Abstract:

The complex structural evolution of the Frontal Areas of the Altun Mountains involves early faulted depression stage, middle depression stage and late uplift stage, in which continental lacustrine shale developed in the Early Jurassic faulted depression. The whole area is north/westoriented graben or halfgraben defined by syndepositional structure, therefore, laminated lacustrine shale interbeded with siltstone and stone deposite in the Xiaoliangshan sag, Qigequan fault nose zone and Tomorlog bugle. The shale continuously distributes along a northeast band and gradually pinches out from the front zone of mountain to basin. The cumulative thickness of shale is more than 300 m and the effective thickness is between 50 and 70 m, providing a necessary material condition for shale gas reservoir. The study confirmed that average contents of sapropelinite, vitrinite and inertinite are 60.7%, 33.1% and 6.2%, respectively. According to the terminology of Tissot and Welte, the organic matter is dominated by type IIkerogen. Total organic carbon (TOC) ranges from 1% to 4% and the organic matter maturity varies from 0.8% to 2.5% based on the RockEval pyrolytical data of 25 outcrop samples, indicating that the laminated shale has high organic matter abundance and is in oil and gas generation window, reflecting that the source rock quality is good. The average clay mineral contents and siliceous mineral contents are 52.51% and 37.42%, respectively. Compared with marine shale in the North America and Sichuan Basin, the LowerMiddle Jurassic shale contains higher clay mineral contents and lower siliceous mineral contents. The fracture, microcrack, interparticle pore, intraparticle pore and organic matter pore developed in the shale, providing more adsorption sites for methane. The specific surface area of the shale is between 9.13 m2/g and 18.14 m2/g, with an average of 13.43 m2/g; the pore volume varies from 0.0266 cm3/g to 0.0887 cm3/g with a mean of 0.06541 cm3/g; average pore diameter ranges from 25.76 nm to 72.48 nm, averaged at 47.87 nm. The specific surface area decreases with increasing average pore diameter; the pore volume increases with the increase of average pore diameter, the less the pore diameter, the greater the specific surface. The specific surface area increases with TOC and Ro, indicating that organic matter pore from the conversion of kerogen and generation of hydrocarbon contribute more surface area for methane to be adsorbed. The greater the specific surface of the shale, the more the shale gas to be adsorbed in shale. The lime mudstone, limestone and gypsum rocks above the Jurassic shale widely distribute as cap rock, contributing to the protect of shale gas reservoir from destruction. Moreover, the hydrocarbon generation history involving shallow burial in the early stage, deep burial in the middle stage and several stages of hydrocarbon generation, and uplift in the late stage is favorable for the preservation of shale gas reservoir. According to the geological parameters of the formation of shale gas reservoir, there are four favorable areas for shale gas exploration, i.e., Xiaoliangshan area, Qigequan area, Hongshuigou area and Tomorlog area.

Key words: Lower Middle Jurassic, Shale gas, geological conditions, Frontal Areas of the Altun Mountains