渤海湾盆地沾化凹陷渤南洼陷古近系深层优质储层形成机制

Formation mechanism of high-quality reservoirs in deep strata of Paleogene, Bonan Subsag, Zhanhua Sag, Bohai Bay Basin

随着勘探程度的提高,渤海湾盆地渤南洼陷油气勘探的主要对象逐渐由中浅层转向中深层。深层勘探的关键问题是优质储层,其发育取决于沉积充填和地层流体的相互作用。古近系沙河街组四段沉积时期,该区发育了咸化环境湖相碎屑岩与膏盐层沉积。利用钻井、实验等基础资料,分析储层沉积环境、地层水特征、岩石学特征及成岩作用类型,探讨深层优质储层发育演化机制。研究结果表明:(1)在分布面积广、沉积厚度大的膏盐层的封闭下,咸化湖泊环境的沙四段碎屑岩,早成岩期经历了碱性成岩环境,形成的碳酸盐胶结物充填原生孔隙,并有效阻滞压实作用的发生;(2)随着烃源岩中的有机质趋于成熟,中成岩期有机酸在异常高压的驱动下向地层中运移,早期碳酸盐胶结物以及长石、岩屑普遍被溶蚀,形成次生孔隙,酸碱交替作用是深层优质储层发育的主要机制;(3)优质储层的发育受到膏盐层分布、烃源岩发育、断裂体系与碎屑沉积体展布的控制,受酸碱流体影响强烈的缓坡扇三角洲和陡坡近岸水下扇中前端砂体,是渤南洼陷沙四段优质储层发育区。

With the continuous improvement of exploration degree, the main object for oil and gas exploration in the Bonan Subsag of Bohai Bay Basin gradually changes from medium and shallow layers to medium and deep layers. The key factor for deep exploration is the development of high-quality reservoir, which depends on the interaction between sedimentary filling and formation fluid. During the deposition of the fourth member of Paleogene Shahejie Formation, lacustrine clastic rocks and gypsum salt deposits developed in this area under a saline environment. Based on the comprehensive study of drilling data and experimental results, the sedimentary environment, formation water characteristics, reservoir petrographic characteristics and diagenetic types of the Paleogene reservoirs in the Bonan Subsag were discussed, the development and evolution mechanism of the deep and high-quality reservoirs in Paleogene was also indicated. Results show that:(1) Under the sealing of gypsum salt layer with wide distribution area and large sedimentary thickness, the clastic rocks of Es4member in salty environment experienced alkaline diagenetic environment in the early stage. The carbonate cements formed in this period filled primary pores and effectively prevented the occurrence of compaction.(2) With the increase of the maturity of organic matters in source rocks, significant organic acid migrated to stratum driven by abnormal high pressure during the middle diagenesis stage. The early carbonate cements, feldspar and debris were generally dissolved and secondary pores were subsequently formed. Acidic and alkaline reworks are the main mechanism for the development of deep high-quality reservoirs.(3) The development of high-quality reservoirs was controlled by the distribution of gypsum salt layer, the development of source rock, fault system and the distribution of clastic sedimentary body. The front sand bodies of gentle-slope fan delta and steep-slope nearshore subaqueous fan strongly influenced by acid and alkali fluid are the high-quality reservoir development area of Es4member in Bonan Subsag.