Study On Reservoir Features And Fracture Of Yanchang Formation, Triassic In The Central Region Of Ordos Basin

Ordos Basin, located in the middle-west of China, is a polycycle sedimentary basin after evolution for a long period. The central region of the basin is the main production area of Mesozoic petroleum accumulations, and Yanchang Formation characterized by low permeability, low porosity and well-developed fractures is the main pays. Based on the previous research, and with several methods such as field investigation, core observation, thin section analysis, logging data etc, features of sedimentation, reservoir and it's fractrures of the oil bearing series of Yanchang Fm., upper triassic, in the central region of the Ordos Basin are studied deeply.Reservoir mainly consists of feldspathic sandstone, feldspathic lithic sandstone in the central region. And diagenesis of it, including compaction, cementation, metasomatism and dissolution, primarily in middle stage of diagenetic A~B, during which fissure develops easily. Different types of pores, such as macro pores, micro pores, mixed pores, intergranular remainder pores and intergranular dissolved pores, have been composing the main room of oil & gas accumulation together. The reservoir property grouped into the one with low porosity and low permeability is not good enogh entirely in this region. macro heterogeneity controled by sedimentary facies and diagenesis is obvious.Tensile fractures of high-angle develops well within the reservoir of Yanchang Fm. in the central region. The strike of them mainly extend from NE to SW, secondly from NW to SE; and a set of fractures, whose development degree inferior to the former, trend approximately from north to south and from west to east. The opening extent of all the fractures distribute from 60-180μm, the line density is between 0.052bar/m-0.44bar/m. respectively. A part of fractures is kept full-filled or semi-filled by calcite and mudstone. It is evidence that the development of fracture is in accord with regional tectonic background of the whole basin. According to the cutting relationships, all the fractures have been formed by two stage of tectonic movements: different stress directions during the forming of fractures are coincident with tectonic stress fields of Yanshanian and himalayanian. The asymmetric distribution of all fractures is mainly effected by lithology, layer thickness, basement fault. After fracures studied, it is proved that the effect of fracture toward reservoir mainly on improving permeability of reservoir, secondly on enlarging the accumulation. On the basis of oil-water displacement experiment, it is thought that fractures have effect on oil displacement efficiency and play an improtant role on waterflooding. Based on the data about tectonic evolution of the basin, stress feature, and favorable zone of oil reservoir together, tectonic fracture was preliminarily predicted.