Lithology identification of complex volcanic rock, reservoir prediction with seismic data and identification of volcanic rock lithology and prediction of fluid property in reservoirs using wireline logging are key techniques for volcanic gas reservoir exploration. Up to now, many exploration wells and a large amount of 3D seismic are performed in volcanic gas reservoirs. Practice and theoretical research has establish a series of effective methods of lithology identification and reservoir and fluid prediction by seismic and wireline logging, with much achievement in gas field exploration evaluation and gas reservoir characterization of Xushen Gas Field.
Songliao Basin, located in northern China, is the largest and most prolific non-marine oil-bearing basin. It develops two sets of different Cretaceous formations. The lower rifted formation has alluvial fan to deep lake siliclastic sediments and coal beds, and widely developed volcanic rock. The upper depression formation is mainly fluvial-lucustrine sediments. Being a huge oilfield, Daqing Oilfield has oil reservoirs discovered in sandbodies in fluvial and deltaic facies of upper depression formation in 1959, and large gas reservoirs discovered in volcanic rock in lower rifted formations in 2002. Volcanic gas reservoirs have burial depth of 3000m~5500m. High pressured fluid history of the basin causes difficulty for tight glutinite in this depth to form gas reservoir. Compared with siliclastics, volcanic rock has not only variety of lithology types and pore types, but also little research result. Study on field crops and modern volcanoes process establishes structural patterns for different types of volcanoes. Based on core analysis data of large amount of volcanic rock, and combined with wireline logging and gas testing data, a series of exploration techniques including lithology identification and reservoir and fluid prediction are developed for volcanic gas reservoirs.
Difference of eruption periods and scales cause variety of lithology and complexity of depositional structures. Weathering and host rock also cause variety of seismic structures of volcanic rock.