Mechanical properties of Indonesian organic-rich shales from the Central Sumatra and North Sumatra Basin, and comparison with U.S. shales

Mechanical characterization of shales is a fundamental requirement in optimizing shale resource development and transferring development strategies between shale plays. In the Indonesian context, where presently limited subsurface data exist, such mechanical insights are essential for evaluating the viability of organic-rich shales as potential hydrocarbon resource. Here we report laboratory-based mechanical properties of several Indonesian organic-rich shales with hydrocarbon potential located within Tertiary basins in an active margin settings. Samples from surface-outcrops of lacustrine sediments in the Central Sumatra Basin (CSB) and subsurface-cores of marine deposits in the North Sumatra Basin (NSB) were examined in a series of experiments for their mechanical properties, including rock strength, elastic properties, and creep compliance. Results show that the Indonesian shales studied here are generally more compliant than Mesozoic/Paleozoic U.S. shales reported in the literature. Within the context of a binary mixture model of stiff and soft components used to explain shale elastic properties, these differences in mechanical properties imply that the soft and stiff end-members are more compliant in Indonesian shales than in U.S. shales. We interpret these differences as the difference in mechanical maturity, which reflects the degree to which mechanical properties have evolved over time due stress-driven diagenesis. CSB and NSB samples are early mature, mechanically, compared to the mechanically mature U.S. shales because of their younger age and limited time for stress-driven compaction to take place. Particularly, the Baong Formation samples from the NSB studied here come from a hard-overpressured interval and are mechanically immature, despite their moderate thermal maturity, because of their exposure to lower effective stress. Laboratory results also show no significant differences in mechanical properties between shales deposited in lacustrine (CSB samples) and marine (NSB samples) environments. Our interpretation provides a reason for why brittleness could vary between shale rocks even when mineral composition and thermal maturity are similar, and emphasizes the importance of geological age in determining the mechanical properties of organic-rich shales.