Study Of Sedimentary Microfacies In Fuyu Oil Formation In North Qijia Area Of Songliao Basin

Songliao basin is located in Northeast China which is a vast Mesozoic- Cenozoic basin., and covers the area of E119°40′～E128°24′, N42°25′～N49°23′and stretches in NNE direction, with length of 750km, width of 350km and total area of 260,000 km2. Songliao basin is divided into six first-order structural units including western slope region, northern plunge region, central depression region, northeast uplift region, southwest uplift region and southeast uplift region. The central depression region includes Heyupao sag, Mingshui terrace, Longlvpao-Honggang terrace, Qijia-Gulong sag, Daqing placanticline, Sanzhao sag and Chaoyanggou terrace.North area of Qijia is located in north Qijia-Gulong sag of central depression region in the north of Songliao basin. It is located in the urban of Da Qing and the Duerbote Mongol borough which belongs to Da Qing City, in Hei Longjiang Province。This area is adjacent to the east of Da Qing placanticline, The west of the area reaches Qijia oil field, the south border of the area is Lamadianmiao village and the north border is Zhangqi village belonging to Lidian town. The total area of North Qijia is 304.5 km2. Fuyu oil formation is one of main oil formation in this area and it belongs to forth segment of Quantou Formation in lower Cretaceous. We divide Fuyu oil formation into seven sublayers according to its lithology, electrical property and gas-oil characteristic and the three upper sublayers is the main oil sublayers.The main lithology of forth segment of Quantou Formation is interbedded sand, silt and mudstone. The grain composition of sand is quartz, feldspar and debris, and includes a little steady heavy mineral. The content of Quartz is more than others and the content of feldspar and debris in the samples are equivalent. The main composition of debris is granite intermediate-acidic volcanics and metamorphic rock. This composition is similar with the lithology of north provenance.By comprehensive study of deposition characteristic(lithology, thickness ratio between sand and strata, sedimentary structure, framework characteristic), electrometry characteristic and reservoir characteristic(physical property), It shows that is shallow water lake delta sediments and the subfacies delta front and delta plain is main facies. Delta front include microfacies of underwater distributary channel, underwater interdistributary, mouth bar and distal bar Delta plain chiefly include distributary channel microfacies and interdistributary microfacies.From bottom to top, the subfacies of forth segment in Quantou Formation is from delta plain to delta plain. The FI1-FI3 sublayers mainly develop delta front subfacies, while the FI5-FI7 sublayers mainly develop delta plain subfacies.The mouth bar and distal bar sedimentary microfacies of delta front always appear in the FI1 sublayer,while we can see mouth bar in the FI2 sublayer in south parts of area. The logging traces of mouth bar and distal bar always take on obviously dental finger-like and funnel-shaped. Distal bar microfacies is above the mouth bar microfacies according to facies sequence on the top of in this area. The overlying layer of forth segment of Quantou Formation is the first segment in Qingshankou formation which develops predelta subfacies.Underwater distributary channel microfacies always appears on FI1-FI3 sublayers and is under the mouth bar and distal bar microfacies. The logging traces of underwater distributary channel microfacies always take on obviously campaniform. The isogram map of thickness ratio between sand and strata show that underwater distributary channel stretches in NS direction whichis is similar to the direction of provenance. Between underwater distributary channel microfacies is underwater interdistributary which is composed of deep gray mudstone or siltston and always develop in the FI2-FI3 sublayers.FI4 sublayer is transitional layer between delta front subfacies and delta plain subfacies. Jin76, Jin73, Gu72, Gu705 well etc of south mainly develop delta front subfacies, but FI4 sublayer in north area mainly develop delta plain subfacies which are composed of amaranth, celadon mudstone and siltstone.Distributary channel and interdistributary microfacies of delta plain mainly develop in FI5-FI7 sublayers. According to the isogram map of thickness ratio between sand and strata of delta plain show that the thickness ratio between sand and strata of delta plain is lower than it is in delta front. At the same time the lithology of delta plain subfacies including amaranth mudstone and a little gypsum vein show that the weather in this period is dry and torrid. The provenance is also inadequate and the scale of water body is small.The reservoir conditions of sandbody of underwater distributary channel are best in all sandbody and its porosity and permeability is the highest in all sandbody. Distributary sanbody is comparative lower than it. However, (underwater) interdistributary have the lowest porosity and permeability in this region because its content of argillized is much higher.The underwater distributary channel sand body is also the best oil-bearing formation in all sandbody which is not only owing to reservoir condiditions, but also due to sand body in the top of forth segment of Quantou formation that is much closer to generating rock of first segmen of Qingshankou formationt.