Study On Sequence Stratigraphy And Evolvement Of Sediment In The Central Depression Of The Northern Songliao Basin

In recent years, with the deepening of geological studies such as sequence stratigraphy and fine sedimentary facies, Fuyu oil layer exploration has made significant breakthroughs in the north of Songliao Basin. Large lithology reservoir was found in Sanzhao sag. In order to make a breakthrough in more places, there is an urgent need to deepen the understanding of petroleum geologic characteristics of the member 4 of the Quantou Formation in the central Depression of the northern Songliao Basin. The article study on the sequence and sedimentary facies, which is based on the petroleum geology theory, analysis technology of high resolution sequence stratigraphy and core, well drilling , well logging and seismic data.Based on a large number of core observations, the types of rock facies and sedimentary facies are highlighted. It indicates that middle-scale trough cross stratification fine sandstone facies and small-scale trough cross stratification siltstone facies, totally 12 types of rock facies are developed, different types of rock facies combined with a certain way to constitute different types of sedimentary facies. The study area developed meandering river, shallow-water delta and shallow lake, totally 3 types of sedimentary facies, including 8 kinds of intrafacies, 20 kinds of microfacies.Rely on the core, logging and seismic data of the study area, the recognizable indicators of high-resolution sequence interface of the member 4 of the Quantou were established. Through the single well sequence identitication and cross well stratigraphy comparison, the strata framework of high-resolution sequence stratigraphy was built. The member 4 of the Quantou was divided into three forth-order sequences, which were named Q4-sq1, Q4-sq2 and Q4-sq3 from the bottom to top. Each forth-order sequences reflected once medium-term base level cycles.The sediment source of the study area come from the northern, eastern, western and southern. There are three main types of stone, such as Fine sandstone, siltstone and mudstone . Shallow-water delta was the main sedimentary facies. In the vertical, the sedimentary of the member 4 of the Quantou shown an obviously retrogradational sequence from the bottom to the top.The range of shallow-water delta and shallow lake expanded gradually. The range of meandering river gradually shrank to the fringe of the study area. In the plane, meandering river, shallow-water delta and shallow lake have a circularity distribution from margin to center. The shallow-water delta was mainly distributed in the central Depression . The shallow lakes was mainly distributed in Qijia-Gulong Sag.Following the study on sequence and sedimentary, combining all conditions of hydrocarbon accumulation, we analyzed the rule petroleum distribution. Hydrocarbon source rocks and the cover of Fuyu oil layer was the member 1 of Qingshankou Formation, which controls the distribution of oil and gas in macroscopy. The areas where reservoir physical property is good, fault-T2 developed well and shallow burial, are best area to oil and gas storage . The map of sand thickness isogram and sandstone percentage isogram revealed that the sandstone mainly distributed around the study area. The thickness of sandstone is thin and sandstone percentage is small in the center of the study area. Sandstone percentage determines the type of reservoir. The mainly type of reservoir is lithologic oil pool in study area. The subsidiary reservoir is structural- lithologic oil pool. Through statistical distribution regularity of commercial oil and gas well in different sedimentary microfacies, the member 4 of the Quantou, oil and gas pools in study area are mainly allocated in channel sandstones, which including point bar of meandering river, delta plain distributary channel sand body and delta front underwater distributary channel sand body. All of three sedimentary microfacies are the most favorable reservoir to oil and gas storage.Considering all the factors,compared with Sanzhao Sag, the beneficial exploration area was forecasted in Q4-sq1 and Q4-sq2 sequence, in Qijia-Gulong Sag. The burial of the forecastable area is shallower,foult-T2 developed well, the thicker sandstone and higher sandstone percentage accorded with the request of the lithologic and structural- lithologic oil pool, two exploration wells indicate that the oil exist is inevitabily. This place is the transition zone of delta plain and delta front, where the distributary channels are developed well. So, we believe that this is a good place for oil storage. Qijia-Gulong Sag have the largest lake in Q4-sq3 seqence. Around the lake, there are many commercial oil well distribute in the delta front subaqueous distributary channel . It is benefit for oil accumulation in these place. Future, we should strengthen exploration around the lake.