关键词: 富有机质页岩, 地球化学特征, 生物标志物, 藻类化石, 中深湖相, 北部湾盆地

Abstract:

Mid-deep lacustrine organic-rich shales in Member 2 of the Liushagang Formation (LS2) are the most important high-quality source rock in the Beibuwan Basin. These dark-grey or-brown shales with developed lamellation contain abundant non-marine dinoflagellate, rugasphaera and leiosphaeridia fossils, they are rich in organic matter, with TOC content ranging between 3.07%～10.35%, type II1 and I kerogen and Pr/Ph ratios varying from 0.6 to 2.5. The shales are characterized by a high abundance of 4-methyl steranes that are probably related to certain dinoflagellates thriving in lakes, relatively abundant pentacyclic triterpanes, tricyclic diterpanes and tetracyclic terpanes but a very tiny amount of oleanane indicative of terrigenous higher-plant input, and a “V”-shaped distribution of C₂₇, C₂₈ and C₂₉ regular steranes. All these characteristics indicated that the organic matter in those shales was mainly of algal origin. The data of various algal fossils, especially plenty of non-marine dinoflagellates and amorphous organic macerals suggested that they were formed in an anoxic and stratified environment of mid-deep brackish lakes in humid climate, where organic-rich shales of the upper LS2 occurred at the top of a high-stand system tract and those of the lower LS2 were distributed at the bottom of a transgressive system tract. In this area the sedimentation rate for Member 2 of the Liushagang Formation (LS2) was relatively low and the lake remained as a starved basin, meanwhile the bloom and death of algae near surface supplied a great deal of organic matter to lacustrine sediments, and the oxygen-depleted bottom water of palaeo-lakes, due to the water-column stratification, favored the accumulation and preservation of sedimentary organic matter, resulting in the formation of organic-rich shales, a high-quality source rock.

Key words: organic-rich shale, geochemical characteristics, biomarker, algal fossil, mid-deep lake facies, Beibuwan Basin