Abstract:

X-ray diffraction (XRD) was used to qualitatively and quantitatively analyze Middle Jurassic lacustrine shale samples from
the northern Qaidam Basin. Results revealed that clay minerals were dominant with a range of 33%~79.7% and averaged in 54.3%.
Quartz mineral ranged from 14.5% to 57.8% with an average of 35.4%. Carbonate mineral ranged from 0 to 15%. Clay mineral
compositions show a middle diagenetic stage, corresponding to a mature evolutionary stage of hydrocarbons with a suitable maturity
condition for shale oil and gas formation. Three types of pores were found in the study area including intragranular pore, intergranular
pore, and marco fracture. Compared with marine shales, Middle Jurassic lacustrine shales in the northern Qaidam Basin have higher
content of clay minerals, less sensitive mineral, and relatively lower content of brittle minerals such as quartz and feldspar but can meet
criteria for the lacustrine shale gas reservoir exploitation. The average index of mineral brittleness was 42.6% suggesting a good

property of brittleness and compressibility which is good for fracturing.

Key words: shale reservoir, mineral compositions, brittleness, X-ray diffraction, Middle Jurassic, Northern Qaidam Basin