Sedimentary Differences Of Typical Depression During Fault Depression Period In Beibu Gulf Basin And Its Impact On Source Rock

Beibuwan basin has great potential of oil and gas resources and is the key object of offshore oil and gas exploration and development.In the basin,the Weixinan and Wushi are typical depressions.The liushagang formation of paleogene Eocene series is the main study interval in the basin,and the most important set of source rocks in the study area developed during this period.However,the development scale and quality of source rocks in different depressions are different,and the sedimentary patterns of the same period are different.This paper will combine the core,well logging,seismic,paleontology,elements,clay mineral and hydrocarbon source rock geochemistry analysis data,further study on Weixinan and Wushi sag of sedimentary environment,sedimentary system mode,and the development of hydrocarbon source rock distribution characteristics,discusses sedimentary difference on effects on the formation of high-quality hydrocarbon source rocks,provide a reference basis for the oil and gas resources constituency.This paper uses well seismic data to divide the stratigraphic sequence of typical depressions,and combines core,spore powder,algae,mesogen,element,clay minerals and other data to analyze the climate,water environment,and redox conditions of each depression.Then,the characteristics of major source rocks in each depression period are analyzed and compared with the analytical data,and finally,the effects of different sedimentary patterns on the formation of high-quality source rocks are summarized.The main insights are as follows:(1)The paleogene in the fault depression period of the study area is the first-order sequence SQ1.The Oligocene and Eocene series are the secondary sequences SQ2 and SQ3.The Weizhou formation and the Liushagang formation are three level sequences SQ4 and SQ5 respectively.Wei1 segment,Wei2 segment,Wei3 segment,Liu 1 segment,Liu 2 segment,Liu 3 segment are four level sequence SQ6,SQ7,SQ8,SQ9,SQ10.(2)The period of the liushagang formation in the study area had a warm and humid climate,mainly dominated by lacustrine sediments,and developed sedimentary systems such as delta and underwater fan.The size of the lake basin is expanded,the semi-deep lake-deep lake reaches the largest area,the water body is the deepest,the algae is flourishing,the nutrients are abundant,the water body is dominated by the fresh-brackish water environment during Liu 2.In the Liu 3 period,the range of semideep lake-deep lake gradually decreased,with shore-shallow lake deposition as the main feature.In the period of weizhou formation,the lake basin shrank,with delta deposition as the main feature.The climate was relatively dry.(3)The main source rocks of the Beibuwan Basin were developed during the Liushagang Formation.The Liu 1 and Liu 3 are generally-better source rocks.Although the distribution range is large and the thickness is large,their hydrocarbon generation capacity is average.Good-high-quality source rock of Liu 2 is the most important set of source rocks in the Beibuwan Basin,with a small distribution range and relatively thin thickness,but strong hydrocarbon generation ability.The quality of source rocks in Weixinan sag is better than that in Wushi sag on the whole,and it is easy to develop high-quality source rocks under the conditions of warm and humid climate,large lake basin area,deep lake water,relatively light to salty water,strong reducing property and algae development.