Characteristics of breccia lava reservoirs and analysis of their dominant factors: case study of the Xushen gas field located in the Songliao Basin

Breccia lava is a type of rock in transition between volcanic clastic rock and lava. Its physical properties are compara-tively special, while the nature of its genesis and the formation of reservoirs, deposits and concentrations vary considerably. In order to discover the different genesis for breccia lava reservoirs, our study has been carried out on breccia lava for its rock constituents, structures, diagenesis and reconstruction from tectonic movements to mineral depositions. From our analysis, we conclude that the main type of reservoir is a secondary emposieu, where rock compositions, structures, diagenesis and tectonic action are the major contributing factors to the deposition of breccia lava and its properties. Diagenesis constrained the extent of conservation of the primary pore of breccia lava and produced secondary porosity. The roles of tectonics were played out through the formation controlling tectonic fissures inside breccia lava and affected the impermeability of breccia lava fluids. The constituents of lava rock determine directly the type of diagenesis and its response to tectonic action. At the time of appropriate rock composition and structures, constructive formation occurred to breccia lava. Breccia lava also responded proactively to tectonic movements, forming the best quality collective reservoir agglomerations of vast spaces and strong permeability.

Deformation Mechanism and Vertical Sealing Capacity of Fault in the Mudstone Caprock

The petrophysical property of mudstone often transforms from ductile to brittle in the process of burial-uplift. The deformation mechanism of fault in brittle and ductile mudstone caprock is different, which leads to the formation of different types of fault zone structure. Different methods are required to evaluate the sealing mechanism of those fault zones. Based on the caprock deformation mechanism, fault sealing mechanism, quantitative evaluation method of vertical fault sealing capacity is put forward in this study. Clay smear is formed in the process of plastic deformation and its continuity controls the sealing capacity of fault. The outcrop and oil field data have confirmed that when sealing parameter SSF is less than 4–7, the clay smear becomes discontinuous and then oil and gas go through the caprock and migrate vertically. Quantities of fractures are formed in mudstone in the process of brittle deformation. The fracture density increases with the increase of the fault displacement. When the fractures are connected, oil and gas go through the caprock and migrate vertically. The connectivity of fault depends on the displacement and the thickness of caprock. On the basis of the above, a method is put forward to quantify the connectivity of fault with the juxtaposition thickness of caprock after faulting. The research on the juxtaposition thickness of caprock after faulting of the member II of Dongying Formation in Nanpu depression and the distribution of oil and gas indicates when the juxtaposition thickness of caprock is less than 96.2 m, the fault becomes leaking vertically. In the lifting stage, with the releasing and unloading of the stress, the caprock becomes brittle generally and then forms through going fault which will lead to a large quantity of oil and gas migrate vertically.