Paleoenvironment evolution and organic matter enrichment mechanisms in the first member of the Qingshankou Formation, Songliao Basin, China

Abstract

Organic matter is the basis for oil and gas generation, and the depositional environment controls its enrichment. The first member of the Qingshankou Formation (K₂qn¹) in Songliao Basin has a thick organic-rich shale and so is an important target section for shale oil exploration and development. In the Gulong Sag, shale samples from this unit were collected over the full length of the section. The characterization of the environments of deposition (EOD) of K₂qn¹ was improved by utilizing lithological characteristics, thin section observations, elemental compositions, and organic carbon concentrations. Combined with the normalization coefficients proposed in this paper, an organic matter correlation model was established to elucidate the factors that influence organic matter enrichment. From the bottom to the top of K₂qn¹, the lake depth gradually becomes shallower, the primary productivity first decreases and then increases, the reducing conditions become stronger and then weaker, the water salinity gradually decreases, the climate first becomes semi-humid and then warm and humid, and the input of terrigenous debris first decreases and then increases. A major marine transgression at the base of the K₂qn¹’s brought in nutrients to increase primary productivity, and the density-stratified reducing environment preserved and enriched organic matter. High primary productivity occurred during the middle of the deposition of the K₂qn¹, while terrigenous input is low. Organic matter is preserved in reduced deep lake environments, resulting in organic matter-rich black shale. The lake became shallower, and the salinity decreased in the upper part of K₂qn¹. Benthic organisms rapidly multiplied, consuming large amounts of oxygen and destroying the previously depositional environment, resulting in a reducing environment disturbed by benthic organisms with poor preservation conditions and the lowest organic matter content.