Analysis Of The Remaining Strata,Late Reformation And Hydrocarbon Generation Potential Of Carboniferous-Permian In The Western Bohai Sea

The oil and gas exploration in the middle and shallow layers of the Bohai Bay Basin has reached a high level after decades of oil and gas exploration.In the face of the severe oil and gas exploration situation,the exploration and development of oil and gas in deep formations has been paid more and more attention as a realistic replacement field.A set of coal measure strata under the background of sea-land transition has been widely developed in the Late Paleozoic,which is an important coal accumulation period in geological history.The prospect of oil and gas exploration is broad.There have been many exploration breakthroughs in the Ordos Basin in the western part of the North China Craton and the land part of the Bohai Bay Basin.This also confirms the exploration potential.However,the Bohai Sea of Bohai Bay Basin,which is in the eastern part of the North China Craton,has a relatively low level of research.This paper takes the Carboniferous-Permian in the western Bohai Sea as the research object.On the basis of previous research,combined with drilling,3D seismic and related geochemical data,the characteristics of the remaining Carboniferous-Permian in the area,the late stage Reform and hydrocarbon generation potential of source rocks are studied.The main insights gained are as follows:（1）Based on drilling and 3D seismic data,it is revealed that the Carboniferous-Permian remains are mainly distributed in Qinan Step Belt,Chengbei Sag,Chengbei Low Uplift and East Slope area.The Shaletian Uplift is missing in the whole area.The maximum thickness of the residual is over 1 200 m,generally concentrated between 200～800 m.It shows the distribution characteristics of being thick in the southwest and thin in the northeast.Gradually thinning or even missing from the uplift,slope to depression,with the structural characteristics of"thin bottom"or"bare bottom".The main body of Carboniferous-Permian is controlled by fault cutting.Most of them are fault blocks of different sizes and burial depths.（2）The analysis of Mesozoic-Cenozoic tectonic evolution in the Western Bohai Sea shows that the Carboniferous-Permian in this area has experienced four stages of transformation:Indosinian,early-middle Yanshanian,late Yanshanian and Himalayan.The Indosinian and Late Yanshanian were dominated by compression uplift,while the early-middle Yanshanian and Himalayan periods were dominated by extensional rifts.Among them,the transformation of the Carboniferous-Permian in the Indosinian period was mainly due to strong denudation.While the transformation of the Carboniferous-Permian in the early-middle Yanshan,late Yanshan and Himalayan mainly changed its occurrence.The differential evolution transformation also makes the hydrocarbon generation evolution of the Upper Paleozoic source rocks in this area can be divided into two types.It can be described the early uplift and late burial（early subsidence,strong uplift,late deep burial,represented by Chengbei area）and intermittent burial（early subsidence,middle stage shallow burial,subsidence burial,represented by Qikou-Shanan area）.（3）The analysis of the sedimentary environment and the geochemical characteristics of related source rocks shows that the late Paleozoic in the western Bohai Sea was mainly developed in the transitional environment of sea and land.The source rocks were mainly developed in the middle and lower parts of the Upper Paleozoic.While the dark mudstone,carbonaceous mudstone and coal rock being the main hydrocarbon-generating lithology.The types of organic matter are mainly II₂ to III,mainly in the ability to generate gas.Combined with the development characteristics of regional source rocks.It is inferred that the thickness of dark mudstone,carbonaceous mudstone and coal rock is concentrated at 50～200 m,15～90 m and 10～50 m,respectively.（4）The thermal history evolution analysis shows that it is low for the Ro of the Upper Paleozoic coal-measure source rocks in the Qikou-Shanan area at the end of the Mesozoic,mainly between 0.5～1.1%.Although the Ro during the Cenozoic generally increases,but the magnitude is small.The current Ro is basically less than 1.3%.The Upper Paleozoic source rock in Chengbei area has a relatively high degree of thermal evolution at the end of the Mesozoic,with the maximum Ro exceeding 1.5%.However,during the Cenozoic,only the Upper Paleozoic source rock in the sag area has increased Ro,with the maximum Ro is about 2.0%.There is secondary hydrocarbon generation,while Ro does not increase in other regions and secondary hydrocarbon generation does not occur.Comprehensive analysis shows that the central and eastern part of Chengbei Low Uplift and the eastern slope are relatively favorable areas with great hydrocarbon generation potential for source rocks.