Genesis and reservoir significance of lamellar sparry calcites of Shahejie Formation shale in Dongying Sag
The upper Submember of the fourth Member of the Eocene Shahejie Formation(Es4U)and the Lower Submember of the third Member of the Eocene Shahejie Formation(Es3L)shales in Dongying Sag are the main horizons of shale oil in Shengli Oil-field.There are three reservoir types:carbonate-rich,mixed and felsic-rich,among which the carbonate-rich type is the most devel-oped.The sections of carbonate-rich shale where lamellar sparry calcites are well-developed exhibit favorable characteristics in reser-voir properties,oil bearing,permeability,and compressibility,and the peak daily oil production of many horizontal well exceeds 100 tons in Niuzhuang,Minfeng,and Lijin sags.In this paper,the genetic mechanism of lamellar sparry calcite and its influence on reservoirs were further analyzed through the thin section,cathodoluminescence,inclusion,isotope geochemistry,and argon ion polishing scanning electron microscope.The results showed that the lamellar sparry calcites could be divided into recrystallized granular calcite laminae and lamellar sparry calcite veins crystallized by hydrocarbon generating fluids.The recrystallized granular calcite laminae were formed by the in-situ recrystallization of the lacustrine authigenic micritic calcite laminae during the early dia-genesis stage,and the formation process was closely related to the reduction of sulfate bacteria during the shallow burial period.The calcite crystals are granular with irregular shapes,the laminae are equally thick and the distribution is continuous.The formation of lamellar sparry calcite veins was closely related to the hydrocarbon generation and expulsion during the thermal evolution maturity of the organic matter and the organic acids dissolved the micrite calcite in shale to form carbonate-rich fluids.These fluids migrated along the lamellar fractures under the action of hydrocarbon generation pressures,then crystallized and precipitated again to form calcite veins.The laminae are thicker,lenticular,and distributed intermittently on the plane.In sparry calcite lamellae,the inter-layer fractures(bedding fractures),intergranular,intragranular pores,and dissolved pores are well-developede,the proportion of macropores is high and fractures and pores have good connectivity.Sparry calcite laminae and organic-rich laminae are interbedded in"couplets",which constitute two lithofacies:lamellar organic-rich sparry caly-lime shale and lamellar organic-rich sparry limy-caly shale.The"source-reservoir"shale has high-quality which is the most favorable lithofacies in the carbonate-rich shale of Jiyang shale oil.
shale oillamellar sparry calcitelaminareservoir spaceDongying Sag
万方数据
维普
