Empirical plot of gas generation from oil-prone marine shales at different maturity stages and its application to assess gas preservation in organic-rich shale system

Abstract

Abstract Determining the degree of gas preservation and loss in shale gas system is critical to evaluate unconventional shale gas potential, particularly in the tectonically faults developed areas. In this study, an empirical plot of gas yields from marine shales at different stages of maturity and residual TOC content is derived from pyrolysis results and applied to evaluate the extent of shale gas loss. Miniature core plugs drilled from immature Barnett mudstone, Eagle Ford limestone, Woodford chert and mudstone samples were isothermally heated at 130 °C, 300 °C, 310 °C, 333 °C, 367 °C, 400 °C, and 425 °C for 72 h under a confining pressure of 68.0 MPa, corresponding to immature, early stage of oil window, main stage of oil window, late stage of oil window, main stage of oil cracking to wet gas, and late stage of oil cracking, respectively. The generated gas yields from marine shales with increasing thermal maturity are evaluated quantitatively by anhydrous closed-system pyrolysis. Experimental results show that gaseous hydrocarbon yields (C1 C5) from type II kerogen-dominated marine shales are linearly proportional to their residual TOC content at each stage of petroleum formation. The regression lines in the plot of generated gas yield and residual TOC content thus represent different gas-generation stages of type II kerogen-dominated marine shales. In the application to three geologic cases, the measured total gas content and present-day TOC content from a variety of shale gas wells with varying thermal maturity levels were incorporated into the empirical plot. And the comparative results show that, the gas contents of these core samples investigated for the low thermal maturity New Albany Shale in the Illinois Basin are somewhat higher than the estimated yield of generated gas from type II kerogen-dominated marine shales at the early stage of oil window, suggesting that microbial gas made contribution to the excess gas content in the New Albany shale gas system and it represents a typical gas mixture containing both early thermogenic gas and biogenic gas. In contrast, significant gas loss was observed in both the high thermal maturity Barnett Shale in the Fort Worth Basin and the overmature Silurian Longmaxi Shale in the Sichuan Basin as compared to each the estimated yield of generated gas. This phenomenon is probably caused by either petroleum expulsion or tectonic dissipation, or coupling of two processes. As for the Silurian Longmaxi Shale, it is estimated that approximately 60%–90% of total generated gas in quantities has been lost in the overmature Silurian Longmaxi shale gas system where industrial gas flow had been achieved. This provides an upper-limit measure to shale gas loss caused by petroleum expulsion and tectonic dissipation throughout geological time.