Sequence stratigraphic analysis of Devonian organic-rich shales in northern Guangxi

Abstract

Deep-water sediments are widely developed in Devonian sedimentary strata around the world. The Devonian's unique climate change, relative sea-level fluctuations, biological evolutionary stages and tectonic activities resulted in the widespread deposition of organic-rich shales across the globe in this era. The Devonian deep-water facies shales in northern Guangxi are widely distributed along the syn-sedimentary faults, such as the Ziyun–Luodian–Nandan–Du'an ancient fault zone and the Yishan fault, forming a series of special depositional successions under the deep-water system. Based on the stratigraphic cyclicity and spatial distribution of the Devonian strata, in conjunction with the total organic carbon content (TOC) of the dark shales at the Tonggong, Luzhai, Dongxing and Shuiyuan sections, it is determined that the Devonian system in northern Guangxi can be divided into 13 third-order sequences. Additional, the organic-rich shales, which are mainly composed of carbonaceous Tentaculitic shales in SQ5–SQ11 (1) developed in the TSTs of SQ5–SQ9 and mainly distributed in the Nandan–Hechi region in the west of the study area, and (2) developed in the HSTs of SQ10–SQ11 and extended along the fault zone from Nandan to the Luzhai region in the east of the study area. The spatial and temporal distribution of these Devonian organic-rich shales indicate that their formation is controlled by a relative sea-level change. The current study provides important clues for future unconventional oil and gas exploration and development in northern Guangxi.