The Mechanism and Enrichment Mode of Tight Sand Oil Formation in the Southwest Ordos Basin

area which provides a large amount of hydrocarbon for the large area tight sand oil formation, the delta tight sand widely developed next to the source rock vertically which provides a large amount of reservoir space for the large area tight sand oil formation, and hydrocarbon generation synchronizes with reservoir rock densification process, furthermore, the fault and fracture system which formed during the late Yanshan tectonic movement connected the source rock with reservoir rock, all of these provide the favorable conditions for a large area of tight sand oil formation. The anatomical results of typical hydrocarbon enrichment zones show that high-quality source rocks are in control of oil and gas enrichment, the favorable sand bodies are in control of oil and gas distribution, and the physical properties of reservoir rock are in control of hydrocarbon enrichment, the North-east-east fractures are in control of oil enrichment and high yield. Based on all the above, this paper establishes tight sandstone oil enrichment mode of the southwest Ordos basin edge, that is, "the source rock was next to the reservoir rock, the pressure difference between the source rock and the reservoir rock served as oil-filling driving force, the source rock and the reservoir phase jointly controlled tight sand oil formation, the reservoir phase and the fault-fissure jointly controlled tight sand oil enrichment ". The high quality source rock widely developed in the area which provides a large amount of hydrocarbon for the large area tight sand oil formation. In recent years, the most scholars agreed that the tight sand oil of Ordos Basin mainly comes from Zhangjiatan shale which often developed in the bottom of the Chang 7 formation other than the Chang 7 formation dark mudstone. Due to the block data limitations, many scholars agreed that Zhangjiatan shale had bad development, preservation and evolution condition in Zhenjing and Bingchang blocks which were located in the southwest edge of the Ordos lake basin, But in recent years the Sinopec`s drilling results indicate that the Zhangjiatan shale are more developed in the blocks of Zhenjing and Binchang which also had high abundance of organic matter, hydrocarbon generation potential, moderate maturity and provided large amount of hydrocarbon for the large area tight sand oil accumulation in these areas. Among these, the Zhangjiatan shale widely distributed in Zhenjing block, in which the shale is more thicker in the northeast part close to the sedimentation centre of the lake basin and is gradually thinning towards southwest part, the thickness of the shale is about 8 to 16 m, and average 10 meters (see Fig 1, black line in Figure 1 was Zhangjiatan shale thickness contour which is on behalf of 10 m). The features of Zhangjiatan shale in Zhenjing block are high amount of potential hydrocarbon, high S2 / S3 Type Index (the value of most samples was larger than 40), high hydrogen index, for short IH (mainly distributed between 200 and 400 mg/g) and low hypoxic index, for short Io (the value of most samples was less than 5 mg/g), all above indicated that the type of organic materials was mainly as type I; The average value of TOC (Total Organic Carbon) equaled to 8 percent, the value of Ro(vitrinite reflectance) ranged from 0.7 to 1.0 percent, the value of S1+S2 equaled to 53.13 mg/g, all above data showed the Zhangjiatan shale was high-quality source rocks with high abundance of organic matter and high hydrocarbon generation potential. Zhangjiatan shale in Binchang block distributed as well as that of Zhenjing block, which was thicker in the northeast part and became gradually thinner towards southwest, the thickness ranged from 12.5m to 22.5m, with an average value of thickness equalled to 15 meters. The Zhangjiatan shale in Binchang block possessed the similar geochemical characteristics with that of Zhenjing block, namely, high organic matter abundance, mainly as type I, high hydrocarbon generation potential, so it was a set of high-quality source rocks. YIN Wei, CHEN Chunfang, XU Shilin, ZHANG Ying, QIU Guiqiang, MA Liyuan and XU Mei’er, 2015. The Mechanism and Enrichment Mode of Tight Sand Oil Formation in Southwestern Ordos Basin. Acta Geologica Sinica (English Edition), 89(supp.): 175177.