苏北盆地金湖凹陷古近系戴南组孔隙演化及次生孔隙成因分析

金湖凹陷戴南组储层总体为一套中低孔-中低渗碎屑储层,次生孔隙是本区最有效、最重要的孔隙类型,而搞清次生孔隙的分布规律及其成因成为下一步有利储层预测的关键。文中综合利用岩心、普通薄片、铸体薄片、扫描电镜及物性参数等资料,对戴南组储层的岩石类型、物性特征、孔隙类型及次生孔隙分布规律进行了综合分析,并从溶蚀作用的发育条件入手探讨了次生孔隙的成因,以期为下一步有利储层预测和勘探开发提供科学依据。研究认为,金湖凹陷戴南组碎屑岩储层主要发育于三角洲、扇三角洲和滨浅湖,受沉积相带控制,储层类型复杂,以不等粒砂岩、含砾不等粒砂岩、细砂岩及粉砂岩为主,不同地区岩性存在差异。根据岩性三角图,砂岩类型以长石岩屑质石英砂岩为主。砂岩成分成熟度中等偏低,结构成熟度中等。戴南组砂岩孔隙度峰值分布在12.0%~14.0%,渗透率峰值分布在1~10 mD。不同沉积环境和地区储层物性也存在一定差异。三角洲前缘亚相的储层物性最好,平均孔隙度约19.4%,平均渗透率约134.2 mD,岩性以细砂岩为主,主要分布于研究区西部和中部。扇三角洲物性次之,平均孔隙度约12.5%,平均渗透率约6.2 mD,岩性以含砾不等粒砂岩为主,位于桐城断裂带东南部便1井附近。滨浅湖亚相的储层物性最差,岩性以粉砂岩为主,平均孔隙度约8.3%,平均渗透率约2.3 mD。戴南组储层孔隙分为原生孔隙和次生孔隙两大类,以次生粒间溶蚀孔隙为主。溶蚀孔隙包括粒间溶孔、粒内溶孔和铸模孔。粒间溶孔多由粒间碳酸盐胶结物如方解石、白云石、铁方解石等溶蚀后形成,粒内溶孔主要是长石及碳酸盐岩屑被选择性溶解而形成,可见白云石晶粒溶解留下的铸模孔。戴南组储层中的原生孔隙相对较少,主要以石英次生加大后的残余粒间孔的形式存在,发育于埋藏较浅的井如新庄1井、关1-1井。戴南组储层孔隙经历了由原生到次生的演化过程。总体上,在浅于约1 100 m,储层主要处于早成岩A阶段,以原生孔隙为主。在1 100~1 500 m,储层处于早成岩B阶段,形成混合孔隙段。超过1 500 m,储层进入中成岩A阶段,原生孔隙消失殆尽,基本上以次生孔隙为主。戴南组储层存在3个次生孔隙发育带:第一次生孔隙发育带在1 200~1 600 m,分布于埋藏相对较浅-中等的井区;第二次生孔隙发育带分布在1 800~2 800 m;第三次生孔隙发育带在2 900~3 000 m左右。第一、二次生孔隙发育带次生孔隙的绝对值较大,说明溶蚀作用较强。戴南组储层次生孔隙发育与方解石、白云石胶结物的溶蚀及长石碎屑和碳酸盐岩岩屑的溶蚀有关。烃源岩成熟排烃是次生孔隙发育的主控因素;碳酸盐胶结物发育提供了次生孔隙发育的物质基础;长石的溶蚀对次生孔隙发育有一定的贡献;次生孔隙的形成与黏土矿物的相互转化有一定的关系;断裂活动进一步促使了次生孔隙的发育。

Pore evolution and genesis of secondary pores in the Paleogene Dainan Formation,Jinhu Sag,Subei Basin

The reservoir of the Dainan Formation, Jinhu Sag is a set of clastic reservoirs with medium-low porosity and permeability. The secondary pore is the most important and effective pore type in the study area, and the determination of its genesis and distribution is the key step for predicting favorable reservoirs. Based on core, thin section and cast thin section data, scanning electron microscope imaging, and physical property measurements, we analyzed the rock type, reservoir property, pore type and distribution of secondary pores, and discussed the genesis of secondary pores in association with the dissolution conditions in this area, which will provide scientific guidance for predicting favorable reservoir and for reservoir exploration. Our study showed that clastic reservoirs mainly deposited in delta, fan delta and show-shallow lakes. Controlled by sedimentary facies, the reservoir rocks are complex and include mainly unequal-grained sandstone and pebbly sandstone, fine sandstone and siltstone, showing varying lithologies in different areas. According to the triangular plot, the sandstone is mainly feldspathic debris quartz sandstone. The compositional maturity is medium to low and the textural maturity is medium. The porosity and permeability peak values were 12.0%-14.0% and 1-10 mD, respectively. There are some differences in reservoir properties associated with different sedimentary facies and regions. The reservoir properties of delta front are most favorable, with an average ~19.4% porosity and ~134.2 mD permeability, and the lithofacies is mainly fine sandstone distributing in the western and central areas. The fan delta near Well Bian-1 in the southeast of the Tongcheng fault zone cames in second. The fan sandstone is gravel bearing mix-grained sandstone with an average porosity of ~12.5% and permeability of ~6.2 mD. The reservoir property of shore-shallow lake is least favorable. It deposited mainly siltstone with an average porosity of ~8.3% and permeability of ~2.3 mD. There are two types of pores, primary and secondary pores, in the Dainan Formation. The dissolution pores included inter and intragranular dissolution pores and mold pores. The intergranular dissolution pores such as calcite, dolomite, iron calcite, etc., formed after dissolution of intergranular carbonate cements. The dissolution pores in grains mainly formed by selective dissolution of feldspar and carbonate debris. The mold pores left by the dissolution of dolomite grains could be seen. There are few primary pores in the Dainan Formation, mainly in the form of residual intergranular pores after quartz enlargement and developed in shallow buried wells, such as Wells Xinzhuang-1 and Guan-1-1. The pores evolved from primary to secondary pores in the Dainan Formation. On the whole, reservoir buried less than 1100 m is mainly in the early diagenetic stage A and mainly developed primary pores; reservoir buried between 1100-1500 m is in the early diagenetic stage B with mixed pore section; and reservoir buried below 1500 m entered the middle diagenetic stage A when secondary pores replaced primary pores. There are three secondary pore developing zones in the reservoir. The first one is between 1200-1600 m and distributed in the relatively shallow to moderately buried well area; the second one is between 1800-2800 m; and the third one is between 2900-3000 m. Among them, the absolute secondary porosity of the first and second secondary pore developing zones is relatively high, indicating strong dissolution. The secondary pore development in the Dainan Formation was related to the dissolutions of calcite and dolomite cements, feldspar and carbonate debris. Mature hydrocarbon from source rocks charging into the reservoir was the main controlling factor for the secondary pore development, as carbonate cements provided the material basis for the dissolution, and the dissolution of feldspar in turn contributed to the development of secondary pore. The formation of secondary pore was also related to the transformation of clay minerals, and fault activity further promoted the development of secondary pore.