Introduction
Structural fractures play an important role in developing trap space or/and conduit channels for hydrocarbon in reservoir rocks (Cao, et al., 1999; Wu, et al., 2006), as well as enlarging and/or connecting the pre-existing fissures and cracks to improve both the porosity and the permeability of the rocks (Zeng, et al., 2007). According to many researchers (e.g. Chen et al.,2001), oil and gas from fracture-related reservoirs account for over 50% of the total hydrocarbon production in the world. Thus, more attention should be paid to fracture-related reservoirs in petroleum exploration.
Figure 1. Structural units of the Liaohe basin
a. Structural units of the Liaohe basin and the location of the study area (Modified after Gu, et al. 2002); b. Distribution and lithology of the volcanic rocks (Es3) in the study area (derived from well data and seismic interpretation from Liaohe Petroleum Company, unpublished data). O8 is the crater of the volcanic eruptions during the period of Es3-the third member of Shahejie Formation, Eocence. Trachyte is the main rock type that bears the potential to be reservoir.
Extensive work has been done by employing finite element methods (FEM) to simulate the stress field and predicted fracture development [e.g. Wang et al. (1999), Maerten L. and Maerten F. (2006), and Maerten et al. (2006)], unfortunately forgetting the fact that strain energy also plays a significant role in producing fractures. A very few of researchers [e.g. Tromans (2008)] discussed the strain energy required to produce new fracture. Wang et al. (1999) proposed that "Crack initiation from a flow occurs when both stress and energy criteria are fulfilled simultaneously," which is better because both the two factors (stress and strain energy) are taken into account. Bio-Factor Method is just based on this concept and is employed to predict structural fractures in volcanic rocks in Olituoz area, Liaohe Oilfield, Northeast China.
The traditional concept of the reservoir has been challenged by the discovery of fracture related volcanic reservoir in Olituoz in 1997 when well O26 started producing oil at a rate of 150 t/day (Zhang et al., 2005). Many researchers have studied the formation mechanism of this unusual reservoirs (Gu et al., 2002; Wu et al.,2006). According to them, the hydrocarbon occupies mainly in trachyte porphyry of Es3 Formation (the third member of Shahejie Formation, Eocene), and the trapping spaces are with fractures, such as cryptoexplosive breccia, structural fractures and primary fractures. Among these fractures, structural fractures play a key role. It is meaningful and helpful to predict structural fractures in volcanic trachyte reservoirs (Es3) of the Olituoz area, for the exploration of the volcanic reservoirs.