Study On The Control Of Pre-Jurassic Paleogeomorphology On The Differential Enrichment Of Hydrocarbon In The Yan’an Formation Of Ding-Jing-Wu Area

Ding-Jing-Wu area is located in the central part of the Ordos Basin,at the junction of Dingbian,Jingbian,and Wuqi counties.Its Jurassic strata were directly deposited on the erosion surface caused by the Indosinian movement at the end of the Late Triassic.Compared with the southwestern part of the basin,the erosion in the study area was weaker,except for the Chang 2 strata exposed at the bottom of the Inner Mongolian-Shaanxi paleochannel at the western edge.Therefore,previous research on the control of pre-Jurassic palaeogeomorphology in Yan’an Formation oil reservoirs has different views under this background and lacks a systematic analysis of the control differences of palaeogeomorphology on hydrocarbon accumulation in different oil reservoirs.With the continuous increase in the density of exploration wells,more oil reservoirs have been discovered in paleo-highlands and paleochannel palaeogeomorphology units,and oil and gas differences are enriched from west to east.The palaeogeomorphology-controlled reservoir theory proposed by previous researchers,which is mainly based on the "palaeogeomorphology-covered river channel sand trap" oil exploration model,can no longer effectively guide exploration.In this setting,it is imperative to deepen the fine characterization of pre-Jurassic palaeogeomorphology and comprehensively analyze the differences in the control of paleogeomorphology on oil reservoirs in different layers.This paper focuses on the Yan 10 and Yan 9 oil reservoirs of the Yan’an Formation in the Ding-Jing-Wu region distributed along the eastern edge of the Inner Mongolian-Shaanxi paleochannel.Based on field outcrop observations,laboratory analysis,and testing,combined with abundant logging data,the paper analyzes the geological structure,sedimentation,and reservoir characteristics of the region.Utilized various methods,the research finely depicts the pre-Jurassic paleogeomorphology.The reservoir configuration analysis theory of Miall was used to dissect thick channel sand bodies in the braided river and meandering river sedimentary systems within the study area,analyzing the relationship between sand body internal configuration and oil and gas accumulation.The relationship between paleogeomorphology and the conditions for hydrocarbon accumulation in the Yan 10 and Yan 9 oil formations was studied,revealing the differences in control that paleogeomorphology has on the enrichment and accumulation of oil and gas in these formations.The models of hydrocarbon accumulation are established.The paper also analyzes the causes of differences in oil and gas enrichment in different regions and layers from both lateral and vertical perspectives.The main achievements and insights gained are as follows:(1)There are several gullies at the edge of the pre-Jurassic Inner Mongolian-Shaanxi paleochannel paleogeomorphology unit in the research area.The entire Jingbian paleo-highland is dissected and renamed as two paleogeomorphology units: the erosion paleo-slope and the residual paleo-mound.(2)The internal configuration of thick channel sand bodies controls the oil and gas migration pathway,thereby controlling the distribution range and reservoir type of the Yan 10 oilfield;as well as controlling the extent of enrichment and location differences in the Yan 9 reservoir.(3)The paleogeomorphology directly controls the formation and distribution of the Yan 10 reservoir,while controlling the Yan 9 oil reservoir indirectly.The favorable conditions for the control of paleogeography on the enrichment and accumulation of oil and gas in the Yan 10 and Yan 9 formations,as well as the main controlling factors of the distribution of the Yan 10 and Yan 9 reservoir,were summarized separately.(4)A Jurassic Yan 10 hydrocarbon accumulation model of "far-source hydrocarbon supply,multi-element transport,paleogeomorphology dominance,and three-dimensional scattered distribution" is proposed,as well as a Yan 9 hydrocarbon accumulation model of "Yan 10 cap layer diversion,connected sand body transport,sedimentary facies control,and paleogeomorphology influence".It was proposed that "paleogeomorphology dominance,cap rock and sand body internal configuration controls,oil and gas migration pathways,and dynamic driving forces of oil and gas migration" are the mechanisms that cause the differences in oil and gas enrichment and accumulation in the Jurassic of Dingjing-Wu area.This study to some extent supplements and improves the theory of paleogeomorphology control reservoir in the Jurassic,providing theoretical support and guidance for the next stage of fine exploration of Jurassic oil reservoirs and having certain practical value.