Characteristics Of Chang 7 Reservoir Formation In Hujianshan Area Of ​​Ordos Basin

Hujianshan area is located in the central and western regions of Yishan slope, which belongs to the sub-tectonic units of Ordos basin. The development of the lake basin reached its culmination in the sedimentary period of the Chang7group of Yanchang Formation of Triassic, and it belongs to meare-abyss sedimentary environment, which developed a set of high-quality source rocks. Chang7layers have been regarded as a source rock, which generally has a poor physical property and belongs to typical tight oil reservoir. With the development of tight oil exploration, commercial oil flow wells were found in Chang7. Based on the research of the reservoir characteristics, we analyzed the effect of hydrocarbon generating characteristics of source rocks in different layers and fuel sources on dense reservoir features, accumulation period, accumulation time, accumulation of power and migration pathways. Accumulation mode of tight oil was built, while distribution of the reservoir and its controlling factors were clarified.The content of feldspar was the highest of the component of sandstone clastics followed by quartz with high content of interstitial material mainly includes kaolinite, hydromica, chlorite and ferrocalcite, etc. The general class of sandstone reservoir is fine sandstone with good sorting and sub angular rounded, mainly belongs to porous cementation.The type of pores mainly are intergranular pore, intergranular dissolved pore, intragranular dissolved pores and a small amount of intergranular pores and fissures, which are very narrow, The reservoir porosity and permeability of Chang7mainly distributed in6%-12%and (0.05-0.3)×10-3μm2, respectively, where the reservoir properties of Chang71are better than Chang72.Through evaluation of source rocks and correlation of oil-source about Chang7reservoir group, it is observed that Chang71, Chang72and Chang73have the characteristics of hydrocarbon supplying ability of source rocks. The crude oil and extracts of this area are basically the same yellow-green fluorescence. The homogenization temperature frequency distribution of fluid inclusion showed a single peak, indicating that which is the product of first stage accumulation. By analyzing the maturity of crude extract and fluid inclusions characteristic of the reservoir, Chang7in Hujianshan area is the first phase product of reservoir and accumulation period for the Early Cretaceous. It is considered that hydrocarbon accumulation in the reservoir occurs mainly after densification by observing the fluorescence chip of reservoir. In the process of hydrocarbon accumulation, rock capillary force causes the resistance of hydrocarbon migration, however, flat the layer of work area is, made the buoyancy insufficient to overcome the capillary force, hence, abnormal pressure and Hydrocarbon generation pressurization generated by under compaction effect is the power of hydrocarbon accumulation. The geologic structure of the Chang7in Hujianshan area is stable and fossil erosion surface and fault zone are undeveloped. Continuous superimposed sand bodies and fractures are the main channels for hydrocarbon migration. There are two types of accumulation modes, which are Vertical migration and short-distance lateral migration.According to the exploration results, the enrichment of oil&gas in Chang72is obviously better than that in Chang71, while it turns to be bad in Wangwazi-Xinanbian. It has been proved that the mother rocks in Chang73, Chang72and Chang71can produce hydrocarbon because of the mudstone which can provide hydrocarbon for the reservoir. Among the mother rocks, Chang72owns relatively more abundant oil sources. However, it appears to be interbed between sandstone and mudstone in Chang71, at the bottom of which there is a set of thin and continuous mudstone, resulting in weak hydrocarbon producing ability and hindering the hydrocarbon from Chang72and Chang73. Thus, the hydrocarbon source of Chang71is strictly restricted, leading to small reserve. The physical properties of Chang72is better than Chang71, besides, it owns wide plane distribution which provides benefit for oil short-distance lateral migration and behaves better than Chang71in thickness, connectivity and vertical superimposition that is good for oil vertical migration and storage. These are the main reasons that Chang72behaves more effective than Chang71.By contrast to the properties of high productivity well and low productivity well, it is observed that high productivity wells share the performances of favorable physical properties, thick mudstone, abundant oil source, short distance for migration and without barrier bed; while in the low productivity wells or non-behavior wells, it is common to observe bad physical properties, thin mudstone or thick mudstone with barrier bed to hinder oil storage. The main pattern of reservoir formation in Chang7is vertical migration and short-distance lateral migration. Generally speaking, the properties of reservoir distribution are mainly controlled by through-going of source rock, physical properties of the reservoir, through-going of sand body and contact relationship between source rock and reserve.