The Development Mechanism of the Secondary Pores in the Reservoirs of the Down-Faulted Basins of the Late Mesozoic Era in the Northeast of China

Key words: 
• down-faulted basin /
• reservoir secondary /
• pores

Abstract: The secondary pores account for a great proportion in the late Jurassic to the early Cretaceous (J₃-K₁) reservoir. The main types of secondary pores in the reservoirs are divided into four types as follows: inter-granular pores, intragranula pores, moldic pores, intracement pores and fracture and so on. The development mechanisms may have four kinds: the first one relates to the thermal evolution of organic matter in mudstones. R.C.Surdam and T.J.Crossey's experiments indicate that the thermal degradation makes kerogens seperate from the outer oxygen-functional groups such as carboxy groups, keto-groups, phenol and so on to produce a large amount of organic acid before it goes into the oil windows. When temperature arrives at 80℃—120℃, the consistency of the organic acid is in the highest degree. The secondary pores are developed after the organic acid solved in the water removing with the pore fluids from the source rocks, and entered into the sandstone reservoirs, and dissolved the unstable elements. This process mainly occurs at the stage of low maturation to maturation of the organic matter. Most of the secondary pores in our area firstly developed at A period of the late diagenesis stage and secondly at B period of early or late diagenesis stage, relating to the thermal evolution of the organic matter, and being identical with the expelling stage of organic acid. The second one relates to the infiltration of the surface and subsurface water. This can be seen in the north of Wuerxun down-faulted depression and Majiapu buildup of the Luxi depression. The distribution was directly controlled by the deposition break, also related to the composition of source region. The conditions of the reservoir of arkose derived from part of the granite are better than the lithic sandstone or lithic subarkose from volcanic rocks or sedimentary rocks. When the buried depth is deep enough, the porosity and permeability will be improved because of the seperating of organic acid and piling up of the secondary pores. The third one relates to the late volcanism. the volcanic movement makes the geothermal gradient rise and so it accelarates the speed of organic matter evolution of the source rocks and sandstone diagenesis. Therefore, we can hope to look for the development of the secondary pore zones in shallow depth,to develop a favourable reservoir with the primary pores piling up in the shallow depth. Volcanic eruption or magma intrusion are always accompanied by the hydrothermal movement. On the one hand, the hydrothermal movement made the veins inject the inter granular pore spaces and reduce the porosity and permeability of the sandstone greatly, on the other hand, the CO ₂ given off by the hydrothermal liquid might be the factor of the development of the secondary pores in the deep buried zones with high geotemperature The forth one relates to the tectonic movement and diagenesis. The low porosity and permeability may be the main characteristics of the reservoirs in the late Jurassic to the early Cretaceous down-faulted basins in the northeastern China. The fissure plays an important rol in the reservoirs, especially in the compacted sandstone reservoirs and some special reservoirs, and become not only the effective paths of fluids but accumulative spaces as well. According to the fissure occurence, the fissures can be divided into microfissures and macrofissures, and by the origins, the fissures may be divided into tectonic fissures and diagenetic fissures.The tectonic fissures mainly possess a certain direction, penetrating (or not) the strata and extending faraway, and the fissure planes may be flat and straight.The observed diagenetic fissures of core are mainly irregular, and in the shape of the root of a tree. From the observation of a thin section, the fractur system appears multidimensional, including the grain fissures formed in the c