Seismic-Sedimentologic Study of Lacustrine Gravity-flow Channels: Oligocene ES1 Formation, Huanghua Depression, China

Liu, Huaqing; Hong, Zhong
Research Institute of Petroleum Exploration & Development- Northwest,PetroChina, Lanzhou, China.

In this study, seismic sedimentology was applied to characterize a lacustrine gravity-flow depositional system in Oligocene Es1 Formation in Qi'nan area, Huanghua faulted-depression.Ninety-degree phasing of seismic traces tied log lithologies (sandstone and shale) to seismic signal in a relative impedance sense. Sedimentary texture from core description and wireline-log facies analysis helped convert seismic facies into depositional facies. A stratal slicing created geologic-time equivalent seismic display that revealed seismic geomorphologic patterns of depositional systems, which then was employed to study the plane view pattern and vertical evolution of the depositional systems. The study suggests that there are at least three types of gravity-flow deposits developed in the Oligocene fault-depressed lacustrine basin, including sandy-gravelly debris flow, slump and turbidite. Among them the sandy-gravelly debris flow is the main and most important depositional type. The C-M plots from core analysis demonstrate that the sample points are distributed along the QR part and approximately parallel the base line of C=M, while the cumulative granularity probability curve exhibits circular-arc shape, with the suspension content exceeding 30%-40% or even more. The seismic facies of lacustrine gravity-flow channels can be divided into U-shaped (or V-shaped), saucer-like, worm-like, and spindle-shaped facies according to the shape of the seismic reflections, which probably relate to the change of fluid energy from strong to weak. The stratal slices demonstrate that during the period when tectonic movement was active and the maximum lake flooding was developed, the slope of the lacustrine basin was steep, and the gravity-flow channels developed along the valley were narrow and straight, and were almost parallel to each other. While at the beginning of lake flooding and in the late period of high standard (HST), the width and sinuosity of the channels increased, and channels became braided downstream, resulting in less erosion to underlain strata.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012