A saline lacustrine depositional environment enhances organic matter enrichment in the Permian Lucaogou shales

Abstract

The Permian Lucaogou Formation shales are world-class source rocks with diverse biological, environmental, and economic properties. Considerable attention has been given to depositional environmental controls on organic matter enrichment. Here, we present a systematic organic and inorganic geochemical analysis of fresh core samples taken from the Lucaogou Formation in the Malu 1 well, located in the Santanghu Basin. The organic matter source of Lucaogou shale is dominated by prokaryotic organisms. γ- and β-carotanes are present in high abundance; lower carbon number carotenoids are also detected. Based on the distribution of n-alkanes, terpanes, and steranes, it is believed that γ-carotane and β-carotane mainly originate from bacteria. Low-carbon carotenoid-derived alkanes could result from the thermal decomposition products of γ-carotane and β-carotane. The minor input of terrigenous clastic material brought elemental nutrients that likely enhanced biotic productivity. Based on organic and inorganic data, the depositional environment can be defined by two stages (I and II). The contribution of green algae increases from Stage I to Stage II. The Stage I organic matter enrichment is controlled by the state of preservation, and the Stage II organic matter is interpreted to be controlled by biotic productivity. The depositional environment and the productivity of the lake have changed synergistically, together controlling the enrichment of organic matter in the Lucaogou shales.