The Study On Sedimentary Microfacies And Prediction Of Favorable Reservoir Zones Of Yingcheng Formation In Shiwu Oil-field Of Songliao Basin

Shi Wu has become one of the most important oil fields after years of exploration in Songliao basin. With deepgoing of the exploration, many basic geological problems become very prominent, which affect the production and sustainable development of the oil field. The types of the microfacies and source areas of the depositional system are complex, which involves comparison of the thin layers. Therefore, it is necessary to identify the microfacies and their distribution, depositional system type of each sandset and the distribution of depositional systems, then to find the accurate correlation of the thin layers, in order to achieve favorable exploration belts and targets.1. Method of the ResearchThere are excellent well logging and core data to identify sedimentary facies, subfacies and microfacies in this area.For each sequence (sandset), statistics of the thickness of sandstone and mudstone, stratal thickness and sand content rate are used for mappings, to cycle the distributional area of mudstone and sandstone. Combining with the analysis of seismic attribute slices, the preponderant microfacies are defined to realize their distribution and go to a step further to unveil the spatial and temporal distribution of the sedimentary facies. The character of sand content rate and particle size distribution are analyzed and types of rock are counted to define the distribution of the deposition systems. In the framework of sequence stratigraphy, the oil layers (thin layers of sand) are contrast accurately. On the basis of the research, some thickness statistics of the sandstone show ing hydrocarbon in the study wells are gathered for the purpose of the facies belt which match with favorable reservoir well.2. Sequence Stratigraphical DivisionFive sand members of YingCheng Formation in Shiwu oilfield (YingⅤis considered to be ShaⅠ) can be equivalent to a certain level of stratigraphical unit. According to the chron duration, YingCheng formation in ShiWu oil-field is divided into four third-order sequences, corresponding to the sand member, and the YingⅤ(ShaⅠ) is considered to be another third-order sequence. There are five maximum flooding surfaces, developing between the transgressive system tracts and the highstand system tracts in these five third-order sequences. The larger scale of lake-floodings in Yingcheng formation occured in YingⅢand YingⅣ.The maximum flooding developed during the development of YingⅣ.3. Research on Sedimentary MicrofaciesFour sedimentary facies can be identified in Yingcheng formation (including ShaⅠ),which are alluvial fan,fan delta, delta and lacustrine facies. Alluvial fan developed in YingⅤ(ShaⅠ), middle - distal fan ( subfacies) and three microfacies such as debris flow,braided channel and sheet flow are recognized.Delta( fan delta) developed in the whole Yingcheng formation (including ShaⅠ), including delta (fan delta) front (subfacies) and eight microfacies such as subaqueous channel, subaqueous gravity channel (existing in fan delta), mouth bar, gravity mouth bar (in fan delta), sand sheet, distal bar, inter-lobe and interdistributary. Lacustrine facies developed throughout the whole Yingcheng formation (including ShaⅠ), including lake and semi-deep-lake (subfacies) and three microfacies such as mud of still water,turbidity current and subaqueous fan are identified.These fourteen types of sedimentary microfacies have their corresponding response logging curves.The amplitude attribute can be applied in the recognition of the sedimentary facies.There are four amplitude types of the sand members of Yingcheng formation in Shiwu fault depression,which are the high amplitude seismic facies, high-media amplitude seismic facies, media-poor amplitude seismic facies and poor amplitude seismic facies. High-amplitude seismic facies is explained as the braided channel,mouth bar of short axis delta or alluvial fan. High-media amplitude seismic facies corresponds to the braided channel, mouth bar of long axis delta or subaqueous fan. Media-poor amplitude seismic facies are on behalf of the interdistributary- sand sheet or subaqueous fans.The mud of still water-turbidity current has poor amplitude.4.Distribution of the Preponderant Sedimentary MicrofaciesThe preponderant facies are channel-mouth bar, subaqueous fan and mud of still water-turbidity in Ying V(ShaⅠ). There are subaqueous fans in the north, east and south and deltas in the northwest and southwest.On southeast side of Xiaokuan fault, there is subaqueous fan sand. Mud of still water-turbidity facies covers the rest of the study area.The preponderant facies are distributary channel-mouth bar, subaqueous fan, interdistributary-sand sheet and mud still water–turbidity in Ying IV. The prograded deltas are in the northwest and southwest, and the prograded fan deltas are in the north, east and south. Interdistributary-sand sheet occupies most of the study area. Mud still water -turbidity distributes in the center of the lake.The preponderant facies are distributary channel-mouth bar, subaqueous fan, interdistributary-sand sheet and mudstone of still water- turbidity in Ying III. There are four deltas or fan deltas in the north,west,south and southwest.The interdistributary -sand sheet is between the northern delta and fan delta in the east (including two lobes of delta in the north). There are three subaqueous fans in the center, and the left large area is mudstone of still water-turbidity facies zone.The preponderant facies are distributary channel-mouth bar, subaqueous fan, interdistributary-sand sheet and mudstone of still water-turbidity in Ying II. Fan deltas develop in the east and south, and deltas develop in the north and southwest. Interdistributary-sand sheet distributes mainly around the north delta. Most of the region is covered by mudstone of still water-turbidity. There is a subaqueous fan in the northeast.The preponderant facies are distributary channel-mouth bar, subaqueous fan, interdistributary-sand sheet and mudstone of still water-turbidity inYing I. There are fan deltas in the north, east and south, and deltas in the southwest. The interdistributary -sand sheet distributes in the main area of northwest.There are different sizes of subaqueous fans in the central area.5. Types and distribution of depositional systemsBased on the lithological analysis of the Yingcheng Formation (including ShaⅠ), five different types of lithological area can be delineated, which are the volcanic area, recycle clastic area, clastic area affected by granite rock, mixed area and carbonate area.The depositional systems are as follows: the northern fan delta or delta system with volcanic source,the eastern fan delta system with recycle clastic source,the southern fan delta system with volcanic and granite source,the southwest delta system with the granite source,the northwest delta system with recycle clastic source and mixed source. The distribution of the sedimentary facies and systems was reconstructed by the Xiaokuan fault in the study area.6.The Prediction of Favorable Reservoir Facies BeltThe statistical result shows that the first-class facies for reservoir is the delta, which is followed by the subaqueous fan and fan delta, the worst is the alluvial fan. The optimal reservoir rock are mouth-bar sand and distributary channel sand of delta front, subaqueous fan sand and mouth-bar sand of fan delta front, among which the mouth-bar and distributary channel of delta are the most favorable reservoir sands.