Study On Sequence Stratigraphy And The Sedimentary Characteristics In Dongying Fm., Gangdong Oil Field

Beidagang Structure Belt, which lying between the Banqiao and Qikou source depression, is in the south of Dagang district, Tianjin City, locating in the middle of Huanghua depression, with the Dazhangtuo fault to the north, Qikou depression to the south, and connecting with Qian'an and Qijiawu, Dazhongwang to the west, 5m water depth line in Bohai to the east. Beidagang Structure Belt, with the NE-trending, is one of the most important gas exploration and development areas in bohai bay basin. Gangdong Oil Field Development Area which has rich hydrocarbon resources, is in the east of Beidagang Oil Field, connecting with Qikou source depression, there are some lithologic reservoirs in Minghuazhen Fm., Guantao Fm. and Dongying Fm.. This paper mainly plans to take Dongying Fm. in Gangdong Oil Field Development Area for example, and make the research on sequence stratigraphy and the sedimentary characteristics. The purpose is through applying the seismic and logging information, combining with the basic idea of the sequence stratigraphy, to build the isochronous stratigraphic framework by the third sequence as residiential unit, and by investigating the lateral distribution of each formation unit and reservoir sand, to make the foundation for the sedimentary facies division.The seismic and logging information is indicaed that there are five unconformity surfaces in Dongying Fm., Gangdong Oil Field. With these unconformity surfaces and the corresponding conformity surfaces, the research area can be divided into four stratigraphic sequences, Sequenceâ… (the third member in Dongying Fm.), Sequenceâ…¡(the second member in Dongying Fm.), Sequenceâ…¢(lower of the first member of Dongying Fm.) and Sequenceâ…£(upper of the first member of Dongying Fm.)The analysis for sequence stratigraphy in single well is base on the characters of logging and lithology, and the standard of the discriminant feature for system tract, this is called"point"analysis; then, making the well tie correlation with the single well to study the distribution in the line of the sequence; then building the isochronous stratigraphic framework through combining the profile of the connected well and the seismic.On the base of the isochronous stratigraphic framework, three kinds of sedimentary pattern are recognized, they are subaqueous fan, lake facies and delta facies. The subaqueous fan contents fan-root, fan-middle and fan-end, the lake facies is the deep-moderately deep lake facies, and delta front and pro-delta constitute the delta facies. With these sub-phase, several tens species micro-phase are recognized.With the depth of strata, sandstone and mudstone in each member which statisticaled from the logging data, we can discuss the sedimentary character in various periods though the contour map made by surfer. The principle to compile the plane distributive maps of sedimentary facies is"thickness map of the sandstone controls the scope; percentage map of the sandstone controls the provenance; analysis of the single well controls the micro-phase". The third member mainly develops the subaqueous fan and lake facies; the second member develops the subaqueous fan, pro-delta and lake facies; lower of the first member's sedimentary facies contains delta and lake facise, and upper of the first member is the same with the lower one.The third member of Dongying Fm(Sq1)., which is in the early stage of uplifting and shrinkage, developed with lake facies, and Cangxian uplift provided the mainly provenance; The second member of Dongying Fm(Sq2) is in a intense fault depression stage, the deposition rate rise, the water body became deeper, as lacking the supply of the clastic materials, lake facies became the most develop sedimentary system; lacustrine basin began to atrophy in the lower of the first member(Sq3) ,with the less strong uplifting, the shore line started to shrink, the delta scale were extending; lower of the first member(Sq4) is in the end stage of uplifting and shrinkage, as the increase of degree for uplifting, the scale of the water body reduced, and the delta developed more completely.