Hydrocarbon Saturation Evaluation of Volcanic Reservoirs Using MICP Experiments, Petroleum Migration and Accumulation Dynamics, and NMR Logging

Hydrocarbon saturation is a critical parameter to estimate from well logging interpretation, directly influencing the net pay thickness. In conventional clastic reservoirs, hydrocarbon saturation can be accurately obtained using the Archie's formula or its variations. However, volcanic reservoirs of the Bohai Sea, formed during multiple eruption periods, exhibit significant lithological variations, complex pore structures, and intricate accumulation patterns, often accompanied by reservoir overpressure. These characteristics render the Archie's formula and saturation height modeling methods based on Mercury Injection Capillary Pressure (MICP) less reliable for saturation estimation. Thus, it is a challenge to figure out the mechanism of hydrocarbon accumulation and accurately estimate the hydrocarbon saturation. This study collects MICP data from over 143 core samples obtained from more than 11 exploratory and appraisal wells drilled into the Bohai Sea Mesozoic volcanic formations over the past two years. The MICP experiments shows that the hydrocarbon saturation ranges from 15% to 35% when the hydrocarbon column height is between 100 m and 500 m in the study area, which obviously contradicts to the high productivity observed in DST tests. The pressure analysis of formation testing suggests that the reservoirs are overpressured, with a pressure coefficients ranging from 1.4 to 1.6. Comprehensive pressure and geological analysis indicate that the hydrocarbon accumulation of volcanic reservoirs in the study area are not only buoyancy driven, but also overpressure injected during the accumulation period. Based on this understanding, this study proposes a novel method to calculate hydrocarbon saturation of volcanic reservoirs of Bohai Sea. Firstly, we fully examine the MICP experiments of the volcanic reservoirs together with the macroscopic dynamics of petroleum migration and accumulation. Then we comprehensively analyze the influence factors of hydrocarbon saturation variations and propose the saturation model. The mercury injection saturation determined using the saturation model is converted to oil/gas saturation and used to calibrate the NMR logging at each sample depth. Finally, we calculate the hydrocarbon saturation of the whole logging interval using the calibrated NMR logging. Analysis and calculation of hydrocarbon saturation in volcanic reservoir of the LK Structure of Bohai Sea shows that when the NMR logging T2 cutoff values of volcanic reservoirs in the Bohai Sea Mesozoic formations range between 10 ms and 15 ms, the hydrocarbon saturation calculated from NMR logging is consistent with the hydrocarbon saturation determined from the MICP experiments, which further validates our method.