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.
During the sedimentary period of the Ordovician Yingshan Formation, the carbonate platform of the Gucheng area in the Tarim basin was characterized by a distally steepened ramp. Relative sea-level changes exerted a strong influence on the shoal facie dolomite reservoirs of the 3rd Member of the Ordovician Yingshan Formation (the Ying 3 member), sedimented in the context of a shallow water environment on the carbonate ramp. However, previous studies that lacked high-frequency sea-level changes in the Gucheng area prevent further dolomite reservoir characterization. The current work carries out systematic sampling based on the continuous core from the upper and middle parts of the Ying 3 member in two newly drilled exploration wells (GC17 and GC601) and a series of geochemistry analyses, such as C-O isotope, Sr isotope, and rare earth elements (REE), which helps to investigate the features of the shoal facies dolomite reservoir development against high-frequency sea-level changes. With the help of Fischer plots of these two wells, high-density δ13C data (sample interval is about 0.272 m) were merged to construct a comprehensive curve, contributing to characterizing the high-frequency sea-level changes of the upper and middle parts of the Ying 3 member in the Gucheng area and validating the relationship between the pore-vug vertical distribution and high-frequency sea-level changes. Results revealed that the porosity of dolomite reservoirs increased when the high-frequency sea-level fell and decreased when it rose. Furthermore, the karst surface can be found at the top of the upward-shallowing cycle during the high-frequency sea-level falling; the pore-vug reservoirs are concentrated below the karst exposure surface, and porous spaces are more developed closer to the top of the cycle. The high frequency sea-level curve built in this study can be used as a standard for further research of regional sea-levels in the Gucheng area, and this understanding is highly practical in the prediction of shoal facies carbonate reservoir in carbonate ramp.
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