Characteristics Of Differential Superposition Between Meso-Cenozoic Basins And Mechanism Of Hydrocarbon Enrichment In The Linhe Depression,Hetao Basin

The Hetao Basin is a mid-small continental oil-bearing basin of Mesozoic-Cenozoic in the mid-western region of China,characterized by its distinctive tectonic background and intricate evolutionary history.However,limited progress had been made in the Linhe depression,which served as the primary exploration area within this basin.Previous research on the evolution process,accumulation conditions,patterns,and enrichment factors of the Linhe Depression was relatively inadequate,leading to an unclear direction for future exploration.To address these challenges comprehensively,we conducted an integrated investigation into the accumulation rules and enrichment mechanisms of hydrocarbons in the Linhe Depression using various data sources including outcrop analysis,core samples examination,well logging interpretation,seismic data analysis as well as basin evolution analysis.As a result of our study efforts,we had obtained four significant findings and insights.(1)The transition from depression to rift and the differential superposition in the Meso-Cenozoic Hetao Basin were proposed as significant phenomena.The classification of the Hetao Basin changed from a depression basin during the Early Cretaceous to a rift basin in the Oligocene.The stress field controlling the basin shifted from weak compression in the Early Cretaceous to weak extension in the Oligocene.During the Early Cretaceous,the basin exhibited a gently concave shape resembling a pot’s bottom,while in the Oligocene it displayed wedge-shaped geometry with greater thickness towards west and thinner towards east.Sedimentation rate was slow during Early Cretaceous but rapid during Oligocene.In terms of subsidence center,it was located in southern region of basin during Early Cretaceous but shifted towards northern part during Oligocene.Based on differences in basin types,field properties,sedimentation rate,and subsidence center,it can be concluded that Meso-Cenozoic Hetao Basin belonged to a differential superimposed basin rather than an inherited rift basin.(2)A set of source rock prediction methodologies suitable for data-scarce well conditions was developed.According to the basin type,the distribution of source rocks was predicted.It was hypothesized that the Early Cretaceous depression basin extensively overlaid source rocks,while only deep sag contained source rocks in the Oligocene rift basin.The distribution characteristics of organic facies for three types of source rocks were predicted using a method that transformed seismic facies,sedimentary facies,and organic facies.The thermal maturity of the source rock was estimated through basin simulation.The thickness of the source rock was determined by quantitatively predicting seismic velocity-lithology analysis.The results from this methodology indicated highquality source rocks in the high structural area,which were confirmed by new drilling in both Jilantai shallow sag and central fault barrier zone areas.This finding challenged previous pessimistic assumptions that these two sets of source rocks are exclusively distributed in deep sag troughs in the northwest.(3)The model of late near-source accumulation in the differential superimposed basin was established,revealing that hydrocarbon accumulation in Linhe Depression exhibits characteristics such as delayed hydrocarbon generation from source rock,late activity of oil-source faults,and late hydrocarbon charging.Three distinct types of late near-source accumulation models were identified and established: Jilantai,Nalinhu,and Xinglong.Furthermore,two separate hydrocarbon accumulation systems were delineated within the western and central regions of the Linhe depression.This achievement significantly deepened our understanding of the rules governing accumulation in Linhe Depression and challenged conventional notions regarding long-distance accumulation in Jilantai large oilfield.(4)A novel hydrocarbon enrichment model in the Linhe Depression had been established,taking into account the impact of late-stage strike-slip transformation.It was emphasized that fault cycle superposition mode,characteristics of strike-slip transformation,and local inversion strength are pivotal factors influencing oil and gas accumulation in the Linhe Depression.A new hydrocarbon enrichment model of "late weak fault cycle-tension strike-slip-strong reversal" was established.The southwest and central hydrocarbon enrichment zones were predicted.Two million tons of oil fields,Nalinhu and Xinglong,had been discovered in the central hydrocarbon enrichment zone selected in this paper,which proved the validity of the evaluation and prediction.The aforementioned findings had deepened our understanding of the fundamental conditions of petroleum geology in the Linhe Depression,enhanced our investigation into hydrocarbon accumulation rules and enrichment mechanisms,provided guidance for future oil and gas exploration endeavors,and furnished theoretical references for exploring oil and gas resources in small to medium-sized Mesozoic-Cenozoic basins in western China.