The Mixed-Sources Characteristics And Mechanism Of Oil Accumulation In Triassic Baikouquan Formation From Circum-Mahu Depression

There are multiple source rocks developing in superimposed basins,which would lead to oil mixing.The distribution of these oils are mainly controlled by source rocks,so it is important to clarify the contributions of different source rocks to mixed oils and reveal the accumulation mechanism of these oils.The crude oils in the Mahu depression in Junggar Basin show a characteristic of oil mixing and there are three sets of Permian source rocks developed under the Triassic Baikouquan(T1b)reservoirs,which provide us a good example to illustrate the features of hydrocarbons generating from different source rocks,oil-source correlation,migration and accumulation mechanisms and models.Basing on the analysis,the controlling factors on formation of mixed oils are proposed.Among the three sets of source rocks,the Permian Fengcheng Formation(P1f)are good source rocks with high abundance and oil-prone organic matter,their tricyclic terpanes(C20?C21 and C23)are displaying a “Going-up” distribution;the Wuerhe Formation(P2w)are intermediate source rocks with moderate oil-generating ability and their tricyclic terpanes(TTs)show a “Going-down” distribution.Jiamuhe Formation(P1j)have high thermal maturity and organic matters are mainly type III,so they can hardly being an effiecient oil-generating source rock.Based on abundance of C25 tricyclic terpanes and lithological difference,the P1 f source rock were divided into two categories,one type is muddy carbonates and the other type is pure mudstones.On the basis of observation of organic facies and hydrous thermal simulation of rock samples,it is concluded that the distribution of TTs are mainly controlled by kerogen type and sedimentary environment,the maturation process will increase the total amount of TTs but will not change their form of distribution and the expulsion effect on the distribution of TTs is negligible.Then,take account of biomarkers,stable carbon isotopes and trace element data,three groups and five subgroups of crude oils have been classified and oil-source relationships have been clarified,i.e.the A1 subgroup oils originate from the P1 f carbonates and A2 subgroup comes from the P1 f mudstones,the B group is mainly from the P2 w mudstones,while the group C oils are mainly mixed oils and subgroup C1 oils are from both the P1 f carbonates and P2 w mudstones,and C2 oils orginate from the P1 f mudstone and P2 w mudstones.In order to quantify the contribution of different source rocks to mixed oils,material balance method were used to calculate the proportion of different oils and artificial mixing experiments were conducted to confirm the contributions.For example,the oils in fault belts mainly originated from the P1 f source rocks and some mixed oils have oil contributions from the P2 w source rocks,but they are limited and mostly below 30%;the different oil samples in slope area show a various proportion of mixed oils;and the oil in deep sag are mainly from the P2 w mudstones,except that some mixed oil samples show a little contribution from the P1 f source rock.The oil mixing process are mainly multi-stage oil mixing whch can be determined by fluid inclusion studies and FTIR parameters,and the distribution of homogenization temperatures proved that the T1 b reservoirs with mixed oils commonly developed two stages of fluid inclusion.The influence of facies and maturity on nitrogen compound parameters were evaluated to select the migration-indicated parameters and the migration directions were confirmed,the oils in deep sag area migrated laterally,the oils in fault belts mainly migrated vertically,and the migration happened in slope area were combination of two ways.In conclusion,three accumulation models have been proposed,the two-sourced two stages accumulation with fault-unconformity transpoting,the two-sourced two stages accumulation with fault-sandbody transporting,and single-sourced one stage accumulation with sandbody transporting.The controlling factors on mixed oil formation are identified,the types and distribution of mixed oils are controlled by spatial distribution of source rocks,and secondly the source facies,maturity and the residual bitumens of early stage could influence the geochemical characteristics of crude oils,and the combination of carrier system will determine the mixing process of mixed oils.