Reservoir Properties of Lacustrine Deep-Water Gravity Flow Deposits in the Late Triassic–Early Jurassic Anyao Formation, Paleo-Ordos Basin, China

The development of gravity flow sedimentology has improved our understanding of the physical properties of different types of gravity flow deposits, especially the advancement of various gravity flow models. Although studies of gravity flows have developed greatly, the linkage between different sub-facies and their reservoir properties is hindered by a lack of detailed sedimentary records. Here, integrated test data (including thin-section petrology, high-pressure mercury injection experiments, capillary pressure curve analysis, and scanning electron microscopy) are used to evaluate links between different types of gravity flows and their reservoir properties from the Late Triassic–Early Jurassic Anyao Formation, southeastern Paleo-Ordos Basin, China. The petrological and sedimentological data reveal two types of deep-water gravity flow deposits comprising sandy debris flow (SDF) and turbidity current (TC) deposits. Both are fine-grained lithic sandstones and form low-porosity and ultra-low permeability reservoirs. Secondary porosity, formed by the dissolution of framework grains, including feldspars and lithic fragments, dominates the pore types. This secondary porosity is widely developed in the Anyao Formation and formed by reaction with organic acids during burial (early mesodiagenesis). The associated mud rocks have reached the early mature stage of the oil window with Tmax of 442–448 °C. Compared with the turbidites, the sandy debris flows have higher framework grain content (87.9 vs. 84.8%), framework grain size (0.091 vs. 0.008 mm), porosity (6.97 vs. 3.44%), pore throat radius (0.102 vs. 0.025 μm), and permeability (0.025 vs. 0.005 mD) but are relatively poor in the sorting of framework grains and pore throat radii. The most important petrological factors affecting porosity and permeability of the SDF reservoirs are framework grain size and feldspar grain content, respectively, but those of the TC reservoirs are feldspar grain content and the maximum pore throat radius. Diagenetic dissolution of framework grains is the most important porosity-affecting factor for both SDF and TC reservoirs. Our multi-proxy study provides new insights into the links between gravity flow sub-facies and reservoir properties in the lacustrine deep-water environment.