Research On Sedimentary Facies And Petroleum Accumulation Regularity Of The Main Oil-bearing Layers Of The Lower Yanchang Formation In Ansai Area

While the exploration and development of the middle-upper part of the Triassic Yanchang Formation in Ansai area is coming to an end,the exploration of new layer began to shift to the lower of Yanchang oil-bearing formation.Due to the short exploration and development process,the small exploration area,many basic geological problems such as sedimentary facies,distribution of sand body and hydrocarbon source rock,etc,have not been solved.Consequently,it is necessary to analyze the distribution of sedimentary facies and petroleum accumulation in new layer,and to provide theoretical support for the further exploration and development.This study is based on the study of the main oil-bearing formation of logging data and analysis data,combining with the core observation record,to draft the characteristics of the top surface structure and paleotectonic,and then to define the distribution of the main oil-bearing formation's sand body,sand body structure,sand ratio and sedimentary facies.Finally,according to the distribution characteristics of the abnormal pressure and the reservoir,summarize the petroleum accumulation regularity and favorable exlopration areas of the main oil-bearing layers of Xiazuhe.Using the results of stratigraphic division and the attributes of the wellhead to draft the top serface structure,it has the lower tectonic relief,as a west dipping monocline,gradient per kilometer between 6-10m,developing six nose uplift structures.And then,using the settlement compensation principle to recovery paleotectonic of the main oil-bearing formation in the late of middle Jurassic,Its tendency is opposite to the present structure,as the West High East low monoclinic structure.Developing three ancient tectonic ridges in the area,its existence indicates the direction of the oil and gas accumulation.The study area developed delta-plain facies in the Chang 101 depositional period,which the main sedimentary microfacies are distributary channel,interdistributary bay,crevasse splay,peat bog.The delta-front facies developed in Chang 91 and Chang 82,which the main sedimentary microfacies are underwater distributary channel,underwater distributary Bay,channel mouth bar and sheet sand.The study area's main oil-bearing formation are mainly lithologic reservoirs.The approved reserve of Chang 82 is in the Xing28 well area,and the approved reserves of Chang 91 and Chang 101 are in the south region of Zhuanyaowan.By analyzing the effects of sand body distribution,hydrocarbon source rock distribution,abnormal pressure characteristics and paleotectonic characteristics on oil accumulation,combined with oil exploration data of exploration wells in Changqing Oil Field,the results show that the petroleum accumulation regularity is near hydrocarbon source rock,superior sedimentary facies,low abnormal pressure and palaeotectonic ridge.According to the results of this study to predict the following six favorable areas:west of Xing28 well in Pingqiao district,southeastern Huaziping of X6173 well district,S383 well district in northwest of Zhenwudong,Zhuanwayao district,S279 well district in west of Yanhewan and XZ6 well district in southwest of Louping.