Quantitative modelling and application of volcanic facies of Lower Cretaceous Yingcheng Formation of the Songliao Basin
松辽盆地营城组火山岩相量化表征与应用

Seismic interpretation of volcanic rocks and corresponding reservoirs in petroliferous basins need to be disciplined by quantitative geological models. Three-dimensional sections outcroped on basin margin provide us with the best natural lab for us to construct quantitative models of volcanic facies. 3-D quantitative modelling of the major volcanic facies including explosive, effusive and extrusive has been carried out in the stratotype succession of Yingcheng Formation (K1y) of southeast uplift of the Songliao Basin. Explosive facies were developed in the early volcanic eruption cycles. Lateral-extension and vertical-thickness of the explosive facies is some thousand and several hundred meters respectively. They are wedge-shaped with a steeper upper slope and gentler lower slope showing an average slope angle of 15°. Successions of the explosive subfacies, from bottom to top, include base surge deposits, interbedding base surge and air-fall, and pyroclastic flow deposits. Effusive facies generally occured in the middle of an eruption cycle and covered on top of the explosive facies. They can extend up to some thousand meters and get a thickness up to several hundred meters. They are lenticular-shaped and pinch out laterally, with a gentler upper slope and steeper lower slope showing an average slope angle of 20°. Successions of the effusive subfacies from bottom to top include lower, middle and upper units. Extrusive facies generally occured at the end of an eruption cycle and are composed of inner, middle and outer units. They often cut through and cover other lithofacies. Their stretches and thickness are usually no more than 1km. They are mushroom-shaped with a large flat roof and sharp border. Applying the above volcanic facies models to borehole-pass-by seismic interpretation, we established the respondence between volcanic facies and seismic information, with the help of which we may understand the meanings of seismic profiles of non-well areas. Erosion of a volcanic edifice will destroy parts of its facies but can not change the facies sequence, so that, it will not disturb the application of the facies models mentioned. On the other hand, if a volcanic body is composed of materials derived from different sources, called superimposed volcanic ediffice, application of these facies models may become problematic.