东营凹陷民丰洼陷北带沙四下亚段深层天然气储层成岩作用

Diagenesis Characteristics of Deep Natural Gas Reservoirs in Es_4x in the North Zone of Minfeng Sag in Dongying Depression

由于对东营凹陷民丰洼陷北带深层天然气储层成岩作用特征及其对储层的控制作用认识不清,给该地区天然气勘探带来了极大的风险。综合运用铸体薄片、扫描电镜观察、流体包裹体分析及岩心物性分析等多种技术方法,对东营凹陷民丰洼陷北带沙四下亚段深层天然气储层成岩作用特征及其对储层物性的影响进行了系统的研究。结果表明,该区北带沙四下亚段深层天然气储层为近岸水下扇砂砾岩,成岩作用具有强压实、欠压实、多期溶解、多期胶结、复杂交代和灰泥组分重结晶的特征。主要胶结-溶解作用序列为长石溶解/石英次生加大→碳酸盐胶结/石英溶解→碳酸盐胶结物溶解/石英次生加大→黄铁矿胶结/沥青充填。近岸水下扇不同亚相(微相)沉积特征的差异性导致了埋藏过程中成岩作用的差异性,并决定了储集物性的差异性。扇根砾岩在埋藏过程中成岩作用以压实作用和灰泥杂基重结晶作用为主,溶解作用微弱,物性持续降低。在中深层灰泥杂基的重结晶作用使扇根砾岩物性快速降低而形成致密层,扇根砾岩全部为干层。由于早期异常高压和早期油气充注对储层保护以及酸性溶解对储层改造的综合作用,扇中辫状水道远离泥岩、砂砾岩储层在中深层仍发育大量原生孔隙和少量的次生孔隙,储层物性较好,可作为良好的天然气储层。扇中辫状水道靠近泥岩层储层、扇中水道间以及扇缘部位因发生强烈胶结作用,在砂体顶底形成致密的胶结壳,可作为较好的盖层。

The diagenetic characteristics of deep natural gas reservoirs and the control of diagenesis on reservoirs in the lower part of the Fourth Member of the Shahejie Formation （Es 4 x）in the north zone of the Minfeng sag in the Dongying depression haven’t been clearly understood,which have led to great risks to hydrocarbon exploration in this area.Based on a combination of thin-section identification, scanning electron microscope （SEM） observation,fluid inclusion analysis and an analysis of core properties,it has been systematically studied about the diagenetic characteristics and its influence on the reservoir property. Nearshore subaqueous fan glutenite is the main regional reservoirs, and the diagenesis is characterized by strong compaction,poor compaction,dissolution and cementation of many stages,complex metasomatism and lime-mud matrix recrystallization.The main cementation-dissolution sequence is feldspar dissolution/quartz overgrowth → carbonate cementation / quartz dissolution →carbonate dissolution /quartz overgrowth → pyrite cementation and bitumen filling.The difference of sedimentary characteristics in different sub/micro-facies of nearshore subaqueous fan controls the difference of diagenetic characteristics, which also determines the difference of reservoir physical property. The conglomerate in inner fan mainly experiences compaction and lime-mud matrix recrystallization,with weak dissolution,and its physical properties keep reducing.The lime-mud matrix recrystallization results in a rapid reduction of physical properties in middle-deep layers.The present physical property of inner fan becomes extremely poor and mainly develops dry layers.Because acid dissolution reconstruct the reservoirs and early hydrocarbon filling and abnormal high pressure can protect reservoirs,the reservoirs far away from mudstone layers in the braided channels of middle fan develop a great number of primary pores and a few secondary pores,have a higher porosity and permeability,and can be effective reservoirs for natural gas.The parts in interdistributaries of middle fan and outer fan develop intensively cementation,it forms dense cemented crusts at the top of the sandstone,and therefore they mainly act as cap rocks.