Pattern of Dual Sequence Structures and Their Forming Mechanism in A Compressional Basin: A Case Study of the Junggar Basin, West China

Abstract

The Junggar Basin is a typical foreland system in China that is characterized by multi-lateral steep thrusts, extrusions, and twisted depressions. It is one of the best natural laboratories in the Central Asian Orogenic Belt to investigate the subduction–accretion process. To date, geologists have been most interested in the basin’s tectonic characteristics and evolution. However, comprehensive and systematic research on sequence stratigraphy for the whole basin has not been conducted. Based on our comprehensive analyses of field outcrops, compound logging and seismic reflection data, we present a systematic study of the sequence stratigraphy pattern for the Junggar Basin. Herein, we identify the Junggar Basin as having a “dual” characteristic sequence framework composed of a lowstand system tract (LST) and a lake expansion system tract (EST), distinguishing it from the traditional ternary system tract. In each sequence unit, the lower segment consists of conglomerate or pebbled sandstone deposits, displaying the characteristics of braided channel. The upper segment consists of siltstone, argillaceous siltstone and mudstone, displaying fine-grained, shore-shallow lacustrine deposits. Furthermore, well logging curves and seismic reflection profiles display a rhythm: a low-energy value with weak reflection in the LST and a high-energy value with strong reflection in the EST.

After careful analysis, we conclude that the combination of multiphasic thrust extrusion and steady deposition after extrusion is likely responsible for the dual-system tracts. The dual-system tract structure forming process can be divided into three stages: the beginning stage, active stage, and weakened stage. The sequence of formation and system tract changes are more sensitive to relaxation and episodic thrust, especially in the slope belt of the central uplift and margin uplift of the Junggar Basin. Additionally, two reservoir predictive models are proposed: the diagonal sandbody forecasting model and truncated and updip-pinchout sandbody forecasting model. We conclude that the coarse clastic unit of the LST has bright prospects for oil and gas exploration within the foreland system in Western China.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018