Lithological and physical characteristics and their controlling factors of the Archaean metamorphic rocks: A case study of the Damintun Sag, in Liaohe depression of Bohai Bay Basin, China

The metamorphic rock hydrocarbon reservoir, located in Damintun Sag, Liaohe Basin, China, is one of the earlies reservoirs in buried hill exploration and development. An Archean metamorphic rock with deep burial adjacent to the Rongshengpu depression in the south of the sag still has great potential for the oil and gas exploration. Based on cores, logs, seismic data, and laboratory measurements, the lithology types, spatial distribution, physical characteristics, lithology controlling factors， and physical properties of the Archean metamorphic buried hill reservoirs are systematically analyzed. The result shows that the lithology of Archean metamorphic rocks in this area can be divided into three types and 19 species, including migmatitic granite, migmatitic gneiss, migmatite, plagioclase amphibole gneiss, leptite, amphibolite, granulite, inhomogeneous basic dike rock, and cataclasite,etc. The metamorphic rock reservoirs in the study area are composed of multiple lithological sections vertically (from bottom to top), ranging from migmatitic granite to gneiss. granulite. and top weathering crust, different types of lithological assemblages can appear. The characteristics of lithology in vertical are comparable, and the lithology is also partitioned from the horizontal view. Metamorphic rock reservoirs can be divided into two types according to their location and physical properties: weathering crust type and inner-type. The former is dominated by dissolved pores and dissolved fractures, while the latter is controlled mainly by structural fractures. Weathering & leaching, rock and mineral components, and tectonic movement control the Archean metamorphic rock reservoirs. Weathering crust reservoirs are controlled mainly by weathering & leaching and lithology types, while the inner reservoirs are controlled by tectono-genesis, lithological assemblage and petrological evolution. Metamorphic rocks with high content of felsic minerals in inner-type reservoirs are likely to be cracked; the storage properties of rocks depend on the content variety of dark minerals, and the appropriate amount of dark minerals is conducive to the formation of fractures. Clastic migmatitic granite and migmatitic gneiss, leptite, and some dynamic metamorphic rocks could be regarded as high-quality reservoirs.