Geological controls on the pore system of lacustrine unconventional shale reservoirs: The Triassic Chang 7 member in the Ordos Basin, China

Abstract

The complex pore system of lacustrine organic-rich shales poses challenges to the assessment of shale oil and gas resources. To better understand the pore system and its controls of the lacustrine unconventional shale reservoirs, 12 oil mature shale samples with varying organic matter contents from the Triassic Chang 7 Member in the Ordos Basin were analyzed using elemental, petrographical, petrophysical, and fractal methods. The results showed that macro- and meso-pores are the main pore space in the lacustrine Chang 7 Shale. Pore diameter peaks are dominantly distributed around 0.7 and 37 nm. The intragranular pores in clays and intergranular pores between organic matter (OM) and minerals are well developed. Whereas the pores in OM are rarely observed. The fractal dimensions of the lacustrine shale's pore structure suggest that D2 is better than D1 in reflecting pore heterogeneity. OM shows negative effects on the total pore volume, while quartz promotes the development of macro- and meso-pores. Total pore volume shows an oscillation with clay mineral content (pore volume increases first up to 45% clay content and decreases thereafter). Both quartz and clays show negative effects on the heterogeneity of meso- and macro-pores, and positive effects on the micro-pore heterogeneity. A new critical maturity point of 0.8% Ro is observed for the change of the pore system of the lacustrine shale reservoirs during the early oil window. Both the fractal dimensions (D1 and D2) and pore structure proxies show the inverse trends before and after this critical value. These observations are probably controlled by the compaction processes coupled with hydrocarbon generation and expulsion, which helps better understand the pore system evolution in the lacustrine shales. The positive role of quartz, 45% clay mineral content and 0.8% Ro as the critical points contribute to the high-quality reservoir or sweet spot identification in lacustrine shales.