Formation Mechanisim And Accumulation Pattern Of Tight Oil Of Chang 7 Oil Reservoir In The Yangchang Formation, P Area, Ordos Basin

There are abundant resources of tight oils in the Chang 7 oil Group of Yanchang Formation in the Ordos basin, which lie in the center of the prototype lacustrine basin. The centers of the prototype lacustrine basin become the favorable targets of prospecting evaluation of tight oil. In recent years, exploration for the tight oil in the Chang 7 oil Group get a bigger breakthrough. Some favorable oil-bearing areas in the sourtheast of Ordos basin were developmented, there were many basic databases of well logging of exploration wells and development wells, logging, formation testing, producing test, analysis and test and so on, where are the ideal study area for research on the genesis of tight oil and hydrocarbons richment rules. The formation conditions, formation mechanism and key controlling factors of tight oils were studied taking the P area in the southeast of the Ordos basin as an example, which were helped to understand the formation process of tight oils and guide the tight oil explormation.The sedimentary facies types are determined and the distribution characteristics of sedimentary facies are analyzed by using of the data of core, well logging, analysis and test, regional background and so on. The petrologic features of reservoir, microcosmic pore-throat characteristics of reservoir are analyzed using the data of casting thin sections, cathode luminescence, scanning electron microscope and high pressure hg injection and so on, and then diagenetic evolution sequences are developmented, the densification process of reservoir are determined by using of property inversion. The four-property relationships of reservoir in the Chang 7 oil Group are analyzed using the data of well logging, logging, formation testing, producing test, analysis and test and so on and also minimum standards of net thickness are determined, the distribution of oil layers of the Chang 7 oil Group are comprehensively interpretated, the space distribution features of tight oil are summarized. The pool-forming periods are analyzed by using of analytical techniques of fluid inclusion, and then the relationships between oil-charging and reservoir densification are analyzed. Fluid dynamics processes of tight oil migration and accumulation are comprehensively analyzed using the combination of paleopressure calculate from the data of the interval transit time and the capillary resistances. The oil migration channel types of Chang 7 oil Group in the study area are summarized using the previous research results, the core, micro chips and so on. Lastly, by using of the coupling in the time and space of geological conditions of tight oil formation, forming process, distribution of tight oil, the main controlling factors of tight oil are optimized, and formation mode of the tight oil type are summarize. Exploration directions of tight oil of Chang 7 oil Group in Ordos basin are pointed up.The research results show that:(1)The reservoir lithology of Chang 7 oil Group in the study area is mainly grayish fine-grained feldspar sandstone or lithic feldspar sandstone, and main mineral composition of the reservoir sandstone is feldspar, secondly quartz, thirdly rock debris. The content of interstitial material is high, which is mainly composed of mudstone matrix and cement including carbonate rock, clay rocks, siliceous, etc. The reservoir of Chang 7 oil Group in the study area is in the diagenetic stage of A now. The reservoir porosity is mainly distributed in 3%-10%, average 6.29%, and the reservoir permeability is mainly distributed in (0.03?0.70)×10-3?m2, with a mean of 0.30×10-3?m2. Areal porosity is generally small, which mainly distributes between 0.5% and 2.5%, average 1.2%. The main reservoir space is residual intergranular pore, second for dissolution pore, total for ultra-low permeability to tight sandstone reservoir. High content of the plastic material such as matrix, smaller grain size with some sedimentation, another fierce compaction, strong carbonate, clay rocks with cementation and relative weakness dissolution have been the mainly controlled factors of reservoir densification. (2)Most of reservoir (mainly IV b and V reservoir) porosity in the late Jurassic began to reduce into 12%, and reservoir began to densify in a large area; most part outside the reservoir had formed the densification in the early stage of early cretaceous in addition to the part of best ?a reservoir of the study area. In the middle and late stage cretaceous and early and middle stage of late cretaceous, the reservoir physical property of ?a reservoir became lower slightly.Ini the late stage of late cretaceous, reservoir physical property was still falling, reservoir porosity lowered 1?2% generally. When oil and gas began to fill in the early cretaceous, general reservoir had appeared the densification. While the large amounts of oil and gas filled in the middle stage of early cretaceous and late stage of the late cretaceous, most of the reservoirs were in a state of densification. The reservoir of Chang 7 oil Group in the study area has the characteristic of densification before accumulation. (3)Provided that oil migration is powered by buoyancy in key accumulation period, and when oil move massively in the ?a and IVb reservoir, it need the oil column height of about 152.8m and 180.5m respectively to overcome capillary resistance corresponding to average pore-throat. While the strata in this period are relatively flat, forming oil column heights of 180.5 m need distribution of sand body of 10.3 km. Real data indicates that the single sand body in the reservoir of Chang 7 oil Group in the study area is generally in 5-25m, the maximum thickness is only about 40 m, and the distribution of sand body is far short of 10.3km. Therefore, it is difficult to only depend on buoyancy to form the current reservoirs. The study found that the most important source of migration dynamics in the study area should be overpressure, the effect of buoyancy is limited. Under the influence of the power, oil and gas can only have a short secondary migration to form the reservoirs closing to the oil source. (4)The main controlling factors of tight oil enriching in the reservoir of Chang 7 are widely distributed oil source rocks with high quality, well allocation relationship between source and storage, relatively high quality sand body and cracks, good sealing conditions, etc. The formation model of the reservoir of Chang 7 oil Group in the study area is summarized as: high quality oil source rocks with extensive development in Chang 73 constantly discharge oil, which directly enter into Chang 72 reservoir upon it along large area resource and storage contacting surface or cracks of Chang 73 and Chang 72.Then, it will continue going straight along the high porosity and permeability as well as high quality sand body or cracks which both have a relatively high continuity until entering into Chang 71 reservoir. It will accumulate when meet more tight sand body or mudstone barrier at the migration channels, form the quasi continuous distribution of many thin oil layers which superpose and connect together. The important exploration target of Chang 7 tight oil in Ordos basin should focus on the composite superimposed area which has the main controlling factors of high quality source rocks, sedimentary facies, ect.