Formation Mechanism And Distribution Of Lacustrine Source Rocks In The Raoyang Sag And Baxian Sag

At present,the fine exploration of oil-rich basins is the main field of oil and gas reserves growth.The development and distribution of hydrocarbon-rich sags in the Bohai Bay Basin are the realistic area for the growth of oil and gas reservers which controls the distribution of major oil and gas.Therefore,the determination of the development and distribution of hydrocarbon-rich sags is the basis for the future prediction of favorable oil and gas exploration zones in the high mature exploration area of Raoyang Sag and Baxian Sag.In order to determine the development and distribution of high-quality source rocks in the Raoyang and Baxian sags,this paper clarifies the formation mechanism of high quality source rocks with the analysis of geological and geochemical markers of lacustrine basin,the distribution of sedimentary facies and organic facies.According to the mechanism of favorable facies belts in the development of high quality source rocks,the distribution of high quality source rocks is predicted by comprehensive geological,geochemical and geophysical techniques.The following main understandings have been obtained:Three sets of palaeolake basin models of high quality source rocks have been established.During the Es₃^L deposition,the lakes were deposited under humid climate,fresh water,water bottom anoxia and middle enrichment of the organic matter.The lakes were hydrological open and the depth can reach 20 meters with the wave base of9.5 meters.During the Es₃^U deposition,the lakes were deposited under relatively arid climate,saline water,reducing conditions and high enrichment of the organic matter.The lakes were hydrological close and the depth can reach 16 meters with the wave base of 9.5 meters.During the Es₁^L deposition,the lakes were deposited under arid climate,saline water,strong reducing conditions and middle enrichment of the organic matter.The lakes were hydrological close and the depth can reach 12 meters with the wave base of 7.5 meters.The sedimentary facies of source rocks deposition in the study area mainly include shallow lakes and semi-deep lakes,and deep lake facies do not develop.The results suggest that the sedimentary organic facies A and B with strong reducing environment,low terrestrial organic matter contribution and high algae organic matter under the background of semi-deep lake controlled the development and distribution of source rocks.During Es₄ and Es₃^M+L deposition,the organic facies“B”were mainly distributed in the area of Maozhou and Bazhou in the Baxian Sag.During Es₃^U and Es₁^L deposition,the organic facies“A and B”were mainly distributed in the Raoyang Sag.The organic facies"A and B"have a southward migration phenomenon with the change of the geological age.Three types of lacustrine basins show that the medium productivity can satisfy the development of source rocks.But the reducing condition is the key factor.In the Baxian Sag,the high quality source rocks occur in Es₄,Es₃^M+L and Es₁^L of northern Bazhou subsag and southern Maozhou subsag.High quality source rocks are developed in Es₄ and Es₃^M+L of the Bazhou subsag.The thickness of the 3 sets of high quality source rocks in the main zone ranges from 30 m to 90 m,40 m to 100 m and 30m to 80 m,respectively.The high quality source rocks in Es₁^L of Maozhou subsag are developed with the thickness of 20 m to 50 m.The high quality source rocks in the Raoyang Sag are developed in Es₃^L,Es₃^U and Es₁^L,which are mainly distributed in Maxi,Renxi,Hejian,Liuxi and Wuqiang subsags.The Maxi subsag developed source rocks in Es₃^L and Es₁^L,with ranges of 20 m to 110m and 45m to 100 m,respectively.The thickness of Es₁^L of the Renxi subsag ranges from 20m to 70m.The source rocks of Es₁^L and Es₃^U range from 30 m to 80 m,10 m to 50 m in thickness in the Hejian subsag,respectively.And the source rocks thickness of Es₃^U in the Liuxi subsag ranges from 10m to 30 m.The source rocks of Es₃^L are developed in Wuqiang subsag with a thickness ranging from 20 m to 70 m.