| The Ordos Basin is an important energy base in China. With the rapid development in recent years, the total oil and gas output of Changqing oilfield exceeded40million tonnes of oil and gas equivalent in2011. Its output is expected to rise to50million tonnes of oil and gas equivalent in2013, with proven oil reserves of2.4billion tons in Mesozoic. To achieve the target of50million tonnes of oil and gas equivalent for the realization of oil and gas equivalent and the continued rapid growth of the resource requirements, it is urgent to seek and find new areas to succeed.The study area is located in the southwest of the Ordos Basin, north to Huanxian, south to Zhenyuan, west to the edge of the basin, east to Maling. Total area is approximately12,000km. The reservoir is located at Yishan slope and Tianhuan depression, portion in Qingyang nose. Preliminary exploration practices and studies have shown that the distribution of sand body is thick, deep buried in Zhenbei-Huanxian area. They are well exploration prospects and efficiency reserves that looking for high equivalent. The reservoir analysis of Chang8formation which has been found in Xifeng Oilfield shows that, as the fracturing technology advances and well production is constantly refreshed, exploration prospects is optimistic despite of poor physical property and low level of equivalent. In recent years, Chang7reservoir exploration constantly found predict the prospect is good in Chang6-Chang8reservoir which are important exploration areas in Mesozoic oil.This article aims to restore paleoporosity on basis of pressure and paleostructure, using "Philippone" formula to strike palaeopressure, simulating the evolution of formation pressure, providing a preliminary reference for hydrocarbon migration and possession. Summary oil accumulation and reservoir formation, select favorable exploration targets and guide further exploration.Using logging, core analysis, physical testing and other data, analysis super-low permeability reservoir, restore palaeopressure and paleoporosity of the accumulation period in specific, combine with oil accumulation and reservoir formation, select favorable exploration targets and guide further exploration. The concrete application of research methods including dual function of porosity evolution, paleopressure restoring method and the interval transit time method.Paleoporosity and its evolution of sandstone reservoir is a hard and key task for studying the mechanism of hydrocarbon accumulation. Current methodology has some limitations. It can not take all controlling factors into consideration, the application process is too complicated, the precision of result is low and it can not embody the whole process of porosity evolution. Based on the project which studies on the mechanism of hydrocarbon accumulation within Chang Formation in Zhenjing area of Ordos Basin, a newly methodology to restore the paleoporosiy of sandstone is presented. This methodology is based on diagenetic history of strata and takes constructive and deconstructive diagenesis of porosity into account. With the effect-oriented simulation as the principle, the simulation process is divided into porosity decrease process and increase process. With the geological time as variable and constrained by the current porosity, the model demonstrates dynamic and reliable characteristics. The process of porosity evolution of sandstone is a key control factor for a high-production hydrocar-bon accumulation.The shale within Chang7of Yanchang formation in Triassic is an important source rock of Mesozoic reservoirs in Ordos Basin. The paleopressure in the period of accumulation plays a vital role for the hydrocarbon migration, and the changes of paleoporosity represent the characteristics of the paleopressure evolution. So according to the mechanism of hydrocarbon fluid expulsion and overpressure release from the period of accumulation to the maximum burial depth, this article presents a new method that can extrapolate the paleoporosity of shale within Chang7in the period of accumulation on the basis of the relationship between porosity increment and the maximum depth. Through analyzing the characteristics of present shale compaction profiles, there is an exponent relationship between the porosity increment and the maximum depth of under-compacted shale within Chang7. Then by using this relationship, the increment of shale porosity in the period of accumulation can be extrapolated and by combining the porosity of normal compaction trend, the total porosity of under-compacted shale within Chang7in the period of accumulation can be restored effectively.Studies suggest that:1In order to analysis pressure of Chang6-Chang8in Zhenbei-Huanxian area, the interval transit time method is used for calculating porosity. The pressure of Chang6-Chang8is less than hydrostatic pressure, coefficient nowadays is less than1while Chang7Member is high pressure, coefficient is greater than1, in line with the actual geological data.2The research of sandstone and mudstone compaction model shows that there are mainly four denudation in this study districts. Because of the differences between compaction model in eastern and western regions, we separate the study area into two parts.3Determine the deepest geological age is100Ma through the analysis of burial history in multiple wells.4The reservoir sandstone is typical low porous and low permeable reservoir rock with characteristic of fine grain size, low composition maturity, poor roundness and middling sorting. The analyses on core, grain size, thin section, shape of the log curves show that the area has been to middle A diagenesis stage.5The relationship of nowadays porosity and depth shows that porosity simulation process is divided into porosity decrease process and increase process. Establish a dual function model to simulate the development process and restore paleoporpsity in specific period, then calculate paleopressure by using "Philippone" formula, combined with paleostructure and source rock to select favorable exploration zones. |
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