Search

The Longtan Formation in the Sichuan Basin contains abundant organic-rich mudstones and coals, however, few studies have been conducted about their gas potential. A comprehensive study has been conducted on the Longtan sediments in order to determine their critical properties and gas potential. The Longtan samples, especially the coals, contain abundant organic matter, whereas S1 and S2 values are generally very low due to high thermal maturity. The organic component is dominated by the vitrinites, which comprises more than 95% of dispersed organic matter. The high vitrinite reflectance values (∼1.88–2.14% Ro) indicate a post thermal maturity of these sediments, which is consistent with the reliable Tmax values. The predominant mineral components are clay minerals and quartzs, and the brittle nature of the studied samples can be confirmed by the high contents of the brittle minerals and high brittleness indexes. The porosity positively correlates with total organic carbon (TOC), and abundant organic-matter, intraparticle, interparticle pores and fractures have been detected in these studied samples. The Longtan Formation in the study area was mainly deposited in an oxic environment. According to the 1D basin modelling, the Longtan sediments reached the maximum Ro after they have been buried to the maximum depth at late Cretaceous, and they began to generate gas at middle Jurassic. Attention shall be paid on the co-exploration and co-exploitation of shale gas and coalbed methane of the Longtan Formation of eastern Sichuan Basin in the future.

We performed low-pressure N2 physisorption experiments to investigate and compare the fractal characteristics of bulk-mudrock, washed, and kerogen samples of Chang 7 mudrocks from the Ordos Basin, China. Their fractal dimensions D were obtained from adsorption data using the Frenkel–Halsey–Hill (FHH) method. The D values for the bulk-mudrock samples were slightly lower than those of the washed samples, while the kerogen samples showed lowest values. The results suggest that the surfaces of the washed samples were rougher than the bulk-mudrock samples and that the kerogen samples had the smoothest surfaces. For the bulk-mudrock and washed samples, D decreased with increasing total organic carbon content, suggesting that organic matter has lower fractal dimension than other fractions in mudrocks. D decreased with increasing free oil, indicating that the fractal dimension can be used to characterize the residual oil content in lacustrine mudrocks approximately. For the bulk-mudrock and washed samples, there are positive trends between fractal dimension and surface area and pore volume. However, similar trends cannot been found in kerogen samples, indicating that the fractal dimensions obtained from low-pressure N2 adsorption experiments cannot describe the fractal characteristics of the kerogen samples well. The fractal dimension decreased with the increasing average pore diameter, which can be attributed to that the samples with higher fractal dimensions contained more small pores and had higher surface area.