Properties and shale gas potential of continental shales in the Jurassic Mohe Foreland Basin, northern China

Abstract

Reservoir properties and shale gas potential of the Upper Jurassic continental shales from the Mohe Foreland Basin were investigated based on organic geochemical analysis, X-ray diffraction, scanning electron microscopy, low pressure nitrogen adsorption, high pressure mercury injection, and porosity and permeability analyses. The results indicate that the shallow lacustrine shales of the Emuerhe Formation are characterized by high total organic carbon content (0.43-22.83 wt.%), low hydrocarbon generative potential (less than 5.0 mg HC/g rock), type II kerogen, and a moderate thermal maturity stage (R o of 1.05-1.46%). The inorganic pores dominate the pore system, and pores associated with clay minerals largely contribute to the surface area. The inorganic pore-dominated lacustrine shale reservoir is characterized by dense, low porosity, and poor connectivity. The shales of the Emuerhe Formation show strong heterogeneity in mineralogy and are relatively enriched in clay minerals (33-66 wt.%). Due to the lack of biogenic silica, the effect of brittle minerals on hydraulic stimulation is ambiguous in the shallow lacustrine shales. The widely distributed, extensionally unfilled fractures can not only enhance hydrocarbon storage and migration capacity, but also play an important role in the generation of artificial reservoir fracturing. The pore network of lacustrine shales contributed by inorganic pores and unfilled micro-fractures could also generate a relatively high porosity and permeability. The high-mature thick shales in the northwest area of the Xingan Town would be the favourable zone for the shale gas exploration and exploitation in the Mohe Foreland Basin.

Comparison of Marine and Continental Shale Gas Reservoirs and Their Gas-Bearing Properties in China: The Examples of the Longmaxi and Shahezi Shales

“…Many studies have been made on continental shale gas in terms of mineral composition, pore structure, and gas-bearing properties, [15][16][17]19,23 as well as the comparison of these aspects between continental and marine shale. 24−26 However, most researches have been focused on the continental shale gas in the high mature stage, 15−17,19,24−26 and few focused on that in the HOMS.…”

“…China is rich in shale gas resources, , distributed across marine, − continental, − and transitional environments . Among them, marine shale gas has been commercially developed and is currently concentrated in the upper Ordovician Wufeng Formation–lower Silurian Longmaxi Formation (LMX) strata in the Sichuan Basin. ,, The marine strata are characterized by stable spatial distribution, good roof-floor preservation conditions, high total organic carbon content (TOC), appropriate maturity (in high-over-mature stage (HOMS)), and high content of gas and quartz. , However, continental shale gas exploration is still in the initial stage, and some breakthroughs have been made in the Jurassic Ziliujing Formation in the Sichuan Basin, , and the Triassic Yanchang Formation in the Ordos Basin, ,, showing great exploration and development potential.…”

“…Among them, marine shale gas has been commercially developed and is currently concentrated in the upper Ordovician Wufeng Formation–lower Silurian Longmaxi Formation (LMX) strata in the Sichuan Basin. ,, The marine strata are characterized by stable spatial distribution, good roof-floor preservation conditions, high total organic carbon content (TOC), appropriate maturity (in high-over-mature stage (HOMS)), and high content of gas and quartz. , However, continental shale gas exploration is still in the initial stage, and some breakthroughs have been made in the Jurassic Ziliujing Formation in the Sichuan Basin, , and the Triassic Yanchang Formation in the Ordos Basin, ,, showing great exploration and development potential. There are many differences between continental and marine shale gas, such as the following for continental shale: (1) The sedimentary face belts are diverse, − with dark mudstones also being developed in plain marsh ,,, in addition to lacustrine. (2) The types of kerogen in organic matter (OM) are complex, ,, including types II 1 , II 2 , and III.…”

“…There are many differences between continental and marine shale gas, such as the following for continental shale: (1) The sedimentary face belts are diverse, − with dark mudstones also being developed in plain marsh ,,, in addition to lacustrine. (2) The types of kerogen in organic matter (OM) are complex, ,, including types II 1 , II 2 , and III. (3) Water frequent changes lead to unstable shale distribution , and the continental shale is often interbedded with tight sandstone and coal. , (4) The maturity is relative low (vitrinite reflectance R o < 2.0%), − mainly in the high-mature stage, with a small number of organic pores ,, and dominant inorganic pores, such as clay-related pores .…”

“…(2) The types of kerogen in organic matter (OM) are complex, ,, including types II 1 , II 2 , and III. (3) Water frequent changes lead to unstable shale distribution , and the continental shale is often interbedded with tight sandstone and coal. , (4) The maturity is relative low (vitrinite reflectance R o < 2.0%), − mainly in the high-mature stage, with a small number of organic pores ,, and dominant inorganic pores, such as clay-related pores . Although the evaluation indicator of marine shale gas , cannot be directly employed in continental shale gas, the comparison between them can reveal the advantages and disadvantages of continental shale gas to better guide its potential evaluation and “sweet spot” identification.…”