桂中坳陷及周缘海相地层油气保存条件

Hydrocarbon preservation conditions in marine strata of the Guizhong Depression and its margin

桂中坳陷及周缘原始生油条件优越,但经历多期构造运动,油气保存条件受到严重破坏,因此油气保存条件成为制约本区勘探决策的关键因素之一。笔者从动态演化角度对盖层、构造作用以及盆地构造演化过程中油气保存条件演变的地下水化学—动力学行为响应特征等与油气保存条件有关的几个方面进行了综合研究。桂中坳陷及周缘主要发育下、中、上泥盆统和下石炭统4套泥质岩盖层,盖层封闭性属中等—好,且坳陷东部泥质岩更发育。燕山运动与喜马拉雅运动使得桂中坳陷东部、东南部遭受强烈抬升和剥蚀,断裂发育,大气水下渗强烈且范围广、持续时间长,而对坳陷中西部影响较弱。现今坳陷中西部地层埋深小于800m的浅部地层,大气水下渗作用强烈,地层水普遍为低矿化度水,属于自由交替带;800m以下地层进入交替阻滞带,油气保存条件逐渐变好,推测交替停滞带在2 000m以下。坳陷东部和东南部大气水下渗深度更大。总体上,桂中坳陷及周缘现今油气保存条件整体已受到了一定的破坏,坳陷中西部相对较好。

There are favorable conditions to petroleum generation in the Guizhong Depression and its margin,but the condition of hydrocarbon preservation has been greatly damaged due to experiences of multi-phase tectonic actions in the past.The present paper comprehensively and dynamically studied caprocks,tectonic actions,conditions of hydrocarbon preservation,and characteristics of groundwater chemical-kinetic behaviors responding to alterations of the hydrocarbon preservation condition during tectonic evolution of the basin.There developed four suits of argillaceous caprocks in the Guizhong Depression and its margin,namely the Lower,Middle and Upper Devonian and Lower Carboniferous covers with moderate to good sealing,in addition,mudstones were thicker in the eastern depression.The Yanshan-Himalayan movement caused the periphery of the Guizhong Depression,especially the eastern and southeastern depression,to have intensively uplifted and been denuded since Jurassic,where faults developed well and more meteoric water infiltrated widely and persistently,however,comparably the central and western depression was less affected by this movement.Strata shallower than 800 meters in the present central and western depression generally have formation water with low salinity and belong to a free-exchanging zone due to the strong infiltration of meteoric water.The hydrocarbon preservation condition meliorates gradually as the strata with burial depth over 800 meters enter into an exchanging-retarded zone,and the exchanging dead zone is presumed to be buried below 2000 meters.The depth of meteoric water infiltration is much greater in the periphery of the depression,especially in the eastern and southeastern depression.As a whole,the present condition of hydrocarbon preservation in the Guizhong Depression and its margin has been destroyed to some extent,while that in the central and western depression is comparatively good.