Shallow-Water, Nonclinoformal Deltaic Systems in Lacustrine Qijia Depression, Songliao Basin, China
Zeng, Hongliu; Zhu, Xiaomin; Zhu, Rukai; Zhang, Qingshi
In seismic-stratigraphic literature, clinoform refers to a stratal package with oblique internal layering, which can be best imaged on seismic reflection profiles as a seismic facies typically composed of three basic geometric elements: topset, foreset, and bottomset. Oblique and shingled clinoforms in particular are used as indicators of sand-rich deltas. However, seismic detection of clinoformal seismic facies is limited by seismic resolution. For thin or low-angled deltaic systems, inclined topsets are not apparent, and interpretation can be difficult. As a result, seismic studies on deltaic sequences that are not associated with seismic clinoforms have been few.
A well and seismic data set in the Qijia Depression, Songliao Basin, China, provides an excellent opportunity for studying nonclinoformal deltaic systems. Depositional sequences were developed in a lacustrine environment in the Cretaceous Qingshankou Formation. The deltaic sediments consist of gray and dark-gray mudstone-interbedded sandstone and siltstone. Although large-scale clinoforms are identified in a major deltaic system in adjacent Daqing Oilfield to the east, reflection events in the 3-D survey are dominantly parallel, lacking clinoforms. Each pair of seismic events roughly represents a high-frequency sequence of 40-50 m thick that is composed of a relative LST at the bottom and a relative HST at the top, both deltaic deposits.
We investigated the high-frequency deltaic systems using core, wireline logs, and seismic stratal slices. In conventional core, typical distributary channel fill, mouth bar, and delta front sand sheet facies were observed. Multiple thin sand bodies of various facies comprise upward-coarsening wireline-log patterns that represent separate deltaic depositional cycles. Without showing obvious clinoformal depositional style, stratal slices revealed numerous distributary channels and deltaic lobes. The thinnest channel sandstone detected by seismic signal is about 1 m. Modern analog from Poyang Lake delta in China and Mossy delta in Canada indicate the sediments were probably deposited in shallow-water environment. Average depositional slope was probably less than 0.5°, with a maximum water depth not exceeding 10 m.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013