-resolution and conventional well logs from the Nanpu oil field. The spectral analysis was performed by calculation of the Fast Fourier Transforms. Energy spectra from 14 wells were compared and analyzed on the basis of their hierarchy, consistency, and energy. The energy spectra of the Eocene Shahejie Formation show thickness peaks at 28.6, 12.0, 7.1, 5.7 and 3.7 meters. Using radiometric dates on interbedded volcanics to calibrate the sedimentation rates, these peaks were estimated to correspond to periodicities in time of roughly 84, 39, 21, 17, and 11 ky. These cycles are superimposed in a hierarch of five levels. The stacking patterns of the cycles display bundling ratios of approximately 1:2, 1:5, and 1:10 that represent respectively precession (20-ky) to obliquity (40-ky), precession to eccentricity (100-ky), and obliquity to eccentricity (400-ky). We believe, therefore, that sedimentary sequences a few to a hundred meters thick all record Milankovitch periodicities. These sequences were produced by the rise and fall of lake level driven by periodic climate changes. The obliquity cycles are most prominent and consistent in all spectra and were probably dominant in the early Tertiary in the Bohai basin. This study suggests that Milankovitch cycles exert a major control on lacustrine sedimentation in rift settings.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995