Geochemical Characteristics And Favorable Target Evaluation Of Chang 7 Shale Gas

Lacustrine shale is highly developed in the Chang 7 Member of the Triassic Yanchang Formation within the central Ordos Basin.Driven by both economic and scientific interests,studies on Chang 7 shale have increased exponentially in recent years.However,Chang 7 shale,with the thermal degree of oil-window,has a complicated oil-water distribution,resulting in extremely complicated occurrence characteristics of the shale gas.Based on geochemical testing technology and gas desorption technology,this research fully reveals the geochemical characteristics of Chang 7 shale and Chang 7shale gas,and comprehensively analyzes the gas occurrence process.Meanwhile,the geological parameters are optimized to establish the target evaluation system,and the favorable exploration targets is determined.The average organic carbon content of Chang 7 shale is 4.52%,the free hydrocarbon content is 4.09 mg/g,the pyrolysed hydrocarbon content is 8.70 mg/g;the organic types are type I and type II₁;the thermal degree is in oil-window.The gas contents mainly ranges from 1.5 m³/t³to 2.5 m³/t³.Chang 7 shale gas is oil prone pyrolysis gas generated from humic matter.Chang 7 shale gas mainly occurs as free gas,with less adsorbed gas and dissolved gas.The contents of free gas,dissolved gas and adsorbed gas keeps on transforming and adjusting throughout the geological history.The migration capabilities of different gas components are different.Nitrogen is the easiest component to migrate,and carbon dioxide is the most difficult component to migrate.In hydrocarbon gases,the molecular diameter affects the gas migration ability.Gases with small molecules migrate faster and easier than those composed of larger molecules.The research area has experienced four tectonic movements,and the the erosion or denudation is intensively affected by the intensity of tectonic movements.Since the Mesozoic,the thermal evolution of the research area has changed from high thermal degree to low thermal degree.Compared with the Chang 8 and Chang 9 source rocks,the Chang 7 source rock has the largest gas generation intensity and gas expulsion intensity,and it provides the largest quantity of gas for shale gas.The Organic carbon content,vitrinite reflectance,shale thickness,burial depth,gas phases,brittle mineral content,hydrocarbon expulsion ratio and gas content are reliable parameters for establishing the target prediction model.Block A is the favorable exploration target in the research area of Ordos Basin.