| The Mesozoic strata are the main oil producing reservoir in the Ordos Basin.With the continuous progress of exploration technology and methods,a series of major discoveries of oil and gas exploration have been made in the basin in recent years,and some new understandings are formed of the basin structure.However,there are some new problems.At present,the research on the formation and distribution of Mesozoic reservoirs in the basin is mainly based on sedimentary facies,and most of them are believed that the Mesozoic oil reservoirs are mainly distributed in the development parts of delta sand bodies and ancient river channel sand bodies.The types of oil reservoir are mainly lithologic oil reservoir and stratigraphic lithologic oil reservoir,which lack the understanding of the influence of structure on oil reservoir distribution.In addition,the structural division of the basin is based on geographical and geomorphic structure,which is insufficient in the study of oil and gas resource distribution.A large number of latest research data and actual production data show that there are faults,nose uplifts,depressions and other structures in the basin,which have played an important role in oil and gas migration and accumulation.This paper studies the reservoir forming structures related to Mesozoic reservoirs in the Ordos Basin,which is based on depression-uplift combination structure,deposition deformation characteristics of lithologic combination structure and fault fracture rock combination structure.By analyzing the relationship between discovered oil pools and related structures,the Mesozoic oil pool combination model is proposed,and the formation and distribution characteristics of different types of oil pools are analyzed.These are the main understandings obtained:(1)The depression-uplift combination structure of Chang 7 source rock evolved from point depression-side uplift type to side depression-side uplift type in the three key periods of oil and gas accumulation and migration(end of Early Cretaceous,end of Late Cretaceous Neogene and now).The evolution of the depression-uplift combination structural unit of source rock can change the direction of oil and gas migration.In addition,the driving mechanism of oil and gas forming in the same depression-uplift combination structural unit is the same.(2)Mesozoic lithologic combination includes fine-grained rock sandstone combination and fine-grained rock combination.Under the same stress,the fractures developed in each lithologic combination are different,and the interface between fine-grained rock and lithologic combination is more likely to produce fractures.Through the study of the lithologic combination structural deformation characteristics after deposition at the end of Early Cretaceous in the period of maximum burial depth of the basin,it is confirmed that the lithologic combination of the basin is roughly divided into plastic section,elastic-plastic section and fracture section with burial depth.The divide depth of the boundary is about 1700 m.(3)A large number of fault fracture zones in the Mesozoic in the Ordos Basin are identified by using the two-dimensional seismic profile and three-dimensional seismic data.It is found that most of these faults in the Mesozoic are characterized by high angle and unobvious fault distance.The faults are continuously distributed in strips on the plane,and most of them are affected by the later activity of the basement fault.The later activity of the bottom fault can cause fault fractures along the direction of the fault in the overlying strata.(4)According to the sand body shape in one type of the depression-uplift combination structural unit,the Mesozoic sand bodies are divided into four types: along slope type,reverse slope type,lens type and flat slope type.There are certain differences in the reservoir and sealing conditions of these four sand body types,among which the sealing of along slope type is the worst,and reverse slope type and lens type are the best.(5)It has been found that there is a good matching relationship between Chang 4 + 5-J oil pools and the distribution zone of fault fracture in the Ordos Basin.A large oil-bearing range appears more at the intersection of multiple fault fracture zones,and is generally far away from the hydrocarbon generation center.By analyzing the distribution of Chang 4 +5-J oil pools and Chang 6 oil pools,it can be found that the large oil-bearing range of Chang 4 + 5-J often connects a Chang 6 oil pool around the fault fracture zone;The discovered Chang 6 oil reservoir is distributed in large areas.Through the seismic data analysis,it is found that Chang 6 is close to the lake basin,and the foreset is obvious.The oil reservoir distribution is also far away from the source rock along the sand body source direction.In the area with small thickness and low maturity of the source rock,the overlying Chang 6 oil reservoir is enriched in a large area.Combined with the sand body configuration,it shows that Chang 6 oil reservoir has the characteristics of lateral migration as a whole;It has been found that there are both fine-grained rock reservoirs and sandstone fine-grained rock combination reservoirs in Chang 7 reservoir,the formation of the former is mainly controlled by fault fracture transformation,and the formation of the latter is due to its physical properties are better than source rocks and it’s direct contacting with source rocks;It has been found that there are few Chang 10-8 oil pools,and the relationship between the distribution of oil pools and fracture zone is more obvious.The distribution trend of some oil pools is consistent with that of fracture zone.The fracture development of well area revealed by core data and water injection development data is consistent with that of basin fracture zone.(6)According to the space characteristics of source rock and fault in the reservoir,the Mesozoic oil and gas pools can be divided into source rock oil and gas pools,adjacent source rock oil and gas pools and fault-controlled oil and gas pools.Source rock oil and gas pools,in which the Chang 7 source rock itself as the reservoir,have the characteristics of widespread oil-bearing and the fracture area is easy to produce high yield in Chang 7source rock;The adjacent oil and gas pools of source rocks include the oil-bearing sand body in Chang 7 source rock and the overlying Chang 6 reservoir in direct contact with the source rock;Fault-controlled oil and gas pools include Chang 4 + 5-J reservoir and Chang10-8 reservoir.Its characteristic is that the distribution of this oil pool type is constrained by fault fracture zone,and oil and gas need to migrate to these reservoirs through faults.(7)The favorable accumulation zones can be divided and predicted by using the depression-uplift combination structure of source rock,the lithologic combination structural deformation after the deposition and the fault fracture-reservoir combination structure.The more favorable range should be defined as the range that the depth of sedimentation greater than 1700 m at the end of Early Cretaceous.It this situation,the discovered Chang 4 + 5-J oil pools can be used to calibrate the fault on the source rock,and the discovered Chang 10-8 oil pools can be used to calibrate the fault under the source rock.Combined with the sand body type,the fault-controlled oil pools can be found.For source rock reservoirs,the development of fractures within the range of lithologic assemblage structural deformation needs to be considered.For reservoirs adjacent to source rock,the sand body in Chang 7 is favorable for accumulation,and Chang 6 needs to be combined with the matching relationship between foreset surface and sand body type.The matching of slope and foreset is more conducive to oil and gas migration.In addition,new favorable strata can be found according to the correlation of oil pools distribution.For example,using the discovered fault-controlled reservoir,new favorable strata in the known oil area can be found along the fault fracture zone. |
PDF Full Text Request