Study On Hydrocarbon Accumulation Conditions Of Upper Cretaceous Nenjiang Formation In Chang-ling Depression,Southern Songliao Basin

This paper takes the Chang-ling Depression in the south of Songliao Basin(central depression)as the study area.Base on the studies of fine sequence stratigraphic division and comparison,effective source rocks,sedimentary facies and reservoir characteristics,reservoir conditions and main control factors of the Upper Cretaceous Nenjiang Formation,we have established a high-precision sequence stratigraphic framework in the Changling Depression,and elucidated the internal composition and spatial distribution of sedimentary systems within the sequence framework.Combining the processing and fine interpretation of seismic and well logging data,the research on the target zone and favorable reservoir zone of the favorable zone is carried out within the isochronous sequence stratigraphy framework.The enrichment regularity of oil and gas in Nenjiang Formation and the hydrocarbon accumulation of complex reservoirs have been revealled.The results of this study will be helpful to find lithologic and low-amplitude structural reservoirs in Nenjiang Formation,to explore and discover new reserves and successional succession zones,and provide practical guidance for the exploration of complex reservoirs in Changling area.In this paper,based on the drilling cores,logging and seismic data,the principle of “circular comparison and classification control” is adopted to identify,classify,and contrast the multi-sub-level cyclic sequence of the Nenjiang Formation in the study area.Finally,the Nenjiang formation is divided into two second-order sequences,five third-order sequences and twelve fourth-order sequences,and the corresponding relations with the sections and sand groups are better.The thickness of Nenjiang formation has a little change,showing the east thick and west thin,north thick and south thin features,baselap develops at the bottom,gradually changes to onlap toward west and south.By the comparative tracking of structural reflection layers in the study area,it is found that there are mainly four types of faults developed in the area.Three NE-trending faults are formed at the end of the Mingshui Formation,with more and more oil-source faults,which can vertically communicate source rocks and reservoirs.On the basis of well logging and test data,the source rock of the whole region was analyzed.Mudstone is well developed in the Nenjiang Formation.Among them,the source rocks of the first and second member of Nenjiang Formation are especially developed.The maturity in the west of Yaoyingtai and Suotu area is relatively high,which is an effective source rock development zone.Through the study of drilling cores and analysis data in the study area,combined with interpretation of logging data,the characteristics of the sedimentary system in the area were analyzed,and the depositional system model was established to identify the sub-facies,microfacies and lithology characteristics of favorable reservoirs.The Nenjiang Formation was mainly controlled by the northern delta sedimentary system.The reservoir mainly developed in the fourth member of Nenjiang Formation,which is the delta front-front delta deposition.The horizontal distribution of sand body is relatively stable and widely distributed,but it is generally thin(2-4m)with siltstone as the main body,which is characterized by the medium-low porosity and low-low permeability.The physical properties change rapidly longitudinally and has strong heterogeneity.Through the implementation of oil and gas sources and stages of reservoir formation,the reservoirs in the region and its adjacent areas are dissected,the characteristics and vertical and horizontal distribution and change regulrities of oil and gas reservoirs are studied,the main controlling factors for reservoir formation are identified,and the reservoir accumulation models of different types of reservoirs are established.The source rocks of the first and second member of the Nenjiang Formation in the study area are in low maturity-maturity stage,and the oil and gas migrate upward by the fault and accumulate in the high part near the fault.The Nenjiang Formation is mainly composed of lithologic reservoirs and structural reservoirs.The longitudinal distribution of oil and gas mainly occurs in the ?,? and ? sand groups in the fourth member of Nenjiang Formation,with sporadic distribution of structural reservoirs in the plane.The Nenjiang Formation main control factors can be summarized as follows: Firstly,the reservoir controls the oil content,the oil well is controlled by sedimentary facies and microfacies obviously,and the oil layers in each sand group are distributed at the pinch out of the sand body,ie,the phase change place.At the same time,physical properties have a certain impact on the oil content,the worse the physical properties,the lower the oil content.Secondly,the fracture controls the oil content.The mudstones of the Nenjiang Formation are developed.In addition to communicating oil sources,the fault also play a role in shielding oil and gas,which can block the horizontal migration of oil and gas.Therefore,in the effective hydrocarbon source area,the effective oil source fault zone and the favorable reservoir area are favorable exploration zones.