Lateral changes of organic matter preservation in the lacustrine Qingshankou Formation (Cretaceous Songliao Basin, NE China): Evidence for basin segmentation

Abstract

Organic matter-rich intervals in the Cretaceous succession of the large non-marine Songliao Basin in northeastern China (covered area ~ 87 × 103 km2), including the Turonian–Coniacian Qingshankou Formation (K2qn), offer unique records to study the role of global (e.g., climate, “limno-eustasy”) and regional (tectonics, basin geometry) controls on organic matter accumulation. A wealth of scientific papers concentrate on the vertical variations in organic matter (OM) preservation along the Southeastern Uplift of the basin, which hosts its most prolific source rock intervals. However, the corresponding K2qn interval in the Central Depression is less well studied, and no lateral correlation between both parts of the basin based on direct information from well logs and drill cores exists to date. Hence, the dominant influencing factors causing lateral changes organic-richness and source potential are not yet fully understood. Basin geometry effects in particular could not be evaluated in detail, as highly resolved seismic data is lacking for the transitional zone between the Southeastern Uplift and Central Depression. This paper uses Integrated Prediction Error Filter Analysis of wireline logs together with petrographical and geochemical data from drill cores of two key wells in the Central Depression and Southeastern Uplift areas for a detailed correlation of the K2qn1 interval and its three sub-units (Sq1 to Sq3). Bulk geochemical (e.g., total organic carbon vs. total sulphur), biomarker (ratios of pristane vs. phytane, dibenzothiophene vs. phenanthrene, and total steroid vs. hopanoid compounds; Gammacerane Index; abundance of 4-methylsteranes and C29 steranes), and petrographical (e.g., maceral composition; framboidal pyrite size) data to trace changes in the depositional conditions. All data sources support a higher degree of salinity stratification and more intense anoxia at the Southeastern Uplift compared to the Central Depression particularly during accumulation of the lowermost and most organic-rich Sq1 sub-unit. These observations indicate that two depocenters may have existed during times of Sq1 deposition, and that the pronounced basin separation inhibited marine incursions from the Southeastern Uplift to the Central Depression. During deposition of the overlying Sq2 and Sq3 sub-units, a low relative lake level and dry climatic conditions likely prevailed. The geochemical data suggest similar dysoxic brackish-fresh water conditions in both depocenters. Moreover, the well log correlation indicates that a gradually connected basin may have formed at this time, reducing the effects of two separate depocenters on sediment accumulation and organic matter preservation. Thus, this study provides the first direct evidence that apart from global factors, basin segmentation strongly controlled the distribution of organic matter-rich rocks within the Qingshankou Formation.