准噶尔盆地玛湖凹陷异常高压分布和形成机理

Abnormal Pressure Distribution and Formation Mechanism in Mahu Sag,Junggar Basin

准噶尔盆地玛湖凹陷三叠系及其以下中—深部地层异常高压发育,为分析其形成机理,利用实测地层压力、声波时差和地震等资料,结合沉积和构造背景,借助单井压实曲线分析、单井地层压力预测和盆地模拟技术,研究了玛湖凹陷地层压力的分布特征、成因机理和保存条件。研究表明,玛湖凹陷不同地区超压顶面出现的层位不同,凹陷西北部侏罗系及以上地层多未出现超压,凹陷东南部和达巴松凸起地区侏罗系已经开始出现超压;异常高压的主要成因是快速沉积欠压实作用,生烃增压作用贡献小,凹陷边缘断裂带附近异常高压形成的主要因素是断裂活动引起的他源超高压;三叠纪至今剩余压力演化可以分为4个阶段:无剩余压力段、剩余压力缓慢增加段、剩余压力快速增加段和剩余压力缓慢递减段;玛湖凹陷独特的构造和沉积演化,具备了异常高压流体封存箱的顶板、底板和边板条件,异常高压得以保存至今。

The Triassic and deeper formations in Mahu sag in Junggar basin are of abnormal high pressure which is one of the important factor controlling the oil and gas accumulation.This paper studied the distribution,origin and preservation condition of the abnormal high pressure using measured formation pressure,interval transit time and seismic data,combined with the sedimentary and tectonic settings as well as well compaction curve and well pressure prediction and basin simulation techniques.The result shows that the top surface of overpressure appears in different formations in different areas in Mahu sag.Its Jurassic and newer strata may not have overpressure in northwestern Mahu sag,but the overpressures are found in the Jurassic of southeastern Mahu sag and Dabasong swell.The main formation mechanism of the abnormal high pressure is undercompaction due to rapid sedimentation,while the contribution from organic material maturing is not big.The major factor of the abnormal high pressure formation near the fault zones in margin of the sag attributes to the other source overpressure that may create from the fault activity.The residual pressure evolution since the Triassic can be divided into four stages：free residual pressure stage,slowly increasing stage of it,rapid increasing stage and slowly decreasing stage.It is the unique sedimentary and tectonic evolution of Mahu sag that forms the conditions of top,bottom and edge plates of the abnormally pressured fluid compartment,which allows the abnormal high pressure to be kept to present.