Sedimentary System And Exploration Prospect Analysis In Chang-9Oil Reservoir Of Ansai Oilfield

Ansai Oilfield is located in the middle part of Yishan slope (including the junction of2Provinces and1Autonomous Region such as Shaanxi, Gansu and Ningxia Hui Autonomous Region) of Ordos Basin, spanning several counties such as Ansai, Zichang, and Zhidan that belongs to Shaanxi Province. Tectonically, it is southerly middle east of northern Shaanxi slope, west dipping and homocline sloping.Ordos Basin is a sedimentary basin with multi-directional provenances. In late Triassic epoch, the main sources of the depositional stage of Chang9oil-bearing formation were the five provenance systems located in northeast, north, northwest, southwest and south. The provenance systems of the northern basin mainly developed from the three directions of the northeast, north and northwest. The research area lies in the intersection of the three provenance systems, in which there has been controversial issues over the developmental state and extended distance as well as no clear understanding over the distribution of intersection. Some think only the provenance in the northeast can be developed but others believe the same thing can be done in the north and northeast. The confirmation will not only have a big impact on the study of sedimentary systems in Ansai-Nanliang areas but provide a basis for decision-making for the choice of reservoir forming and exploration direction as well.This study takes the Chang9oil-bearing formation as an end which includes the big areas such Ansai, Nanliang, Jingwu, Baibao, Dingbian and Yanding as backgrond research, but Ansai and Nanliang are the target areas of research. We have referred to the research results of those predecessors over the extended set of stratigraphic subdivision standard, sedimentary facies, reservoir stratum and reservoir forming. We need to carry through stratigraphic subdivision and make a comparison based on the existing drillings according to sequence stratum and accumulated sequence patterns, ascertaining the distribution characteristics of stratum. Taking the heavy minerals as initial value, we combine clastic rocks and development characteristic of coal leads to determine the direction of provenance systems; using lithology and logging information to further analyze the distribution of the sedimentary system within the framework of sequence research so as to find out the law for sandstone development and distribution. We may employ drilling data to conduct the tectonic analysis for sandstone top surface and make entrapment analysis and evaluation. By means of dissecting typical oil deposit, we establish the relationship among the oil, sedimentary sand body and entrapment, ascertaining the law of petroleum entrapment to set up a pattern. On the basis of finding out the law of rich accumulation oil and gas, at last we use the research results of deposition, entrapment and reservoir forming to summarize the prediction model of favorable block and again use the model to make a prediction to the favorable block in the range of research area, pointing out the next advantageous exploration target. We have come to a conclusion and had a main understanding via the research on the Chang9oil-bearing formation of triassic system in the research area as follows:1、The southeast part in Ansai area is the delta front of distributary channel deposit that belongs to the northeastern provenance system and the partial layers in the northwest belongs to deposition of northern provenance system. The delta front of distributary channel deposit for the northwestern and northern provenance system is found out.2、As a whole, there are feldspar lithic sandstone and lithic feldspar sandstone in Ansai and Nanliang areas, arkose, lithic sandstone and quartz sandstone do not develop; The clastic particle size distribution has similar regularities of distribution as the depositional system; the cement is mainly hydromica, ferrocalcite and laumontite, siliceous and calcite taking the second place; poor reservoir property. In Ansai area there mainly is the feldspar dissolved pore, next the intergranular hole and other type of pores with a small amount of development while there is no development of pores in Nanliang area, but the intergranular holes and feldspar dissolved pores can be seen.3、There are total33entrapment developing in the research area, in which there are10actual entrapments and11evaluating entrapments and12perspective entrapments. Ansai developed2entrapment forms that include1evaluating entrapment and1perspective entrapment. And also there are4developing entrapment forms in Nanliang area,2actual entrapments,1evaluating entrapment and1perspective entrapment.4、The "theory of loop bay" proposed said that the dead color mudstone in the arm of lake grows hydrocarbon, nearby migration gathering surrounding the arm of bay or favorable lithologic trap in the arm of bay; more arms of lake form multiple spot hydrocarbon generation and reservoir forming model within the source accumulation. The sand belt of provenance system in the northwest research area can be accumulated entirely, the provenance system east branch in the north can be accumulated, but the sand belt of provenance system in the northeast may not be easily accumulated.5、The Nanliang area is rich in oil and gas that are lithologic deposit, which mainly distribute in the Chang92sand group and Chang91taking the second place. From northwest to southeast the north line of noon38well-noon219well-noon39well is the most favorable area, and suggesting Nanliang area along the northwest line of noon38well-noon219well-noon39well to develop towards the northwest, make a roll prospecting towards the southeast. The main part of Ansai lies below the northeastern provenance system, in which the area goes against the oil and gas accumulation. Among the areas the most unfavorable area lies in the southeast, and relatively favorable area is in partial layering of northwest, suggesting the exploration should be focused on the northwest.