Study On The Formation Mechanism Of Low Efficiency Oil Layers In The Chang 6 Reservoir In The Middle And Eastern Ordos Basin

The Chang 6 period in the Ordos Basin was the peak period of delta construction,and it is the main oil-bearing layer of the Yanchang Formation.Previous studies have conducted extensive research on the lithological characteristics,physical properties,and reservoir formation mechanisms of the Chang 6 reservoir.Many consensus conclusions have been reached.However,as oilfield development continues,the problem of stable production of conventional oil reservoirs becomes increasingly prominent.With the increasing level of development,the study of inefficient oil layers has gradually become a research direction,and the study of inefficient oil layers,especially poor oil layers,is the focus of research.However,there are still some issues that need to be further addressed in the study of inefficient oil layers in the Chang 6 reservoir,such as what aspects make them inefficient,how they differ from conventional reservoirs,what their formation mechanisms and reservoir models are,how to identify and evaluate them,and how to choose the development method for these inefficient oil layers.Only by clarifying these issues can better replacement capacity be obtained.This paper mainly studies the characteristics of low efficiency oil layers in the Chang 6 reservoir in the eastern part of the Ordos Basin,clarifies their formation mechanism and reservoir formation mode,establishes identification standards,and provides theoretical support and practical guidance for deepening the evaluation and development of low efficiency oil layers in Chang 6,and solving the problem of increasing oil and gas production,storage,and production in the Ordos Basin.This paper uses drilling data in the eastern part of the Ordos Basin,combined with geological data,to conduct in-depth research on the distribution of inefficient oil layers in the Chang 6 reservoir.The physical characteristics of inefficient oil layers in the Chang 6reservoir are clarified,and the classification evaluation results are determined.Using logging data,analysis and testing data,the formation mechanism and reservoir model of inefficient oil layers in the Chang 6 reservoir are determined.The definition and characteristics of inefficient oil layers in the Chang 6 reservoir are summarized,and identification standards for these reservoirs are established.The research in this paper is of great significance for oil and gas exploration in inefficient oil layers of the Chang 6 reservoir in the eastern part of the Ordos Basin.By organizing and analyzing drilling data,logging data,geological data,and analysis and testing data,the formation mechanism of inefficient oil layers in the Chang 6 reservoir in the eastern part of the Ordos Basin is revealed.The sedimentary facies of inefficient oil layers in the Chang 6 reservoir in the study area are mainly submarine distributary channel deposits in the delta front of deltaic sedimentation.Due to the continuous lateral migration of submarine distributary channels and the interaction with lake water,various facies belts have undergone uninterrupted transformation and alteration.As a result,tinefficient oil layers are mainly distributed in the marginal areas of the channels,and some are distributed in the central areas of the channels.By analyzing the physical characteristics of inefficient oil layers in the Chang6 reservoir and combining with historical development and core analysis data,the Chang 6reservoir is classified into I,II,III,and IV categories,among which inefficient oil layers in the Chang 6 reservoir belong to the IV category,with an average porosity of only 7.12%and an average permeability of only 0.27×10^-3μm².By comprehensively using the Walsh inversion method,resolution matching method,longitudinal response discrete method,and other methods,the four-property relationship,reservoir mud content,porosity,permeability,saturation,and other parameters of inefficient oil layers in the Chang 6 reservoir are analyzed to obtain the logging parameter evaluation.Then,combined with qualitative-quantitative joint identification,comprehensive identification chart,and other methods,the identification criteria of inefficient oil layers in the Chang 6 reservoir in the eastern part of the Ordos Basin are clarified,which is the AC between 221μs/m and 225μs/m,and Rt between 32Ω·m and 48Ω·m.Meanwhile,through comprehensive studies on the distribution of source rocks,reservoir characteristics,migration conditions,and obstruction conditions,the depositional conditions of the Chang 6 reservoir in the research area were summarized,and the definition of inefficient oil layers in the Chang 6 reservoir of the Ordos Basin was clarified.The inefficient oil layers in the Chang 6 reservoir has a good oil and gas source,that is,the Chang 7 source rock.Under the influence of abnormally high pressure during the hydrocarbon generation process of the source rock,the Chang 7 source rock in the research area reached its hydrocarbon generation peak at the end of the Early Cretaceous,and continuously charged oil and gas to the Chang 6 reservoir.The stacked sand bodies formed by multiple periods of subaqueous distributary channels provide a certain thickness of storage space for oil and gas.The thick mudstone sedimentation of the Chang 4+5 reservoir provides a good cap rock,effectively preventing oil and gas from escaping upwards.The inefficient oil layers in the Chang 6 reservoir of the Triassic in the research area is influenced by sedimentation and diagenesis,and is poorer in terms of rock granularity and physical properties than conventional reservoirs.Its granularity is mainly fine sandstone,with a porosity range of 3%to 11%,averaging 7.12%,and a permeability range of 0.1×10^-3μm²to 0.7×10^-3μm²,averaging0.27×10^-3μm².Natural fractures develop within inefficient oil layers of the Chang 6 reservoir,with a brittleness index greater than 60%,which is conducive to forming a fracture network during development,improving the pore-permeability structure,and enhancing permeability.Finally,through analysis of reservoir types and main controlling factors,the depositional model of inefficient oil layers in the Chang 6 reservoir is summarized as a sub-lower overpressure-driven lithological oil reservoir formed under the joint influence of sedimentation and diagenesis.