Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Early Miocene Cyclostratigraphy and Sea-Level Changes from the Pearl River Mouth Basin, South China Sea
(1) School of Geo-Resource and Information, China University of Petroleum, Dongying, China.
(2) School of Earth and Environment, The University of Western Australia, Perth, WA, Australia.
The Pearl River Mouth Basin is situated at offshore shelf of northern part of South China Sea. Recent hydrocarbon exploration indicates that the basin is extremely rich in hydrocarbon resources. However, stratigraphic correlation of the subsurface successions has long been disputed due to absence of their counterparts at outcrop exposures. To improve the stratigraphic resolutions for hydrocarbon prospecting and exploration in the basin, this study attempts to undertake detailed cyclostratigraphical analysis of the lower Miocene succession, including 1) to determine the dominant frequencies of high-frequency cycles based on gamma-ray well-logging data, 2) to test whether Milankovitch orbital signals are registered in the rhythmic succession, 3) to estimate the duration and the net accumulation rate of sequences; and 4) to identify and characterize the depositional responses to global and local sea-level changes in a mixed carbonate and siliciclastic setting. A spectral analysis will be carried out in these wells using the Fourier transform algorithm approach.
Our study indicates that three spatial periods exist in the Lower Miocene successions. They vary vertically in the boreholes but cycle ratios are stable and similar to orbital cycle ratios (i.e., Milankovitch cycles). These three periods correspond to eccentricity, obliquity and precession, respectively. Since the duration of these orbital cycles are known, depth intervals in the studied wells were converted into time intervals (duration) to establish a high-resolution astrochronologic time scales which agree with established biostratigraphic chronology and the International Stratigraphic Chart. The net accumulation rate was generated after calculating the decompaction thickness with regional experienced algorithm. The Miocene sea-level change curves were reconstructed based on integration of biostratigraphic study of relative abundance of planktonic foraminifer and ratio of planktonic/benthic foraminifera, sedimentary facies, astrochronologic, and sequence stratigraphic analyses. At third-order sequence scale, the relative sea-level change curves during the early Miocene in the studied area appear similar trends to the global eustatic curves (Haq et al., 1987); both have five 3rd-order rise-fall cycles. However, at higher-order cycle scale, the direction and amplitudes of rise/fall change curves are different from their global equivalents. This disparity may be due to local or regional tectonisms.