Characteristic Investigation Of High-quality Source Rocks And Hydrocarbon Accumulation In The Saline Lake Basin

The development mechanism of source rocks in the salinized lake basin of Dongpu depression,the relationship between main source rocks and crude oil,oil and gas migration and oil and gas accumulation stages lack systematic research,and the quantitative evaluation of crude oil cracking is insufficient.In view of the above situation,this paper adopts the method of combining geology and inorganic-organic geochemistry to comprehensively analyze the molecular geochemical characteristics of source rocks,crude oil and reservoir extracts in the study area.On this basis,the main source rock identification and oil-source rock correlation were carried out,and the controlling factors for the differential development of source rocks in the southern and northern Yellow River regions were analyzed.Analytical techniques are used to study the migration direction of crude oil,hydrocarbon accumulation stage and accumulation time in Huqing area.Finally,the paper discusses the relationship between maturity index and crude oil cracking rate.The main results achieved are as follows:（1）Combining the measured geochemical values of source rocks and the predicted values of logging data,it is determined that there are 3 medium-good source layers in the lower Es3,namely Es3x3,Es3x5 and Es3x8.They are the main source layer in the South Yellow River area.（2）The controlling factors for the differential development of source rocks in the South Yellow River area and the North Yellow River area in the Dongpu depression were compared and analyzed.The North Yellow River area is a fluctuating deep water-environment controlled source rock development model,and the good source rocks are controlled by the water environment.The South Yellow River area is a river-lake-generation-controlled source rock development model,and good source rocks are controlled by the input of biogenesis.（3）The composition of crude oil molecular markers was analyzed by gas chromatography-mass spectrometry-mass spectrometry and other testing methods.It is found that there are significant differences in the composition of special molecular markers in crude oil in the southern and northern regions of the Yellow River,which are mainly reflected in:（1）The long-chain tricyclic terpenes(especially>C₃₃sequences)of crude oil in the northern Yellow River are abnormally abundant,reflecting the algal blooms in the salt lake environment,and the content of long-chain tricyclic terpenes in crude oil decreased significantly.（2）The crude oil from North Yellow River and South Yellow River has 4-methyl sterane series and C30 dinoflagellate series,reflecting some bacteria and dinoflagellates in the lake phase;（3）The crude oil from North Yellow River is rich in short-chain aromatic isoprenes,which reflects the depositional environment of the anoxic light-transmitting layer in the stagnant basin in the study area,and controls the development of high-quality source rocks.In contrast,the short-and medium-chain aromatic isoprenoids were not detected in the Machang crude oil from South Yellow River,revealing that a major factor for the low paleoproductivity of the freshwater lacustrine facies in the Yellow River South is likely to be the lack of an oxygen-deficient light-transmitting layer depositional environment in the Yellow River North salt lake facies.The oil-rock correlation reveals that the crude oil of Machang oilfield in the Yellow River South is mainly derived from the high abundance source rock of the main hydrocarbon source of Es3.Two types of crude oil sources in huqing area of the North Yellow River are determined.Huqing crude oil mainly comes from the source rock of the third part of the sand in the second step,and the second part may be contributed by the third part of the sand in the first step.The crude oil of Es1 member in well Qing 67comes from source rock of Es1 member.（4）Based on the distribution characteristics of the four nitrogen-containing compound migration parameters,it is found that the crude oil in the Huzhuangji area shows three directions of charging in three directions.The oil and gas are generally generated from the Haitongji sag,migrated along the NW direction,and then accumulated and accumulated in the high part of the Huzhuangji fault block along the NEE and SW directions.The Qingzuji crude oil was charged in a single direction from southeast to northwest.（5）Based on the continuous extraction experiment analysis of reservoir sandstone samples,the molecular geochemical characteristics of different extracts were analyzed,and the process of different oil and gas charging was revealed from the perspective of molecular geochemistry.Based on the distribution characteristics of C₂₄tetracyclic terpenes,gammaceranes and hopanes,it is concluded that there may be at least two petroleum charging processes in the Huqing area.Molecular geochemical characteristics of the Es3 oil sands in Qing 12 and Qing 6 wells reveal that they have experienced multiple oil and gas charging processes,while Es3 oil sands of Qing 11 has experienced one oil and gas charging.The oil sands of the Es1 in Qing 67 experienced a process of oil and gas charging,and the oil and gas came from the Es1 strata and migrated vertically over a short distance.There may be no fewer than two petroleum charging processes in the Huqing area.Step I oil charging is divided into two phases:the first phase and the second phase are19-10 Ma and 6-3 Ma,respectively.Step II oil charging is divided into two phases:the first phase and the second phase are 30-25 Ma,and about 3 Ma,respectively.Through the PVT simulation analysis of oil and gas inclusions in the oil-sands,the paleo-pressure history of the Huqing area are clarified.The pore pressure of the reservoir was overpressured in the early hydrocarbon charging period,and the paleo-pressure coefficient was 1.04-1.78 in the oil charging period of the I bench and 1.10-1.41 in the second bench oil charging period.It is supposed that there is a first-stage petroleum degradation process for the petroleum in the II step:there is a first-stage capture of carbon dioxide and methane inclusions in the II step,which is presumed to be a rapid uplift period,and the crude oil is exposed to degradation and oxidation.（6）Based on the gas-oil ratio of crude oil cracking,the typical crude oil cracking rates under different sedimentary backgrounds（salt water,brackish water and fresh water）in Dongpu depression were analyzed,and the average cracking curve of crude oil in Dongpu depression was summarized.With the increase of burial depth,the crude oil cracking rate increases exponentially.The relationship between various maturity indicators and crude oil cracking rate is established,and various molecular marker parameters that can be used for organic matter maturity evaluation and crude oil cracking degree analysis are selected.High temperature（>150℃）and low pressure（<40MPa）are the most important factors to promote the cracking of crude oil in Dongpu depression.