Analysis Of Sedimentary Facies And Filling Succession Of Lower Cretaceous Strata In Wuerxun Depression In Hailaer Basin

Wuerxun depression is one of two major oil-generating depressions in Hailar Basin, and there are more and more wells having obtained oil and gas in industrial value up to now. Although Wuerxun has got a great a chievement in the exploration and development, it still has great potential. The complexity of the lithology in the working area make logging interpretation difficult in some ways,which mainly is the rocks contented with certain pyroclastic materials. The tectonic setting of the study area was complicated with many provenances,influencing the distribution of sedimentary facies,which is useful in searching for targets of petroleum exploration. It is necessary to study the core accurately and to analyze sedimentary facies and depositional systems based on lithology.1.MethologyThe research is based on the study of lithology and sedimentary characteristics on well cores accurately. The lithology of the objective layer is studied through the detailed description of the core and identification of thin section. The description of sedimentary features and facies types is in precision to microfacies. The grain size analysis is able to show the size characteristics of various types of sedimentary facies in the research area. The dependable analysis of core sedimentary facies is foundmental to determine the preponderant facies of a uint which is used in mapping the plane distribution of sedimentary facies.2. Types of RocksThe lithology of Tongbomiao formation- Damoguaihe formation in Wuerxun depressions of Hailaer Basin is very complex, controlled by volcanic clasts and normal terrigenous clasts. It is a set of rocks between volcanic lava and normal sedimentary calstic rock, which are lava, pyroclastic rock, sed pyroclastic rock, pyroclastic sedimentary rock and normal sedimentary rock. The abundance of rocks reduces in turn as follow: pyroclastic sedimentary rock, sedimentary rock, pyroclastic rock, sed pyroclastic rock and lava. The abundance of pyroclastic sedimentary occupies the superiority absolutely. 3. Basin Filling SuccessionTongbomiao formation is a sequence of volcanism - coarse clastic accumulation– volcanism from bottom to top, reflecting the initial stage of fault depression development. The lower part of the first member of Nantun formation is conglomerates and coarse sandstones,fining upward,reflecting the stage of tectonic transition from active to stable.The second member of Nantun formation is similar to the first member. The basin was still in the stage of fault depression during development of Nantun formation. Damoguaihe formation is a set of medium and fine sandstones interbeded with coal beds. The basin was in the stable stage of fault depression during development of Damoguaihe formation.4. Sedimentary facies and its plane distributionThere are four types of sedimentary facies: alluvial fan, fan delta, delta and lake in the objective layers in this research area. The provenances of Tongbomiao formation were in the northwest, the northeast, the southwest and the southeast. In the first member of Nantun formation, there are three types of sedimentary facies:fan delta facies, delta facies and lake facies (shallow lake facies and semi-deep lake facies),while alluvial fan facies was not developed in this period and lacustrine area expanded. The provenances were in the northwest, in north, in southwest, in southeast and in northeast. The scales of deltas in Southwest and northwest are small,reflecting the terrigenous clastic supply was poor or provenances were far distant from depositional area. In the second member of Nantun formation, there are four types of sedimentary facies: alluvial fan, fan delta, delta and lake (shallow lake facies and semi-deep lake) and lacustrine area expanded further. Alluvial fan facies was developed in the western sedimentary system. Semi-deep facies lays in the middle of the depression in trend of nearly south-north with a shape of irregular ellipse. The provenances were in the northeast,in north and in west. In Damoguaihe formation, there are two types of sedimentary facies: delta and lake. Alluvial fan and fan delta were not developed in this period.Provenances were in the northwest and the north.