A Study Of The Differences Of Hydrocarbon Accumulation Of Cenozoic In The Southeast Of Bohai Sea

The continental basins in the east of China are rich in oil and gas resources,in which are located such main petroliferous provinces as Songliao Basin,Bohai Bay Basin and East China Sea shelf basin.Through exploration of more than 60 years,many oil and gas fields have been found in the southeast area of Bohai Sea.Apart from such ten-million-ton-level oilfields as Penglai 25-F oilfield,Penglai 20-B oilfield,Kenli 12-B oilfield,Kenli 10-D oilfield and Kenli 6-D oilfield,some hundred-million-ton-level oil fields have also been found in this area,such as Penglai19-C oilfield on the northeast corner of Bonan uplift,Penglai 9-A oilfield and Penglai15-B oilfield on the of north Miaoxi uplift,Kenli 10-A oilfield,Kenli 16-A oilfield and Kenli 6-A oilfield in Laizhou bay sag.Compared with the large hydrocarbon-rich sags,the sags in the southeast area of Bohai Sea are relatively small.The exploration risk in these sags is relatively high as these sags are close to the eastern edge of Bohai Bay Basin and the strata erosion is relatively serious and the transport system is not clear due to the multi-stage structural transformation and the destruction of the traps after the multi-stage structural transformation.However,in recent years,some exploration achievements have been made in this area,which displays good exploration prospects.In an attempt to promote the effective development of the valuable resources contained in these sags,the differences of oil and gas accumulation in this area are explored in our study,in which the following main aspects are focused.First of all,the geological conditions of oil and gas accumulation are explored.Secondly,the elaborate analysis of typical oil fields is made.Thirdly,the affection of the superimposed extension and strike-slip fault system of the forming of the oil field are probed into.Finally,the mechanism of oil and gas enrichment is researched into.In the research,we adhere to the combination of tradition with innovation and integration of theory with practice.Some new attempts are made in the application of traditional theory and observation and analysis of phenomena,on the basis of which the following findings are generated.1.The Cenozoic Oil and gas reservoirs in the southeast of Bohai Sea area possess the basic conditions to form large-scale oil fields.Generally speaking,there are two sets of high-quality main source rocks developed in the southeast area of Bohai Sea.They are the fourth member and the third member of the Shahejie Formation.The simulation results of source rock evolution show that the Paleogene source rocks in Laizhouwan Sag and Huanghekou Sag have reached the peak of hydrocarbon generation at the late depositional stage of the Guantao Formation（12Ma to now）.Five sets of source-reservoir-cap assemblages have been found in the study area,including the deep assemblage of Mesozoic buried hill and Shahejie Formation（cap formation is on the top,and reservoir is below the oil generation formation）;the deep assemblage of the third member of shahejie Formation and the first and second members of Shahejie Formation（Cap formation is on the top and the third member of shahejie Formation plays a role as the self source-reservoir formation）;the deep assemblage of Shahejie Formation and Dongying Formation（Oil generation formation is below,and Dongying Formation plays a role as the self-reservoir-capping formation）;the shallow assemblage of Guantao Formation and Minghuazhen Formation（Cap formation is on the top and reservoirs is above the oil generation formation）,the shallow assemblage of upper part of Minghuazhen Formation（Oil generation formation is below,and the upper part of Minghuazhen Formation plays a role as the self-reservoir-capping formation）.Many types of traps have been found in the study area.They are mainly structural traps like fault block traps and anticline-fault traps.Locally,lithologic traps and lithologic-structural traps are also disclosed.There are many combinations of carrier system in the study area.The three most common combinations are as follows:the combination of“T-shaped”carrier system,the combination of“ladder-shaped”carrier system,in which fault,unconformity,and sand bodies are included,and the combination of“Y-shaped”carrier system.2.The origins of crude oil in the study area are diverse,for their maturity,sedimentary environment and organic matter sources are different.The oil in the study area can be divided into three oil families.One is family A,with medium maturity,which is similar to the source rock samples of the third member of Shahejie Formation.It is E₂s³ type oil,mainly produced in the northern sag-belt and uplift-belt of Laizhouwan sag.Another is Family B and it is with low maturity,in which gas chromatography displays no biodegradation and the analysis of oil fractions shows low saturated hydrocarbon content,and high non-hydrocarbon content,and the four maturity related biomarker parameters show low value.The characteristics of the samples are similar to those from the fourth member of Shahejie Formation.They belong to the E₂s⁴ type oil.They are mainly from the southern sag-belt of Laizhouwan sag.The last one is Family C,which is with low to medium maturity and represents the mixtures of the third member of Shahejie Formation and the fourth member of Shahejie Formation.They belong to the E₂s³-E₂s⁴-mixed type oil.They are mainly produced in the southern sag-belt of Laizhouwan sag,the eastern sag-belt of the Huanghekou sag and strike-slip-belt of the east branch of Tan-lu fault.3.The typical oil fields in different structural belts in the study area are elaborately analyzed and the hydrocarbon accumulation forming processes of these oilfields are recovered.The results show the following differences in the hydrocarbon accumulation,which manifests the characteristics as follows:in the early-period,the hydrocarbon accumulation occurs in the deep strata and in the late-period,the hydrocarbon accumulation takes place in the shallow layer;the composite of late-period hydrocarbon accumulation occurs in both deep and shallow strata and adjustment takes place in the late-period in the shallow strata;in the late-period,the hydrocarbon accumulation occurs in the shallow strata.The elaborate analysis and recovering of hydrocarbon accumulation process of Kenli 10-A oilfield in the uplift-belt have been carried out.The characteristics of the oilfield are displayed as follows:the oil is preferentially captured by the deep reservoir near the source and deep accumulation is formed;due to the late-period fault activity,carrier system consisting of the fault and sand body help the oil to adjust from deep strata to shallow strata and the shallow hydrocarbon accumulation.is formed.The elaborate analysis and recovering of hydrocarbon accumulation forming process of Kenli 16-A oilfield in the sag-belt has been carried out.The east block of Kenli 16-A oilfield manifests the following characteristics:the carrier system of fault and unconformity help oil reach Mesozoic buried hill reservoir and warehouse hydrocarbon accumulation takes shape.The middle block shows the following characteristics:the oil is preferentially captured by the deep reservoir near the source and deep Shahejie Formation accumulation is formed;late-period fault activity,the carrier system of fault,unconformity and sand body help oil reach the shallow Guantao Formation and the‘ladder-shaped’hydrocarbon accumulation is formed.The elaborate analysis and recovering of hydrocarbon accumulation forming processes of Penglai 31-C oilfield in strike-slip-belt has been carried out.The oilfield has the following characteristics:the oil transport experiences a medium-long distance;a rapid charge occurs in the late-period and a dynamic shallow hydrocarbon accumulation is formed.4.The control of the superimposed extension and strike-slip fault system on the hydrocarbon accumulation is quite different,which embodies the evolution of the fault system controlling the source,the combination of fault system controlling traps and the activity of fault system controlling migration of oil.The“evolution of the fault system controlling the source”suggests that the evolution of superimposed extension and strike-slip fault system and the activity intensity in different periods control the distribution and quality of the source rocks.“The combination of fault system controlling traps”shows that the combination of the fault system in different structural belts controls the combination and scale of these structural traps."The activity of fault system controlling transportation of oil”reveals that the activity intensity of the long-term active source-fault and the late-period formed source-fault during main accumulation-formation-stage controls the time effectiveness of the carrier system.5.Combined with the factors of hydrocarbon enrichment,the enrichment model with vertical carrier system in near-source area and the enrichment model with compound carrier system in far-source area are established.The enrichment model with vertical carrier system in near-source area suggests that in near-source area,shallow source-reservoir-cap assemblage develops（with cap rock on the top and reservoirs above the oil-generation strata）and the vertical carrier system,including long-term active extensional faults and petal-shaped faults formed in the late period,enables oil to migrate to shallow strata,and forms the late-period hydrocarbon accumulation during main accumulation-formation-stage,which provides conditions to form medium-sized oilfields.The enrichment model with compound carrier system in far-source area means that in far-source area,both deep source-reservoir-cap assemblage（with cap rock on the top and the young strata generating oil and the old strata storing the oil）and shallow source-reservoir-cap assemblage develop（cap rock on the top and reservoirs above the oil-generation strata）,the compound carrier system consisting of the lateral migration carrier system（including connected sand body and unconformity）and the vertical migration carrier system（source-fault）enable deep-seated oil to migrate to reservoirs.The medium-deep reservoir is charged in priority and hydrocarbon accumulation is formed.The shallow reservoir is charged during main accumulation-formation-stage,and late-period hydrocarbon accumulation takes shape.The study of the above two enrichment models reveals that the mechanism of hydrocarbon enrichment in this area is:sufficient hydrocarbon source,highly effective carrier system and large traps in both deep and shallow strata.6.Analysis of the factors of the failed wells in the study area reveals the characteristics in the causes of the failures,which are characterized by“three poors”,i.e.the poor effectiveness of vertical transport system,poor reservoir condition and poor trap sealing.“The poor effectiveness transport time of vertical transport system”is manifested in the lack of effective fault（either due to fault throw of T₀² being small or the juxtaposition thickness of faulted cap rock being large,or both cases in existence）;“poor reservoir condition”,resulting in low oil and gas charging degree.Due to poor sealing trap,or lack of effective trap or small scale of trap,it is difficult to form a large-scale oil and gas reservoir.In short,the causes of the failures in oil and gas exploration can be summarized as follows:poor transportation,poor reservoir and small trap scale.7.The theory of“two-stage and three-dimension”joint control of hydrocarbon accumulation distribution in Cenozoic is proposed,in which the special control of superimposed extension and strike-slip fault system is regarded as the core.In this theory,the first stage is zone evaluation and the three-dimension of this stage include the development of the source rock,the superimposed extension and strike-slip fault system and the carrier system conditions,among which the special control of the superimposed extension and strike-slip fault system is the core controlling factor of the hydrocarbon accumulation distribution in this area;the second stage is trap or the reservoir evaluation and the three-dimension of this stage contain trap closure height,hydrocarbon charging degree and effectiveness of source-faults.The controlling factor of first stage is the prerequisite for those of the second stage.Only when the two stage factors and the three elements in each stage coexist and match in time and space can large-scale hydrocarbon accumulation in the Cenozoic be formed.The above research results can provide a geological basis for further exploration and development in the southeast of Bohai Sea area,which has important reference value and provides a useful reference for similar research.