No abstract

The current study was carried out in one of the most economically viable gas-bearing reservoirs in Japan, the Minami-Nagaoka volcanic field, located to the southwest of Nagaoka. Petroleum deposits occur in deep-seated underwater volcanic rocks of Miocene age, and these deposits contribute significant amount of Japan's total natural gas production. Because of the steep rise in domestic gas demand, re-examination of the existing data sets was performed to understand the geological reason for the extreme lateral facies variation, the complex facies distribution pattern, and production uncertainties. The study revealed a very interesting geological phenomenon of multiple eruption vents that is responsible for the complex facies association and distribution pattern within the study area. A robust workflow was designed through maximizing utilization of existing high-resolution datasets to solve the current challenges in the field. At the beginning of the workflow, different complex underwater volcanic facies were recognized in image, core, and log data. After volcanic facies reconstruction in each well, facies associations and their distribution pattern across wells were visualized. Finally, using background geological knowledge and well-based facies identification and correlation, a model with multiple volcanic eruption vents was conceptualized within the study area from north to south. In addition, the probable location of different drilled wells relative to the location of the multiple volcanic vents was defined based on the facies assemblages of each well. The present work demonstrates a unique way to conceptualize and model the paleovolcanic eruption events within a volcanic field. Understanding the paleovolcanic eruption model is key for proper understanding of the facies distribution pattern and facies variation across wells and for finding the best drilling location for hydrocarbon exploration wells. It was observed that the facies with a high amount of lava are located close to the volcanic vent, and wells with a higher amount of brecciated facies are located farther from the volcanic vent. Another type of facies, which is unique for underwater volcanism, is the hyaloclastite (glassy type) facies; this facies is located towards the margin of the volcanic vents. In addition, lateral facies variation was clearly understood after visualizing the eruption model, which was no possible prior to the present study. The current workflow demonstrates a novel approach for better understanding the paleogeological condition for a unique type of gas-bearing submarine volcanic reservoir, which is a very rare example in the present oil and gas industry. The technique in this work represents a new way of looking high-resolution well data for geological modeling and evaluation of any volcanic field.