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Reservoir Characteristics of High Abundance and Low Thermal Stage Lacustrine Shale

Abstract

The lacustrine shale in the oil window is quite different from marine shale. This study use quantities of testing data from cores and outcrops to analyze the reservoir characteristics in Lacustrine Shale of Middle Jurassic, North Qaidam Basin. Measures such as geochemical analysis, XRD-mineral composition analysis, low temperature nitrogen adsorption, FE-SEM and methane isothermal adsorption were used to research reservoir characteristics about geochemical characteristics, mineral compositional, micro pore structure and gas-bearing properties of the samples from the study area. Compared with marine shale, shale in the study area has the characteristics of “ two high, two low, two types”, high organic matter(distribution from 0.3% to 38.7%, at the average of 5.3%) and clay mineral contents(with values from 33% to 79.7%, and average in 54.3%), low thermal maturity(the vitrinite reflectance distribution from 0.4% to 1.3%) and brittle mineral contents(silicate minerals contents average in 41.7%, and with low content of carbonate minerals), organic matter type are mainly in type II and type III. Low thermal maturity lead to low development of organic pores in the samples, dissolution and mineral transformation are the main factors that contribute to secondary pores. Different from marine shale, shale samples in the study area have the character that maximum methane adsorption contents have negative linear correction with surface area. High organic matter contents and low thermal maturity lead to high soluble organic matter contents(chloroform bitumen “A” values from 41.4ppm to 17712ppm, average in 1648.5ppm), which influence the pore structure and the occurrences of shale gas of the samples. Soluble organic matter are adsorbed on the surface of grains or plug between pores and throat, samples have high soluble organic matter contents that have smaller surface area; according to similar compatible principle, nonpolar CH4 are easy dissolved into soluble organic matter, and solubility of CH4 increases with pressure increasing, the measured maximum CH4 adsorption contents of shale samples in high pressure contains large content of dissolved CH4, higher Langmuir maximum CH4 adsorption contents with higher soluble organic matter contents.