Petrology of sandstone reservoirs in Longdong Area, Ordos Basin

The Yanchang Formation of Triassic System of Longdong Area in Ordos Basin is the main productive bed of this area, which is characterized by low porosity and low permeability. Rock properties and the law of its distribution are studied in this paper, based on core description, thin section study and quantitative statistics. Technical analysis and measurement such as X-diffraction analysis, scanning election microscope observation, cathodoluminescence are also used. The research shows that the main rock types of Yanchang Formation are lithic feldspathic sandstones and feldspathic litharenite sandstones with low compositional maturity. The maturity increases gradually from west to east. The grain sizes of the lower members such as Chang8 - ChanglO and the upper members, Changl-Chang4 + 5 are coarse, and the lithology of these members is mainly composed of medium-fine grained sandstones; while the particle sizes of Chang6-Chang7 in the middle members are relatively fine, and the lithology is mainly composed of siltstones. The debris content is high and the types of debris are various. They include mudstone, dolomite and other sedimentary rock debris; the volcanic rock, granite and other magmatic rock debris; and slate, phyllite, schist and some other metamorphic rock debris. The debris type of Chang8 - ChanglO oil beds is mainly volcanic rock. The type of Changl - Chang7 is dolomite debris and the content is high in the west than that in the east. Metamorphic detritus exists for all the oil beds of Yanchang Formation, and the content of mica is higher in the east. By considering the roles of detritus and mica and the role of them playing in diagenesis, the detritus can be divided into two categories, the rigidity and plasticity, and the rigid debris can be further divided into soluble and insoluble, while the plastic debris can also be divided into two types, one is easy to be altered, the other is easy to be deformed. The above research is important for the subsequent researches such as the evolution of Ordos Basin, provenance analysis, diagenesis and porosity evolution in the Upper Triassic Yanchang stage.