Evaluation of a Large Volcanic Gas Play in the Northern Songliao Basin, Northeast China

Luthi, Stefan M.¹, Li Guo Xin², Wang Yu Hua³, Yang Feng Ping³, Zhao Jie³, Jeff Meisenhelder⁴, Sherif Farag⁴, Yang Xing Wang⁵, Zhu You Qing⁵, Hou Hui Jun⁵, Zhang Shu Pin⁵, Wu Chuan⁵, Wu Jie⁵, Thomas J. Neville⁵, Michael Conefrey⁶, Huang Wei³ (1) Delft University of Technology, 2628 RX Delft, Netherlands (2) PetroChina, Beijing, China (3) PetroChina, DaQing, China (4) Schlumberger Oilfield Services, Beijing, China (5) Schlumberger Data and Consulting Services, Beijing, China (6) Consultant, Winchester, VA

Discovery of a gas accumulation in Lower Cretaceous volcanic rocks in the Songliao Basin, Northeast China, has led to a thorough evaluation of the prospect with the goal of assessing its economic viability. The gas is contained in rhyolitic pyroclastics and lava flows, and subordinately in the overlying conglomerates. The volcanic sequence structure forms a saddle along a fault where the volcanic vents were located. Trapping conditions are best described as combined stratigraphic-structural, since no full closure exists. Coal-bearing strata in the underlying sediments are considered the source rock. The volcanics consist in about equal parts of lavas and pyroclastics, and their vertical and lateral distribution patterns allowed reconstructing the sequence of volcanic events. Porosity is vesicular and fractured in the lava flows, and microporous, vuggy and leached in the pyroclastics. Petrophysical analysis indicated that the volcanics are throughout their entire thickness within a capillary transition zone, and that saturations have to be obtained from a saturation-height function above free water level as well as the capillary pressure curves from the various lithologies. A logging suite that includes geochemical logging, NMR measurements and borehole imaging has been found to be best suited for evaluating these unusual reservoir rocks. A semi-deterministic reservoir model was used to calculate reserves in-place and showed the accumulation to be economic. Field development has started in 2006 and shows promising results.