Characteristics and Geological Implications of Lacustrine Carbonate of Nantun Formation in Hailar Basin

Abstract Globally, lacustrine carbonate rocks are often found in rift lake basins. Because they often do not form large-scale conventional or unconventional reservoirs, they have not received enough attention in previous oil and gas exploration. Recent years, exploration examples in China show that lacustrine carbonate has close relationship with unconventional oil and gas. Mudstone with carbonate deposits always has a good exploration effect in the area. However, the genesis of lacustrine carbonate is still controversial. The sediment environment difference between carbonate and mudstone and its effects on unconventional hydrocarbon accumulation are not yet clear. This paper focuses on lacustrine carbonate found in the Nantun Formation in Hailar Basin. Through the analysis of lithofacies, geochemistry, and logging data, several findings have been obtained in the current study. (a) Two kinds of lacustrine carbonates, micrite dolomite and marlstone, which represent semideep to deep lake environments, have been identified in the research area. (b) Lacustrine dolomite (LD), whose genesis may relate to magmatic movement and deep hydrothermal processes, shows an extremely dry, hot paleoclimate and a saline, anaerobic ancient water condition. (c) Carbonate deposition in mudstone is related to the condensation section, and marlstone and micrite dolomite represent the beginning and end of the condensation section, respectively. Compared with mudstone around carbonate rocks, LD represents a drier, hotter, and saltier environment, which is of significance for sedimentary environment identification. In addition, the mudstone with lacustrine carbonate deposits has better organic matter characteristics and can form good unconventional oil and gas reservoirs that are of great significance for further exploration.