Study On Sedimentary Facies Of Cretaceous Formation In Chepaizi Area

The Chepaizi area is located on the Chepaizi uplift in the northwestern margin of junggar basin.Cretaceous is the key exploration horizon at this area,but there are still many problems in the study of sedimentary facies.the types of sedimentary facies are different,the spatial distribution rules are not clear,and the paleogeomorphologic characteristics are not clear.To solve these problems,The study is guided by the theory of sequence stratigraphy and sedimentology,To study cretaceous sequence strata,sedimentary facies,sedimentary facies distribution,sedimentary evolution model,sedimentary relative oil distribution control in Chepaizi area by using seismic,well logging,drilling and other data,the main achievements and experiences has the following several aspects:(1)Established the cretaceous sequence stratigraphic framework again,From the bottom up,It is divided into four three-level sequences: Qingshuihe formation,Hutubi formation,Shengjinkou formation and Lianmuqin formation,and each three-level sequence was further divided into lake invasion system domain and lake recession system domain.A total of 17 sand formations were divided in the system domain.(2)Assured the paleogeomorphologic characteristics and source direction of the Cretaceous sedimentary period in the study area,Which is high in the northwest and low in the southeast,and the northwest area is uplift,which is the area where cretaceous denudation and pinching occurred;Adjacent to the study area is a slope zone,which is the area of rolling topography.East-Southeast for the depression zone,the terrain is relatively flat,has reached the shallow lake-half deep lake area.(3)Defined The signs and types of cretaceous sedimentary facies,and expounded the spatial distribution of sedimentary facies.the rock facies indicate that the study area is a set of lacustrine transgression sedimentary system with mudstone as the main body,the logging facies and the seismic facies indicate the typical fan delta deposits and beach bar deposits.lake facies and fan delta facies are mainly developed in the study area.The fan delta facies is dominated by the sub-facies in the front of the fan delta.Lake facies includes bar-sand subfacies,beach-sand subfacies and shallow lake subfacies,and there are microfacies such as main body of bar side edge of bar,beach ridge,beach mat and shallow lake mud.Fan delta is mainly developed in the lower part of qingshuihe formation in Pai63 and Pai607 well area.The beach bar sandbody is developed in the well area of Pai639,Pai611 etc,and is distributed in a strip along the lake coastline in intermittent distribution.(4)Revealed the evolution of Cretaceous sediments and the corresponding sedimentary models in the study area.the sedimentary models is the fan delta-beach bar sedimentation model,the period of Qingshuihe group,alluvial fan enter to lake basin to form fan delta deposit,The beach bar sandbody is formed by the transformation of the fan body and the clastic material by the lake wave;From Hutubi formation to Shengjinkou formation,the fan delta disappearanced,and large scale beach and dam deposits were formed along the lake shoreline.During the Lianmuqin formation period,the lake basin shrank and the beach-bar sand bodies were distributed in small scale and isolated in the lake basin.(5)Expounded the controlling effect of sedimentation on the distribution of oil and gas,pointed out the prospect block.the underwater distributary channel in the front of fan delta and beach bar sand body are considered to be the most prospect reservoir in the study area.Underwater distributary channel sand bodies and unconformity interface in the high position of the structure formed stratigraphic-lithologic reservoirs.In the low position of the structure,the microfacies of the main body of the bar and the side margin of the bar controlled the formation of lithologic pinchout and sand lens reservoirs,among which the favorable areas are Pai607 and Pai611 well area.