Geochemical Characteristics And Resources Evaluation Of The Paleogene Source Rocks In The Southern Bohai Sea

The Bohai Bay Basin is not only a Mesozoic-Cenozoic rifting basin formed on the basis of Archean Eonothem-Paleoproterozoic Erathem crystalline basemen,but also the most petroliferous continental basin in East China.The southern Bohai Sea,located in the southeastern margin of the Bohai Bay Basin,experience complicated tectonic activities.Previous outcomes of petroleum explorations were poor in southern Bohai Sea although the petroleum explorations have been going on for decades.Thus,it was always thought that the brink sags in the southern Bohai Sea were unable to form high-quality source rocks in large scale.While in recent years,several large-scale oilfileds were discovered in the southern Bohai Sea,which demonstrated the petroleum generation potential of these brink sags.However,the depositional environment,forming mechanism,spatial distribution and petroleum resources potential of source rocks were lack of comprehensive and deep understanding.In this dissertation,a series of expriments（e.g.,Total organic carbon,Rock-Eval pyrolysis,maceral analysis,measurements of vitrinite reflectance,major and trace elements,gold-tube pyrolysis）were employed to detemined these issues.This dissertation aims to re-evaluate the geochemical characteristics of paleogene source rock,reconstruct the paleoenvironment and reveal the controlling factors of organic accumulation.The spatial distributions of source rocks are predicted by log-seismic method and the impacts of superimposed facults on source rocks are also discussed.Moreover,the kinetic model of source rocks in southern Bohai Sea are established and the hydrocarbon generation history is recovered by Petromod software.The genetic method is used to assess the petroluem resources quantity in the southern Bohai Sea.The main achievements of this dissretation are as follows.（1）The brink sags in the souther Bohai Sea are able to form high-quality paleogene source rocks.Source rocks from Es₃^M-Land Es₄^Uare of the best quality,mainly composed by the source rocks in medium,good and excellent quality.While source rokcs from the other layers are in relatively poor quality.The migrations of geochemiacal parameters during Es_1-2and Ed are observed in the souther Bohai Sea.The abundances of organic matter increased from south to north and the type of organic matter gradually transformed from type II₂-III to type II₁.Moreover,there exists good response relationship between depositional environment and source rock qualtiy.The source rock qualtiy successively decreased as the change of depositional environments（semi-deep to deep lake→Shore shallow lake→delta→salt lake）.（2）The paleoenvironmental evolution in the southern Bohai Sea are divided into the following three stages:（1）Es₄^Lto the bottom of Es4^U:sediments in this stage were deposited under an arid and cool climate.The paleolake was characterized by modest productivity in the surface water and oxidizing（oxic to suboxic）conditions in the bottom water,with a relatively high detrital flux and high salinity.（2）The top of Es₄^Uto Es₃^M-L:sediments formed under a warm and humid climate,which resulted in the high productivity in surface water and fresh water body.The bottom water was under suboxic condition,and the detrital flux was relatively low.（3）Es₃^U-Ed:paleoclimate during this stage was warm and humid but exhibited a trend towards an arid and cool climate,leading to the decrease in productivity.The sediments were deposited in oxic bottom water with high detrital inputs.The paleolake was characterized by fresh water body except for the east sub-sag of Huanghekou Sag during Es₁.（3）Primary productivity,detrital flux and tectonic preservation are considered as the major factors controlling organic matter enrichments in the southern Bohai Sea.The disadvantages resulted from the oxygen-rich bottom water can be offset by the high productivity and low detrital inputs.Moreover,the weakly tectonic destruction is good for the preservation of organic matter.The organic matter enrichments in Laizhou Bay Sag and Huanghekou Sag are controlled by primary productivity,detrital flux and tectonic preservation,but for Qingdong Sag,the primary productivity and tectonic preservation are the controlling factors.（4）The superimposed facults in the brink sags of the southern Bohai Sea exert great effects on the paleogene source rocks.For one hand,the scale and thermal evolution of source rocks are determined by the extensional faults and strike-slip extensional faults.For the other hand,the spatio-temporal migrations of the source rocks are controlled by strike-slip faults and strike-slip extensional faults.As the intensification of dextral strike-slip during Es₂-Ed,the high-value regions of TOC,Ro and effective thickness of source rocks gradually move northeastward.（5）The oil yields of source rocks in different types of organic matter exhibit huge differences,while the gas yields are similar.The kinetic models of hydrocarbon generation in the southern Bohai Sea are established and the history of hydrocarbon generation in different sub-sags are recovered.The hydrocarbon generation of Es₄^Uand Es₃^M-Lare significant in the Laizhou Bay Sag and Qingdong Sag and the peak period of hydrocarbon generation occurred before 20 Ma.While the hydrocarbon yields of Ed,Es_1-2and Es₃^M-Lare in high values in the east sub-sag of the Huanghekou Sag.Hydrocarbon generation of the three sets of source rocks exhibit bimodal patterns.The peaks of hydrocarbon generation occurred at 25 Ma and 2 Ma.The assessments of petroleum resources show that the brink sags in study area are of abundant petroleum resources with 28.46×10⁸t oil and 2.49×10¹¹m³natural gas.