The Characteristics And Reservoir Forming Pattern Of Jurassic Oil Sands In The Sourthern Of Ordos Basin

Oil sand is an important resource of energy after conventional oil. Nowadays, it hasrepresented a huge prospect for exploration and development. On the one hand, the oilsands resources are very abundant and widely distributed in China; on the other hand, weare still lack of the awareness of the geological background, resource potential and otheraspects for oil sands. Ordos Basin as an important energy base in China, has been provedrich in oil, natural gas, coal, uranium and other resources, while the exploration of oilsands resources in basin is still in a relatively blank slate. This paper relies on "Nationaloil sands mineralization vision and constituency research" and "Oil sands resources inNorthwest China accumulation potential research" from China Geological Survey Bureauof Oil and Gas resource survey Center. The main object of study is the Jurassic oil sandsof Zhiluo formation and Yan’ an formation in southern Ordos Basin.On the base of the survey of field geological, oil sands outcrops and drilled well,combine additional observation of microscope of oil sands; we clear the geologicalcharacteristics, distribution of oil sands in study area. We can study the source rocks forJurassic oil sands in Miaowan-Yijun region by collecting previous studies’ data andanalysis of geochemical testing laboratories on oil sands samples. We evaluate the oilsands reservoirs by observation of microscope and geochemical testing laboratories. Basedon the systematic study of oil sands deposits formation, temporal and spatial distributionof mineralization and host factors, we can establish the reservoir forming pattern ofJurassic oil sands in southern Ordos Basin.We found that Jurassic Yan’an formation and Zhiluo formation are widely distributedin study area, the strata expose the formation to270°-300°,dip15-25°, the main oil sandstone lithologies are feldspathic sandstone and feldspar quartz sandstone. usually,coarse-grained sandstones have better oiliness than fine-grained sandstones, the bestoiliness can up to oil immersion.Through the measured profiles, lithology combination of drill integrated histogram,mineral microscopy and particle sizes, we found that Zhiluo formation was mainly ofbraided river facies sediment and lacustrine sediment partly, it could further subdividedinto4kinds of subfacies: the floodplain, the san dam, the lakeshore and the shallow lake.Yan’an formation was mainly of meandering river facies and lacustrine sediment, it couldfurther subdivided into5kinds of subfacies: the floodplain, the channel, the semi-deeplake, the lakeshore and the shallow lake.Oil sands have sffered serious oxidation, weathering,and biodegradation.The main porosity types of oil sandstone bodies in study region are intergranularpores, intergranular dissolution pores, fissure, and intercrystalline micropores. Thereservoirs of Yan’an formation and Zhiluo formation are low porosity and lowpermeability reservoir through laboratory experimental test.There are several favorable reservoir conditions for the Jurassic oil sands insouthern Ordos Basin. By comparing the physical properties, carbon isotopes andbiomarkers, we found Jurassic oil sands crude mainly came from lacustrine source rock ofYanchang formation. The fluvial sandstones provide good reservoirs for crude oil. Thestructural contrl the formation of oil sands deposits.The process of oil sands accumulation is closely related to tectonic evolution inWeibei Uplift, Ordos Basin. The reservoir froming pattern belongs to tectonic uplift type.Triassic source rocks began to generate oil in the Late Jurassic, then the oil was migratedto Jurassic reservoir. The effect of Characterized by nappe made tectonic uplift in EarlyCretaceous, it caused large scale oil and gas accumulation. After the Cretaceous, the upliftand tilting movement of Weibei Uplift made the oil reservoir uplift to the surface, thengradually formed oil sands.