| The study on the cause of overpressure is the basis of pressure prediction and hydrocarbon migration and accumulation.At present,the identification and quantitative evaluation methods for the cause of overpressure are becoming more and more experienced,but the identification of different types of overpressure and the quantitative evaluation of their contribution under the action of multiple overpressure mechanisms still need to be studied comprehensively.Overpressure is common in all reservoirs in Kuqa depression.A series of tectonic activities since the Himalayan period have complicated the causes of overpressure,including overpressure caused by the imbalance between compaction and drainage,overpressure caused by tectonic compression,and overpressure caused by fault transfer.The identification and quantitative evaluation of different types of overpressure are still in the exploratory stage.The Kuqa depression is located in the foreland compression background of the southern margin of Tianshan mountains.The overpressure caused by the above three causes in Kuqa depression is identified and evaluated by various methods,and a more practical overpressure evaluation model is established for quantitative evaluation.In this paper,the overpressure caused by the imbalance of compaction and drainage in Kuqa depresssion,tectonic compression and fracture transfer are identified and quantitatively evaluated by using the basic data of logging,drilling,analysis and testing and the previous research results.In order to make up for the fact that the measured data is insufficient,the distribution characteristics of the present pressure are analyzed accurately with the corrected mud data.The overpressure caused by the unbalance between compaction and drainage,tectonic compression and fracture transfer were identified by various methods.Application of equivalent depth method to evaluate the overpressure caused by unbalanced compaction and drainage;Application of new model that only consider line strain under lateral shortening tectonic compression to evaluate the overpressure caused by the tectonic compression;Application of the improved Bowers model to evaluate fracture transfer overpressure.The above method was applied to YN 2 Well to quantitatively evaluate the overpressure formation.According to the study,(1)the current pressure distribution in different regions of Kuqa depression is different,and the maximum residual pressure in keshen,kela and dabai can reach 60 MPa,80MPa and 50 MPa respectively.(2)the contribution of overpressure caused by the imbalance between compaction and drainage to overpressure of the reservoir varies in different regions.The undercompaction is weak in the kela,keshen and dabei regions,generally around 10 MPa,while the residual pressure range in the dina region is 10-50 mpa,and the residual pressure range in the dibei-yinan region is 0-30 Mpa.(3)the overpressure range produced by the extrusion of the Ahe formation is 0-30Mpa;(4)the fault-transfer overpressure is obvious in the areas of keshen,kela,dabei and dina,and the range of fault transfer overpressure is 10-30 Mpa,but no obvious transfer phenomenon is found in the areas of dibei and yinan.(5)Dina area suweiyi group due to the overlying gypsum salt layer high rupture strength,make almost no pressure loss in the historical process,the amount of residual stress formation overpressure and current error within 5% of the residual stress,which caused by unbalanced compaction and drainage the contribution of the overpressure in the range of 29%— 53%,the contribution of overpressure caused by fracture transmission range is 23% —34%,tectonic compression caused by overpressure contribution under 10%;However,the total residual pressure of the overpressure of the ahe formation in the north-yinan area is smaller than that of the present,which is due to the loss of pressure in the geological history,and this phenomenon is closely related to the rupture of the top seal of the ahe formation. |
PDF Full Text Request