Depositional System Of Chang 6&7 Oil Formation Of The Triassic Yanchang Formation In Ordos Basin

In this paper, Chang 6&7 oil formation of Triassic Yanchang Formation in Ordos basin is elected as the object of study, using advanced theories of sedimentology, geophysics and geochemistry, based on outcrops and core observations, combining with drilling and logging data, the source directions, microfacies types, sedimentary system characters and their planar distribution rules is systematically researched. Taking the oil layer group as a mapping unit; and adopting the single factor analysis and multifactor comprehensive mapping method to protray the individual well histogram, well-tie profiles contrast map and planar distribution map of the sedimentary facies. At the same time, sedimentary evolution history is resumed, sedimentary pattern is established and the controlling factors of sedimentary systems over oil reservoirs are finally discussed.The general stratum thickness of Chang 6&7 oil formation in Ordos basin is about 150 to 260m, which were thicker in the north and thinner in the south. The thickest stratum lay mainly in the range of Wangpanshan, Wuqi and Luochuan, with stretching direction of from northwest to southeast and thinning towards each side of northwest-southeast trending axis.The study area was mainly affected by four sedimentary source regions, northeast, southwest, northwest and south. Among them, the major proveniences were northeast and southwest trending. The source rocks of northeast proveniences were the Archaean high grade metamorphic rock coming from Daqing and Yin mountains, while southwest source rock were the medium-low grade metamorphic rock, igneous rock and some carbonatite coming from Longxi ancient land. Alxa ancient land in the northwest and Qinling in the south were also the proveniences.Controlled by source directions, two sedimentary systems-northeast and southwest were developed of Chang6&7 oil formation in Ordos basin. The meandering river delta sedimentary system was mainly distributed in northeast portion, and the braided river delta sedimentary system in southwest portion. Meanwhile deep-lake turbidite fan was developed in the centre of the lake basin. The distribution pattern of sand bodies, extending in the linear or pedate way, was controlled by sedimentary facies.The development features of sedimentary systems were closely related with the evolution of the basin. The Chang73 Age was the peak period for the lake basin, the area of deep lake was more than 6 X 104km2. From the age of Chang73 to Chang71, lake transgression weakened gradually, therefore the area of the lake dwindled accordingly. The deltas around the periphery of the basin began to prograde to the centre of the lake; and the area of the turbidite sandbodies expanded gradually. Since Chang63 Age, the basin's subsidence rate has decreased while sediment supply rate has increased, which led to the suppression of the lake and the intensive progradation of the deltas. The turbidite sandbodies coming from different provenances superimposed in the central lake, and extended widely. From the age of Chang63 to Chang61, the area of deep lake dwindled, the turbidite shrank to the central lake accordingly. The contiguous distributed turbidite sandbodies disintegrated into many small-sized ones gradually.Oil reservoirs were controlled by sedimentary systems. The deep lake sediments of Chang7 formation supplied the reservoir-forming with high quality source rocks; meanwhile the delta front subaqueous distributary channel and turbidite sands in the central lake provided high quality reservoirs for the oil reservoir-forming.