地质科技通报 (May 2022)
Characteristics and geological significance of biomarkers from the Paleogene source rocks in Bozhong Sag, Bohai Bay Basin
Abstract
The Bozhong Sag is located in the central and eastern part of the Bohai Bay Basin, and source rocks of the first and third Member of the Paleogene Shahejie Formation (E2s1 and E2s3) and the third Member of the Donying Formation (E3d3) are mainly developed, indicative of good oil and gas exploration prospects. Previous studies on organic geochemical characteristics of the Bozhong Sag are mainly based on cutting samples, and study on biomarker characteristics using core samples is insufficient. By utilizing both core and cuttings samples, this study integrates organic carbon, rock pyrolysis, chloroform bitumen "A", saturated hydrocarbon chromatography-mass spectrometry and other organic geochemical analysis.Organic matter abundance, type, maturity, source materials and depositional environment of the E3d3, E2s1 and E2s3 source rocks in the Bozhong Sag were studied, and the geochemical characteristics and depositional factors of the three source rocks were studied.The combination characteristics of typical biomarker compounds to distinguish the three source rocks are established, in order to provide a theoretical basis for further clarifying the development pattern and spatial distribution of high-quality source rocks in this area, which is beneficial to the exploration of oil and gas in this area.The results show that the hydrocarbon generation potential of the three source rocks is different, where the generation potential is as E2s3>E2s1>E3d3. The E2s3 source rock has the highest organic matter abundance, typeⅠand Ⅱ1 kerogen, and is currently in the mature stage. The E2s1 is medium to good source rock, with organic matter type Ⅱ1 and Ⅱ2, and is also inmature stage. The E3d3 is a medium source rock, with organic matter typeⅡ1 and Ⅱ2, and is in the stage of low maturity to medium maturity. The biomarker parameters, including Pr/Ph value, gammacerane index, C27-C28-C29 regular sterane content, 4-methylsterane index and other parameters indicate that the source material input and depositional environment are the main factors controlling the different development of three lacustrine source rocks. The input of freshwater algae sources and good preservation conditions are the keys to the development of high-quality source rocks in the E2s3. The input of low-level aquatic organisms and strong reducing preservation conditions are the keys to the development of high-quality source rocks in the E2s1. Mixed source input and good preservation conditions are the key to the development of high-quality source rocks in the E3d3.
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