Characteristics and formation mechanism of the Cretaceous intermediate and mafic volcanic reservoirs in Songliao Basin, NE China.

Numerous intermediate and mafic volcanic rocks of the Cretaceous Yingcheng Formation (K(1)y) are revealed by drill holes in Anda-Wangjiatun area of northern Songliao Basin and have been yielding commercial gas productions. Eight lithology types are indicated by alkali versus silica (TAS) diagram and they are further categorized to six reservoir rock types. Basalt and trachyte provided the most of effective reservoirs that amount to over 80% of the total. This paper summarizes four categories and eleven subcategories of reservoir spaces and their characteristics, considering that primary pore spaces and fractures are determined by volcanic eruption types and emplacing environment while their distribution was controlled by volcanic facies, and secondary porosity and permeability are effected by in-filling, dissolution, weathering, leaching and tectonic fracturing. Cooling units which are composed of single or several lava flow units have better reservoir properties in the top and bottom due to primary porosity distributions. Each of the cooling units behaves as a relatively independent reservoir unit. Favorable reservoirs are developed in the upper part of volcanic sequences, near vent. facies and proximal facies as well as adjacencies to faults. Basaltic lava is characterized as widely extended with length of 10 to 22km and thin layered with thickness of 7 to 39m for each reservoir unit, which indicates that petroleum exploration in basalt should firstly aim at the reservoir unit on top of volcanic sequences. Comparatively, trachytic lava is featured with short extension (3.1 to 6.3km) and thick layered (100 to 200m), so that exploration should tend to look for reservoir rock bodies with favorable source and seal conditions.