Sedimentary Characteristics And Tectonic Control Of Dark Mudstone Of Shahejie Formation In Cangdong Sag

This paper is based on the dark mudstone of Shahejie formation in Cangdong Sag of Bohai bay basin.This study reclassified the Shahejie formation in Cangdong Sag as a three-level sequence,so this study of dark mudstone will be carried out in three-level sequence,and this paper is focuses on the study of sedimentary,geochemical characteristics and hydrocarbon generation characteristics of dark mudstone,and study he control effect of structure on the development of dark mudstone,which will provide a strong support for the delineation of favorable play.In this paper,the organic geochemical characteristics of the dark mudstone are described in detail in the light of the abundance,type and maturity of organic matter,through combine core observation and sampling,sequence,sedimentary,construction,seismic exploration and other aspects of research to evaluate the dark mudstone of Shahejie formation in Cangdong Sag and we found that the dark mudstone has a high organic matter abundance,achieved a good standard.In this paper,we study four three-level sequence of Shahejie formation and find the thickness of dark mudstone of middle3^rd and 1^st Member of Shahejie Formation is large,organic matter abundance is high,organic matter type is well??₁-??and in the stage of immature-low mature stage.Based on the study of dark mudstone,this paper establishes the classification standard and predict the spreading characteristics of high quality dark mudstone of Shahejie formation in Cangdong sag,and summarized three controlling factors of high quality dark mudstone in this area:?1?Palaeoclimate and sedimentary environmental factors;?2?Tectonic activities factor;?3?Fault activities factor.Secondly,the genetic method is used to estimating oil and gas resources in Cangdong sag.The oil and gas accumulation in 3^rd Member of Shahejie Formation is 0.662?0.993×10⁸t,and the oil and gas accumulation 1st Member of Shahejie Formation is 0.605?0.907×10⁸t.Thirdly,we studied the hydrocarbon generation mechanism of dark mudstone in Cangdong Sag.The low-mature dark mudstone in this area has the following characteristics:?1?Abundance of organic matter is high and type of organic matter is well;?2?In a reductive sedimentary environment for a long stage;?3?Long-term in the stage of low maturity;?4?Has a high conversion rate.So,because of these four characteristics,the dark mudstone of Shahejie formation in Cangdong Sag has a formation of immature-low mature oil capacity.There are wo types of hydrocarbon generation modes of the low-mature dark mudstone in this region:?1?The hydrocarbon generation mode of the?saline?semi-saline lacustrine?strong?reductive phase represented by the source rocks in 1st Member of Shahejie Formation;?2?The hydrocarbon generation mode of the light-micro saline lacustrine?weak?reductive phase of the represented by the source rocks in 3rd Member of Shahejie Formation.Finally,the development of dark mudstone of Shahejie formation in Cangdong Sag is analyzed from the structural perspective:?1?The Cangdong fault and Xuxi fault control the development of source rock.And in the late of Shahejie period,the Xuxi fault significantly strengthened the control of the development of dark mudstone;?2?There are 6 tectonic units in Cangdong Sag,but the negative tectonic units,namely Cangdong low-lying area and Nanpi low-lying area are developed better;?3?The tectonic evolution and tensile action in Cangdong Sag in Shahejie period was weak first and then strong,and the tectonic evolution more intense,the organic matter type is better;?4?The subsidence rate and duration affect the sediment thickness and organic matter input of dark mudstone.